Detection of DCO+ in a circumstellar disk

We report the first detection of DCO+ in a circumstellar disk. The DCO+ J=5-4 line at 360.169 GHz is observed with the 15m James Clerk Maxwell Telescope in the disk around the pre-main sequence star TW Hya. Together with measurements of the HCO+ and H13CO+ J=4-3 lines, this allows an accurate determination of the DCO+/HCO+ ratio in this disk. The inferred value of 0.035+-0.015 is close to that found in cold pre-stellar cores and is somewhat higher than that measured in the envelope around the low-mass protostar IRAS 16293 -2422. It is also close to the DCN/HCN ratio obtained for pristine cometary material in the jet of comet Hale-Bopp. The observed DCO+/HCO+ ratio for TW Hya is consistent with theoretical models of disks which consider gas-phase fractionation processes within a realistic 2-D temperature distribution and which include the effects of freeze-out onto grains

H_2 and CO Emission from Disks around T Tauri and Herbig Ae Pre-Main-Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas

We present ISO Short-Wavelength Spectrometer observations of H_2 pure-rotational line emission from the disks around low- and intermediate-mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. The pre-main-sequence sources have been selected to be isolated from molecular clouds and to have circumstellar disks revealed by millimeter interferometry. We detect "warm" (T ≈100-200 K) H_2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~ 10^(-4) M_☉ up to 8 x 10^(-3) and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish ^(12)CO 3-2/^(13)CO 3-2, and ^(12)CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T ≈ 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Keplerian rotation, consistent with interferometer data on the lower J lines. The ratios of the ^(12)CO 3-2/^(13)CO 3-2 integrated fluxes indicate that ^(12)CO 3-2 is optically thick but that ^(13)CO 3-2 is optically thin or at most moderately thick. The ^(13)CO 3-2 lines have been used to estimate the cold gas mass. If a H_2/CO conversion factor of 1 x 10^(-4) is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100, in accordance with previous studies. These findings confirm that CO is not a good tracer of the total gas content in disks since it can be photodissociated in the outer layers and frozen onto grains in the cold dense part of disks, but that it is a robust tracer of the disk velocity field. In contrast, H_2 can shield itself from photodissociation even in low-mass "optically thin" debris disks and can therefore survive longer. The warm gas is typically 1%-10% of the total mass deduced from millimeter continuum emission, but it can increase up to 100% or more for the debris-disk objects. Thus, residual molecular gas may persist into the debris-disk phase. No significant evolution in the H_2 CO, or dust masses is found for stars with ages in the range of 10^6-10^7 yr, although a decrease is found for the older debris-disk star β Pictoris. The large amount of warm gas derived from H_2 raises the question of the heating mechanism(s). Radiation from the central star as well as the general interstellar radiation field heat an extended surface layer of the disk, but existing models fail to explain the amount of warm gas quantitatively. The existence of a gap in the disk can increase the area of material influenced by radiation. Prospects for future observations with ground- and space-borne observations are discussed

The gas temperature in flaring disks around pre-main sequence stars

A model is presented which calculates the gas temperature and chemistry in the surface layers of flaring circumstellar disks using a code developed for photon-dominated regions. Special attention is given to the influence of dust settling. It is found that the gas temperature exceeds the dust temperature by up to several hundreds of Kelvins in the part of the disk that is optically thin to ultraviolet radiation, indicating that the common assumption that Tgas=Tdust is not valid throughout the disk. In the optically thick part, gas and dust are strongly coupled and the gas temperature equals the dust temperature. Dust settling has little effect on the chemistry in the disk, but increases the amount of hot gas deeper in the disk. The effects of the higher gas temperature on several emission lines arising in the surface layer are examined. The higher gas temperatures increase the intensities of molecular and fine-structure lines by up to an order of magnitude, and can also have an important effect on the line shapes.Comment: 14 pages, 10 figures, accepted for publication in A&

Warm molecular layers in protoplanetary disks

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Submillimeter lines from circumstellar disks around pre-main sequence stars

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Warm Molecular Layers in Protoplanetary Disks

We have investigated molecular distributions in protoplanetary disks, adopting a disk model with a temperature gradient in the vertical direction. The model produces sufficiently high abundances of gaseous CO and HCO+ to account for line observations of T Tauri stars using a sticking probability of unity and without assuming any non-thermal desorption. In regions of radius R > 10 AU, with which we are concerned, the temperature increases with increasing height from the midplane. In a warm intermediate layer, there are significant amounts of gaseous molecules owing to thermal desorption and efficient shielding of ultraviolet radiation by the flared disk. The column densities of HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement with the observations of DM Tau, but are smaller than those of LkCa15. Molecular line profiles from our disk models are calculated using a 2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative transfer code for a direct comparison with observations. Deuterated species are included in our chemical model. The molecular D/H ratios in the model are in reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and Astrophysic

H_2 Emission From Disks Around Herbig Ae and T Tauri Stars

We present the initial results of a deep ISO-SWS survey for the low J pure rotational emission lines of H2 toward a number of Herbig Ae and T Tauri stars. The objects are selected to be as isolated as possible from molecular clouds, with a spectral energy distribution characteristic of a circumstellar disk. For most of them the presence of a disk has been established directly by millimeter interferometry. The S (1) line is detected in most sources with a peak flux of 0.3-1 Jy. The S(0) line is definitely seen in 2 objects: GG Tau and HD 163296. The observations suggest the presence of "warm" gas at T_(kin) ≈ 100 K with a mass of a few % of the total gas+ dust mass, derived assuming a gas-to-dust ratio of 100:1. The S(1) peak flux does not show a strong correlation with spectral type of the central star or continuum flux at 1.3 millimeter. Possible origins for the warm gas seen in H_2 are discussed, and comparisons with model calculations are made

Vertical structure models of T Tauri and Herbig Ae/Be disks

In this paper we present detailed models of the vertical structure (temperature and density) of passive irradiated circumstellar disks around T Tauri and Herbig Ae/Be stars. In contrast to earlier work, we use full frequency- and angle-dependent radiative transfer instead of the usual moment equations. We find that this improvement of the radiative transfer has strong influence on the resulting vertical structure of the disk, with differences in temperature as large as 70 %. However, the spectral energy distribution (SED) is only mildly affected by this change. In fact, the SED compares reasonably well with that of improved versions of the Chiang & Goldreich (CG) model. This shows that the latter is a reasonable model for the SED, in spite of its simplicity. It also shows that from the SED alone, little can be learned about the vertical structure of a passive circumstellar disk. The molecular line emission from these disks is more sensitive to the vertical temperature and density structure, and we show as an example how the intensity and profiles of various CO lines depend on the adopted disk model. The models presented in this paper can also serve as the basis of theoretical studies of e.g. dust coagulation and settling in disks.Comment: 12 pages, 15 figures, accepted for publication in A&