Discovery of a stripped red giant core in a bright eclipsing binary star

Discovery of a stripped red giant core in a bright eclipsing binary star

P. F. L. Maxted, D. R. Anderson, M. R. Burleigh, A. Collier Cameron, U. Heber, B. T. Gänsicke, S. Geier, T. Kupfer, T. R. Marsh, G. Nelemans, S. J. O’Toole, R. H. Østensen, B. Smalley, R. G. West, S. Bloemen Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, D 96049 Bamberg, Germany
Abstract

We report the serendipitous discovery from WASP archive photometry of a binary star in which an apparently normal A-type star (J024725 A) eclipses a smaller, hotter subdwarf star (J024725 B). The kinematics of J024725 A show that it is a blue-straggler member of the Galactic thick-disk. We present follow-up photometry and spectroscopy from which we derive approximate values for the mass, radius and luminosity for J024725 B assuming that J024725 A has the mass appropriate for a normal thick-disk star. We find that the properties of J024725 B are well matched by models for a red giant stripped of its outer layers and currently in a shell hydrogen-burning stage. In this scenario, J024725 B will go on to become a low mass white dwarf () composed mostly of helium. J024725 B can be studied in much greater detail than the handful of pre helium white dwarfs (pre-He-WD) identified to-date. These results have been published by 2011arXiv1107.4986M. We also present a preliminary analysis of more recent observations of J024725 with the UVES spectrograph, from which we derive much improved masses for both stars in the binary. We find that both stars are more massive than expected and that J024725 A rotates sub-synchronously by a factor of about 2. We also present lightcurves for 5 new eclipsing pre-He-WD subsequently identified from the WASP archive photometry, 4 of which have mass estimates for the subdwarf companion based on a pair of radial velocity measurements.

Department of Physics, University of Warwick, Coventry, CV4 7AL

Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

Australian Astronomical Observatory, PO Box 296, Epping, NSW, 1710, Australia

Institute of Astronomy, K.U.Leuven, Celestijnenlaan 200D, 3001, Heverlee, Belgium

Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH

1 Discovery of J024725

The WASP survey (Wide Angle Search for Planets, 2006PASP..118.1407P) uses two instruments to monitor the brightness of millions of stars in both hemispheres. Each instrument has 8 e2V CCD cameras with 200 mm f/1.8 Canon lenses to produce images covering approximately on the sky per camera. Two 30s exposures are obtained on selected fields every 5 – 10 minutes every clear night. The strategy is optimised for the detection of planetary transits for stars with V 9-13. The techniques used to identify planetary transits in the WASP data are also very effective at identifying eclipsing binary stars. One star flagged as an eclipsing binary star as part of this process was 1SWASP J024743.37251549.2 (J024725 hereafter). The WASP lightcurve of this star is shown as a function of orbital phase with the period 0.6678 d in Fig.1. The shape and depths of the eclipses in this lightcurve show that the feature at phase 0 is the total eclipse of a smaller but hotter star by its larger and cooler companion. The catalogue photometry available show that the larger star (J024725 A), which contributes 90% of the optical light, is an A-type star, so we obtained follow-up observations to determine the nature of the smaller, hotter star. Photometry with the SAAO 1.0-m telescope (Fig.1) confirmed our interpretation of the WASP lightcurve. Spectroscopy with a variety of instruments was used to confirm the mid-A spectral type of J024725 A and to measure its the spectroscopic orbit (Fig. 2). We used the lightcurve model EBOP (1981psbs.conf..111E; 1981AJ.....86..102P) to analyse the WASP and SAAO 1.0-m lightcurves. The surface brightness ratio we derive from the lightcurve models can be combined with the observed V and K magnitudes of J024725 to estimate the effective temperatures TK and TK. The surface gravity of J024725 B, , can be derived directly from the parameters of the lightcurve model and the mass function. In Fig. 3 we compare these values of T and to the effective temperatures and surface gravities of 298 faint blue stars observed by 1997ApJ...491..172S. It is clear that J024725 B is unusually cool given its surface gravity and sits well below the main sequence (long-dashed lines) and the zero-age horizontal branch (short-dashed lines).

Figure 1.: Lightcurves of J024725. From bottom-to-top: WASP white-light photometry with lightcurve model fit, SAAO 1.0-m I-band and V-band.
Figure 2.: Left panel: GMOS-S spectrum of J024725. Right panel: Radial velocities of J024725 A with a circular orbit fit. The spectrograph used is indicated as follows: filled circles – EFOSC2; triangles – ISIS; diamonds – GMOS.
Figure 3.: Location of the J024725 B in the T plane (solid diamond) compared to 298 faint blue stars from the survey of 1997ApJ...491..172S.
Figure 4.: Location of the J024725 A in the T – density () plane compared to evolutionary models for normal stars with masses as noted and [Fe/H]= 0.65 from 2000A&AS..141..371G.
Figure 5.: Location of the J024725 B and related objects in the Hertzsprung–Russell diagram. Models for the formation of low mass white dwarfs (with final masses as noted, bottom-to-top) are also shown as follows: 1999A&A...350...89D – solid lines (0.195M and 0.234M); 2004ApJ...616.1124N – dotted lines (0.205M and 0.215M); 2010ApJ...715...51V – dashed lines (0.21M).

The kinematics of J024725 show that it is a member of the Galactic thick disk, which suggests that it is likely to be old ( Gyr), metal poor () and have enhanced -element abundance ([Mg/Fe] ). The density of J024725 A, , can be derived directly from the parameters of the lightcurve model and the mass function. We compare the values of T and to stellar models for the appropriate composition in Fig. 4. This comparison leads to an estimate of for the mass of J024725 A and, via the mass function, a mass estimate of for J024725 B.

In Fig. 5 we compare the position of J024725 B in the Hertzsprung-Russell diagram to evolutionary tracks for the formation of low mass white dwarfs (M) as a result of drastic mass loss from low mass red giant stars. The observed properties of J024725 B are well matched by such models during the phase when the star is evolving bluewards at almost constant luminosity due to p-p shell-hydrogen burning in the thin hydrogen envelope. In this scenario J024725 B will become a low mass white dwarf composed almost entirely of helium, so we dub it a pre helium white dwarf (pre-He-WD). Also shown in Fig. 5 are other He-WD and pre-He-WD. The parameters of the related objects are listed in Table 1.

A complete description of the discovery and characterisation of J024725 has been accepted for publication in MNRAS (2011arXiv1107.4986M).

Figure 6.: A small section of our UVES spectra of J024725 displayed as greyscale images in phase order.
Figure 7.: Spectra of J024725 A (upper spectrum) and J024725 B (lower spectrum) around H.

2 UVES spectroscopy of J024725

We obtained high resolution, high signal-to-noise spectroscopy of J024725 with the UVES echelle spectrograph on the VLT 8.2-m UT2 telescope. Service mode observations were used to obtain 46 spectra covering the quadrature phases of the orbit and 12 spectra during the total eclipse, i.e., spectra of J024725 A alone. A small section of these spectra around the Mg II 4481Å line is shown in Fig. 6. The narrow Mg II line from J024725 B can be seen moving in anti-phase to the broader and stronger spectral lines of J024725 A. The mean spectrum obtained during total eclipse was used as a template to measure the radial velocity of J024725 A by cross correlation. We then subtracted the mean spectrum of J024725 A from the 46 out-of-eclipse spectra after shifting it by the appropriate radial velocity and scaling it by an estimate of the luminosity ratio at this wavelength. This process revealed the underlying spectrum of J024725 B. We measured the radial velocity of J024725 B using a gaussian fit to the Mg II 4481Å line in these spectra. We then shifted and added these spectra to produce the mean spectrum of J024725 B shown in Fig. 7. Also visible in this spectrum are a weak He I 4471Å line and the broad H line.

The radial velocites measured from these UVES spectra combined with the inclination from the lightcurve model imply masses of and . The coverage of the UVES spectra is greater than the limited results presented here and several other spectral lines from J024725 B are visible, so it will be possible to further improve these mass estimates. Even so, it is clear that the masses of both J024725 A and J024725 B are larger than expected based on the stellar models we have used above. For J024725 A the discrepancy between the mass observed and that expected based on stellar models is similar to that observed by 2007AJ....133.2457K for V209  Cen A, the companion to the pre-He-WD V209  Cen B in an eclipsing binary member of the globular cluster  Cen.

We have measured a projected rotational velocity of for J024725 A from the rotational broadening of its spectral lines. With our improved mass estimates from the UVES spectroscopy we find that this is approximately half the rotational velocity expected if J024725 A rotates synchronously with the orbit.

Name Period [d] Mass[M] Source

NGC 6121-V46 0.087 2006BaltA..15...61O
HD 188112 0.607 2003A+A...411L.477H
J024725 B 0.668 This paper
PC1-V36 0.794 2007AJ....133.2457K
V209  Cen B 0.834 2007AJ....133.2457K
KIC 10657664 3.274 2011ApJ...728..139C
KOI-75 5.189 2010ApJ...715...51V
KOI-81 23.89 2010ApJ...715...51V
Regulus B 40.11 2008ApJ...682L.117G
Table 1.: Masses and periods for low mass white dwarfs and pre-He-WDs in binary systems.

3 New eclipsing pre-He-WD

We have inspected several thousand lightcurves of stars flagged as eclipsing binary stars in the WASP archive to look for new examples of eclipsing pre-He-WD similar to J024725. The features we looked for in the lightcurve were: a total eclipse with a depth of about 10%; sharp ingress/egress to the total eclipse; a visible secondary eclipse. The lightcurves of 6 stars satisfying these criteria, including J024725, are shown in Fig. 8. The properties of these stars are given in Table 2. The spectral type of the stars has been estimated from the catalogue photometry available for these stars. Only J1323+43 (EL CVn) has been previously identified as an eclipsing binary star (2002MNRAS.331...45K).

For 4 of these stars we have obtained two spectra with the TWIN spectrograph on the CAHA 3.5-m telescope, one at each of the quadrature phases. Observations were obtained with a low resolution grating on the blue arm covering the wavelength range 3290 – 5450Å. These spectra have been used to confirm that the spectral types given in Table 2 are approximately correct but have not yet been analysed any further. The red arm observations have a resolution of approximately 1.5Å and cover the H lines. We used least squares fitting to determine an empirical line profile for the H line in each star composed of the sum of 3 gaussian functions. This empirical line profile was then used to measure the radial velocity of the star at the two quadrature phases observed. We then assumed a mass for the brighter component of each binary based on its spectral type and used the mass function to estimate the masses for the fainter pre-He-WD components given in Table 2.

4 Discusssion

J024725 B is an ideal system for testing in detail models for the formation of low mass helium white dwarfs. It is a bright star, much brighter than the more distant examples of pre-He-WD found in globular clusters. It is a double-lined eclipsing binary star and so it is possible to measure precise, model-independent masses and radii for both stars in the binary. This is not possible for most of the other pre-He-WD listed in Table 1. The total eclipses and moderate luminosity ratio of this binary make it possible to recover a high quality spectrum of the pre-He-WD in this binary, as we have shown for our UVES spectra. This will make it possible to measure properties of J024725 B such as it rotational velocity, effective temperature and surface composition. This may make it possible to test the prediction of some evolutionary models that objects such as J024725 B should be hydrogen deficient. Some low mass white dwarfs are expected to undergo a number of unstable flashes of CNO hydrogen burning during their early evolution. The occurance of these flashes depends critically on the mass of hydrogen that remains on the surface of the star, which in turn depends on the mass loss history of the star. Understanding these hydrogen shell flashes is crucial for a better understanding of all low mass white dwarfs, particularly the low mass white dwarf companions to millisecond pulsars. It may be possible to put useful constraints on the hydrogen envelope mass in J024725 B by comparing its total mass to the core mass inferred from its luminosity. The sub-synchronous rotation of J024725 A is rather surprising given that it is expected to have gained rather a lot of mass and angular momentum from the the red giant progenitor of J024725 B. It may be that this star is currently far from equilibrium. A detailed reconstruction of the evolutionary history of J024725 will lead to a much better understanding of how stars react to mass accretion. This will obviously be interesting for improving our understanding of binary star evolution, but may have wider implications, e.g., episodic accretion may be the process that dominates the observed properties of pre main-sequence stars (2009ApJ...702L..27B). The kinematics of J024725 also imply useful constraints on the composition and age of this binary star.

The discovery of several other eclipsing pre-He-WD opens up the possibility of exploring how the formation of these objects varies with parameters such as the initial masses and orbital periods of the binary. It also makes the tests of the evolution models for these objects much stronger because fine tuning of parameters or extraordinary evolutionary scenarios that might be invoked to explain the formation of a single object cannot be justified when several similar examples exist. It may also be possible to put useful constraints on the space density of such objects since the WASP survey seems to be very effective at detecting these short period eclipsing binaries.

Figure 8.: WASP lightcurves of eclipsing pre-HE-WD.

Star Spectral V Period M
Type [mag] [d] [M]

J024725 A6 11.9 0.668 0.29
J1323+43 A1 9.4 0.795 0.2
J162504 A7 10.4 1.526 0.15
J1628+10 F6 12.9 0.720 0.05
J210106 A2 11.5 1.290 0.2
J232839 A6 13.3 0.769
Table 2.: Newly identified eclipsing pre-He-WD

Acknowledgments

Funding for WASP comes from consortium universities and from the UK’s Science and Technology Facilities Council (STFC). The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement n227224 (PROSPERITY), as well as from the Research Council of K.U.Leuven grant agreement GOA/2008/04.

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