453 research outputs found
V838 Monocerotis: the central star and its environment a decade after outburst
Aims. V838 Monocerotis erupted in 2002, brightened in a series of outbursts,
and eventually developed a spectacular light echo. A very red star emerged a
few months after the outburst. The whole event has been interpreted as the
result of a merger. Methods. We obtained near-IR and mid-IR interferometric
observations of V838 Mon with the AMBER and MIDI recombiners located at the
Very Large Telescope Interferometer (VLTI) array. The MIDI two-beam
observations were obtained with the 8m Unit Telescopes between October 2011 and
February 2012. The AMBER three-beam observations were obtained with the compact
array (Bm) in April 2013 and the long array (B140m) in May 2014,
using the 1.8m Auxiliary Telescopes. Results. A significant new result is the
detection of a compact structure around V838 Mon, as seen from MIDI data. The
extension of the structure increases from a FWHM of 25 mas at 8 {\mu}m to 70
mas at 13 {\mu}m. At the adopted distance of D = 6.1 0.6 kpc, the dust is
distributed from about 150 to 400 AU around V838 Mon. The MIDI visibilities
reveal a flattened structure whose aspect ratio increases with wavelength. The
major axis is roughly oriented around a position angle of -10 degrees, which
aligns with previous polarimetric studies reported in the literature. This
flattening can be interpreted as a relic of the 2002 eruption or by the
influence of the currently embedded B3V companion. The AMBER data provide a new
diameter for the pseudo-photosphere, which shows that its diameter has
decreased by about 40% in 10yrs, reaching a radius R = 750 200
R (3.5 1.0 AU). Conclusions. After the 2002 eruption,
interpreted as the merging of two stars, it seems that the resulting source is
relaxing to a normal state. The nearby environment exhibits an equatorial
over-density of dust up to several hundreds of AU.Comment: Astronomy and Astrophysics (2014) Will be set by the publishe
The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample
The mass-loss process from evolved stars is a key ingredient for our
understanding of many fields of astrophysics, including stellar evolution and
the chemical enrichment of the interstellar medium via stellar yields. One the
main unsolved questions is the geometry of the mass-loss process. Taking
advantage of the results from the Herschel Mass loss of Evolved StarS (MESS)
programme, we initiated a coordinated effort to characterise the geometry of
mass loss from evolved red giants at various spatial scales. For this purpose
we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner
envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this
contribution we present an overview of the interferometric data collected
within the frame of our Large Programme, and we also add archive data for
completeness. We studied the geometry of the inner atmosphere by comparing the
observations with predictions from different geometric models. Asymmetries are
detected for five O-rich and S-type, suggesting that asymmetries in the N band
are more common among stars with such chemistry. We speculate that this fact is
related to the characteristics of the dust grains. Except for one star, no
interferometric variability is detected, i.e. the changes in size of the shells
of non-mira stars correspond to changes of the visibility of less than 10%. The
observed spectral variability confirms previous findings from the literature.
The detection of dust in our sample follows the location of the AGBs in the
IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in
region II and in the carbon stars in region VII. The SiC dust feature does not
appear in the visibility spectrum of UAnt and SSct, which are two carbon stars
with detached shells. This finding has implications for the theory of SiC dust
formation.Comment: 43 pages, 31 figures; accepted for publication in Astronomy &
Astrophysics. Abstract shortened for compilation reasons. Metadata correcte
Estimation of the dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
To study the effect of metallicity on the mass-loss of AGB stars, we have conducted mid-infrared photometric measurements of AGB stars in the Sagittarius and Fornax Dwarf Spheroidal Galaxies ([Fe/H]=-1.1 and -1.3) with the 10-micron camera of VISIR at the VLT. These observations combined with previous near-infrared photometric measurements allow us to estimate mass-loss rates in these galaxies. We show here that the observed AGB display dust-driven mass-loss. Dust mass-loss rate are found to be in the range 0.2 Myr for the observed AGB stars in SgrD and around 5 Myr for the observed star in Fornax
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL
We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study
the photosphere, the warm molecular layer, and the inner wind of the close-by
oxygen-rich AGB star R Doradus. We present observations in filters V,
cntH, and cnt820 and investigate the surface brightness distribution of
the star and of the polarised light produced in the inner envelope. Thanks to
second-epoch observations in cntH, we are able to see variability on
the stellar photosphere. We find that in the first epoch the surface brightness
of R Dor is asymmetric in V and cntH, the filters where molecular
opacity is stronger, while in cnt820 the surface brightness is closer to being
axisymmetric. The second-epoch observations in cntH show that the
morphology of R Dor changes completely in a timespan of 48 days to a more
axisymmetric and compact configuration. The polarised intensity is asymmetric
in all epochs and varies by between a factor of 2.3 and 3.7 with azimuth for
the different images. We fit the radial profile of the polarised intensity
using a spherically symmetric model and a parametric description of the dust
density profile, . On average, we find exponents of
that correspond to a much steeper density profile than that of
a wind expanding at constant velocity. The dust densities we derive imply an
upper limit for the dust-to-gas ratio of at 5.0
. Given the uncertainties in observations and models, this value is
consistent with the minimum values required by wind-driving models for the
onset of a wind, of . However, if the steep density
profile we find extends to larger distances from the star, the dust-to-gas
ratio will quickly become too small for the wind of R Dor to be driven by the
grains that produce the scattered light.Comment: 10 pages, 8 figures, 4 table
Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy
We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal
(dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space
Telescope. The stars were selected from a near-infrared survey of Fornax and
include the three reddest stars, with presumably the highest mass-loss rates,
in that galaxy. Such carbon stars probably belong to the intermediate-age
population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary
aim of this paper is to investigate mass-loss rate, as a function of luminosity
and metallicity, by comparing AGB stars in several galaxies with different
metallicities. The spectra of three stars are fitted with a radiative transfer
model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun
yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We
find no evidence that these rates depend on metallicity, although we do suggest
that the gas-to-dust ratio could be higher than at solar metallicity, in the
range 240 to 800. The C2H2 bands are stronger at lower metallicity because of
the higher C/O ratio. In contrast, the SiC fraction is reduced at low
metallicity, due to low silicon abundance. The total mass-loss rate from all
known carbon-rich AGB stars into the interstellar medium of this galaxy is of
the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf
irregular galaxy WLM, which has a similar visual luminosity and metallicity.
The difference is attributed to the younger stellar population of WLM. The
suppressed gas-return rate to the ISM accentuates the difference between the
relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies.
Our study will be useful to constrain gas and dust recycling processes in low
metallicity galaxies.Comment: MNRAS accepte
Circumstellar CO in metal-poor stellar winds: the highly irradiated globular cluster star 47 Tucanae V3
We report the first detection of circumstellar CO in a globular cluster.
Observations with ALMA have detected the CO J=3-2 and SiO v=1 J=8-7 transitions
at 345 and 344 GHz, respectively, around V3 in 47 Tucanae (NGC 104; [Fe/H] =
-0.72 dex), a star on the asymptotic giant branch. The CO line is detected at 7
sigma at a rest velocity v_LSR = -40.6 km/s and expansion velocity of 3.2 +/-
~0.4 km/s. The brighter, asymmetric SiO line may indicate a circumstellar
maser. The stellar wind is slow compared to similar Galactic stars, but the
dust opacity remains similar to Galactic comparisons. We suggest that the
mass-loss rate is set by the levitation of material into the circumstellar
environment by pulsations, but that the terminal wind-expansion velocity is
determined by radiation pressure on the dust: a pulsation-enhanced dust-driven
wind. We suggest the metal-poor nature of the star decreases the grain size,
slowing the wind and increasing its density and opacity. Metallic alloys at
high altitudes above the photosphere could also provide an opacity increase.
The CO line is weaker than expected from Galactic AGB stars, but its strength
confirms a model that includes CO dissociation by the strong interstellar
radiation field present inside globular clusters.Comment: 5 pages, accepted MNRAS Letter
The expanding dusty bipolar nebula around the nova V1280 Sco
V1280 Sco is one of the slowest dust-forming nova ever historically observed.
We performed multi-epoch high-spatial resolution observations of the
circumstellar dusty environment of V1280 Sco to investigate the level of
asymmetry of the ejecta We observed V1280 Sco in 2009, 2010 and 2011 using
unprecedented high angular resolution techniques. We used the NACO/VLT adaptive
optics system in the J, H and K bands, together with contemporaneous VISIR/VLT
mid-IR imaging that resolved the dust envelope of V1280 Sco, and SINFONI/VLT
observations secured in 2011. We report the discovery of a dusty
hourglass-shaped bipolar nebula. The apparent size of the nebula increased from
0.30" x 0.17" in July 2009 to 0.64" x 0.42" in July 2011. The aspect ratio
suggests that the source is seen at high inclination. The central source shines
efficiently in the K band and represents more than 56+/-5% of the total flux in
2009, and 87+/-6% in 2011. A mean expansion rate of 0.39+/-0.03 mas per day is
inferred from the VISIR observations in the direction of the major axis, which
represents a projected upper limit. Assuming that the dust shell expands in
that direction as fast as the low-excitation slow ejecta detected in
spectroscopy, this yields a lower limit distance to V1280 Sco of 1kpc; however,
the systematic errors remain large due to the complex shape and velocity field
of the dusty ejecta. The dust seems to reside essentially in the polar caps and
no infrared flux is detected in the equatorial regions in the latest dataset.
This may imply that the mass-loss was dominantly polar
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