Hot subluminous stars in GALEX

A selection of hot subluminous stars in the Galex survey
I. Correlation with the Guide Star Catalog thanks: Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programmes 82.D-0750, 83.D-0540, and 085.D-0866.

S. Vennes , A. Kawka22footnotemark: 233footnotemark: 3, and P. Németh22footnotemark: 2
Astronomický ústav AV ČR, Fričova 298,CZ-251 65 Ondřejov, Czech Republic
Department of Physics and Space Sciences, 150 W. University Blvd, Florida Institute of Technology, Melbourne, FL 32901, USA
E-mail: vennes@sunstel.asu.cas.cz (SV); kawka@sunstel.asu. cas.cz (AK); nemeth@sunstel.asu.cas.cz (PN) Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.
Abstract

We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical, and infrared source list. The selection is secured using colour criteria that correlate well with effective temperatures K. We built a versus diagram for bright sources using the Galaxy Evolution Explorer (GALEX) magnitude (), and the associated Guide Star Catalog (GSC2.3.2) photographic quick-V magnitude and the Two Micron All Sky Survey (2MASS) J and H magnitudes. This distillation process delivered a catalogue of sources with comprising known hot subdwarf stars and another known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional new hot subdwarf candidates and hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than . Our sample of spectroscopically confirmed hot subdwarfs comprises ten sdO stars and 42 sdB stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical, and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix.

keywords:
stars: fundamental parameters – subdwarfs – ultraviolet: stars – white dwarfs
pagerange: A selection of hot subluminous stars in the GALEX survey I. Correlation with the Guide Star Catalog thanks: Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programmes 82.D-0750, 83.D-0540, and 085.D-0866.Bpubyear: 2010

1 Introduction

Surveys of blue- or ultraviolet (UV)-excess objects are a rich source of hot subluminous stars and are beneficial to the study of hot subdwarf and white dwarf stars. Following the pioneering work of Humason & Zwicky (1947), colourimetric surveys, such as the northern and southern Tonantzintla survey (TON/TON-S, see Iriarte & Chavira, 1957) and the Palomar-Haro-Luyten survey (Haro & Luyten, 1962) provided for many years the source lists for spectroscopic observations and analysis of faint blue stars at high-Galactic latitude (e.g., Greenstein, 1966; Greenstein & Sargent, 1974).

The spectroscopic observation and spectral classification of samples of subluminous stars (e.g., Humason & Zwicky, 1947; Feige, 1958; Cowley, 1958; Slettebak, Bahner, & Stock, 1961; Berger, 1963) helped Greenstein & Sargent (1974) define the extended (or extreme) horizontal branch (EHB) in the HR diagram. Their diagram comprising 189 faint blue stars shows a sequence of hot white dwarfs ranging from to 56,000 K and overlapping with a sequence of sdO stars in the higher range of temperatures and luminosities (). The evolutionary tracks of Paczyński (1971a, b) suggested for the first time that helium stars with masses in the range and without hydrogen envelopes evolve from the EHB toward the white dwarf cooling sequence. However, based on EHB space density and birthrate estimates, Heber (1986) concluded that the EHB channel contributes only a small percentage of objects on the white dwarf cooling sequence with the majority following the channel connecting planetary nebula nuclei to the white dwarfs (Drilling & Schoenberner, 1985).

Greenstein & Sargent (1974) also found that most EHB stars have weak helium and weak heavy element lines, hence the label population II or “halo”, and are classified as sdB stars in contrast with the helium-rich sdO stars. In addition, the luminosity-to-mass ratio , or its reciprocal (where ), appeared approximately constant among these objects. Applying a model atmosphere analysis to their sample, Greenstein & Sargent extracted temperature () and surface gravity () measurements and, since , they estimated , or for a representative mass of . The amount of hydrogen present in the star was unknown although they surmised that the nearly constant luminosity-to-mass ratio implied a thin hydrogen envelope.

The ground-based blue-excess surveys were soon followed by space-borne UV surveys. The TD-1 satellite conducted a systematic, all-sky survey (Thompson et al., 1978; Carnochan & Wilson, 1983) that included the Galactic plane. The survey was conducted in four bands, one photometric band centred on 2700 Å and three low-resolution spectroscopic bands centred on 1565, 1965, and 2365 Å. The original catalogue of Thompson et al. (1978) includes 31215 entries with the catalogue prefix “TD1”, out of which Carnochan & Wilson (1983) listed 464 objects with UV colours of spectral types B4V or earlier. Among these objects, 47 were labelled as subdwarfs (“sd”). The spectroscopic identifications were secured optically. For example, Berger & Fringant (1980) listed 28 hot subluminous stars, thirteen of them new objects listed with the “UVO” catalogue prefix.

The sdB and sdO stars detected in the TD-1 have visual magnitudes ranging from 8 to 12 and are, to this day, the brightest stars of their class. For example, the brightest star in the sample is the sdO HD 49798 (Jaschek & Jaschek, 1963; Mereghetti et al., 2009) with a UV flux at 2740 Å of erg cm s Å, or mag in the AB magnitude system111.. The next two brightest evolved stars are HD 76431 (Berger & Fringant, 1980; Ramspeck, Heber, & Edelmann, 2001) and BD+75 325 (Gould, Herbig, & Morgan, 1957) with mag. Few hot white dwarfs like UVO2309+10 (BPM 97895, GD 246; Luyten, 1963; Giclas, Burnham, & Thomas, 1965) were originally listed by Carnochan & Wilson (1983), but TD-1 UV photometry eventually led to discoveries such as the white dwarf companions to the F7 II star HR 3643 and the F4 V star 56 Persei (Landsman, Simon, & Bergeron, 1996). In summary, the TD-1 UV catalogue includes objects with mag, and, after early-type main-sequence stars, it is populated mostly with hot sdB and sdO stars.

Furthermore, the Orbiting Astronomical Observatory-2 (OAO-2, Code, Holm, & Bottemiller, 1980) delivered a catalogue of 531 pointed UV sources that contained few white dwarfs (e.g., Feige24, Holm, 1976) and hot subdwarfs. The Astronomical Netherlands Satellite (ANS) also produced a catalogue of 3573 pointed UV sources (Wesselius et al., 1982) listing hot white dwarfs, analysed by Wesselius & Koester (1978), and about three times as many hot subdwarfs. These experiments pre-selected bright subdwarfs with and erg cm s Å corresponding to and 14.4, respectively. Although the OAO-2 and ANS reached fainter magnitudes, the all-sky coverage of the TD-1 survey allowed for the systematic identification of new subluminous stars. Ultraviolet flux measurements of subluminous stars are also listed in the Midcourse Space Experiment (MSX) UV catalogue (Newcomer et al., 2004).

Large catalogues of subluminous stars were first built based on extensive blue and UV-excess surveys such as the Palomar-Green (PG) survey (Green, Schmidt, & Liebert, 1986), the Kitt-Peak-Downes (KPD) survey (Downes, 1986), the Edinburgh-Cape (EC) survey (Kilkenny et al., 1997), the Montreal-Cambridge-Tololo (MCT) survey (Lamontagne et al., 2000), or the First Byurakan Survey (FBS, Mickaelian, 2008). These and similar catalogues provided the basis for our current understanding of EHB and post-EHB stars, as well as post-asymptotic giant branch (AGB) stars.

Detailed spectroscopic studies based on the PG survey (Moehler, Heber, & de Boer, 1990a; Saffer et al., 1994) placed hot sdB stars in the diagram close to the evolution tracks of Caloi (1989) for core helium-burning stars with very thin hydrogen envelopes and masses near . Recent spectroscopic investigations of the Hamburg-Schmidt (HS) subdwarf catalogue (Edelmann et al., 2003) using the evolutionary models of Dorman, Rood, & O’Connell (1993) confirmed the basic picture. High mass-loss rates are possibly responsible for thinning the hydrogen envelope and preventing the star from climbing the asymptotic giant branch (D’Cruz et al., 1996). Other formation scenarios examined by Han et al. (2002, 2003) involve a common-envelope phase or an episode of Roche lobe overflow that may help remove the hydrogen envelope and direct the star toward the EHB as originally proposed by Mengel et al. (1976), or involve the merger of two helium white dwarfs. Indeed, Maxted et al. (2001), Morales-Rueda et al. (2003), and Edelmann et al. (2005) found that a significant fraction of these stars reside in spectroscopic binary systems in support of the scenario for the formation of sdB stars through close binary evolution.

The evolutionary paths leading to the formation of helium rich subdwarfs (sdO) are less certain although the helium double-degenerate mergers (Han et al., 2002, 2003) or helium flash/mixing (Sweigart, Mengel, & Demarque, 1974; Sweigart, 1997a, b) may account for the paucity of hydrogen. Analysis of helium-rich subdwarfs (sdO) from the PG survey (Dreizler et al., 1990; Thejll et al., 1994) suggest that sdO stars evolved away from the EHB. Recently, Stroeer et al. (2007) draws further distinction between helium-deficient sdO stars and helium-enriched sdO stars, with the former linked to sdB stars and the latter possibly formed in the merger of helium white dwarfs, or via helium mixing. Detailed abundance studies (e.g., Lanz et al., 2004; Ahmad et al., 2007) offers insights into the formation of helium-rich subdwarf stars involving helium mixing. Based on a population study Zhang, Chen, & Han (2009) conclude that many sdO stars evolve directly from sdB stars, and that only the more massive sdO stars () are the outcome of white dwarf mergers.

The majority of white dwarfs are formed through the post-AGB channels (Drilling & Schoenberner, 1985; Heber, 1986). Initial estimates of the death-rate of main-sequence stars and of the birth-rate of white dwarf stars show broad agreement between the rates although they were still affected by considerable uncertainties (see Drilling & Schoenberner, 1985). Post-AGB stars are powered by a hydrogen-burning shell in thermal equilibrium interspersed with thermal pulses (helium-shell flash). The evolutionary tracks of Schoenberner (1983) connect the AGB stars to the white dwarfs by invoking a phase of enhanced mass-loss (or super-wind) that occurs between pulses, i.e., during the hydrogen shell-burning phase. Higher mass-loss rates imply shorter evolutionary time-scales and, therefore, the assumed rates affect the growth of the carbon/oxygen core and determine the final mass of the nascent white dwarf. Weidemann (2000) compared available data to the results of model calculations (e.g., Vassiliadis & Wood, 1993) linking the initial main-sequence mass to the final white dwarf mass. Observations of white dwarfs in stellar systems where evolutionary time-scales are measurable, i.e., clusters (e.g., Kalirai et al., 2008) or evolved binaries, provide the data and constrain the initial-mass to final-mass problem (Weidemann, 2000). Properties of white dwarfs in spectroscopic and colourimetric studies are parametrized by and that are transformed into ages () and masses (), hence radii (), using evolutionary models (e.g., Wood, 1995; Benvenuto & Althaus, 1999).

Extensive colourimetric surveys such as the PG survey helped establish the character of the white dwarf population such as its local space density and formation rate (Green, 1980; Liebert, Bergeron, & Holberg, 2005) while deeper surveys such as the Two-Degree Field (2dF) survey allowed to probe the spatial distribution of the white dwarf population and determine its Galactic scale-height (Vennes et al., 2002). Deep and extensive colourimetric surveys such as the Sloan Digital Sky Survey (SDSS, e.g., Eisenstein et al., 2006) and, potentially, Galaxy Evolution Explorer (GALEX) all-sky survey combined with kinematical data (e.g., Salim & Gould, 2003; Lépine & Shara, 2005) should help retrace the Galactic history, i.e., birth rates and time scales, of the white dwarf population.

We have initiated a program aimed at identifying the white dwarf population in the GALEX all-sky survey. As a first instalment, we present a new sample of bright subluminous stars based on GALEX and matched with optical/infrared source catalogues (Section 2). The sample is particularly suitable for GAIA Calibration (Soubiran et al., 2008) and adds material for surveys of pulsating, hot subdwarfs (Billères et al., 2002; Østensen et al., 2010). We describe the source selection in Section 2.1, the spectroscopic follow-up in Section 2.2, and the model atmospheres employed in the analysis in Section 2.3. Next, we present the bright () catalogue in Section 3.1 and a model atmosphere analysis of the new subdwarfs and white dwarfs in sections 3.2 and 3.3, respectively. We conclude in Section 4. Photometric ultraviolet, optical, and infrared absolute magnitudes, suitable for the study of hot white dwarfs in multiwavelength surveys, are listed in Appendix A.

2 A Galex/gsc survey

2.1 Source selection

Figure 1: Distribution of the GALEX/GSC selection in Galactic coordinates. The selection of 63691 stars with GALEX mag is restricted to Galactic latitudes - and occupies an area of 25000 square degrees.

We obtained UV photometry from the Galaxy Evolution Explorer (GALEX) all-sky survey. GALEX provides photometry in two bands, and . The bandpass is Å (defined at % peak value) with an effective wavelength of 1539 Å, and the bandpass is Å with an effective wavelength of 2316 Å (see Morrissey et al., 2007). We searched the GALEX GR4+5 release using the CasJobs batch query services with the following criteria:

and we extracted the parameters ra, dec, nuv_mag, nuv_magerr, fuv_mag, fuv_magerr, fov_radius, and e_bv. The right ascension and declination (ra, dec) supplied by GALEX are J2000. The and magnitudes (nuv_mag, fuv_mag) and associated errors (nuv_magerr, fuv_magerr) are defined in the AB system. We opted for magnitudes calculated using the SExtractor “mag_auto” with a variable elliptical aperture rather than a fixed aperture. The distance of the source from the image (or tile) centre (foc_radius) is measured in degrees, and each tile has a field of view of 1.25 in the or 1.27 in the . Finally, the total extinction in the line-of-sight (e_bv) is based on the maps of Schlegel, Finkbeiner, & Davis (1998). However, reddening corrections were not initially applied because we lack a priori knowledge of the distance and spectral type.

Figure 2: Colour indices ( versus ) of 60379 stars from the joint GALEX/GSC survey. The locations of the main-sequence (full line) and white dwarf cooling sequence (dashed line, ) are shown. Contour lines (dotted lines) show the number density of stars (per square magnitude) at 1%, 3%, 10%, and 30% of the peak density. The corresponding white dwarf temperature scale is shown on the left, and the main sequence spectral types on the right. No corrections are applied for interstellar reddening.

A selection of bright stars with GALEX suffers potential photometric inaccuracies. First, the imaging quality degrades toward the edge of the tile (Morrissey et al., 2007) altering the point spread function (PSF). Next, the corrections for photometric nonlinearity are uncertain and depend on the aperture size. This effect, compounded with the broadening of the PSF for off-centre sources, introduces a systematic offset between predicted and measured magnitudes. In practice, we applied the nonlinearity correction for a 3 aperture and labelled as potentially inaccurate the photometry of sources located more than 0.4 from the tile centre. The requirement that the detection be included could also result in many bright objects being dropped from the list. A -only selection that may potentially triple the membership to the bright source list will be examined elsewhere.

Entries with coordinates matching within degrees were merged and the search resulted in the selection of individual sources. Figure 1 shows the source locations in Galactic coordinates. The GR45 release covers square degrees and excludes the Galactic plane.

Figure 3: Reduced proper-motion diagram of a sample of UV-excess objects with . The scales on the right-hand side show the corresponding absolute V magnitudes for transverse velocities of, from inside to outside, 10, 25, and 50 km s. The samples of known hot subdwarfs (left) and white dwarfs (right) are shown with open circles and comprise 159 and 61 objects, respectively. The white dwarf sequence at (dashed lines) is shown for tangential velocities of, from top to bottom, 10, 25, and 50 km s. No corrections are applied for interstellar reddening.

Next, we cross-correlated the source list with the Guide Star Catalog version 2.3.2 (GSC2.3.2, Lasker et al., 2008) using VizieR at the Centre de Donnés astronomiques de Strasbourg222CDS available at http://cdsweb.u-strasbg.fr/CDS.html (Ochsenbein, Bauer, & Marcout, 2000).. The closest GSC entry found within a radius of 10 arcseconds of the GALEX coordinates is thereby associated with the UV source. The joint GALEX/GSC survey comprises sources, or 96% of the total, in the 3 sample and sources in the 10 sample. For each object we recorded both UV magnitudes (, ) and we adopted the “quick-V” () magnitude which is close to the Johnson magnitude, (Russell et al., 1990). Following the same positional criteria we also obtained the infrared photometry from the Two Micron All Sky Survey (2MASS; Skrutskie et al., 2006), which is available at VizieR on CDS.

Figure 2 shows a versus diagram of our GALEX/GSC selection. The data are compared to synthetic main-sequence and white dwarf colours. The procedure selects objects with temperatures above 12,000 K, or spectral types earlier than B8.

Finally, we built a sample of UV-excess objects by restricting the index . The choice of the index as a primary selection criterion is motivated by the need to mitigate the effect of binary companions on the selection of white dwarf stars. Although the 2MASS magnitude is generally more reliable than the GSC magnitude it is potentially contaminated by a putative main-sequence companion to the target white dwarf star (see Section 3.3).

We obtained proper-motion for each object from the Naval Observatory Merged Astrometric Dataset (NOMAD Zacharias et al., 2004) accessed at VizieR. Then, we calculated the reduced proper-motion index :

where is the proper-motion in units of arcsecond yr, and is the transverse velocity in km s. Figure 3 shows the index versus the colour index for the sample of 694 objects with, on the left panel, the subset of known subdwarfs marked, and, on the right, the known white dwarfs also marked. A white dwarf sequence at (see Appendix A) is drawn covering the temperature between and 84000 K at three representative transverse velocities. Figure 4 shows the corresponding H distribution functions for the known white dwarfs and subdwarfs. Hot white dwarfs dominate the number count at while hot subdwarfs are predominant in the range . The H distribution functions are converted into a transverse velocity distribution by calculating the absolute magnitude for each object and solving for the velocity.

Greenstein & Sargent (1974) already suggested that stars on the EHB have similar luminosities. Based on values of 2.35 (Greenstein & Sargent, 1974), 2.28 (Moehler, Heber, & de Boer, 1990a)333Edelmann et al. (2003) state that the Strömgren calibration used by Moehler, Heber, & de Boer (1990a) may have been inappropriate for sdB stars which may explain their lower mean value for . Nonetheless, we include the value in the average and account for the possible uncertainties., 2.64 (Saffer et al., 1994), and 2.73 from the population synthesis of Han et al. (2003), we adopted a mean for the subdwarfs. We found that the transverse velocities follow a distribution of the form with km s. Based on a similar sample of hot subdwarf stars, Thejll et al. (1997) concluded that these objects belong to the old thick Galactic disc. Similarly, Altmann, Edelmann, & de Boer (2004) found that 87% of their sample of sdB stars belong to the thick disc with the remainder in the Galactic halo.

Next, we estimated for each white dwarf by adopting a mean mass of and by estimating using the colour index. We used the mass-radius relations of Wood (1995) and Benvenuto & Althaus (1999). The resulting white dwarf velocities follow the same relation but with km s. The kinematics of hot white dwarfs also suggest that they belong to the old disc (Sion et al., 1988).

The apparent deficit in blue (), luminous () stars in the upper-left corner of Figure 3 is caused in part by interstellar extinction affecting more distant stars and in part by the relative rarity of luminous blue stars. Using the parametrization of Cardelli, Clayton, & Mathis (1989), we estimate while . For a hypothetical, distant star with , the ultraviolet-optical colour correction would be . The corresponding shift in colour would displace an intrinsically blue object in Figure 3 close to a magnitude toward the right. This effect, not apparent at lower luminosities, helps draw further distinctions between luminous and subluminous stars that are not affected to the same degree by interstellar extinction.

Figure 4: Distribution of (left) and corresponding transverse velocities (right) for hot subdwarfs (top) and white dwarfs (bottom). The velocity distribution functions follow , with the best-fit values given in the upper right corner above the distributions.

The information gathered from the sample of known objects is useful in distinguishing the white dwarf and subdwarf populations based on the observed distribution.

2.2 Follow-up optical spectroscopy

We obtained low dispersion spectra of subluminous candidates using the EFOSC2 (ESO Faint Object Spectrograph and Camera) attached to the New Technology Telescope (NTT) at La Silla Observatory, and using the RC Spectrograph (Ritchey-Chretien Focus Spectrograph) attached to the 4-m telescope at Kitt Peak National Observatory (KPNO).

At the NTT on UT 2008 October 19-22, 2009 March 2-4, and 2009 August 23-27 we employed the grism #11 (300 lines per mm) with a dispersion of Å per binned pixel (). The slit width was set at resulting in a resolution of Å. At the NTT on UT 2010 March 2-4 we employed grism #7 (600 lines per mm) with a dispersion of Å per pixel. The slit width was set at resulting in a resolution of Å. Finally, at KPNO on UT 2010 March 23-26 we employed the KPC-10A grating (316 lines per mm) with a dispersion of 2.75 Å per pixel in first order and centred on 5300 Å. The slit width was set at resulting in a spectral resolution of Å.

Our spectroscopic observations are progressing toward decreasing values. The sample is completed and the sample with is 80% completed. We obtained high signal-to-noise spectra suitable for detailed line profile analysis. This analysis is based on the calculation of model grids and detailed synthetic hydrogen and helium line profiles.

2.3 Model atmospheres

We computed a grid of high-gravity models in local thermodynamic equilibrium (LTE) suitable for the analysis of hydrogen-rich white dwarfs. The models are in convective/radiative equilibrium and assume plane parallel geometry. The model opacities include hydrogen bound-bound, bound-free, and free-free absorptions, as well as H bound-free and free-free absorptions at lower temperatures. The model opacities also include the hydrogen Rayleigh scattering and the electron scattering. The grid covers a range of parameters suitable for the analysis of hydrogen-rich white dwarfs with from 7000 to 100000 K, from 5.5 to 9.5, and a pure hydrogen composition. Detailed hydrogen line profiles are computed using the tables of Lemke (1997). Kawka & Vennes (2006) provide additional details of the model calculations444Recent improvements in hydrogen line broadening theory taking into account non-ideal effects (Tremblay & Bergeron, 2009) could result in upward revisions of our and measurements by up to 10% and 0.15 dex, respectively.. We describe in Appendix A and employ in Section 3.3 a set of absolute magnitudes computed using these white dwarf models.

Next, we computed a grid of non-LTE model atmospheres and synthetic spectra covering a range of parameters () suitable for hydrogen and helium-rich subdwarf stars using the codes Tlusty-Synspec (Hubeny & Lanz, 1995; Lanz & Hubeny, 1995). The models are regrouped in three overlapping grids with grid number 1 suitable for sdB stars covering from 21000 to 35000 in steps of 1000 K, from 4.5 to 6.25 in steps of 0.25 dex, and from -4 to -1 in steps of 0.5 dex. Similarly, grid number 2, suitable for sdB and sdO stars, covers from 25000 to 45000, from 5.0 to 6.25, and from -3 to 0. Finally, grid number 3, suitable for helium-rich stars, covers from 34000 to 60000 in steps of 2000 K, from 5.0 to 6.25, and from 0 to 2. The hydrogen, neutral helium, and ionized helium models comprise 9, 24, and 20 energy levels, respectively. Figure 5 shows model spectra representative of the sdB and sdO classes.

Figure 5: Model spectra for sdB stars (left panel), He-sdB stars (middle panel), and He-sdO stars (right panel) extracted from the main model grid.
Figure 6: NTT and KPNO spectra of bright sdB and sdO stars from the GALEX selection ranked according to temperatures from the coolest star (lower left) to the hottest star (upper right).

3 A Catalogue of subluminous stars

3.1 Sample of bright () subluminous candidates

RA (J2000) Dec (J2000) 555From NOMAD (Zacharias et al., 2004). Name Type Telescope
(mag) (mag) (mas yr)
0 13 01.0 +72 31 19 11.97 11.49 NGC 40 PNN
2 10 22.9 +29 27 27 12.32 11.91 uvby98 610242080 B 3.58m
2 45 24.7 +32 45 29 11.45 11.33 TYC 2329-941-1 B 3.58m
2 45 56.8 +11 50 15 11.55 11.87 B 3.58m
3 15 08.3 +14 53 49 11.45 10.97 US 3783,HIP 15137 B 666The sdB classifications of Mitchell (1998), Wegner & Swanson (1990), and Wagner et al. (1988) for these objects are not confirmed. 3.58m
3 21 39.8 +47 27 17 11.91 11.70 Cl Melotte 20 488 sdB 777New subdwarfs in close binaries, see Kawka et al. (2010a). 4.0m
4 01 05.4 32 23 48 11.03 11.20 CD-32 1567 sdB 3.58m
4 24 41.2 20 07 12 12.13 11.92 IM Eri NL
4 47 04.2 67 06 52 11.61 11.26 HD 270754 B1.5Ia 888B1.5 Ia star in the LMC (Feast, Thackeray, & Wesselink, 1960) with a B2Ia ultraviolet classification (Smith Neubig & Bruhweiler, 1999).
5 05 30.7 +52 49 50 10.63 11.74 G191-B2B DA WD 999DA white dwarf WD 0505+527.
6 06 13.3 20 21 07 11.81 11.80 CPD-20 1123, Albus 1 He-sdB 101010The blue star Albus 1 (Caballero & Solano, 2007) was identified as a sdB-He star by Vennes, Kawka, & Smith (2007).
6 36 46.5 +47 00 25 11.91 11.88 B 4.0m
6 39 21.4 +33 23 52 11.77 11.63 B 4.0m
6 39 52.1 +51 56 58 12.47 11.99 sdB 4.0m
6 57 36.8 73 24 47 10.99 11.90 CPD-73 420 sdB 3.58m
7 02 22.2 +18 40 32 12.29 11.98 B 3.58m
7 37 25.2 +37 14 01 10.57 10.81 KUV 07341+3721,TD1 31205 B 4.0m
7 47 21.9 +62 25 42 11.67 11.91 FBS 0742+625 sdB 4.0m
8 08 37.6 +29 08 40 11.90 11.44 TYC 1939-499-1 A 4.0m
8 25 13.5 +73 06 38 12.27 11.78 Z Cam DN
8 28 32.9 +14 52 05 11.65 11.78 TD1 31206,UVO 0825+15 He-sdB 111111Classified as a sdO by Berger & Fringant (1980). 3.58m
8 41 38.5 07 26 02 12.08 11.82 TYC 4875-465-1 B 3.58m
9 23 00.2 67 53 14 11.49 11.27 TYC 9196-1935-1 B 3.58m
10 00 59.0 +02 48 05 11.62 11.23 TYC 247-190-1 B 3.58m
10 35 53.2 39 04 14 11.32 11.04 TYC 7710-2503-1 B 3.58m
10 39 07.7 +36 45 33 11.23 11.25 CBS 129 B 4.0m
10 44 10.6 +48 19 02 12.20 11.79 A 4.0m
11 05 41.5 14 04 24 11.82 11.54 EC 11031-1348 sdB+GV 121212Reed & Stiening (2004) noted the composite optical-infrared colours. 3.58m
12 11 56.5 46 55 47 11.53 11.31 B 3.58m
12 40 51.6 10 54 13 11.26 10.89 BD-10 3529 B 3.58m
12 41 51.8 +17 31 19 10.67 11.64 Feige 67,TD1 30996 sdO
12 45 58.8 43 05 20 10.86 10.46 CD-42 7878, HD 110942 B 3.58m
12 57 52.7 35 27 18 11.26 11.55 B 3.58m
13 32 59.7 41 12 17 11.72 11.43 TYC 7792-881-1 B 3.58m
13 46 47.2 07 07 31 12.15 11.81 B 3.58m
13 48 45.0 +43 37 58 11.32 11.35 FB 140 B 4.0m
14 11 16.0 30 53 07 11.63 11.90 CD-30 11223, FAUST 3993 sdB 3.58m
14 27 08.4 +72 57 50 11.51 11.21 B 4.0m
15 09 48.5 38 44 54 11.20 10.93 HD 134199,CPD-38 6054 B 3.58m
16 29 32.9 +80 16 55 11.00 10.53 B 4.0m
17 02 28.3 +63 53 31 11.58 11.99 B 4.0m
17 23 26.6 +46 19 02 11.88 11.55 TYC 3508-387-1 B 4.0m
17 36 51.3 +28 06 35 11.53 11.44 TYC 2084-448-1 sdB+GV 3.58m
18 01 52.5 +37 42 09 11.72 11.37 TYC 3102-1109-1 B 4.0m
18 15 13.0 +39 42 31 11.93 11.51 B 4.0m
19 11 09.3 14 06 54 11.77 11.77 He-sdO 3.58m
20 47 42.0 +08 46 56 11.12 10.79 TYC 1089-1800-1 B 3.58m
20 15 04.8 40 05 44 12.02 11.85 EC 20117-4014 sdBV
23 19 58.4 09 56 11.26 11.50 Feige 110 sdO
23 49 47.8 +38 44 40 11.31 11.71 FBS 2347+385 sdB 2.0m
Table 1: Sample of bright () subluminous candidates.

The bright selection potentially harbours new nearby subdwarf or white dwarf stars. Table 1 lists bright UV-selected stars () showing the predominance of massive B stars, followed by hot subdwarfs (sdO, sdB), white dwarfs (DNdwarf nova, PNNplanetary nebula nucleus, DA), and a nova-like star (NL). The content of this list resembles and complements the TD-1 catalogue of UV-bright stars. Only about 10% of UV sources listed by Carnochan & Wilson (1983) are subluminous stars, compared to % in Table 1. The magnitudes range from 10.5 to 12.5 while in the TD-1 survey, suggesting, as expected, that subluminous stars should dominate fainter selections such as the present GALEX selection.

The bright source list includes a single white dwarf, G191-B2B, and 15 hot subdwarfs. No new white dwarfs were found in the bright source list. Six of the subdwarfs were previously known: the hot sdO stars Feige 67 and Feige 110, the He-sdB star CPD-20 1123 (Albus 1), the TD-1 discovery UVO 0825+15, the sdB plus GV binary EC 110311348, and the pulsating sdB EC 201174014 (O’Donoghue et al, 1997). An additional nine new subdwarfs complete the bright source list, including the newly identified binaries GALEX J0321+4727 and GALEX J2349+3844 (Kawka et al., 2010a). The sdB GALEX J0321+4727 is in a 0.26584 d binary with a late-type companion showing a large reflection effect, while the sdB GALEX J2349+3844 is in a 0.46249 d binary with, most probably, a white dwarf companion. The brightest new subdwarf in our sample is CD32 1567 with .

Samples of bright, nearby white dwarfs are largely based on the Lowell proper-motion survey (Giclas, Burnham, & Thomas, 1980), listed with the GD and GR prefixes, or the New Luyten Two-Tenth survey (Luyten, 1979, 1980; Salim & Gould, 2003), listed with the NLTT prefix. Of the ten known white dwarfs brighter than , only RE 2214492 is a relatively recent discovery (Holberg et al., 1993) in the ROSAT/Wide Field Camera extreme-UV survey (see Mason et al., 1995). Based on their expected UV flux output, eight out of the ten known white dwarfs brighter than were eligible for detection, while the remaining two objects (WD 1142645 and WD 0839327) have low UV flux output. However, the fields surrounding WD 1142645, WD 0839327, Procyon B (WD 0736+053), Sirius B (WD 0642-166), WD 2032+248 (Wolf 1346), and WD 1620391 (CD38 10980) are close to the Galactic plane in areas not covered in the GALEX survey. Three of the remaining objects suffered various defects mainly caused by their excessive UV brightness: WD 0413077 (40 Eri B) is in fact not included in GSC2.3.2 although it would still have been excluded from our selection because of the poor quality of the and photometry; WD 0310688 was also excluded because of the unreliability of the photometry; RE 2214492 was excluded from our selection because of the lack of photometry. Because it met all the criteria, WD 0505+527 (G191-B2B) is the only white dwarf retrieved in the bright source list.

3.2 Properties of the hot subdwarfs

Figure 6 shows our spectroscopy of subdwarf stars sorted by effective temperatures. Helium line strengths show large variations from one object to the next with the noteworthy cases of J0747+6225 and J0827+1753 which display strong Hei line spectra. The spectra of GALEX J09342512 and J1632+0752 (PG 1629+081) show peculiar Hei4471/Heii4686 line ratios that are too weak compared to other objects with similar Balmer line spectra. The discrepancy is likely to result in helium abundance inconsistencies. Two of our targets showed composite sd+MS spectra and are not shown here (but see Section 3.2.1). Ten objects show He-rich spectra with the remainder showing H-rich spectra, or a 1:5 number ratio.

We fitted the observed spectra from H (or H) to H with the non-LTE model grid using minimization techniques. The quoted errors are 1- statistical errors. Figure 7 shows an example of an analysis of hydrogen/helium line profiles. The hot sdB star GALEX J0639+5156 is part of our bright sample () and is typical of its class. Table 2 lists the atmospheric parameters of the hot subdwarfs observed in this program. We listed the results of our analysis for GALEX J09342512 and J1632+0752 excluding the Heii4686 line. A detailed analysis of these peculiar subdwarfs will be presented elsewhere after high-dispersion red and blue spectra are obtained. A preliminary analysis of the composite sdB+GV spectra is presented in Section 3.2.1.

Figure 7: Model atmosphere analysis of the low-dispersion KPNO spectrum of GALEX J0639+5156. The top panels show the line profiles and best-fit models, while the lower panels show the contours at 60, 90, and 99%.

The spectral classification follows a simple scheme (see a discussion in Heber, 2009): spectra with dominant hydrogen Balmer lines along with weaker Hei or Heii lines are labelled sdB or sdO, respectively. Spectra with dominant Heii lines are labelled He-sdO, whereas spectra with unusually strong Hei lines are labelled He-sdB. The newly identified He-sdO GALEX J19111406 is a bright example of its class. The low-dispersion spectrum shows strong lines of the Heii Pickering series, Heii, Hei, and a strong Ciii-iv blend, and Niii multiplet. All He-sdO stars share these features to varying degrees and are characterized with K, and . Among their cooler counterparts, such as the He-sdB star GALEX J0828+1452, some have higher hydrogen abundance than He-sdO stars with weaker carbon and nitrogen lines. Overall, our subdwarf selection displays abundance diversities often noted in sdB stars (Edelmann et al., 2003) and sdO stars (Stroeer et al., 2007).

    GALEX J Type           Other names
(mag)      (K) (c.g.s.)
025023.8040610 sdB 13.02 HE 02470418, PB 9286
031737.9144622131313Strömgren photometry from Wesemael et al. (1992): PG 0314+146 (), PG 0838+133 (), PG 0920+029 (), PG 1257+171 (), PG 1355+071 (), PG 1408+098 (), PG 1432+004 (), PG 1629+081 (). He-sdO 12.67 PG 0314+146
032139.8472716141414Also in the bright () sample.151515Parameters taken from Kawka et al. (2010a). sdB 11.79
040105.3322348 sdB 11.20 CD32 1567
042034.8012041 He-sdO 12.52
050018.9091203 sdB 14.63
050720.3280225 sdB 12.39 CD28 1974, HE 05052806
050735.7034815 sdB 14.42
063952.0515658 sdB 11.97
065736.8732447 sdB 11.90 CPD73 420
074617.1061006 sdB 12.85
074721.9622542 sdB 11.91 FBS 0742+625
075147.0092526 sdB 14.12
075234.2161604 He-sdO 14.23
080510.9105834 sdB 12.21
080656.7152718 sdB 14.69
081203.8070352 sdB 14.78
081233.6160121 sdB 13.57
082734.9175358 sdB 14.62
082832.8145205 He-sdB 11.78 TD1 31206
083412.3071211 sdB 14.86
084143.8130431 He-sdO 13.66 PG 0838+133
085649.3170115 sdB 13.17
090540.9121228 sdB 14.67 PG 0902+124
092308.3024208 sdB 14.48 PG 0920+029
092856.1061634 sdB 14.29 PG 0926+065
093448.2251248 sdO 13.03
095256.6371940 He-sdO 12.69
100752.0292435 sdB 12.87 CD28 7922
104122.8504420 He-sdO 15.31 PG 1038+510
104130.4184210 sdB 13.24
104148.9073031 sdB 12.14
110055.9105542 sdB 14.20
110541.4140423 sdB+GV 13.0161616Based on spectral decomposition (see Section 3.2.1). (28000) (5.50) (3.0) EC 110311348, FAUST 2814
111422.0242130 sdB 12.68 EC 111192405
125941.6164827 sdB 14.37 PG 1257+171
131512.4024531 sdO 15.10
135629.2493403 sdB 12.30 CD48 8608
135824.6065137 sdB 14.50 PG 1355+071
141055.8093256 sdB 14.10 PG 1408+098
141115.9305307 sdB 11.90 CD30 11223, FAUST 3993
143519.8001350 sdB 12.50 PG 1432+004
145928.5190350 sdB 14.17 PG 1457+193
160131.3044027 He-sdB 14.56 PG 1559+048
163201.4075940 sdO 13.01 PG 1629+081
173153.7064706 sdB 13.74
173651.2280635 sdB+GV 12.6 (28000) (5.50) (3.0)
181032.0053909 sdB 13.81
184559.8413827 He-sdB 14.63
190302.5352829 sdB 14.34
191109.3140654 He-sdO 11.77
234947.7384440 sdB 11.71 FBS 2347+385
Table 2: Properties of evolved subluminous stars.

3.2.1 The sdBG2V binaries EC 110311348 and GALEX J1736+2806 and four binary candidates

Companions to hot subdwarf stars are often detected from radial velocity variations (Morales-Rueda et al., 2003; Kawka et al., 2010a), while more luminous companions contribute to composite spectra or colours (Aznar Cuadrado & Jeffery, 2002; Stark & Wade, 2003; Reed & Stiening, 2004). For example, Figure 8 shows a preliminary spectral decomposition of EC 110311348 and GALEX J1736+2806. Based on infrared excess measurements, Ulla & Thejll (1998) found that out of 41 hot subdwarfs, 13 may have G-type companions. The fraction is somewhat lower than found by Thejll, Ulla, & MacDonald (1995) who measured a 50% incidence of binaries in their sample. Figure 9 shows the location of our own sample of hot subdwarf stars in a versus diagram. Most objects cluster close to a sequence defined by single, hot subdwarfs with . A small group of six objects, two of them analysed here (EC 110311348, GALEX J1736+2806), are located at and indicating the presence of a G-type companion in of our sample.

The subdwarf stars with IR excess are EC 110311348 (Kilkenny et al., 1997) which shows composite optical-infrared colours (Reed & Stiening, 2004), and the newly spectroscopically identified sdB+GV GALEXJ 1736+2806. The four other objects with likely F to late G companions are CD-28 1974 (GALEXJ 05072802), CPD-73 420 (GALEX J06577324), CD-28 7922 (GALEX J10072924) and CD-48 8608 (GALEX J13564934). The two subdwarfs with marked composite spectra (EC 110311348 and GALEXJ 1736+2806) are possibly less luminous, hence more easily contaminated in the optical, than the other four objects that show infrared colours typical of G-type stars, but that do not show evidence of a companion in blue spectroscopy.

Figure 8: Spectral energy distribution of the sdB+G2V binaries EC 110311348 and GALEX J1736+2806 and spectral decompositions. The hot subdwarfs are represented with a model at K and the companion by a G2V template from Pickles (1998). The photometry for EC 110311348 is from Kilkenny et al. (1997).

3.2.2 Overlap with other catalogues of blue stellar objects

Our selection recovered thirteen hot subdwarfs from the Palomar-Green survey (Green, Schmidt, & Liebert, 1986): the sdB stars PG 0902+124, PG 0920+029, PG 0926+065, PG 1257+171, PG 1335+071, PG 1408+098, PG 1432+004 (Moehler, Heber, & de Boer, 1990a; Aznar Cuadrado & Jeffery, 2001), PG 1457+193, PG 1559+048, and PG 1629+081, and the sdO stars PG 0314+146 (Aznar Cuadrado & Jeffery, 2001), PG 0838+133 (Dreizler et al., 1990; Thejll et al., 1994), and PG 1038+510. A detailed spectroscopic analysis for eight out of ten sdB stars from the PG survey, and two out of three sdO stars from the same survey is presented here for the first time. Wesemael et al. (1992) list Strömgren photometry for eight PG objects from our selection (see Table 2).

Now, we compare the results of our analysis of four PG subdwarfs with the results of previous studies.

Figure 9: Optical-infrared versus diagram for new hot subdwarfs (Table 2) showing confirmed or likely binaries in the lower-right corner. Confirmed binaries are shown with open squares and suspected binaries with open circles. The companion spectral types range from late F to early G. Most objects cluster near the subdwarf sequence shown with synthetic colours for models at  K and (thick line). Note the narrow range of optical-IR colours of hot, single subdwarf stars.

PG 0314+146 Using hydrogen models to describe the spectral energy distribution of PG 0314+146 Aznar Cuadrado & Jeffery (2001) derive a relatively low temperature of 20800K. Moreover, Moehler et al. (1990b) observed narrow hydrogen Balmer lines, in contrast to our spectrum which shows broad and shallow Heii lines typical of helium-rich sdO stars. However, Wesemael et al. (1992) noted a discrepancy between their own photometry and that of Moehler et al. (1990b) and concluded that Moehler et al. observed the wrong object. Our GALEX target is located at the coordinates listed by Wesemael et al. The International Ultraviolet Explorer (IUE) low-dispersion large-aperture spectrum SWP51740L (Aznar Cuadrado & Jeffery, 2001) is peculiar with a broad Ly line and strong Civ1550 line, but weaker Heii1640 line. The Ly line strength is inconsistent with a photospheric origin. Indeed, the equivalent width of Å implies a neutral hydrogen column density in the line of sight cm. Following the dust-to-gas relation cm, the extinction coefficient is predicted to be , or % of the total extinction in the line-of-sight toward PG 0314+146 (, Schlegel, Finkbeiner, & Davis, 1998). At a Galactic latitude and distance of 300-500 pc (assuming ), PG 0314+146 lies 170-290 pc above the plane and the extinction coefficient is expected to be closer to the total extinction in the line-of-sight. In summary, PG 0314+146 is a hot He-sdO with a UV flux distribution showing large interstellar extinction.

PG 0838+133 Our new parameters confirm the analysis of Thejll et al. (1994), K, and 90% helium (), and Dreizler et al. (1990), , , . We find that the atmosphere is He-rich, as previously determined, and that our new temperature and surface gravity measurements are consistent with the average of the published values.

PG 1432+004 Our new measurements agree with the parameters measured by Moehler, Heber, & de Boer (1990a) that are based on spectroscopy and Strömgren photometry ( K, ). However, Aznar Cuadrado & Jeffery (2001) measured K at based on IUE spectra. The IUE temperature is significantly higher than the optical measurements possibly because of uncertainties in the interstellar reddening index () determined by Aznar Cuadrado & Jeffery (2001).

PG 1629+081 Allard et al. (1994) and Aznar Cuadrado & Jeffery (2001) noted that PG 1629+081 has a composite optical-UV spectrum. However, their temperature measurements, (Allard et al., 1994) and K at (Aznar Cuadrado & Jeffery, 2001), are lower than expected for a subdwarf showing dominant Heii4686 line. We could not verify the presence of a companion because of the low quality of the 2MASS J photometry.

We recovered two objects from the First Byurakan Survey of blue stellar objects (Mickaelian, 2008): FBS 0742+625 classified as B1, and FBS 2347+385 classified as sdB. Our analysis establishes sdB classifications for both objects. We also note from the Hambourg-ESO survey the objects HE 02470418 (PB 9286, Berger & Fringant, 1984) and HE 05052806 (see Frebel et al., 2006). Berger & Fringant (1984) previously classified PB 9286 as a sdB. Finally, two objects, EC 110311348 and EC 111192405, are found in the Edinburgh-Cape survey (Kilkenny et al., 1997). Several objects are also included in the Cape Photographic Durchmusterung (CPD) or Cordoba Durchmusterung (CD) catalogues and we listed these names in Table 2 as long as magnitudes and positions () are reasonably matched.

Three hot subdwarfs are also listed in the UV catalogues TD-1 and Far-Ultraviolet Space Telescope (FAUST; Bowyer et al., 1995). The source TD1 31206171717The TD-1 catalogue number 32707 used in Simbad is an error. (Thompson et al., 1978), also known as UVO 0825+15 (Carnochan & Wilson, 1983), is listed as an sdO by Berger & Fringant (1980) and in the catalogue of spectroscopically identified subdwarfs of Kilkenny, Heber, & Drilling (1988). The sources FAUST 2814 and FAUST 3993 are also known as EC 110311348 (Kilkenny et al., 1997) and CD30 11223, respectively.

Figure 10: Parameters of the GALEX sample of sdO (open square), He-sdO (open triangles), sdB (full squares), and He-sdB stars (full triangles). (Top) Helium abundance versus effective temperature measurements showing clear class distinctions. Upper or lower limits are shown with arrows. (Bottom) Surface gravity versus effective temperature measurements compared to evolutionary sequences from Dorman, Rood, & O’Connell (1993) labelled with the corresponding masses. The ZAEHB and TAEHB are marked with the labels “a” and “b”, respectively. The HeMS from (Paczyński, 1971a) and Divine (1965) are marked with the labels “c” and “d”, respectively. Typical error bars are shown in the upper left corner.
    GALEX J           Notes
(mag) (mag) (mag) (mag) (mas yr)     (K)     (cgs)
041051.6+592501181818Extreme UV source identified by Mason et al. (1995). 14.69 29990140 2RE J0410+592
045219.3+251935 14.93 19620370
081237.8+173701 13.46 15260160
193156.8+011745191919From Vennes, Kawka, & Németh (2010). 14.18 20890120
215947.0+293640 15.12 528501850
232358.7+014025202020Haro & Luyten (1962); photometry available from Kilkenny (1995). 14.28 743003600 PHL 497
Table 3: New white dwarfs selected from colours () and proper motion ().

3.2.3 Hot subdwarf evolution

Figure 10 shows the measured parameters in the versus plane and the versus planes. The temperature and surface gravity measurements are compared to EHB sequences at 0.471, 0.473, 0.475, and 0.480 (Dorman, Rood, & O’Connell, 1993). The zero-age extreme horizontal-branch and the terminal-age extreme horizontal-branch that marks the exhaustion of helium in the core are labelled ZAEHB and TAEHB, respectively. The location of the helium main sequence (HeMS), covering from 0.33 to is from Divine (1965), Hansen & Spangenberg (1971), and Paczyński (1971a). A class distinction is drawn at . The 10 hot objects () are on average more luminous than the 38 cooler objects () and are predominantly He-rich. The average value of is 1.9 in the former group while it is 2.5 in the latter. Some He-sdO stars are close to the He-burning main-sequence with masses ranging from 0.75 to as found by Thejll et al. (1994) and are possibly more massive than the average sdB star ().

GALEX J08041058 is possibly following an evolutionary path similar to the sdB plus WD binary HD 188112 (Heber et al., 2003), and will eventually evolve as an extremely low-mass white dwarf (see Kawka, Vennes, & Vaccaro, 2010b). A radial velocity study and search for a potential close companion is under way.

3.3 Properties of the hot white dwarfs

We analysed the white dwarf spectra from H (or H) to H using the same minimization techniques employed in the analysis of the hot subdwarfs. The results of our analysis using the pure-H model grid are summarized in Table 3. Close inspection of the spectrum of GALEX J1931+0117 suggested that the atmosphere of the star is polluted with heavy elements not normally observed in these objects. Vennes, Kawka, & Németh (2010) presented a detailed analysis of this peculiar white dwarf.

Figure 11 (left) shows hot white dwarfs in a versus diagram. Synthetic hot white dwarf colours are listed in Appendix A and the sample of known white dwarfs is described in Appendix B. Composite colours were computed by combining absolute infrared magnitudes for M dwarfs (Kirkpatrick & McCarthy, 1994) with white dwarf absolute magnitudes. The white dwarf stars with notable IR excess are located in the lower left corner of the diagram. These objects are: PG0205+134, which was misclassified as a sdO and reclassified as a hot DA (Liebert, Bergeron, & Holberg, 2005) with a cool companion (Greenstein, 1986; Williams, McGraw, & Grashuis, 2001a; Williams et al., 2001b), PG 0824+289 (DA+dC, Heber et al., 1993), GD 123 (DA+dM4.5, Farihi, Becklin, & Zuckerman, 2005), PG 1114+187 (DA+dMe, Hillwig, Honeycutt, & Robertson, 2000), PG 1123+189 (DA+dM, Green, Ali, & Napiwotzki, 2000), HZ 43 which is a hot DA with a crowded dM companion (Greenstein, 1986), and the post common-envelope system GD 245 (DA+dMe Schmidt et al., 1995). Most objects fall along single hot white dwarf colours, but the objects with large infrared excess fall close to the WD+dM2 sequence.

Notable IR excess is also apparent in EUVE J1847-223 and EUVE J2124+284 and to a lesser extent in GALEX J0410+5925 and J1931+0117. Vennes, Kawka, & Németh (2010) found a possible link between the infrared excess in GALEX J1931+0117 and the high heavy element abundance in its atmosphere. The infrared excess may be the signature of a debris disc being accreted onto the white dwarf surface. The infrared flux excess in the other three objects remains to be investigated. The case of EUVE 0623376 illustrates potential problem with the brightest member from the sample. The index is affected by inaccurate non-linearity correction of the GALEX magnitude measurement. Figure 11 (right) shows the same sample but as a function of white dwarf effective temperatures. Additional objects, that lay hidden in the main white dwarf cooling sequence, show a index apparently contaminated by late M dwarf companion (dM4-dM6). The majority of the known DA white dwarfs closely follow the predicted UV-infrared colour index. About 10% of the white dwarfs have early M dwarf companions (M0-M4) with possibly another 10% having late M dwarf companions (M4-M9). Early-type companions are only detectable in far and extreme ultraviolet selections such as that of TD-1 (Landsman, Simon, & Bergeron, 1996) or EUVE (Vennes, Christian, & Thorstensen, 1998).

Figure 11: (Left) UV-infrared versus diagram for 6 new hot DA white dwarfs (Table 3) and 50 known DA white dwarfs (Table 7) from the present GALEX selection. Objects in the lower left corner are known white dwarf plus late-type pairs. The bulk of measurements are close to the hot white dwarf sequence that covers from, top to bottom, 84000 to 12000 K (full line). Composite colours for DA plus dM2, dM4, dM6, and dM9 are shown with dashed lines. (Right) same as left panel but as a function of effective temperatures.

4 Summary and conclusions

We presented a new selection of subluminous stars based on the GALEX all-sky survey and UV-optical-IR colour indices built using the GSC2.3.2 and 2MASS J and H magnitudes. A reduced proper-motion diagram allowed to segregate new white dwarf and subdwarf candidates and spectroscopic follow-up observations uncovered 48 previously unknown or poorly studied hot subdwarf stars, and six new white dwarf stars. Two of the new subdwarfs have been found to be in short-period binaries (Kawka et al., 2010a) while one of the white dwarfs is a rare high-metallicity DAZ white dwarf (Vennes, Kawka, & Németh, 2010).

The content of the bright source list () is typical of bright UV surveys such as the TD-1 survey. The list contains primarily blue stars, but also 15 hot subdwarf stars. Nine of the bright subdwarfs were previously unstudied while the sdB star GALEX J0639+5156 and the He-sdO star GALEX J19111406 were previously uncatalogued. The bright subdwarf stars are part of a larger list of 52 objects, with 48 of them analysed here for the first time. We found that six of the hot subdwarfs show composite UV-IR colours indicative of F to K type companions, with two of them showing composite sdB plus GV optical spectra. The incidence of binaries in our sample is low (%). Thejll, Ulla, & MacDonald (1995) and Ulla & Thejll (1998) found a higher fraction of binaries in their mixed sample of hot subdwarfs (20%). Williams, McGraw, & Grashuis (2001a); Williams et al. (2001b) also conclude based on IR and optical colour criteria that % of sdO stars have cooler companions, while Jeffery & Pollacco (1998) and Reed & Stiening (2004) found a 20% incidence of binaries among samples of hot sdB stars. Relatively inaccurate and colour indices are possibly responsible for the lower incidence of binaries in our study.

The stellar parameters and locate the sample of hot subdwarfs along post-EHB evolutionary tracks from the zero-age to the He main-sequence. The abundance of helium in the sample of sdB stars does not correlate with effective temperature K, but a shallow trend is observed at K (see Edelmann et al., 2003). A clear break in the helium abundance at K separates He-sdO stars from the other classes. The He-sdB GALEX J18454138 is an outstanding case among hot sdB stars. Its helium abundance is comparable to He-sdO stars but with a temperature K cooler.

Although we were able to report the discovery of several new hot subdwarfs, the bright source list does not contain new white dwarfs. The six new white dwarfs uncovered in this study were selected amongst fainter objects with higher proper-motions (). New white dwarfs may yet be uncovered amongst unobserved candidates in the range .

Future work on this program will involve a detailed abundance study, in particular for carbon and nitrogen that offer clues to the origin of subdwarfs (e.g., Lanz et al., 2004; Ahmad et al., 2007). The Niii/Ciii4634-4651 blend is seen in several He-sdO stars. High-dispersion optical and ultraviolet spectroscopy would also enable abundance measurements in sdB stars. For example, Ohl, Chayer, & Moos (2000) measured a metallicity of solar in the sdB star PG 0749+658: A more complete abundance pattern would also contribute in improving our / measurements that are based on H/He model atmospheres (see Edelmann et al., 2003).

Spectroscopic observations and model atmosphere analyses of a second list of objects based on GALEX GR6 photometry will be reported in a forthcoming paper.

Acknowledgments

S.V. and A.K. acknowledge support from the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 300030908, IAA 301630901) and from the Grant Agency of the Czech Republic (GA ČR P209/10/0967). A.K. also acknowledges support from the Centre for Theoretical Astrophysics (LC06014). We thank C. Latham and E. Snape for their assistance with the initial catalogue selection, and R. Østensen for helpful comments on the paper.

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Appendix A Ultraviolet, visual, and infrared magnitudes for white dwarfs

Figure 12: (Left) versus colour indices for white dwarfs at, from top to bottom, 0.4, 0.5, 0.6, 0.8, 1.0, and 1.2   and K, and (right) versus for the same masses and temperatures.

Table 4 lists absolute GALEX and magnitudes, Johnson magnitude, and the 2MASS and magnitudes for white dwarfs of 0.4, 0.5, 0.6, 0.8, 1.0 and 1.2 . The absolute calibration employs the mass-radius relations of Wood (1995) and Benvenuto & Althaus (1999). We calculated the absolute ultraviolet magnitudes and in the AB system and using the post-launch response curves212121Available at http://galexgi.gsfc.nasa.gov/docs/galex/
Documents/PostLaunchResponseCurveData.html
.
. Next, we calculated the Johnson and 2MASS magnitudes using the response curves and zero-points of Bessell (1990) and Cohen, Wheaton, & Megeath (2003), respectively. Figure 12 shows colour indices for all masses and between 12000 and 84000 K. The index is sensitive to surface gravity, hence mass, at lower temperatures. With increasing surface gravity, the extended Ly line wing and the Lyman satellites act to depress the continuum in the band. The synthetic colours are computed with pure hydrogen models and do not take into account the effect of metallicity. The colours should also be corrected for the effect of interstellar reddening as needed.

( K) (mag) (mag) (mag) (mag) (mag) ( K) (mag) (mag) (mag) (mag) (mag)
10.0 15.103 12.631 11.622 11.574 11.530 10.0 15.538 12.912 11.921 11.862 11.813
11.0 13.381 12.067 11.289 11.405 11.401 11.0 13.819 12.353 11.591 11.699 11.688
12.0 12.127 11.644 11.079 11.297 11.310 12.0 12.566 11.910 11.368 11.588 11.599
13.0 11.372 11.273 10.908 11.198 11.221 13.0 11.710 11.565 11.217 11.506 11.526
14.0 10.902 10.928 10.739 11.082 11.115 14.0 11.210 11.226 11.055 11.401 11.431
15.0 10.526 10.629 10.595 10.978 11.018 15.0 10.827 10.929 10.912 11.300 11.338
16.0 10.192 10.356 10.461 10.877 10.924 16.0 10.501 10.668 10.789 11.210 11.255
18.0 9.607 9.867 10.210 10.682 10.740 18.0 9.937 10.203 10.560 11.038 11.095
20.0 9.104 9.443 9.982 10.503 10.570 20.0 9.455 9.801 10.353 10.880 10.946
22.0 8.658 9.065 9.770 10.335 10.410 22.0 9.029 9.443 10.161 10.730 10.804
24.0 8.246 8.711 9.561 10.165 10.248 24.0 8.643 9.114 9.976 10.584 10.666
26.0 7.869 8.378 9.357 9.998 10.088 26.0 8.292 8.806 9.795 10.441 10.530
28.0 7.522 8.058 9.151 9.825 9.923 28.0 7.971 8.511 9.612 10.291 10.388
30.0 7.210 7.758 8.944 9.645 9.749 30.0 7.687 8.237 9.430 10.134 10.238
32.0 6.940 7.493 8.751 9.470 9.579 32.0 7.442 7.997 9.260 9.982 10.089
36.0 6.505 7.071 8.424 9.165 9.277 36.0 7.054 7.622 8.978 9.721 9.833
40.0 6.166 6.748 8.162 8.916 9.031 40.0 6.757 7.341 8.757 9.513 9.627
44.0 5.879 6.476 7.934 8.697 8.813 44.0 6.502 7.101 8.561 9.325 9.441
48.0 5.626 6.236 7.729 8.498 8.615 48.0 6.272 6.885 8.378 9.149 9.266
52.0 5.388 6.011 7.531 8.307 8.424 52.0 6.051 6.676 8.197 8.974 9.091
56.0 5.159 5.793 7.337 8.117 8.236 56.0 5.841 6.476 8.021 8.802 8.920
60.0 4.933 5.576 7.141 7.925 8.044 60.0 5.629 6.274 7.838 8.624 8.743
64.0 4.700 5.352 6.934 7.723 7.842 64.0 5.426 6.079 7.661 8.451 8.570
68.0 4.455 5.116 6.715 7.507 7.627 68.0 5.220 5.882 7.480 8.272 8.392
72.0 4.193 4.861 6.475 7.270 7.391 72.0 5.018 5.687 7.299 8.095 8.216
76.0 3.912 4.587 6.215 7.013 7.135 76.0 4.806 5.482 7.107 7.905 8.026
80.0 3.593 4.274 5.914 6.715 6.838 80.0 4.594 5.276 6.912 7.714 7.835
84.0 3.175 3.862 5.514 6.318 6.441 84.0 4.374 5.062 6.709 7.513 7.636
10.0 15.898 13.158 12.182 12.115 12.062 10.0 16.545 13.633 12.681 12.600 12.540
11.0 14.195 12.603 11.854 11.953 11.938 11.0 14.902 13.077 12.349 12.435 12.411
12.0 12.952 12.142 11.617 11.836 11.843 12.0 13.606 12.606 12.100 12.309 12.309
13.0 12.016 11.813 11.477 11.763 11.780 13.0 12.634 12.259 11.946 12.232 12.244
14.0 11.478 11.482 11.327 11.673 11.700 14.0 11.990 11.950 11.818 12.161 12.182
15.0 11.088 11.188 11.186 11.577 11.613 15.0 11.566 11.655 11.679 12.074 12.105
16.0 10.758 10.926 11.061 11.487 11.530 16.0 11.231 11.398 11.560 11.992 12.031
18.0 10.202 10.473 10.843 11.325 11.380 18.0 10.676 10.953 11.346 11.836 11.887
20.0 9.729 10.081 10.645 11.176 11.240 20.0 10.211 10.572 11.157 11.695 11.757
22.0 9.311 9.730 10.458 11.032 11.105 22.0 9.810 10.239 10.987 11.567 11.638
24.0 8.939 9.415 10.287 10.899 10.980 24.0 9.448 9.933 10.822 11.442 11.521
26.0 8.601 9.119 10.117 10.766 10.854 26.0 9.123 9.648 10.662 11.317 11.404
28.0 8.294 8.837 9.945 10.627 10.723 28.0 8.832 9.380 10.501 11.188 11.283
30.0 8.025 8.577 9.776 10.482 10.585 30.0 8.577 9.132 10.341 11.053 11.153
32.0 7.797 8.354 9.620 10.344 10.451 32.0 8.364 8.923 10.197 10.925 11.029
36.0 7.438 8.007 9.366 10.110 10.221 36.0 8.038 8.609 9.973 10.720 10.829
40.0 7.171 7.756 9.174 9.931 10.044 40.0 7.800 8.386 9.808 10.567 10.679
44.0 6.952 7.551 9.013 9.778 9.893 44.0 7.606 8.208 9.672 10.439 10.553
48.0 6.758 7.372 8.867 9.639 9.755 48.0 7.443 8.058 9.555 10.329 10.445
52.0 6.583 7.209 8.731 9.509 9.626 52.0 7.299 7.925 9.450 10.229 10.345
56.0 6.419 7.055 8.601 9.383 9.501 56.0 7.169 7.807 9.354 10.138 10.255
60.0 6.261 6.907 8.472 9.259 9.377 60.0 7.049 7.696 9.263 10.051 10.169
64.0 6.105 6.760 8.343 9.133 9.252 64.0 6.936 7.592 9.176 9.968 10.087
68.0 5.951 6.614 8.212 9.005 9.125 68.0 6.830 7.493 9.093 9.888 10.007
72.0 5.796 6.466 8.078 8.874 8.995 72.0 6.726 7.398 9.011 9.809 9.929
76.0 5.636 6.313 7.938 8.737 8.858 76.0 6.628 7.306 8.931 9.732 9.853
80.0 5.470 6.153 7.789 8.592 8.713 80.0 6.533 7.217 8.854 9.657 9.778
84.0 5.300 5.989 7.637 8.441 8.563 84.0 6.437 7.127 8.774 9.580 9.702
Table 4: Absolute ultraviolet, optical, and infrared magnitudes as a function of white dwarf effective temperatures and masses.
( K) (mag) (mag) (mag) (mag) (mag) ( K) (mag) (mag) (mag) (mag) (mag)
10.0 17.215 14.156 13.228 13.134 13.069 10.0 18.175 14.949 14.055 13.944 13.872
11.0 15.660 13.603 12.896 12.968 12.937 11.0 16.757 14.403 13.726 13.780 13.741
12.0 14.297 13.129 12.638 12.837 12.829 12.0 15.337 13.931 13.460 13.643 13.627
13.0 13.346 12.745 12.457 12.744 12.751 13.0 14.341 13.530 13.261 13.539 13.538
14.0 12.587 12.460 12.344 12.683 12.698 14.0 13.568 13.213 13.123 13.464 13.473
15.0 12.106 12.171 12.219 12.612 12.636 15.0 12.967 12.958 13.026 13.412 13.429
16.0 11.749 11.911 12.097 12.533 12.567 16.0 12.560 12.700 12.917 13.349 13.375
18.0 11.185 11.468 11.885 12.382 12.430 18.0 11.970 12.255 12.705 13.210 13.252
20.0 10.724 11.094 11.701 12.246 12.305 20.0 11.504 11.883 12.520 13.075 13.129
22.0 10.326 10.765 11.532 12.120 12.187 22.0 11.111 11.560 12.356 12.953 13.017
24.0 9.972 10.465 11.372 11.999 12.075 24.0 10.766 11.269 12.202 12.837 12.910
26.0 9.658 10.189 11.219 11.881 11.965 26.0 10.460 10.998 12.051 12.721 12.803
28.0 9.377 9.928 11.063 11.756 11.848 28.0 10.193 10.748 11.902 12.601 12.691
30.0 9.135 9.693 10.911 11.627 11.726 30.0 9.964 10.524 11.758 12.478 12.574
32.0 8.932 9.493 10.775 11.506 11.609 32.0 9.774 10.338 11.630 12.364 12.465
36.0 8.624 9.197 10.565 11.315 11.423 36.0 9.488 10.063 11.438 12.190 12.296
40.0 8.397 8.986 10.410 11.172 11.283 40.0 9.276 9.866 11.296 12.059 12.169
44.0 8.220 8.822 10.289 11.058 11.172 44.0 9.110 9.715 11.185 11.957 12.069
48.0 8.068 8.684 10.183 10.959 11.074 48.0 8.971 9.588 11.091 11.869 11.983
52.0 7.934 8.562 10.088 10.870 10.985 52.0 8.849 9.478 11.007 11.791 11.907
56.0 7.819 8.457 10.006 10.792 10.909 56.0 8.741 9.380 10.932 11.720 11.837
60.0 7.710 8.358 9.926 10.716 10.834 60.0 8.642 9.291 10.862 11.654 11.772
64.0 7.611 8.267 9.853 10.646 10.765 64.0 8.555 9.212 10.800 11.596 11.714
68.0 7.519 8.183 9.784 10.581 10.700 68.0 8.475 9.140 10.743 11.542 11.662
72.0 7.432 8.104 9.718 10.518 10.637 72.0 8.402 9.074 10.690 11.492 11.613
76.0 7.349 8.028 9.654 10.457 10.577 76.0 8.333 9.012 10.640 11.445 11.566
80.0 7.270 7.955 9.592 10.398 10.519 80.0 8.268 8.953 10.593 11.401 11.522
84.0 7.193 7.883 9.531 10.339 10.460 84.0 8.207 8.897 10.547 11.357 11.479
Table 5: continued

Appendix B A sample of bright white dwarfs in the Galex all-sky survey

Table 6 lists colours and effective temperatures of known non-DA white dwarfs (DAB, DAO, DB, DO) recovered in our GALEX-GSC selection. The reference for the tabulated effective temperature and the usual name are also listed. Table 7 provides the same information as Table 6 but for the DA white dwarfs.

Table 6: Colours and temperatures of known non-DA white dwarfs in the GALEX selection.
R.A.(J2000) Dec.(J2000) Name Sp. Type Reference 222222 References: (1) Werner, Rauch, & Kruk (2008); (2) Dreizler & Werner (1996); (3) Jordan et al. (1993); (4) Heber, Dreizler, & Hagen (1996); (5) Koester, Liebert, & Saffer (1994); (6) Thejll, Vennes, & Shipman (1991); (7) Bergeron et al. (1994).
(mag) (mag) ( K)
00 08 18.2 51 23 15.1 1.06 0.19 200.0 KPD 0005+5106 DO (1)
00 41 35.4 20 09 17.4 2.42 0.09 115.0 PG 0038+199 DO (2)
01 13 46.7 00 28 30.9 2.24 0.17 65.0 HS 0111+0012 DO (2)
02 12 04.8 08 46 49.8 2.13 0.21 36.0 HS 0209+0832 DAB (3)
05 08 31.0 01 16 41.3 2.68 0.18 80: HS 0505+0112 DAO (4)
12 13 56.3 32 56 33.1 2.18 0.21 53.0 HZ 21 DO (2)
13 04 32.0 59 27 33.8 1.27 0.08 28.8 GD 323 DAB (5)
16 47 18.5 32 28 31.8 1.45 0.08 25.0 GD 358 DB (6)
23 45 02.9 80 56 59.0 1.92 0.30 65.3 GD 561 DAO (7)
R.A.(J2000) Dec.(J2000) Name Sp. Type Reference 232323 References: (1) Vennes et al. (1997); (2) Vennes (1999); (3) Bergeron, Saffer, & Liebert (1992); (4) Liebert, Bergeron, & Holberg (2005); (5) Jordan et al. (1998); (6) Hillwig, Honeycutt, & Robertson (2000); (7) Dupuis et al. (1998); (8) Barstow et al. (1994); (9) Kawka et al. (2007); (10) Vennes, Korpela, & Bowyer (1997); (11) Schmidt et al. (1995).
(mag) (mag) ( K)
00 07 32.1 33 17 27.2 1.97 0.14 50.8 GD 2 DA (1)
00 39 52.2 31 32 26.4 2.05 0.12 52.0 GD 8 DA (2)
00 53 40.5 36 01 16.6 1.74 0.12 27.4 EUVE 0053+360 DA (1)
01 04 41.2 09 49 42.7 1.40 0.05 25.6 PG 0102+096 DA (2)
01 41 28.7 83 34 56.9 1.01 0.23 18.7 GD 419 DA (3)
02 08 03.4 13 36 24.6 0.94 0.60 57.4 PG 0205+134 DA (4)
02 18 48.0 14 36 06.2 1.49 0.02 27.2 PG 0216+144 DA (1)
03 04 37.2 02 56 58.0 1.68 0.40 35.6 GD 41 DA (1)
03 11 49.1 19 00 56.1 0.72 0.11 17.9 PG 0308+188 DA (4)
04 12 43.5 11 51 47.7 0.51 0.20 20.8 HZ 2 DA (3)
04 28 39.4 16 58 12.4 1.51 0.02 24.4 EGGR 37 DA (2)
05 05 30.7 52 49 50.1 1.91 0.13 61.2 G191 B2B DA (1)
05 10 14.0 04 38 37.3 0.99 0.20 20.0 HS 0507+0434A DA (5)
06 23 12.8 37 41 25.0 1.51 0.11 61.7 EUVE 0623376 DA (1)
08 27 05.1 28 44 02.2 1.32 0.62 50.7 PG 0824+289 DA (4)
08 42 53.0 23 00 25.9 1.45 0.39 25.0 PG 0839+232 DA (4)
09 42 50.7 26 00 58.6 2.36 0.21 67.9 PG 0824+289 DA (4)
09 46 39.1 43 54 54.9 0.05 0.06 12.8 PG 0943+441 DA (4)
09 48 46.7 24 21 25.4 0.84 0.04 14.5 PG 0945+246 DA (4)
10 36 25.2 46 08 27.9 0.71 0.53 30.2 GD 123 DA (1)
10 44 45.7 57 44 35.7 2.01 0.19 30.8 PG 1041+580 DA (1)
10 54 43.4 27 06 58.4 1.32 0.05 23.1 PG 1051+274 DA (4)
11 00 34.4 71 38 03.5 2.18 0.11 41.2 PG 1057+719 DA (1)
11 07 42.6 59 58 28.3 0.87 0.05 17.9 EGGR 75 DA (3)
11 17 03.7 18 25 58.1 0.65 0.51 50: PG 1114+187 DA+dMe (6)
11 26 19.1 18 39 14.8 0.47 0.54 56.9 PG 1123+189 DA (1)
11 32 27.4 15 17 29.4 0.68 0.05 16.9 PG 1129+156 DA (4)
11 37 05.2 29 47 57.6 1.26 0.11 21.3 PG 1134+301 DA (4)
11 43 59.5 07 29 04.4 2.48 0.03 61.8 PG 1141+078 DA (4)
11 45 56.7 31 49 29.5 0.52 0.03 14.9 PG 1143+321 DA (4)
11 48 03.2 18 30 46.3 1.58 0.06 26.6 PG 1145+188 DA (1)
12 12 33.9 13 46 26.6 1.73 0.12 31.9 PG 1210+141 DA (4)
12 36 44.8 47 55 20.6 2.21 0.02 56.1 PG 1234+481 DA (1)
13 16 21.8 29 05 57.8 0.81 0.57 50.0 HZ 43 DA (7)
13 46 01.9 57 00 33.7 0.18 0.12 13.4 PG 1344+573 DA (4)
14 10 27.1 32 08 33.8 0.89 0.09 18.1 PG 1408+324 DA (4)
16 02 42.1 57 58 16.0 0.53 0.17 14.7 PG 1601+581 DA (4)
16 05 21.1 43 04 36.0 2.16 0.05 37.0 PG 1603+432 DA (4)
16 38 26.4 35 00 12.3 1.80 0.01 37.2 EUVE 1638+349 DA (1)
16 59 48.3 44 01 04.1 1.92 0.08 30.5 PG 1658+441 DA (4)
17 13 05.8 69 31 23.2 0.06 0.10 15.6 EGGR 370 DA (3)
17 38 02.8 66 53 46.7 2.28 0.30 88.0 RE 1738+665 DA (8)
18 00 09.7 68 35 52.7 2.23 0.43 46.0 KUV 18004+6836 DA (1)
18 47 56.6 22 19 40.6 0.47 0.62 31.6 EUVE 1847223 DA (1)
19 43 43.8 50 04 38.9 1.64 0.01 34.4 EUVE 1943+500 DA (1)
20 10 56.7 30 13 09.9 0.18 0.06 14.8 LTT 7987 DA (9)
21 16 53.1 73 50 41.0 2.21 0.41 54.7 KUV 21168+7338 DA (1)
21 24 58.1 28 26 03.2 1.25 0.21 53.0 EUVE 2124+284 DA (10)
21 52 25.2 02 23 18.6 0.97 0.08 18.2 EGGR 150 DA (3)
22 58 48.2 25 15 43.4 1.29 0.49 22.2 GD 245 DA+dMe (11)
Table 7: Colours and temperatures of known DA white dwarfs in the GALEX selection.
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