S. Guilloteau

Dynamical Masses of T Tauri Stars and Calibration of Pre-Main-Sequence Evolution

We have used the high sensitivity and resolution of the IRAM interferometer to produce subarcsecond 12CO J = 2-1 images of nine protoplanetary disks surrounding T Tauri stars in the Taurus-Auriga cloud (seven singles and two binaries). The images demonstrate the disks are in Keplerian rotation around their central stars. Using the least-square fit method described in the 1998 work by Guilloteau & Dutrey, we derive the disks properties, in particular its inclination angle and rotation velocity, hence the dynamical mass. Since the disk mass is usually small, this is a direct measurement of the stellar mass. Typically, we reach an internal precision of 10% in the determinations of stellar mass. The overall accuracy is limited by the uncertainty in the distance to a specific star. In a distance-independent way, we compare the derived masses with theoretical tracks of pre-main-sequence evolution. Combined with the mean distance to the Taurus region (140 pc), for stars with mass close to 1 M☉, our results tend to favor the tracks with cooler photospheres (higher masses for a given spectral type). We find that in UZ Tau E, the disk and the spectroscopic binary orbit appear to have different inclinations.

Probing the structure of protoplanetary disks: a comparative study of DM Tau, LkCa 15, and MWC 480

Context. The physical structure of proto-planetary disks is not yet well constrained by current observations. Millimeter interferometry is an essential tool to investigate young disks. Aims. We study the vertical and radial temperature distribution in a few well-known disks from an observational perspective. The surface density distribution of CO and HCO + and the scale-height are also investigated. Methods. We report CO observations at sub-arcsecond resolution with the IRAM array of the disks surrounding MWC 480, LkCa 15, and DM Tau, and simultaneous measurements of HCO + J = 1 → 0. To derive the disk properties, we fit a standard disk model in which all parameters are power laws of the distance to the star to the data. Possible biases associated with the method are detailed and explained. We compare the properties of the observed disks with similar objects. Results. We find evidence for a vertical temperature gradient in the disks of MWC 480 and DM Tau, as in AB Aur, but not in LkCa 15. The disk temperatures increase with stellar effective temperature. Except for AB Aur, the bulk of the CO gas is at temperatures smaller than 17 K, below the condensation temperature on grains. We find the scale height of the CO distribution to be larger (by 50%) than the expected hydrostatic scale height. The total amount of CO and the isotopologue ratio depends globally on the star. The more UV luminous objects appear to have more CO, but there is no simple dependency. The [ 13 CO]/[HCO + ] ratio is ∼600, with substantial variations between sources, and with radius. The temperature behavior is consistent with expectations, but published chemical models have difficulty reproducing the observed CO quantities. Changes in the slope of the surface density distribution of CO, compared to the continuum emission, suggest a more complex surface density distribution than is usually assumed in models. Vertical mixing seems an important chemical agent, as does photo-dissociation by the ambient UV radiation at the disk’s outer edge.

A dual-frequency sub-arcsecond study of proto-planetary disks at mm wavelengths: first evidence for radial variations of the dust properties

Context. Proto-planetary disks are thought to provide the initial environment for planetary system formation. The dust and gas distribution and its evolution with time is one of the key elements in the process. Aims. We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view, and, when possible, in a model-independent way, by using parametric laws. Methods. We used the IRAM PdBI interferometer to provide very high angular resolution (down to 0.4 �� in some sources) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region. The sample includes single and multiple systems, with a total of 23 individual disks. We used track-sharing observing mode to minimize the biases. We fitted these data with two kinds of models: a “truncated power law” model and a model presenting an exponential decay at the disk edge (“viscous” model). Results. Direct evidence for tidal truncation is found in the multiple systems. The temperature of the mm-emitting dust is constrained in a few systems. Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index β. In most disks that are sufficiently resolved at two different wavelengths, we find a radial dependence of β, which appears to increase from low values (as low as 0) at the center to about 1.7−2 at the disk edge. The same behavior could apply to all studied disks. It introduces further ambiguities in interpreting the brightness profile, because the regions with apparent β ≈ 0 can also be interpreted as being optically thick when their brightness temperature is high enough. Despite the added uncertainty on the dust absorption coefficient, the characteristic size of the disk appears to increase with a higher estimated star age. Conclusions. These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks. Constraints of the surface density distributions and their evolution remain ambiguous because of a degeneracy with the β(r )l aw.

Molecular gas and dust around a radio-quiet quasar at redshift 4.69

GALAXIES are believed to have formed a large proportion of their stars in giant bursts of star formation early in their lives, but when and how this took place are still very uncertain. The presence1–6 of large amounts of dust in quasars and radio galaxies at redshifts z > 4 shows that some synthesis of heavy elements had already occurred at this time. This implies that molecular gas—the building material of stars—should also be present, as it is in galaxies at lower redshifts (z ≈ 2.5, refs 7–10). Here we report the detection of emission from dust and carbon monoxide in the radio-quiet quasar BR1202 – 0725, at redshift z = 4.69. Maps of these emissions reveal two objects, separated by a few arc seconds, which could indicate either the presence of a companion to the quasar or gravitational lensing of the quasar itself. Regardless of the precise interpretation of the maps, the detection of carbon monoxide confirms the presence of a large mass of molecular gas in one of the most distant galaxies known, and shows that conditions conducive to huge bursts of star formation existed in the very early Universe.

Resolving the inner dust disks surrounding LkCa 15 and MWC 480 at mm wavelengths

Aims. We constrain the dust distribution and its properties (temperature, emissivity) in inner proto-planetary disks Methods. We performed sub-arcsecond high-sensitivity interferometric observations of the thermal dust emission at 1.4 mm and 2.8 mm in the disks surrounding LkCa 15 and MWC 480, with the new 750 m baselines of the IRAM PdBI array. This provides a linear resolution of ∼60 AU at the distance of Taurus. Results. We report the existence of a cavity of ∼50 AU radius in the inner disk of LkCa 15. Whereas LkCa 15 emission is optically thin, the optically thick core of MWC 480 is resolved at 1.4 mm with a radius of ∼35 AU, constraining the dust temperature. In MWC 480, the dust emission is coming from a colder layer than the CO emission, most likely the disk mid-plane. Conclusions. These observations provide direct evidence of an inner cavity around LkCa 15. Such a cavity most probably results from the tidal disturbance created by a low-mass companion or large planet at ∼30 AU from the star. These results suggest that planetary system formation is already at work in LkCa 15. They also indicate that the classical steady-state viscous disk model is too simplistic a description of the inner 50 AU of “proto-planetary” disks and that the disk evolution is coupled to the planet formation process. The MWC 480 results indicate that a proper estimate of the dust temperature and size of the optically thick core are essential for determining the dust emissivity index β.

Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. The large scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial motions. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motions indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.

Oxygen depletion in dense molecular clouds: a clue to a low O2 abundance?

Context. Dark cloud chemical models usually predict large amounts of O2, often above observational limits. Aims. We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion. Methods. We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts. Results. The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H + + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10 −4 . Conclusions. The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.

Disks around CQ Tauri and MWC 758: dense PDR or gas dispersal?

The Herbig Ae stars are the massive analogs of the TTauri stars. Very few disks surrounding these kind of stars have been studied in detail. To better constraint the disks parameters (temperature and density) we observed the disks around CQ Tau and MWC 758 with the IRAM array in continuum and CO line emissions. The disks properties are derived using a standard parametric model. The two sources show a surprising low CO abundance (assuming a standard gas-to-dust ratio). We use the Meudon PDR code to study the chemistry. For CQ Tau we find that photodissociation of CO is a viable mechanism to explain the CO depletion without modifying the gas-to-dust ratio. However, we find in both sources that the temperature of large grains can be low enough to prevent CO from being released from the grain surfaces. In addition the low inclination of the CQ Tau disk challenges the UX Ori classification of this star. We conclude that CO does not appear as a direct tracer of the gas-to-dust ratio.

Cold CO in circumstellar disks - On the effects of photodesorption and vertical mixing

Aims. We attempt to understand the presence of gas phase CO below its sublimation temperature in circumstellar disks. We study two promising mechanisms to explain this phenomenon: turbulent mixing and photodesorption. Methods. We compute the chemical evolution of circumstellar disks including grain surface reactions with and without turbulent mixing and CO photodesorption. Results. We show that photodesorption significantly enhances the gas phase CO abundance, by extracting CO from the grains when the visual extinction remains below about 5 magnitudes. However, the resulting dependence of column density on radial distance is inconsistent with observations so far. We propose that this inconsistency could be the result of grain growth. On the other hand, the influence of turbulent mixing is not found to be straightf orward. The effi ciency of turbulent mixing depends upon a variety of parameters, including the disk structure. For the set of par ameters we chose, turbulent mixing is not found to have any significant influence on the CO column density.

Chemistry in disks. I. Deep search for N2H+ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tauri

Aims.To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N2H+ in three disks surrounding DM Tau, LkCa 15, and MWC 480. Methods: We used the IRAM PdBI array to observe the N2H+ J=1-0 line and applied a ?^2-minimization technique to estimate corresponding column densities. These values are compared, together with HCO+ column densities, to results of a steady-state disk model with a vertical temperature gradient coupled to gas-grain chemistry. Results: We report two N2H+ detections for LkCa 15 and DM Tau at the 5 ? level and an upper limit for MWC 480. The column density derived from the data for LkCa 15 is much lower than previously reported. The [ N2H^+/HCO^+] ratio is on the order of 0.02-0.03. So far, HCO+ remains the most abundant observed molecular ion in disks. Conclusions: .All the observed values generally agree with the modelled column densities of disks at an evolutionary stage of a few million years (within the uncertainty limits), but the radial distribution of the molecules is not reproduced well. The low inferred concentration of N2H+ in three disks around low-mass and intermediate-mass young stars implies that this ion is not a sensitive tracer of the overall disk ionization fraction. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Research partially supported by PCMI, the French national program for the Physics and Chemistry of the Interstellar Medium.