616 research outputs found
Evolution and Nucleosynthesis of Zero Metal Intermediate Mass Stars
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are
presented. The models have been evolved from the pre Main Sequence up to the
Asymptotic Giant Branch (AGB). At variance with previous claims, we find that
these updated stellar models do experience thermal pulses in the AGB phase. In
particular we show that: a) in models with mass larger than 6 Mo, the second
dredge up is able to raise the CNO abundance in the envelope enough to allow a
"normal" AGB evolution, in the sense that the thermal pulses and the third
dredge up settle on; b) in models of lower mass, the efficiency of the CNO
cycle in the H-burning shell is controlled by the carbon produced locally via
the 3alpha reactions. Nevertheless the He-burning shell becomes thermally
unstable after the early AGB. The expansion of the overlying layers induced by
these weak He-shell flashes is not sufficient by itself to allow a deep
penetration of the convective envelope. However, immediately after that, the
maximum luminosity of the He flash is attained and a convective shell
systematically forms at the base of the H-rich envelope. The innermost part of
this convective shell probably overlaps the underlying C-rich region left by
the inter-shell convection during the thermal pulse, so that fresh carbon is
dredged up in a "hot" H-rich environment and a H flash occurs. This flash
favours the expansion of the outermost layers already started by the weak
thermal pulse and a deeper penetration of the convective envelope takes place.
Then, the carbon abundance in the envelope rises to a level high enough that
the further evolution of these models closely resembles that of more metal rich
AGB stars. These stars provide an important source of primary carbon and
nitrogen.Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in Ap
The Herschel view of the nebula around the luminous blue variable star AG Carinae
Far-infrared Herschel PACS imaging and spectroscopic observations of the
nebula around the luminous blue variable (LBV) star AG Car have been obtained
along with optical imaging in the Halpha+[NII] filter. In the infrared light,
the nebula appears as a clumpy ring shell that extends up to 1.2 pc with an
inner radius of 0.4 pc. It coincides with the Halpha nebula, but extends
further out. Dust modeling of the nebula was performed and indicates the
presence of large grains. The dust mass is estimated to be ~ 0.2 Msun. The
infrared spectrum of the nebula consists of forbidden emission lines over a
dust continuum. Apart from ionized gas, these lines also indicate the existence
of neutral gas in a photodissociation region that surrounds the ionized region.
The abundance ratios point towards enrichment by processed material. The total
mass of the nebula ejected from the central star amounts to ~ 15 Msun, assuming
a dust-to-gas ratio typical of LBVs. The abundances and the mass-loss rate were
used to constrain the evolutionary path of the central star and the epoch at
which the nebula was ejected, with the help of available evolutionary models.
This suggests an ejection during a cool LBV phase for a star of ~ 55 Msun with
little rotation.Comment: accepted for publication in A&
High Resolution X-ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association
The Cygnus OB2 Association contains some of the most luminous OB stars in our
Galaxy, the brightest of which are also among the most luminous in X-rays. We
have obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS)
observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in the
Association. Although our analysis will focus on the X-ray properties of Cyg
OB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2
Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous
studies of early-type stars, we find that the X-ray properties of Cyg OB2 No.
8a are very similar to those of other OB stars that have been observed using
high-resolution X-ray spectroscopy. From analyses of the He-like ion "fir"
emission lines, we derive radial distances of the He-like line emission sources
and find these fir-inferred radii are consistent with their corresponding X-ray
continuum optical depth unity radii. Contrary to other O-star results, the
emission lines of Cyg OB2 No. 8a show a large range in line centroid shifts
(roughly -800 to +250 km/s). We discuss the implications of our results in
light of the fact that Cyg OB2 No. 8a is a member of a rather tight stellar
cluster, and shocks could arise at interfaces with the winds of these other
stars.Comment: 36 pages (including 4 tables and 12 figures). LaTeX. Submitted to Ap
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations
We have modeled the emission of H2O rotational lines from the extreme C-rich
star IRC+10216. Our treatment of the excitation of H2O emissions takes into
account the excitation of H2O both through collisions, and through the pumping
of the nu2 and nu3 vibrational states by dust emission and subsequent decay to
the ground state. Regardless of the spatial distribution of the water
molecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the Submillimeter
Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the
absorption of dust-emitted photons at 6 m in the nu2 band. As noted by
previous authors, the inclusion of radiative pumping lowers the ortho-H2O
abundance required to account for the 557 GHz emission, which is found to be
(0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization of
orbiting comets or Fischer-Tropsch catalysis. Predictions for other
submillimeter H2O lines that can be observed by the Herschel Space Observatory
(HSO) are reported. Multitransition HSO observations promise to reveal the
spatial distribution of the circumstellar water vapor, discriminating among the
several hypotheses that have been proposed for the origin of the H2O vapor in
the envelope of IRC+10216. We also show that, for observations with HSO, the
H2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searching
for H2O in other C-rich AGB stars.Comment: 35 pages, 12 figures, to be published in The Astrophysical Journa
Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds
Context : A 1000-s flare from a new hard X-ray transient, IGR J08408-4503,
was observed by INTEGRAL on May 15, 2006 during the real-time routine
monitoring of IBIS/ISGRI images performed at the INTEGRAL Science Data Centre.
The flare, detected during a single one-hour long pointing, peaked at 250 mCrab
in the 20-40 keV energy range.
Aims : Multi-wavelength observations, combining high-energy and optical data,
were used to unveil the nature of IGR J08408-4503.
Methods : A search in all INTEGRAL public data for other bursts from IGR
J08408-4503 was performed, and the detailed analysis of another major flare is
presented. The results of two Swift Target of Opportunity observations are also
described. Finally, a study of the likely optical counterpart, HD 74194, is
provided.
Results : IGR J08408-4503 is very likely a supergiant fast X-ray transient
(SFXT) system. The system parameters indicate that the X-ray flares are
probably related to the accretion of wind clumps on a compact object orbiting
about 1E13 cm from the supergiant HD 74194. The clump mass loss rate is of the
order of 1E-6 solar mass/yr.
Conclusions : Hard X-ray flares from SFXTs allow to probe the stellar winds
of massive stars, and could possibly be associated with wind perturbations due
to line-driven instabilities.Comment: 5 pages with 5 figures. Published as a Letter in Astronomy &
Astrophysic
A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um
with a spectral resolution of about 4 km s^-1. We have identified a forest of
lines in six bands of C2H2 involving the vibrational states from the ground to
3nu5 and in two bands of HCN, involving the vibrational states from the ground
up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We
have estimated the kinetic, vibrational, and rotational temperatures, and the
abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16
cm) by fitting about 300 of these ro-vibrational lines. The envelope can be
divided into three regions with approximate boundaries at 0.019 arcsec (the
stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4
arcsec (outer dust formation zone). Most of the lines might require a large
microturbulence broadening. The derived abundances of C2H2 and HCN increase by
factors of 10 and 4, respectively, from the innermost envelope outwards. The
derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational
states up to 3000 K above ground are populated, suggesting pumping by
near-infrared radiation from the star and innermost envelope. Low rotational
levels can be considered under LTE while those with J>20-30 are not
thermalized. A few lines require special analysis to deal with effects like
overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the
Astrophysical Journa
Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
Context. The recent detection of warm HO vapor emission from the outflows
of carbon-rich asymptotic giant branch (AGB) stars challenges the current
understanding of circumstellar chemistry. Two mechanisms have been invoked to
explain warm HO vapor formation. In the first, periodic shocks passing
through the medium immediately above the stellar surface lead to HO
formation. In the second, penetration of ultraviolet interstellar radiation
through a clumpy circumstellar medium leads to the formation of HO
molecules in the intermediate wind.
Aims. We aim to determine the properties of HO emission for a sample of
18 carbon-rich AGB stars and subsequently constrain which of the above
mechanisms provides the most likely warm HO formation pathway.
Methods, Results, and Conclusions. See paper
Mean angular diameters, distances and pulsation modes of the classical Cepheids FF Aql and T Vul - CHARA/FLUOR near-infrared interferometric observations
We report the first angular diameter measurements of two classical Cepheids,
FF Aql and T Vul, that we have obtained with the FLUOR instrument installed at
the CHARA interferometric array. We obtain average limb-darkened angular
diameters of \theta_LD = 0.878 +/- 0.013 mas and \theta_LD = 0.629 +/- 0.013
mas, respectively for FF Aql and T Vul. Combining these angular diameters with
the HST-FGS trigonometric parallaxes leads to linear radii R = 33.6 +/- 2.2
Rsol and R = 35.6 +/- 4.4 Rsol, respectively. The comparison with empirical and
theoretical Period-Radius relations leads to the conclusion that these Cepheids
are pulsating in their fundamental mode. The knowledge of the pulsation mode is
of prime importance to calibrate the Period-Luminosity relation with a uniform
sample of fundamental mode Cepheids
Large-scale environments of binary AGB stars probed by Herschel. II: Two companions interacting with the wind of pi1 Gruis
Context. The Mass loss of Evolved StarS (MESS) sample observed with PACS on
board the Herschel Space Observatory revealed that several asymptotic giant
branch (AGB) stars are surrounded by an asymmetric circumstellar envelope (CSE)
whose morphology is most likely caused by the interaction with a stellar
companion. The evolution of AGB stars in binary systems plays a crucial role in
understanding the formation of asymmetries in planetary nebul{\ae} (PNe), but
at present, only a handful of cases are known where the interaction of a
companion with the stellar AGB wind is observed.
Aims. We probe the environment of the very evolved AGB star Gruis on
large and small scales to identify the triggers of the observed asymmetries.
Methods. Observations made with Herschel/PACS at 70 m and 160 m
picture the large-scale environment of Gru. The close surroundings of
the star are probed by interferometric observations from the VLTI/AMBER
archive. An analysis of the proper motion data of Hipparcos and Tycho-2
together with the Hipparcos Intermediate Astrometric Data help identify the
possible cause for the observed asymmetry.
Results. The Herschel/PACS images of Gru show an elliptical CSE whose
properties agree with those derived from a CO map published in the literature.
In addition, an arc east of the star is visible at a distance of
from the primary. This arc is most likely part of an
Archimedean spiral caused by an already known G0V companion that is orbiting
the primary at a projected distance of 460 au with a period of more than 6200
yr. However, the presence of the elliptical CSE, proper motion variations, and
geometric modelling of the VLTI/AMBER observations point towards a third
component in the system, with an orbital period shorter than 10 yr, orbiting
much closer to the primary than the G0V star.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
- …