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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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 free gas. However this is not an instantaneous event, and a period exists where a planetary system is forming accompanied by one or more disks of dust and gas; these evolving disks in forming planetary systems are called 'transition disks.

In a paper published on the arXiv database at Cornell University Library on 5 January 2015 and accepted for publication in the Publications of the Astronomical Society of Japan, a team of scientists led by Daehyun Oh of the Department of Astronomical Science of the Graduate University for Advanced Studies and the National Astronomical Observatory of Japan discuss the results of a study of the LkCa 15 stellar system and its transition disks made with the Subaru Telescope.

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 a well documented transition disk as well as two candidate planets, with a second inner disk having been discovered in 2015.

Oh et al. were able to clearly resolve two elliptical disks around LkCa 15, these are apparently in the same plane, tilted at a angle of 44˚ from our perspective. However the gap separating these disks, which forms an apparent third component to the system, appears to be tilted at a greater angle, 52˚. Since this gap is only an apparent member of the disk system, not a genuine object, this apparent difference in angle can only be a product of the structure of the two actual rings.

PI and overlapped polarization vector map images (2.0′′ × 2.0′′) before (a) and after (b) halo subtraction. The saturated region is occulted by a software mask (r∼0.1′′ ), the vectors are binned with spatial resolution, and the lengths are arbitrary for presentation purposes. (a): The effect of a polarized halo appears to have a tendency toward the minor axis of the disk. (b): The polarization tendency to the minor axis was removed, and the disk-origin polarization along the disk surface was revealed. Bottom: The radial Stokes Qr (c) and Ur (d) images. In the Qr image, both the outer and inner disks are significantly detected as expected from the PI image. On the other hand, the Ur image shows no disk-like component. Oh et al. (2015).

In order to resolve this Oh et al. examined the brightness asymmetries of the disks in order to glean further information about their inclination. This works because the side of the disk behind the star is reflecting light directly back towards us, whereas light reaching us from the near side has to be scattered through the disk. Using this method they found that the inner portions of the disks were misaligned by about 13˚, the best explanation for this being that the inner disk is significantly warped.

Elliptical fitting results of the inner disk (purple), the gap (yellow), and the outer disk (red). The image has been smoothed by a gaussian with r=2 pixels to reduce the effects of speckles on the inferred structure of the disk. The central region is also shown in the right top panel. White star indicates the location of LkCa 15. Green and orange stars indicate where the planet candidates LkCa 15 b and c were detected in 2014, respectively (Sallum et al. 2015). Empty green and orange circles indicate the locations of two infrared sources seen in 2009-2010 (Kraus & Ireland 2012), which are assumed as LkCa 15 b and c, respectively. Oh et al. (2015).

This study greatly adds to the evidence for the presence of one or more planets or protoplanets in the LkCa 15 system. The warping of the inner disk observed would require the presence of a planet with a planet having a mass at least equivalent to that of Jupiter, while the gap between the two disks is about 27 AU (27 times the distance at which the Earth orbits the Sun), which would require multiple such large planets to clear.

See also...

Detecting debris disks around small nearby stars in old Hubble images.                        Debris disks are rings of dust, rock and icy material left surrounding stars after planet formation has occurred (unlike protoplanetary disks, which are present around very young stars only, and which are thought to be largely consumed by planetary formation). Our Solar System has two such debris disks, the Asteroid Belt and the Kuiper Belt, and in recent years...

The outer disk of T Chamaeleontis.                 T Chamaeleontis is a T Tauri star (a very young star which has not yet began to generate heat by hydrogen fusion, but which produces considerable energy through gravitational heating) estimated to be about 7 million years old, roughly 350 light years from Earth in the constellation of Chamaeleontis. It is known to be surrounded by two transition disks (disks of dust and gas surrounding very young stars, thought to...

 

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...

  

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J160421.7-213028: A young star in the Upper Scorpius Association with a possible massive planet within a protoplanetary disk.

J160421.7-213028 is a young star roughly 145 parsecs (473 light years) from Earth, which forms part of the Upper Scorpius Association, making it five to ten million years old. Observations of the star with the SubmillimeterArray and Atacama Large Millimeter/submillimeter Array have shown that the star is surrounded by a protoplanetary disk (disk of dust and gas around a young star from which planets are thought to form) with a radius of 63 AU (63 times the distance at which the Earth orbits the Sun) positioned almost face-on to us.

In a paper published in the journal Astronomy &Astrophysics on 22 October 2015 and on the online arXiv database at Cornell University Library on 1 October 2015, a group of astronomers led by Paola Pinilla of the Leiden Observatory at Leiden University discuss new observations of J160421.7-213028 made in June 2015 using the SPHERE instrument of the Very Large Telescope at a wavelength of 0.626 µm (observations of remote objects at different wavelengths tend to reveal different features, as different molecules reflect light at different wavelengths, so it is important to make observations at precise wavelengths).

The Very Large Telescope observations show J160421.7-213028 to have a disk extending to 120 AU from the star, with an inner disk reaching to 15 AU, a gap between 15 and 40 AU, and a bright ring between 40 and 120 AU, which reaches peak brightness at 59 AU. This outer ring has a distinct dip in brightness, also at 59 AU.

Very Large Telescope image of J160421.7-213028 at a wavelength of 0.626 µm. Pinilla et al. (2015).

A gap in the outer ring of J160421.7-213028 was previously seen in an image taken with the High Contrast Instrument on the Subaru Telescope of the system taken in April 2012. If these two features are the same object then it appears to be moving around the disk in a clockwise direction at a rate of 12.3º per year, though it is possible that the dips represent two different temporary features, or that the object is in fact moving much faster and has completed one or more revolutions about the star in addition to its apparent movement in the time between the two images.

An object orbiting a star at a distance of 59 AU would be expected to travel at a rate of ~0.8º per year, considerably slower than the observed feature. However a dip in the brightness of the ring does not necessarily imply an object within the ring; all the light seen in the ring is in fact produced by the star and only reflected by objects in the ring, so it is quite likely that any dimming of the light seen coming from the ring actually reflects an object closer to the star casting a shadow onto the ring.

J160421.7-213028 has a gap in its disk between 15 and 40 AU from the star. A gap this large would could be caused by a very large planet forming within the disk. Such a planet would be expected to have a mass 5-10 times that of Jupiter (still to small to be directly observed at the distances involved) and to orbit at a distance of 20-40 AU. However such a body would still be to far from the star to move at a speed of 12.3º per year.

Calculating from the speed of the perceived movement of the dark area, if it is a shadow then it would need an object orbiting the star at a distance of 9.6 AU (within the inner disk of the system) to cast a shadow moving at the appropriate speed.

Pinella et al. suggest that future observations of the J160421.7-213028 system (which are planned) should reveal whether the feature seen in the two images is the same and is moving on a predictabel trajectory, or alternatively unrelated temporary features.

See also...

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...

The outer disk of T Chamaeleontis.

T Chamaeleontis is a T Tauri star (a very young star which has not yet began to generate heat by hydrogen fusion, but which produces considerable energy through gravitational heating) estimated to be about 7 million years old, roughly 350 light years from Earth in the constellation of Chamaeleontis. It is known to be surrounded by two transition disks (disks of dust and gas surrounding very young stars, thought to be...

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 initial protostars (Class 0 Protostars) are embedded in envelopes of gas and dust up to 0.1 parsecs (0.3...

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The outer disk of T Chamaeleontis.

T Chamaeleontis is a T Tauri star (a very young star which has not yet began to generate heat by hydrogen fusion, but which produces considerable energy through gravitational heating) estimated to be about 7 million years old, roughly 350 light years from Earth in the constellation of Chamaeleontis. It is known to be surrounded by two transition disks (disks of dust and gas surrounding very young stars, thought to be associated with planet formation), with a possible substellar companion (planet or brown dwarf) between the two. The inner disk is known to extend from 0.13 to 0.17 AU from the star (i.e. 0.13-0.17 times the distance at which the Earth orbits the Sun), while the outer disk has proved harder to analyse, though it is thought either to comprise either a very compact disk of material at about 40 AU, or a more diffuse disk reaching from 40-80 AU but with a very steep density gradient and most of its mass close to its inner surface.

In a paper published in the journal Astronomy & Astrophysics on 15 February 2015 a team of scientists led by  Nuria Huélamo of the Centro de Astrobiología at the European Space Agency Center in Villanueva de la Cañada describe the results of a study of the outer disk of T Chamaeleontis using the Atacama Large Millimeter Array in Chile, which looked specifically for the molecules CO (carbon monoxide), ¹³CO (carbon monoxide molecules in which the carbon molecule is the Carbon-13 isotope), CS (carbon sulphite – check) and SO₂ (sulphur dioxide). This is possible because all molecules will absorb light energy at a range of frequencies, but can only absorb so much before they must emit it again, which occurs at a specific set of frequencies for each molecule (this is why sodium lights are orange, neon lights are red and the sky is blue – the colour of nitrogen), enabling astronomers and astrophysicists to look for specific molecules in distant objects.

The CO, ¹³CO and CS molecules were detected in the disk, but SO₂ was not found. The CO content of the disk appeared to stretch to a distance of 230 AU from the star, considerably more than has been previously suggested, which the ¹³CO molecule was found at distances of up to 170 AU and CS at 100 AU.

Integrated emission maps of the CO(3–2), ¹³CO(3–2), and the CS(7–6) transitions (from left to right). The black contours represent the continuum emission at 850 μm at 5, 15, 30, 45, 60, 75, 90, and 110σ where 1σ is 0.7 mJy beam¯¹. We detect two emission bumps separated by 40 AU and an outer dust radius of 79 AU. The white ellipses are the synthesized beams for the spectral emission lines and the green ellipse is the synthesized beam for the continuum map. The white dashed line in the left panel represents the axis where the position–velocity has been obtained. Huélamo et al. (2015).

Huélamo et al. were also able to measure the velocity at which the molecules were moving towards or away from the Earth by measuring the Doppler shift on the light they emitted. This works because an object moving towards us catches up a bit with light it emits (the speed of light is fixed), compressing the light waves (making them closer together), which from our point of view makes them slightly more blue (blue-shifting, which indicates an object is coming towards us), while objects moving away from us stretch out the distance between waves (making them further apart) and making them slightly more red from our point of view (red shifting, which indicates an object is getting further away). The CO component of the disk was found to be moving at between -5.0 and 16.5 kilometres per second, the ¹³CO component at between -3.0 and -15.0 kilometres per second and the CS component at between 0.0 and 11.0 kilometres per second.

Intensity-weighted mean velocity maps (first-order moment, 2σ cut for CO(3–2) and ¹³CO(3–2), and 1.5σ cut for CS(7–6)). Huélamo et al. (2015).

From this Huélamo et al. calculate that the outer disk is tilted at an angle of 67˚ from our perspective, and that it comprises an inner dusty portion reaching from 40 to 80 AU, but with most of its mass inside of 50 AU from the star, and an outer gassy portion which reaches 230 AU from the star. They further calculate that the rotation of this disk implies the star has a mass equivalent to about 1.5 times that of the Sun and is about 10 million years old.

See also…

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...

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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