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The Nature of Faint Blue Stars in the Halo. II

Greenstein, Jesse L. and Sargent, Anneila I. (1974) The Nature of Faint Blue Stars in the Halo. II. Astrophysical Journal Supplement Series, 28 (259). pp. 157-209. ISSN 0067-0049.

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Spectra and colors of 189 hot (FB) stars selected colorimetrically and mostly within 30° of the galactic poles are analyzed quantitatively for surface gravity and effective temperatures. Palomar spectra give line intensities and Balmer line profiles, found in the Appendix tables. Using a network of model atmospheres, the g and θ so determined give directly the light-to-mass ratio, and eventually the luminosities. The high-latitude hot stars cover a range of 10^6 in luminosity, and are classifiable into various groups. Spectroscopically normal B stars make up 26 percent of the sample; they rotate and have nearly normal line spectra. Interpreted as Population I stars, on the mass-luminosity relation, they have relatively high luminosities and large z-coordinates. For some, the nuclear lifetimes at present luminosities are less than transit times from the plane. Their high velocities present a problem for galactic kinematics. A few are slightly helium rich, while others are highly evolved Population II stars, from details of the spectroscopy. Some of the Population I stars may be in not-understood, short-lived evolutionary phases of high luminosity and low mass. The (g, θ)-relation can be transformed into luminosity-temperature diagrams if masses are known. Many fall along a relation log gθ^4 = 2.35, have common properties of halo stars, and T_(eff) from 9500° to 40,000° K. The light-to-mass ratio for these is 68 (in solar units) ; most HB and sdB stars have weak metal and weak helium lines, i.e., are halo stars. We call this the extended horizontal branch (EHB). Quantitative classification gives 17 percent of the FB stars as horizontal-branch B and early A (HB) stars, and 16 percent subdwarf B (sdB) stars. There are 16 percent subdwarf O (sdO) stars, including planetary nuclei; these have strong, or very strong, helium lines. Assuming a constant mass, 0.66 m_⊙, the observed L/m gives Mb = +0.5. The EHB for field stars fits the globular cluster HB, and extends it to high T_(eff). The bolometric correction produces the change of M_v with color. The hot white dwarfs are 11 percent of the FB stars and appear as a bridge between the sdO stars and the ordinary DA white dwarfs. Composite stars are unresolved, noninteracting binaries with strange UBV colors; they require a faint Mv for the primary (e.g., sdO or sdB), and an unevolved G companion. The helium deficiency and the evolutionary problems are sharpest for the EHB stars. In a few, slightly stronger helium lines are accompanied by lines of peculiar elements (S III, P II seen at high dispersion. While the L/m ratio is closely the same for sdO as for sdB and HB, the He lines become strong. Evolutionary tracks avoid the region of the hot EHB stars; the hottest sdO stars approach the helium main sequence. The low surface helium is almost certainly not cosmological in origin. Gravitational diffusion in a nonrotating star in the absence of convection is plausible; in the sdO and the peculiar sdB stars convection and helium flashes may have occurred. Numerous radial velocities were measured. Where possible, proper motions, luminosities, and radial-velocity dispersions were used for space motions. The Population I normal stars have abnormally high space motions, not greatly different from Population II stars. The luminosities derived from spectroscopy are consistent with those from peculiar motions.

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Sargent, Anneila I.0000-0002-4633-5098
Additional Information:© 1974 American Astronomical Society • Provided by the NASA Astrophysics Data System. Received 1973 December 3. I appreciate very much the generous spans of observing time allocated me at the Aerospace Corporation by E. E. Epstein. Observing time at the Millimeter Wave Observatory and the National Radio Astronomy Observatory is also gratefully acknowledged. I am indebted to G. R. Knapp for introducing me to the technicalities of millimeter radio astronomy and to S. Beckwith, T. Boroson, P. Vanden Bout, J. Elias, N. J. Evans, P. Goldreich, G. Neugebauer, R. N. Snell, J. D. White, and W. J. Wilson for assistance at various stages of the project. In part, presented as the Henry Norris Russell Lecture of the American Astronomical Society, delivered at Boulder, Colorado, 1970 June. It would be impossible to thank adequately the numerous associates for whose advice and suggestions we wish to express our deep gratitude. Without the enormous advances in stellar atmosphere computations, this quantitative classification method would not have been possible; we are grateful to those who developed the ATLAS program, to Dimitri Mihalas for frequent advice, to Deane Peterson and Harry Shipman for specially computed models, and to Daniel Klinglesmith for his unpublished models. We are obviously indebted to many who helped refine the temperature scale, or worked on similar problems. We must also express our gratitude to Marilynne Rice for her skill and patience in preparing this manuscript. And finally, we are grateful to the memory of Henry Norris Russell, who always insisted on the primacy of quantitative observations in astrophysics. J. L. G. wishes to express his gratitude for the leisure and instruction provided by his visit to NORDITA (Copenhagen) in 1972, and to the Associates of the California Institute of Technology for their support of the Lee A. DuBridge Professorship. A. I. S. is grateful for the opportunity to work at IOTA (Cambridge) during the summers of 1969 and 1970.
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Lee A. DuBridge FoundationUNSPECIFIED
Subject Keywords:faint blue stars — high-velocity stars — horizontal-branch stars — subdwarfs — stellar evolution
Issue or Number:259
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98735
Deposited By: George Porter
Deposited On:18 Sep 2019 23:21
Last Modified:03 Oct 2019 21:43

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