1,153 research outputs found
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Rest-Frame Ultraviolet Spectra of z~3 Lyman Break Galaxies
We present the results of a systematic study of the rest-frame UV
spectroscopic properties of Lyman Break Galaxies (LBGs). The database of almost
1000 LBG spectra proves useful for constructing high S/N composite spectra. The
composite spectrum of the entire sample reveals a wealth of features
attributable to hot stars, HII regions, dust, and outflowing neutral and
ionized gas. By grouping the database according to galaxy parameters such as
Lyman-alpha equivalent width, UV spectral slope, and interstellar kinematics,
we isolate some of the major trends in LBG spectra which are least compromised
by selection effects. We find that LBGs with stronger Lyman-alpha emission have
bluer UV continua, weaker low-ionization interstellar absorption lines, smaller
kinematic offsets between Lyman-alpha and the interstellar absorption lines,
and lower star-formation rates. There is a decoupling between the dependence of
low- and high-ionization outflow features on other spectral properties. Most of
the above trends can be explained in terms of the properties of the large-scale
outflows seen in LBGs. According to this scenario, the appearance of LBG
spectra is determined by a combination of the covering fraction of outflowing
neutral gas which contains dust, and the range of velocities over which this
gas is absorbing. Higher sensitivity and spectral resolution observations are
still required for a full understanding of the covering fraction and velocity
dispersion of the outflowing neutral gas in LBGs, and its relationship to the
escape fraction of Lyman continuum radiation in galaxies at z~3.Comment: 28 pages including 17 figures. Accepted for publication in Ap
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
Mass-luminosity relation and pulsational properties of Wolf-Rayet stars
Evolution of Population I stars with initial masses from 70M_\odot to
130M_\odot is considered under various assumptions on the mass loss rate \dot
M. The mass-luminosity relation of W-R stars is shown to be most sensitive to
the mass loss rate during the helium burning phase \dot M_{3\alpha}. Together
with the mass-luminosity relation obtained for all evolutionary sequences
several more exact relations are determined for the constant ratio
f_{3\alpha}=\dot M/\dot M_{3\alpha} with 0.5 \le f_{3\alpha} \le 3.
Evolutionary models of W-R stars were used as initial conditions in
hydrodynamic computations of radial nonlinear stellar oscillations. The
oscillation amplitude is larger in W-R stars with smaller initial mass or with
lower mass loss rate due to higher surface abundances of carbon and oxygen. In
the evolving W-R star the oscillation amplitude decreases with decreasing
stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects
allow us to calculate the integral of the mechanical work W done over the
pulsation cycle in each mass zone of the hydrodynamical model. The only
positive maximum on the radial dependence of W is in the layers with
temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump
kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot
are shown to be also excited by the kappa-mechanism but the instability driving
zone is at the bottom of the envelope and pulsation motions exist in the form
of nonlinear running waves propagating outward from the inner layers of the
envelope.Comment: 15 pages, 10 figures, submitted to Astronomy Letter
Mitigation and Adaptation Strategies â Organic Agriculture
This psoter presents information on the following
- The concept of organic farming in the context of climate change
- Carbon sequestration on organic farms
- Consequences of an area-wide conversion to organic agricultur
Pregalactic LiBeB Production by Supernova Cosmic Rays
I calculate the evolution of Be and B abundances produced by cosmic rays
generated by massive stars in the pregalactic phase of the universe. The inputs
for calculation, i.e. the star formation rate and the nuclear abundances of
cosmic rays, which I assume to be the same as those of the ISM, are taken from
the results of a detailed cosmic chemical evolution model with its parameters
best fitted from several items of observational information including an early
reionization of the IGM by . I found that when the Li plateau
abundance observed in metal-poor halo stars originated in the pregalactic
cosmological cosmic ray nucleosynthesis, Be and B simultaneously produced with
Li amount to the lowest levels ever detected in metal-poor halo stars. It
is desirable to observe Be and B abundances in metal-poor halo stars with
[Fe/H] in order to elucidate the possibility of early LiBeB
production by pregalactic supernova cosmic ray nucleosynthesis.Comment: 13 pages, 8 figures, ApJ accepte
Dust-to-Gas Ratio and Metallicity in Dwarf Galaxies
We examine the dust-to-gas ratio as a function of metallicity for dwarf
galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies
(BCDGs)]. Using a one-zone model and adopting the instantaneous recycling
approximation, we prepare a set of basic equations which describes processes of
dust formation and destruction in a galaxy. Four terms are included for the
processes: dust formation from heavy elements ejected by stellar mass loss,
dust destruction in supernova remnants, dust destruction in star-forming
regions, and accretion of heavy elements onto preexisting dust grains. Solving
the equations, we compare the result with observational data of nearby dIrrs
and BCDGs. The solution is consistent with the data within the reasonable
ranges of model parameters constrained by the previous examinations. This means
that the model is successful in understanding the dust amount of nearby
galaxies. We also show that the accretion rate of heavy element onto
preexisting dust grains is less effective than the condensation of heavy
elements in dwarf galaxies.Comment: 14 pages LaTeX, 4 figures, to appear in Ap
New Models for Wolf-Rayet and O Star Populations in Young Starbursts
Using the latest stellar evolution models, theoretical stellar spectra, and a
compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we
construct evolutionary synthesis models for young starbursts. We explicitly
distinguish between the various WR subtypes (WN, WC, WO), and we treat O and Of
stars separately. We provide detailed predictions of UV and optical emission
line strengths for both the WR stellar lines and the major nebular hydrogen and
helium emission lines, as a function of several input parameters related to the
starburst episode. We also derive the theoretical frequency of WR-rich
starbursts. We then discuss: nebular HeII 4686 emission, the contribution of WR
stars to broad Balmer line emission, techniques used to derive the WR and O
star content from integrated spectra, and explore the implications of the
formation of WR stars through mass transfer in close binary systems in
instantaneous bursts. The observational features predicted by our models allow
a detailed quantitative determination of the massive star population in a
starburst region (particularly in so-called "WR galaxies") from its integrated
spectrum and provide a means of deriving the burst properties (e.g., duration,
age) and the parameters of the initial mass function of young starbursts.
(Abridged abstract)Comment: Accepted by ApJ Supplements. LaTeX using aasmp4, psfigs macros. 49
pages including 23 figures. Paper (full, or text/figures separated) and
detailed model results available at
http://www.stsci.edu/ftp/science/starburst/sv97.htm
Intermediate mass stars: updated models
A new set of stellar models in the mass range 1.2 to 9 is
presented. The adopted chemical compositions cover the typical galactic values,
namely and . A comparison among
the most recent compilations of similar stellar models is also discussed. The
main conclusion is that the differencies among the various evolutionary results
are still rather large. For example, we found that the H-burning evolutionary
time may differ up to 20 %. An even larger disagreement is found for the
He-burning phase (up to 40-50 %). Since the connection between the various
input physics and the numerical algorithms could amplify or counterbalance the
effect of a single ingredient on the resulting stellar model, the origin of
this discrepancies is not evident. However most of these discrepancies, which
are clearly found in the evolutionary tracks, are reduced on the isochrones. By
means of our updated models we show that the ages inferred by the theory of
stellar evolution is in excellent agreement with those obtained by using other
independent methods applied to the nearby Open Clusters. Finally, the
theoretical initial/final mass relation is revised.Comment: 35 pages, 24 figures, 4 tables, accepted for publication in the
Astrophisycal Journa
Multiwavelength study of the starburst galaxy NGC7714. I: Ultraviolet-Optical spectroscopy
We have studied the physical conditions in the central 300 pc of the
proto-typical starburst galaxy NGC 7714. Our analysis is based on ultraviolet
spectroscopy with the HST+GHRS and ground-based optical observations.The data
are interpreted using evolutionary models optimized for young starburst
regions. The massive stellar population is derived in a self-consistent way
using the continuum and stellar absorption lines in the ultraviolet and the
nebular emission line optical spectrum.
The central starburst has an age of about 4.5 Myr, with little evidence for
an age spread. Wolf-Rayet features at the ultraviolet indicates a stellar
population of 2000 Wolf-Rayet stars. The overall properties of the newly
formed stars are quite similar to those derived, e.g., in 30 Doradus. A
standard Salpeter IMF is consistent with all observational constraints. We find
evidence for spatial structure within the central 300 pc sampled. Therefore it
is unlikely that the nucleus of NGC 7714 hosts a single star cluster exceeding
the properties of other known clusters. Contrary to previous suggestions, we
find no evidence for a nuclear supernova rate that would significantly exceed
the total disk-integrated rate. About one supernova event per century is
predicted.Comment: 19 pages, 9 figures in a tar file. Accepted for publication in ApJ,
1999, March, issue 51
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