4,928 research outputs found
The Wolf-Rayet Content of M33
Wolf-Rayet stars (WRs) are evolved massive stars, and the relative number of
WC-type and WN-type WRs should vary with metallicity, providing a sensitive
test of stellar evolutionary theory. The observed WC/WN ratio is much higher
than that predicted by theory in some galaxies but this could be due to
observational incompleteness for WN-types, which have weaker lines. Previous
studies of M33's WR content show a galactocentric gradient in the relative
numbers of WCs and WNs, but only small regions have been surveyed with
sufficient sensitivity to detect all of the WNs. Here we present a sensitive
survey for WRs covering all of M33, finding 55 new WRs, mostly of WN type. Our
spectroscopy also improves the spectral types of many previously known WRs,
establishing in one case that the star is actually a background quasar. The
total number of spectroscopically confirmed WRs in M33 is 206, a number we
argue is complete to approximately 5%, with most WRs residing in OB
associations, although approximately 2% are truly isolated. The WC/WN ratio in
the central regions (<2 kpc) of M33 is much higher than that predicted by the
current Geneva evolutionary models, while the WC/WN ratios in the outer regions
are in good accord, as are the values in the SMC and LMC. The WC/WN ratio and
the WC subtype distribution both argue that the oxygen abundance gradient in
M33 is significantly larger than found by some recent studies, but are
consistent with the two-component model proposed by Magrini et al.Comment: ApJ, in pres
Implications of the metallicity dependence of Wolf-Rayet winds
Aims: Recent theoretical predictions for the winds of Wolf-Rayet stars
indicate that their mass-loss rates scale with the initial stellar metallicity
in the local Universe.We aim to investigate how this predicted dependence
affects the models of Wolf-Rayet stars and their progeny in different chemical
environments. Methods: We compute models of stellar structure and evolution for
Wolf-Rayet stars for different initial metallicities, and investigate how the
scaling of the Wolf-Rayet mass-loss rates affects the final masses, the
lifetimes of the WN and WC subtypes, and how the ratio of the two populations
vary with metallicity. Results: We find significant effects of metallicity
dependent mass-loss rates for Wolf-Rayet stars. For models that include the
scaling of the mass-loss rate with initial metallicity, all WR stars become
neutron stars rather than black holes at twice the solar metallicity; at lower
, black holes have larger masses. We also show that our models that include
the mass-loss metallicity scaling closely reproduce the observed decrease of
the relative population of WC over WN stars at low metallicities.Comment: 8 pages, 9 figures, accepted by Astronomy & Astrophysic
Limitations for shapelet-based weak-lensing measurements
We seek to understand the impact on shape estimators obtained from circular
and elliptical shapelet models under two realistic conditions: (a) only a
limited number of shapelet modes is available for the model, and (b) the
intrinsic galactic shapes are not restricted to shapelet models.
We create a set of simplistic simulations, in which the galactic shapes
follow a Sersic profile. By varying the Sersic index and applied shear, we
quantify the amount of bias on shear estimates which arises from insufficient
modeling. Additional complications due to PSF convolution, pixelation and pixel
noise are also discussed.
Steep and highly elliptical galaxy shapes cannot be accurately modeled within
the circular shapelet basis system and are biased towards shallower and less
elongated shapes. This problem can be cured partially by allowing elliptical
basis functions, but for steep profiles elliptical shapelet models still depend
critically on accurate ellipticity priors. As a result, shear estimates are
typically biased low. Independently of the particular form of the estimator,
the bias depends on the true intrinsic galaxy morphology, but also on the size
and shape of the PSF.
As long as the issues discussed here are not solved, the shapelet method
cannot provide weak-lensing measurements with an accuracy demanded by upcoming
missions and surveys, unless one can provide an accurate and reliable
calibration, specific for the dataset under investigation.Comment: 8 pages, 5 figures, submitted to A&
Massive runaway stars in the Small Magellanic Cloud
Using archival Spitzer Space Telescope data, we identified for the first time
a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the
detection of their bow shocks. The geometry of detected bow shocks allows us to
infer the direction of motion of the associated stars and to determine their
possible parent clusters and associations. One of the identified runaway stars,
AzV 471, was already known as a high-velocity star on the basis of its high
peculiar radial velocity, which is offset by ~40 km/s from the local systemic
velocity. We discuss implications of our findings for the problem of the origin
of field OB stars. Several of the bow shock-producing stars are found in the
confines of associations, suggesting that these may be "alien" stars
contributing to the age spread observed for some young stellar systems. We also
report the discovery of a kidney-shaped nebula attached to the early WN-type
star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure
created owing to the interaction between the stellar wind and the ambient
interstellar medium.Comment: Accepted by A&
Limitations of model fitting methods for lensing shear estimation
Gravitational lensing shear has the potential to be the most powerful tool
for constraining the nature of dark energy. However, accurate measurement of
galaxy shear is crucial and has been shown to be non-trivial by the Shear
TEsting Programme. Here we demonstrate a fundamental limit to the accuracy
achievable by model-fitting techniques, if oversimplistic models are used. We
show that even if galaxies have elliptical isophotes, model-fitting methods
which assume elliptical isophotes can have significant biases if they use the
wrong profile. We use noise-free simulations to show that on allowing
sufficient flexibility in the profile the biases can be made negligible. This
is no longer the case if elliptical isophote models are used to fit galaxies
made up of a bulge plus a disk, if these two components have different
ellipticities. The limiting accuracy is dependent on the galaxy shape but we
find the most significant biases for simple spiral-like galaxies. The
implications for a given cosmic shear survey will depend on the actual
distribution of galaxy morphologies in the universe, taking into account the
survey selection function and the point spread function. However our results
suggest that the impact on cosmic shear results from current and near future
surveys may be negligible. Meanwhile, these results should encourage the
development of existing approaches which are less sensitive to morphology, as
well as methods which use priors on galaxy shapes learnt from deep surveys.Comment: 10 pages, 8 figure
Stellar evolution with rotation XI: Wolf-Rayet star populations at different metallicities
Grids of models of massive stars ( 20 ) with rotation are
computed for metallicities ranging from that of the Small Magellanic Cloud
(SMC) to that of the Galactic Centre. The hydrostatic effects of rotation, the
rotational mixing and the enhancements of the mass loss rates by rotation are
included. The evolution of the surface rotational velocities of the most
massive O--stars mainly depends on the mass loss rates and thus on the initial
value. The minimum initial mass for a star for entering the Wolf--Rayet
(WR) phase is lowered by rotation. For all metallicities, rotating stars enter
the WR phase at an earlier stage of evolution and the WR lifetimes are
increased, mainly as a result of the increased duration of the eWNL phase.
Models of WR stars predict in general rather low rotation velocities (
km s) with a few possible exceptions, particularly at metallicities
lower than solar where WR star models have in general faster rotation and more
chance to reach the break--up limit.The properties of the WR populations as
predicted by the rotating models are in general in much better agreement with
the observations in nearby galaxies. The observed variation with metallicity of
the fractions of type Ib/Ic supernovae with respect to type II supernovae as
found by Prantzos & Boissier (\cite{Pr03}) is very well reproduced by the
rotating models, while non--rotating models predict much too low ratios.Comment: 20 pages, 16 figure, Astronomy and Astrophysics, in pres
The mass-loss rates of red supergiants and the de Jager prescription
Mass loss of red supergiants (RSG) is important for the evolution of massive
stars, but is not fully explained. Several empirical prescriptions have been
proposed, trying to express the mass-loss rate (Mdot) as a function of
fundamental stellar parameters (mass, luminosity, effective temperature). Our
goal is to test whether the de Jager et al. (1988) prescription, used in some
stellar evolution models, is still valid in view of more recent mass-loss
determinations. By considering 40 Galactic RSGs presenting an infrared excess
and an IRAS 60-mu flux larger than 2 Jy, and assuming a gas-to-dust mass ratio
of 200, it is found that the de Jager rate agrees within a factor 4 with most
Mdot estimates based on the 60-mu signal. It is also in agreement with 6 of the
only 8 Galactic RSGs for which Mdot can be measured more directly through
observations of the circumstellar gas. The two objects that do not follow the
de Jager prescription (by an order of magnitude) are mu Cep and NML Cyg. We
have also considered the RSGs of the Magellanic Clouds. Thanks to the works of
Groenewegen et al. (2009) and Bonanos et al. (2010), we find that the RSGs of
the SMC have Mdots consistent with the de Jager rate scaled by
(Z/Zsun)**(alpha), where Z is the metallicity and alpha is 0.7. The situation
is less clear for the LMC RSGs. In particular, for luminosties larger than
1.6E+05 Lsun, one finds numerous RSGs (except WOH-G64) having Mdot
significantly smaller than the de Jager rate, and indicating that Mdot would no
longer increase with L. Before this odd situation is confirmed through further
analysis of LMC RSGs, we suggest to keep the de Jager prescription unchanged at
solar metallicity in the stellar evolutionary models and to apply a
(Z/Zsun)**0.7 dependence.Comment: 13 pages, 9 figures. Accepted by Astronomy and Astrophysic
A Spectroscopic Search for the non-nuclear Wolf-Rayet Population of the metal-rich spiral galaxy M83
We present a catalogue of non-nuclear regions containing Wolf-Rayet stars in
the metal-rich spiral galaxy M83 (NGC5236). From a total of 283 candidate
regions identified using HeII 4686 imaging with VLT-FORS2, Multi Object
Spectroscopy of 198 regions was carried out, confirming 132 WR sources. From
this sub-sample, an exceptional content of 1035 +/- 300 WR stars is inferred,
with N(WC)/N(WN) approx 1.2, continuing the trend to larger values at higher
metallicity amongst Local Group galaxies, and greatly exceeding current
evolutionary predictions at high metallicity. Late-type stars dominate the WC
population of M83, with N(WC8-9)/N(WC4-7)=9 and WO subtypes absent, consistent
with metallicity dependent WC winds. Equal numbers of late to early WN stars
are observed, again in contrast to current evolutionary predictions. Several
sources contain large numbers of WR stars. In particular, #74 (alias region 35
from De Vaucouleurs et al. contains 230 WR stars, and is identified as a Super
Star Cluster from inspection of archival HST/ACS images. Omitting this
starburst cluster would result in revised statistics of N(WC)/N(WN) approx 1
and N(WC8-9)/N(WC4-7) approx 6 for the `quiescent' disk population. Including
recent results for the nucleus and accounting for incompleteness in our
spectroscopic sample, we suspect the total WR population of M83 may exceed 3000
stars.Comment: 39 pages, 13 figures, 17 finding charts, accepted for Astronomy &
Astrophysics. Version will full resolution images available at
ftp://astro1.shef.ac.uk/pub/pac/m83.ps.g
Long-Term V-Band Monitoring of the Bright Stars of M33 at the Wise Observatory
We have conducted a long-term V-band photometric monitoring of M33 on 95
nights during four observing seasons (2000 - 2003). A total number of 6418
lightcurves of bright objects in the range of 14 - 21 mag have been obtained.
All measurements are publicly available. A total of 127 new variables were
detected, of which 28 are periodic. Ten previously known non-periodic variables
were identified as periodic, 3 of which are Cepheids, and another previously
known periodic variable was identified as an eclipsing binary. Our derived
periods range from 2.11 to almost 300 days. For 50 variables we have combined
our observations with those of the DIRECT project, obtaining lightcurves of up
to 500 measurements, with a time-span of ~7 years. We have detected a few
interesting variables, including a 99.3 day periodic variable with a 0.04 mag
amplitude, at the position of SNR 19.Comment: 29 pages, accepted for publication in MNRAS. Additional material is
available at http://wise-obs.tau.ac.il/~shporer/m33
Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
The star complexes (large scale star forming regions) of NGC 6822 were traced
and mapped and their size distribution was compared with the size distribution
of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial
distributions of different age stellar populations were compared with each
other. The star complexes of NGC 6822 were determined by using the isopleths,
based on star counts, of the young stars of the galaxy, using a statistical
cutoff limit in density. In order to map them and determine their geometrical
properties, an ellipse was fitted to every distinct region satisfying this
minimum limit. The Kolmogorov-Smirnov statistical test was used to study
possible patterns in their size distribution. Isopleths were also used to study
the stellar populations of NGC 6822. The star complexes of NGC 6822 were
detected and a list of their positions and sizes was produced. Indications of
hierarchical star formation, in terms of spatial distribution, time evolution
and preferable sizes were found in NGC 6822 and the MCs. The spatial
distribution of the various age stellar populations has indicated traces of an
interaction in NGC 6822, dated before 350 +/- 50 Myr.Comment: 10 pages, 7 figures, accepted by A&A; minor typeface correction
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