We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0″. 075 (10 AU) to 0″. 025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 .72 0 .05 ± • •) and position angle (138 .02 0 .07).
We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of C 18 O (J = 2 − 1), SO (J N = 6 5 − 5 4 ) and 1.3 mm dust continuum toward L1527 IRS, a class 0 solar-type protostar surrounded by an infalling and rotating envelope. C 18 O emission shows strong redshifted absorption against the bright continuum emission associated with L1527 IRS, strongly suggesting infall motions in the C 18 O envelope. The C 18 O envelope also rotates with a velocity mostly proportional to r −1 , where r is the radius, while the rotation profile at the -2innermost radius (∼54 AU) may be shallower than r −1 , suggestive of formation of a Keplerian disk around the central protostar of ∼ 0.3 M ⊙ in dynamical mass. SO emission arising from the inner part of the C 18 O envelope also shows rotation in the same direction as the C 18 O envelope. The rotation is, however, rigid-body like which is very different from the differential rotation shown by C 18 O. In order to explain the line profiles and the position-velocity (PV) diagrams of C 18 O and SO observed, simple models composed of an infalling envelope surrounding a Keplerian disk of 54 AU in radius orbiting a star of 0.3 M ⊙ are examined. It is found that in order to reproduce characteristic features of the observed line profiles and PV diagrams, the infall velocity in the model has to be smaller than the free-fall velocity yielded by a star of 0.3 M ⊙ . Possible reasons for the reduced infall velocities are discussed.
The instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated by linear analysis. In the case where the dust to gas ratio is enhanced and turbulence is week, the instability grows, even in gravitationally stable disks, on a timescale of order 10 4−5 yr at a radius of order 100AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the instability reduces to the so-called "secular gravitational instability", which was investigated previously as an instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular gravitational instability in the simplified treatment. This may indicate that we should not neglect small terms in equation of motion if the growth rate is small. The instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the instability is the accumulation of dust grains, and hence the instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii ∼100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.
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