1,454 research outputs found
Tomography of the red supergiant star {\mu} Cep
A tomographic method, aiming at probing velocity fields at depth in stellar
atmospheres, is applied to the red supergiant star {\mu} Cep and to snapshots
of 3D radiative-hydrodynamics simulation in order to constrain atmospheric
motions and relate them to photometric variability.Comment: 2 pages, 2 figures, accepted as Proceedings of IAU Symposium No. 343,
201
Properties of the CO and HO MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations
Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is
well suited for studying the complex processes in its atmosphere that lead to
the chemical enrichment of the interstellar medium. Aims. We intend to
investigate the shape and composition of the close molecular layer (also known
as the MOLsphere) that surrounds the star. This analysis is part of a wider
program that aims at understanding the dynamics of the circumstellar envelope
of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed
using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large
Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral
resolution of the instrument (R 1500), we were able to investigate the
carbon monoxide band heads and the water-vapor bands. We used two different
approaches to analyse our data: a model fit in both the continuum and
absorption lines and then a fit with a Radiative HydroDynamics (RHD)
simulation. Results. Using the continuum data, we derive a uniform disk
diameter of ~mas, a power law type limb-darkened disk diameter
of ~mas and a limb-darkening exponent of .
Within the absorption lines, using a single layer model, we obtain parameters
of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in
the visibilities. Conclusions. We derived a new value of the angular diameter
of Betelgeuse in the K band continuum. Our observations in the absorption lines
are well reproduced by a molecular layer at 1.2 stellar radii containing both
CO and HO. The visibilities at higher spatial frequencies are matching a
convection pattern in a RHD simulation.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics; Language editin
An edge-on translucent dust disk around the nearest AGB star L2 Puppis - VLT/NACO spectro-imaging from 1.04 to 4.05 microns and VLTI interferometry
As the nearest known AGB star (d=64pc) and one of the brightest (mK-2), L2
Pup is a particularly interesting benchmark object to monitor the final stages
of stellar evolution. We report new lucky imaging observations of this star
with the VLT/NACO adaptive optics system in twelve narrow band filters covering
the 1.0-4.0 microns wavelength range. These diffraction limited images reveal
an extended circumstellar dust lane in front of the star, that exhibits a high
opacity in the J band and becomes translucent in the H and K bands. In the L
band, extended thermal emission from the dust is detected. We reproduce these
observations using Monte-Carlo radiative transfer modeling of a dust disk with
the RADMC-3D code. We also present new interferometric observations with the
VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the
limb-darkened angular diameter of theta_LD = 17.9 +/- 1.6 mas, converting to a
maximum linear radius of R = 123 +/- 14 Rsun. Considering the geometry of the
extended K band emission in the NACO images, this upper limit is probably close
to the actual angular diameter of the star. The position of L2 Pup in the
Herzsprung-Russell diagram indicates that this star has a mass around 2 Msun
and is probably experiencing an early stage of the asymptotic giant branch. We
do not detect any stellar companion of L2 Pup in our adaptive optics and
interferometric observations, and we attribute its apparent astrometric wobble
in the Hipparcos data to variable lighting effects on its circumstellar
material. We however do not exclude the presence of a binary companion, as the
large loop structure extending to more than 10 AU to the North-East of the disk
in our L band images may be the result of interaction between the stellar wind
of L2 Pup and a hidden secondary object. The geometric configuration that we
propose, with a large dust disk seen almost edge-on, appears particularly
favorable to test and develop our understanding of the formation of bipolar
nebulae.Comment: 16 pages, 15 figure
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
Chiral molecule formation in interstellar ice analogs: alpha-aminoethanol NH 2 CH(CH 3 )OH
International audienceAims. Aminoalcohol molecules such as alpha-aminoethanol NH 2 CH(CH 3)OH may be aminoacid precursors. We attempt to charac-terize and detect this kind of molecules which is important to establish a possible link between interstellar molecules and life as we know it on Earth. Methods. We use Fourier transform infrared (FTIR) spectroscopy and mass spectrometry to study the formation of alpha-aminoethanol NH 2 CH(CH 3)OH in H 2 O:NH 3 : CH 3 CHO ice mixtures. Isotopic substitution with 15 NH 3 and ab-initio calculation are used to confirm the identification of alpha-aminoethanol. Results. After investigating the thermal reaction of solid NH 3 and acetaldehyde CH 3 CHO at low temperature, we find that this reac-tion leads to the formation of a chiral molecule, the alpha aminoethanol NH 2 CH(CH 3)OH. For the first time, we report the infrared and mass spectra of this molecule. We also report on its photochemical behavior under VUV irradiation. We find that the main photo-product is acetamide (NH 2 COCH 3). Data provided in this work indicates that alpha-aminoethanol is formed in one hour at 120 K and suggests that its formation in warm interstellar environments such as protostellar envelopes or cometary environments is likely
On the close to threshold meson production in neutron-neutron collisions
A method of measuring the close to threshold meson production in
neutron-neutron collisions is described where the momenta of the colliding
neutrons can be determined with the accuracy obtainable for the proton-proton
reaction. The technique is based on the double quasi-free nn --> nn X^0
reaction, where deuterons are used as a source of neutronsComment: 6 pages, 2 figures, to be published in Phys. Lett.
The close circumstellar environment of Betelgeuse - II. Diffraction-limited spectro-imaging from 7.76 to 19.50 microns with VLT/VISIR
Context: Mass-loss occurring in red supergiants (RSGs) is a major contributor
to the enrichment of the interstellar medium in dust and molecules. The
physical mechanism of this mass loss is however relatively poorly known.
Betelgeuse is the nearest RSG, and as such a prime object for high angular
resolution observations of its surface (by interferometry) and close
circumstellar environment. Aims: The goal of our program is to understand how
the material expelled from Betelgeuse is transported from its surface to the
interstellar medium, and how it evolves chemically in this process. Methods: We
obtained diffraction-limited images of Betelgeuse and a PSF calibrator
(Aldebaran) in six filters in the N band (7.76 to 12.81 mic) and two filters in
the Q band (17.65 and 19.50 mic), using the VLT/VISIR instrument. Results: Our
images show a bright, extended and complex circumstellar envelope at all
wavelengths. It is particularly prominent longwards of 9-10 mic, pointing at
the presence of O-rich dust, such as silicates or alumina. A partial circular
shell is observed between 0.5 and 1.0" from the star, and could correspond to
the inner radius of the dust envelope. Several knots and filamentary structures
are identified in the nebula, one of the knots, located at a distance of 0.9"
west of the star, being particularly bright and compact. Conclusions: The
circumstellar envelope around Betelgeuse extends at least up to several tens of
stellar radii. Its relatively high degree of clumpiness indicates an
inhomogeneous spatial distribution of the material lost by the star. Its
extension corresponds to an important intermediate scale, where most of the
dust is probably formed, between the hot and compact gaseous envelope observed
previously in the near infrared and the interstellar medium.Comment: 10 page
Fundamental properties of the Population II fiducial stars HD 122563 and Gmb 1830 from CHARA interferometric observations
We have determined the angular diameters of two metal-poor stars, HD 122563
and Gmb 1830, using CHARA and Palomar Testbed Interferometer observations. For
the giant star HD 122563, we derive an angular diameter theta_3D = 0.940 +-
0.011 milliarcseconds (mas) using limb-darkening from 3D convection simulations
and for the dwarf star Gmb 1830 (HD 103095) we obtain a 1D limb-darkened
angular diameter theta_1D = 0.679 +- 0.007 mas. Coupling the angular diameters
with photometry yields effective temperatures with precisions better than 55 K
(Teff = 4598 +- 41 K and 4818 +- 54 K --- for the giant and the dwarf star,
respectively). Including their distances results in very well-determined
luminosities and radii (L = 230 +- 6 L_sun, R = 23.9 +- 1.9 R_sun and L = 0.213
+- 0.002 L_sun, R = 0.664 +- 0.015 R_sun, respectively). We used the CESAM2k
stellar structure and evolution code in order to produce models that fit the
observational data. We found values of the mixing-length parameter alpha (which
describes 1D convection) that depend on the mass of the star. The masses were
determined from the models with precisions of <3% and with the well-measured
radii excellent constraints on the surface gravity are obtained (log g = 1.60
+- 0.04, 4.59 +- 0.02, respectively). The very small errors on both log g and
Teff provide stringent constraints for spectroscopic analyses given the
sensitivity of abundances to both of these values. The precise determination of
Teff for the two stars brings into question the photometric scales for
metal-poor stars.Comment: accepted A&A, 8 dbl-column pages, incl. 7 tables and 4 figure
Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry
Context. Red giant branch (RGB) stars are very bright objects in galaxies and
are often used as standard candles. Interferometry is the ideal tool to
characterize the dynamics and morphology of their atmospheres. Aims. We aim at
precisely characterising the surface dynamics of a sample of RGB stars.
Methods. We obtained interferometric observations for three RGB stars with the
MIRC instrument mounted at the CHARA interfer- ometer. We looked for
asymmetries on the stellar surfaces using limb-darkening models. Results. We
measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas,
HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas.
We detected departures from the centrosymmetric case for all three stars with
the tendency of a greater effect for lower logg of the sample. We explored the
causes of this signal and conclude that a possible explanation to the
interferometric signal is the convection-related and/or the magnetic-related
surface activity. However, it is necessary to monitor these stars with new
observations, possibly coupled with spectroscopy, in order to firmly establish
the cause.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics,
section 1. Letters to the Editor. The official date of acceptance is
06/03/201
Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH2NH), ammonia (NH3), and hydrogen cyanide (HCN)
International audienceAstronomy & Astrophysics Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) ABSTRACT Context. Studing chemical reactivity in astrophysical environments is an important means for improving our understanding of the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. Laboratory simulations of the reactivity of ice analogs provide important insight into the reactivity in these environments. Here, we use these experimental simulations to investigate the Strecker synthesis leading to the formation of aminoacetonitrile in astrophysical-like conditions. The aminoacetonitrile is an interesting compound because it was detected in SgrB2, hence could be a precursor of the smallest amino acid molecule, glycine, in astrophysical environments. Aims. We present the first experimental investigation of the formation of aminoacetonitrile NH 2 CH 2 CN from the thermal processing of ices including methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) in interstellar-like conditions without VUV photons or particules. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy to monitor the ice evolution during its warming. Infrared spec-troscopy and mass spectroscopy are then used to identify the aminoacetonitrile formation. Results. We demonstrate that methanimine can react with − CN during the warming of ice analogs containing at 20 K methanimine, ammonia, and [NH + 4 − CN] salt. During the ice warming, this reaction leads to the formation of poly(methylene-imine) polymers. The polymer length depend on the initial ratio of mass contained in methanimine to that in the [NH + 4 − CN] salt. In a methanimine excess, long polymers are formed. As the methanimine is progressively diluted in the [NH + 4 − CN] salt, the polymer length decreases until the aminoacetonitrile formation at 135 K. Therefore, these results demonstrate that aminoacetonitrile can be formed through the second step of the Strecker synthesis in astrophysical-like conditions
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