Chemical Evolution in Protostellar Envelopes: Cocoon Chemistry

et al. 2013

A&A

Context. Over the last few years, the chemistry of molecules other than CO in the planet-forming zones of disks is starting to be explored with Spitzer and high-resolution ground-based data. However, these studies have focused only on a few simple molecules. Aims. The aim of this study is to put observational constraints on the presence of more complex organic and sulfur-bearing molecules predicted to be abundant in chemical models of disks and to simulate high resolution spectra in view of future missions. Methods. High signal-to-noise ratio (S/N) Spitzer spectra of the near edge-on disks IRS 46 and GV Tau are used to search for midinfrared absorption bands of various molecules. These disks are good laboratories because absorption studies do not suffer from low line/continuum ratios that plague emission data. Simple local thermodynamic equilibrium (LTE) slab models are used to infer column densities (or upper limits) and excitation temperatures. Results. Mid-infrared bands of HCN, C 2 H 2 and CO 2 are clearly detected toward both sources. The HCN and C 2 H 2 absorption arises in warm gas with excitation temperatures of 400−700 K, whereas the CO 2 absorption originates in cooler gas of ∼250 K. Column densities and their ratios are comparable for the two sources. No other absorption features are detected at the 3σ level. Column density limits of the majority of molecules predicted to be abundant in the inner disk -C 2 H 4 , C 2 H 6 , C 6 H 6 , C 3 H 4 , C 4 H 2 , CH 3 , HNC, HC 3 N, CH 3 CN, NH 3 and SO 2 -are determined and compared with disk models. Conclusions. The inferred abundance ratios and limits with respect to C 2 H 2 and HCN are roughly consistent with models of the chemistry in high temperature gas. Models of UV irradiated disk surfaces generally agree better with the data than pure X-ray models. The limit on NH 3 /HCN implies that evaporation of NH 3 -containing ices is only a minor contributor. The inferred abundances and their limits also compare well with those found in comets, suggesting that part of the cometary material may derive from warm inner disk gas. The high resolution simulations show that future instruments on the James Webb Space Telescope (JWST), the Extremely Large Telescopes (ELTs), the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) can probe up to an order of magnitude lower abundance ratios and put important new constraints on the models, especially if pushed to high S/Ns.

Section: Warm Chemistrymentioning

confidence: 87%

Exploring organic chemistry in planet-forming zones

Bast

Lahuis

et al. 2013

A&A

“…This in contrast with high and intermediate mass stars (Lahuis & van Dishoeck 2000;Thi et al, in prep. ) where these molecules are characteristic of hot-core chemistry (Doty et al 2002;Rodgers & Charnley 2003). Toward ULIRGs absorption of C 2 H 2 , HCN and CO 2 has been observed by Lahuis et al (2007a) suggesting the presence of pressure confined massive star formation.…”

Section: Molecular Emission and Absorptionmentioning

confidence: 97%

c2dSpitzerIRS spectra of embedded low-mass young stars: gas-phase emission lines

Lahuis

Jørgensen

et al. 2010

A&A

Context. A survey of mid-infrared gas-phase emission lines of H 2 , H 2 O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer "Cores to Disks" (c2d) legacy program. Aims. The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks) and the physical processes (accretion, irradiation by UV and/or X-rays, excitation through slow and fast shocks) which take place. The mid-IR spectral range hosts a suite of diagnostic lines which can distinguish them. A key point is to spatially resolve the emission in the Spitzer-IRS spectra to separate extended PDR and shock emission from compact source emission associated with the circumstellar disk and jets.Methods. An optimal extraction method is used to separate both spatially unresolved (compact, up to a few hundred AU) and spatially resolved (extended, thousand AU or more) emission from the IRS spectra. The results are compared with the c2d disk sample and literature PDR and shock models to address the physical nature of the sources. Conclusions. The observed emission on ≥1000 AU scales is characteristic of PDR emission and likely originates in the outflow cavities in the remnant envelope created by the stellar wind and jets from the embedded young stars. Weak shocks along the outflow wall can also contribute. The compact emission is likely of mixed origin, comprised of optically thick circumstellar disk and/or jet/outflow emission from the protostellar object.

“…A key result from the pre-stellar core models is the depletion of many carbon-bearing species towards the centre of the core (Bergin & Langer 1997;Lee et al 2004). Ceccarelli et al (1996) modelled the chemistry in the collapse phase, and others have done so more recently (Rodgers & Charnley 2003;Doty et al 2004;Lee et al 2004;Garrod & Herbst 2006;Aikawa et al 2008;Garrod et al 2008). All of these models are one-dimensional, and thus necessarily ignore the circumstellar disk.…”

Section: Introductionmentioning

confidence: 99%

“…This can lead to further chemical enrichment, especially in the warmer parts of the disk (Aikawa et al 1997;Aikawa & Herbst 1999;Willacy & Langer 2000;van Zadelhoff et al 2003;Rodgers & Charnley 2003;Aikawa et al 2008). The interior of the disk is shielded from direct irradiation by the star, so it is colder than the disk's surface and the remnant cloud.…”

Section: Introductionmentioning

confidence: 99%

The chemical history of molecules in circumstellar disks

Visser

Doty

et al. 2009

A&A

226

319

Context. Many chemical changes occur during the collapse of a molecular cloud to form a low-mass star and the surrounding disk. One-dimensional models have been used so far to analyse these chemical processes, but they cannot properly describe the incorporation of material into disks. Aims. The goal of this work is to understand how material changes chemically as it is transported from the cloud to the star and the disk. Of special interest is the chemical history of the material in the disk at the end of the collapse. Methods. A two-dimensional, semi-analytical model is presented that, for the first time, follows the chemical evolution from the pre-stellar core to the protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H 2 O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps towards illustrating the full chemistry. Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ∼18 K. Water remains solid almost everywhere during the infall and disk formation phases and evaporates within ∼10 AU of the star. Mixed CO-H 2 O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet-and comet-forming zones of the disk (∼5−30 AU from the star) is predicted to spend enough time in a warm zone (several 10 4 yr at a dust temperature of 20−40 K) during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.