1,424 research outputs found
The molecular envelope of CRL 618: A new model based on Herschel/HIFI observations
We study the physical properties and molecular excitation of the different
warm gas components found in the protoplanetary nebula CRL 618. We revise our
previous Herschel/HIFI observations, which consist of several 12CO and 13CO
lines in the far-infrared/sub-mm band. These data have been re-analyzed in
detail by improving calibration, the signal-to-noise-ratio, and baseline
substraction. We identify the contributions of the different nebular components
to the line profiles. We have used a spatio-kinematical model to better
constrain the temperature, density, and kinematics of the molecular components
probed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, and
J=6-5 transitions are detected in this source. The line profiles present a
composite structure showing spectacular wings in some cases, which become
dominant as the energy level increases. Our analysis of the high-energy CO
emission with the already known low-energy J=2-1 and J=1-0 lines confirms that
the high-velocity component, or fast bipolar outflow, is hotter than previously
estimated with a typical temperature of ~300 K. This component may then be an
example of a very recent acceleration of the gas by shocks that has not yet
cooled down. We also find that the dense central core is characterized by a
very low expansion velocity, ~5 km/s, and a strong velocity gradient. We
conclude that this component is very likely to be the unaltered circumstellar
layers that are lost in the last AGB phase, where the ejection velocity is
particularly low. The physical properties of the other two nebular components,
the diffuse halo and the double empty shell, more or less agrees with the
estimations derived in previous models.Comment: Accepted for publication in Astronomy & Astrophysics. 8 pages, 3
figure
Mapping the circumstellar SiO maser emission in R Leo
The study of the innermost circumstellar layers around AGB stars is crucial
to understand how these envelopes are formed and evolve. The SiO maser emission
occurs at a few stellar radii from the central star, providing direct
information on the stellar pulsation and on the chemical and physical
properties of these regions. Our data also shed light on several aspects of the
SiO maser pumping theory that are not well understood yet. We aim to determine}
the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in
the oxygen-rich evolved star R Leo. We have imaged with milliarcsecond
resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28SiO
v=1, 2 J=1-0 and 29SiO v=0 J=1-0) and 86 GHz (28SiO v=1 J=2-1 and 29SiO v=0
J=2-1) masing regions. We confirm previous results obtained in other
oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting
regions, the 28SiO v=2 transition is produced in an inner layer, closer to the
central star. On the other hand, the 86 GHz line arises in a clearly farther
shell. We have also mapped for the first time the 29SiO v=0 J=1-0 emission in R
Leo. The already reported discrepancy between the observed distributions of the
different maser lines and the theoretical predictions is also found in R Leo.Comment: accepted for publication in A&
New flaring of an ultraluminous X-ray source in NGC 1365
We have studied a highly variable ultraluminous X-ray source (ULX) in the
Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations
between 2002 and 2006. In 2006 April, the source peaked at a luminosity ~ 3 x
10^{40} erg/s in the 0.3-10 keV band (similar to the maximum luminosity found
by ASCA in 1995), and declined on an e-folding timescale ~ 3 days. The X-ray
spectrum is always dominated by a broad power-law-like component. When the
source is seen at X-ray luminosities ~ 10^{40} erg/s, an additional soft
thermal component (which we interpret as emission from the accretion disk)
contributes ~ 1/4 of the X-ray flux; when the luminosity is higher, ~ 3 x
10^{40} erg/s, the thermal component is not detected and must contribute < 10%
of the flux. At the beginning of the decline, ionized absorption is detected
around 0.5-2 keV; it is a possible signature of a massive outflow. The
power-law is always hard, with a photon index Gamma ~ 1.7 (and even flatter at
times), as is generally the case with bright ULXs. We speculate that this
source and perhaps most other bright ULXs are in a high/hard state: as the
accretion rate increases well above the Eddington limit, more and more power is
extracted from the inner region of the inflow through non-radiative channels,
and is used to power a Comptonizing corona, jet or wind. The observed thermal
component comes from the standard outer disk; the transition radius between
outer standard disk and Comptonizing inner region moves further out and to
lower disk temperatures as the accretion rate increases. This produces the
observed appearance of a large, cool disk. Based on X-ray luminosity and
spectral arguments, we suggest that this accreting black hole has a likely mass
~ 50-150 Msun (even without accounting for possible beaming).Comment: 14 pages, to appear in MNRA
Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars
We present the results of SiO maser observations at 43GHz toward two AGB
stars using the VLBA. Our preliminary results on the relative positions of the
different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO
maser pumping could be wrong at some fundamental level. A deep revision of the
SiO pumping models could be necessary.Comment: poster, 2 pages, 2 figures, Proc. IAU Symp. 287 "Cosmic Masers: from
OH to H0", R.S. Booth, E.M.L. Humphreys and W.H.T. Vlemmings, ed
SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states
The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense
in AGB stars and have been mapped using VLBI showing ring-like distributions.
Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely
coincident, while the v=1 J=2-1 maser arises from a well separated region
farther out. These relative locations can be explained by models tools that
include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not
directly affected by any line overlap and its spot structure and position,
relative to the other lines, is a good test to the standard pumping models. We
present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3
J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution
of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is
systematically composed of a series of spots that occupy a ring-like structure.
The overall ring structure is extremely similar to that found in the other 43
GHz transitions and is very different from the structure of the v=1 J=2-1
maser. The positions of the individual spots of the different 43 GHz lines are,
however, very rarely coincident, which in general is separated by about 0.3 AU
(between 1 and 5 mas). These results are very difficult to reconcile with
standard pumping models, which predict that the masers of rotational
transitions within a given vibrational state require very similar excitation
conditions, while the transitions of different vibrational states should appear
in different positions. However, models including line overlap tend to predict
v=1, v=2, v=3 J=1-0 population inversion to occur under very similar
conditions, while the requirements for v=1 J=2-1 appear clearly different, and
are compatible with the observational results.Comment: 9 pages, 4 figures accepted by A&
Flow control in S-Shaped Air Intake using Zero-Net-Mass-Flow
Flow control using zero-net-mass-flow jets in a twodimensional model of an S-Shaped air intake diffuser was investigated. Experiments were conducted in a channel flow facility at a Reynolds number of Re = 8×104 with particular image velocimetry measurements in the symmetry plane of the duct. In the natural configuration, separation of the boundary layer occurs in a region of the duct with a high degree of curvature. A stability analysis of the wall normal profile at the location of the applied control is presented and estimates the most effective frequency of the actuator. Time-averaged velocity fields show total reattachment of the boundary layer using active flow control
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