1,364 research outputs found
Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models
The heavy mass loss observed in evolved asymptotic giant branch stars is
usually attributed to a two-step process: atmospheric levitation by
pulsation-induced shock waves, followed by radiative acceleration of newly
formed dust grains. Detailed wind models suggest that the outflows of M-type
AGB stars may be triggered by photon scattering on Fe-free silicates with grain
sizes of about 0.1 - 1 m. Due to the low grain temperature, these Fe-free
silicates can condense close to the star, but they do not produce the
characteristic mid-IR features that are often observed in M-type AGB stars.
However, it is probable that the silicate grains are gradually enriched with Fe
as they move away from the star, to a degree where the grain temperature stays
below the sublimation temperature, but is high enough to produce emission
features. We investigate whether differences in grain temperature in the inner
wind region, which are related to changes in the grain composition, can be
detected with current interferometric techniques, in order to put constraints
on the wind mechanism. To investigate this we use radial structures of the
atmosphere and wind of an M-type AGB star, produced with the 1D
radiation-hydrodynamical code DARWIN. The spectral energy distribution is found
to be a poor indicator of different temperature profiles and therefore is not a
good tool for distinguishing different scenarios of changing grain composition.
However, spatially resolved interferometric observations have promising
potential. They show signatures even for Fe-free silicates (found at 2-3
stellar radii), in contrast to the spectral energy distribution. Observations
with baselines that probe spatial scales of about 4 stellar radii and beyond
are suitable for tracing changes in grain composition, since this is where
effects of Fe enrichment should be found.Comment: Accepted for publication in Section 8. Stellar atmospheres of
Astronomy and Astrophysics. The official date of acceptance is 07/09/2017. 9
pages, 7 figures, 4 figures in appendi
Interferometry of chemically peculiar stars: theoretical predictions vs. modern observing facilities
By means of numerical experiments we explore the application of
interferometry to the detection and characterization of abundance spots in
chemically peculiar (CP) stars using the brightest star eps~Uma as a case
study. We find that the best spectral regions to search for spots and stellar
rotation signatures are in the visual domain. The spots can clearly be detected
already at a first visibility lobe and their signatures can be uniquely
disentangled from that of rotation. The spots and rotation signatures can also
be detected in NIR at low spectral resolution but baselines longer than 180~m
are needed for all potential CP candidates. According to our simulations, an
instrument like VEGA (or its successor e.g., FRIEND) should be able to detect,
in the visual, the effect of spots and spots+rotation, provided that the
instrument is able to measure , and/or closure phase. In
infrared, an instrument like AMBER but with longer baselines than the ones
available so far would be able to measure rotation and spots. Our study
provides necessary details about strategies of spot detection and the
requirements for modern and planned interferometric facilities essential for CP
star research.Comment: Accepted by NMRAS, 18 pages, 11 figures, 2 table
Modelling the atmosphere of the carbon-rich Mira RU Vir
Context. We study the atmosphere of the carbon-rich Mira RU Vir using the
mid-infrared high spatial resolution interferometric observations from
VLTI/MIDI. Aims. The aim of this work is to analyse the atmosphere of the
carbon-rich Mira RU Vir, with state of the art models, in this way deepening
the knowledge of the dynamic processes at work in carbon-rich Miras. Methods.
We compare spectro-photometric and interferometric measurements of this
carbon-rich Mira AGB star, with the predictions of different kinds of modelling
approaches (hydrostatic model atmospheres plus MOD-More Of Dusty,
self-consistent dynamic model atmospheres). A geometric model fitting tool is
used for a first interpretation of the interferometric data. Results. The
results show that a joint use of different kind of observations (photometry,
spectroscopy, interferometry) is essential to shed light on the structure of
the atmosphere of a carbon-rich Mira. The dynamic model atmospheres fit well
the ISO spectrum in the wavelength range {\lambda} = [2.9, 25.0] {\mu}m.
Nevertheless, a discrepancy is noticeable both in the SED (visible), and in the
visibilities (shape and level). A possible explanation are intra-/inter-cycle
variations in the dynamic model atmospheres as well as in the observations. The
presence of a companion star and/or a disk or a decrease of mass loss within
the last few hundred years cannot be excluded but are considered unlikely.Comment: 15 pages. Accepted in A&
The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
Context: Stars on the asymptotic giant branch (AGB) show broad evidence of
inhomogeneous atmospheres and circumstellar envelopes. These have been studied
by a variety of methods on various angular scales. In this paper we explore the
envelope of the well-studied carbon star TX Psc by the technique of
spectro-astrometry. Aims: We explore the potential of this method for detecting
asymmetries around AGB stars. Methods:We obtained CRIRES observations of
several CO v=1 lines near 4.6 m and HCN lines near 3 m in
2010 and 2013. These were then searched for spectro-astrometric signatures. For
the interpretation of the results, we used simple simulated observations.
Results: Several lines show significant photocentre shifts with a clear
dependence on position angle. In all cases, tilde-shaped signatures are found
where the positive and negative shifts (at PA 0deg) are associated with blue
and weaker red components of the lines. The shifts can be modelled with a
bright blob 70 mas to 210 mas south of the star with a flux of several percent
of the photospheric flux. We estimate a lower limit of the blob temperature of
1000 K. The blob may be related to a mass ejection as found for AGB stars or
red supergiants. We also consider the scenario of a companion object.
Conclusions: Although there is clear spectro-astrometric evidence of a rather
prominent structure near TX Psc, it does not seem to relate to the other
evidence of asymmetries, so no definite explanation can be given. Our data thus
underline the very complex structure of the environment of this star, but
further observations that sample the angular scales out to a few hundred
milli-arcseconds are needed to get a clearer picture
Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations
Stellar parameter determination is a challenging task when dealing with
galactic giant stars. The combination of different investigation techniques has
proven to be a promising approach. We analyse archive spectra obtained with the
Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric
observations from the Very Large Telescope MID-infrared Interferometric
instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The
aim of this work is to determine stellar parameters using spectroscopy and
interferometry. The observations are used to constrain the model atmosphere,
and eventually the stellar evolutionary model in the region where the tracks
map the beginning of the carbon star sequence. Two different approaches are
used to determine stellar parameters: (i) the 'classic' interferometric
approach where the effective temperature is fixed by using the angular diameter
in the N-band (from interferometry) and the apparent bolometric magnitude; (ii)
parameters are obtained by fitting a grid of state-of-the-art hydrostatic
models to spectroscopic and interferometric observations. We find a good
agreement between the parameters of the two methods. The effective temperature
and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the
H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star
just recently, which means that the star is still in a 'quiet' phase compared
to the subsequent strong-wind regime. This is in agreement with the C/O ratio
being only slightly larger than 1.Comment: 11 pages, 9 figures, 5 table
Una usina de conocimiento para la resoluciĂłn de problemas prĂĄcticos
Fil: Boveris, Alberto. Universidad de Buenos Aires. Facultad de Farmacia y BioquĂmica; ArgentinaFil: Paladini, Alejandro C. Universidad de Buenos Aires. Facultad de Farmacia y BioquĂmica; ArgentinaEn el 190Âș aniversario de la UBA, la Facultad de Farmacia y BioquĂmica renueva el compromiso que\nestableciĂł en su creaciĂłn hace 54 años: "Construir una unidad acadĂ©mica cientĂfica y tĂ©cnica adaptada\nal requerimiento de profesionales farmacĂ©uticos y bioquĂmicos necesarios en la segunda mitad del\nsiglo XX". Hoy, en pleno siglo XXI se ubica entre las 100 mejores facultades de Farmacia del mundo
An allelic variant in the intergenic region between ERAP1 and ERAP2 correlates with an inverse expression of the two genes
The Endoplasmatic Reticulum Aminopeptidases ERAP1 and ERAP2 are implicated in a variety of immune and non-immune functions. Most studies however have focused on their role in shaping the HLA class I peptidome by trimming peptides to the optimal size. Genome Wide Association Studies highlighted non-synonymous polymorphisms in their coding regions as associated with several immune mediated diseases. The two genes lie contiguous and oppositely oriented on the 5q15 chromosomal region. Very little is known about the transcriptional regulation and the quantitative variations of these enzymes. Here, we correlated the level of transcripts and proteins of the two aminopeptidases in B-lymphoblastoid cell lines from 44 donors harbouring allelic variants in the intergenic region between ERAP1 and ERAP2. We found that the presence of a G instead of an A at SNP rs75862629 in the ERAP2 gene promoter strongly influences the expression of the two ERAPs with a down-modulation of ERAP2 coupled with a significant higher expression of ERAP1. We therefore show here for the first time a coordinated quantitative regulation of the two ERAP genes, which can be relevant for the setting of specific therapeutic approaches
Trade-off between angular resolution and straylight contamination in CMB anisotropy experiments. II. Straylight evaluation
Satellite CMB anisotropy missions and new generation of balloon-borne and
ground experiments, make use of complex multi-frequency instruments at the
focus of a meter class telescope. Between 70 GHz and 300 GHz, where foreground
contamination is minimum, it is extremely important to reach the best trade-off
between the improvement of the angular resolution and the minimization of the
straylight contamination mainly due to the Galactic emission. We focus here, as
a working case, on the 30 and 100 GHz channels of the Planck Low Frequency
Instrument (LFI). We evaluate the GSC introduced by the most relevant Galactic
foreground components for a reference set of optical configurations. We show
that it is possible to improve the angular resolution of 5-7% by keeping the
overall GSC below the level of few microKelvin. A comparison between the level
of straylight introduced by the different Galactic components for different
beam regions is presented. Simple approximate relations giving the rms and
peak-to-peak levels of the GSC are provided. We compare the results obtained at
100 GHz with those at 30 GHz, where GSC is more critical. Finally, we compare
the results based on Galactic foreground templates derived from radio and IR
surveys with those based on WMAP maps including CMB and extragalactic source
fluctuations.Comment: Submitted to A&A. Quality of the figures was degraded for
size-related reason
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