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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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…

 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...

 Water and Hydroxides in the Circumstellar Disk around HD 163296.

HD 163296 is a young Herbig Ae star (a star producing heat by gravitational collapse, which is expected will fuse Hydrogen in the future, but which has not reached this stage yet) slightly under 400 light years from Earth. It is surrounded by a fairly well documented circumstellar disk, which reaches slightly over 900 AU from the star...

 The formation of a Keplerian Disk in the L1551 NE Protostellar System.

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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