Monday, 27 April 2015

Imaging the potential protoplanet in the Gomez’s Hamburger system.


Many young stars are surrounded by extensive disks of dust and gas. These disks are thought to be where planets are formed, and are therefore known as protoplanetary disks. Recent discoveries of large planets orbiting young stars at tens or even hundreds of AU (i.e. tens or hundreds of times the distance at which the Earth orbits the Sun) has led astronomers to speculate that such planets could form in the outer parts of protoplanetary discs as a result of gravitational instabilities in the rotating disk. However direct evidence of this process is hard to come by.

Gomez’s Hamburger (or IRAS 18059-3211) is a young A-type star (a star with 1.4 to 2.1 times the mass of the Sun) approximately 900 light years from Earth in the constellation of Sagitarius. It is known to be surrounded by an extensive protoplanetary disk, with an estimated mass equivalent to between 2% and 30% of that of the Sun, which is seen almost edge on when viewed from the Earth. This has previously been shown to have a dense area located about 330 AU to the south of the central star (as seen from Earth), which has a mass of at least that of Jupiter, and which has been speculated to be a protoplanet forming through the collapse of a gravitational instability.

In a paper published in the journal Astronomy & Astrophysics on 13 April 2015, and on the online arXiv database at Cornell University Library on 10 April 2015, a team of scientists led by Oliver Berné of the Université deToulouse and the Centre national de la recherche scientifique present the results of a series of observations of Gomez’s Hamburger made with the VISIR (VLT Imager and Spectrometer for mid-Infrared) spectrograph at the Very Large Telescope (VLT), which provide insight into the structure of the disk and the potential protoplanet.

Molecules will absorb light as energy across a broad part of the spectrum, but can only absorb a finite amount of light before being forced to re-emit some of this energy. However this energy is not released in random bursts, but radiated at specific frequencies determined by the atoms present in the molecule, which atoms are bound to which other atoms, and even which isotopes of each element are present. This gives each molecule its own unique spectrographic signature, which can be used by astronomers to detect different molecules in distant objects such as protoplanetary disks.

Berné et al. observed the disk of Gomez’s Hamburger with filters for specific molecules, notably PAHs (Poly Aromatic Hydrocarbons), and combined the new data with previously obtained data on the system obtained by the Submillimeter Array (SMA), which looked at the spectra for 12CO and 13CO (Carbon Monoxide molecules containing the isotopes 12Carbon and 13Carbon.

VLT-VISIR 8.6 μm (PAH1 filter) image of GoHam in colour, the scale is in Jy/arcsec2. In contours : velocity integrated 12CO(2-1) emission observed with the SMA. Berné et al.(2015).

The edge on disk of Gomez’s Hamburger was clearly resolved at PAH wavelengths, with the two halves of the disk separated by a broad, dark like calculated to be about 375 AU thick. This is because PAHs are escaping from the surface of the disk, making them visible above and below it, but are not clearly visible within the disk where they are hidden by other molecules. The radius of the disk seen at PAH wavelengths is about 750 AU, much smaller than that observed at CO wavelengths, about 1650 AU, but the PAH emissions could be seen far higher above the disk than the CO emissions, about 770 AU as opposed to about 450 AU.

VLT-VISIR 8.6 μm (PAH1 filter, same as left panel) in colour. Contours show the emission of 13CO (2-1) emanating from fromGoHam b after subtraction of the best fit disk model. This region also corresponds to the local decrease of mid-IR emission seen in the VISIR image. Berné et al. (2015).

The putative protoplanet, Gomez’s Hamburber b, or GoHam b for short (when naming bodies in other stellar systems stars are given an upper case letter and planets a lower case letter) was detected in these observations as an area or denser material with a radius of about 155 AU, and a mass of between 0.8 and 11.4 times that of Jupiter (depending on the density of the dust in this region, which cannot be directly measured).

VLT-VISIR 11.2 μm (PAH2 filter) image of GoHam in colour, the scale is in Jy/arcsec2. Berné et al. (2015).

An area of slightly denser gas and dust over 100 AU across is of course a long way short of being a planet. Nevertheless Berné et al. feel that a planet beginning to form on the edge of the disk is the most likely explanation for this structure. Another possibility might be a spiral arm within the disk (such structures have been seen within other protoplanetary disks), though a single spiral arm observable at only one place within the disk would be difficult to explain, as such arms usually come in groups and are usually extensive. Alternatively it could be an asymmetric horseshoe structure, which have also been observed in some protoplanetary disks, but previously observed horseshoe structures have comprised denser areas of dust only, whereas the Gomez’s Hamburger structure appears to contain both dust and gas.

See also…

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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...

Protoplanetary disks around Class I Protostars in the ρ Ophiuchi Star Forming Region.
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