Using lab-based experiments to estimate the composition of the HR 4796 debris disk.

The appearence of debris disks is an important stage in the evolution of young planetary systems, occurring after the dissipation of the gas-rich protoplanetary disk has left an optically thin disk of planetesimals that are the leftovers from the planetary formation. This disk in turn depletes with time, as the number of planitesimals declines, which 75% of young stars in the 18-million-year-old β Pictoris moving group having a detectable debris disk, compared to 20% of billion-year-old stars. Debris disks can be detected through their infrared emmissions, which are in excess of those produced by the stellar photosphere.

The A-type star HR 4796 is 231 light years from our Solar System in the constelation of Centurus. It is estimated to be 10 million years old, and has one of the best known debris disks, which has been studied with numerous different instruments. The ring is about 80 AU from the star (i.e. 80 times as far from the star as the Earth is from the Sun) and extends for about 10 AU. 

In a paper posted on the Arxiv database at Cornell University on 4 December 2023, Julien Milli and Olivier Poch of the Université Grenoble Alpes, Jean-Baptiste Renard of the Université d'Orléans, Jean-Charles Augereau and Pierre Beck, also of the Université Grenoble Alpes, Elodie Choquet and Jean-Michel Geffrin of Aix Marseille Université, Edith Hadamcik of Sorbonne Université, Jérémie Lasue of the Université de Toulouse, François Ménard and Arthur Péronne, again of the Université Grenoble Alpes, Clément Baruteau, also of the Université de Toulouse, and Ryo Tazaki and Vanesa Tobon Valencia, once again of the Université Grenoble Alpes, present the results of a laboratory experiment intended to use dust analogues, to determine the nature of the material in the debris disk of HR 4796.

Examination of the HR 4796 system with the SPHERE instrument at the European Southern Observatory and the GPI instrument at the International Gemini Observatory has enabled polemetric analysis of the brightness of the ring at almost all azimuth angles. Every point on the ring forms a distinct angle between the light source (i.e. the star at the centre of the system) and the viewer, giving a different light scattering angle. The ring is angles at 76.5° to the star from our perspective, giving it minor axes (shortest apparent circumferances) with scattering angles of  13.5° to the northwest (brightest point) and 166.5° to the southeast (faintest point).

The HR 4796 disc at various wavelengths. The semi-major axis of the disc is roughly 1”. North is up, East to the left. Milli et al. (2023).

Most models of debris disks have assumed that they are made up of compact spheres of material similar to that found in comets, i.e. silicates, amorphous carbon, water ice and pore spaces. However, such models cannot reproduce the refractive properties of the HR 4796 disk. Changing the model to assume that the disk contained a high volume of metalic iron, as well as amorphous carbon and silicates, can reproduce the light scattering of the debris disk, but not the degree of polarisation. This suggests that either the particles in the disk are not spheres, or that the presumed size distribution of those spheres is very wrong. For most possible compositions of material, the degree of polarisation could be achieved only with milimetre sized particles, while the spectral energy distribution requires micron-scale particles.

The PROGRA² instrument is dedicated to the study of light scattered by solid particles deposited on a surface or lifted in clouds in microgravity conditions during parabolic flights that took place between 2018 and 2021. Samples were contained in a vial, and targetted with laser light; the scattered light produced by the particles was then split using a polarised beam splitter in two channels, and measured by two different detectors. 

Examination of the database of results produced by this experiment revealed that the iron sulphide mineral pyrrhotite produced a degree of polarisation similar to that seen in the HR 4796 debris disk with particle sizes in the 1-200 µm range. The experiment did not record the degree of light scattering in the near infra-red, which would be necessary for a rigorous comparison, but the scattering at visible wavelengths was a good match for the HR 4796 disk.

Milli et al. postulate that pyrrhotite is an interesting candidate for the material of the HR 4796 debris disk. Because the data comes from an archived result from an experiment not designed to model the disk, it was not adjusted to try to gat a better fit. Nevertheless, it does appear that a cloud of pyrrhotite particles dominated by particles smaller than 100 µm, would produce a good match for the optical properties of the HR 4796 disk. Iron sulphide minerals are an unsurprising component for a circumstellar dust ring. Stratospheric dust particles, Antarctic Micro-Meteorites, and sampled material from Comet Wild 2 have all produced iron sulphide minerals, and comet 67P/Churyumov-Gerasimenko has been shown to contain about 7.5% iron, probably in the form of iron sulphides or iron-nickel alloys. These are dark minerals, responsible for the dark reflectance at optical and near infrared wavelengths from  cometary and primitive asteroids surfaces, and would therefore be plausible components of the HR 4796 debris disk. 

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Observing the Elias 2-24 Protoplanetary Disk with the Atacama Large Millimeter/Submillimeter Array.

The Ophiuchus Molecular Cloud is a dense molecular cloud roughly 125 parsecs (408 light years) from Earth in the constellation of Ophiuchus, that forms one of the closest areas of star-formation to the Earth. This region contains over 200 known T Tauri stars (very young stars which have not yet begun to generate heat by hydrogen fusion, but which produce considerable energy through gravitational heating) and at least 16 protostars (stars which are still gaining mass by accretion from a surrounding disk, the accretion disk, and are emitting ionised material in jets from their poles). Elias 2-24 is a T Tauri star within the Ophiuchus Molecular Cloud with an estimated age of 400 000 years and mass roughly equal to that of the Sun. This star is surrounded by a protoplanetary disk (a dense structure from which planets are thought to form) from which matter is still actively accreting onto the star, and which is roughly edge on when seen from Earth, making it a good candidate for observation by astronomers trying to understand these structures.

In a paper published on the arXiv database at Cornell University Library on 18 November 2017, and accepted for publication in the Astrophysical Journal Letters, a team of scientists led by Lucas Cieza of the Facultad de Ingenier a y Ciencias, N ucleo de Astronom a at the Universidad Diego Portales, and the Millennium Nucleus Center of Protoplanetary Disks in ALMA Early Science, describe the results of a study of the Elias 2-24 Protoplanetary Disk made with the Atacama Large Millimeter/Submillimeter Array (ALMA) on 13 and 14 July 2017.

Cieza et al. immaged the Elias 2-24 Protoplanetary Disk at a range of wavelengths, intended to detect the densities of different molecules. 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.

Using this method Cieza et al. were able to detect three distinct gaps in the protoplanetary disk around Elias 2-24, at distances of 20, 52, and 87 AU from the star (i.e. 20, 52 and 87 times as far from the star as Earth is from the Sun), and have widths of 6, 28 and 11 AU, respectively. Such gaps in protoplanetaty disks are thought to be caused by the formation of planets, as matter from the disk accretes onto the forming protoplanetary body. Calculating the amount of matter that would be missing from the disks in this gap, Cieza et al. suggest that enough material has been used to form planets with masses of 4, 20 and 10 times that of Jupiter, though they do not believe that all of the missing material would have been used up by planetary formation; much of it is likely to have been ejected from these gaps by tidal forces generated by the forming protoplanets.

 Composite ALMA image of the Elias 2-24 Protoplanetary Disk, assembled from averaged images at different wavelengths. Cieza et al. (2017).

Cieza et al. further note that the temperatures at the inner two gaps, 23 and 15 K corresponds closely to those predicted for the snow-lines of Carbon Monoxide (23-28 K) and Nitrogen (12-15 K), i.e. the temperatures at which these molecules with cease to be disassociated gases and start to accrete into snow, suggesting that such snow-formation plays a role in the early stages of planetary formation.

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Understanding the disk around HL Tauri.

HL Tauri is a young protostar (star which is still gaining mass by accretion from a surrounding disk, the accretion disk, and is emitting ionized material in jets from its poles) 140 parsecs (457 light years) from our Solar System in the constellation of Taurus. It's disk has been known and observed for a number of years, however recent observations by the Atacama Large Millimeter/submillimeter Array (ALMA) revealed this disk to be divided into a series of light and dark bands. This is surprising, as such banding is usually associated with planet formation in a more developed disk (a protoplanetary disk) with darker zones forming where (invisible) planets have formed and either accreted or tidally pushed away the (visible) dust in a the dark bands.
 

In a paper published on the arXiv database at Cornell University Library on 11 March 2016, a team of astronomers led by Carlos Carrasco-González of the Instituto de Radioastronomíay Astrofísica discuss a series of new observations of HL Tauri made with the Very Large Array (VLA) of the National Radio Astronomy Observatory at wavelengths of 6.7, 7.0 and 7.3 mm (observations of remote objects at different wavelengths tend to reveal different features, as different molecules reflect light at different wavelengths) between December 2014 and September 2015.

Atacama Large Millimeter/submillimeter Array of HL Tauri at a wavelength of 1.3 mm. The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) are shown. Carrasco-González et al. (2016).

Carrasco-González et al. were able to observe many of the features seen in the ALMA images of HL Tauri, including the innermost dark and light bands. However the much longer wavelengths used by the enabled a much more detailed study of the inner part of the disk, an area essentially opaque in the ALMA images. This enabled Carrasco-González et al. to make estimations of the density of particles and average grain sizes within different parts of the disk. 

Left: VLA image at 7.0 mm with an angular resolution of ~20 au (0.”15; tapered image). Right: Close-up to the center of the disk. VLA image at 7.0 mm with an angular resolution of ~10 au (~0.”7; natural weighting). The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) from the ALMA images are shown. The inner disk and the first pair of dark (D1) and bright (B1) rings are clearly seen in the 7.0 mm images. Carrasco-González et al. (2016).

The VLA observations showed showed structure within the dust distribution in the innermost bright ring of HL Tauri. Here the dust was found to form clumps or knots, most of which were of a temporary nature, but with one particularly dense clump on the northeastern part of the limb being more permanent in nature. Carrasco-González et al. suggest that this is possibly a protoplanet in the earliest stages of formation, and that therefore planet formation around of HL Tauri is beginning in the brighter areas of the disk, not the lighter, with the bright areas representing parts of the disk with higher densities due to early stage planetary accretion rather than areas from which dust has been cleared by later stage planetary formation.

(a) Superposition of the VLA 7.0 mm image (contours; naturally weighted image; beam size ≈0:”067) over the ALMA 1.3 mm image (colour scale). Contour levels are -4, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, 100, and 150 times the rms of the 7.0 mm map, 3.5 μJy beam^-1. The two arrows mark the direction of the collimated jet at a P.A. of ~45º. (b) A close-up to the center of the disk. Colour scale is the ALMA 1.3 mm image and contours are from the high angular resolution VLA 7.0 mm image (robust 0 weighted image; beam size ≈0:”04). Contour levels are 3, 4, 5, 6, 7, 8, 12, 16, 20, 24, 28, 32, 40, 48, and 56 times 7 μJy beam^-1. (c) and (d): Comparison between sub-bands contour images at 7.3 mm and 6.7 mm (robust 0.4 weighting; beam sizes ≈0:”053) over the 7.0 mm colour scale image. Contours in both panels are 8, 9, 10, 11, 12, 13, 14, 15, and 16 times 6 μJy beam^-1.

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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...

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 (i.e. over 900 times as far from the star as Earth is from the...

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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.

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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.

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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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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 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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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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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).

See also…

 Discovering new debris disks in old Hubble images.

 The debris disk of 49 Ceti.

 Water and Hydroxides in the Circumstellar Disk around HD 163296.

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