461 research outputs found
A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars
We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in
42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region.
CO J=2-1 is observed simultaneously to indicate the level of confusion
with the surrounding molecular cloud. The bandpass also contains two
transitions of ortho-HCO, one of SO and the CO J=2-1 line which
provide complementary information on the nature of the emission.
While CO is in general dominated by residual emission from the cloud,
CN exhibits a high disk detection rate % in our sample. We even report CN
detection in stars for which interferometric searches failed to detect
CO, presumably because of obscuration by a foreground, optically thick,
cloud. Comparison between CN and o-HCO or SO line profiles and intensities
divide the sample in two main categories. Sources with SO emission are bright
and have strong HCO emission, leading in general to [HCO/CN].
Furthermore, their line profiles, combined with a priori information on the
objects, suggest that the emission is coming from outflows or envelopes rather
than from a circumstellar disk. On the other hand, most sources have
[HCO/CN], no SO emission, and some of them exhibit clear
double-peaked profiles characteristics of rotating disks. In this second
category, CN is likely tracing the proto-planetary disks. From the line flux
and opacity derived from the hyperfine ratios, we constrain the outer radii of
the disks, which range from 300 to 600 AU. The overall gas disk detection rate
(including all molecular tracers) is , and decreases for fainter
continuum sources.
This study shows that gas disks, like dust disks, are ubiquitous around young
PMS stars in regions of isolated star formation, and that a large fraction of
them have AU.Comment: 31 pages (including 59 figures
Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in Oph and upper Scorpius
We attempt to determine the molecular composition of disks around young
low-mass stars in the Oph region and to compare our results with a
similar study performed in the Taurus-Auriga region. We used the IRAM 30 m
telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars
located in the Oph and upper Scorpius regions. CO J=2-1 is
observed simultaneously to provide an indication of the level of confusion with
the surrounding molecular cloud. The bandpass also contains two transitions of
ortho-HCO, one of SO, and the CO J=2-1 line, which provides
complementary information on the nature of the emission. Contamination by
molecular cloud in CO and even CO is ubiquitous. The CN detection
rate appears to be lower than for the Taurus region, with only four sources
being detected (three are attributable to disks). HCO emission is found
more frequently, but appears in general to be due to the surrounding cloud. The
weaker emission than in Taurus may suggest that the average disk size in the
Oph region is smaller than in the Taurus cloud. Chemical modeling shows
that the somewhat higher expected disk temperatures in Oph play a direct
role in decreasing the CN abundance. Warmer dust temperatures contribute to
convert CN into less volatile forms. In such a young region, CN is no longer a
simple, sensitive tracer of disks, and observations with other tracers and at
high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&
Dynamical Masses of Low Mass Stars in the Taurus and Ophiuchus Star Forming Regions
We report new dynamical masses for 5 pre-main sequence (PMS) stars in the
L1495 region of the Taurus star-forming region (SFR) and 6 in the L1688 region
of the Ophiuchus SFR. Since these regions have VLBA parallaxes these are
absolute measurements of the stars' masses and are independent of their
effective temperatures and luminosities. Seven of the stars have masses
solar masses, thus providing data in a mass range with little data, and of
these, 6 are measured to precision . We find 8 stars with masses in the
range 0.09 to 1.1 solar mass that agree well with the current generation of PMS
evolutionary models. The ages of the stars we measured in the Taurus SFR are in
the range 1-3 MY, and MY for those in L1688. We also measured the
dynamical masses of 14 stars in the ALMA archival data for Akeson~\&~Jensen's
Cycle 0 project on binaries in the Taurus SFR. We find that the masses of 7 of
the targets are so large that they cannot be reconciled with reported values of
their luminosity and effective temperature. We suggest that these targets are
themselves binaries or triples.Comment: 20 page
VINCI / VLTI observations of Main Sequence stars
Main Sequence (MS) stars are by far the most numerous class in the Universe.
They are often somewhat neglected as they are relatively quiet objects (but
exceptions exist), though they bear testimony of the past and future of our
Sun. An important characteristic of the MS stars, particularly the solar-type
ones, is that they host the large majority of the known extrasolar planets.
Moreover, at the bottom of the MS, the red M dwarfs pave the way to
understanding the physics of brown dwarfs and giant planets. We have measured
very precise angular diameters from recent VINCI/VLTI interferometric
observations of a number of MS stars in the K band, with spectral types between
A1V and M5.5V. They already cover a wide range of effective temperatures and
radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as
well as the asteroseismic information available for some of these stars, the
angular diameters put strong constraints on the detailed models of these stars,
and therefore on the physical processes at play.Comment: 5 pages, 3 figures. To appear in the Proceedings of IAU Symposium
219, "Stars as Suns", Editors A. Benz & A. Dupree, Astronomical Society of
the Pacifi
Direct quartz-coesite transformation in shocked porous sandstone from Kamil Crater (Egypt)
Coesite, a high-pressure silica polymorph (pressure 3–10 GPa,
temperature <3000 K), is a diagnostic feature of shock metamorphism
associated with impact cratering on quartz-bearing target rocks. It
is preserved as a metastable phase in sedimentary target rocks that
experienced peak pressures in excess of ~10 GPa, where it typically
occurs as intergranular polycrystalline aggregates of microcrystals
embedded in silica glass known as “symplectic regions.” The presence
of coesite in the symplectic regions of rocks experiencing shock
conditions beyond the limits of the coesite stability field is a controversial
issue. Through a combined scanning and transmission electron
microscopy and Raman spectroscopy study of shocked quartzarenites
from the 45-m-diameter Kamil Crater (southwest Egypt), we show
that coesite in symplectic regions forms through direct subsolidus
transformation from quartz, in contrast with the prevailing hypothesis
for crystalline targets. The quartz-to-coesite transformation takes
place during localized shock-wave reverberation at the beginning of
the pore collapse process. Complete pore collapse generates the high
temperature regimes responsible for the subsequent production of
the embedding silica melts, in part at the expense of the previously
formed coesite. This work documents the role of pore collapse in
producing localized pressure-temperature-time gradients in shocked
porous targets, as predicted by numerical models in the literature
3D electron diffraction in nano-geology: present and perspectives
When working on advanced research topics in geosciences, one must often deal with small yields and cryptocrystalline polyphasic samples. Conventional optical and X-ray crystallographic tools may not be sufficient for the proper characterization of these samples. The development of efficient probes able to investigate the nanoworld becomes
therefore crucial for pushing forward our understanding about the geochemical and mineralogical processes that regulate Earth and extraterrestrial environments.
In the last ten years, electron diffraction (ED) evolved from a qualitative method restricted to few dedicated TEM users, to a robust protocol for phase identification and abinitio structure determination [1]. Such change has been mostly propelled by the development of routines for 3D data collection. This methodology is in principle equivalent to
single-crystal X-ray diffraction, but allows sampling crystals of few tens on nanometers. We will show here some examples of recent applications of ED in geosciences, namely how to achieve an easy and relatively fast characterization of minor and cryptocrystalline phases in natural and experimental samples. We were able identify and characterize modulated phases able to carry hydrogen at upper-mantle conditions, to follow aragonite growth from the first nucleation seeds [2] and to identify mineralogical phases and polytypes in non-equilibrated extraterrestrial samples and in impact rocks [3]
VLTI/VINCI diameter constraints on the evolutionary status of delta Eri, xi Hya, eta Boo.
other location: http://www.obs-nice.fr/pichon/science.html ; Accepted for publication in Astron. Astrophys.International audienceUsing VLTI/VINCI angular diameter measurements, we constrain the evolutionary status of three asteroseismic targets: the stars Eri, Hya, Boo. Our predictions of the mean large frequency spacing of these stars are in agreement with published observational estimations. Looking without success for a companion of Eri we doubt on its classification as an RS CVn star
The Frontier Mountain meteorite gap (Antarctica)
The Frontier Mountain blue ice field is an important Antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984. Remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow, ice thickness, bedrock topography, ice ablation and surface mass transport by wind, along with detailed descriptions of the field situation at the trap. This solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism. In particular, we observe that the meteorite trap forms in a blue ice field i) located upstream of an absolute and a shallow subice barriers; ii) characterized by compressive ice flow with horizontal velocities decreasing from 100 to <10 cm a-1 on approaching the obstacle; iii) undergoing mean ablation rates of 6.5 cm a-1; iv) nourished by a limited snow accumulation zone extending ~20 km upstream of the blue ice area. We also draw the following conclusions: i) the origin of the meteorite trap can be explained according to the present-day glaciological situation; ii) the meteorite concentration develops according to the general principles of the “ice flow model”; iii) the accumulation model can be described as “stagnant ice or slow-moving ice against an absolute and submerged barriers”, according to the descriptive schemes present in literature; iv) the Frontier Mountain ice field is an effective trap for meteorites weighing more than ~200 g; for smaller masses, the combination of wind and glacial drift may remove meteorites in less than a few tens of ka; v) although the activation age of the FM trap is not yet constrained, we infer that one of the most important findsite may be as old as 50 ka, i.e. older than the Last Glacial Maximum
Eating and feeding disorders in pediatric age
Eating and feeding disorders are common in pediatric age and may be important to discover and recover the early symptoms in order to optimize the treatment and management
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