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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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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