2,255 research outputs found
Dust Formation By Failed Supernovae
We consider dust formation during the ejection of the hydrogen envelope of a
red supergiant during a failed supernova (SN) creating a black hole. While the
dense, slow moving ejecta are very efficient at forming dust, only the very
last phases of the predicted visual transient will be obscured. The net grain
production consists of ~0.01 solar masses of very large grains (10 to 1000
microns). This means that failed SNe could be the source of the very large
extrasolar dust grains identified by Ulysses, Galileo and radar studies of
meteoroid re-entry trails rather than their coming from an ejection process
associated with protoplanetary or other disks.Comment: submitted to MNRA
Dust Formation in the Presence of Photons I: Evaporation Rates for Small Dust Grains
The temperature of newly forming dust is controlled by the radiation field.
As dust forms around stars, stellar transients, quasars or supernovae, the
grains must grow through a regime where they are stochastically heated by
individual photons. Since evaporation rates increase exponentially with
temperature while cooling times decrease only as a power law, the evaporation
rates for these small grains are dominated by the temperature spikes. We
calculate effective evaporation temperatures for a broad range of input spectra
that are encapsulated in a series of simple interpolation formulae for both
graphitic and silicate grains. These can be easily used to first determine if
dust formation is possible and then to estimate the radius or time at which it
commences for a broad range of radiation environments. With these additional
physical effects, very small grains may form earlier than in standard models of
AGB winds. Even for very high mass loss rates, the hottest stars that can form
dust are G and F stars particularly in the case of silicate dusts. For hotter
stars, the higher fluxes of ultraviolet photons prevent dust formation. Thus,
episodic dust formation by OH/IR stars and LBVs is primarily driven by
fluctuations in their apparent temperatures rather than changes in luminosity
or mass loss rates.Comment: 13 pages, 10 figures, submitted to MNRA
Failed Supernovae Explain the Compact Remnant Mass Function
One explanation for the absence of higher mass red supergiants (16.5 Msun < M
< 25Msun) as the progenitors of Type IIP supernovae (SNe) is that they die in
failed SNe creating black holes. Simulations show that such failed SNe still
eject their hydrogen envelopes in a weak transient, leaving a black hole with
the mass of the star's helium core (5-8Msun). Here we show that this naturally
explains the typical masses of observed black holes and the gap between neutron
star and black hole masses without any fine-tuning of the SN mechanism beyond
having it fail in a mass range where many progenitor models have density
structures that make the explosions more likely to fail. There is no difficulty
including this ~20% population of failed SNe in any accounting of SN types over
the progenitor mass function. And, other than patience, there is no
observational barrier to either detecting these black hole formation events or
limiting their rates to be well below this prediction.Comment: Submitted to Ap
The Evolution and Structure of Early-type Field Galaxies: A Combined Statistical Analysis of Gravitational Lenses
We introduce a framework for simultaneously investigating the structure and
luminosity evolution of early-type gravitational lens galaxies. The method is
based on the fundamental plane, which we interpret using the aperture
mass-radius relations derived from lensed image geometries. We apply this
method to our previous sample of 22 lens galaxies with measured redshifts and
excellent photometry. Modeling the population with a single mass profile and
evolutionary history, we find that early-type galaxies are nearly isothermal
(logarithmic density slope n = 2.06 +/- 0.17, 68% C.L.), and that their stars
evolve at a rate of dlog(M/L)_B/dz = -0.50 +/- 0.19 (68% C.L.) in the rest
frame B band. For a Salpeter IMF and a concordance cosmology, this implies a
mean star formation redshift of > 1.5 at 95% confidence. While this model
can neatly describe the mean properties of early-type galaxies, it is clear
that the scatter of the lens sample is too large to be explained by
observational uncertainties alone. We therefore consider statistical models in
which the galaxy population is described by a distribution of star formation
redshifts. We find that stars must form over a significant range of redshifts
(Delta z_f > 1.7, 68% C.L.), which can extend as low as z_f = 1 for some
acceptable models. However, the typical galaxy will still have an old stellar
population ( > 1.5). The lens sample therefore favors early star formation
in field ellipticals -- even if we make no a priori assumption regarding the
shape of the mass distribution in lenses, and include the range of possible
deviations from homology in the uncertainties. Our evolution results call into
question several recent claims that early-type galaxies in low-density
environments have much younger stars than those in rich clusters.Comment: 36 pages including 6 figures, (re-)submitted to Ap
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