37 research outputs found
Constraining the abundances of complex organics in the inner regions of solar-type protostars
The high abundances of Complex Organic Molecules (COMs) with respect to
methanol, the most abundant COM, detected towards low-mass protostars, tend to
be underpredicted by astrochemical models. This discrepancy might come from the
large beam of the single-dish telescopes, encompassing several components of
the studied protostar, commonly used to detect COMs. To address this issue, we
have carried out multi-line observations of methanol and several COMs towards
the two low-mass protostars NGC1333-IRAS2A and -IRAS4A with the Plateau de Bure
interferometer at an angular resolution of 2 arcsec, resulting in the first
multi-line detection of the O-bearing species glycolaldehyde and ethanol and of
the N-bearing species ethyl cyanide towards low-mass protostars other than IRAS
16293. The high number of detected transitions from COMs (more than 40 methanol
transitions for instance) allowed us to accurately derive the source size of
their emission and the COMs column densities. The COMs abundances with respect
to methanol derived towards IRAS2A and IRAS4A are slightly, but not
substantitally, lower than those derived from previous single-dish
observations. The COMs abundance ratios do not vary significantly with the
protostellar luminosity, over five orders of magnitude, implying that low-mass
hot corinos are quite chemically rich as high-mass hot cores. Astrochemical
models still underpredict the abundances of key COMs, such as methyl formate or
di-methyl ether, suggesting that our understanding of their formation remains
incomplete.Comment: 60 pages, 10 figures, 17 tables. Accepted for publication in Ap
First measurement of the 14N/15N ratio in the analogue of the Sun progenitor OMC-2 FIR4
We present a complete census of the 14N/15N isotopic ratio in the most
abundant N-bearing molecules towards the cold envelope of the protocluster
OMC-2 FIR4, the best known Sun progenitor. To this scope, we analysed the
unbiased spectral survey obtained with the IRAM-30m telescope at 3mm, 2mm and
1mm. We detected several lines of CN, HCN, HNC, HC3N, N2H+, and their
respective 13C and 15N isotopologues. The lines relative fluxes are compatible
with LTE conditions and moderate line opacities have been corrected via a
Population Diagram method or theoretical relative intensity ratios of the
hyperfine structures. The five species lead to very similar 14N/15N isotopic
ratios, without any systematic difference between amine and nitrile bearing
species as previously found in other protostellar sources. The weighted average
of the 14N/15N isotopic ratio is 270 +/- 30. This 14N/15N value is remarkably
consistent with the [250-350] range measured for the local galactic ratio but
significantly differs from the ratio measured in comets (around 140).
High-angular resolution observations are needed to examine whether this
discrepancy is maintained at smaller scales. In addition, using the CN, HCN and
HC3N lines, we derived a 12C/13C isotopic ratio of 50 +/- 5.Comment: Accepted for publication in ApJ ; 19 pages, 5 tables, 12 figure
Water ice deuteration: a tracer of the chemical history of protostars
Context. Millimetric observations have measured high degrees of molecular
deuteration in several species seen around low-mass protostars. The Herschel
Space Telescope, launched in 2009, is now providing new measures of the
deuterium fractionation of water, the main constituent of interstellar ices.
Aims. We aim at theoretically studying the formation and the deuteration of
water, which is believed to be formed on interstellar grain surfaces in
molecular clouds. Methods. We used our gas-grain astrochemical model GRAINOBLE,
which considers the multilayer formation of interstellar ices. We varied
several input parameters to study their impact on water deuteration. We
included the treatment of ortho- and para-states of key species, including H2,
which affects the deuterium fractionation of all molecules. The model also
includes relevant laboratory and theoretical works on the water formation and
deuteration on grain surfaces. In particular, we computed the transmission
probabilities of surface reactions using the Eckart model, and we considered
ice photodissociation following molecular dynamics simulations. Results. The
use of a multilayer approach allowed us to study the influence of various
parameters on the abundance and the deuteration of water. Deuteration of water
is found to be very sensitive to the ortho-to-para ratio of H2 and to the total
density, but it also depends on the gas/grain temperatures and the visual
extinction of the cloud. Since the deuteration is very sensitive to the
physical conditions, the comparison with sub-millimetric observation towards
the low-mass protostar IRAS 16293 allows us to suggest that water ice is formed
together with CO2 in molecular clouds with limited density, whilst formaldehyde
and methanol are mainly formed in a later phase, where the condensation becomes
denser and colder.Comment: Accepted for publication to A&A. 24 pages, 10 figure
Subarcsecond Analysis of Infalling-Rotating Envelope around the Class I Protostar IRAS 04365+2535
Sub-arcsecond images of the rotational line emission of CS and SO have been
obtained toward the Class I protostar IRAS 043652535 in TMC-1A with ALMA. A
compact component around the protostar is clearly detected in the CS and SO
emission. The velocity structure of the compact component of CS reveals
infalling-rotating motion conserving the angular momentum. It is well explained
by a ballistic model of an infalling-rotating envelope with the radius of the
centrifugal barrier (a half of the centrifugal radius) of 50 AU, although the
distribution of the infalling gas is asymmetric around the protostar. The
distribution of SO is mostly concentrated around the radius of the centrifugal
barrier of the simple model. Thus a drastic change in chemical composition of
the gas infalling onto the protostar is found to occur at a 50 AU scale
probably due to accretion shocks, demonstrating that the infalling material is
significantly processed before being delivered into the disk.Comment: 15 March 2016, ApJ, accepte
Introduction d’une plateforme numérique dans un dispositif APP – quelles adaptations, quelle influence sur la coopération/collaboration étudiante ?
Nous présentons dans cet article une analyse des gains constatés et des difficultés rencontrées lors de l’introduction d’une plateforme numérique dans un dispositif d’apprentissage sous forme d’Apprentissage Par Problèmes (APP). L’étude croise le bilan des enseignants, des retours d’étudiants et des traces d’activité enregistrées sur la plateforme. Une attention particulière est portée à l’influence de la plateforme sur la coopération/collaboration entre étudiants
Vertical Structure of the Transition Zone from Infalling Rotating Envelope to Disk in the Class 0 Protostar, IRAS04368+2557
We have resolved for the first time the radial and vertical structure of the
almost edge-on envelope/disk system of the low-mass Class 0 protostar L1527.
For that, we have used ALMA observations with a spatial resolution of
0.250.13 and
0.370.23 at 0.8 mm and 1.2 mm,
respectively. The L1527 dust continuum emission has a deconvolved size of 78 au
21 au, and shows a flared disk-like structure. A thin
infalling-rotating envelope is seen in the CCH emission outward of about 150
au, and its thickness is increased by a factor of 2 inward of it. This radius
lies between the centrifugal radius (200 au) and the centrifugal barrier of the
infalling-rotating envelope (100 au). The gas stagnates in front of the
centrifugal barrier and moves toward vertical directions. SO emission is
concentrated around and inside the centrifugal barrier. The rotation speed of
the SO emitting gas is found to be decelerated around the centrifugal barrier.
A part of the angular momentum could be extracted by the gas which moves away
from the mid-plane around the centrifugal barrier. If this is the case, the
centrifugal barrier would be related to the launching mechanism of low velocity
outflows, such as disk winds
The Herschel and IRAM CHESS Spectral Surveys of the Protostellar Shock L1157-B1: Fossil Deuteration
We present the first study of deuteration toward the protostellar shock L1157-B1, based on spectral surveys performed with the Herschel-HIFI and IRAM 30 m telescopes. The L1157 outflow is driven by a low-mass Class 0 protostar and is considered the prototype of the so-called chemically active outflows. The young (2000 yr), bright blueshifted bow shock, B1, is an ideal laboratory for studying the gas chemically enriched by the release of dust mantles due to the passage of a shock. A total of 12 emission lines (up to E_u = 63 K) of CH_(2)DOH, HDCO, and DCN are detected. In addition, two lines of NH_(2)D and HDO are tentatively reported. To estimate the deuteration, we also extracted from our spectral survey emission lines of non-deuterated isotopologues (^(13)CH_(3)OH, H_2 ^(13)CO, H^(13)CN, H_2 ^(13)CO, and NH_3). We infer higher deuteration fractions for CH_(3)OH (D/H = 0.2-2 × 10^(–2)) and H_(2)CO (5-8 × 10^(–3)) than for H_(2)O (0.4-2 × 10^(–3)), HCN (~10^(–3)), and ammonia (≤3 × 10^(–2)). The measurement of deuteration of water, formaldehyde, and methanol in L1157-B1 provides a fossil record of the gas before it was shocked by the jet driven by the protostar. A comparison with gas-grain models indicates that the gas passed through a low-density (≤10^3 cm^(–3)) phase, during which the bulk of water ices formed, followed by a phase of increasing density, up to 3 × 10^4 cm^(–3), during which formaldehyde and methanol ices formed
Pierre in the lab’s everyday life
I shall evoke, through some memories of the pioneers years of the Grenoble Astrophysics Group, how Pierre played a decisive role in the lab’s everyday life, as a computer wizard, as an insatiable inquiring mind and as an always careful and warm colleague
Observations millimetriques de molecules circumstellaires : de la recherche de nouvelles especes, a la mesure d'abondances isotopiques
CNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc
Etude de nuages moléculaires : le rapport d'abondances ortho/para du formaldéhydre : observations millimétriques de quatre régions de formation d'étoiles
De l'observation des transitions à 140 et 150 GHz de l'ortho formaldéhyde et à 72 et 145 GHz du para formaldéhyde se déduit un rapport d'abondances ortho/ para voisin de 3 dans le nuage moléculaire interstellaire Orion A et apparemment plus faible dans les nuages sombres TMC1 et L183. L'étude des mécanismes chimiques en phase gazeuse succeptibles de régir les abondances des deux espèces conduit à un rapport théorique de 3 dans les nuages chauds et denses connue Orion; ce rapport reste indéterminé mais pourrait être plus faible dans les nuages froids connue TMC1 et L183. Quatre nuages moléculaires géants (S147/S153, S184, S158/S159(NGC7538) et W3) ont été cartographiés avec l'antenne de 2,50 m de l'Observatoire de Bordeaux en émissions 13C0 (J=1-0) et HC0+ (J=1-0), dont les étendues se révèlent tout à fait comparables. Quelques caractéristiques des nuages (dimensions, masse, dynamique ...) sont déduites des observations 13 COpas de résum