1,028 research outputs found
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
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
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
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