F. Palla’s scientific contributions

The quasi-static contraction of primordial stars composed of pure hydrogen and helium gas is studied by following numerically the evolution of a star of five solar masses from the end of protostellar accretion to the onset of hydrogen burning. Although the protostellar core of this mass is radiatively stable and undergoing nonhomologous contraction, its large surface area and luminosity force the star to a partially convective, homologously contracting state within only 100 yr. Deuterium later ignites at an off-center temperature maximum but fails to produce interior convection. The star follows a conventional premain sequence track in the HR diagram, reaching the ZAMS after 1.2 million yr, with a luminosity of 880 solar luminosities and a radius of 1.2 solar radii.

Contents: 1. Introduction. 2. The search for primordial stars. 3. Chemistry in the early universe. 4. The evolution of collapsing gas clouds. 5. From clouds to stars.

The structure and evolution of a protostar forming from a cloud composed of pure hydrogen and helium gas are calculated. Using an accretion rate of 0.0044 solar mass/yr, the collapse of the cloud is followed numerically as a sequence of steady state accretion flows onto the hydrostatic core, which grows from an initial mass of 0.01 solar mass to 10.5 solar masses. The core is surrounded by an optically thick radiative precursor for most of its evolution. The core radius reaches 47 solar radii when the mass is 1 solar mass. For sufficiently massive cores, the deep interior contracts strongly, driving out a 'luminosity wave' which reaches the surface when the mass is 8 solar masses. This results in a large increase in core radius, the establishment of surface convection, and the disappearance of the radiative precursor. The dependence of core radius on the mass and accretion rate is analytically derived, and a new table or Rosseland mean opacities for metal-free gas is presented.

The thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas is investigated. It is assumed that the cloud is in pressure-free collapse. Over a broad range of initial conditions, virtually all the gas is converted to molecular form by a density n = 10 to the 12th/cu cm. The reactions found to be most effective are the three-body ones: H + H + H yielding H2 + H; and H + H + H2 yielding 2H2. As a consequence of significant cooling from the molecules, the temperature rise is slowed, and the Jeans mass eventually falls below 0.1 solar mass for clouds less massive than 100 solar masses. Such clouds should thus be capable of fragmenting into low-mass stars. This conclusion is even more valid if angular momentum slows the collapse. Also included in a heuristic manner is the effect of shock heating from colliding fragments in a turbulent collapsing cloud. Owing to the early destruction of hydrogen molecules, the Jeans mass cannot drop as far with substantial heating.The primordial stellar mass spectrum may thus be a sensitive function of the degree and effectiveness of intercloud collisions.