Astron. Astrophys. 364, 102-136 (2000)

A spectroscopic study of field BHB star candidates ^{*  **}

T. Kinman ¹, F. Castelli ², C. Cacciari ³, A. Bragaglia ³, D. Harmer ¹ and F. Valdes <sup>1

1</sup> Kitt Peak National Observatory, National Optical Astronomy Observatories, Box 26732, Tucson, AZ 85726-6732, USA (kinman@noao.edu)
² CNR-Gruppo Nazionale Astronomia and Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy (castelli@ts.astro.it)
³ Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy (cacciari@bo.astro.it; angela@bo.astro.it)

Received 23 February 2000 / Accepted 7 June 2000

Abstract

New spectroscopic observations are presented for a sample of thirty-one blue horizontal branch (BHB) star candidates that are sufficiently nearby to have reliable proper motions. Comments are given on a further twenty-five stars that have previously been suggested as BHB star candidates but which were not included in our sample. Moderately high-resolution spectra (/ 15 000) of twenty five of our program stars were taken with the coudé feed spectrograph at Kitt Peak. Twelve of the program stars were also observed with the CAT spectrograph at ESO. Six of these program stars were observed from both hemispheres. IUE low-resolution spectra are available for most of our candidates and were used, in addition to other methods, in the determination of their  and reddening. A compilation of the visual photometry for these stars (including new photometry obtained at Kitt Peak) is also given. Abundances were obtained from these spectra using models computed by Castelli with an updated version of the ATLAS9 code (Kurucz 1993a).

All thirty one candidates are halo stars. Of these, twenty eight are classified as BHB stars because:

1. they lie close to the ZAHB (in a similar position to the BHB stars in globular clusters) in the  versus  plot. For all but one of these stars, far-UV data were available which were consistent with other data (Strömgren photometry, energy distributions, H profiles) for deriving  and .

2. they have a distribution of  (40 km s) that is similar to that found for the BHB in globular clusters. Peterson et al. (1995) and Cohen & McCarthy (1997) have shown that the BHB stars in the globular clusters M13 and M92 have a higher  ( 40 km s) than those in M3 and NGC 288 (20 km s). The mean deprojected rotational velocity () was calculated for both the two globular clusters and the nearby BHB star samples. A comparison of these suggests that both globular cluster  types are present in our nearby sample. No obvious trend is seen between  and either  or [Fe/H].

3. they have -0.99[Fe/H]-2.95 (mean [Fe/H] -1.67; dispersion 0.42 dex), which is similar to that found for field halo RR Lyrae and red HB stars. These local halo field stars appear (on average) to be more metal-poor than the halo globular clusters. The local sample of red giant stars given by Chiba & Yoshii (1998) contains a greater fraction of metal-poor stars than either our halo samples or the halo globular clusters. The stars in our sample that have a  that exceeds about 8 500 K show the He I ( 4471) line with a strength that corresponds to the solar helium abundance.

4. they show a similar enhancement of the -elements ( = +0.430.04 and also = +0.440.02) to that found for other halo field stars of similar metallicity.

Key words: stars: abundances stars: fundamental parameters stars: horizontal-branch stars: AGB and post-AGB stars: white dwarfs Galaxy: halo

* Based on observations obtained at KPNO, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation, and the European Southern Observatory, Chile.
** Table 4 and Table 5 are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/364/102 or at http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/364/102

Send offprint requests to: T. Kinman

SIMBAD Objects

Contents

• 1. Introduction
• 2. The selection of candidates: notes on the individual objects
• 3. Photometric observations in the visible spectrum
• 4. Spectroscopic observations
  • 4.1. KPNO observations
  • 4.2. ESO-CAT observations
  • 4.3. The measurement of the Kitt Peak (KPNO) and ESO-CAT spectra
• 5. The interstellar reddening for the program stars
  • 5.1. The interstellar reddening for the program stars from whole-sky maps
  • 5.2. Reddening from the intrinsic colour calibration
  • 5.3. Reddening from spectrophotometric data
  • 5.4. Reddening from IUE ultraviolet data
• 6. The model atmosphere analysis
• 7. Stellar parameters
  • 7.1. Stellar parameters from Strömgren photometry
  • 7.2. Stellar parameters from spectrophotometry in the visible
  • 7.3. Stellar parameters from H
  • 7.4. Stellar parameters from IUE data
  • 7.5. The effective temperatures from
  • 7.6. The comparison of the stellar parameters determined by the different methods
• 8. Abundances
  • 8.1. Abundances from KPNO and ESO-CAT spectra
  • 8.2. The [Fe/H] abundance as a function of the equivalent width of Mg II  4481 line and the colour index
• 9. Uncertainties
  • 9.1. Uncertainty in the the stellar parameters ,  
  • 9.2. Uncertainty in
  • 9.3. Errors in
  • 9.4. Non-LTE effects
  • 9.5. Convection
• 10. Discussion
  • 10.1. Comparison with previous observers
  • 10.2. Comparison of BHB abundances with those of other types of halo stars
  • 10.3. Comparison with ZAHB models
  • 10.4. Projected rotational velocities ()
  • 10.5. Abundances of the -elements
  • 10.6. BHB binaries and HD 130095
  • 10.7. The RR Lyrae variable CS Eri (HD 16456)
• 11. Summary and conclusions
• Acknowledgements
• Appendices
  • A: Comments on other possible BHB star candidates
• References

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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