1,026 research outputs found
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
Instability of LBV-stars against radial oscillations
In this study we consider the nonlinear radial oscillations exciting in
LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric
luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <=
49.1 M_odot. Hydrodynamic computations were carried out with initial conditions
obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with
initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show
instability against radial oscillations with amplitude growth time comparable
with dynamical time scale of the star. Radial oscillations exist in the form of
nonlinear running waves propagating from the boundary of the compact core to
the upper boundary of the hydrodynamical model. The velocity amplitude of outer
layers is of several hundreds of km/s while the bolometric light amplitude does
not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism
and are due to the instability of the gas with adiabatic exponent close to the
critical value Gamma_1 = 4/3 due to the large contribution of radiation in the
total pressure. The range of the light variation periods (6 day <= P <= 31 day)
of hydrodynamical models agrees with periods of microvariability observed in
LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter
Some features of the reproduction of fishes in parts of the Volga before and after flow regulation.
Conditions, times, duration and success of reproduction of fish in the Volga around Koz'modem'yansk (in the zone of the former Cheboksary reservoir) and in the upper part of Kuybyshev reservoir were studied. In years with a low spring runoff, spawning success is greater in the river than in the reservoir; in high-water years with a relatively stable water level in spring it may be higher in the reservoir than in the river. Differences were noted in the yield of young of different species depending on the spawning period.-from Sport Fishery Abstract
Shock-Induced Polarized Hydrogen Emission Lines in the Mira Star omicron Ceti
In the spectra of pulsating stars, especially Mira stars, the detection of
intense hydrogen emission lines has been explained by a radiative shock wave,
periodically propagating throughout the atmosphere. Previous observation of the
Mira star omicron Ceti around a bright maximum of light led to the detection of
a strong linear polarization associated to Balmer emissions, although the
origin of this phenomenon is not fully explained yet. With the help of
spectropolarimetry, we propose to investigate the nature of shock waves
propagating throughout the stellar atmosphere and present, for omicron Ceti
(the prototype of Mira stars), a full observational study of hydrogen emission
lines formed in the radiative region of such a shock. Using the instrument
NARVAL, we performed a spectropolarimetric monitoring of omicron Ceti during
three consecutive pulsation cycles. All Stokes parameters were systematically
collected, with a particular emphasis on the maxima of luminosity, i.e. when a
radiative shock wave is supposed to emerge from the photosphere and propagate
outward. On Balmer lines, over a large part of the luminosity cycle, we report
detections in Stokes spectra which are evolving with time. These signatures
appear to be strongly correlated to the presence of an intense shock wave
responsible for the hydrogen emission lines. We establish that those lines are
polarized by a process inherent to the mechanism responsible for the emission
line formation: the shock wave itself. Two mechanisms are considered: a global
one that implies a polarization induced by convective cells located around the
photosphere and a local one that implies a charge separation due to the passage
of the shock wave, inducing an electrical current. Combined with the existing
turbulence, this may generate a magnetic field, hence polarization.Comment: 11 pages, 9 figures; Astronomy and Astrophysics 2011, preprint onlin
Rest-Frame R-band Lightcurve of a z~1.3 Supernova Obtained with Keck Laser Adaptive Optics
We present Keck diffraction limited H-band photometry of a z~1.3 Type Ia
supernova (SN) candidate, first identified in a Hubble Space Telescope (HST)
search for SNe in massive high redshift galaxy clusters. The adaptive optics
(AO) data were obtained with the Laser Guide Star facility during four
observing runs from September to November 2005. In the analysis of data from
the observing run nearest to maximum SN brightness, the SN was found to have a
magnitude H=23.9 +/- 0.14 (Vega). We present the H-band (approximately
rest-frame R) light curve and provide a detailed analysis of the AO photometric
uncertainties. By constraining the aperture correction with a nearby (4"
separation) star we achieve 0.14 magnitude photometric precision, despite the
spatially varying AO PSF.Comment: 11 pages, 8 figures, Accepted for Publication in AJ Updated the
citations, fixed typo
UBVJHKLM photometry and modeling of R Coronae Borealis
We present the results of UBVJHKLM photometry of R CrB spanning the period
from 1976 to 2001. Studies of the optical light curve have shown no evidence of
any stable harmonics in the variations of the stellar emission. In the L band
we found semi-regular oscillations with the two main periods of ~3.3 yr and
11.9 yr and the full amplitude of ~0.8 mag and ~0.6 mag, respectively. The
colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be
remarkably stable in contrast to its brightness. This indicates that the inner
radius is a constant, time-independent characteristic of the dust shell. The
observed behavior of the IR light curve is mainly caused by the variation of
the optical thickness of the dust shell within the interval \tau(V)= 0.2-0.4.
Anticorrelated changes of the optical brightness (in particular with P ~ 3.3
yr) have not been found. Their absence suggests that the stellar wind of R CrB
deviates from spherical symmetry. The light curves suggest that the stellar
wind is variable. The variability of the stellar wind and the creation of dust
clouds may be caused by some kind of activity on the stellar surface. With some
time lag, periods of increased mass-loss cause an increase in the dust
formation rate at the inner boundary of the extended dust shell and an increase
in its IR brightness. We have derived the following parameters of the dust
shell (at mean brightness) by radiative transfer modeling: inner dust shell
radius r_in ~ 110 R_*, temperature T_dust(r_in) ~ 860 K, dust density
\rho_dust(r_in) ~ 1.1x10^{-20} g cm^-3, optical depth \tau(V) ~ 0.32 at 0.55
micron, mean dust formation rate [dM/dt]_dust ~ 3.1x10^-9 M_sun / yr, mass-loss
rate [dM/dt]_gas ~ 2.1x10^-7 M_sun / yr, size of the amorphous carbon grains
<(~) 0.01 micron, and B-V ~ -0.28.Comment: 9 pages, 6 figures, accepted for publication in A&
Subaru FOCAS Spectroscopic Observations of High-Redshift Supernovae
We present spectra of high-redshift supernovae (SNe) that were taken with the
Subaru low resolution optical spectrograph, FOCAS. These SNe were found in SN
surveys with Suprime-Cam on Subaru, the CFH12k camera on the
Canada-France-Hawaii Telescope (CFHT), and the Advanced Camera for Surveys
(ACS) on the Hubble Space Telescope (HST). These SN surveys specifically
targeted z>1 Type Ia supernovae (SNe Ia). From the spectra of 39 candidates, we
obtain redshifts for 32 candidates and spectroscopically identify 7 active
candidates as probable SNe Ia, including one at z=1.35, which is the most
distant SN Ia to be spectroscopically confirmed with a ground-based telescope.
An additional 4 candidates are identified as likely SNe Ia from the
spectrophotometric properties of their host galaxies. Seven candidates are not
SNe Ia, either being SNe of another type or active galactic nuclei. When SNe Ia
are observed within a week of maximum light, we find that we can
spectroscopically identify most of them up to z=1.1. Beyond this redshift, very
few candidates were spectroscopically identified as SNe Ia. The current
generation of super red-sensitive, fringe-free CCDs will push this redshift
limit higher.Comment: 19 pages, 26 figures. PASJ in press. see
http://www.supernova.lbl.gov/2009ClusterSurvey/ for additional information
pertaining to the HST Cluster SN Surve
A New Determination of the High Redshift Type Ia Supernova Rates with the Hubble Space Telescope Advanced Camera for Surveys
We present a new measurement of the volumetric rate of Type Ia supernova up
to a redshift of 1.7, using the Hubble Space Telescope (HST) GOODS data
combined with an additional HST dataset covering the North GOODS field
collected in 2004. We employ a novel technique that does not require
spectroscopic data for identifying Type Ia supernovae (although spectroscopic
measurements of redshifts are used for over half the sample); instead we employ
a Bayesian approach using only photometric data to calculate the probability
that an object is a Type Ia supernova. This Bayesian technique can easily be
modified to incorporate improved priors on supernova properties, and it is
well-suited for future high-statistics supernovae searches in which
spectroscopic follow up of all candidates will be impractical. Here, the method
is validated on both ground- and space-based supernova data having some
spectroscopic follow up. We combine our volumetric rate measurements with low
redshift supernova data, and fit to a number of possible models for the
evolution of the Type Ia supernova rate as a function of redshift. The data do
not distinguish between a flat rate at redshift > 0.5 and a previously proposed
model, in which the Type Ia rate peaks at redshift >1 due to a significant
delay from star-formation to the supernova explosion. Except for the highest
redshifts, where the signal to noise ratio is generally too low to apply this
technique, this approach yields smaller or comparable uncertainties than
previous work.Comment: Accepted for publication in Ap
The Hubble Space Telescope Cluster Supernova Survey: II. The Type Ia Supernova Rate in High-Redshift Galaxy Clusters
We report a measurement of the Type Ia supernova (SN Ia) rate in galaxy
clusters at 0.9 < z < 1.45 from the Hubble Space Telescope (HST) Cluster
Supernova Survey. This is the first cluster SN Ia rate measurement with
detected z > 0.9 SNe. Finding 8 +/- 1 cluster SNe Ia, we determine a SN Ia rate
of 0.50 +0.23-0.19 (stat) +0.10-0.09 (sys) SNuB (SNuB = 10^-12 SNe L_{sun,B}^-1
yr^-1). In units of stellar mass, this translates to 0.36 +0.16-0.13 (stat)
+0.07-0.06 (sys) SNuM (SNuM = 10^-12 SNe M_sun^-1 yr^-1). This represents a
factor of approximately 5 +/- 2 increase over measurements of the cluster rate
at z < 0.2. We parameterize the late-time SN Ia delay time distribution with a
power law (proportional to t^s). Under the assumption of a cluster formation
redshift of z_f = 3, our rate measurement in combination with lower-redshift
cluster SN Ia rates constrains s = -1.41 +0.47/-0.40, consistent with
measurements of the delay time distribution in the field. This measurement is
generally consistent with expectations for the "double degenerate" scenario and
inconsistent with some models for the "single degenerate" scenario predicting a
steeper delay time distribution at large delay times. We check for
environmental dependence and the influence of younger stellar populations by
calculating the rate specifically in cluster red-sequence galaxies and in
morphologically early-type galaxies, finding results similar to the full
cluster rate. Finally, the upper limit of one host-less cluster SN Ia detected
in the survey implies that the fraction of stars in the intra-cluster medium is
less than 0.47 (95% confidence), consistent with measurements at lower
redshifts.Comment: 29 pages, 14 figures. Accepted for publication in ApJ on 16 February
2011. See the HST Cluster Supernova Survey website at
http://supernova.lbl.gov/2009ClusterSurvey for a version with full-resolution
images and a complete listing of transient candidates from the survey. This
version fixes a typo in the metadata; the paper is unchanged from v
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