Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

4.4. Population III Remnants

Even if a large fraction of the baryons are processed through Population III stars, this does not necessarily guarantee dark matter production. However, most stars ultimately produce dark remnants and we now list the various possibilities.

LOW MASS OBJECTS    We will see in Section 9.3 that stars in the range 0.08-0.8 M (which are still on the main-sequence) are probably excluded from explaining any of the dark matter problems. However, objects in the range 0.001-0.08 M would never burn hydrogen and would certainly be dim enough to escape detection. [Note that Salpeter (1993) argues that the critical mass for hydrogen burning could be higher for Population III stars because slow protostellar accretion could lead to degenerate cores with lower central temperatures than usual.] Such brown dwarfs (BDs) represent a balance between gravity and degeneracy pressure. Those above 0.01 M could still burn deuterium; Shu et al (1987) have argued that this may represent a lower limit to a BD's mass but this conclusion is not definite. The evidence for stars in the brown dwarf mass range (e.g. Simons & Becklin 1992, Steele et al 1993) is controversial, but this merely reflects the fact that they are hard to find (Stevenson 1991) and it would be very surprising if the IMF happened to cut off just above 0.08 M. Most searches have focused on BDs in binary systems with M-dwarfs; however, we already know that the BDs making up the dark matter could not be in such binaries else the M-dwarfs would have more than the dark density (cf McDonald & Clarke 1993). Objects below 0.001 M are held together by intermolecular rather than gravitational forces (i.e. they have atomic density) and may be described as snowballs. We will see in Section 10.1 that such objects are unlikely to constitute the dark matter.

INTERMEDIATE MASS OBJECTS    Stars in the range 0.8-4 M would leave white dwarf remnants, while those between 8 M and some mass M_BH would leave neutron stars remnants. In either case, the remnants would eventually cool and become dark. (Stars in the mass range 4-8 M could be disrupted entirely during their carbon-burning stage.) Stars more massive than M_BH could evolve to black holes: the value of M_BH is uncertain but it may be as high as 50 M (Schild & Maeder 1985) or as low as 25 M (Maeder 1992). Only intermediate mass remnants definitely form at the present epoch; this is why some theorists favor them as dark matter candidates (Silk 1991, 1992, 1993). However, we will see in Section 5.2 that their nucleosynthetic consequences may make them poor dark matter candidates.

VERY MASSIVE OBJECTS    Stars in the mass range above 100 M, which are termed "Very Massive Objects" or VMOs, would experience the pair-instability during their oxygen-burning phase (Fowler & Hoyle 1964). This would lead to disruption below some mass M_c but complete collapse above it (Woosley & Weaver 1982, Ober et al 1983, Bond et al 1984). VMO black holes may therefore be more plausible dark matter candidates than ordinary stellar black holes. In the absence of rotation, M_c 200M; however, M_c could be as high as 2 × 10⁴ M if rotation were maximal (Glatzel et al 1985). Note that stars with an initial mass above 100 M are radiation-dominated and therefore unstable to pulsations during hydrogen burning. These pulsations would lead to considerable mass loss but are unlikely to be completely disruptive. Nevertheless there is no evidence that VMOs form at the present epoch, so they are invoked specifically to explain dark matter.

SUPERMASSIVE OBJECTS    Stars larger than 10⁵ M are termed "Supermassive Objects" or SMOs. If they are metal-free, they would collapse directly to black holes on a timescale 10⁴(M / 10⁵ M)^-1 y before any nuclear burning (Fowler 1966). They would therefore have no nucleosynthetic consequences, although they could explode in some mass range above 10⁵ M if they had nonzero metallicity (Fricke 1973, Fuller et al 1986). SMOs would also generate very little radiation, emitting only 10^-11 of their rest-mass energy in photons. The existence of SMOs is rather less speculative than that of VMOs since supermassive black holes are thought to reside in some galactic nuclei and to power quasars (Blandford & Rees 1991). However, these would only have a tiny cosmological density.

Note that Population III stars are likely to span a range of masses, so the remnants need not be confined to one of the candidates listed above. From the point of view of the dark matter problem, one is mainly interested in where most of the mass resides. However, the other components could also have important observational consequences, as in the Salpeter & Wasserman (1993) scenario, where the small number of neutron stars is invoked to explain gamma-ray bursts.