1,360 research outputs found
Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR
We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red
supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The
VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar
radii = 136--306 AU) away from the star. We also detected compact emission
within a radius of 0.5" from the star. Comparison of our VISIR image taken in
2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals
the outward motions of four dust clumps. The proper motions of these dust
clumps amount to 0.2"--0.6" in 12 years. This translates into expansion
velocities (projected onto the plane of the sky) of 13--40 km/s with an
uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained
by a simple accelerating outflow, implying the possible random nature of the
dust cloud ejection mechanism. The inner compact emission seen in the 2010
VISIR image is presumably newly formed dust, because it is not detected in the
image taken in 1998. The mass of the dust clouds is estimated to be
(3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount
of dust ejected in one year estimated from the (gas+dust) mass-loss rate of
2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on
the clumpy dust cloud ejection.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
High spectral resolution spectroscopy of the SiO fundamental lines in red giants and red supergiants with VLT/VISIR
We present high spectral resolution (R = 30000) spectroscopic observations of
the SiO fundamental lines near 8.1 micron in 16 bright red giants and red
supergiants using VLT/VISIR. Our sample consists of seven normal K--M giants,
three Mira stars, three optically bright red supergiants, two dusty red
supergiants, and the enigmatic object GCIRS3 near the Galactic center. We
detected SiO fundamental lines in all of our program stars except for GCIRS3.
The SiO lines in normal K and M giants as well as optically bright red
supergiants do not show P-Cyg profiles or blueshifts, which means the absence
of systematic outflows in the SiO line forming region. On the other hand, we
detected P-Cyg profiles in the SiO lines in the dusty red supergiants VY CMa
and VX Sgr (with the latter being a new detection), which suggest outflow
velocities of 27 and 17 km/s, respectively. We derived basic stellar parameters
(effective temperature, surface gravity, luminosity, and mass) for the normal
K--M giants and optically bright red supergiants in our sample and compared the
observed VISIR spectra with synthetic spectra predicted from MARCS photospheric
models. Most of the SiO lines observed in the program stars warmer than ~3400 K
are reasonably reproduced by the MARCS models, which allowed us to estimate the
silicon abundance as well as the 28Si/29Si and 28Si/30Si ratios. However, we
detected possible absorption excess in some SiO lines. Moreover, the SiO lines
in the cooler red giants and red supergiant cannot be explained by the MARCS
models at all even if the dust emission is taken into account. This
disagreement may be a signature of the dense, extended molecular outer
atmosphere.Comment: 13 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Scaling of the critical slip distance in granular layers
We investigate the nature of friction in granular layers by means of
numerical simulation focusing on the critical slip distance, over which the
system relaxes to a new stationary state. Analyzing a transient process in
which the sliding velocity is instantaneously changed, we find that the
critical slip distance is proportional to the sliding velocity. We thus define
the relaxation time, which is independent of the sliding velocity. It is found
that the relaxation time is proportional to the layer thickness and inversely
proportional to the square root of the pressure. An evolution law for the
relaxation process is proposed, which does not contain any length constants
describing the surface geometry but the relaxation time of the bulk granular
matter. As a result, the critical slip distance is scaled with a typical length
scale of a system. It is proportional to the layer thickness in an
instantaneous velocity change experiment, whereas it is scaled with the total
slip distance in a spring-block system on granular layers.Comment: 4 papge
Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 micron CO lines
The outer atmosphere of K giants shows thermally inhomogeneous structures
consisting of the hot chromospheric gas and the cool molecular gas. We present
spectro-interferometric observations of the multicomponent outer atmosphere of
the well-studied K1.5 giant Arcturus (alpha Boo) in the CO first overtone lines
near 2.3 micron. We observed Arcturus with the AMBER instrument at the Very
Large Telescope Interferometer (VLTI) at 2.28--2.31 micron with a spectral
resolution of 12000 and at projected baselines of 7.3, 14.6, and 21.8 m. The
high spectral resolution of the VLTI/AMBER instrument allowed us to spatially
resolve Arcturus in the individual CO lines. Comparison of the observed
interferometric data with the MARCS photospheric model shows that the star
appears to be significantly larger than predicted by the model. It indicates
the presence of an extended component that is not accounted for by the current
photospheric models for this well-studied star. We found out that the observed
AMBER data can be explained by a model with two additional CO layers above the
photosphere. The inner CO layer is located just above the photosphere, at 1.04
+/- 0.02 stellar radii, with a temperature of 1600 +/- 400 K and a CO column
density of 10^{20 +/- 0.3} cm^-2. On the other hand, the outer CO layer is
found to be as extended as to 2.6 +/- 0.2 stellar radii with a temperature of
1800 +/- 100 K and a CO column density of 10^{19 +/- 0.15} cm^-2. The
properties of the inner CO layer are in broad agreement with those previously
inferred from the spatially unresolved spectroscopic analyses. However, our
AMBER observations have revealed that the quasi-static cool molecular component
extends out to 2--3 stellar radii, within which region the chromospheric wind
steeply accelerates.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER II. Time variations between pre-maximum and minimum light
Our recent visible polarimetric images of the well-studied AGB star W Hya
taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed
clumpy dust clouds close to the star at ~2 Rstar. We present second-epoch
SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) in the
continuum (645, 748, and 820 nm), in the Halpha line (656.3 nm), and in the TiO
band (717 nm) as well as high-spectral resolution long-baseline interferometric
observations in 2.3 micron CO lines with the AMBER instrument at the Very Large
Telescope Interferometer (VLTI). The high-spatial resolution polarimetric
images have allowed us to detect clear time variations in the clumpy dust
clouds as close as 34--50~mas (1.4--2.0 Rstar) to the star. We detected the
formation of a new dust cloud and the disappearance of one of the dust clouds
detected at the first epoch. The Halpha and TiO emission extends to ~150 mas
(~6 Rstar), and the Halpha images reveal time variations. The degree of linear
polarization is higher at minimum light (13--18%) than that at pre-maximum
light. The power-law-type limb-darkened disk fit to the AMBER data in the
continuum results in a limb-darkened disk diameter of 49.1+/-1.5 mas and a
limb-darkening parameter of 1.16+/-0.49, indicating that the atmosphere is more
extended with weaker limb-darkening compared to pre-maximum light. Our Monte
Carlo radiative transfer modeling suggests the predominance of small (0.1
micron) grains of Al2O3, Mg2SiO4, and MgSiO3 at minimum light, in marked
contrast to the predominance of large (0.5 micron) grains at pre-maximum light.
The variability phase dependence of the grain size implies that small grains
might just have started to form at minimum light in the wake of a shock, while
the pre-maximum light phase might have corresponded to the phase of efficient
grain growth.Comment: 14 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Vigorous atmospheric motion in the red supergiant star Antares
Red supergiant stars represent a late stage of the evolution of stars more
massive than about nine solar masses, in which they develop complex,
multi-component atmospheres. Bright spots have been detected in the atmosphere
of red supergiants using interferometric imaging. Above the photosphere of a
red supergiant, the molecular outer atmosphere extends up to about two stellar
radii. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas
(less than 3,500 kelvin) of a red supergiant coexist at about three stellar
radii. The dynamics of such complex atmospheres has been probed by ultraviolet
and optical spectroscopy. The most direct approach, however, is to measure the
velocity of gas at each position over the image of stars as in observations of
the Sun. Here we report the mapping of the velocity field over the surface and
atmosphere of the nearby red supergiant Antares. The two-dimensional velocity
field map obtained from our near-infrared spectro-interferometric imaging
reveals vigorous upwelling and downdrafting motions of several huge gas clumps
at velocities ranging from about -20 to +20 kilometres per second in the
atmosphere, which extends out to about 1.7 stellar radii. Convection alone
cannot explain the observed turbulent motions and atmospheric extension,
suggesting that an unidentified process is operating in the extended
atmosphere.Comment: 27 pages, 8 figures, published in Natur
Exploring the water and carbon monoxide shell around Betelgeuse with VLTI/AMBER
We present the results of the analysis of our recent interferometric
observations of Betelgeuse, using the AMBER instrument of the VLTI. Using the
medium spectral resolution mode () we detected the presence of the
water vapour and carbon monoxide (CO) molecules in the H and K bands. We also
derived the photospheric angular diameter in the continuum. By analysing the
depth of the molecular lines and the interferometric visibilities, we derived
the column densities of the molecules, as well as the temperature and the size
of the corresponding regions in the atmosphere of Betelgeuse (the MOLsphere)
using a single shell model around the photosphere. Our results confirm the
findings by Perrin \et al\ (\cite{Perrin2004}) and Ohnaka \et al\
(\cite{Ohnaka2011}) that the HO and CO molecules are distributed around
Betelgeuse in a MOLsphere extending to approximately 1.3 times the star's
photospheric radius.Comment: Betelgeuse Workshop, November 2012, Paris. To be published in the
European Astronomical Society Publications Series, 2013, Editors: Pierre
Kervella, Thibaut Le Bertre \& Guy Perri
Temperature and pore pressure effects on the shear strength of granite in the brittle-plastic transition regime
Currently published lithospheric strength profiles lack constraints from experimental data for shear failure of typical crustal materials in the brittle-plastic transition regime in wet environments. Conventional triaxial shear fracture experiments were conducted to determine temperature and pore pressure effects on shear fracture strength of wet and dry Tsukuba granite. Experimental conditions were 70MPa < P-C < 480MPa, 10MPa < P-p < 300MPa, 25 A degreesC < T < 480 degreesC, at a constant strain rate of 10(-5)s(-1). An empirical relation is proposed which can predict the shear strength of Tsukuba granite, within the range of experimental conditions. Mechanical pore pressure effects are incorporated in the effective stress law. Chemical effects are enhanced at temperatures above 300 degreesC. Below 300 degreesC wet and dry granite strengths are temperature insensitive and wholly within the brittle regime. Above 400 degreesC, semi-brittle effects and ductility are observed
Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER
Aim: We present a high-spatial and high-spectral resolution observation of
the well-studied K giant Aldebaran with AMBER at the Very Large Telescope
Interferometer (VLTI). Our aim is to spatially resolve the outer atmosphere
(so-called MOLsphere) in individual CO first overtone lines and derive its
physical properties, which are important for understanding the mass-loss
mechanism in normal (i.e., non-Mira) K--M giants. Methods: Aldebaran was
observed between 2.28 and 2.31 micron with a projected baseline length of 10.4m
and a spectral resolution of 12000. Results: The uniform-disk diameter observed
in the CO first overtone lines is 20--35% larger than is measured in the
continuum. We have also detected a signature of inhomogeneities in the
CO-line-forming region on a spatial scale of ~45 mas, which is more than twice
as large as the angular diameter of the star itself. While the MARCS
photospheric model reproduces the observed spectrum well, the angular size in
the CO lines predicted by the MARCS model is significantly smaller than
observed. This is because the MARCS model with the parameters of Aldebaran has
a geometrical extension of only ~2% (with respect to the stellar radius). The
observed spectrum and interferometric data in the CO lines can be
simultaneously reproduced by placing an additional CO layer above the MARCS
photosphere. This CO layer is extended to 2.5 +/- 0.3 stellar radii with CO
column densities of 5x10^{19}--2x10^{20} cm^-2 and a temperature of 1500 +/-
200 K. Conclusions: The high spectral resolution of AMBER has enabled us to
spatially resolve the inhomogeneous, extended outer atmosphere (MOLsphere) in
the individual CO lines for the first time in a K giant. Our modeling of the
MOLsphere of Aldebaran suggests a rather small gradient in the temperature
distribution above the photosphere up to 2--3 stellar radii.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
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