Wednesday, 2 July 2014

Protoplanetary 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 Regions. The initial protostars (Class 0 Protostars) are embedded in envelopes of gas and dust up to 0.1 parsecs (0.3 light years) across. Over time this dust envelope begins to rotate and collapse under the influence of the protostars gravity, a stage referred to as a Class I Protostar. Eventually most of this material is accreted into rotating protoplanetary disk about a Class II Protostar through the conservation of angular momentum (put in simple terms, material is able to remain in a disk on a single plain around the equator of a fast rotating object, but on any other plain is thrown away from the object). Within these disks dust size particles accrete over time into larger bodies, eventually forming asteroids even planets thousands of kilometres across. Finally the majority of the material in the protoplanetary disk is either accreted into larger bodies or blown away by stellar radiation from the new star, leaving a Class III Protostar, surrounded by a system of planets and debris disks.

In a paper published on the online arXiv database at Cornell University Library on 5 May 2014, and accepted for publication in the journal Astronomy & Astrophysics a team of scientists led by Anna Miotello of the European Southern Observatory and the Dipartimento di Fisica at the Universita’ degli Studi di Milano describe the results of a study of two Class I Protostars in the ρ Ophiuchi Star Forming Region with the Australian Telescope Compact Array at wavelengths of 3 mm, 3 cm and 6 cm, and the results obtained from attempts to build models that fit these observations.

The protostars of the ρ Ophiuchi Star Forming Region are about 815 light years from Earth in the constellation of Ophiuchus. The two bodies studied were Elias29 and WL12. Elias29 is calculated to have a mass 3 times that of the Sun, an effective radius 5.9 times that of the Sun and a surface temperature of 4786 k (compared to 5778 K for the Sun), while WL12 is thought to have a mass 0.6 times that of the Sun, an effective radius 3.5 times that of the Sun and a surface temperature of 3980 K.

Elias29 map: detection of the source at 3 mm. The total flux of the source at 3 mm is 10.36 mJy, with a 3σ rms of 0.18 mJy. Miotello et al. (2014).

Miotello et al. calculate that Elias29 either has a dense protoplanetary disk reaching to about 15 AU (15 times the distance at which the Earth orbit’s the Sun) from the central star or a thinner disk reaching 50-200 AU from the star. The properties of the smaller, thicker disk were impossible to model, but the larger, thinner disk would almost certainly contain pebbles with sizes that could be measured in centimetres. WL12 appears to have an optically thick disk with a mass at least 30% of that of our Sun, reaching to about 30 AU from the star.

WL12 map: detection of the source at 3 mm. The total flux of the source at 3 mm is 17.48 mJy, with a 3σ rms of 0.22 mJy. Miotello et al. (2014).

Both stars appear to have mineral grains in the millimetre range already forming within their disks. The disks appear to be relatively compact, but also rather thicker than models would predict; Miotello et al. suggest that this may be due to magnetic fields generated by the protostars countering the angular momentum of the spinning disk. They further suggest that this mechanism is only likely to be efficient during the early part of the Class I Protostar stage, while the disk is relatively massive.

See also…


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