365 research outputs found
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&
Detection of an Inner Gaseous Component in a Herbig Be Star Accretion Disk: Near- and Mid-Infrared Spectrointerferometry and Radiative Transfer modeling of MWC 147
This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.We study the geometry and the physical conditions in the inner (AU-scale) circumstellar region around the young Herbig Be star MWC 147 using long-baseline spectrointerferometry in the near-infrared (NIR) K-band, VLTI/AMBER observations, and PTI archive data, as well as the mid-infrared (MIR) N-band, VLTI/MIDI observations. The emission from MWC 147 is clearly resolved and has a characteristic physical size of ~1.3 and ~9 AU at 2.2 and 11 μm, respectively (Gaussian diameter). The MIR emission reveals asymmetry consistent with a disk structure seen under intermediate inclination. The spectrally dispersed AMBER and MIDI interferograms both show a strong increase in the characteristic size toward longer wavelengths, much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. We model the interferometric data and the spectral energy distribution of MWC 147 with two-dimensional, frequency-dependent radiation transfer simulations. This analysis shows that models of spherical envelopes or passive irradiated Keplerian disks (with vertical or curved puffed-up inner rim) can easily fit the SED, but predict much lower visibilities than observed; the angular size predicted by such models is 2-4 times larger than the size derived from the interferometric data, so these models can clearly be ruled out. Models of a Keplerian disk with optically thick gas emission from an active gaseous disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the absolute level and the spectral dependence of the NIR and MIR visibilities. We conclude that the NIR continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the MIR emission also contains contributions from the outer, irradiated dust disk.S. K. was
supported for this research through a fellowship from the International
Max Planck Research School ( IMPRS) for Radio and
Infrared Astronomy at the University of Bonn
Asymmetric silicate dust distribution toward the silicate carbon star BM Gem
Silicate carbon stars show the 10 micron silicate emission, despite their
carbon-rich photospheres. They are considered to have circumbinary or
circum-companion disks, which serve as a reservoir of oxygen-rich material shed
by mass loss in the past. We present N-band spectro-interferometric
observations of the silicate carbon star BM Gem using MIDI at the Very Large
Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution
of oxygen-rich dust with high spatial resolution. BM Gem was observed with
VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline
configurations. The N-band visibilities observed for BM Gem show a steep
decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron,
reflecting the optically thin silicate emission feature emanating from
sub-micron-sized amorphous silicate grains. The differential phases obtained at
baselines of ~44--46 m show significant non-zero values (~ -70 degrees) in the
central part of the silicate emission feature between ~9 and 11 micron,
revealing a photocenter shift and the asymmetric nature of the silicate
emitting region. The observed N-band visibilities and differential phases can
be fairly explained by a simple geometrical model in which the unresolved star
is surrounded by a ring with azimuthal brightness modulation. The best-fit
model is characterized by a broad ring (~70 mas across at 10 micron) with a
bright region which is offset from the unresolved star by ~20 mas at a position
angle of ~280 degrees. This model can be interpreted as a system with a
circum-companion disk and is consistent with the spectroscopic signatures of an
accretion disk around an unseen companion recently discovered in the violet
spectrum of BM Gem.Comment: 7 pages, 3 figures, accepted for publication in A&
Properties of the CO and HO MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations
Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is
well suited for studying the complex processes in its atmosphere that lead to
the chemical enrichment of the interstellar medium. Aims. We intend to
investigate the shape and composition of the close molecular layer (also known
as the MOLsphere) that surrounds the star. This analysis is part of a wider
program that aims at understanding the dynamics of the circumstellar envelope
of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed
using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large
Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral
resolution of the instrument (R 1500), we were able to investigate the
carbon monoxide band heads and the water-vapor bands. We used two different
approaches to analyse our data: a model fit in both the continuum and
absorption lines and then a fit with a Radiative HydroDynamics (RHD)
simulation. Results. Using the continuum data, we derive a uniform disk
diameter of ~mas, a power law type limb-darkened disk diameter
of ~mas and a limb-darkening exponent of .
Within the absorption lines, using a single layer model, we obtain parameters
of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in
the visibilities. Conclusions. We derived a new value of the angular diameter
of Betelgeuse in the K band continuum. Our observations in the absorption lines
are well reproduced by a molecular layer at 1.2 stellar radii containing both
CO and HO. The visibilities at higher spatial frequencies are matching a
convection pattern in a RHD simulation.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics; Language editin
Temporal variations of the outer atmosphere and the dust shell of the carbon-rich Mira variable V Oph probed with VLTI/MIDI
We present the first multi-epoch N-band spectro-interferometric observations of the carbon-rich Mira variable V Oph using MIDI at the ESO's Very Large Telescope Interferometer. Our MIDI observations were carried out at three different phases 0.18, 0.49, and 0.65, using three different baselines configurations (UT2-UT4, UT1-UT4, and UT2-UT3) with projected baseline lengths of 42-124 m. The wavelength dependence of the uniform-disk diameters obtained at all epochs is characterized by a roughly constant region between 8 and 10 micron with a slight dip centered at ~9.5 micron and a gradual increase longward of 10 micron. These N-band angular sizes are significantly larger than the estimated photospheric size of V Oph. The angular sizes observed at different epochs reveal that the object appears smaller at phase 0.49 (minimum light) with uniform-disk diameters of ~5-12 mas than at phases 0.18 (~12-20 mas) and 0.65 (~9-15 mas). We interpret these results with a model consisting of optically thick C2H2 layers and an optically thin dust shell. Our modeling suggests that the C2H2 layers around V Oph are more extended (~1.7-1.8 Rstar) at phases 0.18 and 0.65 than at phase 0.49 (~1.4 Rstar) and that the C2H2 column densities appear to be the smallest at phase 0.49. We also find that the dust shell consists of amorphous carbon and SiC with an inner radius of ~2.5 Rstar, and the total optical depths at phases 0.18 and 0.65 are higher than that at phase 0.49. Our MIDI observations and modeling indicate that carbon-rich Miras also have extended layers of polyatomic molecules as previously confirmed in oxygen-rich Miras
Imaging the dynamical atmosphere of the red supergiant Betelgeuse in the CO first overtone lines with VLTI/AMBER
We present the first 1-D aperture synthesis imaging of the red supergiant
Betelgeuse in the individual CO first overtone lines with VLTI/AMBER. The
reconstructed 1-D projection images reveal that the star appears differently in
the blue wing, line center, and red wing of the individual CO lines. The 1-D
projection images in the blue wing and line center show a pronounced,
asymmetrically extended component up to ~1.3 stellar radii, while those in the
red wing do not show such a component. The observed 1-D projection images in
the lines can be reasonably explained by a model in which the CO gas within a
region more than half as large as the stellar size is moving slightly outward
with 0--5 km s^-1, while the gas in the remaining region is infalling fast with
20--30 km s^-1. A comparison between the CO line AMBER data taken in 2008 and
2009 shows a significant time variation in the dynamics of the CO line-forming
region in the photosphere and the outer atmosphere. In contrast to the line
data, the reconstructed 1-D projection images in the continuum show only a
slight deviation from a uniform disk or limb-darkened disk. We derive a
uniform-disk diameter of 42.05 +/- 0.05 mas and a power-law-type limb-darkened
disk diameter of 42.49 +/- 0.06 mas and a limb-darkening parameter of (9.7 +/-
0.5) x 10^{-2}. This latter angular diameter leads to an effective temperature
of 3690 +/- 54 K for the continuum-forming layer. These diameters confirm that
the near-IR size of Betelgeuse was nearly constant over the last 18 years, in
marked contrast to the recently reported noticeable decrease in the mid-IR
size. The continuum data taken in 2008 and 2009 reveal no or only marginal time
variations, much smaller than the maximum variation predicted by the current
3-D convection simulations.Comment: 21 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The molecular and dusty composition of Betelgeuse's inner circumstellar environment
The study of the atmosphere of red supergiant stars in general and of
Betelgeuse (alpha Orionis) in particular is of prime importance to understand
dust formation and how mass is lost to the interstellar medium in evolved
massive stars. A molecular shell, the MOLsphere (Tsuji, 2000a), in the
atmosphere of Betelgeuse has been proposed to account for the near- and
mid-infrared spectroscopic observations of Betelgeuse. The goal is to further
test this hypothesis and to identify some of the molecules in this MOLsphere.
We report on measurements taken with the mid-infrared two-telescope beam
combiner of the VLTI, MIDI, operated between 7.5 and 13.5 m. The data are
compared to a simple geometric model of a photosphere surrounded by a warm
absorbing and emitting shell. Physical characteristics of the shell are
derived: size, temperature and optical depth. The chemical constituents are
determined with an analysis consistent with available infrared spectra and
interferometric data. We are able to account for the measured optical depth of
the shell in the N band, the ISO-SWS spectrum and K and L band interferometric
data with a shell whose inner and outer radii are given by the above range and
with the following species: H2O, SiO and Al2O3. These results confirm the
MOLsphere model. We bring evidence for more constituents and for the presence
of species participating in the formation of dust grains in the atmosphere of
the star, i.e. well below the distance at which the dust shell is detected. We
believe these results bring key elements to the understanding of mass loss in
Betelgeuse and red supergiants in general and bring support to the dust-driven
scenario.Comment: 11 pages, 10 figures, accepted for publication in A&
Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution
We study the circumstellar environment of the carbon-rich star R Scl using
the near- and mid-infrared high spatial resolution observations from the
ESO-VLTI instruments VINCI and MIDI. These observations aim at increasing our
knowledge of the dynamic processes in play within the very close circumstellar
environment where the mass loss of AGB stars is initiated. Data are interpreted
using a self-consistent dynamic model. Interferometric observations do not show
any significant variability effect at the 16 m baseline between phases 0.17 and
0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97
and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find
fairly good agreement between the dynamic model and the spectrophotometric data
from 0.4 to 25 m. The model agrees well with the time-dependent flux data
at 8.5 m, whereas it is too faint at 11.3 and 12.5 m. The VINCI
visibilities are reproduced well, meaning that the extension of the model is
suitable in the K-band. In the mid-infrared, the model has the proper extension
to reveal molecular structures of C2H2 and HCN located above the stellar
photosphere. However, the windless model used is not able to reproduce the more
extended and dense dusty environment. Among the different explanations for the
discrepancy between the model and the measurements, the strong nonequilibrium
process of dust formation is one of the most probable. The complete dynamic
coupling of gas and dust and the approximation of grain opacities with the
small-particle limit in the dynamic calculation could also contribute to the
difference between the model and the data
Further detections of OH masers in carbon stars with silicate features
A sample of J-type carbon stars was searched for OH maser emission. The new
detection of three OH lines towards two silicate carbon stars is reported. In
V778 Cyg, previously known as the main-lines (1665 and 1667 MHz) maser source,
the satellite 1612 MHz emission was discovered while in NSV 2814 the main OH
lines were detected. The presence of OH maser lines confirms the former
suggestion that oxygen-rich material is located in the vicinity (
cm) of silicate carbon stars.Comment: LaTeX2e, 4 pages with 2 figure
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