52 research outputs found
Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red supergiants in the LMC
A radiative transfer code is used to model the spectral energy distributions
of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants
(RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and
photometric data are available. As a result we derive mass-loss rates and
bolometric luminosities.
A gap in the luminosity distribution around M_bol = -7.5 mag separates AGB
stars from RSGs. The luminosity distributions of optically bright carbon stars,
dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little
overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of
the AGB and will not evolve significantly in luminosity before mass loss ends
their AGB evolution.
Derived mass-loss rates span a range from Mdot about 10^-7 to 10^-3 M_sun/yr.
More luminous and cooler stars are found to reach higher mass-loss rates. The
highest mass-loss rates exceed the classical limit set by the momentum of the
stellar radiation field, L/c, by a factor of a few due to multiple scattering
of photons in the circumstellar dust envelope. Mass-loss rates are lower than
the mass consumption rate by nuclear burning, Mdot_nuc, for most of the RSGs.
Two RSGs have Mdot >> Mdot_nuc, however, suggesting that RSGs shed most of
their stellar mantles in short phases of intense mass loss. Stars on the
thermal pulsing AGB may also experience episodes of intensified mass loss, but
their quiescent mass-loss rates are usually already higher than Mdot_nuc.Comment: 15 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysics Main Journa
A dearth of OH/IR stars in the Small Magellanic Cloud
We present the results of targeted observations and a survey of 1612-, 1665-,
and 1667-MHz circumstellar OH maser emission from asymptotic giant branch (AGB)
stars and red supergiants (RSGs) in the Small Magellanic Cloud (SMC), using the
Parkes and Australia Telescope Compact Array radio telescopes. No clear OH
maser emission has been detected in any of our observations targeting luminous,
long-period, large-amplitude variable stars, which have been confirmed
spectroscopically and photometrically to be mid- to late-M spectral type. These
observations have probed 3 - 4 times deeper than any OH maser survey in the
SMC. Using a bootstrapping method with LMC and Galactic OH/IR star samples and
our SMC observation upper limits, we have calculated the likelihood of not
detecting maser emission in any of the two sources considered to be the top
maser candidates to be less than 0.05%, assuming a similar pumping mechanism as
the LMC and Galactic OH/IR sources. We have performed a population comparison
of the Magellanic Clouds and used Spitzer IRAC and MIPS photometry to confirm
that we have observed all high luminosity SMC sources that are expected to
exhibit maser emission. We suspect that, compared to the OH/IR stars in the
Galaxy and LMC, the reduction in metallicity may curtail the dusty wind phase
at the end of the evolution of the most massive cool stars. We also suspect
that the conditions in the circumstellar envelope change beyond a simple
scaling of abundances and wind speed with metallicity
IRAS04496-6958: A luminous carbon star with silicate dust in the Large Magellanic Cloud
We describe ISO observations of the obscured Asymptotic Giant Branch (AGB)
star IRAS04496-6958 in the Large Magellanic Cloud (LMC). This star has been
classified as a carbon star. Our new ISOCAM CVF spectra show that it is the
first carbon star with silicate dust known outside of the Milky Way. The
existence of this object, and the fact that it is one of the highest luminosity
AGB stars in the LMC, provide important information for theoretical models of
AGB evolution and understanding the origin of silicate carbon stars.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter
The mass-loss, expansion velocities, and dust production rates of carbon stars in the Magellanic Clouds
The properties of carbon stars in the Magellanic Clouds (MCs) and their total dust production rates are predicted by fitting their spectral energy distributions (SED) over pre-computed grids of spectra reprocessed by dust. The grids are calculated as a function of the
stellar parameters by consistently following the growth for several dust species in their circumstellar envelopes, coupled with a stationary wind. Dust radiative transfer is computed taking as input the results of the dust growth calculations. The optical constants for amorphous carbon are selected in order to reproduce different observations in the
infrared and optical bands of Gaia Data Release 2. We find a tail of extreme mass-losing carbon stars in the Large Magellanic Cloud (LMC) with low gas-to-dust ratios that is not present in the Small Magellanic Cloud (SMC). Typical gas-to-dust ratios are around 700 for the extreme stars, but they can be down to similar to 160-200 and similar to 100 for
a few sources in the SMC and in the LMC, respectively. The total dust production rate for the carbon star population is similar to 1.77 +/- 0.45 x 10(-5) M-circle dot yr(-1), for the LMC, and similar to 2.52 +/- 0.96 x 10(-6) M-circle dot yr(-1), for the SMC. The extreme carbon stars
observed with the Atacama Large Millimeter Array and their wind speed are studied in detail. For the most dust-obscured star in this sample the estimated mass-loss rate is similar to 6.3 x 10(-5) M-circle dot yr(-1). The grids of spectra are available at:(1) and included in the
SED-fitting python package for fitting evolved stars.(2
ISO observations of obscured Asymptotic Giant Branch stars in the Large Magellanic Cloud
We present ISO photometric and spectroscopic observations of a sample of 57
bright Asymptotic Giant Branch stars and red supergiants in the Large
Magellanic Cloud, selected on the basis of IRAS colours indicative of high
mass-loss rates. PHOT-P and PHOT-C photometry at 12, 25 and 60 m and CAM
photometry at 12 m are used in combination with quasi-simultaneous
ground-based near-IR photometry to construct colour-colour diagrams for all
stars in our sample. PHOT-S and CAM-CVF spectra in the 3 to 14 m region
are presented for 23 stars. From the colour-colour diagrams and the spectra, we
establish the chemical types of the dust around 49 stars in this sample. Many
stars have carbon-rich dust. The most luminous carbon star in the Magellanic
Clouds has also a (minor) oxygen-rich component. OH/IR stars have silicate
absorption with emission wings. The unique dataset presented here allows a
detailed study of a representative sample of thermal-pulsing AGB stars with
well-determined luminosities.Comment: 19 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysics Main Journa
The Isaac Newton Telescope monitoring survey of Local Group dwarf galaxies--V. The star formation history of Sagittarius dwarf irregular galaxy derived from long period variable stars
We conducted an optical monitoring survey of the Sagittarius dwarf irregular
galaxy (SagDIG) during the period of June 2016 -- October 2017, using the 2.5-m
Isaac Newton Telescope (INT) at La Palama. Our goal was to identify Long Period
Variable stars (LPVs), namely asymptotic giant branch stars (AGBs) and red
supergiant stars (RSGs), to obtain the Star Formation History (SFH) of
isolated, metal-poor SagDIG. For our purpose, we used a method that relies on
evaluating the relation between luminosity and the birth mass of these most
evolved stars. We found LPV candidates within two half-light radii of
SagDIG. LPV candidates were in common with previous studies, including one
very dusty AGB (x-AGB). By adopting the metallicity for older
population and for younger ages, we estimated that the star
formation rate changes from Myrkpc
( Gyr ago) to Myrkpc (
Gyr ago). Like many dwarf irregular galaxies, SagDIG has had continuous star
formation activity across its lifetime, though with different rates, and
experiences an enhancement of star formation since . We also
evaluated the total stellar mass within two half-light radii of SagDIG for
three choices of metallicities. For metallicity and we
estimated the stellar mass M = () and () M, respectively. Additionally, we
determined a distance modulus = mag, using the tip of the
red giant branch (TRGB).Comment: 16 pages, Accepted for publication in Ap
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands
We present the initial results from the Spitzer Survey of the Small
Magellanic Cloud (S3MC), which imaged the star-forming body of the Small
Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the
F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and
takes a wide range of values that are unrelated to metallicity but
anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that
photodestruction is primarily responsible for the low abundance of PAHs
observed in star-forming low-metallicity galaxies. We use the S3MC images to
compile a photometric catalog of ~400,000 mid- and far-infrared point sources
in the SMC. The sources detected at the longest wavelengths fall into four main
categories: 1) bright 5.8 um sources with very faint optical counterparts and
very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2)
Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6),
identified as carbon stars. 3) Bright mid-infrared sources with neutral colors
and bright optical counterparts, corresponding to oxygen-rich evolved stars.
And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This
excess is reminiscent of debris disks, and is detected in only a small fraction
of these stars (<5%). The majority of the brightest infrared point sources in
the SMC fall into groups one to three. We use this photometric information to
produce a catalog of 282 bright YSOs in the SMC with a very low level of
contamination (~7%).Comment: Accepted for publication in The Astrophysical Journal. Given the
draconian figure file-size limits implemented in astro-ph, readers are
encouraged to download the manuscript with full quality images from
http://celestial.berkeley.edu/spitzer/publications/s3mcsurvey.pd
Detailed maps of interstellar clouds in front of omega Centauri: Small-scale structures in the Galactic Disc-Halo interface
We used the multiplex capabilities of the AAOmega spectrograph at the
Anglo-Australian Telescope to create a half-square-degree map of the neutral
and low-ionized ISM in front of the nearby (~5 kpc), most massive Galactic
globular cluster, omega Centauri. Its redshifted, metal-poor and hot horizontal
branch stars probe the medium-strong Ca II K and Na I D2 line absorption, and
weak absorption in the lambda5780 and lambda5797 Diffuse Interstellar Bands
(DIBs), on scales around a parsec. The kinematical and thermodynamical picture
emerging from these data is that we predominantly probe the warm neutral medium
and weakly-ionized medium of the Galactic Disc-Halo interface, ~0.3-1 kpc above
the mid-plane. A comparison with Spitzer Space Telescope 24-micron and
DIRBE/IRAS maps of the warm and cold dust emission confirms that both Na I and
Ca II trace the overall column density of the warm neutral and weakly-ionized
medium. Clear signatures are seen of the depletion of calcium atoms from the
gas phase into dust grains. Curiously, the coarse DIRBE/IRAS map is a more
reliable representation of the relative reddening between sightlines than the
Na I and Ca II absorption-line measurements, most likely because the latter are
sensitive to fluctuations in the local ionization conditions. The behaviour of
the DIBs is consistent with the lambda5780 band being stronger than the
lambda5797 band in regions where the ultraviolet radiation level is relatively
high, as in the Disc-Halo interface. This region corresponds to a sigma-type
cloud. In all, our maps and simple analytical model calculations show in
unprecedented detail that small-scale density and/or ionization structures
exist in the extra-planar gas of a spiral galaxy. (abridged)Comment: Accepted for publication in MNRA
Measuring Dust Production in the Small Magellanic Cloud Core-Collapse Supernova Remnant 1E 0102.2-7219
We present mid-infrared spectral mapping observations of the core-collapse
supernova remnant 1E 0102.2-7219 in the Small Magellanic Cloud using the
InfraRed Spectrograph (IRS) on the Spitzer Space Telescope. The remnant shows
emission from fine structure transitions of neon and oxygen as well as
continuum emission from dust. Comparison of the mid-IR dust emission with
observations at x-ray, radio and optical wavelengths shows that the dust is
associated with the supernova ejecta and is thus newly formed in the remnant.
The spectrum of the newly formed dust is well reproduced by a model that
includes 3x10^-3 solar masses of amorphous carbon dust at 70 K and 2x10^-5
solar masses of Mg2SiO4 (forsterite) at 145 K. Our observations place a lower
limit on the amount of dust in the remnant since we are not sensitive to the
cold dust in the unshocked ejecta. We compare our results to observations of
other core-collapse supernovae and remnants, particularly Cas A where very
similar spectral mapping observations have been carried out. We observe a a
factor of ~10 less dust in E 0102 than seen in Cas A, although the amounts of
amorphous carbon and forsterite are comparable.Comment: submitted to Ap
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