Monday, 5 September 2016

Imaging the inner disk of LkCa 15.

LkCa 15 is a young (2-5 million-year-old) K5-type orange dwarf star, roughly 547 light years from Earth in the Taurus-Auriga star-forming region 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 a one of the best known transition disks (a structure on the way from being a protoplanetary disk, a dense structure from which planets are thought to form, to a debris disk, a relict of earlier planet-formation, such as the Main Asteroid Belt and Kuiper Belt in our Solar System), which comprises an inner disk close to the star, a gap with three candidate planets and an outer disk which begins at about 50 AU from the star (i.e. 50 times as far from the star as the Earth is from the Sun). This is recognizably similar to our Solar System, with an outer disk in a similar position to our Kuiper Belt, three large candidate planets in a similar position to the four giant planets of our Solar System (Jupiter, Saturn, Uranus and Neptune) and an inner disk in the region occupied by our inner planets (Mercury, Venus, Earth and Mars), however while the outer disk of LkCa 15 has been well studied and the planets imaged several times, resolving the inner disk has proven problematic.

In a paper published on the arXiv database at Cornell University Library on 2 September 2016, a team of scientists led by Christian Thalmann of the Institute for Astronomy at ETH Zurich describe the results of a new study of the LkCa 15 system using the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on the European Southern Observatory’s Very Large Telescope, and discuss the implications of this study.

Thalmann et al. made two rounds of observations with the instrument, which makes long exposure still images, the first making images with an exposure time of 32 seconds, using a coronagraph to block out the light from the star (the DEEP images), and the second making images with an exposure time of 0.85 seconds and not using a coronagraph (the FAST images.

The DEEP images were able to resolve both disks of LkCa 15 and the gap between them in much better detail than has previously been possible. These show the inner disk to be a roughly elliptical structure, comparable in shape and orientation to the outer disk, but approximately half the size. The FAST images show similar structures to the DEEP images, though at a lower resolution.

SPHERE IRDIS J-band imaging polarimetry of LkCa 15. Each panel shows the DEEP and FAST images side-by-side at the same scale, with insets showing the shape of the PSF core (a) Polarized flux of Deep at linear stretch (arb. units). The inner disk saturates the color scale. (b) The corresponding S/N map at a stretch of [-10σ , 10σ ]. (c) Polarized flux of DEEP after scaling with an inclined r² map to render the faint disk structures visible (arb. units). (d-f) The same three images for FAST. While overall sensitivity is lower in these data, they a fford an unobstructed view onto the inner disk. In all panels, the star’s location is marked with a white disk. The black wedges on the color scales mark the zero level. Thalmann et al. (2016).

The two arms of the inner disk appear to be asymmetrical, with the western arm trending outward and the eastward arm curling inward, and there appears to be a local brightening along the far side of the minor axis. There is also an apparent darkening on the inner part of the disk, possibly indicating a gap within it, though Thalmann et al. are cautious of over-interpreting these results, which are at the very limit of the telescope's operating capacity. The three candidate planets were also resolved, though again Thalmann et al. advise caution, but in this case they do feel the evidence for the best understood planet (LkCa 15b), is particularly strong, and note that such a planet could cause some of the apparent structures seen in the inner disk.

Thalmann et al. resolve the outer disk as being tilted at an angle of 60° seen by an Earth-based observer. They could not resolve any spirals or structural asymmetries within this outer disk, but did note four dimmed radial lines at 50° , 135° , 200°and 325°. The nature of these lines is unclear, though they could be shadows cast by inner disk regions or magnetospheric accretion columns.

Analysis of the outer disk structure of LkCa 15. (a) Ellipse fits to the maximum gradient (solid blue line) and the flux minimum (dotted blue line) in the r²-scaled DEEP image. (b) Comparison of the best-fit gap edge in J-band (blue solid line) with those in RI-band (red long-dashed line) and sub-millimeter interferometry (green short-dashed line). (c) Full-intensity KLIP image (5 subtracted modes) of the Full data in the K1K2 filter for comparison. The gap edge derived from the DEEP image coincides very well with the edge of the bright crescent in the KLIP image. (d) The image in panel (a) at a harder stretch, emphasizing the surface brightness variations in the outer disk. Four position angles with reduced brightness are marked, possibly indicating transient shadowing from the inner disk. Thalmann et al. (2016).

See also...

Transition disks around LkCa 15.             Planets are thought to form in protoplanetary disks, which is to say disks of gas and dust around young stars. However not all the material in a protoplanetary disk is likely to be used up in the formation of planets, leaving one or more debris disks, such as the Main Asteroid Belt and Kuiper Belt in our own Solar System. These debris disks typically contain rocky and icy bodies, but not...
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....
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