2,082 research outputs found
Vigorous atmospheric motion in the red supergiant star Antares
Red supergiant stars represent a late stage of the evolution of stars more
massive than about nine solar masses, in which they develop complex,
multi-component atmospheres. Bright spots have been detected in the atmosphere
of red supergiants using interferometric imaging. Above the photosphere of a
red supergiant, the molecular outer atmosphere extends up to about two stellar
radii. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas
(less than 3,500 kelvin) of a red supergiant coexist at about three stellar
radii. The dynamics of such complex atmospheres has been probed by ultraviolet
and optical spectroscopy. The most direct approach, however, is to measure the
velocity of gas at each position over the image of stars as in observations of
the Sun. Here we report the mapping of the velocity field over the surface and
atmosphere of the nearby red supergiant Antares. The two-dimensional velocity
field map obtained from our near-infrared spectro-interferometric imaging
reveals vigorous upwelling and downdrafting motions of several huge gas clumps
at velocities ranging from about -20 to +20 kilometres per second in the
atmosphere, which extends out to about 1.7 stellar radii. Convection alone
cannot explain the observed turbulent motions and atmospheric extension,
suggesting that an unidentified process is operating in the extended
atmosphere.Comment: 27 pages, 8 figures, published in Natur
Evidence for bipolar jets in late stages of AGB winds
Bipolar expansion at various stages of evolution has been recently observed
in a number of AGB stars. The expansion is driven by bipolar jets that emerge
late in the evolution of AGB winds. The wind traps the jets, resulting in an
expanding, elongated cocoon. Eventually the jets break-out from the confining
spherical wind, as recently observed in W43A. This source displays the most
advanced evolutionary stage of jets in AGB winds. The earliest example is
IRC+10011, where the asymmetry is revealed in high-resolution near-IR imaging.
In this source the jets turned on only ~200 years ago, while the spherical wind
is ~4000 years old.Comment: 6 pages, to appear in "Asymmetrical Planetary Nebulae III" editors M.
Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series
IRC+10216 in Action: Present Episode of Intense Mass-Loss Reconstructed by Two-Dimensional Radiative Transfer Modeling
We present two-dimensional (2D) radiative transfer modeling of IRC+10216 at
selected moments of its evolution in 1995-2001, which correspond to three
epochs of our series of 8 near-infrared speckle images (Osterbart et al. 2000,
Weigelt et al. 2002). The high-resolution images obtained over the last 5.4
years revealed the dynamic evolution of the subarcsecond dusty environment of
IRC+10216 and our recent time-independent 2D radiative transfer modeling
reconstructed its physical properties at the single epoch of January 1997
(Men'shchikov et al. 2001). Having documented the complex changes in the
innermost bipolar shell of the carbon star, we incorporate the evolutionary
constraints into our new modeling to understand the physical reasons for the
observed changes. The new calculations imply that during the last 50 years, we
have been witnessing an episode of a steadily increasing mass loss from the
central star, from Mdot ~ 10^-5 Msun/yr to the rate of Mdot ~ 3x10^-4 Msun/yr
in 2001. The rapid increase of the mass loss of IRC+10216 and continuing
time-dependent dust formation and destruction caused the observed displacement
of the initially faint components C and D and of the bright cavity A from the
star which has almost disappeared in our images in 2001. Increasing dust
optical depths are causing strong backwarming that leads to higher temperatures
in the dust formation zone, displacing the latter outward with a velocity v_T ~
27 km/s due to the evaporation of the recently formed dust grains. This shift
of the dust density peak in the bipolar shell mimics a rapid radial expansion,
whereas the actual outflow has probably a lower speed v < v_inf ~ 15 km/s. The
model predicts that the star will remain obscured until Mdot starts to drop
back to lower values in the dust formation zone.Comment: 10 pages, 6 figures, accepted by Astronomy and Astrophysics, also
available at
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm
Radiative Transfer Modeling of Three-Dimensional Clumpy AGN Tori and its Application to NGC 1068
Recent observations of NGC 1068 and other AGN support the idea of a
geometrically and optically thick dust torus surrounding the central
supermassive black hole and accretion disk of AGN. In type 2 AGN, the torus is
seen roughly edge-on, leading to obscuration of the central radiation source
and a silicate absorption feature near 10 micron. While most of the current
torus models distribute the dust smoothly, there is growing evidence that the
dust must be arranged in clouds. We describe a new method for modeling near-
and mid-infrared emission of 3-dimensional clumpy tori using Monte Carlo
simulations. We calculate the radiation fields of individual clouds at various
distances from the AGN and distribute these clouds within the torus region. The
properties of the individual clouds and their distribution within the torus are
determined from a theoretical approach of self-gravitating clouds close to the
shear limit in a gravitational potential. We demonstrate that clumpiness in AGN
tori can overcome the problem of over-pronounced silicate features. Finally, we
present model calculations for the prototypical Seyfert 2 galaxy NGC 1068 and
compare them to recent high-resolution measurements. Our model is able to
reproduce both the SED and the interferometric observations of NGC 1068 in the
near- and mid-infrared.Comment: 16 pages, 16 figures, 6 tables (figures reduced due to astro-ph
limitations); accepted by A&
HD 85567: A Herbig B[e] star or an interacting B[e] binary
Context. HD 85567 is an enigmatic object exhibiting the B[e] phenomenon, i.e.
an infrared excess and forbidden emission lines in the optical. The object's
evolutionary status is uncertain and there are conflicting claims that it is
either a young stellar object or an evolved, interacting binary.
Aims. To elucidate the reason for the B[e] behaviour of HD 85567, we have
observed it with the VLTI and AMBER.
Methods. Our observations were conducted in the K-band with moderate spectral
resolution (R~1500, i.e. 200 km/s). The spectrum of HD 85567 exhibits Br gamma
and CO overtone bandhead emission. The interferometric data obtained consist of
spectrally dispersed visibilities, closure phases and differential phases
across these spectral features and the K-band continuum.
Results. The closure phase observations do not reveal evidence of asymmetry.
The apparent size of HD 85567 in the K-band was determined by fitting the
visibilities with a ring model. The best fitting radius, 0.8 +/- 0.3 AU, is
relatively small making HD 85567 undersized in comparison to the
size-luminosity relationship based on YSOs of low and intermediate luminosity.
This has previously been found to be the case for luminous YSOs, and it has
been proposed that this is due to the presence of an optically thick gaseous
disc. We demonstrate that the differential phase observations over the CO
bandhead emission are indeed consistent with the presence of a compact (~1 AU)
gaseous disc interior to the dust sublimation radius.
Conclusions. The observations reveal no sign of binarity. However, the data
do indicate the presence of a gaseous disc interior to the dust sublimation
radius. We conclude that the data are consistent with the hypothesis that HD
85567 is a YSO with an optically thick gaseous disc within a larger dust disc
that is being photo-evaporated from the outer edge.Comment: Accepted for publication in A &
Spatially resolved H_2 emission from a very low-mass star
Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have
been studied very little. So far, only a few CO outflows have been observed,
allowing us to map the immediate circumstellar environment. We present the
first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun
Class I source. By means of VLT SINFONI K-band observations, we probed the H2
emission down to the first ~50 AU from the source. The molecular emission shows
a complex structure delineating a large outflow cavity and an asymmetric
molecular jet. Thanks to the detection of several H2 transitions, we are able
to estimate average values along the jet-like structure (from source position
to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17
cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the
warm H2 component . In addition, from the total flux of the Br Gamma line, we
infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7
Msun/yr, respectively. The outflow structure is similar to those found in
low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%),
and the mass accretion rate is about one order of magnitude higher when
compared to objects of roughly the same mass, pointing to the young nature of
the investigated source.Comment: accepted as a Letter in A&
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