We report new dynamical masses for five pre-main sequence (PMS) stars in the L1495 region of the Taurus star-forming region (SFR) and six in the L1688 region of the Ophiuchus SFR. Since these regions ...have VLBA parallaxes, these are absolute measurements of the stars' masses and are independent of their effective temperatures and luminosities. Seven of the stars have masses , thus providing data in a mass range with little data, and of these, six are measured to precision . We find eight stars with masses in the range 0.09-1.1 that agree well with the current generation of PMS evolutionary models. The ages of the stars we measured in the Taurus SFR are in the range 1-3 Myr, and Myr for those in L1688. We also measured the dynamical masses of 14 stars in the ALMA archival data for Akeson & Jensen's Cycle 0 project on binaries in the Taurus SFR. We find that the masses of seven of the targets are so large that they cannot be reconciled with reported values of their luminosity and effective temperature. We suggest that these targets are themselves binaries or triples.
Aims. We aim to obtain a spatially resolved measurement of velocity dispersions in the disk of TW Hya. Methods. We obtained images with high spatial and spectral resolution of the CO J = 2–1, CN N = ...2–1 and CS J = 5–4 emission with ALMA in Cycle 2. The radial distribution of the turbulent broadening was derived with two direct methods and one modelling approach. The first method requires a single transition and derives Tex directly from the line profile, yielding a vturb. The second method assumes that two different molecules are co-spatial, which allows using their relative line widths for calculating Tkin and vturb. Finally we fitted a parametric disk model in which the physical properties of the disk are described by power laws, to compare our direct methods with previous values. Results. The two direct methods were limited to the outer r > 40 au disk because of beam smear. The direct method found vturb to range from ≈130 m s-1 at 40 au, and to drop to ≈50 m s-1 in the outer disk, which is qualitatively recovered with the parametric model fitting. This corresponds to roughly 0.2−0.4 cs. CN was found to exhibit strong non-local thermal equilibrium effects outside r ≈ 140 au, so that vturb was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed line widths only within r ≲ 100 au, within which vturb was found to drop from 100 m s-1 (≈0.4 cs) to zero at 100 au. The parametric model yielded a nearly constant 50 m s-1 for CS (0.2−0.4 cs). We demonstrate that absolute flux calibration is and will be the limiting factor in all studies of turbulence using a single molecule. Conclusions. The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require reaching an absolute calibration precision of about 3%, or finding a suitable combination of light and heavy molecules that are co-located in the disk.
We present ALMA Cycle 2 observations at 0 5 resolution of TW Hya of CS emission. The radial profile of the integrated line emission displays oscillatory features outward of 1 5 ( au). A dip-like ...feature at 1 6 is coincident in location, depth, and width with features observed in dust scattered light at near-infrared wavelengths. Using a thermochemical model indicative of TW Hya, gas-grain chemical modeling, and non-LTE radiative transfer, we demonstrate that such a feature can be reproduced with a surface density depression, consistent with the modeling performed for scattered-light observations of TW Hya. We further demonstrate that a gap in the dust distribution and dust opacity only cannot reproduce the observed CS feature. The outer enhancement at 3 1 is identified as a region of intensified desorption due to enhanced penetration of the interstellar far-UV radiation at the exponential edge of the disk surface density, which intensifies the photochemical processing of gas and ices.
Context. Determining the gas density and temperature structures of protoplanetary disks is a fundamental task in order to constrain planet formation theories. This is a challenging procedure and most ...determinations are based on model-dependent assumptions. Aims. We attempt a direct determination of the radial and vertical temperature structure of the Flying Saucer disk, thanks to its favorable inclination of 90 degrees. Methods. We present a method based on the tomographic study of an edge-on disk. Using ALMA, we observe at 0.5″ resolution the Flying Saucer in CO J = 2–1 and CS J = 5–4. This edge-on disk appears in silhouette against the CO J = 2–1 emission from background molecular clouds in ρ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity provide a direct measure of the gas temperature as a function of radius and height above the disk mid-plane. Results. The overall thermal structure is consistent with model predictions, with a cold (<12−15 K) CO-depleted mid-plane and a warmer disk atmosphere. However, we find evidence for CO gas along the mid-plane beyond a radius of about 200 au, coincident with a change of grain properties. Such behavior is expected in the case of efficient rise of UV penetration re-heating the disk and thus allowing CO thermal desorption or favoring direct CO photo-desorption. CO is also detected at up to 3–4 scale heights, while CS is confined to around 1 scale height above the mid-plane. The limits of the method due to finite spatial and spectral resolutions are also discussed. Conclusions. This method appears to be a very promising way to determine the gas structure of planet-forming disks, provided that the molecular data have an angular resolution which is high enough, on the order of 0.3−0.1″ at the distance of the nearest star-forming regions.
Context.
Grain surface chemistry is fundamental to the composition of protoplanetary disks around young stars.
Aims.
The temperature of grains depends on their size. We evaluate the impact of this ...temperature dependence on the disk chemistry.
Methods.
We modeled a moderately massive disk with 16 different grain sizes. We used the 3D Monte Carlo POLARIS code to calculate the dust grain temperatures and the local
uv
flux. We modeled the chemistry using the three-phase astrochemical code NAUTILUS. Photo processes were handled using frequency-dependent cross sections and a new method to account for self and mutual shielding. The multi-grain model outputs are compared to those of single-grain size models (0.1 μm); there are two different assumptions for their equivalent temperature.
Results.
We find that the Langmuir-Hinshelwood mechanism at equilibrium temperature is not efficient to form H
2
at 3–4 scale heights (
H
), and we adopt a parametric fit to a stochastic method to model H
2
formation instead. We find the molecular layer composition (1–3
H
) to depend on the amount of remaining H atoms. Differences in molecular surface densities between single and multi-grain models are mostly due to what occurs above 1.5
H
. At 100 au, models with colder grains produce H
2
O and CH
4
ices in the midplane, and those with warmer grains produce more CO
2
ices; both of these allow for an efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules production is enhanced by the presence of warmer grains in the multi-grain models. Using a single-grain model mimicking grain growth and dust settling fails to reproduce the complexity of gas-grain chemistry.
Conclusions.
Chemical models with a single-grain size are sensitive to the adopted grain temperature and cannot account for all expected effects. A spatial spread of the snowlines is expected to result from the ranges in grain temperature. The amplitude of the effects depends on the dust disk mass.
Aims. We attempt to determine the masses of single or multiple young T Tauri and HAeBe stars from the rotation of their Keplerian disks. Methods. We used the IRAM PdBI interferometer to perform ...arcsecond resolution images of the CN N = 2−1 transition with good spectral resolution. Integrated spectra from the 30 m radiotelescope show that CN is relatively unaffected by contamination from the molecular clouds. Our sample includes 12 sources, among which isolated stars like DM Tau and MWC 480 are used to demonstrate the method and its accuracy. We derive the dynamical mass by fitting a disk model to the emission, a process giving M/D, the mass-to-distance ratio. We also compare the CN results with higher resolution CO data, that are however affected by contamination. Results. All disks are found in nearly perfect Keplerian rotation. We determine accurate masses for 11 stars, in the mass range 0.5 to 1.9 M⊙. The remaining one, DG Tau B, is a deeply embedded object for which CN emission partially arises from the outflow. With previous determinations, this leads to 14 (single) stars with dynamical masses. Comparison with evolutionary tracks, in a distance independent modified HR diagram, show good overall agreement (with one exception, CW Tau), and indicate that measurement of effective temperatures are the limiting factor. The lack of low mass stars in the sample does not allow to distinguish between alternate tracks.
The accuracy of masses of pre-main-sequence stars derived from their locations on the Hertzsprung-Russell diagram (HRD) can be tested by comparison with accurate and precise masses determined ...independently. We present 29 single stars in the Taurus star-forming region (SFR) and 3 in the Ophiuchus SFR with masses measured dynamically to a precision of at least 10%. Our results include 9 updated mass determinations and 3 that have not had their dynamical masses published before. This list of stars with fundamental, dynamical masses, Mdyn, is drawn from a larger list of 39 targets in the Taurus SFR and 6 in the Ophiuchus SFR. Placing the stars with accurate and precise dynamical masses on HRDs that do not include internal magnetic fields underestimates the mass compared to Mdyn by about 30%. Placing them on an HRD that does include magnetic fields yields mass estimates in much better agreement with Mdyn, with an average difference between Mdyn and the estimated track mass of 0.01 0.02 M . The ages of the stars, 3-10 MY on tracks that include magnetic fields, is older than the 1-3 MY indicated by the nonmagnetic models. The older ages of T Tauri stars predicted by the magnetic models increase the time available for evolution of their disks and formation of the giant gas exoplanets. The agreement between our Mdyn values and the masses on the magnetic field tracks provides indirect support for these older ages.
Context. Deuterium fractionation is a valuable tool for understanding the chemical evolution during the process that leads to the formation of a Sun-like planetary system. Aims. Methanol is thought ...to be mainly formed during the prestellar phase, and its deuterated form keeps a memory of the conditions at that epoch. The unique combination of high angular resolution and sensitivity provided by ALMA enables us to measure methanol deuteration in the planet formation region around a Class 0 protostar and to understand its origin. Methods. We mapped both the 13CH3OH and CH2DOH distribution in the inner regions (~100 au) of the HH212 system in Orion B. To this end, we used ALMA Cycle 1 and Cycle 4 observations in Band 7 with angular resolution down to ~0.̋15. Results. We detected 6 lines of 13CH3OH and 13 lines of CH2DOH with upper level energies of up to 438 K in temperature units. We derived a rotational temperature of (171 ± 52) K and column densities of 7 × 1016 cm-2 (13CH3OH) and 1 × 1017 cm-2 (CH2DOH), respectively. This yields a D/H ratio of (2.4 ± 0.4) × 10-2, which is lower by an order of magnitude than previously measured values using single-dish telescopes toward protostars located in Perseus. Our findings are consistent with the higher dust temperatures in Orion B with respect to the temperature derived for the Perseus cloud. The emission traces a rotating structure extending up to 45 au from the jet axis, which is elongated by 90 au along the jet axis. So far, the origin of the observed emission appears to be related with the accretion disc. Only higher spatial resolution measurements will be able to distinguish between different possible scenarios, however: disc wind, disc atmosphere, or accretion shocks.
Chemistry in disks Guilloteau, S; Reboussin, L; Dutrey, A ...
Astronomy and astrophysics (Berlin),
8/2016, Letnik:
592
Journal Article
Recenzirano
Odprti dostop
Aims. We attempt to determine the molecular composition of disks around young low-mass stars. Methods. We used the IRAM 30 m radio telescope to perform a sensitive wideband survey of 30 stars in the ...Taurus Auriga region known to be surrounded by gaseous circumstellar disks. We simultaneously observed HCO super(+)(3-2), HCN(3-2), C sub(2) H(3-2), CS(5-4), and two transitions of SO. We combined the results with a previous survey that observed super(13) CO (2-1), CN(2-1), two o-H sub(2) CO lines, and another transition of SO. We used available interferometric data to derive excitation temperatures of CN and C sub(2) H in several sources. We determined characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200-400 au from the stars. We compared the derived column densities to the predictions of an extensive gas-grain chemical disk model under conditions representative of T Tauri disks. Results. This survey provides 20 new detections of HCO super(+) in disks, 18 in HCN, 11 in C sub(2) H, 8 in CS, and 4 in SO. HCO super(+) is detected in almost all sources and its J= 3-2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Large variations of the column density ratios are observed, but do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C sub(2) H ratios, are well reproduced by gas-grain chemistry in cold disks. Conclusions. This study provides a comprehensive census of simple molecules in disks of radii >200-300 au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e., NOEMA and ALMA.
Context. Dust determines the temperature structure of protoplanetary disks, however, dust temperature determinations almost invariably rely on a complex modeling of the Spectral Energy Distribution. ...Aims. We attempt a direct determination of the temperature of large grains emitting at mm wavelengths. Methods. We observe the edge-on dust disk of the Flying Saucer, which appears in silhouette against the CO J = 2−1 emission from a background molecular cloud in ρ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity allows us to directly measure the dust temperature. The dust opacity can then be derived from the emitted continuum radiation. Results. The dust disk absorbs the radiation from the CO clouds at several velocities. We derive very low dust temperatures, 5 to 7 K at radii around 100 au, which is much lower than most model predictions. The dust optical depth is >0.2 at 230 GHz, and the scale height at 100 au is at least 8 au (best fit 13 au). However, the dust disk is very flat (flaring index −0.35), which is indicative of dust settling in the outer parts.
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