Rotation, activity, and lithium abundance in cool binary stars††thanks: Based on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated with IAC, and the Automatic Photoelectric Telescopes in Arizona, jointly operated with Fairborn Observatory.
We have used two robotic telescopes to obtain time-series high-resolution optical echelle spectroscopy and and/or photometry for a sample of 60 active stars, mostly binaries. Orbital solutions are presented for 26 double-lined systems and for 19 single-lined systems, seven of them for the first time but all of them with unprecedented phase coverage and accuracy. Eighteen systems turned out to be single stars. The total of 6,609 =55,000 echelle spectra are also used to systematically determine effective temperatures, gravities, metallicities, rotational velocities, lithium abundances and absolute H-core fluxes as a function of time. The photometry is used to infer unspotted brightness, and/or colors, spot-induced brightness amplitudes and precise rotation periods. An extra 22 radial-velocity standard stars were monitored throughout the science observations and yield a new barycentric zero point for our STELLA/SES robotic system. Our data are complemented by literature data and are used to determine rotation-temperature-activity relations for active binary components. We also relate lithium abundance to rotation and surface temperature. We find that 74% of all known rapidly-rotating active binary stars are synchronized and in circular orbits but 26% (61 systems) are rotating asynchronously of which half have and . Because rotational synchronization is predicted to occur before orbital circularization active binaries should undergo an extra spin-down besides tidal dissipation. We suspect this to be due to a magnetically channeled wind with its subsequent braking torque. We find a steep increase of rotation period with decreasing effective temperature for active stars, , for both single and binaries, main sequence and evolved. For inactive, single giants with d, the relation is much weaker, . Our data also indicate a period-activity relation for H of the form for binaries and for singles. Its power-law difference is possibly significant. Lithium abundances in our (field-star) sample generally increase with effective temperature and are paralleled with an increase of the dispersion. The dispersion for binaries can be 1–2 orders of magnitude larger than for singles, peaking at an absolute spread of 3 orders of magnitude near 5000 K. On average, binaries of comparable effective temperature appear to exhibit 0.25 dex less surface lithium than singles, as expected if the depletion mechanism is rotation dependent. We also find a trend of increased Li abundance with rotational period of form but again with a dispersion of as large as 3–4 orders of magnitude.
In a previous paper (Strassmeier et al. ; paper I) we reported on radial and rotational velocities, chromospheric emission-line fluxes, lithium abundances, and rotation periods of a total sample of 1,058 G5–K2 dwarfs, subgiants, and giants based on 1,429 moderate-resolution KPNO coudé spectra and 8,038 Strömgren photometric data points. The aim of this survey was to detect new candidates for Doppler imaging but, besides the discovery of 170 new variable stars and 36 new spectroscopic binaries, the more intriguing result was that 74% of the G-K stars with Ca ii H&K emission also showed significant lithium on their surface. However, G-K giants should have very few lithium on their surface because of convective mixing. Theoretical models predict that surface lithium has to be diluted by many factors once a star arrives at the bottom of the red giant branch (Iben , Charbonnel & Balachandran ). Out of the 21 Doppler imaging candidates found, just four stars were single stars, three of them evolved, the rest were spectroscopic binaries, but all four single stars had very strong lithium.
Despite that it is generally acknowledged that higher than normal lithium abundance is common among magnetically active stars, no unique correlation with rotation rate was found after Skumanich’s () original discovery. Recently, White et al. () revisited this issue in their sample of solar-type dwarfs but no such correlation was found. Almost all surveys just revealed trends, if at all, and even these appear to be of different quality (e.g. Lébre et al. ; Böhm-Vitense ; do Nascimento et al. , ; De Medeiros et al. ; De Laverny et al. ; Randich et al. ). The comprehensive survey of nearby giants by Luck & Heiter () did not even show a trend. However, the line broadening in their stellar sample was just 3–7 km s and likely too narrow a range to see a trend. Böhm-Vitense () suggested that the steep decrease of in early G giants as well as in Hyades F dwarfs at effective temperatures cooler than 6,450 K, i.e. at their lithium dip at about the same temperature, are the result of deep mixing and related to the merging of the hydrogen and the helium convection zones. More recently, Takeda et al. () announced evidence for a (positive) correlation of Li abundance with rotational velocity in a sample of solar-analog stars.
The spectroscopic survey of 390 solar-like dwarf stars by White et al. () included 28 of the stars in our survey. Relating Ca ii H&K radiative losses to stellar rotation, White et al. () found a saturation of chromospheric emission for rotational velocities above approximately 30 km s. In an earlier paper, Strassmeier et al. () verified that evolved stars obey qualitatively the same scaling of Ca ii-K-line flux with stellar rotational velocity or period as do main-sequence stars (see, e.g., Mamajek & Hillenbrand , Pace & Pasquini , or Pizzolato et al.  for a summary). No qualitative difference was found between single evolved stars and their equally rapidly rotating counterparts in a spectroscopic binary. However, large scatter indicated that rotation might not be the only relevant parameter. Based on a sample of 22 intermediate-mass G and K giants in close binaries, Gondoin () not only verified the rotational dependency of (coronal) X-ray surface flux but also found a dependency on surface gravity. Such a dependence could stem from the effect of gravity on coronal electron density and on the overall sizes of coronal loops.
Massarotti et al. () reported rotational and radial velocities for 761 giants within 100 pc of the Sun. They found that all binaries in their sample with periods less than 20 days have circular orbits while about half the orbits with periods between 20–100 days still showed significant eccentricity. They also found evidence that the rotational velocity of horizontal branch stars is larger than that of first-ascend giants by a few km s. Earlier, De Medeiros et al. () presented a study of 134 late-type giants in spectroscopic binaries and found a considerable number of G-K giant stars with moderate to moderately-high rotation rates. These rotators have orbital periods shorter than 250 days and circular or nearly circular orbits and appear to be synchronized with the orbit.
The present paper follows up on the newly identified spectroscopic binaries with active components from our paper I. Its direct aim is to determine their orbits on the basis of high-precision radial velocities and to separate their component’s rotation and activity tracers along with other absolute astrophysical parameters. Only with precise stellar parameters can we directly compare binary components with single stars and then be aware of the spectrum contamination from unknown secondaries or even tertiary stars. We recall that an unknown continuum contribution from a secondary star impacts on the determination of the effective temperature, gravity etc. and could together drastically alter the derived lithium abundances and thereby mask any relation if present. In Sect. 2 we restate our sample selection criteria and give a summary of the target stars. In Sect. 3 we describe the new observations and in Sect. 4 we derive basic quantities from the spectra and the light curves. These include radial velocities, orbital parameters, rotational velocities and photometric periods, stellar atmospheric parameters like temperature, gravity and metallicity, lithium abundances, and absolute H-core fluxes. Sect. 5 lists notes to individual stars. Sect. 6 presents the analysis in terms of rotation, temperature, activity, and lithium-abundance relations. Finally, Sect. 7 summarizes our findings and conclusions.
2 Sample selection
Our sample selection is based on the 1,058 stars from the KPNO Doppler-imaging candidate survey in paper I. It itself was drawn from a total of 6,440 stars from the Hipparcos catalog (ESA , van Leeuwen ) for the brightness range 7\fm0–9\fm5 and declination through +70, colors between 0\fm67 and 1\fm0 for stars with parallaxes mas (i.e. G5–K3 dwarfs) and between 0\fm87 and 1\fm2 for mas (i.e. G5–K2 giants and subgiants). These criteria were imposed to select stars with a significant outer convective envelope where the likelihood of detecting magnetic activity is highest. Out of the 1,058 stars, 371 (35%) were found with Ca ii H&K emission but only 78 (7.3%) with 10 km s. On the contrary, a lithium line was detected in 283 (74%) of all stars that had Ca ii emission (with 58% of the stars with lithium above 10 mÅ). Out of a subsample of 172 stars with moderate to strong Ca ii emission, 168 (97.7%) turned out to be photometric variable and for 134 a photometric (rotational) period could be obtained. Finally, 36 targets were single-lined spectroscopic binaries (SB1), of which 17 were new detections. A further 16 targets were found “possible SB1s”. An additional 30 targets were double-lined spectroscopic binaries (SB2), of which 19 were new detections. Two targets were even triple lined (SB3) of which one was a new detection and another four were new candidates. All along, there were a few misidentifications and misinterpretations as well as unrecognized literature entries. Whenever recognized, we try to clarify these in the present paper.
We now present time-series observations of all those targets in the survey that were previously unknown or suspected spectroscopic binaries in 2000. This sample comprised 59 stars presented in this paper. One additional star was added because of its comparable uncertain orbit despite being a fourth magnitude star ( UMi). During the final observational stages for the present paper we learned that Griffin (), Griffin () and Griffin & Filiz Ak () had picked up many of our original SB candidate stars from paper I that found their way into the new edition of the CABS catalog (Eker et al. ), and independently determined orbital elements. We compare our results whenever possible. Stellar identifications and some basic observable properties for all our target stars are summarized in Table 1. Note that three of the stars actually do not show significant Ca ii H&K emission and would not be dubbed magnetically active but are left in the sample because they were monitored initially in order to search for signs of binarity. All three turned out to be single stars though. Altogether, 18 of the SB1 candidates were found to be single with some of them still members of a visual-binary system. These stars are also in Table 1 but are summarized in a separate table later in the paper. In one case the two components of a wide visual binary (BD+11 2052AB = ADS 7406AB) were treated as separate stars throughout the paper. One double-lined binary (HD 16884) turned out to be actually a quadruple system with both pairs being SB1. First orbit determinations are presented for seven systems (HD 50255, HD 82841, HD 106855, HD 147866, HD 190642, HD 199967, and HD 226099).
3 New observations and data reductions
3.1 High-resolution optical spectroscopy
Time-series high-resolution echelle spectroscopy was taken with the 1.2 m STELLA-I telescope between June 2006 and May 2012. Most spectra were exposed just long enough to measure a precise radial velocity and had S/N of between 40–80:1 but several spectra per target were exposed to reach S/N well above 100:1. A total of 6,609 spectra for a total of 60 stars were obtained over the course of approximately six years. STELLA-I is a fully robotic telescope that, together with STELLA-II, makes up the STELLA observatory at the Izãna ridge on Tenerife in the Canary islands (Strassmeier et al. , ). The fiber-fed STELLA Echelle Spectrograph (SES) is the telescope’s only instrument. It is a white-pupil design with an R2 grating with two off-axis collimators, a prism cross disperser and a folded Schmidt camera with an E2V 2k2k CCD as the detector. All spectra have a fixed format on the CCD and cover the wavelength range from 388–882 nm with increasing inter-order gaps near the red end starting at 734 nm towards 882 nm. The resolving power is =55,000 corresponding to a spectral resolution of 0.12 Å at 650 nm (3-pixel sampling). An example spectrum is shown in Fig. 1. We note that the SES received a major upgrade in summer 2012 with a new cross disperser, a new optical camera, and a new CCD. A bit earlier, the SES fiber was moved to the prime focus of the second STELLA telescope in 2011. Further details of the performance of the system were reported by Weber et al. () and Granzer et al. ().
|\objectHD 553||V741 Cas||834||11013||8.17||1.03||K0||SB2||121||1320||s,34|
|\objectHD 8997||EO Psc||6917||74742||7.74||0.96||K2/K1-K6V||SB2||165||1453||b|
|\objectHD 9902||BG Psc||74827||8.71||0.63||F5-6V/G9-K0IV||SB2||95||748||a,c|
|\objectHD 18645||FU Cet||13968||130230||7.86||0.75||G2III-IV||S||70||784||f|
|\objectHD 18955||IR Eri||14157||148731||8.45||0.82||K0V/K2-3V||SB2||67||1446||g|
|\objectHD 23551||MM Cam||18012||12924||7.11||0.91||K0III||S||101||836|
|\objectHD 45762||V723 Mon||30891||133321||8.30||0.87||G0||SB2+1||89||1231||z|
|\objectHD 62668||BM Lyn||38003||41995||7.73||1.10||K0III||SB1||137||2000||a,f,i|
|\objectHD 82159||GS Leo||46637||98615||8.85||0.92||G9V||VB,SB1||107||1458||u,32|
|\objectHD 82286||FF UMa||46919||14919||7.89||0.96||G5||SB2||161||1446||i,31,35|
|\objectHD 82841||OS Hya||46987||136965||8.45||1.08||K2||SB1||61||1447|
|\objectHD 95188||XZ LMi||53747||81610||8.45||0.74||G8V||S||85||891||v,j|
|\objectHD 95559||GZ Leo||53923||81634||8.83||0.96||K0V/K2V||SB2||113||1162||k,30|
|\objectHD 95724||YY LMi||54028||62375||8.96||0.94||G5V||S||76||856||v|
|\objectHD 105575||QY Hya||59259||180519||9.04||0.93||(K5/M1)/G4||SB3||51||1164||29|
|\objectHD 106855||UV Crv||59914||180648||9.59||0.81||K1V||SB2,VB||88||1227|
|\objectHD 109011||NO UMa||61100||28414||8.10||0.94||K2V||SB2||184||1629||o|
|\objectHD 111487||IM Vir||138983||9.69||0.64||G5||SB2||106||1292||p|
|\objectHD 112859||BQ CVN||63368||44410||8.09||0.92||F5V/K0III-IV||SB2||93||1249||a,w|
|\objectHD 127068||HK Boo||70826||101044||8.43||0.89||G8V/G5-8IV||SB2||99||1257||a|
|\objectHD 138157||OX Ser||75861||101580||7.14||1.02||K0III||SB1||148||1086||u,32|
|\objectHD 142680||V383 Ser||77963||101816||8.71||0.95||K0-2V/K7V||SB2||90||781||a,v|
|\objectHD 143937||V1044 Sco||78708||184077||8.65||0.91||G9V/M0V||SB2||86||1131||l|
|\objectHD 147866||V894 Her||80302||84343||8.1||1.1||K0||SB1||115||1269|
|\objectHD 150202||GI Dra||81284||30015||7.97||0.93||K0III||SB1||179||1112||a|
|\objectHD 153525||V1089 Her||83006||46403||7.88||1.04||K0V||S||196||1349||x|
|\objectHD 190642||V4429 Sgr||99042||163264||8.08||0.99||F6-8V/K1III-IV||SB1||196||1274|
|\objectHD 197913||OR Del||102490||106466||7.55||0.76||G6V/G8V||SB2||194||1848||m|
References. a: Griffin (2009), b: Griffin (1987), c: Cutispoto et al. (2003), d: Climenhaga et al. (1951), e: Favata et al. (1993), f: Fekel (1997), g: Fekel et al. (2004), h: Latham et al. (2002), i: Henry et al. (1995), j: Wright et al. (2004), k: Karatas et al. (2004), l: Cutispoto et al. (1999), m: Griffin (2005), n: Griffin & Keenan (1992), o: Halbwachs et al. (2003), p: Morales et al. (2009), q: Otero & Dubovsky (2004), r: Fekel et al. (1999), s: Duemmler et al. (2002), t: Duquennoy & Mayor (1988), u: Gálvez et al. (2006), v: Barnes (2007), w: Montes et al. (2000), x: Strassmeier (1994), y: Gálvez et al. (2005), z: Griffin (2010), 29: Szalai et al. (2007), 30: Gálvez et al. (2009), 31: Gálvez et al. (2007), 32: Griffin & Filiz Ak (2010), 33: Goldberg et al. (2002), 34: Griffin (2003), 35: Griffin (2012).
|\objectHD 3712||Cas||K0 II-III||230||–3.863||0.094||–4.31||+0.447|
|\objectHD 4128||Cet||K0 III||143||+13.686||0.107||+13.1||+0.586|
|\objectHD 12929||Ari||K2 III||440||–14.138||0.118||–14.64||+0.502|
|\objectHD 18884||Cet||M2 III||60||–25.688||0.293||–26.08||+0.392|
|\objectHD 20902||Per||F5 Ib||152||–1.800||0.138||–2||+0.200|
|\objectHD 25025||Eri||M0 III||26||+61.159||0.217||+61.1||+0.059|
|\objectHD 29139||Tau||K5 III||503||+54.646||0.188||+54.26||+0.386|
|\objectHD 36673||Lep||F0 Ib||53||+25.871||0.413||+25.2||+0.671|
|\objectHD 62509||Gem||K0 III||442||+3.783||0.076||+3.23||+0.553|
|\objectHD 81797||Hya||K3 III||203||–4.033||0.076||–4.7||+0.667|
|\objectHD 84441||Leo||G0 II||165||+5.001||0.046||+4.5||+0.501|
|\objectHD 107328||16 Vir||K0.5 III||332||+37.117||0.160||+36.4||+0.717|
|\objectHD 109379||Crv||G5 II||49||-6.947||0.069||–7.6||+0.653|
|\objectHD 124897||Boo||K1.5 III||787||–4.871||0.149||–5.3||+0.429|
|\objectHD 146051||Oph||M0.5 IIIab||146||–19.133||0.146||–19.6||+0.467|
|\objectHD 161096||Oph||K2 III||763||–11.844||0.095||–12.53||+0.686|
|\objectHD 168454||Sgr||K2 III||18||–19.798||0.035||–20.4||+0.602|
|\objectHD 186791||Aql||K3 II||538||–2.430||0.261||–2.79||+0.360|
|\objectHD 204867||Aqr||G0 Ib||80||+6.808||0.151||+6.3||+0.508|
|\objectHD 206778||Peg||K2 Ib||572||+3.588||0.262||+3.39||+0.198|
|\objectHD 212943||35 Peg||K0 III||402||+54.740||0.111||+54.16||+0.580|
Double star with the A component being a “semiregular pulsating variable”.
SES spectra are automatically reduced using the IRAF111The Image Reduction and Analysis Facility is hosted by the National Optical Astronomy Observatories in Tucson, Arizona at URL iraf.noao.edu.-based STELLA data-reduction pipeline (Weber et al. ). The images were corrected for bad pixels and cosmic-ray impacts. Bias levels were removed by subtracting the average overscan from each image followed by the subtraction of the mean of the (already overscan subtracted) master bias frame. The target spectra were flattened by dividing by a nightly master flat which has been normalized to unity. The nightly master flat itself is constructed from around 50 individual flats observed during dusk, dawn, and around midnight. After removal of the scattered light, the one-dimensional spectra were extracted with the standard IRAF optimal extraction routine (Horne ). The blaze function was then removed from the target spectra, followed by a wavelength calibration using consecutively recorded Th-Ar spectra. Finally, the extracted spectral orders were continuum normalized by dividing them with a flux-normalized synthetic spectrum of the same spectral classification as the target in question.
3.2 VI and by photometry
Johnson-Cousins and/or Strömgren photometry of 56 of the targets were obtained with the Amadeus (T7) and/or the Wolfgang (T6) automatic photoelectric telescope (APT) at Fairborn Observatory in southern Arizona, respectively. These 56 stars are listed in Table 6 along with the analysis results. A total of 16,591 observations in either or pairs were obtained. Typically, one APT observation consists of three ten-second (30 s for Strömgren) integrations on the variable, four integrations on the comparison star, two integrations on the check star, and two integrations on the sky. A 30\arcsec diaphragm was used for all targets. The standard error of a nightly mean for Amadeus from the overall seasonal mean changed over the past decade but was typically 4–6 mmag in and 6–8 mmag in for the brightness range of stars in this paper. The observing seasons between 2007–2009 showed increasing scatter due to a slowly but systematic malfunction of the acquisition CCD camera. By late 2009 it got so bad that we had to exchange the entire camera plus its CCD and after that, starting with HJD 2,455,143, the scatter was back to the original values quoted above. The standard error of a nightly mean for Wolfgang was between 1.2–3 mmag, depending on system brightness. For further details we refer to Strassmeier et al. () and Granzer et al. ().
From concurrent observations of Johnson standards in , we also deduce an all-sky solution and apply it to the differential values whenever feasible. Its accuracy never significantly exceeds 0\fm01 in though. In case data were taken, can be determined only relative to the comparison star because our all-sky standards were not observed in the band due to time constraints. Absolute errors are typically around 0\fm01 for except for the time period 2007–2009, as mentioned above, and was then likely 0\fm02.
4 Data products
4.1 Radial velocity precision and zeropoint
Radial velocities were determined from an order-by-order cross correlation with a synthetic template spectrum from an ATLAS9 atmosphere (Kurucz ) that roughly matches the target spectral classification. A two-dimensional cross correlation is performed in case the target is double lined. Each of 19 selected echelle orders gives one relative radial velocity. These velocities are then weighted according to the spectral region depending on the amount of available spectral lines and are then averaged. The true internal error is likely close to the rms of these relative velocities. Numerical simulations of the cross correlation errors from synthetic spectra and various two-Gaussian fits to its peaks are discussed in the paper by Weber & Strassmeier (). Note that the external rms values were significantly larger during the initial year of STELLA operation in 2006/07 (120 m s), compared to thereafter (30 m s). The final radial velocities of our program stars are barycentric and corrected for Earth rotation. No gravitational redshift corrections are applied.
|\objectHIP 46634||1129||+27.9270.047||Var. but no|
Without Ca ii H&K emission according to our paper I and thus not an active star.
Table 2 lists our standard-star measurements. A total of 22 radial-velocity standards were monitored since mid 2006. Their velocities were derived similarly from cross correlations with a synthetic template spectrum that fitted the star’s classification. Among these standards are often observed bright giant stars like Oph, Ari, Gem, Tau, 16 Vir, a.o., which are all IAU radial-velocity standards (see Scarfe et al. ). These spectra are also used to identify two epochs in our data when all stars on the observing menu showed a constant offset in velocity. These epochs are identified with two of our hardware maintenance episodes where the fiber injection unit had been readjusted. The average offsets determined from above standard stars were km s and km s for the epochs 2,453,902–4,085 and 2,454,192–435, respectively. These corrections were applied to all data in this paper.
The preliminary zero point determination in Strassmeier et al. () from four standard stars ( Oph, Cas, Gem, Cet) showed an offset of the STELLA spectra by +0.46 km s with respect to the CORAVEL system defined by Udry et al. (). In the present paper, we revise this value to +0.503 km s from 22 standard stars. The weights applied for this average shift are the total number of spectra per radial-velocity standard. Note that an absolute heliocentric zero point for STELLA has not been determined yet, e.g. by using asteroids. All velocities in the present paper are on the STELLA system.
Table 3 lists the stars in our sample that were found to exhibit no orbital-induced radial velocity variations, i.e. are presumably single stars. These stars are mostly still very active stars and further investigation with the present time-series data are forthcoming. Several of them, e.g. SAO 150676 or HD 76799, show periodic radial-velocity variations with amplitudes of the order of 1 km s, which we interpret to be due to stellar rotation. Few of our single-star survey targets in paper I were also revisited by Griffin (), most notably HD 136655 and HD 184591, for which we recommend to read his “Notes on the six constant-velocity stars” for their observational history.
4.2 Binary orbits
For single-lined (SB1) as well as double-lined (SB2) binaries we solve for the components using the general least-square fitting algorithm MPFIT (Markwardt ). For solutions with non-zero eccentricity we use the prescription from Danby & Burkardt () to calculate the eccentric anomaly. Usually, we first determine the orbit for both components separately and then combine the two and give rms values for both. Eclipsing binaries were treated slightly separately due to the Rossiter-McLaughlin effect in the radial-velocity curve. Some of the velocities around the two conjunctions were discarded by applying 3- clipping. Most of the orbits are sampled nearly perfectly due to robotic scheduling (Granzer et al. ) but the unexpected high eccentricity in three systems left some phase gaps during periastron. The STELLA time coverage does not exceed 5.9 years, typical values are around 3.3 years. Although partly compensated for by the high velocity precision and the dense sampling, systems with periods longer than, say, 200 days, have comparably uncertain orbital periods. Whenever there were literature data for the eight systems with orbital periods in excess of 100 d that expanded the total time range and improved the orbit, these are included in the period determination. This was the case for HIP 999, HD 105575(ab)c and HD 202109. Otherwise, the orbits are computed from STELLA radial velocities alone. The resulting elements and their errors are summarized in Table 4 and Table 5 for the single-lined and the double-lined systems, respectively. Assuming that the fit is of good quality, we derive the element uncertainties by scaling the formal one-sigma errors from the covariance matrix using the measured values. is a time of periastron or, if the orbit is circular, a time of maximum positive radial velocity. Note that we give orbital periods as observed and not corrected for the rest frame of the system.
The computed radial-velocity curves are compared with our observed velocities in the series of panels in Fig. 2 and Fig. 3 for the single-lined orbits and the double-lined orbits, respectively. Note that two of the four triple systems, HIP 999 and HD 105575, appear in both figures. The latter system is even triple lined. The third triple system, HD 237944, is also triple lined and consists of an SB2 in a short orbit but with the third component in a wide thousands-of-years orbit around the close pair. The fourth triple system is HD 45762 where the secondary is again an SB1 but with a period exactly 1/3 of the wide SB2 pair. HD 106855 is a close visual double with its primary A-component to appear double lined, its bona-fide more massive Ab component being again an SB1 for itself. HD 16884 appears to be a quadruple system with two SB1 pairs in a bound 106.65-d orbit. More comments to individual systems are collected in Sect. 5. Note that these comments do not intend to give a comprehensive overview of CDS/Simbad-collected literature but are meant to draw attention to some specific system features.
|(days)||(HJD 245+)||(km s)||(km s)||(deg)||(10 km)|
Time of periastron, or ascending node for
Triple system. Orbit (ab) around c is given.
Quadruple system with two SB1 pairs in a synchronized orbit.
SB1 orbit of the tertiary component, , around the close (eclipsing) pair.
SB1 with long period. The orbit given is from a combination of STELLA data and selected published values. is 2421363. See individual notes.
At the time of the start of our monitoring program in 2006, a total of 17 stars were already either known binaries or had even an orbit computed, or at least had an orbital period known. However, we accumulated enough observations for all systems to allow for orbital solutions just using our own data for a certain epoch. This was motivated not only by consistency arguments but also by detections of orbital-period variations in active binaries and particularly in one of our targets (FF UMa; Gálvez et al. )222See our individual notes for this star though.. Orbital-period variations of magnetically active binaries are known for a long time (e.g. Hall & Kreiner ; see also Lanza et al. ) and were partly explained by mild mass exchange and mass loss due to a stellar wind. However, Applegate () proposed a connection between the gravitational quadrupole moment and the shape of a magnetically active component in the system as it goes through its magnetic activity cycle. Therefore, it appears rewarding to redetermine the orbits of such binaries from time to time.
|(days)||(HJD245+)||(km s)||(km s)||(deg)||(10 km)||(M)||(km s)|
|(days)||(HJD245+)||(km s)||(km s)||(deg)||(10 km)||(M)||(km s)|
Time of periastron, or ascending node for circular orbits.
SB2, but with a third component determined from the -residuals. The third-component orbit is given in the SB1 list in Table 4. See also the individual notes.
Hierarchial triple system with lines from components and . The orbit is SB1 with =19.933 d. See individual notes.
SB3 with a third component in the spectrum denoted . Its orbit around the (eclipsing) pair is given in Table 4.
SB3. The third component, , has a median velocity of –12.251.3(stdev) km s for the time of our observations.
|HD 18955||19223||51099||50||T6||29||8.46||…||…||…||var at|
|HD 23551||24513||51447||3729||T6+7||242||7.08||0.035||0.94||81.690.03||from T6|
|HD 95559||95242||51551||3686||T6+7||1061||8.83||0.08||0.93||1.5170004 10|
|HD 106855||106991||51551||3685||T7||992||9.35||0.11||1.27||0.67052586 10|
|HIP 63322||113168||51240||65||T6||35||8.89||0.08||…||2.2270.003||FAP 20%|
4.3 Rotational periods and spot amplitudes from photometry
Table 6 is the summary of the results from our photometric analysis. A total of 56 stars out of the total sample of 60 targets were observed, of which 39 have a detectable period. The time coverage and the sampling are very different from target to target, ranging from a minimum of 20 nights for QY Hya (HD 105575) to 4,160 nights (11 yrs) for HY CMa (SAO 151224) with a sampling of all-night-long monitoring (QY Hya) to one observation per night as for most targets. This diversity of data wealth makes a coherent spot-modelling analysis not feasible for the present paper and we just focus on the determination of average light-curve parameters. A more detailed analysis per target is planned for the future whenever feasible (see, e.g. Strassmeier et al.  for the most recent example of HD 123351, excluded from this paper).
Most relevant for this paper is the determination of a precise photometric period, , that can be interpreted as the stellar rotation period. This allows a superior measurement of rotation with respect to spectroscopic measurements because they are independent of the (unknown) inclination of the rotational axis and can be determined much more precisely. To proceed, we first pre-filtered all photometric data by excluding data points with an rms of 0.02 mag in order to remove data grossly affected by clouds. Periods were then searched for either in Johnson or in Strömgren or, for the stars where we have both passbands, in the combined series. For light-curves with a highly harmonic content, the Lomb periodogram (Lomb ) in the formulation of Scargle () was applied. In cases where a period could have been affected by the window function, the CLEAN algorithm (Roberts et al. ) was employed to verify, but not to alter, the period identification from the Lomb periodogram. For light curves with a highly non-harmonic content, i.e. for eclipsing binaries, a string-length minimization, a variant of a Lafler-Kinman (Lafler & Kinman ) statistic described in Dworetsky (), was used to determine the final photometric period. Some of the stars had already had a period determined in our paper I but the period in the present paper supersede these values. In Fig. 4, a phased light curve from all data for each of the variable targets is shown. These light curves are phased with the photometric period indicated in the graph and the respective time of periastron listed in Tables 4 or 5, or ascending node for circular orbits. Single-star zero epochs are arbitrary.
Errors for the periods were obtained by using a method sometimes referred to as “refitting to synthetic data sets” (e.g. Ford ). This method estimates confidence intervals by synthesizing a large number of data sets (typically 10) out from the original data by adding Gaussian random values to the measurements proportional to the actual rms of the data. The respective period-search algorithm is then applied to these synthetic data and the resulting standard deviation assumed to be the standard deviation of the original period. This value is given in Table 6 as the error for .
Note that Table 6 strictly lists photometric periods. We interpret these usually as stellar rotational periods but emphasize that care must be taken because, e.g., in case the star is a close binary, an ellipticity effect could mimic a spot light curve with a photometric period that is easily misinterpreted as the stellar rotation period. For one system, HD 138157, we interpret the 55-mmag amplitude partly due to the ellipticity effect. Its true rotational period is close to the orbital period. For five of our six eclipsing binaries the rotational modulation appears to have the same period as the orbital motion. For one case, HD 190642, a newly discovered eclipsing pair, the spot wave appears doubled humped and gives half of the true rotation period.
The typical error of the magnitude zero point is around 0.01–0.02 mag, but never higher than 0.05 mag. The wave amplitude, , in Table 6 is the maximum observed amplitude within our data coverage. For the eclipsing systems, we cut out the data points during eclipse phases and then redetermined the maximum amplitude. The color index, C.I., is the average color index over all observations. In case of T7 observations it is (-), for T6 it is -, if available. Variations in the C.I. are present but are in general too noisy for period analysis. Most of the observations of HD 136655 had to be discarded due to obvious instrumental problems. Its remaining data shows no evidence for photometric variability. Orbital period and of the eclipsing binary HD 105575 are equally constrainable from photometry than from radial velocities. If =0 is adopted, we obtain =2,454,160.772 and =0.292380.00009 d just from our 20-day long time series. However, because the photometry was obtained around JD2,451,250, i.e. almost 10,000 eclipses prior to the spectroscopic data, the combination of the two data sets still results in a loss of the cycle count and could not be combined. Table 5 gives the orbit computed only with the radial velocities.
4.4 Rotational velocities
Selected spectra of our program stars were subjected to a de-noising procedure based on a principal component analysis developed for Doppler imaging (Carroll et al. , ). This procedure employs typically a total of around 1100 spectral lines in the wavelength range 480–850 nm to determine the noise spectrum. This noise spectrum can then be used to de-noise an arbitrary wavelength section. The only free parameter is the number of principal components. No strict rule can be given for the choice of this value but we followed the prescription laid out in Martínez González et al. (). It suggests 15 components for our typical late spectral types and wavelength coverage. It is then used to de-noise a well-known and unblended spectral line from which we measure under the assumption of a temperature and gravity dependent macroturbulence. For this paper, we have chosen the well-known and unblended Fe i 549.7516 nm line with an excitation energy of 1.011 eV, a transition probability of , and a typical microturbulence of 2.0 km s. The radial-tangential macroturbulences, , are taken from Gray (), as listed in Fekel (). This macroturbulence is then subtracted from the measured total line broadening. At this point, we note that Gray () gave most probable macroturbulent velocities, which are 1.414 times greater than the rms values given here. The total line broadening is measured independently from fits with synthetic spectra over a large range of wavelengths (see next section) and the resulting measures were then compared and found to agree within their estimated errors. The final values of and for all our program stars are tabulated in Table 7 for further reference.
Up to three nightly (and two daily) Th-Ar calibration frames are used to monitor the spectrograph focus throughout the year. The line widths of Th-Ar emission lines are determined automatically and an unweighted average stored in the STELLA data base. No focus drifts were evident during the time in question. However, manual refocusing was done after every maintenance run. The telescope focus is being controlled and adjusted daily by inserting a focus pyramid into the telescope beam. It splits a stellar image into four equally distant images if the telescope is in focus (see Granzer et al. ). The STELLA control system issues a message to the operator in Potsdam if this had to be corrected.
4.5 Stellar atmospheric parameters
Five selected spectral orders (#28, 29, 33, 37, 39) from the STELLA/SES spectra are used to determine the stellar effective temperature, the gravity and the metallicity. These orders cover the wavelength ranges 549–556, 560–567, 583–591, 608–616, and 615–623 nm. Our numerical tool PARSES (“PARameters from SES”; Allende-Prieto ) is implemented as a suite of Fortran programs in the STELLA data analysis pipeline. It is based on the synthetic spectrum fitting procedure described in Allende-Prieto et al. (). Model atmospheres and synthetic spectra were taken from the ATLAS9 CD (Kurucz ). Synthetic spectra are pre-tabulated with metallicities between dex and +0.5 dex in steps of 0.5 dex, logarithmic gravities between 1.5 and 5 in steps of 0.5, and temperatures between 3500 K and 7,250 K in steps of 250 K for a wavelength range of 380–920 nm. All calculations were done with a microturbulence of 2 km s. This grid is then used to compare with the five selected echelle orders of each spectrum. Free parameters are , , Fe/H, and line broadening , where is the radial-tangential macroturbulence, again adopted from Gray () and was fixed during the fit. Internal errors are estimated from the rms of the solutions to the five echelle orders and were for typically 20–30 K, for typically 0.06 dex, and for Fe/H typically 0.03 dex.
Double-lined spectra can not be run automatically through the PARSES routines. We first apply a similar disentangling technique as for the automatic radial-velocity measurement of triple-lined spectra, i.e. we remove once the primary and once the secondary component from the combined spectrum at well-separated phases in order to create a single-star spectrum for each component. The resulting spectra were then subjected to the PARSES synthetic-spectrum fitting. This was done only for the five echelle orders that are employed for the PARSES fit (see above) and not for the entire 80 orders. Errors are generally larger for SB2s than for SB1 and single systems due to continuum cross talk. External errors are again estimated from the rms of the solutions to the five echelle orders and were for typically 100 K, for typically 0.3 dex, and for Fe/H typically 0.3 dex. However, the range of errors is systematically larger for the SB2 systems than for the SB1 and single stars because of the large range of brightness differences between the components in our sample (ranging from 1:1, i.e. nearly equal components, to 12:1 for HIP 999).
Lithium abundances are determined for all individual spectra that had sufficient S/N in the respective echelle order. The time averaged results are listed in Table 7 with respect to the (H) = 12.00 scale for hydrogen. A double Gaussian fit to Li i 6707.8 Å and the nearby Fe i 6707.4-Å line yields an individual equivalent width for the lithium line. Despite that these values are still the combined equivalent width for the close blend from the Li and the Li isotopes, it effectively removes the Fe i blend. Weaker CN features are neglected in this study because of limited S/N and spectral resolution. The listed equivalent widths are the averages of the individual measures with unit weight. The errors in Table 7 are still the O–C error from an individual Gaussian fit but again averaged over all measures in time. Because the rms values of the equivalent widths of an entire time series per star were never larger than 3 of above O–C error, we do not explicitly list them in Table 7. However, these rms are useful to judge eventual long-term variability. SB2’s are measured only at phases of quadrature, where blending is minimized. The curves of growth from Pavlenko & Magazzú () are then used to convert the equivalent width to a logarithmic lithium abundance using the non-LTE transformation and the effective temperatures and gravities from Table 7.
|(mas)||(K)||(m s)||(mÅ)||(H = 12.00)||(km s)|
|(mas)||(K)||(m s)||(mÅ)||(km s)|