Thursday, 14 April 2016

Understanding the disk around HL Tauri.

HL Tauri is a young protostar (star which is still gaining mass by accretion from a surrounding disk, the accretion disk, and is emitting ionized material in jets from its poles) 140 parsecs (457 light years) from our Solar System in the constellation of Taurus. It's disk has been known and observed for a number of years, however recent observations by the Atacama Large Millimeter/submillimeter Array (ALMA) revealed this disk to be divided into a series of light and dark bands. This is surprising, as such banding is usually associated with planet formation in a more developed disk (a protoplanetary disk) with darker zones forming where (invisible) planets have formed and either accreted or tidally pushed away the (visible) dust in a the dark bands.
In a paper published on the arXiv database at Cornell University Library on 11 March 2016, a team of astronomers led by Carlos Carrasco-González of the Instituto de Radioastronomíay Astrofísica discuss a series of new observations of HL Tauri made with the Very Large Array (VLA) of the National Radio Astronomy Observatory at wavelengths of 6.7, 7.0 and 7.3 mm (observations of remote objects at different wavelengths tend to reveal different features, as different molecules reflect light at different wavelengths) between December 2014 and September 2015.

Atacama Large Millimeter/submillimeter Array of HL Tauri at a wavelength of 1.3 mm. The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) are shown. Carrasco-González et al. (2016).

Carrasco-González et al. were able to observe many of the features seen in the ALMA images of HL Tauri, including the innermost dark and light bands. However the much longer wavelengths used by the enabled a much more detailed study of the inner part of the disk, an area essentially opaque in the ALMA images. This enabled Carrasco-González et al. to make estimations of the density of particles and average grain sizes within different parts of the disk. 

Left: VLA image at 7.0 mm with an angular resolution of ~20 au (0.”15; tapered image). Right: Close-up to the center of the disk. VLA image at 7.0 mm with an angular resolution of ~10 au (~0.”7; natural weighting). The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) from the ALMA images are shown. The inner disk and the first pair of dark (D1) and bright (B1) rings are clearly seen in the 7.0 mm images. Carrasco-González et al. (2016).

The VLA observations showed showed structure within the dust distribution in the innermost bright ring of HL Tauri. Here the dust was found to form clumps or knots, most of which were of a temporary nature, but with one particularly dense clump on the northeastern part of the limb being more permanent in nature. Carrasco-González et al. suggest that this is possibly a protoplanet in the earliest stages of formation, and that therefore planet formation around of HL Tauri is beginning in the brighter areas of the disk, not the lighter, with the bright areas representing parts of the disk with higher densities due to early stage planetary accretion rather than areas from which dust has been cleared by later stage planetary formation.

(a) Superposition of the VLA 7.0 mm image (contours; naturally weighted image; beam size 0:”067) over the ALMA 1.3 mm image (colour scale). Contour levels are -4, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, 100, and 150 times the rms of the 7.0 mm map, 3.5 μJy beam-1. The two arrows mark the direction of the collimated jet at a P.A. of ~45º. (b) A close-up to the center of the disk. Colour scale is the ALMA 1.3 mm image and contours are from the high angular resolution VLA 7.0 mm image (robust 0 weighted image; beam size 0:”04). Contour levels are 3, 4, 5, 6, 7, 8, 12, 16, 20, 24, 28, 32, 40, 48, and 56 times 7 μJy beam-1. (c) and (d): Comparison between sub-bands contour images at 7.3 mm and 6.7 mm (robust 0.4 weighting; beam sizes 0:”053) over the 7.0 mm colour scale image. Contours in both panels are 8, 9, 10, 11, 12, 13, 14, 15, and 16 times 6 μJy beam-1.

See also... Keplerian Disk of Class I Protostar L1489 IRS.                                                Recent studies of the Keplerian Disks around other Protostars with the Submillimeter Array (SMA) have suggested that in the early Class 0 Protostar stage little rotation occurs within the Keplerian Disk and the rate of infalling (i.e. the rate at which material falls from... disks around Class I Protostars in the ρ Ophiuchi Star Forming Region.                                                        Stars are thought to form from the aggregation of material from vast clouds of molecules known as Stellar Nurseries or Star Forming... and Hydroxides in the Circumstellar Disk around HD 163296.                                  HD 163296 is a young Herbig Ae star (a star producing heat by gravitational collapse, which is expected will fuse Hydrogen in the future, but which has not reached this stage yet) slightly under 400 light years from Earth. It is surrounded by a fairly well documented circumstellar disk, which reaches slightly over 900 AU from the star (i.e. over 900 times as far from the star as Earth is from the...
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