William R. F. Dent

In recent years, gas has been observed in an increasing number of debris discs, though its nature remains to be determined. Here, we analyse CO molecular excitation in optically thin debris discs, and search Atacama Large Millimeter/submillimeter Array (ALMA) Cycle-0 data for CO J = 3–2 emission in the Fomalhaut ring. No significant line emission is observed; we set a 3σ upper limit on the integrated line flux of 0.16 Jy km s⁻¹. We show a significant dependence of the CO excitation on the density of collisional partners n, on the gas kinetic temperature T_k and on the ambient radiation field J, suggesting that assumptions widely used for protoplanetary discs (e.g. local thermodynamic equilibrium, LTE) do not necessarily apply to their low density debris counterparts. When applied to the Fomalhaut ring, we consider a primordial origin scenario where H₂ dominates collisional excitation of CO, and a secondary origin scenario dominated by e⁻ and H₂O. In either scenario, we obtain a strict upper limit on the CO mass of 4.9 × 10⁻⁴ M_⊕. This arises in the non-LTE regime, where the excitation of the molecule is determined solely by the well-known radiation field. In the secondary scenario, assuming any CO present to be in steady state allows us to set an upper limit of ∼55 per cent on the CO/H₂O ice ratio in the parent planetesimals. This could drop to ∼3 per cent if LTE applies, covering the range observed in Solar system comets (0.4–30 per cent). Finally, in light of our analysis, we present prospects for CO detection and characterization in debris discs with ALMA.

We present observations of the Rosette Nebula and its near environment in the CO 3–2 transition obtained with an angular resolution of 20″. The gas dynamics of the region are complex; we find (1) a ring of gas expanding at about 20 km s⁻¹, (2) a number of collimated outflow sources, and (3) a chain of dust clumps having a velocity gradient along its length.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Debris disks are believed to be related to planetesimals left over around stars after planet formation has ceased. The frequency of debris disks around M-dwarfs which account for 70% of the stars in the Galaxy is unknown while constrains have already been found for A- to K-type stars. We have searched for cold debris disks around 32 field M-dwarfs by conducting observations at lambda = 850 microns with the SCUBA bolometerarray camera at the JCMT and at lambda = 1.2mm with the MAMBO array at the IRAM 30-m telescopes. This is the first survey of a large sample of M-dwarfs conducted to provide statistical constraints on debris disks around this type of stars. We have detected a new debris disk around the M0.5 dwarf GJ842.2 at lambda = 850 microns, providing evidence for cold dust at large distance from this star (~ 300AU). By combining the results of our survey with the ones of Liu et al. (2004), we estimate for the first time the detection rate of cold debris disks around field M-dwarfs with ages between 20 and 200 Myr. This detection rate is 13^{+6}_{-8} % and is consistent with the detection rate of cold debris disks (9 - 23 %) around A- to K-type main sequence stars of the same age. This is an indication that cold disks may be equally prevalent across stellar spectral types.

We detect near-infrared H2 emission in DG Tau using the Infrared Camera and Spectrograph (IRCS) on the 8.2-m SUBARU telescope. The spectra obtained along the jet axis show that the centroidal position of the 1–0 S(1) emission is offset by 0.2'' from the star towards the jet, while those obtained perpendicular to the jet axis show a marginal extension, indicating that the emission line region has a typical width of ~0.6''. Their line profiles show a peak velocity of ~15 km s-1 blueshifted from the systemic velocity. These results indicate that the emission originates from a warm molecular wind with a flow length and width of ~40 and ~80 AU, respectively. The line flux ratios ( and an upper limit for ) suggest that the flow is thermalized at a temperature of ~2000 K, and is likely heated by shocks or ambipolar diffusion. The observed velocity and spatial extension suggest that the H2 and forbidden line emission originate from different components of the same flow, i.e., a fast and partially ionised component near the axis and a slow molecular component surrounding it. Such a flow geometry agrees with model predictions of magneto-centrifugal driven winds.