21.09.2021

Using radio data from the ALMA observatory and physical modelling, astronomers led by Kamber Schwarz (Max Planck Institute for Astronomy and University of Arizona) have managed to determine the mass of a potential "planet factory," the protoplanetary disk around the star GM Aurigae. From their reconstruction, which includes a determination of the disk's temperature profile, the astronomers deduce that the factory may just have started production: in an unstable region, conditions should be right to form a giant gas planet. The results showcase the increasing ability of astronomers to probe physical properties of planet formation as it happens.

Planets like our Earth form in so-called protoplanetary disks: gigantic disks of gas and dust that surround young stars. Over the past decades, astronomers have been able to observe such disks in ever finer detail. But finding out whether or not such a disk has already begun to form planets remains a challenge. Now, a group of astronomers led by Kamber Schwarz (MPIA and University of Arizona) has published the most successful attempt yet at industrial espionage in a planet factory: Using the ALMA observatory to observe the disk around the young star ("T Tauri star") GM Aurigae, combined with data from ESA's Herschel Space Observatory, they found evidence that a region within the disk has begun to collapse to form a giant gas planet. That disk and its star are a bit over 500 light-years away from us, which in astronomy counts as our closer galactic neighbourhood.

The work of Schwarz and her colleagues is part of a larger program called MAPS, "Molecules with ALMA at Planet-forming Scales", which is co-led by Karin Oberg, an astronomer at the Center for Astrophysics | Harvard and Smithsonian (CfA). Oberg says: "With ALMA we were able to see how molecules are distributed where exoplanets are currently assembling." The results were published as part of a special edition of the Astrophysical Journal Supplement with a total of 20 articles about MAPS results.

This ability of the ALMA observatory, which operates at millimeter and submillimeter wavelengths, to not only detect molecules, but also to track the detailed structures formed by those molecules within the disk, was crucial for Schwarz's sub-project. A key quantity in determining whether or not a protoplanetary mass is unstable, and likely to begin forming planets, is the disk's mass. This is where it gets difficult: Most of the mass of the disk is in the form of hydrogen molecules, H2. But such molecules are notoriously difficult to detect - at the comparatively low temperatures of a protoplanetary disk, it emits virtually no radiation at all.

In such situations, astronomers try to find "tracer molecules" that can be observed by their characteristic radiation and are typically found in the company of molecular hydrogen. When the presence and amount of those molecules is observed, the presence and the amount of molecular hydrogen can be estimated. For a protoplanetary disk as in this case, a very unusual molecule designated HD, hydrogen deuteride is a useful tracer. HD is a molecule consisting of an ordinary hydrogen atom (whose nucleus is a single proton) and a heavy hydrogen atom - Deuterium (which has a proton and a neutron as its nucleus).

HD is chemically identical to ordinary H2, so it is to be expected that the ratio between abundances of the two are always about the same, even in a protoplanetary disk. HD, in contrast with ordinary H2, can be detected in the far infrared, using radiation the molecule emits when changing between two different states of rotation. For the disk around GM Aurigae, ESA's Herschel Space Observatory, which specialized in far infrared and submillimeter observations, had already observed this characteristic HD radiation.

But the amount of radiation of this type depends on two factors: the amount of HD present, but also the temperature. In order to deduce the amount of HD, and from there estimate the mass of H2 in the disk, Schwarz and her colleagues first had to reconstruct the temperature in the different parts of the disk. This is where MAPS came in, with its thorough survey of many molecular lines.

Both atoms and molecules emit characteristic radiation in many different, very narrow wavelength regions called "spectral lines" - specifically, "emission" lines when molecules emit light in those regions as they are being excited by collisions with other molecules. For molecules, such spectral lines are typically in the infrared or submillimeter/millimeter region of the electromagnetic spectrum. Submillimeter/millimeter wavelength molecular lines are the main target of the MAPS observations.

How much radiation is emitted at what characteristic frequency depends on the energy available, specifically on the temperature of the molecular gas in question - as the temperature rises, new quantum states become accessible, and the main radiation output shifts to different frequencies. For example, the energy of certain kinds of radiation from carbon monoxide molecules decreases with temperature, acting as a natural "cosmic thermometer".

Schwarz and her colleagues put all of this together. Their reference point was an (axisymmetric) physical model they had created for the protoplanetary disk, detailed enough to reproduce the distribution of gas and dust as well as the varying temperatures. They changed their model parameters around until they had achieved the best-possible fit with the various observations - notably the CO distribution and temperature distribution, as reconstructed from a "cosmic thermometer" consisting of eleven emission lines from carbon monoxide, and the Herschel Space Observatory data point for HD.

In the end, the astronomer had not only found the best mass estimate yet for a protoplanetary disk of this kind, namely that the disk contains 0.2 solar masses' worth of material - a surprisingly large value. The disk is also comparatively cold, with 32% of the mass cooler than 20 Kelvin (20 degrees Celsius above absolute zero). Using a parameter whose value indicates the stability or instability of specific regions of such a disk ("Toomre Q parameter"), they were also able to show that while most disk regions were stable, and not in danger of collapsing, there was an exception: Within a certain distance from the central star - between 70 and 100 times the Earth-Sun distance - the disk does appear to be on the verge of instability. That region is visible in observations that show emissions from dust as a bright ring. The presence of significant amounts of dust is likely to shield that region from the star's radiation - which in turn produces lower temperatures that help along with gravitational collapse.

Taken together, the elaborate feat of industrial espionage in a potential planet factory indicates that yes, planet production might well have already started, with the most likely product a future giant gas planet.

But to be certain, additional observations are needed. In particular, in planet formation, one would expect a local region of the ring to collapse. But the physical model on which the present conclusions are based is axisymmetric, modelling the disk as a collection of rings. Also, the present model does not allow for significant motion of the gas, another factor that could help decide whether conditions for local collapse and subsequent planet formation are just right.

To that end, Schwarz and colleagues plan to investigate the nature of the potentially unstable region by using another part of the MAPS data: Specific properties of the spectral lines, notably how wide a wavelength region they cover, allow researchers to reconstruct the velocities at which gas is moving in different regions of the disk. That should allow the astronomers to say for certain whether or not the disk contains a planet-in-the-making.

Overall, the current results, which are being published as one of 20 articles with MAPS results in a special edition of the Astrophysical Journal Supplement Series, are an impressive demonstration of how far astronomers have come in tracing the beginnings of planet formation: from coarse observations of young stars to detailed imaging of the protoplanetary disks around them, and now to tracing diverse species of molecules within such disks - and using the information to deduce the presence of a potential giant-planet-forming instability.

Quelle: SD