26 research outputs found
Chemical composition of Galactic OB stars II. The fast rotator Z Oph
Z Oph, HD149757, is an O9.5 Vnn star with a very high projected rotational
velocity (vsini >= 340 km\s). It is also a classical runaway star due to its
high proper motion. We perform a quantitative analysis of its optical spectrum
in order to measure important observables of the star such as its mass,
effective temperature, luminosity and He, C, N, and O abundances. Comparing
these observed values to those predicted by the rotating evolutionary models of
the Geneva group we find that none of the two sets of models is capable of
reproducing the characteristics of the star. Nevertheless, due to its runaway
nature, the reason for this discrepancy may be that the star is not the result
of the evolution of a single object, but the product of the evolution of a
close binary system.Comment: Accepted for publication in A&
Chemical composition of Galactic OB stars I. CNO abundances in O9 stars
We present NLTE abundances of CNO for a sample of four O9 stars in the
Galaxy, together with new determinations of their stellar parameters, , , (He) and microturbulence. These new analyses take
into account the effect of {\it line--blocking} in the spectral synthesis with
our classical NLTE, plane--parallel and hydrostatic model atmospheres. The
sample includes three O9 He normal stars: two dwarfs, HD 214680 and HD 34078,
and one supergiant, HD 209975, and one fast rotating giant with a preliminary
high He overabundance, HD 191423 with (He)=0.20. We find first that
the consideration of microturbulence in the spectral synthesis for the fast
rotator leads to a considerably lower He abundance, (He)=0.12. The
CNO abundances of the three He normal stars are in good agreement with the
values in the literature for Galactic B dwarfs with no evidence of mixing, and
show that they all have the same chemical composition. We also discuss however
the possible CNO contamination of the supergiant HD 209975. For the fast
rotator we find that the abundances show the trend of the CNO contamination: a
N overabundance together with C and O depletion. The N/C and N/O ratios of our
stars as a function of their projected rotational velocities are consistent
with the predictions of the recent evolutionary models of Meynet & Maeder
[Mey&Mae00].Comment: 19 pages, 12 figures, 13 tables, accepted by A&
The VLT-FLAMES survey of massive stars: Mass loss and rotation of early-type stars in the SMC
We have studied the optical spectra of a sample of 31 O- and early B-type
stars in the Small Magellanic Cloud, 21 of which are associated with the young
massive cluster NGC 346. Stellar parameters are determined using an automated
fitting method. Comparison with predictions of stellar evolution that account
for stellar rotation does not result in a unique age, though most stars are
best represented by an age of 1-3 Myr. The present day v_sini distribution of
the 21 dwarf stars in our sample is consistent with an underlying rotational
velocity (v_r) distribution that can be characterised by a mean velocity of
about 160-190 km/s and an effective half width of 100-150 km/s. The v_r
distribution must include a small percentage of slowly rotating stars. If
predictions of the time evolution of the equatorial velocity for massive stars
within the environment of the SMC are correct, the young age of the cluster
implies that this underlying distribution is representative for the initial
rotational velocity distribution. The location in the Hertzsprung-Russell
diagram of the stars showing helium enrichment is in qualitative agreement with
evolutionary tracks accounting for rotation, but not for those ignoring v_r.
The mass loss rates of the SMC objects having luminosities of log L/L_sun > 5.4
are in excellent agreement with predictions. However, for lower luminosity
stars the winds are too weak to determine M_dot accurately from the optical
spectrum. Two of three spectroscopically classified Vz stars from our sample
are located close to the theoretical zero age main sequence, as expected.Comment: 35 pages, accepted for publication in A&
O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic O dwarfs to
track the conditions under which weak winds (i.e mass loss rates lower than ~
1e-8 Msol/yr) appear. The sample is composed of low and high luminosity dwarfs
including Vz stars and stars known to display qualitatively weak winds.
Atmosphere models including non-LTE treatment, spherical expansion and line
blanketing are computed with the code CMFGEN. Both UV and Ha lines are used to
derive wind properties while optical H and He lines give the stellar
parameters. Mass loss rates of all stars are found to be lower than expected
from the hydrodynamical predictions of Vink et al. (2001). For stars with log
L/Lsol > 5.2, the reduction is by less than a factor 5 and is mainly due to the
inclusion of clumping in the models. For stars with log L/Lsol < 5.2 the
reduction can be as high as a factor 100. The inclusion of X-ray emission in
models with low density is crucial to derive accurate mass loss rates from UV
lines. The modified wind momentum - luminosity relation shows a significant
change of slope around this transition luminosity. Terminal velocities of low
luminosity stars are also found to be low. The physical reason for such weak
winds is still not clear although the finding of weak winds in Galactic stars
excludes the role of a reduced metallicity. X-rays, through the change in the
ionisation structure they imply, may be at the origin of a reduction of the
radiative acceleration, leading to lower mass loss rates. A better
understanding of the origin of X-rays is of crucial importance for the study of
the physics of weak winds.Comment: 31 pages, 42 figures. A&A accepted. A version of the paper with full
resolution figures is available at
http://www.mpe.mpg.de/~martins/publications.htm
Identifying birth places of young isolated neutron stars
Young isolated radio-quiet neutron stars are still hot enough to be
detectable at X-ray and optical wavelengths due to their thermal emission and
can hence probe cooling curves. An identification of their birth sites can
constrain their age. For that reason we try to identify the parent associations
for four of the so-called Magnificent Seven neutron stars for which proper
motion and distance estimates are available. We are tracing back in time each
neutron star and possible birth association centre to find close encounters.
The associated time of the encounter expresses the kinematic age of the neutron
star which can be compared to its characteristic spin-down age. Owing to
observational uncertainties in the input data, we use Monte-Carlo simulations
and evaluate the outcome of our calculations statistically. RX J1856.5-3754
most probably originated from the Upper Scorpius association about 0.3 Myr ago.
RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr
ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to
come from a nearby young association such as the Sco-Cen complex or the
extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is
possible. We also give constraints on the observables as well as on the radial
velocity of the neutron star. Given the birth association, its age and the
flight time of the neutron star, we estimate the mass of the progenitor star.
Some of the potential supernovae were located very nearby (<100pc) and thus
should have contributed to the 10Be and 60Fe material found in the Earth's
crust. In addition we reinvestigate the previously suggested neutron star/
runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely
that both objects were ejected during the same supernova event.Comment: 14 figures, 13 table
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
PREDIKSI KUANTITAS AIR BERSIH DARI PDAM KABUPATEN KARANGFANYAR DI KECAMATAN MATESIH PADA TAHUN 2025
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