1,344 research outputs found
ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris
[abridged] The 12CO/13CO ratio is often used as a measure of the 12C/13C
ratio in the circumstellar environment, carrying important information about
the stellar nucleosynthesis. External processes can change the 12CO and 13CO
abundances, and spatially resolved studies of the 12CO/13CO ratio are needed to
quantify the effect of these processes on the globally determined values.
Additionally, such studies provide important information on the conditions in
the circumstellar environment. The detached-shell source R Scl, displaying CO
emission from recent mass loss, in a binary-induced spiral structure as well as
in a clumpy shell produced during a thermal pulse, provides a unique laboratory
for studying the differences in CO isotope abundances throughout its recent
evolution. We observed both the 12CO(J=3-2) and the 13CO(J=3-2) line using
ALMA. We find significant variations in the 12CO/13CO intensity ratios and
consequently in the abundance ratios. The average CO isotope abundance ratio is
at least a factor three lower in the shell (~19) than that in the present-day
(60). Additionally, variations in the ratio of more
than an order of magnitude are found in the shell itself. We attribute these
variations to the competition between selective dissociation and isotope
fractionation in the shell, of which large parts cannot be warmer than ~35 K.
However, we also find that the 12CO/13CO ratio in the present-day mass loss is
significantly higher than the 12C/13C ratio determined in the stellar
photosphere from molecular tracers (~19). The origin of this discrepancy is
still unclear, but we speculate that it is due to an embedded source of
UV-radiation that is primarily photo-dissociating 13CO. This radiation source
could be the hitherto hidden companion. Alternatively, the UV-radiation could
originate from an active chromosphere of R Scl itself....Comment: 6 pages, 5 figures, online data available at
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/A+A/556/L
From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs
We compare three different models of clumpy gas disk and show that the
Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically
thick, obscuring torus are best explained by a collisional model consisting of
quasi-stable, self-gravitating clouds. Kinetic energy of clouds is gained by
mass inflow and dissipated in cloud collisions. The collisions give rise to a
viscosity in a spatially averaged gas dynamical picture, which connects them to
angular momentum transport and mass inflow. It is found that CND and torus
share the same gas physics in our description, where the mass of clouds is 20 -
50 M_sun and their density is close to the limit of disruption by tidal shear.
We show that the difference between a transparent CND and an obscuring torus is
the gas mass and the velocity dispersion of the clouds. A change in gas supply
and the dissipation of kinetic energy can turn a torus into a CND-like
structure and vice versa. Any massive torus will naturally lead to sufficiently
high mass accretion rates to feed a luminous AGN. For a geometrically thick
torus to obscure the view to the center even super-Eddington accretions rates
with respect to the central black hole are required.Comment: 9 pages, no figures. Accepted for publication in A&
Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
To test the dust torus model for active galactic nuclei directly, we study
the extent and morphology of the nuclear dust distribution in the Circinus
galaxy using high resolution interferometric observations in the mid-infrared
with the MIDI instrument at the Very Large Telescope Interferometer. We find
that the dust distribution in the nucleus of Circinus can be explained by two
components, a dense and warm disk-like component of 0.4 pc size and a slightly
cooler, geometrically thick torus component with a size of 2.0 pc. The disk
component is oriented perpendicular to the ionisation cone and outflow and
seems to show the silicate feature at 10 micron in emission. It coincides with
a nuclear maser disk in orientation and size. From the energy needed to heat
the dust, we infer a luminosity of the accretion disk corresponding to 20% of
the Eddington luminosity of the nuclear black hole. We find that the
interferometric data are inconsistent with a simple, smooth and axisymmetric
dust emission. The irregular behaviour of the visibilities and the shallow
decrease of the dust temperature with radius provide strong evidence for a
clumpy or filamentary dust structure. We see no evidence for dust reprocessing,
as the silicate absorption profile is consistent with that of standard galactic
dust. We argue that the collimation of the ionising radiation must originate in
the geometrically thick torus component. Our findings confirm the presence of a
geometrically thick, torus-like dust distribution in the nucleus of Circinus,
as required in unified schemes of Seyfert galaxies. Several aspects of our data
require that this torus is irregular, or "clumpy".Comment: 20 pages, 16 figures, accepted for publication by A&
Exploring the inner region of Type 1 AGNs with the Keck interferometer
The exploration of extragalactic objects with long-baseline interferometers
in the near-infrared has been very limited. Here we report successful
observations with the Keck interferometer at K-band (2.2 um) for four Type 1
AGNs, namely NGC4151, Mrk231, NGC4051, and the QSO IRAS13349+2438 at z=0.108.
For the latter three objects, these are the first long-baseline interferometric
measurements in the infrared. We detect high visibilities (V^2 ~ 0.8-0.9) for
all the four objects, including NGC4151 for which we confirm the high V^2 level
measured by Swain et al.(2003). We marginally detect a decrease of V^2 with
increasing baseline lengths for NGC4151, although over a very limited range,
where the decrease and absolute V^2 are well fitted with a ring model of radius
0.45+/-0.04 mas (0.039+/-0.003 pc). Strikingly, this matches independent radius
measurements from optical--infrared reverberations that are thought to be
probing the dust sublimation radius. We also show that the effective radius of
the other objects, obtained from the same ring model, is either roughly equal
to or slightly larger than the reverberation radius as a function of AGN
luminosity. This suggests that we are indeed partially resolving the dust
sublimation region. The ratio of the effective ring radius to the reverberation
radius might also give us an approximate probe for the radial structure of the
inner accreting material in each object. This should be scrutinized with
further observations.Comment: accepted for publication in A&A Letter
Improved VLBI astrometry of OH maser stars
Aims: Accurate distances to evolved stars with high mass loss rates are
needed for studies of many of their fundamental properties. However, as these
stars are heavily obscured and variable, optical and infrared astrometry is
unable to provide enough accuracy.
Methods: Astrometry using masers in the circumstellar envelopes can be used
to overcome this problem. We have observed the OH masers of a number of
Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long
Baseline Array (VLBA). We have used the technique of phase referencing with
in-beam calibrators to test the improvements this technique can provide to Very
Long Baseline Interferometry (VLBI) OH maser astrometric observations.
Results: We have significantly improved the parallax and proper motion
measurements of the Mira variable stars U Her, S CrB and RR Aql.
Conclusions: It is shown that both in-beam phase-referencing and a decrease
in solar activity during the observations significantly improves the accuracy
of the astrometric observations. The improved distances to S CrB (418 +21 -18
pc) and RR Aql (633 +214 -128 pc) are fully consistent with published P-L
relations, but the distance to U Her (266 +32 -28 pc) is significantly smaller.
We conclude that for sources that are bright and have a nearby in-beam
calibrator, VLBI OH maser astrometry can be used to determine distances to OH
masing stars of up to ~2 kpc.Comment: 15 pages, 10 figures; accepted for publication in A&A; for a version
with high-resolution figures see
http://www.astro.uni-bonn.de/~wouter/papers/astrom/astrom.shtm
Silicon isotopic abundance toward evolved stars and its application for presolar grains
Galactic chemical evolution (GCE) is important for understanding the
composition of the present-day interstellar medium (ISM) and of our solar
system. In this paper, we aim to track the GCE by using the 29Si/30Si ratios in
evolved stars and tentatively relate this to presolar grain composition. We
used the APEX telescope to detect thermal SiO isotopologue emission toward four
oxygen-rich M-type stars. Together with the data retrieved from the Herschel
science archive and from the literature, we were able to obtain the 29Si/30Si
ratios for a total of 15 evolved stars inferred from their optically thin 29SiO
and 30SiO emission. These stars cover a range of masses and ages, and because
they do not significantly alter 29Si/30Si during their lifetimes, they provide
excellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function of
time. The 29Si/30Si ratios inferred from the thermal SiO emission tend to be
lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya),
and close to an interstellar or solar value of 1.5 for the higher-mass carbon
star IRC+10216 and two red supergiants. There is a tentative correlation
between the 29Si/30Si ratios and the mass-loss rates of evolved stars, where we
take the mass-loss rate as a proxy for the initial stellar mass or current
stellar age. This is consistent with the different abundance ratios found in
presolar grains. We found that older objects (up to possibly 10 Gyr old) in our
sample trace a previous, lower 29Si/30Si value of about 1. Material with this
isotopic ratio is present in two subclasses of presolar grains, providing
independent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derived
from the SiO emission of evolved stars is a useful diagnostic tool for the
study of the GCE and presolar grains.Comment: 7 pages, 4 figure
Unexpectedly large mass loss during the thermal pulse cycle of the red giant R Sculptoris!
The asymptotic giant branch star R Sculptoris is surrounded by a detached
shell of dust and gas. The shell originates from a thermal pulse during which
the star undergoes a brief period of increased mass loss. It has hitherto been
impossible to constrain observationally the timescales and mass-loss properties
during and after a thermal pulse - parameters that determine the lifetime on
the asymptotic giant branch and the amount of elements returned by the star.
Here we report observations of CO emission from the circumstellar envelope and
shell around R Sculptoris with an angular resolution of 1.3 arcsec. What was
hitherto thought to be only a thin, spherical shell with a clumpy structure, is
revealed to contain a spiral structure. Spiral structures associated with
circumstellar envelopes have been seen previously, from which it was concluded
that the systems must be binaries. Using the data, combined with hydrodynamic
simulations, we conclude that R Sculptoris is a binary system that underwent a
thermal pulse approximately 1800 years ago, lasting approximately 200 years.
About 0.003 Msun of mass was ejected at a velocity of 14.3 km s-1 and at a rate
approximately 30 times higher than the prepulse mass-loss rate. This shows that
approximately 3 times more mass is returned to the interstellar medium during
and immediately after a pulse than previously thought.Comment: Accepted by Natur
Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey
The emission of warm dust dominates the mid-infrared spectra of active
galactic nuclei (AGN). Only interferometric observations provide the necessary
angular resolution to resolve the nuclear dust and to study its distribution
and properties. The investigation of dust in AGN cores is hence one of the main
science goals for the MID-infrared Interferometric instrument MIDI at the VLTI.
As the first step, the feasibility of AGN observations was verified and the
most promising sources for detailed studies were identified. This was carried
out in a "snapshot survey" with MIDI using Guaranteed Time Observations. In the
survey, observations were attempted for 13 of the brightest AGN in the
mid-infrared which are visible from Paranal. The results of the three
brightest, best studied sources have been published in separate papers. Here we
present the interferometric observations for the remaining 10, fainter AGN. For
8 of these, interferometric measurements could be carried out. Size estimates
or limits on the spatial extent of the AGN-heated dust were derived from the
interferometric data of 7 AGN. These indicate that the dust distributions are
compact, with sizes on the order of a few parsec. The derived sizes roughly
scale with the square root of the luminosity in the mid-infrared, s ~ sqrt(L),
with no clear distinction between type 1 and type 2 objects. This is in
agreement with a model of nearly optically thick dust structures heated to T ~
300 K. For three sources, the 10 micron feature due to silicates is tentatively
detected either in emission or in absorption. Based on the results for all AGN
studied with MIDI so far, we conclude that in the mid-infrared the differences
between individual galactic nuclei are greater than the generic differences
between type 1 and type 2 objects.Comment: 18 pages, 8 figures, updated to version published in A&A 502, 67-8
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