arXiv subjects

Ewine F. van Dishoeck

Publications and source records attributed to Ewine F. van Dishoeck.

At least 271 records · Page 15Linked to original sources

arXiv2010

The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. IV. NH3 and CH3OH

NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and HCOOCH3. Despite a number of recent studies, little is known about their abundances in the solid state. (...) In this work, we investigate the ~ 8-10 micron region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 micron silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9 micron in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 nu_2 umbrella mode, and derive abundances with respect to water between ~2 and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the nu_4 C-O stretch mode of this molecule at ~9.7 micron in 16 objects, yielding abundances consistent with those derived by Boogert et al. 2008 (hereafter paper I) based on a simultaneous 9.75 and 3.53 micron data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10-20 % of nitrogen is locked up in known ices.

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arXiv2010

A cold complex chemistry toward the low-mass protostar B1-b: evidence for complex molecule production in ices

Gas-phase complex organic molecules have been detected toward a range of high- and low-mass star-forming regions at abundances which cannot be explained by any known gas-phase chemistry. Recent laboratory experiments show that UV irradiation of CH3OH-rich ices may be an important mechanism for producing complex molecules and releasing them into the gas-phase. To test this ice formation scenario we mapped the B1-b dust core and nearby protostar in CH3OH gas using the IRAM 30m telescope to identify locations of efficient non-thermal ice desorption. We find three CH3OH abundance peaks tracing two outflows and a quiescent region on the side of the core facing the protostar. The CH3OH gas has a rotational temperature of ~10 K at all locations. The quiescent CH3OH abundance peak and one outflow position were searched for complex molecules. Narrow, 0.6-0.8 km s-1 wide, HCOOCH3 and CH3CHO lines originating in cold gas are clearly detected, CH3OCH3 is tentatively detected and C2H5OH and HOCH2CHO are undetected toward the quiescent core, while no complex molecular lines were found toward the outflow. The core abundances with respect to CH3OH are ~2.3% and 1.1% for HCOOCH3 and CH3CHO, respectively, and the upper limits are 0.7-1.1%, which is similar to most other low-mass sources. The observed complex molecule characteristics toward B1-b and the pre-dominance of HCO-bearing species suggest a cold ice (below 25 K, the sublimation temperature of CO) formation pathway followed by non-thermal desorption through e.g. UV photons traveling through outflow cavities. The observed complex gas composition together with the lack of any evidence of warm gas-phase chemistry provide clear evidence of efficient complex molecule formation in cold interstellar ices.

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arXiv2010

Grain growth across protoplanetary discs: 10-micron silicate feature versus millimetre slope

Young stars are formed within dusty discs. The grains in the disc are originally of the same size as interstellar dust. Models predict that these grains will grow in size through coagulation. Observations of the silicate features at micron wavelengths are consistent with growth to micron sizes whereas the slope of the SED at longer wavelengths traces growth up to mm sizes. We here look for a correlation between these two grain growth indicators. A large sample of T-Tauri and Herbig-Ae/Be stars was observed with the Spitzer Space Telescope at 5-13 micron; a subsample was observed at mm wavelengths. We complement this subsample with data from the literature to maximise the overlap between micron and mm observations and search for correlations. Synthetic spectra are produced to determine which processes may produce the dust evolution. Dust disc masses in the range <1 to 7 x 10^-4 MSun are obtained. Most sources have a mm spectral slope consistent with grain growth. There is a tentative correlation between the 10-micron silicate feature and the mm slope of the SED. The observed sources seem to be grouped per star-forming region in the micron-vs-mm diagram. The modelling results show that the 10-micron feature becomes flatter and subsequently the mm slope becomes shallower. Grain size distributions shallower than that of the ISM and/or bright central stars are required to explain specific features. Settling of larger grains towards the disc midplane affects the 10-micron feature, but hardly the mm slope. The tentative correlation between the strength of the 10-micron feature and the mm slope suggests that the inner and outer disc evolve simultaneously. Dust with a mass dominated by mm-sized grains is required to explain the shallowest mm slopes. Other processes besides grain growth may also be responsible for the removal of small grains.

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arXiv2010

A Spitzer Survey of Protoplanetary Disk Dust in the Young Serpens Cloud: How do Dust Characteristics Evolve with Time?

We present Spitzer IRS mid-infrared (5-35 micron) spectra of a complete flux-limited sample (> 3 mJy at 8 micron) of young stellar object (YSO) candidates selected on the basis of their infrared colors in the Serpens Molecular Cloud. Spectra of 147 sources are presented and classified. Background stars (with slope consistent with a reddened stellar spectrum and silicate features in absorption), galaxies (with redshifted PAH features) and a planetary nebula (with high ionization lines) amount to 22% of contamination in this sample, leaving 115 true YSOs. Sources with rising spectra and ice absorption features, classified as embedded Stage I protostars, amount to 18% of the sample. The remaining 82% (94) of the disk sources are analyzed in terms of spectral energy distribution shapes, PAHs and silicate features. The presence, strength and shape of these silicate features are used to infer disk properties for these systems. About 8% of the disks have 30/13 micron flux ratios consistent with cold disks with inner holes or gaps, and 3% of the disks show PAH emission. Comparison with models indicates that dust grains in the surface of these disks have sizes of at least a few μ\m. The 20 micron silicate feature is sometimes seen in absence of the 10 micron feature, which may be indicative of very small holes in these disks. No significant difference is found in the distribution of silicate feature shapes and strengths between sources in clusters and in the field. Moreover, the results in Serpens are compared with other well-studied samples: the c2d IRS sample distributed over 5 clouds and a large sample of disks in the Taurus star-forming region. The remarkably similar distributions of silicate feature characteristics in samples with different environment and median ages - if significant - imply that the dust population in the disk surface results from an equilibrium between dust growth and destructive collision processes that are maintained over a few million years for any YSO population irrespective of environment.

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arXiv2010

Water Vapor in the Inner 25 AU of a Young Disk around a Low-Mass Protostar

Water is one of the key molecules in the physical and chemical evolution of star- and planet-forming regions. We here report the first spatially resolved observation of thermal emission of (an isotopologue of) water with the Plateau de Bure Interferometer toward the deeply embedded Class 0 protostar NGC 1333-IRAS4B. The observations of the H2-18-O 3_13-2_20 transition at 203.4 GHz resolve the emission of water toward this source with an extent of about 0.2" corresponding to the inner 25 AU (radius). The H2-18-O emission reveals a tentative velocity gradient perpendicular to the extent of the protostellar outflow/jet probed by observations of CO rotational transitions and water masers. The line is narrow, about 1 km/s (FWHM), significantly less than what would be expected for emission from an infalling envelope or accretion shock, but consistent with emission from a disk seen at a low inclination angle. The water column density inferred from these data suggests that the water emitting gas is a thin warm layer containing about 25 M_Earth of material, 0.03% of the total disk mass traced by continuum observations.

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arXiv2009

Sun-bathing around low-mass protostars: APEX-CHAMP+ observations of high-J CO

We present the first maps of high-excitation CO J=6-5 and 7-6 and isotopologue lines over 2'-5' regions at 10" resolution toward low-mass protostars to probe the origin of the warm gas in their surroundings. The data were obtained using the CHAMP+ 650/850 GHz heterodyne array receiver on APEX. Surprisingly strong quiescent extended narrow-line high-J 12CO 6-5 and 7-6 emission is seen toward all protostars, suggesting that heating by UV photons along the outflow cavity dominates the emission. At the source position itself, passive heating of the collapsing inner envelope by the luminosity of the source also contributes. The UV photons are generally not energetic enough to dissociate CO since the [C I] 2-1 emission, also probed by our data, is weak except at the bow shock at the tip of the outflow. The extended UV radiation is produced by the star-disk boundary layer as well as the jet- and bow-shocks, and will also affect the chemistry of species such as H2O and HCN around the outflow axis. Shock-heated warm gas characterized by broad CO line profiles is seen only toward the more massive Class 0 outflows. Outflow temperatures, estimated from the CO 6-5/3-2 line wing ratios, are ~100 K. These data illustrate the importance of getting spatial information to characterize the physical processes in YSO surroundings, important to interpret future Herschel and ALMA data.

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arXiv2009

PROSAC: A Submillimeter Array survey of low-mass protostars. II. The mass evolution of envelopes, disks, and stars from the Class 0 through I stages

The key question about early protostellar evolution is how matter is accreted from the large-scale molecular cloud, through the circumstellar disk onto the central star. A sample of 20 Class 0 and I protostars has been observed in continuum at (sub)millimeter wavelengths at high angular resolution with the Submillimeter Array. Using detailed dust radiative transfer models, we have developed a framework for disentangling the continuum emission from the envelopes and disks, and from that estimated their masses. For the Class I sources in the sample, HCO+ 3-2 line emission has furthermore been observed with the Submillimeter Array. Four of these sources show signs of Keplerian rotation, constraining the masses of the central stars. Both Class 0 and I protostars are surrounded by disks with typical masses of about 0.05 M_sun. No evidence is found for a correlation between the disk mass and evolutionary stage of the young stellar objects. This contrasts the envelope mass, which decreases sharply from 1 M_sun in the Class 0 stage to <0.1 M_sun in the Class I stage. Typically, the disks have masses that are 1-10% of the corresponding envelope masses in the Class 0 stage and 20-60% in the Class I stage. For the Class I sources for which Keplerian rotation is seen, the central stars contain 70-98% of the total mass in the star-disk-envelope system, confirming that these objects are late in their evolution through the embedded protostellar stages. Theoretical models tend to overestimate the disk masses relative to the stellar masses in the late Class I stage. The results argue in favor of a picture in which circumstellar disks are formed early during the protostellar evolution and rapidly process material accreted from the larger scale envelope onto the central star.

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arXiv2009

Quantification of segregation dynamics in ice mixtures

(Abridged) The observed presence of pure CO2 ice in protostellar envelopes is attributed to thermally induced ice segregation, but a lack of quantitative experimental data has prevented its use as a temperature probe. Quantitative segregation studies are also needed to characterize diffusion in ices, which underpins all ice dynamics and ice chemistry. This study aims to quantify the segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures covering a range of astrophysically relevant ice thicknesses and mixture ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions. Segregation is then monitored at 23-70 K as a function of time, through infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate sequentially through surface processes, followed by an order of magnitude slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk process. The thick ices must therefore be either more porous or segregate through a different mechanism, e.g. a phase transition. The segregation dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations of surface hopping and pair swapping. The experimentally determined surface-segregation rates for all mixture ratios follow the Ahrrenius law with a barrier of 1080[190] K for H2O:CO2 and 300[100] K for H2O:CO mixtures. During low-mass star formation H2O:CO2 segregation will be important already at 30[5] K. Both surface and bulk segregation is proposed to be a general feature of ice mixtures when the average bond strengths of the mixture constituents in pure ice exceeds the average bond strength in the ice mixture.

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arXiv2009

High-precision C17O, C18O and C16O measurements in young stellar objects: analogues for CO self-shielding in the early solar system

Using very high resolution (lambda / Dlambda ~ 95000) 4.7 micron fundamental and 2.3 micron overtone ro-vibrational CO absorption spectra obtained with the CRIRES infrared spectrometer on the Very Large Telescope (VLT), we report detections of four CO isotopologues -- C16O, 13CO, C18O and the rare species, C17O -- in the circumstellar environment of two young protostars, VV CrA and Reipurth 50. We argue that the observed CO absorption lines probe a protoplanetary disk in VV CrA, and a protostellar envelope in Reipurth 50. All CO line profiles are spectrally resolved, permitting direct calculation of CO oxygen isotopologue ratios with 5-10% accuracy. The ro-vibrational level populations for all species can be reproduced by assuming that CO absorption arises in two temperature regimes. For both objects, 12C/13C are on the order of 100, nearly twice the expected interstellar medium (ISM) ratio. The derived oxygen abundance ratios for the VV CrA disk show a significant mass-independent deficit of C17O and C18O relative to C16O compared to ISM baseline abundances. The Reipurth 50 envelope shows no clear differences in oxygen CO isotopologue ratios compared with the local ISM. A mass-independent fractionation can be interpreted as being due to selective photodissociation of CO in the disk surface due to self-shielding. The deficits in C17O$ and C18O$ in the VV CrA protoplanetary disk are consistent with an analogous origin of the 16O variability in the solar system by isotope selective photodissociation, confirmation of which may be obtained via study of additional sources. The higher fractionation observed for the VV CrA disk compared with the Reipurth 50 envelope is likely due to a combination of disk geometry, grain growth, and vertical mixing processes. [Abstract abridged]

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arXiv2009

Multidimensional chemical modelling, II. Irradiated outflow walls

Observations of the high-mass star forming region AFGL 2591 reveal a large abundance of CO+, a molecule known to be enhanced by far UV (FUV) and X-ray irradiation. In chemical models assuming a spherically symmetric envelope, the volume of gas irradiated by protostellar FUV radiation is very small due to the high extinction by dust. The abundance of CO+ is thus underpredicted by orders of magnitude. In a more realistic model, FUV photons can escape through an outflow region and irradiate gas at the border to the envelope. Thus, we introduce the first 2D axi-symmetric chemical model of the envelope of a high-mass star forming region to explain the CO+ observations as a prototypical FUV tracer. The model assumes an axi-symmetric power-law density structure with a cavity due to the outflow. The local FUV flux is calculated by a Monte Carlo radiative transfer code taking scattering on dust into account. A grid of precalculated chemical abundances, introduced in the first part of this series of papers, is used to quickly interpolate chemical abundances. This approach allows to calculate the temperature structure of the FUV heated outflow walls self-consistently with the chemistry. Synthetic maps of the line flux are calculated using a raytracer code. Single-dish and interferometric observations are simulated and the model results are compared to published and new JCMT and SMA observations. The two-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dish observations and also explains the non-detection by the SMA. We conclude that the observed CO+ line flux and its narrow width can be interpreted by emission from the warm and dense outflow walls irradiated by protostellar FUV radiation.

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arXiv2009

Photodesorption of ices I: CO, N2 and CO2

A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. This paper aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO:N2 ices is explored. The ice photodesorption is studied experimentally under ultra high vacuum conditions and at 15-60 K using a hydrogen discharge lamp (7-10.5 eV). The ice desorption during irradiation is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Both the UV photodesorption yields per incident photon and the photodesorption mechanisms are molecule specific. CO photodesorbs without dissociation from the surface layer of the ice. N2, which lacks an electronic transition in this wavelength range, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with or layered on top of CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15-18 K) the derived photodesorption yields are 2.7x10^-3 and <2x10^-4 molecules photon-1 for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2x10^-3x(1-e^(-X/2.9)) + 1.1x10^-3x(1-e^(-X/4.6)) molecules photon-1, where X is the ice thickness in monolayers and the two parts of the expression represent a CO2 and CO photodesorption pathway.

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arXiv2009

Cold gas as an ice diagnostic toward low mass protostars

Up to 90% of the chemical reactions during star formation occurs on ice surfaces, probably including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes. For that purpose the IRAM 30m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined. The observed gas and ice abundances span several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept.

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arXiv2008

Photodesorption of Ices II: H2O and D2O

Gaseous H2O has been detected in several cold astrophysical environments, where the observed abundances cannot be explained by thermal desorption of H2O ice or by H2O gas phase formation. These observations hence suggest an efficient non-thermal ice desorption mechanism. Here, we present experimentally determined UV photodesorption yields of H2O and D2O ice and deduce their photodesorption mechanism. The ice photodesorption is studied under ultra high vacuum conditions and at astrochemically relevant temperatures (18-100 K) using a hydrogen discharge lamp (7-10.5 eV), which simulates the interstellar UV field. The ice desorption during irradiation is monitored using reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed species. The photodesorption yield per incident photon is identical for H2O and D2O and depends on both ice thickness and temperature. For ices thicker than 8 monolayers the photodesorption yield Y is linearly dependent on temperature due to increased diffusion of ice species such that Y(T) = 1E-3(1.3+0.032*T) UV photon-1, with a 60% uncertainty for the absolute yield. The increased diffusion also results in an increasing H2O:OH desorption product ratio with temperature. The yield does not depend on the substrate, the UV photon flux or the UV fluence. The yield is also independent on the initial ice structure since UV photons efficiently amorphize H2O ice. The results are consistent with theoretical predictions of H2O photodesorption and partly in agreement with a previous experimental study. Applying the experimentally determined yield to a Herbig Ae/Be star+disk model shows that UV photodesorption of ices increases the H2O content by orders of magnitude in the disk surface region compared to models where non-thermal desorption is ignored.

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arXiv2008

Detection of CI in absorption towards PKS 1830-211 with the eSMA

We report the first science observations and results obtained with the "extended" SMA (eSMA), which is composed of the SMA (Submillimeter Array), JCMT (James Clerk Maxwell Telescope) and CSO (Caltech Submillimeter Observatory). Redshifted absorptions at z=0.886 of CI (^3P_1 - ^3P_0) were observed with the eSMA with an angular resolution of 0.55"x0.22" at 1.1 mm toward the southwestern image of the remarkable lensed quasar PKS 1830-211, but not toward the northeastern component at a separation of ~1". Additionally, SMA observations of CO, 13CO and C18O (all J=4-3) were obtained toward this object: CO was also detected toward the SW component, but none of the isotopologues were. This is the first time [CI] is detected in this object, allowing the first direct determination of relative abundances of neutral atomic carbon to CO in the molecular clouds of a spiral galaxy at z>0.1. The [CI] and CO profiles can be decomposed into two and three velocity components respectively. We derive C/CO column density ratios ranging from <0.5 (representative of dense cores) to ~2.5 (close to translucent clouds values). This could indicate that we are seeing environments with different physical conditions or that we are witnessing chemical evolution of regions where C has not completely been converted into CO.

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arXiv2008

The Spitzer c2d Legacy Results: Star Formation Rates and Efficiencies; Evolution and Lifetimes

(Abridged) The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star formation rates and efficiencies, numbers and lifetimes for SED classes, and clustering properties. Current star formation efficiencies range from 3% to 6%. Taken together, the five clouds are producing about 260 solar masses of stars per Myr. The star formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, and the current stock of dense cores would be exhausted in 1.8 Myr on average. The derived lifetime for the Class I phase is 0.44 to 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.10 to 0.16 Myr, is longer than early estimates. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 solar mass per cubic pc. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 solar masses, given an efficiency of 0.3. Our data confirm and aggravate the "luminosity problem" for protostars. Our results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

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arXiv2008

Optical Characterization of a New Young Stellar Population in the Serpens Molecular Cloud

We report on the results of an optical spectroscopic survey designed to confirm the youth and determine the spectral types among a sample of young stellar object (YSO) candidates in the Serpens Molecular Cloud. We observed 150 infrared excess objects, previously discovered by the Spitzer Legacy Program "From Molecular Cores to Planet-Forming Disks" (c2d), bright enough for subsequent Spitzer/IRS spectroscopy. We obtained 78 optical spectra of sufficient S/N for analysis. Extinctions, effective temperatures and luminosities are estimated for this sample, and used to construct H-R diagrams for the population. We identified 20 background giants contaminating the sample, based on their relatively high extinction, position in the H-R diagram, the lack of Halpha emission and relatively low infrared excess. Such strong background contamination (25%) is consistent with the location of Serpens being close to the Galactic plane (5degrees Galactic latitude). The remaining 58 stars (75%) were all confirmed to be young, mostly K and M-type stars that are presumed to belong to the cloud. Individual ages and masses for the YSOs are inferred based on theoretical evolutionary models. The models indicate a spread in stellar ages from 1 to 15 Myr, peaking at 2 - 6 Myr, and a mass distribution of 0.2 to 1.2 Msun with median value around 0.8 Msun. Strong H emission lines (EW[Halpha] > 3 A) have been detected in more than half of the sample (35 stars). The mass accretion rates as derived from the H line widths span a broad distribution over 4 orders of magnitude with median accretion rate of 10^-8 Msun/yr. Our analysis shows that the majority of the infrared excess objects detected in Serpens are actively accreting, young T-Tauri stars.

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arXiv2008

The eSMA: description and first results

The eSMA ("extended SMA") combines the SMA, JCMT and CSO into a single facility, providing enhanced sensitivity and spatial resolution owing to the increased collecting area at the longest baselines. Until ALMA early science observing (2011), the eSMA will be the facility capable of the highest angular resolution observations at 345 GHz. The gain in sensitivity and resolution will bring new insights in a variety of fields, such as protoplanetary/transition disks, high-mass star formation, solar system bodies, nearby and high-z galaxies. Therefore the eSMA is an important facility to prepare the grounds for ALMA and train scientists in the techniques. Over the last two years, and especially since November 2006, there has been substantial progress toward making the eSMA into a working interferometer. In particular, (i) new 345-GHz receivers, that match the capabilities of the SMA system, were installed at the JCMT and CSO; (ii) numerous tests have been performed for receiver, correlator and baseline calibrations in order to determine and take into account the effects arising from the differences between the three types of antennas; (iii) first fringes at 345 GHz were obtained on August 30 2007, and the array has entered the science-verification stage. We report on the characteristics of the eSMA and its measured performance at 230 GHz and that expected at 345 GHz. We also present the results of the commissioning and some initial science-verification observations, including the first absorption measurement of the C/CO ratio in a galaxy at z=0.89, located along the line of sight to the lensed quasar PKS1830-211, and on the imaging of the vibrationally excited HCN line towards IRC+10216.

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arXiv2008

Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps

We present velocity-resolved spectro-astrometric imaging of the 4.7 $μ$m rovibrational lines of CO gas in protoplanetary disks using the CRIRES high resolution infrared spectrometer on the Very Large Telescope (VLT). The method as applied to three disks with known dust gaps or inner holes out to 4-45 AU (SR 21, HD 135344B and TW Hya) achieves an unprecedented spatial resolution of $0.1-0.5$ AU. While one possible gap formation mechanism is dynamical clearing by giant planets, other equally good explanations (stellar companions, grain growth, photo-evaporation) exist. One way of distinguishing between different scenarios is the presence and distribution of gas inside the dust gaps. Keplerian disk models are fit to the spectro-astrometric position-velocity curves to derive geometrical parameters of the molecular gas. We determine the position angles and inclinations of the inner disks with accuracies as good as 1-2\degr, as well as the radial extent of the gas emission. Molecular gas is detected well inside the dust gaps in all three disks. The gas emission extends to within a radius of 0.5 AU for HD 135344B and to 0.1 AU for TW Hya, supporting partial clearing by a $< 1-10 M_{\rm Jup}$ planetary body as the cause of the observed dust gaps, or removal of the dust by extensive grain coagulation and planetesimal formation. The molecular gas emission in SR 21 appears to be truncated within $\sim 7 $AU, which may be caused by complete dynamical clearing by a more massive companion. We find a smaller inclination angle of the inner disk of TW Hya than that determined for the outer disk, suggestive of a disk warp. We also detect significant azimuthal asymmetries in the SR 21 and HD 135344B inner disks.

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