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…
The outer disk of T Chamaeleontis. T Chamaeleontis is a T Tauri star
(a very young star which has not yet...
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...
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...
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