The Nature of the Faint Blue Stars

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view Abstract Citations (87) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nature of the Faint Blue Stars Greenstein, Jesse L. Abstract A spectroscopic survey of about one hundred faint blue stars, largely at the galactic poles, has been carried out with the 200-inch telescope of the Palomar Observatory for the past 10 years. The 190-A/mm spectra can be classified so as to distinguish with some confidence among the subdwarfs, halo, or horizontal-branch stars, detect composite spectra of unresolvable hot and cool pairs, and recognize with certainty the white dwarfs. The unpublished colors of the Humason-Zwicky stars, measured by the late Daniel L. Harris III, are here given in Table 1, together with my spectra; the spectra and available colors for the Tonantzintla and Feige stars are in Tables 2 and 3. Few "normal" main-sequence stars are found. Statistical resumes are in Table 4, and in special detail for the faintest stars observed in Table S. Among stars of (m) = +12.5 mag., we find 30 per cent halo (main sequence and horizontal branch), 25 per cent hot subdwarfs, and 15 per cent white dwarfs; the faint group at (m) = + 15.3 mag. has 40 per cent subdwarfs and 40 per cent white dwarfs, for a (M = +7 mag. The eight faintest stars in Table S (including the Lick observations by Kinman) at m> +16.0 mag. include five white dwarfs. Individually interesting stars in Tables 1-3 include many white dwarfs of type DA, and hot subdwarfs resembling type DO. None has continuous or featureless spectra, i.e., all are certainly stars, except Ton 202, which may be a DC or may have the weak emission features of an old nova, supernova, or blue galaxy. A resume of my own unpublished line intensities and half-widths is found in Figures 5-7. Hydrogen lines are enhanced in the subdwarfs but nearly normal in halo stars; He i and He ii lines are greatly enhanced in sdB and sdO by up to a factor of S. They are also greatly widened in subdwarfs. The He I lines are generally weakened by up to a factor of 3 in the halo Bp stars. Section VI includes a first- order astrophysical discussion (excluding white dwarfs) of the behavior of the broadened lines of H, He i, and He ii. The Stark broadening indicates luminosities down to Mv = +6. It is probable that the He/H ratio is high in some subdwarfs, but even more interesting is the fact that it is probably low in the halo stars. No metallic lines are seen, i.e., the group is almost certainly metal poor. The possible extragalactic nature of some faint blue stars is only marginally tested by this survey which is poor in stars fainter than 16 mag. Nevertheless, no rapid transition to a quasi-stellar, super- compact galaxy population is possible at 15.5 mag., nor is it indicated even at 16.5 mag., of which we have only a very small sample. Possibly the faint star, Ton 202, and the star plus galaxy HZ 46 (not here observed spectroscopically) are the only objects suspected to be extragalactic out of over a hundred. A proper motion of Ton 202 would be important. The high percentage of white dwarfs at 15.5 mag. suggests that they would dominate until nearly 19 mag., while the horizontal-branch stars would disappear by 17.5 mag. In Figure 4, the photoelectric colors of QSRS's (Sandage) and BSO's (Sandage and Veron) with (m = +17 mag. overlap those of subdwarfs and white dwarfs. However, about one-third of their colors lie completely outside regions of the B - V, U - B diagram studied here. The most promising candidates for small, distant galaxies are therefore to be found among oblects with positive B - V and large negative U - B. Publication: The Astrophysical Journal Pub Date: May 1966 DOI: 10.1086/148633 Bibcode: 1966ApJ...144..496G full text sources ADS | data products SIMBAD (108) NED (1)