Blue Stragglers in the Solar Vicinity: Newborn or Reborn

Abstract

view Abstract Citations (8) References (95) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Blue Stragglers in the Solar Vicinity: Newborn or Reborn Eggen, Olin J. Abstract A selected sample of 1400 A0/1 to F2/(log Te=3.96 to 3.82) stars with accurate photometric and astrometric data are discussed on the basis of luminosities obtained from both Strömgren+Hβ and Geneva photometry. The sample contains members of the Hyades, Sirius, and HR 1614 superclusters, all of which show an appreciable age spread. The spread of 6 to 20 × 108 yr in the Hyades supercluster is reflected in a spread of about 6 to 10 × 108 yr in the Praesepe and Hyades clusters within the supercluster Strong arguments are presented for the presence of a larger age spread in the noncluster supercluster members than in the contained clusters. The age range for members of the Sirius supercluster is 2 to 6 × 108 yr and nearly all of this range is seen in the UMa nucleus of the Sirius supercluster, contained within the supercluster. The HR 1614 supercluster is near 4 × 109 yr old and may include the old disk cluster NGC 6791. About three-quarters of bright A-star sample consists of young disk stars, on the basis of their (U, V) velocities. The Am, USPC (Sct), and λ Boo stars represent about 10%, 6%, and 1%, respectively, of the total sample as well as of the young disk and old disk constituents of the sample. Although at least 7 USPC (δSct) are also Am stars, the two types, generally, are separated by about 2 × 108 yr. The rotational velocities, at various temperatures intervals, of the Am stars are about one-half those of the USPC(δSct) in the same intervals, whereas these velocities for the USPC and normal stars are nearly identical. At least 50% of the Am stars are spectroscopic binaries, half of which are double lined, whereas only 20% of the USPC are known spectroscopic binaries. The 15 λ Boo stars discussed here contain 8 young disk and 4 old disk stars, with 3 objects of unknown population because of radial velocity variations. Six young disk λ Boo stars are members of the Hyades (3) or Sirius (3) supercluster. The age of the λ Boo stars appears to be between 6 and 10 × 108 yr. The so-called "A2 IV" stars, labeled as possible peculiar objects by Abt (1994), may define the blue edge of the instability region. The USPC in the sample place the blue edge near log Te=3.904 for Mυ>+0.75 mag. Four stars classified as USPC are considerably hotter than the blue edge and are probably a different type of object. The old disk (BS) stars constitute about one-quarter of the bright A-star sample in each of the stages-normal stars, Am stars, USPC, and λ Boo stars. The spectroscopic binary frequency ranges from at least 50% for the Am stars to less than 5% for normal stars. This situation is the strongest argument for new birth as the origin of BS or, if rebirth, that whatever the required conditions they must mimic those for new birth. The strongest argument for rebirth as the origin of BS may be in the Am stars. Despite the fact that Am and USPC appear to be successive stages of evolution, separated by the blue edge of the instability region, and that at least a half-dozen USPC are also Am stars, the large difference in the apparent rotational velocity and large difference in the spectroscopic binary frequency makes such successive evolutionary stages seem unlikely. However, this very difficulty suggests the possibility that forward evolution for Am stars ends near the blue edge and coalescence or merger of the components ensues. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117567 Bibcode: 1995AJ....110..823E Keywords: STARS: EARLY-TYPE; STARS: EVOLUTION full text sources ADS | data products SIMBAD (69)