New results from a survey of faint proper-motion stars - A probable deficiency of very low luminosity degenerates
Spectroscopy classifications and broad-band colors are presented largely for faint Luyten proper-motion stars with µ ≥ 1"/yr. Our primary goal was to find nearby white dwarfs of very low luminosity (VLL) — i.e., with M_bol ≥ +16 — for which recent theoretical calculations predict substantial numbers. The only possible case, LP 131-66, is discussed in detail elsewhere. Furthermore, no additional aritificially red "blanketed" stars like LP 701-29 were found. Questions concerning the completeness of Luyten's catalogs are discussed, specifically for very faint stars of large proper motion; we believe that the catalogs are >~50% complete for stars with m_R ≤ 18.0 in finished Luyten Palomar fields. We also consider the likelihood of finding VLL objects as brighter, very nearby (<5 pc) stars, and as common proper-motion companions to nearby K-M dwarfs. It is concluded that there is probably a large deficiency of VLL degenerates relative to the predicted numbers from (1) the Shaviv-Kovetz 0.6 M_sun cooling curve, (2) normalized to Green's empirical density of hot stars, and (3) with the assumption of a constant white dwarf birthrate. We consider it unlikely that the deficiency in VLL stars will be completely explained by drastic errors in the cooling theory. In particular, we consider the possibilities that Debye cooling might begin at higher luminosities than predicte or that Stevenson's insolubility effects could produce additional gravitational energy in a crystallizing core. Yet the empirical deficiency becomes even worse when one assumes a "conventional" disk star-formation picture — a disk age of ~15×10^9 years with star formation much greater in the past. Alternatively, if most disk star formation ocurred <~10^10 years ago, a curtailment of white dwarfs in the VLL interval can be explained. These results thus support the view that the stellar disk is effectively much younger than the currently accepted age of the halo. In addition to white dwarfs, the search plan uncovered several new M dwarfs of very low luminosity and a substantial number of very cool subdwarfs. Contrary to previous experience, not one M dwarf with probable M_v > +15 showed strong line emission at Hα; most were observed at least twice, though at moderate (6-12 Å) resolution. Individual discussions of white dwarfs include (1) the classification of the fourth Praesepe cluster member LB 1847 (EG 60) as DA, (2) the abundances of GD 401 (Gr 451), (3) two DB stars BPM 70524 and LB 433, and (4) some nonvariable DA stars. Finally, we report that GD 439 has a subdwarf G spectrum with a very strong CH band; the proper motion constrains M_v to be >~ +4, making this another carbon subdwarf candidate.