338 research outputs found
The new carbon symbiotic star IPHAS J205836.43+503307.2
We are performing a search for symbiotic stars using IPHAS, the INT Halpha
survey of the northern Galactic plane, and follow-up observations. Candidate
symbiotic stars are selected on the basis of their IPHAS and near-IR colours,
and spectroscopy and photometry are obtained to determine their nature. We
present here observations of the symbiotic star candidate IPHAS
J205836.43+503307.2. The optical spectrum shows the combination of a number of
emission lines, among which are the high-excitation species of [OIII], HeII,
[Ca V], and [Fe VII], and a red continuum with the features of a star at the
cool end of the carbon star sequence. The nebular component is spatially
resolved: the analysis of the spatial profile of the [NII]6583 line in the
spectrum indicates a linear size of ~2.5 arcsec along the east-west direction.
Its velocity structure suggests an aspherical morphology. The near-infrared
excess of the source, which was especially strong in 1999, indicated that a
thick circumstellar dust shell was also present in the system. The carbon star
has brightened in the last decade by two to four magnitudes at red and
near-infrared wavelengths. Photometric monitoring during a period of 60 days
from November 2010 to January 2011 reveals a slow luminosity decrease of 0.2
magnitudes. From the observed spectrophotometric properties and variability, we
conclude that the source is a new Galactic symbiotic star of the D-type, of the
rare kind that contains a carbon star, likely a carbon Mira. Only two other
systems of this type are known in the Galaxy.Comment: 6 pages, 4 figure
On the difference between type E and A OH/IR stars
The observed SEDs of a sample of 60 OH/IR stars are fitted using a radiative
transfer model of a dusty envelope. Among the whole sample, 21 stars have
reliable phase-lag distances while the others have less accurate distances.
L*-P,Mlr-P and Mlr-L* relations have been plotted for these stars. It is found
that type E (with emission feature at 10um and type A (with absorption feature
at 10um) OH/IR stars have different L*-P and Mlr-L* relations while both of
them follow a single Mlr-P relation. The type E stars are proven to be located
in the area without large scale dense interstellar medium while the type A
stars are located probably in dense interstellar medium. It is argued here that
this may indicate the two types of OH/IR stars have different chemical
composition or zero age main sequence mass and so evolve in different ways.
This conclusion has reinforced the argument by Chen et al.(2001) who reached a
similar conclusion from the galactic distribution of about 1000 OH/IR stars
with the IRAS low-resolution spectra (LRS).Comment: 6 pages, 9 figures, 2 table
Axial Symmetry and Rotation in the SiO Maser Shell of IK Tauri
We observed v=1, J=1-0 43-GHz SiO maser emission toward the Mira variable IK
Tauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resulting
from these observations show that SiO masers form a highly elliptical ring of
emission approximately 58 x 32 mas with an axial ratio of 1.8:1. The major axis
of this elliptical distribution is oriented at position angle of ~59 deg. The
line-of-sight velocity structure of the SiO masers has an apparent axis of
symmetry consistent with the elongation axis of the maser distribution.
Relative to the assumed stellar velocity of 35 km/s, the blue- and red-shifted
masers were found to lie to the northwest and southeast of this symmetry axis
respectively. This velocity structure suggests a NW-SE rotation of the SiO
maser shell with an equatorial velocity, which we determine to be ~3.6 km/s.
Such a NW-SE rotation is in agreement with a circumstellar envelope geometry
invoked to explain previous water and OH maser observations. In this geometry,
water and OH masers are preferentially created in a region of enhanced density
along the NE-SW equator orthogonal to the rotation/polar axis suggested by the
SiO maser velocities.Comment: 17 Pages, 4 figures (2 color); accepted for publication in Ap
Mast cells produce a unique chondroitin sulfate epitope
The granules of mast cells contain a myriad of mediators that are stored and protected by the sulfated glycosaminoglycan (GAG) chains that decorate proteoglycans. Whereas heparin is the GAG predominantly associated with mast cells, mast cell proteoglycans are also decorated with heparan sulfate and chondroitin sulfate (CS). This study investigated a unique CS structure produced by mast cells that was detected with the antibody clone 2B6 in the absence of chondroitinase ABC digestion. Mast cells in rodent tissue sections were characterized using toluidine blue, Leder stain and the presence of mast cell tryptase. The novel CS epitope was identified in rodent tissue sections and localized to cells that were morphologically similar to cells chemically identified as mast cells. The rodent mast cell-like line RBL-2H3 was also shown to express the novel CS epitope. This epitope co-localized with multiple CS proteoglycans in both rodent tissue and RBL-2H3 cultured cells. These findings suggest that the novel CS epitope that decorates mast cell proteoglycans may play a role in the way these chains are structured in mast cells
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
The Unusual Infrared Object HDF-N J123656.3+621322
We describe an object in the Hubble Deep Field North with very unusual
near-infrared properties. It is readily visible in Hubble Space Telescope
NICMOS images at 1.6um and from the ground at 2.2um, but is undetected (with
signal-to-noise <~ 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1um. The
f_nu flux density drops by a factor >~ 8.3 (97.7% confidence) from 1.6 to
1.1um. The object is compact but may be slightly resolved in the NICMOS 1.6um
image. In a low-resolution, near-infrared spectrogram, we find a possible
emission line at 1.643um, but a reobservation at higher spectral resolution
failed to confirm the line, leaving its reality in doubt. We consider various
hypotheses for the nature of this object. Its colors are unlike those of known
galactic stars, except perhaps the most extreme carbon stars or Mira variables
with thick circumstellar dust shells. It does not appear to be possible to
explain its spectral energy distribution as that of a normal galaxy at any
redshift without additional opacity from either dust or intergalactic neutral
hydrogen. The colors can be matched by those of a dusty galaxy at z >~ 2, by a
maximally old elliptical galaxy at z >~ 3 (perhaps with some additional
reddening), or by an object at z >~ 10 whose optical and 1.1um light have been
suppressed by the intergalactic medium. Under the latter hypothesis, if the
luminosity results from stars and not an AGN, the object would resemble a
classical, unobscured protogalaxy, with a star formation rate >~ 100 M_sun/yr.
Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a
space density several hundred times lower than that of present-day L* galaxies.Comment: Accepted for publication in the Astrophysical Journal. 27 pages,
LaTeX, with 7 figures (8 files); citations & references updated + minor
format change
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
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
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
Variability and spectral classification of LMC giants: results from DENIS and EROS
We present the first cross-identifications of sources in the near-infrared
DENIS survey and in the micro-lensing EROS survey in a field of about 0.5
square degrees around the optical center (OC) of the Large Magellanic Cloud. We
analyze the photometric history of these stars in the EROS data base and obtain
light-curves for about 800 variables. Most of the stars are long period
variables (Miras and Semi-Regulars), a few Cepheids are also present. We also
present new spectroscopic data on 126 asymptotic giant branch stars in the OC
field, 30 previously known and 96 newly discovered by the DENIS survey. The
visible spectra are used to assign a carbon- (C-) or oxygen-rich (O-rich)
nature to the observed stars on the basis of the presence of molecular bands of
TiO, VO, CN, C2. For the remaining of the stars we used the (J-Ks) color to
determine whether they are O-rich or C-rich. Plotting Log(period) versus Ks we
find three very distinct period-luminosity relations, mainly populated by
Semi-Regular of type a (SRa), b (SRb) and Mira variables. Carbon-rich stars
occupy mostly the upper part of these relations. We find that 65% of the
asymptotic giant branch population are long period variables (LPVs).Comment: 9 pages, 7 figures, 4 tables (2 via CDS), accepted by A&A journa
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