378 research outputs found
Ices in Star-Forming Regions: First Results from VLT-ISAAC
The first results from a VLT-ISAAC program on L- and M-band infrared
spectroscopy of deeply-embedded young stellar objects are presented. The advent
of 8-m class telescopes allows high S/N spectra of low-luminosity sources to be
obtained. In our first observing run, low- and medium-resolution spectra have
been measured toward a dozen objects, mostly in the Vela and Chamaeleon
molecular clouds. The spectra show strong absorption of H2O and CO ice, as well
as weak features at `3.47' and 4.62 mu. No significant solid CH3OH feature at
3.54 mu is found, indicating that the CH3OH/H2O ice abundance is lower than
toward some massive protostars. Various evolutionary diagnostics are
investigated for a set of sources in Vela.Comment: 8 pages, 4 figures, to appear in The Origins of Stars and Planets:
the VLT View, eds. J. Alves, M. McCaughrean (Springer Verlag
Search for solid HDO in low-mass protostars
We present ground-based 2.1 to 4.2 microns observations of four low-mass
protostars. We searched for the 4.1 microns OD stretch band, characteristic of
solid HDO in grain mantles. We did not detect solid HDO in any of the four
sources, but we derive 3-sigma upper limits from 0.5% to 2% for the HDO/H2O
ratio depending on the source. These ratios provide strong constraints to
solid-state deuteration models when compared to deuterium fractionation values
observed in the gas phase. We discuss various scenarios that could lead to such
a low water deuteration compared to the high formaldehyde and methanol
deuteration observed in the gas phase.Comment: 8 pages, 6 figures Accepted for publication in A&
The circumstellar environment of T Tau S at high spatial and spectral resolution
We have obtained the first high spatial (0.05'') and spectral (R~35000)
resolution 2 micron spectrum of the T Tau S tight binary system using adaptive
optics on the Keck II telescope. We have also obtained the first 3.8 and 4.7
micron images that resolve the three components of the T Tau multiple system,
as well as new 1.6 and 2.2 micron images. Together with its very red
near-infrared colors, the spectrum of T Tau Sb shows that this T Tauri star is
extincted by a roughly constant extinction of Av~15 mag, which is probably the
0.7''x0.5'' circumbinary structure recently observed in absorption in the
ultraviolet. T Tau Sa, which is also observed through this screen and is
actively accreting, further possesses a small edge-on disk that is evidenced by
warm (390 K), narrow overtone CO rovibrational absorption features in our
spectrum. We find that T Tau Sa is most likely an intermediate-mass star
surrounded by a semi-transparent 2-3 AU-radius disk whose asymmetries and short
Keplerian rotation explain the large photometric variability of the source on
relatively short timescales. We also show that molecular hydrogen emission
exclusively arises from the gas that surrounds T Tau S and that its spatial and
kinematic structure, while providing suggestive evidence for a jet-like
structure, is highly complex.Comment: accepted for publication in the Astrophysical Journal; 41 pages, 10
figure
Desorption of CO and O2 interstellar ice analogs
Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds
through freeze-out processes. The aim of this work is to characterize
quantitatively the physical processes that are involved in the desorption
kinetics of CO-O2 ices by interpreting laboratory temperature programmed
desorption (TPD) data. This information is used to simulate the behavior of
CO-O2 ices under astrophysical conditions. The TPD spectra have been recorded
under ultra high vacuum conditions for pure, layered and mixed morphologies for
different thicknesses, temperatures and mixing ratios. An empirical kinetic
model is used to interpret the results and to provide input parameters for
astrophysical models. Binding energies are determined for different ice
morphologies. Independent of the ice morphology, the desorption of O2 is found
to follow 0th-order kinetics. Binding energies and temperature-dependent
sticking probabilities for CO-CO, O2-O2 and CO-O2 are determined. O2 is
slightly less volatile than CO, with binding energies of 912+-15 versus 858+-15
K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but
its binding energies are slightly increased compared with pure ice whereas
those for O2 are slightly decreased. Lower limits to the sticking probabilities
of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20K. The
balance between accretion and desorption is studied for O2 and CO in
astrophysically relevant scenarios. Only minor differences are found between
the two species, i.e., both desorb between 16 and 18K in typical environments
around young stars. Thus, clouds with significant abundances of gaseous CO are
unlikely to have large amounts of solid O2.Comment: 8 pages + 2 pages online material, 8 figures (1 online), accepted by
A&
Nature and evolution of the dominant carbonaceous matter in interplanetary dust particles: effects of irradiation and identification with a type of amorphous carbon
Aims.Interplanetary dust particle (IDP) matter probably evolved under irradiation in the interstellar medium (ISM) and the solar nebula. Currently IDPs are exposed to irradiation in the Solar System. Here the effects of UV and proton processing on IDP matter are studied experimentally. The structure and chemical composition of the bulk of carbon matter in IDPs is characterized. Methods: .Several IDPs were further irradiated in the laboratory using ultraviolet (UV) photons and protons in order to study the effects of such processing. By means of infrared and Raman spectroscopy, IDPs were also compared to different materials that serve as analogs of carbon grains in the dense and diffuse ISM. Results: .The carbonaceous fraction of IDPs is dehydrogenated by exposure to hard UV photons or 1 MeV protons. On the other hand, proton irradiation at lower energies (20 keV) leads to an efficient hydrogenation of the carbonaceous IDP matter. The dominant type of carbon in IDPs, observed with Raman and infrared spectroscopy, is found to be either a form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H), depending on the IDP, consisting of aromatic units with an average domain size of 1.35 nm (5-6 rings in diameter), linked by aliphatic chains. Conclusions: .The D- and 15N-enrichments associated to an aliphatic component in some IDPs are probably the result of chemical reactions at cold temperatures. It is proposed that the amorphous carbon in IDPs was formed by energetic processing (UV photons and cosmic rays) of icy grains, maybe during the dense cloud stage, and more likely on the surface of the disk during the T Tauri phase of our Sun. This would explain the isotopic anomalies and morphology of IDPs. Partial annealing, 300-400°C, is required to convert an organic residue from ice photoprocessing into the amorphous carbon with low heteroatom content found in IDPs. Such annealing might have occurred as the particles approached the Sun and/or during atmospheric entry heating
Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
Context. Methane is among the main components of the ice mantles of
insterstellar dust grains, where it is at the start of a rich solid-phase
chemical network. Quantification of the photon-induced desorption yield of
these frozen molecules and understanding of the underlying processes is
necessary to accurately model the observations and the chemical evolution of
various regions of the interstellar medium. Aims. This study aims at
experimentally determining absolute photodesorption yields for the CH4 molecule
as a function of photon energy. The influence of the ice composition is also
investigated. By studying the methane desorption from layered CH4:CO ice,
indirect desorption processes triggered by the excitation of the CO molecules
is monitored and quantified. Methods. Tunable monochromatic VUV light from the
DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91
nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice
samples. The release of species in the gas phase is monitored by quadrupole
mass spectrometry and absolute photodesorption yields of intact CH4 are
deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV
(~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4,
which confirms a desorption mechanism mediated by electronic transitions in the
ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a
pattern characteristic of CO absorption, indicating desorption induced by
energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from
the pure ice in various interstellar environments is around 2.0 x 10^-3
molecules per incident photon. Results on CO-induced indirect desorption of CH4
provide useful insights for the generalization of this process to other
molecules co-existing with CO in ice mantles
Infrared spectroscopy of HCOOH in interstellar ice analogues
Context: HCOOH is one of the more common species in interstellar ices with
abundances of 1-5% with respect to solid H2O. Aims: This study aims at
characterizing the HCOOH spectral features in astrophysically relevant ice
mixtures in order to interpret astronomical data. Methods: The ices are grown
under high vacuum conditions and spectra are recorded in transmission using a
Fourier transform infrared spectrometer. Pure HCOOH ices deposited at 15 K and
145 K are studied, as well as binary and tertiary mixtures containing H2O, CO,
CO2 and CH3OH. The mixture concentrations are varied from 50:50% to ~10:90% for
HCOOH:H2O. Binary mixtures of HCOOH:X and tertiary mixtures of HCOOH:H2O:X with
X = CO, CO2, and CH3OH, are studied for concentrations of ~10:90% and
~7:67:26%, respectively. Results: Pure HCOOH ice spectra show broad bands which
split around 120 K due to the conversion of a dimer to a chain-structure. Broad
single component bands are found for mixtures with H2O. Additional spectral
components are present in mixtures with CO, CO2 and CH3OH. The resulting peak
position, full width at half maximum and band strength depend strongly on ice
structure, temperature, matrix constituents and the HCOOH concentration.
Comparison of the solid HCOOH 5.9, 7.2, and 8.1 micron features with
astronomical data toward the low mass source HH 46 and high mass source W 33A
shows that spectra of binary mixtures do not reproduce the observed ice
features. However, our tertiary mixtures especially with CH3OH match the
astronomical data very well. Thus interstellar HCOOH is most likely present in
tertiary or more complex mixtures with H2O, CH3OH and potentially also CO or
CO2, providing constraints on its formation.Comment: 11 pages, 10 figures, accepted by A&
Testing grain surface chemistry : a survey of deuterated formaldehyde and methanol in low-mass Class 0 protostars
Context : Despite the low cosmic abundance of deuterium (D/H ~ 1e-5), large
degrees of deuterium fractionation in molecules are observed in star forming
regions with enhancements that can reach 13 orders of magnitude, which current
models have difficulties to account for.
Aims : Multi-isotopologue observations are a very powerful constraint for
chemical models. The aim of our observations is to understand the processes
forming the observed large abundances of methanol and formaldehyde in low-mass
protostellar envelopes (gas-phase processes ? chemistry on the grain surfaces
?) and better constrain the chemical models. Methods : Using the IRAM 30m
single-dish telescope, we observed deuterated formaldehyde (HDCO and D2CO) and
methanol (CH2DOH, CH3OD, and CHD2OH) towards a sample of seven low-mass class 0
protostars. Using population diagrams, we then derive the fractionation ratios
of these species (abundance ratio between the deuterated molecule and its main
isotopologue) and compare them to the predictions of grain chemistry models.
Results : These protostars show a similar level of deuteration as in
IRAS16293-2422, where doubly-deuterated methanol -- and even triply-deuterated
methanol -- were first detected. Our observations point to the formation of
methanol on the grain surfaces, while formaldehyde formation cannot be fully
pined down. While none of the scenarii can be excluded (gas-phase or grain
chemistry formation), they both seem to require abstraction reactions to
reproduce the observed fractionations.Comment: 21 pages, 12 figures, accepted by A&
The gas temperature in flaring disks around pre-main sequence stars
A model is presented which calculates the gas temperature and chemistry in
the surface layers of flaring circumstellar disks using a code developed for
photon-dominated regions. Special attention is given to the influence of dust
settling. It is found that the gas temperature exceeds the dust temperature by
up to several hundreds of Kelvins in the part of the disk that is optically
thin to ultraviolet radiation, indicating that the common assumption that
Tgas=Tdust is not valid throughout the disk. In the optically thick part, gas
and dust are strongly coupled and the gas temperature equals the dust
temperature. Dust settling has little effect on the chemistry in the disk, but
increases the amount of hot gas deeper in the disk. The effects of the higher
gas temperature on several emission lines arising in the surface layer are
examined. The higher gas temperatures increase the intensities of molecular and
fine-structure lines by up to an order of magnitude, and can also have an
important effect on the line shapes.Comment: 14 pages, 10 figures, accepted for publication in A&
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