Star Formation at Zero and Very Low Metallicities

We describe how star formation is expected to proceed in the early metal-free Universe, focusing on the very first generations of stars. We then discuss how the star formation process may change as the effects of metallicity, external radiative feedback, and magnetic and turbulent support of the gas become more important. The very first stars (Pop III.1) have relatively simple initial conditions set by cosmology and the cooling properties of primordial gas. We describe the evolution of these stars as they grow in mass by accretion from their surrounding gas cores and how the accretion process is affected and eventually terminated by radiative feedback processes, especially HII region expansion and disk photoevaporation. The ability of the protostar and its disk to generate dynamically important magnetic fields is reviewed and their effects discussed. Pop III.1 star formation is likely to produce massive (~100-200Msun) stars that then influence their surroundings via ionization, stellar winds, and supernovae. These processes heat, ionize and metal-enrich the gas, thus altering the initial conditions for the next generation of star formation. Stars formed from gas that has been altered significantly by radiative and/or mechanical feedback, but not by metal enrichment (Pop III.2) are expected to have significantly smaller masses than Pop III.1 stars because of more efficient cooling from enhanced HD production. Stars formed from gas that is metal-enriched to levels that affect the dynamics of the collapse (the first Pop II stars) are also expected to have relatively low masses. We briefly compare the above star formation scenarios to what is known about present-day star formation.Comment: 16 pages, including 11 figures, Review paper to appear in "First Stars III", eds. B. O'Shea, A. Heger and T. Abe

Mass Limits to Primordial Star Formation from Protostellar Feedback

How massive were the first stars? This question is of fundamental importance for galaxy formation and cosmic reionization. Here we consider how protostellar feedback can limit the mass of a forming star. For this we must understand the rate at which primordial protostars accrete, how they and their feedback output evolve, and how this feedback interacts with the infalling matter. We describe the accretion rate with an ``isentropic accretion'' model: the rate is initially very large (~0.03 M_sun/yr when m_* =1 M_sun) and declines as m_*^{-3/7}. Protostellar evolution is treated with a model that tracks the total energy of the star. A key difference compared to previous studies is allowance for rotation of the infalling envelope. This leads to photospheric conditions at the star and dramatic differences in the feedback. Two feedback mechanisms are considered: HII region breakout and radiation pressure from Lyman-alpha and FUV photons. Radiation pressure appears to be the dominant mechanism for suppressing infall, becoming dynamically important around 20 M_sun.Comment: 4 pages; To appear in proceedings of the 13th Annual Astrophysics Conference in Maryland: The Emergence of Cosmic Structure, eds. S. Holt and C. Reynolds, (AIP

Trans-Relativistic Supernovae, Circumstellar Gamma-Ray Bursts, and Supernova 1998bw

Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct evidence that supernovae are the progenitors of some GRBs. However, this burst was unusually dim, smooth and soft compared to other bursts with known afterglows. Whether it should be considered a prototype for cosmological GRBs depends largely on whether the supernova explosion and burst were asymmetrical or can be modeled as spherical. We address this question by treating the acceleration of the supernova shock in the outermost layers of the stellar envelope, the transition to relativistic flow, and the subsequent expansion (and further acceleration) of the ejecta into the surrounding medium. We find that GRB 980425 could plausibly have been produced by a collision between the relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model requires no significant asymmetry. This event therefore belongs to a dim subclass of GRBs and is not a prototype for jet-like cosmological GRBs.Comment: 5 pages, 2 figures, to appear in Gamma 2001, eds. S. Ritz, N. Gehrels, and C. Shrade

Metal-Ion Absorption in Conductively Evaporating Clouds

We present computations of the ionization structure and metal-absorption properties of thermally conductive interface layers that surround evaporating warm spherical clouds, embedded in a hot medium. We rely on the analytical formalism of Dalton & Balbus to calculate the temperature profile in the evaporating gas, and explicitly solve the time-dependent ionization equations for H, He, C, N, O, Si, and S in the interface. We include photoionization by an external field. We estimate how departures from equilibrium ionization affect the resonance-line cooling efficiencies in the evaporating gas, and determine the conditions for which radiative losses may be neglected in the solution for the evaporation dynamics and temperature profile. Our results indicate that non-equilibrium cooling significantly increases the value of the saturation parameter at which radiative losses begin to affect the flow dynamics. As applications we calculate the ion fractions and projected column densities arising in the evaporating layers surrounding dwarf-galaxy-scale objects that are also photoionized by metagalactic radiation. We compare our results to the UV metal-absorption column densities observed in local highly-ionized metal-absorbers, located in the Galactic corona or intergalactic medium. Conductive interfaces significantly enhance the formation of high-ions such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially for clouds embedded in a high-pressure corona. However, the enhanced columns are still too low to account for the O VI columns (~1e14 cm^-2) observed in the local high-velocity absorbers. We find that O VI columns larger than ~1e13 cm^-2 cannot be produced in evaporating clouds. Our results do support the conclusion of Savage & Lehner, that absorption due to evaporating O VI likely occurs in the local interstellar medium, with characteristic columns of ~1e13 cm^-2.Comment: Accepted for Publication in Ap

Lyman alpha radiation in external galaxies

The Ly alpha line of atomic hydrogen is often a luminous component of the radiation emitted by distant galaxies. Except for those galaxies which have a substantial central source of non-stellar ionizing radiation, most of the Ly alpha radiation emitted by galaxies is generated within regions of the interstellar medium which are photoionized by starlight. Conversely, much of the energy radiated by photoionized regions is carried by the Ly alpha line. Only hot, massive stars are capable of ionizing hydrogen in the interstellar medium which surrounds them, and because such stars are necessarily short-lived, Ly alpha emission traces regions of active star formation. Researchers argue that the strength of the Ly alpha emission observed from external galaxies may be used to estimate quantitatively the dust content of the emitting region, while the Ly alpha line profile is sensitive to the presence of shock waves. Interstellar dust particles and shock waves are intimately associated with the process of star formation in two senses. First, both dust particles and shock waves owe their existence to stellar activity; second, they may both serve as agents which facilitate the formation of stars, shocks by triggering gravitational instabilities in the interstellar gas that they compress, and dust by shielding star-forming molecular clouds from the ionizing and dissociative effects of external UV radiation. By using Ly alpha observations as a probe of the dust content in diffuse gas at high redshift, we might hope to learn about the earliest epochs of star formation