We report on the analysis of high-resolution optical spectra for 77 subdwarf
B (sdB) stars from the ESO Supernova Ia Progenitor Survey. Effective
temperature, surface gravity, and photospheric helium abundance are determined
simultaneously by spectral line profile fitting of hydrogen and helium lines,
and are found to be in agreement with previous studies of sdB stars. 24 objects
show spectral signs of a cool companion, being either companion absorption
lines or a flux contribution at Halpha. Five stars with relatively high
luminosity show peculiar Halpha profiles, possibly indicating stellar winds.
Our results are compared to recent theoretical simulations by Han et al. (2003)
for the distribution in effective temperature and surface gravity, and are
found to agree very well with these calculations. Finally we present a binary
system consisting of two helium-rich hot subdwarfs.Comment: 8 pages, 6 figures. To appear in "Extreme Horizontal Branch Stars and
  Related Objects", Astrophysics and Space Science, Kluwer Academic Publishers,
  edited by P. F. L. Maxte

Christlieb, N.

Han, Z.

Heber, U.

Homeier, D.

Lisker, T.

Napiwotzki, R.

Reimers, D.

English

arXiv

University of Hertfordshire Research Archive

arXiv:astro-ph/0310816v1   28 Oct 2003Spectroscopic analysis of sdB stars from the ESO SupernovaIa Progenitor Survey ∗T. Lisker, U. Heber and R. NapiwotzkiDr. Remeis-Sternwarte, Bamberg, GermanyN. Christlieb and D. ReimersHamburger Sternwarte, Hamburg, GermanyD. HomeierUniversity of Georgia, Athens, USAAbstract. We report on the analysis of high-resolution optical spectra for 77 subdwarf B (sdB)stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity,and photospheric helium abundance are determined simultaneously by spectral line profilefitting of hydrogen and helium lines, and are found to be in agreement with previous studiesof sdB stars. 24 objects show spectral signs of a cool companion, being either companionabsorption lines or a flux contribution at Hα. Five stars with relatively high luminosity showpeculiar Hα profiles, possibly indicating stellar winds. Our results are compared to recenttheoretical simulations by Han et al. (2003) for the distribution in effective temperature andsurface gravity, and are found to agree very well with these calculations. Finally we present abinary system consisting of two helium-rich hot subdwarfs.Keywords: stars: subdwarfs, stars: horizontal branch, stars: evolution, stars: winds1. IntroductionIn the past years, subdwarf B (sdB) stars were subject to many observationaland also theoretical studies, raising lots of new questions in attempting toanswer the old ones. How are these stars formed? The relative importance ofsingle-star formation as well as binary evolution still needs to be determinedfrom observations. Maxted et al. (2001) showed that many sdB stars reside inclose binaries, indicating a former mass transfer phase to account for the thinenvelope of the star. Han et al. (2003) examined various formation channelsfor simulating sdB formation in this context, combining them to yield the ob-served sdB population. Theoretical studies of this kind need to be comparedto observational data of high quality to evaluate the simulation results andjudge our current understanding of sdB formation. With our sdB sample fromthe ESO Supernova Ia Progenitor Survey (SPY, Napiwotzki et al. 2001), wepresent a homogeneous, high quality dataset of higher resolution and largerwavelength coverage than previous studies of comparable size. Its analysisshould therefore bring new insight into the physics of sdB stars.∗ Based on observations collected at the Paranal Observatory of the European SouthernObservatory for program No. 165.H-0588(A) and 167.D-0407(A).c© 2007 Kluwer Academic Publishers. Printed in the Netherlands.lisker.tex; 25/01/2007; 9:45; p.122. Observation, reduction, and spectral classificationSpectra were obtained at ESO VLT with the UV-Visual Echelle Spectrograph(UVES) at UT2 (Kueyen). A slit width of 2.′′1 was used, resulting in a re-solving power of 18 500 (0.36 Å at Hα) or better. The wavelength coverage is3300−6650 Å, with gaps at about 4500−4600 Å and 5600−5700 Å. For mostof the stars, two spectra in different nights were taken. They were then re-duced with a procedure developed by C. Karl using the ESO MIDAS softwarepackage and parts of the UVES automatic reduction pipeline. The reducedspectra were convolved with a Gaussian of 1.0 Å FWHM and rebinned to0.4 Å pixel size.After a first selection of potential sdB spectra from SPY by visual in-spection, a rough line profile fit was performed, and objects with helium-dominated atmospheres were excluded. Following the common assumptionthat sdB and sdOB stars belong to the same evolutionary type, all selectedstars are named sdB stars, being 77 objects with 20 000 K < Teff < 38 000 K,4.8 < log (g) < 6.0 and −4.0 ≤ log (y) < −0.8 (photospheric helium abun-dance y = NHe/NH). 51 program stars are objects from the Hamburg/ESOsurvey (Wisotzki et al. 1996), 9 from the Hamburg Quasar Survey (Hagen etal. 1995), 14 from McCook & Sion (1999), and 3 from other sources.In many spectra, signs of a cool companion can be seen, e.g. a broadCa K line (3933 Å) as opposed to the frequently occuring narrow interstellarCa K line. The Mg I triplet at 5167 Å, 5173 Å, and 5184 Å turned out to be agood indicator for cool companions, because it reveals even the presence ofcompanions with a very low contribution to the total flux. In the spectra offive stars, though, only a rise in the continuum level at Hα pointed towardsa composite object. Altogether, 24 out of 77 sdB stars show signs of a coolcompanion. These objects are excluded from the following analysis, becausetheir determined parameters cannot be considered reliable.3. Results from spectral analysisThe determination of Teff , log (g), and log (y) was done by fitting simultane-ously each hydrogen and helium line (except Hα) to synthetic model spectra,using a procedure by R. Napiwotzki (Napiwotzki et al. 1999) based on Safferet al. (1994). See Edelmann et al. (2003) for a more detailed description.The final results were calculated as the mean value of the fit results fromindividual exposures of each star. For six objects, only one useful exposurewas available.The statistical 1-σ-errors yielded by the fit procedure are typically lowerthan 100 K, 0.02 dex, and 0.04 dex for Teff , log (g), and log (y), respectively.However, since the differences in the fit results of individual exposures oflisker.tex; 25/01/2007; 9:45; p.23Figure 1. Distribution of our sdB sample in the Teff-log (g)-plane. Grey star symbols denoteobjects with peculiar Hα profiles (see Sect. 3.2).each star are mostly larger, we used the distribution of those differences forestimating the “true errors”, leading to ∆Teff = 360 K, ∆ log (g) = 0.048 dex,∆ log (y) = 0.045 dex.3.1. A PFigure 1 shows the position of our sdB stars in the Teff-log (g)-plane, alongwith the Zero-Age EHB and Terminal-Age EHB for solar metallicity (ZA-EHB and TAEHB, Dorman et al. 1993) and the Zero-Age Main Sequence forpure helium stars (He-ZAMS, Paczyn´ski 1971). Up to effective temperaturesof Teff ≈ 33 000 K, most of our stars lie on the so-called EHB strip (definedby ZAEHB and TAEHB), while for higher Teff , almost all objects lie nearthe TAEHB or above it. This behaviour is reproduced in current theoreticalcalculations, as discussed in Sect. 4.Our distribution of helium abundances (Fig. 2) shows a wide spread fromlog (y) < −4 to slightly supersolar helium abundance. Compared with thevalues from Edelmann et al. (2003) shown in Fig. 2, there is overall agree-ment, but we find a higher fraction of stars with very low helium abundances,lacking an explanation. Theoretical calculations of the photospheric heliumcontent, which include mass loss rates of ˙M = 10−14 to 10−13 M⊙/yr, are ableto explain values of −4 < log (y) < −2 (Fontaine & Chayer 1997; Unglaub& Bues 2001). In our data, 18 out of 53 stars (one third) have log (y) > −2,clearly pointing towards higher mass loss rates in many cases or alternative(additional) physical mechanisms.lisker.tex; 25/01/2007; 9:45; p.34Figure 2. Shaded histogram: Helium abundance distribution of our sdB stars. Open his-togram: Distribution from Edelmann et al. (2003).Figure 3. Two spectra with peculiar Hα profiles, the left with a flat line core, the right with afaint emission. Observed spectra are shown as histograms, model spectra as solid grey lines.3.2. P H Five of our sdB stars show abnormal Hα line core profiles (Fig. 3), having alower depth than the model line core, being broader, and in two cases showinga faint emission. No significant variation is observed between the first andsecond exposure of the stars (time intervals range from 3 d to 1 yr), but slightchanges cannot be excluded. All objects are amongst the hottest program starswith Teff > 34 000 K (Fig. 1), and four lie among the highest luminosityobjects of the sample (the luminosity of the fifth sdB is about average). Thehelium abundances are in the normal range of −3.5 < log (y) < −1.1.lisker.tex; 25/01/2007; 9:45; p.45Figure 4. Luminosity in Eddington luminosities versus cumulative normalized countsfrom this work as well as from three previous studies. ZAEHB and TAEHB are for[Fe/H] = 0.00,−0.47,−1.48,−2.26 from left to right (Dorman et al. 1993).Line broadening could possibly be caused by rotation. However, con-volution of the synthetic spectra with rotational profiles does not lead toacceptable fits for any hydrogen or helium line. Even for Hα itself, the matchis still not satisfying, implying that other physical effects have to be the cause.There is an obvious similarity to the findings of Heber et al. (2003), whoobserved such Hα profiles in four high-luminosity sdB stars and suggestedstellar winds to be the cause. Their profiles also show broadening around thecore region, with the core being flattened in one case and containing emissionin the other cases. As shown by Vink (2003, these proceedings), it is possibleto reproduce the emission when calculating winds of ˙M = 10−11.5 M⊙/yr. Thebroadening, however, is not accounted for, and still remains to be explained.3.3. C   SdB samples of comparable size were presented in the last decade by Safferet al. (1994), Maxted et al. (2001), and Edelmann et al. (2003). They can bestbe compared to our sample by using their respective cumulative luminositydistribution, shown in Fig. 4. It can be seen that the SPY data agree well withthe observations of Maxted et al. (2001) and Edelmann et al. (2003), whereasthere is a larger average offset to the data of Saffer et al. (1994) (whichhas already been discussed in Edelmann et al. 2003). Different resolution,signal-to-noise ratio, homogeneity, and models used for line profile fittingcan probably explain these moderate differences between the samples.lisker.tex; 25/01/2007; 9:45; p.56Figure 5. Teff-log (g)-distribution of our sdB sample (filled circles) along with the simulatedstars from the best-fit model of Han et al. (2003, grey dots).4. Observation versus TheoryIn order to draw conclusions about potential sdB formation scenarios, wenow compare our data with the simulations of Han et al. (2003), which aim todetermine the relative importance of different sdB binary formation channels(stable Roche-lobe overflow channel, common envelope ejection channel,helium white dwarf merger channel). Several poorly known physical parame-ters, e.g. common envelope ejection efficiency, are varied in order to producetwelve simulation sets with different parameter configurations. Out of them,Han et al. (2003) determined a best-fit model by comparing periods and min-imum companion masses for binary sdB stars with observations by Maxtedet al. (2001) and Morales-Rueda et al. (2003). A comparison with our Teff-log (g)-data is interesting because of the homogeneity and high resolution ofour sdB spectra, surpassing previous observational samples.Before comparing theory and observation, one must consider observa-tional selection effects, in the present case mainly due to main sequence (MS)companions, which either outshine the sdB and thus prevent its detection, ormake its analysis unreliable. Han et al. (2003) apply this effect by excludingall systems with a MS companion having Teff > 4000 K, which is matchedby our exclusion of all composite spectra and therefore allows us to compareour data with the theoretical predictions.Figure 5 shows our data along with the simulated sdB stars from thebest-fit model of Han et al. (2003). The simulations match well the shapeof our sdB distribution as well as the Teff-log (g)-area covered. One of ourlisker.tex; 25/01/2007; 9:45; p.67Figure 6. Comparison of first exposure (grey histogram) and second exposure (black his-togram) of HE 0301−3039. Both spectra were brought to the same flux level. Barycentriccorrections have been applied, and the given wavelength is relative to the apparent line center.objects is a potential blue Horizontal Branch star (labeled “1” in Fig. 5) andis therefore not subject to a comparison with calculations for EHB stars. Theobject labeled “2” lies in a region where there is almost no simulated star.However, this object may be in a fast post-EHB phase of evolution (see alsoFig. 1). As long as theory does not predict the formation of such an object tobe impossible, it cannot be considered a disagreement with the simulations toobserve one such star.The positions of our stars at or above the TAEHB at higher tempera-tures are reproduced in the simulations, partially reflecting their prediction ofmasses in the range 0.4 < MsdB < 0.6 M⊙, as opposed to 0.46 < MsdB <0.50 M⊙ from single star evolutionary calculations (which define the ZA-EHB and TAEHB shown in Fig. 5) . Note, however, that the latter predictsome 10 % of the objects to be in the post-EHB stage (Dorman et al. 1993),which could probably explain the observed stars at high temperatures andluminosities above the TAEHB as well.5. A subdwarf+ subdwarf binaryFigure 6 shows parts of two spectra of the helium-rich hot subdwarf HE0301−3039, the time interval being 10.5 months. The lines split into twoapparently equal components, being obviously deeper when combined thanwhen separated, the radial velocity difference being about 160 km/s for alllines. Hence HE 0301−3039 is a double-lined binary system consisting oftwo very similar hot subdwarfs. To our knowledge this is the first such systemdiscovered. Further observations are scheduled to measure its radial velocitycurve and system parameters.lisker.tex; 25/01/2007; 9:45; p.786. ConclusionsWe have reported on the spectral analysis of 77 sdB stars from SPY, as well ason the detection of the first known binary consisting of two hot subdwarfs. 24sdB stars show signs of a cool companion. Of the 53 non-composite objects,five stars with relatively high luminosity show peculiar Hα profiles, possiblyindicating stellar winds. Our data are found to be in good agreement withprevious studies of sdB stars. The best-fit model of the binary populationsynthesis calculations by Han et al. (2003) reproduces the observed sdB dis-tribution very well. Note, however, that at this point no statement can be madeas to the relative importance of single-star formation channels. A detailedcomparison of our observations with all simulation sets of Han et al. (2003)will be presented in a forthcoming paper.AcknowledgementsWe would like to thank Zhanwen Han for providing us with his simulationresults, and Jorick S. Vink for valuable information on stellar winds.ReferencesDorman, B., Rood, R. T., & O’Connell, R. W. 1993, ApJ, 419, 596Edelmann, H., Heber, U., Hagen, H.-J., et al. 2003, A&A, 400, 939Fontaine, G. & Chayer, P. 1997, in The Third Conference on Faint Blue Stars, 169Hagen, H.-J., Groote, D., Engels, D., & Reimers, D. 1995, A&AS, 111, 195Han, Z., Podsiadlowski, P., Maxted, P. F. L., & Marsh, T. R. 2003, MNRAS, 341,669Heber, U., Maxted, P. F. L., Marsh, T. R., Knigge, C., & Drew, J. E. 2003, in ASPConf. Ser. 288: Stellar Atmosphere Modeling, 251Maxted, P. F. L., Heber, U., Marsh, T. R., & North, R. C. 2001, MNRAS, 326, 1391McCook, G. P. & Sion, E. M. 1999, ApJS, 121, 1Morales-Rueda, L., Maxted, P. F. L., Marsh, T. R., North, R. C., & Heber, U. 2003,MNRAS, 338, 752Napiwotzki, R., Christlieb, N., Drechsel, H., et al. 2001, Astron. Nachr., 322, 411Napiwotzki, R., Green, P. J., & Saffer, R. A. 1999, ApJ, 517, 399Paczyn´ski, B. 1971, Acta Astronomica, 21, 1Saffer, R. A., Bergeron, P., Koester, D., & Liebert, J. 1994, ApJ, 432, 351Unglaub, K. & Bues, I. 2001, A&A, 374, 570Wisotzki, L., Koehler, T., Groote, D., & Reimers, D. 1996, A&AS, 115, 227lisker.tex; 25/01/2007; 9:45; p.8

Astrophysics and Space Science

in The Third Conference on Faint Blue Stars,

Spectroscopic analysis of sdB stars from the ESO Supernova Ia Progenitor Survey

T. Lisker

U. Heber

R. Napiwotzki

N. Christlieb

D. Reimers

D. Homeier

Crossref

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.337.5703

Spectroscopic analysis of sdB stars from the ESO Supernova Ia Progenitor
  Survey

Spectroscopic analysis of sdB stars from the ESO Supernova Ia Progenitor Survey

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