512 research outputs found
Rates of Observable Black Hole Emergence in Supernovae
A newly formed black hole may be directly identified if late-time accretion
of material from the base of the ejected envelope generates a luminosity that
is observable in the tail of the supernova light curve. In this work we
estimate the rate at which events where the black hole ``emerges'' in the
supernova light curve can be detected with present capabilities. Our
investigation is based on an analytical model of the accretion luminosity at
emergence as a function of progenitor mass, coupled to the inferred rate of
observed Type II supernovae in nearby galaxies. We find through a parameter
survey that under optimistic assumptions the potential rate of observable
events can be as high as several per year. However, supernovae which produce
black holes are also likely to be low energy explosions and therefore
subluminous, as was the case for the best candidate to date, SN1997D. If black
hole-forming supernovae are underdetected owing to lower luminosities, the rate
of observing black hole emergence is probably not larger than once every few
years. We therefore emphasize the importance of dedicated searches for nearby
supernovae as well as faint supernovae projects for improving the prospects of
observationally certifying the supernova--black hole connection.Comment: ApJ accepted, 13 pages, uses emulateapj
Testing population synthesis models with globular cluster colors
We have measured an extensive set of UBVRIJHK colors for M31 globular
clusters [Barmby et al. 2000]. We compare the predicted simple stellar
population colors of three population synthesis models to the intrinsic colors
of Galactic and M31 globular clusters. The best-fitting models fit the cluster
colors very well -- the weighted mean color offsets are all < 0.05 mag. The
most significant offsets between model and data are in the U and B passbands;
these are not unexpected and are likely due to problems with the spectral
libraries used by the models. The metal-rich clusters ([Fe/H] > -0.8) are best
fit by young (8 Gyr) models, while the metal-poor clusters are best fit by
older (12--16 Gyr) models. If this range of globular cluster ages is correct,
it implies that conditions for cluster formation must have existed for a
substantial fraction of the galaxies' lifetimes.Comment: To appear in ApJ Letters; 8 pages including 3 figures and 1 tabl
On the Sensitivity of Massive Star Nucleosynthesis and Evolution to Solar Abundances and to Uncertainties in Helium Burning Reaction Rates
We explore the dependence of pre-supernova evolution and supernova
nucleosynthesis yields on the uncertainties in helium burning reaction rates.
Using the revised solar abundances of Lodders (2003) for the initial stellar
composition, instead of those of Anders & Grevesse (1989), changes the
supernova yields and limits the constraints that those yields place on the
12C(a,g)16O reaction rate. The production factors of medium-weight elements (A
= 16-40) were found to be in reasonable agreement with observed solar ratios
within the current experimental uncertainties in the triple alpha reaction
rate. Simultaneous variations by the same amount in both reaction rates or in
either of them separately, however, can induce significant changes in the
central 12C abundance at core carbon ignition and in the mass of the supernova
remnant. It therefore remains important to have experimental determinations of
the helium burning rates so that their ratio and absolute values are known with
an accuracy of 10% or better.Comment: Accepted for publication by the Astrophysical Journa
Spectroscopy of Close Companions to QSOs and the Ages of Interaction-Induced Starbursts
We present low-resolution absorption-line spectra of three candidate close (
< 3 arcsec) companions to the low redshift QSOs 3CR 323.1, PG 1700+518, and PKS
2135-147. The spectra were obtained with LRIS on the Keck telescopes and with
the Faint Object Spectrograph on the University of Hawaii 2.2 m telescope. For
3CR 323.1 and PG 1700+518, we measure relative velocities that are consistent
with an association between the QSOs and their companion galaxies. The spectral
features of the companion galaxy to 3CR 323.1 indicate a stellar population of
intermediate age (approx. 2.3 Gyr). In contrast, the spectrum of the companion
object to PG 1700+518 shows strong Balmer absorption lines from a relatively
young stellar population, along with the Mg Ib absorption feature and the 4000
A break from an older population. By modeling the two stellar components of
this spectrum, it is possible to estimate the time that has elapsed since the
end of the most recent major starburst event: we obtain approx. 0.1 Gyr. This
event may have coincided with an interaction that triggered the QSO activity.
Finally, our spectroscopy shows conclusively that the supposed companion to PKS
2135-147 is actually a projected Galactic G star.Comment: 10 pages, 5 Postscript figures. Latex (AASTEX). To appear in ApJ.
Letters, Volume 480 (1997
The Stellar Mass Spectrum in the Young Populous Cluster NGC 1866
The young populous cluster NGC 1866 in the Large Magellanic Cloud LMC), which
is probably one of the most massive object formed in the LMC during the last ~
3 Gyr, appears to have an unexpectedly high mass-to-light ratio. From its
velocity dispersion Fischer et al. (1992) find its mass to be (1.35 " 0.25) x
105 Mu. The luminosity of this cluster is MV = -8.93 " 0.13, corresponding to
LV = (3.2 " 0.4) x 105 LV (u). This yields M/LV = 0.42 " 0.09 in solar units.
For a cluster of age 0.1 Gyr such a relatively high mass-to-light ratio
requires a mass spectrum with an exponent x = 1.72 " 0.09; or x = 1.75 " 0.09
if mass loss by evolving stars is taken into account.Comment: To be published in the October 1999 issue of the Publications of the
Astronomical Society of the Pacifi
The Galactic disk mass-budget : II. Brown dwarf mass-function and density
In this paper, we extend the calculations conducted previously in the stellar
regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte
Carlo calculations taking into account the brown dwarf formation rate, spatial
distribution and binary fraction. Comparison with existing surveys seems to
exclude a power-law MF as steep as the one determined in the stellar regime
below 1 \msol and tends to favor a more flatish behaviour. Comparison with
methane-dwarf detections tends to favor an eventually decreasing form like the
lognormal or the more general exponential distributions determined in the
previous paper. We calculate predicting brown dwarf counts in near-infrared
color diagrams and brown dwarf discovery functions. These calculations yield
the presently most accurate determination of the brown dwarf census in the
Galactic disk. The brown dwarf number density is comparable to the stellar one,
pc. The corresponding brown dwarf mass
density, however, represents only about 10% of the stellar contribution, i.e.
\rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density
determined previously yields the density of star-like objects, stars and brown
dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ,
corrected version with correct figure
Self-Regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-ray Luminosity Functions of Quasars
We postulate that supermassive black-holes grow in the centers of galaxies
until they unbind the galactic gas that feeds them. We show that the
corresponding self-regulation condition yields a correlation between black-hole
mass (Mbh) and galaxy velocity dispersion (sigma) as inferred in the local
universe, and recovers the observed optical and X-ray luminosity functions of
quasars at redshifts up to z~6 based on the hierarchical evolution of galaxy
halos in a Lambda-CDM cosmology. With only one free parameter and a simple
algorithm, our model yields the observed evolution in the number density of
optically bright or X-ray faint quasars between 2<z<6 across 3 orders of
magnitude in bolometric luminosity and 3 orders of magnitude in comoving
density per logarithm of luminosity. The self-regulation condition identifies
the dynamical time of galactic disks during the epoch of peak quasar activity
(z~2.5) as the origin of the inferred characteristic quasar lifetime of ~10
million years. Since the lifetime becomes comparable to the Salpeter e-folding
time at this epoch, the model also implies that the Mbh-sigma relation is a
product of feedback regulated accretion during the peak of quasar activity. The
mass-density in black-holes accreted by that time is consistent with the local
black-hole mass density of ~(0.8-6.3) times 10^5 solar masses per cubic Mpc,
which we have computed by combining the Mbh-sigma relation with the measured
velocity dispersion function of SDSS galaxies (Sheth et al.~2003). Applying a
similar self-regulation principle to supernova-driven winds from starbursts, we
find that the ratio between the black hole mass and the stellar mass of
galactic spheroids increases with redshift as (1+z)^1.5 although the Mbh-sigma
relation is redshift-independent.Comment: 10 pages, 5 figures, submitted to Ap
The CANADA-FRANCE REDSHIFT SURVEY XIII: The luminosity density and star-formation history of the Universe to z ~ 1
The comoving luminosity density of the Universe is estimated from the CFRS
faint galaxy sample in three wavebands (2800A, 4400A and 1 micron) over the
redshift range 0 < z < 1. In all three wavebands, the comoving luminosity
density increases markedly with redshift. For a (q_0 = 0.5, Omega = 1.0)
cosmological model, the comoving luminosity density increases as at 1 micron, as at 4400A and as at 2800A, these exponents being reduced by 0.43 and 1.12 for (0.05,0.1)
and (-0.85,0.1) cosmological models respectively. The variation of the
luminosity density with epoch can be reasonably well modelled by an actively
evolving stellar population with a Salpeter initial mass function (IMF)
extending to 125 M_sun, a star-formation rate declining with a power 2.5, and a
turn-on of star-formation at early epochs. A Scalo (1986) IMF extending to the
same mass limit produces too many long-lived low mass stars. This rapid
evolution of the star-formation rate and comoving luminosity density of the
Universe is in good agreement with the conclusions of Pei and Fall (1995) from
their analysis of the evolving metallicity of the Universe. One consequence of
this evolution is that the physical luminosity density at short wavelengths has
probably declined by two orders of magnitude since z ~ 1.Comment: uuencoded compressed tar file containing 8 page Tex file, 2
postscript figures and 2 tables. Ap J Letters, in press. Also available at
http://www.astro.utoronto.ca/~lilly/CFRS/papers.htm
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