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1992Aj 103.1987B the Astronomical Journal THE ASTRONOMICAL JOURNAL VOLUME 103, NUMBER 6 JUNE 1992 103.1987B A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. II Timothy C. Beers1 2 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 1992AJ George W. Preston2 and Stephen A. Shectman2 The Observatories, Carnegie Institution of Washington, Pasadena, California 91101 Received 27 November 1991; revised 19 February 1992 ABSTRACT We report on spectroscopic observations for 1044 stars, located primarily in the southern Galactic hemisphere, chosen from a list of candidate metal-deficient stars discovered in the HK objective-prism survey. Metal abundances for the program stars, on the [Fe/H] scale, are obtained from a previously published calibration of the variation in the equivalent width of the Ca il K absorption line (A3933 A) as a function of broadband (B—V)0 color. Broadband UBV photometry is available for roughly one-third of the present sample. For the majority of stars without available photometry (primarily stars with an inferred [Fe/H] > — 2.0), we obtain estimated dereddened (B—V)0 colors from an empirical calibration of the variation of Balmer line equivalent width with broadband color. Radial velocities, with accuracies on the order of 10 km s_1, are obtained for all our program stars. Distance estimates, accurate to 10%-15%, are reported for the subsample of program stars with available photometry. Less accurate distances are obtained for the remaining stars by a calibration of apparent magnitude with crude brightness estimates from the original survey plates. The sample reported here includes 734 stars with [Fe/H]< —1.0, 446 stars with [Fe/H]< — 2.0, 70 stars with [Fe/H]< — 3.0, and at least three stars with [Fe/H] < — 4.0. Due to uncertainties in the calibration of our metal-abundance determinations for late-type stars, the actual number of stars with [Fe/H]< —4.0 in the present sample may be on the order of 5-10. Fifty stars in our sample exhibit anomalously strong G bands, characteristic of the subgiant CH stars discussed by Bond. Among the most metal-deficient objects (as inferred from their Can K equivalent widths), we identify six stars with moderate to strong CN bands. The present program stars form the basis for addressing a wide range of questions concerning the formation and evolution of the Galactic halo population, and, by inference, the nature of the first generations of star formation in the universe. The number of extremely metal-poor stars ( [Fe/H] < —2.0) reported in the present sample is roughly quadruple the sample size of nonkinematically selected stars of similar metal abundance re- ported by previous workers, and is on the order of the total previous samples of stars in this abundance regime when all sources are included. The number of stars with [Fe/H]< — 3.0 in the present sample is seven times the number of previously identified stars with spectroscopically determined abundances which are this low. We emphasize, however, that the present sample only represents on the order of 10% of the number of such extreme objects that remain to be discovered upon completion of the follow-up spectroscopy and photometry of metal-deficient candidates identified in our ongoing objective-prism survey. 1. INTRODUCTION however, samples of stars have been too small to place anything but the most general limits on the applicability of A host of longstanding problems concerning the forma- even relatively crude models. In the first paper in this series tion and evolution of stars in the halo of our Galaxy can Beers et al. ( 1985, hereafter referred to as BPS I) identified only be tested with a large sample of metal-deficient stars. 134 stars with inferred abundances [Fe/H] < — 2.0, and The metallicity distribution function (MDF) of low-mass, speculated that the relative numbers of stars at low metal- long-lived stars, for example, provides constraints on mod- licities might be consistent with a differential MDF, dN/ els of chemical evolution and on timescales for the redis- dz, which was flat down to the lowest measurable abun- tribution of elements within the Galaxy (Pagel & Patchett dance, a speculation supported by the analysis of Beers 1975; Hartwick 1976; Pagel 1989). Until very recently, (1987). Recently, analyses of the MDF for large samples of kinematically selected halo stars have confirmed the ex- 'Visiting Astronomer, Kitt Peak National Observatory, which is operated cellent agreement between simple model predictions (as by the Association of Universities for Research in Astronomy, under modified by Hartwick 1976) and observation down to at contract to the National Science Foundation. least [Fe/H] =—3.0 (Laird et al. 1988; Ryan & Norris 2Visiting Astronomer, Cerro Tololo Inter-American Observatory, which is operated by the Association of Universities for Research in Astronomy, 1991). under contract to the National Science Foundation. Other important problems of current interest include: 1987 Astron. J. 103 (6), June 1992 0004-6256/92/061987-48$00.90 © 1992 Am. Astron. Soc. 1987 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1988 BEERS ETAL: STARS OF VERY LOW METAL ABUNDANCE 1988 103.1987B (1) Observations of the light elements Li, Be, and B in sures were 90 min long, using 103a-O emulsion baked in extremely metal-deficient stars (Gilmore et al. 1991; forming gas. The spectra were widened to 0.2 mm by Hobbs et al. 1991; Molaro 1991), which potentially pro- means of a single trail produced by nine discrete offsets of vide sensitive tests of standard and inhomogeneous big the guide telescope. The spectral resolution varies with see- 1992AJ bang cosmological models (Malaney & Alcock 1990; ing and focus quality from plate to plate, but is typically 5 Walker et al. 1991). (2) Measurement of the abundances A FWHM. An interference filter placed near the focal of the CNO elements in field stars (see Sneden 1985, and plane restricts the bandpass of our survey to approximately references therein; Wheeler et al. 1989, and references 150 A centered on A3950 Â. Because of the interference therein; Sneden 1990). Of particular importance to such filter, sky background is suppressed and spectral overlap is studies is the dependence of CNO abundances on the stel- reduced. The faintest stars in our survey reach apparent lar metallicity. (3) Measurement of the relative abun- magnitude 2?^ 16.0, several magnitudes fainter than the dances of heavy elements in metal-deficient stars, which surveys of Bond (1970), Bond (1980), or Bidelman & summarize aspects of the cumulative history of star forma- MacConnell (1973). The prism spectra saturate at B tion, stellar evolution, and nucleosynthesis processes which ~ 11.0. We are in the process of extending the survey in the have taken place in our Galaxy (Gilroy et al. 1988; Grat- northern hemisphere with the 24V26" Burrell-Schmidt at ton & Sneden 1990; Molaro & Bonafacio 1990; Molaro & Kitt Peak National Observatory (using Ila-O plates, and a Castelli 1990; Zhao & Magain 1990; Ryan et al. 1991; similar observing strategy), and are continuing to expand Norris et al. 1992). With a sufficiently large sample of the areal coverage from the south. metal-poor stars, one might attempt to unravel the mass Tables 1(a) and 1(b) list the survey plates which have spectrum of star formation and evolution processes as a been obtained to date. In total, the survey now contains function of chemical time, and perhaps, eventually, real 289 plates, 270 of which are unique, for a present sky time. Thielemann, et al. (1990), and Mathews & Cowan coverage of 6750 square deg. The plates in Tables 1(b) for (1990) concisely summarize a theoretical outline of the which we have at least partial spectroscopic data available nucleosynthesis sites which are expected to dominate the are indicated in boldface type. Figure 1 shows the locations early chemical history of the Galaxy. of the survey plates on polar plots of the north and south In this paper we report radial velocities, metallicity, and Galactic hemispheres. Each square represents a single distance estimates for a sample of 1044 candidate metal- objective-prism plate drawn roughly to scale. The dark poor stars selected from the HK objective-prism/ squares represent plates for which detailed observations of interference-filter survey of Beers et al. Included in this some (but not necessarily all) low-metallicity candidates sample are sufficiently large numbers of stars with extreme are discussed in this paper. As seen in the figure, we pres- abundances to address many of the problems outlined ently have sufficient plates to provide excellent sky cover- above. In Sec. 2 we summarize the present status of the HK age within 20° of the Galactic poles. survey. In Sec. 3 we discuss moderate-resolution (1 Â) The plates are visually inspected with a low-power bin- spectroscopic observations for stars in the present sample. ocular microscope. The vast majority of images exhibit Section 4 summarizes the broadband UBV photometry strong Ca il H and K absorption typical of normal G and available for roughly one-third of the stars in the sample, K stars. A small fraction of stars (roughly 1 in 1000) and discusses the estimation of (B—V)q from Balmer line exhibit weak Ca il H and K lines which identifies them as indices for stars presently without photometry. In Sec. 5 metal-poor (hereafter referred to as MP) candidates. we apply the calibration of Beers et al. (1990) to obtain Three to five times this number of spectra exhibit weak or estimates of [Fe/H] (accurate to <$[Fe/H]~0.15-0.20 dex absent Ca n K and strong He, identifying them to be AB- from observed equivalent widths of the Ca il K line in our or A-type stars.
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