997 research outputs found
Cool carbon stars in the halo: new very red or distant objects
The goal of this paper is to present and analyse a new sample of cool carbon
(C)stars located in the halo. Twenty three new C stars were discovered. Spectra
are typical of N-type stars with C2 and CN bands and sometimes Halpha in
emission. ... Four objects are particularly red with J-K > 3, with 2 located at
more than 5 kpc. from the Galactic plane. Eight additional objects with similar
properties are found in the literature and our previous works. These 12 C stars
could be useful to study mass loss at low metallicity. Two objects are at
distances of 95 and 110 kpc. They are located in the region with galactocentric
Z < -60 kpc in which the model of Law et al. predicts the Sgr stream to have a
loop. (Abstact abridged)Comment: 16 pages, 12 figures, accepted by A
The mass-loss return from evolved stars to the Large Magellanic Cloud III. Dust properties for carbon-rich asymptotic giant branch stars
We present a 2Dust model for the dust shell around a LMC long-period variable
(LPV) previously studied as part of the OGLE survey. OGLE LMC LPV 28579 (SAGE
J051306.40-690946.3) is a carbon-rich asymptotic giant branch (AGB) star for
which we have photometry and spectra from the Spitzer SAGE and SAGE-Spec
programs along with UBVIJHK_s photometry. By modeling this source, we obtain a
baseline set of dust properties to be used in the construction of a grid of
models for carbon stars. We reproduce its spectral energy distribution using a
mixture of AmC and SiC (15% by mass). The grain sizes are distributed according
to the KMH model. The best-fit model has an optical depth of 0.28 for the shell
at the peak of the SiC feature, with R_in~1430 R_sun or 4.4 R_star. The
temperature at this inner radius is 1310 K. Assuming an expansion velocity of
10 km s^-1, we obtain a dust mass-loss rate of 2.5x10^-9 M_sun yr-1. We
calculate a 15% variation in this rate by testing the fit sensitivity against
variation in input parameters. We also present a simple model for the molecular
gas in the extended atmosphere that could give rise to the 13.7 \mu m feature
seen in the spectrum. We find that a combination of CO and C_2H_2 gas at an
excitation temperature of about 1000 K and column densities of 3x10^21 cm^-2
and 10^19 cm^-2 respectively are able to reproduce the observations. Given that
the excitation temperature is close to T_dust(R_in), most of the molecular
contribution probably arises from the inner shell region. The luminosity
corresponding to the first epoch of SAGE observations is 6580 L_sun. For an
effective temperature of about 3000 K, this implies a stellar mass of 1.5-2
M_sun and an age of 1-2.5 Gyr. For a gas:dust ratio of 200, we obtain a gas
mass-loss rate of 5.0x10^-7 M_sun yr^-1, consistent with the gas mass-loss
rates estimated from the period, color and 8 \mu m flux of the source.Comment: 14 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Herschel imaging and spectroscopy of the nebula around the luminous blue variable star WRAY 15-751
We have obtained far-infrared Herschel PACS imaging and spectroscopic
observations of the nebular environment of the luminous blue variable WRAY
15-751. These images clearly show that the main, dusty nebula is a shell of
radius 0.5 pc and width 0.35 pc extending outside the H-alpha nebula. They also
reveal a second, bigger and fainter dust nebula, observed for the first time.
Both nebulae lie in an empty cavity, likely the remnant of the O-star wind
bubble formed when the star was on the main sequence. The kinematic ages of the
nebulae are about 20000 and 80000 years and each nebula contains about 0.05
Msun of dust. Modeling of the inner nebula indicates a Fe-rich dust. The
far-infrared spectrum of the main nebula revealed forbidden emission lines
coming from ionized and neutral gas. Our study shows that the main nebula
consists of a shell of ionized gas surrounded by a thin photodissociation
region illuminated by an "average" early-B star. The derived abundance ratios
N/O=1.0+/-0.4 and C/O=0.4+/-0.2 indicate a mild N/O enrichment. We estimate
that the inner shell contains 1.7+/-0.6 Msun of gas. Assuming a similar
dust-to-gas ratio for the outer nebula, the total mass ejected by WRAY 15-751
amounts to 4+/-2 Msun. The measured abundances, masses and kinematic ages of
the nebulae were used to constrain the evolution of the star and the epoch at
which the nebulae were ejected. Our results point to an ejection of the nebulae
during the RSG evolutionary phase of an ~ 40 Msun star. The presence of
multiple shells around the star suggests that the mass-loss was not a
continuous ejection but rather a series of episodes of extreme mass-loss. Our
measurements are compatible with the recent evolutionary tracks computed for an
40 Msun star with little rotation. They support the O-BSG-RSG-YSG-LBV filiation
and the idea that high-luminosity and low-luminosity LBVs follow different
evolutionary paths.Comment: 19 pages, 13 figures, accepted for publication in A&
The distribution of H13CN in the circumstellar envelope around IRC+10216
H13CN J=8-7 sub-millimetre line emission produced in the circumstellar
envelope around the extreme carbon star IRC+10216 has been imaged at
sub-arcsecond angular resolution using the SMA. Supplemented by a detailed
excitation analysis the average fractional abundance of H13CN in the inner wind
(< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN
fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50.
Multi-transitional single-dish observations further requires the H13CN
fractional abundance to remain more or less constant in the envelope out to a
radius of about 4E16 cm, where the HCN molecules are effectively destroyed,
most probably, by photodissociation. The large amount of HCN present in the
inner wind provides effective line cooling that can dominate over that
generated from CO line emission. It is also shown that great care needs to be
taken in the radiative transfer modelling where non-local, and non-LTE, effects
are important and where the radiation field from thermal dust grains plays a
major role in exciting the HCN molecules. The amount of HCN present in the
circumstellar envelope around IRC+10216 is consistent with predicted
photospheric values based on equilibrium chemical models and indicates that any
non-equilibrium chemistry occurring in the extended pulsating atmosphere has no
drastic net effect on the fractional abundance of HCN molecules that enters the
outer envelope. It further suggests that few HCN molecules are incorporated
into dust grains.Comment: Accepted for publication in ApJ. 20 pages, 7 figure
The ALMA detection of CO rotational line emission in AGB stars in the Large Magellanic Cloud
Context. Low- and intermediate-mass stars lose most of their stellar mass at the end of their lives on the asymptotic giant branch (AGB). Determining gas and dust mass-loss rates (MLRs) is important in quantifying the contribution of evolved stars to the enrichment of the interstellar medium.
Aims: This study attempts to spectrally resolve CO thermal line emission in a small sample of AGB stars in the Large Magellanic Cloud (LMC).
Methods: The Atacama Large Millimeter Array was used to observe two OH/IR stars and four carbon stars in the LMC in the CO J = 2-1 line.
Results: We present the first measurement of expansion velocities in extragalactic carbon stars. All four C stars are detected and wind expansion velocities and stellar velocities are directly measured. Mass-loss rates are derived from modelling the spectral energy distribution and Spitzer/IRS spectrum with the DUSTY code. The derived gas-to-dust ratios allow the predicted velocities to agree with the observed gas-to-dust ratios. The expansion velocities and MLRs are compared to a Galactic sample of well-studied relatively low MLRs stars supplemented with extreme C stars with properties that are more similar to the LMC targets. Gas MLRs derived from a simple formula are significantly smaller than those derived from dust modelling, indicating an order of magnitude underestimate of the estimated CO abundance, time-variable mass loss, or that the CO intensities in LMC stars are lower than predicted by the formula derived for Galactic objects. This could be related to a stronger interstellar radiation field in the LMC.
Conclusions: Although the LMC sample is small and the comparison to Galactic stars is non-trivial because of uncertainties in their distances (hence luminosities), it appears that for C stars the wind expansion velocities in the LMC are lower than in the solar neighbourhood, while the MLRs appear to be similar. This is in agreement with dynamical dust-driven wind models
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
A Submillimeter HCN Laser in IRC+10216
We report the detection of a strong submillimeter wavelength HCN laser line
at a frequency near 805 GHz toward the carbon star IRC+10216. This line, the
J=9-8 rotational transition within the (04(0)0) vibrationally excited state, is
one of a series of HCN laser lines that were first detected in the laboratory
in the early days of laser spectroscopy. Since its lower energy level is 4200 K
above the ground state, the laser emission must arise from the inner part of
IRC+10216's circumstellar envelope. To better characterize this environment, we
observed other, thermally emitting, vibrationally excited HCN lines and find
that they, like the laser line, arise in a region of temperature approximately
1000 K that is located within the dust formation radius; this conclusion is
supported by the linewidth of the laser. The (04(0)0), J=9-8 laser might be
chemically pumped and may be the only known laser (or maser) that is excited
both in the laboratory and in space by a similar mechanism.Comment: 11 pages, 3 figure
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
Silicon carbide absorption features: dust formation in the outflows of extreme carbon stars
Infrared carbon stars without visible counterparts are generally known as
extreme carbon stars. We have selected a subset of these stars with absorption
features in the 10-13 m range, which has been tentatively attributed to
silicon carbide (SiC). We add three new objects meeting these criterion to the
seven previously known, bringing our total sample to ten sources. We also
present the result of radiative transfer modeling for these stars, comparing
these results to those of previous studies. In order to constrain model
parameters, we use published mass-loss rates, expansion velocities and
theoretical dust condensation models to determine the dust condensation
temperature. These show that the inner dust temperatures of the dust shells for
these sources are significantly higher than previously assumed. This also
implies that the dominant dust species should be graphite instead of amorphous
carbon. In combination with the higher condensation temperature we show that
this results in a much higher acceleration of the dust grains than would be
expected from previous work. Our model results suggest that the very optically
thick stage of evolution does not coincide with the timescales for the
superwind, but rather, that this is a very short-lived phase. Additionally, we
compare model and observational parameters in an attempt to find any
correlations. Finally, we show that the spectrum of one source, IRAS
175343030, strongly implies that the 10-13 m feature is due to a solid
state rather than a molecular species.Comment: 13 Figure
An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
Chandra high energy resolution observations have now been obtained from
numerous non-peculiar O and early B stars. The observed X-ray emission line
properties differ from pre-launch predictions, and the interpretations are
still problematic. We present a straightforward analysis of a broad collection
of OB stellar line profile data to search for morphological trends. X-ray line
emission parameters and the spatial distributions of derived quantities are
examined with respect to luminosity class. The X-ray source locations and their
corresponding temperatures are extracted by using the He-like f/i line ratios
and the H-like to He-like line ratios respectively. Our luminosity class study
reveals line widths increasing with luminosity. Although the majority of the OB
emission lines are found to be symmetric, with little central line
displacement, there is evidence for small, but finite, blue-ward line-shifts
that also increase with luminosity. The spatial X-ray temperature distributions
indicate that the highest temperatures occur near the star and steadily
decrease outward. This trend is most pronounced in the OB supergiants. For the
lower density wind stars, both high and low X-ray source temperatures exist
near the star. However, we find no evidence of any high temperature X-ray
emission in the outer wind regions for any OB star. Since the temperature
distributions are counter to basic shock model predictions, we call this the
"near-star high-ion problem" for OB stars. By invoking the traditional OB
stellar mass loss rates, we find a good correlation between the fir-inferred
radii and their associated X-ray continuum optical depth unity radii. We
conclude by presenting some possible explanations to the X-ray source problems
that have been revealed by this study.Comment: Published in 2007, ApJ, 668, 456. An Erratum scheduled for
publication in 2008, ApJ, 680, is included as an Appendix. The Erratum
corrects some tabulated data in 5 tables and 2 figure
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