952 research outputs found
Contribution of Type Ia and Type II Supernovae for Intra-Cluster Medium Enrichment
The origin of the chemical composition of the intracluster medium (ICM) is
discussed in this paper. In particular, the contribution from Type Ia
supernovae (SNe Ia) to the ICM enrichment is shown to exist by adopting the
fitting formulas which have been used in the analysis of the solar system
abundances. Our analysis means that we can use the frequency of SNe Ia relative
to SNe II as the better measure than for
estimating the contribution of SNe Ia. Moreover, the chemical compositions of
ICMs are shown to be similar to that of the solar system abundances. We can
also reproduce the sulfur/iron abundance ratio within a factor of 2, which
means that the abundance problem of sulfur needs not to be emphasized too
strongly. We need more precise observations to conclude whether ICMs really
suffer the shortage problem of sulfur or not.Comment: 20 pages, LaTeX text and 15 postscript figures. Accepted for
publication in Astrophysical Journa
Abundances of Baade's Window Giants from Keck/HIRES Spectra: II. The Alpha- and Light Odd Elements
We report detailed chemical abundance analysis of 27 RGB stars towards the
Galactic bulge in Baade's Window for elements produced by massive stars: O, Na,
Mg, Al, Si, Ca and Ti. All of these elements are overabundant in the bulge
relative to the disk, especially Mg, indicating that the bulge is enhanced in
Type~II supernova ejecta and most likely formed more rapidly than the disk. We
attribute a rapid decline of [O/Fe] to metallicity-dependent yields of oxygen
in massive stars, perhaps connected to the Wolf-Reyet phenomenon. he explosive
nucleosynthesis alphas, Si, Ca and Ti, possess identical trends with [Fe/H],
consistent with their putative common origin. We note that different behaviors
of hydrostatic and explosive alpha elements can be seen in the stellar
abundances of stars in Local Group dwarf galaxies. We also attribute the
decline of Si,Ca and Ti relative to Mg, to metallicity- dependent yields for
the explosive alpha elements from Type~II supernovae. The starkly smaller
scatter of [/Fe] with [Fe/H] in the bulge, as compared to the halo, is
consistent with expected efficient mixing for the bulge. The metal-poor bulge
[/Fe] ratios are higher than ~80% of the halo. If the bulge formed from
halo gas, the event occured before ~80% of the present-day halo was formed. The
lack of overlap between the thick and thin disk composition with the bulge does
not support the idea that the bulge was built by a thickening of the disk
driven by the bar. The trend of [Al/Fe] is very sensitive to the chemical
evolution environment. A comparison of the bulge, disk and Sgr dSph galaxy
shows a range of ~0.7 dex in [Al/Fe] at a given [Fe/H], presumably due to a
range of Type~II/Type~Ia supernova ratios in these systems.Comment: 51 pages, 6 tables, 27 figures, submitte
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
In the two previous papers of this series, we have discussed the importance
of t he -gradients due to helium settling on rotation-induced mixing,
first in a n approximate analytical way, second in a 2D numerical simulation.
We have found that, for slowly rotating low mass stars, a process of ``creeping
paralysis" in which the circulation and the diffusion are nearly frozen may
take place below the convective zone. Here we apply this theory to the case of
lithium and beryll ium in galactic clusters and specially the Hyades. We take
into account the rota tional braking with rotation velocities adjusted to the
present observations. We find that two different cells of meridional
circulation appear on the hot side of the "lithium dip" and that the "creeping
paralysis" process occurs, not dir ectly below the convective zone, but deeper
inside the radiative zone, at the to p of the second cell. As a consequence,
the two cells are disconnected, which ma y be the basic reason for the lithium
increase with effective temperature on thi s side of the dip. On the cool side,
there is just one cell of circulation and t he paralysis has not yet set down
at the age of the Hyades; the same modelisatio n accounts nicely for the
beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
Structure and evolution of rotationally and tidally distorted stars
This paper aims to study the configuration of two components caused by
rotational and tidal distortions in the model of a binary system. The
potentials of the two distorted components can be approximated to 2nd-degree
harmonics. Furthermore, both the accretion luminosity () and the
irradiative luminosity are included in stellar structure equations. The
equilibrium structure of rotationally and tidally distorted star is exactly a
triaxial ellipsoids. A formula describing the isobars is presented, and the
rotational velocity and the gravitational acceleration at the primary surface
simulated. The results show the distortion at the outer layers of the primary
increases with temporal variation and system evolution. Besides, it was
observed that the luminosity accretion is unstable, and the curve of the
energy-generation rate fluctuates after the main sequence in rotation
sequences. The luminosity in rotation sequences is slightly weaker than that in
non-rotation sequences. As a result, the volume expands slowly. Polar ejection
is intensified by the tidal effect. The ejection of an equatorial ring may be
favoured by both the opacity effect and the -effect in
the binary system.Comment: 10 pages, 17 figures,Accepted by Astronomy and Astrophysic
Evolution and appearance of Be stars in SMC clusters
Star clusters are privileged laboratories for studying the evolution of
massive stars (OB stars). One particularly interesting question concerns the
phases, during which the classical Be stars occur, which unlike HAe/Be stars,
are not pre-main sequence objects, nor supergiants. Rather, they are extremely
rapidly rotating B-type stars with a circumstellar decretion disk formed by
episodic ejections of matter from the central star. To study the impact of
mass, metallicity, and age on the Be phase, we observed SMC open clusters with
two different techniques: 1) with the ESO-WFI in its slitless mode, which
allowed us to find the brighter Be and other emission-line stars in 84 SMC open
clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine
accurately the evolutionary phases of Be stars in the Be-star rich SMC open
cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar
evolution / appearance as a function of metallicity and mass / spectral type is
developed, involving the fractional critical rotation rate as a key parameter.Comment: Proceedings of the IAUS266 of the GA200
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
In this paper we develop a calculation code to account for the effects
carried by fast rotation on the observed spectra of early-type stars. Stars are
assumed to be in rigid rotation and the grid of plane-parallel model
atmospheres used to represent the gravitational darkening are calculated by
means of a non-LTE approach. Attention is paid on the relation between the
apparent and parent non-rotating counterpart stellar fundamental parameters and
apparent and true vsini parameters as a function of the rotation rate
Omega/Omega_c, stellar mass and inclination angle. It is shown that omission of
gravitational darkening in the analysis of chemical abundances of CNO elements
can produce systematic overestimation or underestimation, depending on the
lines used, rotational rate and inclination angle. The proximity of Be stars to
the critical rotation is re-discussed by correcting not only the vsini of 130
Be stars, but also their effective temperature and gravity to account for
stellar rotationally induced geometrical distortion and for the concomitant
gravitational darkening effect. We concluded that the increase of the vsini
estimate is accompanied by an even higher value of the stellar equatorial
critical velocity, so that the most probable average rate of angular velocity
of Be stars attains Omega/Omega_c ~ 0.88.Comment: 20 pages, 16 figures. Submitted for publication in A&
Close binary evolution. III. Impact of tides, wind magnetic braking, and internal angular momentum transport
Massive stars with solar metallicity lose important amounts of rotational
angular momentum through their winds. When a magnetic field is present at the
surface of a star, efficient angular momentum losses can still be achieved even
when the mass-loss rate is very modest, at lower metallicities, or for
lower-initial-mass stars. In a close binary system, the effect of wind magnetic
braking also interacts with the influence of tides, resulting in a complex
evolution of rotation. We study the interactions between the process of wind
magnetic braking and tides in close binary systems. We discuss the evolution of
a 10 M star in a close binary system with a 7 M companion using
the Geneva stellar evolution code. The initial orbital period is 1.2 days. The
10 M star has a surface magnetic field of 1 kG. Various initial
rotations are considered. We use two different approaches for the internal
angular momentum transport. In one of them, angular momentum is transported by
shear and meridional currents. In the other, a strong internal magnetic field
imposes nearly perfect solid-body rotation. The evolution of the primary is
computed until the first mass-transfer episode occurs. The cases of different
values for the magnetic fields and for various orbital periods and mass ratios
are briefly discussed. We show that, independently of the initial rotation rate
of the primary and the efficiency of the internal angular momentum transport,
the surface rotation of the primary will converge, in a time that is short with
respect to the main-sequence lifetime, towards a slowly evolving velocity that
is different from the synchronization velocity. (abridged).Comment: 11 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
A study of the B and Be star population in the field of the LMC open cluster NGC2004 with VLT-FLAMES
Observations of hot stars belonging to the young cluster LMC-NGC2004 and its
surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA
mode. 25 Be stars were discovered; the proportion of Be stars compared to
B-type stars is found to be of the same order in the LMC and in the Galaxy
fields. 23 hot stars were discovered as spectroscopic binaries (SB1 and SB2), 5
of these are found to be eclipsing systems from the MACHO database, with
periods of a few days. About 75% of the spectra in our sample are polluted by
hydrogen (Halpha and Hgamma), [SII] and [NII] nebular lines. These lines are
typical of HII regions. They could be associated with patchy nebulosities with
a bi-modal distribution in radial velocity, with higher values (+335 kms^{-1})
preferentially seen inside the southern part of the known bubble LMC4 observed
in HI at 21 cm.Comment: 12 pages, 17 figures, accepted to A&
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