Molecular Processing in the Disks of Massive Stars

By Andrew Barr, Alexander Tielens, and Joan Schmelz

Paper: High-resolution Infrared Spectroscopy of Hot Molecular Gas in AFGL 2591 and AFGL 2136: Accretion in the Inner Regions of Disks around Massive Young Stellar Objects
Barr, Andrew G., et al., 2020/09, ApJ, 900, 104.

Over the last two decades, astronomers have discovered that almost all stars have an associated planetary system. This gives rise to a number of key questions including, "What is the chemical inventory accessible to forming planets?” and "What chemical and physical processes are important in the evolution of the organics delivered to newly forming planets?” To address these questions, we have initiated a survey of the mid-infrared spectra of the disks around massive stars.

While observations of disks around low-mass stars have become routine, even the detection of their high-mass counterparts is rare. So despite the importance of disks to star and planet formation, little is known about the physical and chemical processes that govern the early phases of high-mass stellar evolution.

High resolution mid-infrared spectroscopy with the EXES instrument on SOFIA provides a unique view of these regions. With EXES, molecules can be studied that have no pure rotational transitions in the sub-millimeter, and hence, instruments such as ALMA cannot detect their presence and derive their abundances. This includes such simple organic species as acetylene and methane, the building blocks of complex organic molecules in warm gas.

EXES observations are capable of probing these disks on the scales of planet formation. The spectra show that large amounts of water and simple organics are produced within 50 AU of the protostar, which means this material could eventually be incorporated into planetary systems around these massive stars.

EXES has conducted the first ever full spectral survey at high spectral resolution of the 4.5-13 µm regime towards two massive protostars, AFGL 2591 and AFGL 2136. Across the wavelength range of the spectral survey, absorption lines of CO, H2O, HCN, C2H2, NH3 and CS are detected. All of these molecules are predicted to form in large quantities in the inner regions of circumstellar disks. Temperatures of 600 K are derived towards both sources, consistent with models.

Previous modelling and imaging of sub-millimeter emission lines towards these sources places the absorbing gas close to the central protostar, at a distance of around 50 AU. This is the planet forming zone so this material may eventually be incorporated into planets orbiting these stars. The composition of the gas in these disks is different from that of cold molecular clouds in which these stars are formed, attesting to the importance of chemical processing in the warm, dense environment of these disks. Astronomers think that similar processes played a role in the early solar nebula in which the terrestrial planets were formed.

The fact that the molecules are seen in absorption implies that the disk photosphere behaves like a stellar atmosphere, with a temperature that decreases outwards. Using this approach, we were able to calculate the chemical abundances of each molecule. This study provides a benchmark to guide future JWST/MIRI observations which will be able to carry out this kind of research towards low mass protostars in a similar stage of their evolution as AFGL 2591 and AFGL 2136.

Absorption lines have also been observed towards other massive protostars. Physical conditions are consistent with an accreting disk with a high abundance of material in the planet forming zone. We are continuing the study of these objects with follow-up spectral surveys to each of these known sources.

SOFIA provides a unique view and opportunity to carry out these kinds of studies, piercing through the dusty environs to the hearts of these disks. As we begin to get more data from these rarely observed objects, we will begin to unravel some of the questions surrounding the disks of massive protostars and the formation of planets in them.

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