446 research outputs found
The influence of galaxy surface brightness on the mass-metallicity relation
We study the effect of surface brightness on the mass-metallicity relation
using nearby galaxies whose gas content and metallicity profiles are available.
Previous studies using fiber spectra indicated that lower surface brightness
galaxies have systematically lower metallicity for their stellar mass, but the
results were uncertain because of aperture effect. With stellar masses and
surface brightnesses measured at WISE W1 and W2 bands, we re-investigate the
surface brightness dependence with spatially-resolved metallicity profiles and
find the similar result. We further demonstrate that the systematical
difference cannot be explained by the gas content of galaxies. For two galaxies
with similar stellar and gas masses, the one with lower surface brightness
tends to have lower metallicity. Using chemical evolution models, we
investigate the inflow and outflow properties of galaxies of different masses
and surface brightnesses. We find that, on average, high mass galaxies have
lower inflow and outflow rates relative to star formation rate. On the other
hand, lower surface brightness galaxies experience stronger inflow than higher
surface brightness galaxies of similar mass. The surface brightness effect is
more significant for low mass galaxies. We discuss implications on the
different inflow properties between low and high surface brightness galaxies,
including star formation efficiency, environment and mass assembly history
The influence of rotation in radiation driven winds from hot stars II. CAK topological analysis
The topological analysis from Bjorkman (1995) for the standard model that
describes the winds from hot stars by Castor, Abbott & Klein (1975) has been
extended to include the effect of stellar rotation and changes in the
ionization of the wind. The differential equation for the momentum of the wind
is non--linear and transcendental for the velocity gradient. Due to this
non--linearity the number of solutions that this equation possess is not known.
After a change of variables and the introduction of a new physically
meaningless independent variable, we manage to replace the non--linear momentum
differential equation by a system of differential equations where all the
derivatives are {\it{explicitely}} given. We then use this system of equations
to study the topology of the rotating--CAK model. For the particular case when
the wind is frozen in ionization () only one physical solution is
found, the standard CAK solution, with a X--type singular point. For the more
general case (), besides the standard CAK singular point, we
find a second singular point which is focal--type (or attractor). We find also,
that the wind does not adopt the maximal mass--loss rate but almost the
minimal.Comment: 11 pages, Astronomy and Astrophysics Accepte
Chemical composition of B-type supergiants in the OB8, OB10, OB48, OB78 associations of M31
Absolute and differential chemical abundances are presented for the largest
group of massive stars in M31 studied to date. These results were derived from
intermediate resolution spectra of seven B-type supergiants, lying within four
OB associations covering a galactocentric distance of 5 - 12 kpc. The results
are mainly based on an LTE analysis, and we additionally present a full
non-LTE, unified model atmosphere analysis of one star (OB78-277) to
demonstrate the reliability of the differential LTE technique. A comparison of
the stellar oxygen abundance with that of previous nebular results shows that
there is an offset of between ~0.15 - 0.4 dex between the two methods which is
critically dependent on the empirical calibration adopted for the R23 parameter
with [O/H]. However within the typical errors of the stellar and nebular
analyses (and given the strength of dependence of the nebular results on the
calibration used) the oxygen abundances determined in each method are fairly
consistent. We determine the radial oxygen abundance gradient from these stars,
and do not detect any systematic gradient across this galactocentric range. We
find that the inner regions of M31 are not, as previously thought, very 'metal
rich'. Our abundances of C, N, O, Mg, Si, Al, S and Fe in the M31 supergiants
are very similar to those of massive stars in the solar neighbourhood.Comment: 15 pages, 9 figures and 9 tables. Submitted to A&A April 200
Understanding B-type Supergiants in the Low Metallicity Environment of the SMC
Spectroscopic analyses of 7 SMC B-type supergiants and 1 giant have been
undertaken using high resolution optical data obtained on the VLT with UVES.
FASTWIND, a non-LTE, spherical,line-blanketed model atmosphere code was used to
derive atmospheric and wind parameters of these stars as well as their absolute
abundances. The implications of these results for stellar evolution and line
driven wind theory are discussed.Comment: 19 pages, 6 tables, 10 figures, accepted for publication in Astronomy
and Astrophysics (5/12/2003
Chemical abundances and winds of massive stars in M31: a B-type supergiant and a WC star in OB10
We present high quality spectroscopic data for two massive stars in the OB10
association of M31, OB10-64 (B0Ia) and OB10-WR1 (WC6). Medium resolution
spectra of both stars were obtained using the ISIS spectrograph on the William
Hershel Telescope. This is supplemented with HST-STIS UV spectroscopy and KeckI
HIRES data for OB10-64. A non-LTE model atmosphere and abundance analysis for
OB10-64 is presented indicating that this star has similar photospheric CNO, Mg
and Si abundances as solar neighbourhood massive stars. A wind analysis of this
early B-type supergiant reveals a mass-loss rate of M_dot=1.6x10^-6
M_solar/yr,and v_infty=1650 km/s. The corresponding wind momentum is in good
agreement with the wind momentum -- luminosity relationship found for Galactic
early B supergiants. Observations of OB10W-R1 are analysed using a non-LTE,
line-blanketed code, to reveal approximate stellar parameters of log L/L_solar
\~ 5.7, T~75 kK, v_infty ~ 3000 km/s, M_dot ~ 10^-4.3 M_solar/yr, adopting a
clumped wind with a filling factor of 10%. Quantitative comparisons are made
with the Galactic WC6 star HD92809 (WR23) revealing that OB10-WR1 is 0.4 dex
more luminous, though it has a much lower C/He ratio (~0.1 versus 0.3 for
HD92809). Our study represents the first detailed, chemical model atmosphere
analysis for either a B-type supergiant or a WR star in Andromeda, and shows
the potential of how such studies can provide new information on the chemical
evolution of galaxies and the evolution of massive stars in the local Universe.Comment: 17 pages, 14 figures, MNRAS accepted version, some minor revision
Near-infrared spectroscopy of candidate red supergiant stars in clusters
Clear identifications of Galactic young stellar clusters farther than a few
kpc from the Sun are rare, despite the large number of candidate clusters. We
aim to improve the selection of candidate clusters rich in massive stars with a
multiwavelength analysis of photometric Galactic data that range from optical
to mid-infrared wavelengths. We present a photometric and spectroscopic
analysis of five candidate stellar clusters, which were selected as
overdensities with bright stars (Ks < 7 mag) in GLIMPSE and 2MASS images. A
total of 48 infrared spectra were obtained. The combination of photometry and
spectroscopy yielded six new red supergiant stars with masses from 10 Msun to
15 Msun. Two red supergiants are located at Galactic coordinates
(l,b)=(16.7deg,-0.63deg) and at a distance of about ~3.9 kpc; four other red
supergiants are members of a cluster at Galactic coordinates
(l,b)=(49.3deg,+0.72deg) and at a distance of ~7.0 kpc. Spectroscopic analysis
of the brightest stars of detected overdensities and studies of interstellar
extinction along their line of sights are fundamental to distinguish regions of
low extinction from actual stellar clusters. The census of young star clusters
containing red supergiants is incomplete; in the existing all-sky near-infrared
surveys, they can be identified as overdensities of bright stars with infrared
color-magnitude diagrams characterized by gaps.Comment: 16 pages, 10 figures, accepted to A&A 201
Observational Tests and Predictive Stellar Evolution II: Non-standard Models
We examine contributions of second order physical processes to results of
stellar evolution calculations amenable to direct observational testing. In the
first paper in the series (Young et al. 2001) we established baseline results
using only physics which are common to modern stellar evolution codes. In the
current paper we establish how much of the discrepancy between observations and
baseline models is due to particular elements of new physics. We then consider
the impact of the observational uncertainties on the maximum predictive
accuracy achievable by a stellar evolution code. The sun is an optimal case
because of the precise and abundant observations and the relative simplicity of
the underlying stellar physics. The Standard Model is capable of matching the
structure of the sun as determined by helioseismology and gross surface
observables to better than a percent. Given an initial mass and surface
composition within the observational errors, and no additional constraints for
which the models can be optimized, it is not possible to predict the sun's
current state to better than ~7%. Convectively induced mixing in radiative
regions, seen in multidimensional hydrodynamic simulations, dramatically
improves the predictions for radii, luminosity, and apsidal motions of
eclipsing binaries while simultaneously maintaining consistency with observed
light element depletion and turnoff ages in young clusters (Young et al. 2003).
Systematic errors in core size for models of massive binaries disappear with
more complete mixing physics, and acceptable fits are achieved for all of the
binaries without calibration of free parameters. The lack of accurate abundance
determinations for binaries is now the main obstacle to improving stellar
models using this type of test.Comment: 33 pages, 8 figures, accepted for publication in the Astrophysical
Journa
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