268 research outputs found
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&
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
Electron Paramagnetic Study of Spin Labelled Molecule on Colloidal Surfaces
Spin labelling techniques have been used to determine the
adsorption characteristics of a nitroxide on colloidal gold. The
number of molecules adsorbed per surface atom was determined and
the desorption of adsorbed molecules on coagulation of gold noted
Monitoring subaquatic vegetation using Sentinel-2 imagery in Gallocanta Lake (Aragón, Spain)
Remote sensing allows the study of aquatic vegetation cover in shallow lakes from the different spectral responses of the water as the vegetation grows from the bottom toward the surface. In the case of Gallocanta Lake, its seasonality and shallow depth (less than 2 m) allow us to appreciate the variations in the aquatic vegetation with the apparent color. Six common vegetation indices were tested, and the one with the best response was the so-called NDI45, which uses the normalized ratio between the far red (705 nm) and red (665 nm) bands. Our aims are to show the variations in the surface area covered by vegetation at the bottom of the lagoon, its growth and disappearance when drying occurs, and recolonization in a flooding period. The degree of cover reaches 90% at the most favorable times of the year, generally in summer and coinciding with flooding of the lake. The studied period shows how this method can be used for lacustrine habitat detection and highlights the need for field vegetation inventories in future works, which will allow the spectral measurements to be related to the species present
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&
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
Scaling and universality of critical fluctuations in granular gases
The global energy fluctuations of a low density gas granular gas in the
homogeneous cooling state near its clustering instability are studied by means
of molecular dynamics simulations. The relative dispersion of the fluctuations
is shown to exhibit a power-law divergent behavior. Moreover, the probability
distribution of the fluctuations presents data collapse as the system
approaches the instability, for different values of the inelasticity. The
function describing the collapse turns out to be the same as the one found in
several molecular equilibrium and non-equilibrium systems, except for the
change in the sign of the fluctuations
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