533 research outputs found
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
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&
Interstellar deuterated ammonia: From NH3 to ND3
We use spectra and maps of NH2D, ND2H, and ND3, obtained with the CSO, IRAM
30m and Arecibo telescopes, to study deuteration processes in dense cores. The
data include the first detection of the hyperfine structure of ND2H. The
emission of ND2H and ND3 does not seem to peak at the positions of the embedded
protostars, but instead at offset positions, where outflow interactions may
occur. A constant ammonia fractionation ratio in star-forming regions is
generally assumed to be consistent with an origin on dust grains. However, in
the pre-stellar cores studied here, the fractionation varies significantly when
going from NH3 to ND3. We present a steady state model of the gas-phase
chemistry for these sources, which includes passive depletion onto dust grains
and multiply saturated deuterated species up to five deuterium atoms (e.g.
CD5+). The observed column density ratios of all four ammonia isotopologues are
reproduced within a factor of 3 for a gas temperature of 10 K. We also predict
that deuterium fractionation remains significant at temperatures up to 20 K. ND
and NHD, which have rotational transitions in the submillimeter domain are
predicted to be abundant.Comment: 14 pages, 12 figures, 12 table
Ice emission and the redshifts of submillimeter sources
Observations at submillimeter wavelengths have revealed a population of
sources thought to be at relatively large redshifts. The position of the 850
m passband on the Rayleigh-Jeans portion of the Planck function leads to a
maximum redshift estimate of 4.5 since sources will not retain their
redshift independent brightness close to the peak of the Planck function and
thus drop out of surveys. Here we review evidence that ice absorption is
present in the spectra of local ultraluminous infrared galaxies which are often
taken as analogs for the 850 m source population. We consider the
implication of this absorption for ice induced spectral structure at far
infrared wavelengths and present marginal astronomical evidence that amorphous
ice may have a feature similar to crystalline ice near 150 m. Recent
corroborative laboratory evidence is supportive of this conclusion. It is
argued that early metal enrichment by pair instability SN may lead to a high
ice content relative to refractory dust at high redshift and a fairly robust
detection of ice emission in a quasar is presented. It is further
shown that ice emission is needed to understand the 450 m sources observed
in the GOODS-N field. We are thus encouraged to apply far infrared ice emission
models to the available observations of HDF 850.1, the brightest submillimeter
source in the {\it Hubble Deep Field}. We suggest that a redshift as large as
13 may need to be considered for this source, nearly a factor of three above
the usual top estimate. Inclusion of the possibility of far infrared ice
emission in the spectral energy distributions of model sources generally
broadens the range of redshifts to be considered for submillimeter sources
compared to models without ice emission.Comment: 37 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration
Water is the main component of interstellar ice mantles, is abundant in the
solar system and is a crucial ingredient for life. The formation of this
molecule in the interstellar medium cannot be explained by gas-phase chemistry
only and its surface hydrogenation formation routes at low temperatures (O, O2,
O3 channels) are still unclear and most likely incomplete. In a previous paper
we discussed an unexpected zeroth-order H2O production behavior in O2 ice
hydrogenation experiments compared to the first-order H2CO and CH3OH production
behavior found in former studies on hydrogenation of CO ice. In this paper we
experimentally investigate in detail how the structure of O2 ice leads to this
rare behavior in reaction order and production yield. In our experiments H
atoms are added to a thick O2 ice under fully controlled conditions, while the
changes are followed by means of reflection absorption infrared spectroscopy
(RAIRS). The H-atom penetration mechanism is systematically studied by varying
the temperature, thickness and structure of the O2 ice. We conclude that the
competition between reaction and diffusion of the H atoms into the O2 ice
explains the unexpected H2O and H2O2 formation behavior. In addition, we show
that the proposed O2 hydrogenation scheme is incomplete, suggesting that
additional surface reactions should be considered. Indeed, the detection of
newly formed O3 in the ice upon H-atom exposure proves that the O2 channel is
not an isolated route. Furthermore, the addition of H2 molecules is found not
to have a measurable effect on the O2 reaction channel.Comment: 1 page, 1 figur
ISO spectroscopy of gas and dust: from molecular clouds to protoplanetary disks
Observations of interstellar gas-phase and solid-state species in the 2.4-200
micron range obtained with the spectrometers on board the Infrared Space
Observatory are reviewed. Lines and bands due to ices, polycyclic aromatic
hydrocarbons, silicates and gas-phase atoms and molecules (in particular H2,
CO, H2O, OH and CO2) are summarized and their diagnostic capabilities
illustrated. The results are discussed in the context of the physical and
chemical evolution of star-forming regions, including photon-dominated regions,
shocks, protostellar envelopes and disks around young stars.Comment: 56 pages, 17 figures. To appear in Ann. Rev. Astron. Astrophys. 2004.
Higher resolution version posted at
http://www.strw.leidenuniv.nl/~ewine/araa04.pd
Astrochemical models of interstellar ices: History matters
Ice is ubiquitous in the interstellar medium. We model the formation of the
main constituents of interstellar ices, including H2O, CO2 , CO, and CH3 OH. We
strive to understand what physical or chemical parameters influence the final
composition of the ice and how they benchmark to what has already been
observed, with the aim of applying these models to the preparation and analysis
of JWST observations. We used the Nautilus gas-grain model, which computes the
gas and ice composition as a function of time for a set of physical conditions,
starting from an initial gas phase composition. All important processes
(gas-phase reactions, gas-grain interactions, and grain surface processes) are
included and solved with the rate equation approximation. We first ran an
astrochemical code for fixed conditions of temperature and density mapped in
the cold core L429-C to benchmark the chemistry. One key parameter was revealed
to be the dust temperature. When the dust temperature is higher than 12 K, CO2
will form efficiently at the expense of H2O, while at temperatures below 12 K,
it will not form. Whatever hypothesis we assumed for the chemistry (within
realistic conditions), the static simulations failed to reproduce the observed
trends of interstellar ices in our target core. In a second step, we simulated
the chemical evolution of parcels of gas undergoing different physical and
chemical situations throughout the molecular cloud evolution and starting a few
1e7 yr prior to the core formation (dynamical simulations). Our dynamical
simulations satisfactorily reproduce the main trends already observed for
interstellar ices. Moreover, we predict that the apparent constant ratio of
CO2/H2O observed to date is probably not true for regions of low AV , and that
the history of the evolution of clouds plays an essential role, even prior to
their formation.Comment: Accepted for publication in 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
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