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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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.

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Protoplanetary disks around Class I Protostars in the ρ Ophiuchi Star Forming Region.
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