Aperture Synthesis 12CO and 13CO Observations of DM Tauri: 350 AU Radius Circumstellar Gas Disk

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Abstract

We have made aperture synthesis observations of 12CO (1-0), 13CO (1-0), 2.6 mm, and 2.7 mm continuum emissions from a young star DM Tau with the Nobeyama Millimeter Array (NMA). It has been found from our observations that 12CO gas with a size of 5"9 x 4"6 is associated with DM Tau and the systematic velocity gradient of 12CO exists along P.A. = 160°. The observational results suggest that DM Tau is surrounded by the disk structure of molecular gas ("circumstellar gas disk") with a radius of 350 AU and an inclination of 40° and that the kinematics of the disk is consistent with Keplerian rotation around DM Tau with a mass of 0.48 M_sun. We have also detected the 13CO, 2.6 mm, and 2.7 mm continuum emissions from the disk around DM Tau. The H₂ mass of the gas disk in the outer region (r > 100 AU) is estimated to be 2.3 × 10-3 M_sun from the 12CO and 13CO emissions, and that in the inner region (r < 100 AU) is 0.019 M_sun from 2.6 mm continuum flux. Although the disk around DM Tau is larger than the solar system, the disk may be a protoplanetary disk from the viewpoint that the mass of the disk is comparable to the minimum mass of the solar nebula (∼0.01 M_sun) and the kinematics of the disk is consistent with Keplerian rotation. The radial dependence of H₂ gas surface density of the disk is roughly estimated as Σ(r) ∝ r-2.0±0.3 using the outer and the inner masses of the disk, unless the depletion of 13CO occurs in the outer region. This power-law index of 2.0±0.3 is significantly larger than the value of 1.5 adopted in the model of minimum solar nebula (Kyoto model). DM Tau is one of the young stars located in the L1551 star-forming region, including HL Tau, GG Tau, and a protostar, L1551-IRS 5. The systemic velocities of gas disks found around these stars in the different evolutional stages are in the very narrow range (V_LSR = 5.7 - 6.4 km s-1), and the major axes of the disks are roughly perpendicular to the direction of the local magnetic field. It is likely that these stars were successively formed in the same parent cloud and that the magnetic field might play an important role in the formation of the star-disk systems.

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