Person:

Oberg, Karin

Infrared photometry and spectroscopy (1-25 um) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H2O ices form at extinctions of Ak=0.25+/-0.07 mag (Av=2.1+/-0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H2O ice (2.3+/-0.1*10^-5 relative to Nh) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH3OH abundance (<3-8% relative to H2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 um) continuum extinction relative to Ak increases as a function of Ak. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to Rv~ 3.5 (Ak=0.71) and Rv~5.0 (Ak=1.47). For lines of sight with Ak>1.0 mag, the tau9.7/Ak ratio is a factor of 2 lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the tau9.7/Ak ratio. This process is likely related to grain growth by coagulation, as traced by the A7.4/Ak continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.

Complex molecules have been broadly classified into three generations dependent on the mode of formation and the required formation temperature (<25, 25-100 K, and >100 K). Around massive young stellar objects (MYSOs), icy grain mantles and gas are exposed to increasingly higher temperatures as material accretes from the outer envelope in toward the central hot region. The combination of this temperature profile and the generational chemistry should result in a changing complex molecular composition with radius around MYSOs. We combine IRAM 30 m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection. We find that emission from N-bearing organics and CH3OH present substantial increases in emission around 8000 AU and R < 3000 AU, while unsaturated O-bearing molecules and hydrocarbons do not. The increase in line flux for some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for many of the same molecules at R < 3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase reactions drive the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used together with an adapted gas-grain astrochemical model to constrain the chemical generation(s) to which different classes of molecules belong.

Interstellar complex organic molecules were first identified in the hot inner regions of massive young stellar objects (MYSOs), but have more recently been found in many colder sources, indicating that complex molecules can form at a range of temperatures. Individually these observations provide limited constraints, however, on how complex molecules form, and whether the same formation pathways dominate in cold, warm and hot environments. To address these questions, we use spatially resolved observations from the Submillimeter Array of three MYSOs together with mostly unresolved literature data to explore how molecular ratios depend on environmental parameters, especially temperature. Toward the threeMYSOs, we find multiple complex organic emission peaks characterized by different molecular compositions and temperatures. In particular, CH3CCH and CH3CN seem to always trace a luke-warm (T∼60 K) and a hot (T>100 K) complex chemistry, respectively. These spatial trends are consistent with abundance-temperature correlations of four representative complex organics – CH3CCH, CH3CN, CH3OCH3 and CH3CHO – in a large sample of complexmolecule hostsmined fromthe literature. Together these results indicate a general chemical evolution with temperature, i.e. that new complex molecule formation pathways are activated as a MYSO heats up. This is qualitatively consistent with model predictions. Furthermore, these results suggest that ratios of complex molecules may be developed into a powerful probe of the evolutionary stage of a MYSO, as well as provide information about its formation history.

From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth–Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3–10 million years) star TW Hydrae, for which the range is 0.0005–0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.

Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: (N_2) and (O_2) are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure (N_2) and (O_2) thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic (N_2) and (O_2) isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: (N_2) photodesorption mainly occurs through excitation of the (b^1\sqcap_u) state and subsequent desorption of surface molecules. The observed vibronic structure in the (N_2) photodesorption spectrum, together with the absence of (N_3) formation, supports that the photodesorption mechanism of (N_2) is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, (O_2) photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of (N_2) and (O_2) integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between (10^{-3}) and (10^{-2}) photodesorbed molecules per incoming photon.

We present Submillimeter Array observations of H2CO and N2H+ emission in the disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 at 2"-6" resolution and discuss the distribution of these species with respect to CO freeze-out. The H2CO and N2H+ emission toward HD 163296 does not peak at the continuum emission center that marks the stellar position but is instead significantly offset. Using a previously developed model for the physical structure of this disk, we show that the H2CO observations are reproduced if H2CO is present predominantly in the cold outer disk regions. A model where H2CO is present only beyond the CO snow line (estimated at a radius of 160 AU) matches the observations well. We also show that the average H2CO excitation temperature, calculated from two transitions of H2CO observed in these two disks and a larger sample of disks around T Tauri stars in the DISCS (the Disk Imaging Survey of Chemistry with SMA) program, is consistent with the CO freeze-out temperature of 20 K. In addition, we show that N2H+ and H2CO line fluxes in disks are strongly correlated, indicative of co-formation of these species across the sample. Taken together, these results imply that H2CO and N2H+ are generally present in disks only at low temperatures where CO depletes onto grains, consistent with fast destruction of N2H+ by gas-phase CO, and in situ formation of H2CO through hydrogenation of CO ice. In this scenario H2CO, CH3OH and N2H+ emission in disks should appear as rings with the inner edge at the CO midplane snow line. This prediction can be tested directly using observations from ALMA with higher resolution and better sensitivity.

We report the first detection of c-C3H2 in a circumstellar disk. The c-C3H2 J = 6-5 line (217.882 GHz) is detected and imaged through Atacama Large Millimeter Array (ALMA) Science Verification observations toward the disk around the Herbig Ae star HD 163296 at 0.''8 resolution. The emission is consistent with that arising from a Keplerian rotating disk. Two additional c-C3H2 transitions are also tentatively detected, bolstering the identification of this species, but with insufficient signal-to-noise ratio to constrain the spatial distribution. Using a previously developed model for the physical structure of this disk, we fit a radial power-law distribution model to the c-C3H2 6-5 emission and find that c-C3H2 is present in a ring structure from an inner radius of about 30 AU to an outer radius of about 165 AU. The column density is estimated to be 1012-1013 cm–2. The clear detection and intriguing ring structure suggest that c-C3H2 has the potential to become a useful probe of radiation penetration in disks.

Planets form in the disks around young stars. Their formation efficiency and composition are intimately linked to the protoplanetary disk locations of “snow lines” of abundant volatiles. We present chemical imaging of the carbon monoxide (CO) snow line in the disk around TW Hya, an analog of the solar nebula, using high spatial and spectral resolution Atacama Large Millimeter/Submillimeter Array observations of diazenylium (N2H+), a reactive ion present in large abundance only where CO is frozen out. The N2H+ emission is distributed in a large ring, with an inner radius that matches CO snow line model predictions. The extracted CO snow line radius of ∼30 astronomical units helps to assess models of the formation dynamics of the solar system, when combined with measurements of the bulk composition of planets and comets.

Volatiles, especially CO, are important gas tracers of protoplanetary disks (PPDs). Freeze-out and sublimation processes determine their division between gas and solid phases, which affects both which disk regions can be traced by which volatiles, and the formation and composition of planets. Recently, multiple lines of evidence have suggested that CO is substantially depleted from the gas in the outer regions of PPDs, i.e., more depleted than would be expected from a simple balance between freeze-out and sublimation. In this paper, we show that the gas dynamics in the outer PPDs facilitates volatile depletion through turbulent diffusion. Using a simple 1D model that incorporates dust settling, turbulent diffusion of dust and volatiles, as well as volatile freeze-out/sublimation processes, we find that as long as turbulence in the cold midplane is sufficiently weak to allow a majority of the small grains to settle, CO in the warm surface layer can diffuse into the midplane region and deplete by freeze-out. The level of depletion sensitively depends on the level of disk turbulence. Based on recent disk simulations that suggest a layered turbulence profile with very weak midplane turbulence and strong turbulence at the disk surface, CO and other volatiles can be efficiently depleted by up to an order of magnitude over Myr timescales.