790 research outputs found
Discovery of a detached HI gas shell surrounding alpha Orionis
We report the detection of the HI line at 21 cm in the direction of alpha Ori
with the Nancay Radiotelescope and with the Very Large Array. The observations
confirm the previous detection of HI emission centered on alpha Ori, but
additionally reveal for the first time a quasi-stationary detached shell of
neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200
pc). The detached shell appears elongated in a direction opposite to the star's
space motion. A simple model shows that this detached atomic gas shell can
result from the collision of the stellar wind from alpha Ori with the local
interstellar medium (ISM). It implies that alpha Ori has been losing matter at
a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In
addition, we report the detection of atomic hydrogen associated with the
far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been
suggested to trace the bow shock resulting from the motion of the star through
the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX)
of a far-UV counterpart to this arc.Comment: Accepted for publication in the Monthly Notices; version with full
resolution figures available at
http://aramis.obspm.fr/~lebertre/paper-alphaOri_MNRAS.pd
CO and HI observations of an enigmatic cloud
An isolated HI cloud with peculiar properties has recently been discovered by
Dedes, Dedes, & Kalberla (2008, A&A, 491, L45) with the 300-m Arecibo
telescope, and subsequently imaged with the VLA. It has an angular size of ~6',
and the HI emission has a narrow line profile of width ~ 3 km/s.
We explore the possibility that this cloud could be associated with a
circumstellar envelope ejected by an evolved star.
Observations were made in the rotational lines of CO with the IRAM-30m
telescope, on three positions in the cloud, and a total-power mapping in the HI
line was obtained with the Nancay Radio Telescope.
CO was not detected and seems too underabundant in this cloud to be a
classical late-type star circumstellar envelope. On the other hand, the HI
emission is compatible with the detached-shell model that we developed for
representing the external environments of AGB stars.
We propose that this cloud could be a fossil circumstellar shell left over
from a system that is now in a post-planetary-nebula phase. Nevertheless, we
cannot rule out that it is a Galactic cloud or a member of the Local Group,
although the narrow line profile would be atypical in both cases.Comment: Accepted for publication in Astronomy and Astrophysic
Circumstellar HI and CO around the carbon stars V1942 Sgr and V CrB
Context. The majority of stars that leave the main sequence are undergoing
extensive mass loss, in particular during the asymptotic giant branch (AGB)
phase of evolution. Observations show that the rate at which this phenomenon
develops differs highly from source to source, so that the time-integrated mass
loss as a function of the initial conditions (mass, metallicity, etc.) and of
the stage of evolution is presently not well understood. Aims. We are
investigating the mass loss history of AGB stars by observing the molecular and
atomic emissions of their circumstellar envelopes. Methods. In this work we
have selected two stars that are on the thermally pulsing phase of the AGB
(TP-AGB) and for which high quality data in the CO rotation lines and in the
atomic hydrogen line at 21 cm could be obained. Results. V1942 Sgr, a carbon
star of the Irregular variability type, shows a complex CO line profile that
may originate from a long-lived wind at a rate of ~ 10^-7 Msol/yr, and from a
young (< 10^4 years) fast outflow at a rate of ~ 5 10^-7 Msol/yr. Intense HI
emission indicates a detached shell with 0.044 Msol of hydrogen. This shell
probably results from the slowing-down, by surrounding matter, of the same
long-lived wind observed in CO that has been active during ~ 6 10^5 years. On
the other hand, the carbon Mira V CrB is presently undergoing mass loss at a
rate of 2 10^-7 Msol/yr, but was not detected in HI. The wind is mostly
molecular, and was active for at most 3 10^4 years, with an integrated mass
loss of at most 6.5 10^-3 Msol. Conclusions. Although both sources are carbon
stars on the TP-AGB, they appear to develop mass loss under very different
conditions, and a high rate of mass loss may not imply a high integrated mass
loss.Comment: Accepted for publication in Astron. Astrophy
HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud
We report HI 21-cm line observations of the AGB star X Her obtained with the
Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI
emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope
of the star. The HI distribution exhibits a head-tail morphology, similar to
those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24
pc) in the plane of the sky, along the direction of the star's space motion. We
also detect a velocity gradient of ~6.5 km/s across the envelope, consistent
with the HI tracing a turbulent wake that arises from the motion of a
mass-losing star through the ISM. GBT mapping of a 2x2deg region around X Her
reveals that the star lies (in projection) near the periphery of a much larger
HI cloud that also exhibits signatures of ISM interaction. The properties of
the cloud are consistent with those of compact high-velocity clouds. Using CO
observations, we have placed an upper limit on its molecular gas content of
N_H2<1.3e20 cm^-2. Although the distance to the cloud is poorly constrained,
the probability of a chance coincidence in position, velocity, and apparent
position angle of space motion between X Her and the cloud is extremely small,
suggesting a possible physical association. However, the large HI mass of the
cloud (~>2.4~M_sun) and the blueshift of its mean velocity relative to X Her
are inconsistent with an origin tied directly to stellar ejection. (abridged)Comment: Accepted to AJ; 47 pages, 15 figures; version with full resolution
figures available at
http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_XHer.pd
Discovery of an HI Counterpart to the Extended Tail of Mira
We report the detection of an HI counterpart to the extended,
far-ultraviolet-emitting tail associated with the asymptotic giant branch star
Mira (o Ceti). Using the Nancay Radio Telescope (NRT), we have detected
emission as far as 88' north of the star, confirming that the tail contains a
significant atomic component (M_HI ~ 4x10e-3 M_sun). The NRT spectra reveal a
deceleration of the tail gas caused by interaction with the local interstellar
medium. We estimate an age for the tail of ~1.2x10e5 years, suggesting that the
mass-loss history of Mira has been more prolonged than previous observational
estimates. Using the Very Large Array (VLA) we have also imaged the HI tail out
to ~12' (0.4 pc) from the star. The detected emission shows a ``head-tail''
morphology, but with complex substructure. Regions with detected HI emission
correlate with far-ultraviolet-luminous regions on large scales, but the two
tracers are not closely correlated on smaller scales (<1'). We propose that
detectable tails of HI are likely to be a common feature of red giants
undergoing mass-loss.Comment: accepted to ApJ; version with full-resolution figures available at:
http://www.cfa.harvard.edu/~lmatthew/Mira_HI.pd
Three different glacier surges at a spot: What satellites observe and what not
In the Karakoram, dozens of glacier surges occurred in the past 2 decades, making the region a global hotspot. Detailed analyses of dense time series from optical and radar satellite images revealed a wide range of surge behaviour in this region: from slow advances longer than a decade at low flow velocities to short, pulse-like advances over 1 or 2 years with high velocities. In this study, we present an analysis of three currently surging glaciers in the central Karakoram: North and South Chongtar Glaciers and an unnamed glacier referred to as NN9. All three glaciers flow towards the same small region but differ strongly in surge behaviour. A full suite of satellites (e.g. Landsat, Sentinel-1 and 2, Planet, TerraSAR-X, ICESat-2) and digital elevation models (DEMs) from different sources (e.g. Shuttle Radar Topography Mission, SRTM; Satellite Pour l'Observation de la Terre, SPOT; High Mountain Asia DEM, HMA DEM) are used to (a) obtain comprehensive information about the evolution of the surges from 2000 to 2021 and (b) to compare and evaluate capabilities and limitations of the different satellite sensors for monitoring surges of relatively small glaciers in steep terrain. A strongly contrasting evolution of advance rates and flow velocities is found, though the elevation change pattern is more similar. For example, South Chongtar Glacier had short-lived advance rates above 10 yr-1, velocities up to 30 d-1, and surface elevations increasing by 170 m. In contrast, the neighbouring and 3-times-smaller North Chongtar Glacier had a slow and near-linear increase in advance rates (up to 500 yr-1), flow velocities below 1 d-1 and elevation increases up to 100 m. The even smaller glacier NN9 changed from a slow advance to a full surge within a year, reaching advance rates higher than 1 yr-1. It seems that, despite a similar climatic setting, different surge mechanisms are at play, and a transition from one mechanism to another can occur during a single surge. The sensor inter-comparison revealed a high agreement across sensors for deriving flow velocities, but limitations are found on small and narrow glaciers in steep terrain, in particular for Sentinel-1. All investigated DEMs have the required accuracy to clearly show the volume changes during the surges, and elevations from ICESat-2 ATL03 data fit neatly to the other DEMs. We conclude that the available satellite data allow for a comprehensive observation of glacier surges from space when combining different sensors to determine the temporal evolution of length, elevation and velocity changes
Identification of a novel mechanism of blood-brain communication during peripheral inflammation via choroid plexus-derived extracellular vesicles
Here, we identified release of extracellular vesicles (EVs) by the choroid plexus epithelium (CPE) as a new mechanism of blood-brain communication. Systemic inflammation induced an increase in EVs and associated pro-inflammatory miRNAs, including miR-146a and miR-155, in the CSF. Interestingly, this was associated with an increase in amount of multivesicular bodies (MVBs) and exosomes per MVB in the CPE cells. Additionally, we could mimic this using LPS-stimulated primary CPE cells and choroid plexus explants. These choroid plexus-derived EVs can enter the brain parenchyma and are taken up by astrocytes and microglia, inducing miRNA target repression and inflammatory gene up-regulation. Interestingly, this could be blocked in vivo by intracerebroventricular (icv) injection of an inhibitor of exosome production. Our data show that CPE cells sense and transmit information about the peripheral inflammatory status to the central nervous system (CNS) via the release of EVs into the CSF, which transfer this pro-inflammatory message to recipient brain cells. Additionally, we revealed that blockage of EV secretion decreases brain inflammation, which opens up new avenues to treat systemic inflammatory diseases such as sepsis
Secular dynamics of a planar model of the Sun-Jupiter-Saturn-Uranus system; effective stability into the light of Kolmogorov and Nekhoroshev theories
We investigate the long-time stability of the Sun-Jupiter-Saturn-Uranus
system by considering a planar secular model, that can be regarded as a major
refinement of the approach first introduced by Lagrange. Indeed, concerning the
planetary orbital revolutions, we improve the classical circular approximation
by replacing it with a solution that is invariant up to order two in the
masses; therefore, we investigate the stability of the secular system for
rather small values of the eccentricities. First, we explicitly construct a
Kolmogorov normal form, so as to find an invariant KAM torus which approximates
very well the secular orbits. Finally, we adapt the approach that is at basis
of the analytic part of the Nekhoroshev's theorem, so as to show that there is
a neighborhood of that torus for which the estimated stability time is larger
than the lifetime of the Solar System. The size of such a neighborhood,
compared with the uncertainties of the astronomical observations, is about ten
times smaller.Comment: 31 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1010.260
The low-lying quadrupole collective excitations of Ru and Pd isotopes
Quadrupole excitations of even-even Ru and Pd isotopes are described within
microscopic approach based on the general collective Bohr model which includes
the effect of coupling with the pairing vibrations. The excitation energies and
E2 transition probabilities observed in 104-114Ru and 106-110Pd are reproduced
in the frame of the calculation containing no free parameters.Comment: 11 pages, 18 figures in EPS forma
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