Friday 2 May 2014

Searching for circumplanetary disks around LkCa 15.

Just as young stars are typically surrounded by a disk of material that is accreting onto the star as well as potentially coalescing to form planets (circumstellar or protoplanetary disks), comets and other bodies; young planets, particularly very large ones, ought in theory to be surrounded by smaller disks of material, accreting onto the planet and potentially coalescing to form moons. Such disks should potentially be detectable, although as these disks are smaller and less massive than circumstellar disks, so they will be correspondingly hard to observe.

In a paper published on the arXiv database at Cornell University Library on 22 April 2014, a team of scientists led by Andrea Isella of the Department of Astronomy at the California Institute of Technology, describe the results of a search for circumplanetary disks around the young star LkCa 15, using the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array.

LkCa 15 is a young (2-5 million-year-old) K5-type orange dwarf star, roughly 547 light years from Earth in the constellation of Taurus. It has approximately the same mass as the Sun but only about 74% of its luminosity, new material is still accreting onto the star at a rate of about one Earth mass every 23 years. The system has an observed circumstellar disk with an inner margin about 45 AU from the star (i.e. about 45 times the average distance at which the Earth orbits the Sun), the area starward of this inner edge being thought to have coalesced into a number of planetary bodies. The material beyond 45 AU is unlikely to go on to form planets as it is too diffuse and scattered (hence circumstellar disk rather than protoplanetary disk), but may form comets or similar bodies. A single potential planet has been detected in the LkCa 15 system, LkCa 15b; if this observation is accurate the planet has a mass 6-10 times that of Jupiter and orbits at a distance of 16 AU. Such a large, young planet, or any other similar body in the system, would be likely to have a large circumplanetary disk, which would be amenable to detection.

Isella et al. were able to detect an inner ring of material about the LkCa 15, apparently made up of about 3 Earth masses of dust and fine grains within a few AU of the star. However they were not able to detect any circuplanetary disk about the candidate planet LkCa 15b, or any other body in the system. This non-detection does not mean such disks do not exist, but rather that if they do then they were below the limits for detection by the array. One or more disks comprising about 10% of the mass of Jupiter within 1 AU of a planet could still potentially exist in the LkCa 15 system, although this would imply that, despite the young age of the system and any potential planets, that the majority of planetary accretion has already taken place.

(Top) 1.6” x 1.6” map of the LkCa 15's continuum disk emission observed at the wavelength of 7 mm obtained by reducing the weights of the complex visibilities measured on the longest baselines to increase the sensitivity of the extended structures. The rms noise level in the map is 6.1 µJy beam-1 . The FWHM of the synthesized beam is 0.15”. (Center) Map of the 7 mm emission obtained by adopting natural weighting of the complex visibilities to maximize the angular resolution and the point source sensitivity. The rms noise level is 3.6 µJy beam-1 and the FWHM of the synthesized beam is 0.07”. The green ellipse corresponds to an orbital radius of 45 AU and traces the outer edge of the dust depleted cavity as measured from the observations at 1.3 mm. (Bottom) Map of the innermost 45 AU disk region. Contours are plotted at 2 and 4x the noise level. The white triangle shows the expected position of LkCa 15 b assuming that the star is located at the peak of the 7 mm emission. Isella et al. (2014).

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