Understanding the protoplanetary disk of the eruptive protostar V900 Mon.

Mass accretion is a crucial stage in the development of young stars, with material accreting onto young stellar objects from the circumstellar disks surrounding them. Some (and possibly all) young stars undergo explosive eruptions during this stage of their evolution, linked to episodes of increased mass accretion onto the star. This is best documented in FU Orionis-type protostars, a class of stellar objects with protoplanetary disks more compact than seen in other types of young stellar object.

The FU Orionis-type protostar V900 Mon was discovered in 2012, and is about 4000 light years from Earth within Thommes' Nebula in the constellation of Monoceros, and lying within the Galactic Plane, where light is known to be scattered by higher levels of interstellar dust, making objects appear dimmer. This is likely to be more true for V900 Mon, which is still embedded within the dust cloud from which it formed.

Observations of the distribution of Carbon Monoxide around V900 Mon, made with the Atacama Large Millimeter/submillimeter Array (ALMA), have shown that the star is surrounded by two cones of material, each with an opening angle of 70°, with the western outflow oriented roughly 250° east-of-north. Such wide cones typically eject gas at much lower rates than the bipolar outflow jets seen around T Tauri-type young stars, ejecting material at less than 100 km per second. 

The nebula around V900 Mon is very bright on its western side, and very faint on its eastern side, as is the carbon monoxide emission stream, consistent with us viewing the star pole on, so that material behind the surrounding disk is obscured. It has been calculated that the circumstellar disk around V900 Mon measures 82 AU by 71 AU (where 1 AU is the distance between the Sun and the Earth) with an inclination of about 28°, and can be seen from the Earth at an angle of 169°, almost pole-on. Estimates of the amount of material in the disk range between 1% and 30% of the mass of our Sun; a mass at the larger end of this range would make it one of the largest FU Orionis-type disks known.

The eruptive activity of FU Orionis-type protostars is presumed to have a profound impact on the circumstellar environment, including the crystallization of amorphous silicates, although this has never been observed. Curiously, crystallization via thermal annealing has been detected around the T-Tauri star EX Lupi, which has much cooler outbursts (less than 1000 K). A previous attempt to detect large-sized silicate grains in the circumstellar disk of V900 Mon using the Very Large Telescope Imager and Spectrometer for mid Infrared was inconclusive.

In a paper published on the arXiv database at Cornell University on 13 November 2023, and accepted for publication in the journal Astronomy & Astrophysics, a team of scientists led by Foteini Lykou of the Konkoly Observatory of the Hungarian Research Network Research Centre for Astronomy and Earth Sciences, present the results of a new study of V900 Mon, made using the MATISSE instrument at La Silla Paranal Observatory, with additional data from the Multi Unit Spectroscopic Explorer (MUSE) at the European Southern Observatory, the SpeX: 0.7-5.3 Micron Medium-Resolution Spectrograph and Imager at the Institute for Astronomy on Mauna Kea, and the MID-infrared Interferometric instrument of the Very Large Telescope.

V900 Mon has been observed to brighten steadily for the past 20 years (it has been located in older images since its discovery), with a brief period of dimming in 2018-19. It has also become slightly bluer since the dimming event. It is likely that the brightening has been caused by a diminishing of the dust cloud around the star, rather than by the star actually getting brighter.

Young stellar objects, including FU Orionis-type protostars, tend to have an inner accretion disk which reaches to less than 1 AU from the star, and an outer passive disk, which may reach as far as 100 AU from the star, with the hotter inner disk emitting radiation at much shorter wavelengths than the cooler outer disk. 

Data from the MATISSE instrument suggests that the inner disk of V900 Mon reaches no more than 1.5 AU from the star, and the majority of the material in the outer disk no more than 10 AU. Combined with the ALMA data, this suggests that the disk is inclined at 14°, and that Earth-bound viewers are seeing it at an angle of 158°, almost pole-on. 

The star at the centre of the V900 Mon system cannot actually be seen in the MUSE images, which are dominated by the reflection nebula (Thommes’ Nebula), although the outer disk can be detected. The disk was notably bluer when a larger aperture used, suggesting that the nebula is scattering light more at this end of the spectrum.

White image of V900 Mon and Thommes’ Nebula from MUSE (4700 – 9300Å). The cross sign marks the location of the star and the drawn green line marks the bulk of the emitting region that includes the ellipsoidal component. The field of view is 1′×1′. The image has been stretched at arbitrary levels of the square root of intensity to enhance nebula features. Lykou et al. (2023).

The images show the star+disk system, an ellipsoidal-like component adjacent to the star toward the northwest, and a large-scale source to the west and southwest (Thommes’ Nebula), though Lykou et al. were able to remove much of the interference from the latter two using a corrective algorithm. 

Once this was done, spectrographic analysis revealed what appears to be a jet-like structure emerging from the stellar region, with a knot of material about 27 000 AU (0.43 light years) from the star. The position of this aligns with the previously detected carbon monoxide outflow, being close to 250° east of north,  perpendicular to the disk’s major axis. Taking into account the 14° inclination of the disk, the actual distance from the star to the knot may be 111 600 AU (1.77 light years).

Continuum-subtracted Hydoigen-α linemap of V900 Mon and Thommes’ Nebula. The map has been linearly scaled to enhance the emission features (in white) and designate scattered light continuum from the nebula and background sources (black-shaded regions). The green contours (arbitrary levels) mark the carbon monoxide (2-1) moment zero map, showing the blue-shifted emission component to the west and part of the red-shifted wide-angle lobe to the east. The emission-line knot (E.L.K.) is clearly visible and co-aligned to the blue-shifted carbon monoxide outflow, as indicated by the guiding arrow (position angle roughly 250° east-of-north). The field of view is 50′′×50′′. Lykou et al. (2023).

The spectra of the knot suggest that it is comprised of excited gas, and appears blue shifted compared to the other structures, suggesting that it's motion is towards the viewers at a speed of about 100 km per second. The knot is estimated to have left the star about 5150 years ago, long before the most recent eruption of V900 Mon, which is thought to have happened since the 1960s, although it will be possible to calculate the exact age of the knot only when the true motion of the knot is known, and it will only be possible to calculate this once it has been determined whether the gas in the knot is expanding, something which will require repeated observations.

There also appears to be some sort of structure to Thommes' Nebula, with at least one globule within, which does not appear to be an emission from the star. The nature of this is, however, unclear, and will require further observations.

MATISSE observations of the V900 Mon system show a structure at the centre of the system less than 3 AU in diameter, which Lykou et al. interpret as an inner accretion disk, something expected in a FU Orionis-type protostar system. Modelling of the system suggests that the inner radius of this disk is equal to about 2 stellar radii, though this is poorly constrained. This inner disk is estimated to be emitting 314 times as much light as our Sun.

Material is thought to be accreting from the disk onto the star at a rate of about 0.000 04 times the mass of our Sun each year, or roughly the mass of Jupiter over 30 years. 

The protoplanetary disks of FU Orionis-type protostars are thought to be silicate-rich, although silicates could only be detected in the outer disk of V900 Mon at distances of greater than 10 AU, suggesting that silicates are either absent from the inner part of the disk, or are sheilded from observation by other material. 

Crystalline silicates are commonly detected in the protoplanetary disks of Herbig and T Tauri stars, but are apparently absent in the disks of FU Orionis-type protostars, which also seems to be the case for V900 Mon, with the detected silicates in the disk being more consistent with large amorphous grains.

Theoretically, eruptive outbursts from protostars might be caused by the presence of a companion body, with outbursts occurring when this body passes through the cirumstellar disk, causing a peak in the amount of material accreting onto the protostar. A possible companion body has been detected in the Z CMa FU Orionis-type protostar system, but none of the observations of V900 Mon suggest the presence of such a body. 

It is possible that a flyby star could have interacted with the circumstellar disk of V900 Mon within the last century. If such a star was travelling at less than 10 km per second then it should still be within the field of view of Lykou et al.'s observations, but no such star could be detected.

Because we are essentially looking down upon the V900 Mon system, the obscuring of the inner part of the disk could be due to the presence of either a clump of dust above the system, or at the origin of the molecular outflow emanating from the disk. A similar phenomenon has been observed in the Herbig Ae star HD163296, which is known to have associated Herbig-Haro objects (bright patches of nebulosity formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second), with a dust cloud having apparently caused the star to dim.

Cartoon indicating the presumed geometry of the system. The drawing is not to scale. Lykou et al. (2023).

The nature of the dust causing this dimming cannot be directly determined, though Lykou et al. reason that it is likely to be mostly small grains of amorphous silica, with a grain size of less than 0.1 μm. If this dust is constrained within a radius of less than 5 AU, then it will have a mass in the region of 990 000 000 000 000 kilotonnes, roughly equivalent to the mass of the Dwarf Planet Ceres, although this is based upon a very rough analysis and cannot be assumed to be an accurate figure. Future imaging of the system at different wavelengths should allow a more accurate estimate to be made.

Examination of historic images of V900 Mon taken at a variety of wavelengths show that one feature consistently recovered is a 'helicoidal' tail fanning out to more than 20'' west-southwest of the star. The emission-line knot discovered by Lykou et al. is not visible in any of the previous images of the system, suggesting it is either not visible at the wavelengths utilised by those studies, or is to dim at those wavelengths to be detected.

Lykou et al. also looked for any additional knot formed by a more recent eruption, within the past 30 years. If such a knot was assumed to move at about 25 km per second, then it would be no more than 13'' from the star in the images. It was not possible to detect such a knot in the data, but this may reflect the proximity of the knot to the star, and it is possible that such a knot will be detected in future observations, now that it is being looked for.

Drawing of the individual nebular features at the V900 Mon reflection nebula.  Lykou et al. (2023).

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First protein of extraterrestrial origin found within the Acfer 086 Meteorite.

Although individual amino acids have been found in abundance in carbonaceous meteorites there have been only two reports of polymers of amino acids The first of these was di-glycine, from the Murchison Meteorite, a carbonaceous chondrite which fell in Victoria, Australia in 1969, and the Yamato-791198, a second carbonaceous chondrite collected from Antarctica in 1979. More recently a range of large polymers of mainly glycine was reported in the Allende Meteorite, a 17.3 kg carbonaceous chondrite which fell in Mexico in February 1969.

In a paper published on the arXiv Database at Cornell University on 22 February 2020, Malcolm McGeogh of the PLEX Corporation, Sergei Dikler of Bruker Scientific, and Julie McGeogh of the Department of Molecular and Cellular Biology at Harvard University, present the discovery of the first known protein of extraterrestrial origin, found within the Acfer 086 Meteorite, a 17.5 kg carbonaceous chondrite found in Algeria in 1989.

In follow-on work to the discovery of large polymers in the Allende Meteorite high molecular mass organic molecules were discovered in the Allende and Acfer 086 meteorites, with extra-terrestrial isotope enhancement that confirmed that these unexpected molecules were not artifacts due to terrestrial contamination.

Studying molecules from the Acfer 086 Meteorite with a state-of-the-art mass spectrometer generated much-improved signal-to-noise ratios that allowed McGeogh et al. to discover via the iron⁵⁴ isotope satellite (isotope satellites are small peaks that can be seen shouldering the main peaks in an Nuclear Magnetic Resonance spectrum) that iron was also present and bonded to the glycine and hydroxy-glycine, in specific arrangements. Also, via its lithium⁶ satellite, lithium was found to be a standard component.  When the prior observations are also taken into account, the most probable core molecule has a molecular mass of 2320, containing two glycine strands each of length 16 residues, but variants with 15 and 17 residues also appear to be present. This molecule is given the name 2320 hemolithin, derived from its molecular weight and iron and lithium contents.

Model of the 2320 hemolithin molecule after Merck Molecular Force Field energy minimisation. Top: in space-filling mode; Centre: ball and stick; Bottom: enlarged view of iron, oxygen and lithium termination. White, hydrogen; orange, lithium; grey, carbon; blue, nitrogen; red, oxygen; and green, iron. Hydrogen bonds are shown by dotted lines. McGeogh et al. (2020).

This is the first report of a protein from any extra-terrestrial source. Room temperature extracts from micron-sized meteorite particles contain polymers of amino acids with a definite chain length centered at 16 residues. Analysis via iron and lithium isotope satellites in mass spectrometry reveals a novel protein motif with iron atoms closing out the ends of anti-parallel peptide chains composed of glycine.

In a review of the known binding geometries for iron within proteins such as hemoglobin or hemerythrin, this new motif was not found. Its overwhelming simplicity and the new association with beta sheet proteins make its future study very important.

The molecule was found to be highly enriched in deuterium (hydrogen²) compared to terrestrial hydrogen sources. Such high deuterium enhancements are well documented in molecular clouds where low temperature isotopic selection has operated for timespans of more than 10 million years. McGeogh et al. estimate that simple amino acids should be able to slowly polymerise in the conditions of 'warm dense molecular clouds', without necessarily requiring surfaces for the reaction. Furthermore, the elements hydrogen, lithium, carbon, nitrogen, oxygen, and iron, comprising this molecule were initially the most abundant when the first massive stars released them about 13 billion years ago.

Deuterium is also enriched in molecules of proto-planetary discs. A previous study applied complete molecular cloud chemistry models to the disc and one of the predictions from this relates to the amount of deuterium/hydrogen enrichment expected at 30AU radius (i.e 30 timse as far from the Sun as the Earth), which is the likely region for the origin of comets, which was again consistent with the level of enhancement found in the hemolithin molecule. Interestingly, the Acfer 086 Meteorite has also been shown to be enriched in the nitrogen isotope nitrogen¹⁵ to an extent consistent with cometary values. However, both Acfer 086 and the Allende Meteorite are considered to be CV3 chondritic meteorites, thought to derive from a parent body in the asteroid belt that is only between 2.2 and 3.3AU from the Sun.

It appears that three common variants of Hemolithin with chain lengths 15, 16 and 17 may co-exist. If this molecular type grows spontaneously when its glycine, iron, oxygen, lithium and hydrogen components are present together in a particular environment then it is difficult to imagine such a tight length distribution being the result. A function yet to be identified could determine the length and maintain such a tight distribution. If successful in that function, energy could be harvested, which is a thermodynamic requirement, to aid in the creation of molecular copies of identical length.

One-step room temperature solvent extraction from micron scale particles of the Acfer 086 Meteorite, a CV3 chondrite, yielded a relatively simple spectrum that is dominated by a single protein type. This is composed of anti-parallel beta strands of glycine, each of 15 to 17 -residue length, with about 20% oxidation to hydroxy-glycine, and termination at each end with an iron atom directly bonded to carbon and nitrogen terminals of the peptide strands. There are additional tri-oxygen/iron groups at each end, and lithium adducts. The principal indicator of extra-terrestrial origin is an extreme raised deuterium/hydrogen ratio that is revealed by close quantitative fitting of isotopic satellite peaks. The average molecular deuterium excess above terrestrial is comparable to cometary levels, interstellar levels and also equal to the highest prior report in micro-meteorites. The iron oxide grouping at the tips of the molecule is of a type studied in other contexts for the photo-splitting of water.

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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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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).

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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...

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.

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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.

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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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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