Monday 28 July 2014

The 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 the Disk onto the Protostar) is high. In late Class 0 stage and early Class 1 Stage Protostars, infalling continues and the Keplerian Disk begins to rotate due to angular momentum. Eventually in later stage Class 1 Protostars the Keplerian Disk begins to rotate  so fast that centrifugal force prevents further accretion of material onto the Protostar, which then ceases the main stage of its growth (though it will still need to undergo several stages of evolution before being regarded as a true star). The remaining material in the Keplerian Disk can then form a Protoplanetary Disk, from which planets and other bodies can form around the star.

In a paper published on the arXiv database at Cornell University Library on 10 July 2014, a team of scientists led by Hsi-Wei Yen of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei describe the results of a study of the Class I Protostar L1489 IRS with the Atacama Large Millimeter/submillimeter Array (ALMA), and the conclusions drawn from this.

L1489 IRS is located in the Taurus Molecular Cloud, roughly 140 parsecs from Earth (456.6 light years). It is surrounded by a protostellar envelope containing about 3% of the Sun’s mass which reaches about 4200 AU from the central Protostar (i.e. 4200 times the distance at which the Sun orbits the Earth), within which a faint molecular outflow, reaching several thousand AU from the Protostar and flowing both north and south, has been observed. Within this envelope a Keplerian Disk has been observed, which SMA observations suggest reaches about 200 AU from the Protostar and contains about 0.4% of the Sun’s mass, while Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations suggest this disk reaches 250-450 AU from the Protostar and contains about 0.5% of the Sun’s mass. The Protostar itself is estimated to be 1.4 times as massive as the Sun (SMA) or 1.8 times as massive (CARMA).

Yen et al. determined a Keplerian Disk reaching 700 AU from the Protostar, containing approximately 0.5% of the Sun’s mass. The central Protostar has a mass equivalent to 1.6 time that of the Sun and has an angle of inclination of 66˚. The disk is 15-25 AU deep with a temperature of 30-50 K (-243 to -223˚C) at a distance of 100 AU. 

Moment 0 map (contour) overlaid on the moment 1 map (color) of the C¹⁸O (2–1) emission in L1489 IRS. A filled ellipse at the bottom-right corner denotes the beam size. A cross shows the protostellar position. Contour levels are from 3σ to 15σ in steps of 3σ, from 15σ to 50σ in steps of 5σ, and then from 50σ to 90σ in steps of 10σ, where 1σ is 10 mJy Beamˉ¹ km sˉ¹. Yen et al. (2014).

Yen et al. also detected the presence of significant areas of blue- and red-shifted material beyond this disk and offset from it at angles of about 65˚ to the north and 35˚to the south. In astronomy red- and blue-shifting are taken to be signs of movement away ant towards the observer; this is because the speed of light is absolute, so an object moving towards the observer will compress the lightwaves slightly and an object moving away extend them slightly.

These areas of red- and blue-shifted material are reversed with regard to the known areas of outflow from the protostar (i.e. blue to the north, where material outflowing from the Protostar will appear red and red to the south where material outflowing from the Protostar will appear blue), so Yen et al. conclude that these are areas where material is inflowing onto the Keplerian Disk, and from there accreting onto the Protostar.

Schematic figure demonstrating the configuration of Yen et al.’s model of a flared Keplerian disk with two streams of infalling flows toward the disk. Blue and red arrows show the directions of the blueshifted and redshifted outflows, respectively. Yen et al. (2014).

Yen et al.  further conclude that the protostellar envelope around L1489 IRS still contains about 2% of the Sun’s mass, and that material is infalling from this to the Keplerain disk at a rate of 0.00004-7% of the Sun’s mass per year.

See also…

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Protostars are areas within molecular clouds where stars are forming, areas of increased density surrounded by dusty envelopes of gas hundreds of AU across (AU stands for Astronomical Unit, 1 AU being the average distance between the Earth and the Sun). As these...

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