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 $\mu$m passband on the Rayleigh-Jeans portion of the Planck function leads to a maximum redshift estimate of $z\sim$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 $\mu$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 $\mu$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 $z=6.42$ quasar is presented. It is further shown that ice emission is needed to understand the 450 $\mu$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&

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

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