Protostars to Planets — The Chemical Pathway

Title: Complex organic molecules in low-mass protostars on solar system scales — I. Oxygen-bearing species

Authors: M. L. van Gelder, B. Tabone, Ł. Tychoniec, E. F. van Dishoeck, H. Beuther, A. C. A. Boogert, A. Caratti o Garatti, P. D. Klaassen, H. Linnartz, H. S. P. Müller, V. Taquet

First Author’s Institution: Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands

Status: Published in Astronomy & Astrophysics [closed access]

Complex Organic Molecules in Protostellar Systems

Life, as we know it, is contingent upon complex organic molecules (COMs), or molecules containing carbon and at least 6 atoms. These molecules have been detected in high- and low-mass star-forming regions which generally consist of a “baby” star, or protostar, and associated protostellar outflows. Studying the rich chemistry in protostellar systems involves placing constraints on the abundances of COMs, which are dependent on the environmental properties, or evolutionary stage. Figure 1 illustrates the different stages of these systems.

Figure 1:  Cartoon depicting the evolution of a stellar and planetary system, starting from initial accretion at the Class 0 stage to a fully developed system with a central star and associated planets at the Class III stage. Image adapted from Williams, J. P. Star Formation Course.

Protostars are usually considered to be in the Class 0 or Class I stage, where the star is still accreting, or accumulating, most of its mass in the form of particles, debris, and gas. Class 0 sources are deeply-embedded in a circumstellar dusty envelope making direct observations of the central protostar difficult. Class I objects begin to emerge but are still surrounded by an envelope, with an accretion disk beginning to form. At the Class II stage, a pre-main sequence star has reached its final mass and is surrounded by a massive protoplanetary disk whereas Class III objects have low-mass disks with little to no accretion. Though protoplanetary disks during the Class II stage were considered to be the birthplace of planets, planet formation can already begin in the Class 0/I phase! Detailing the chemical inventory and abundances of COMs in Class 0/I protostars allows us to not only study early stages of star formation, but also probe the initial conditions of planetary formation. 

COMs are thought to form in cold protostellar envelopes on icy mantles of dust grains where molecules ranging from methanol (CH3OH) to glycerol (HOCH2CH(OH)CH2OH) can be found. As COMs move towards the protostar, the ultraviolet (UV) radiation leads to dissociation of these large molecules into their simpler constituents, which can react with other COMs. The rich gas-phase chemistry enabled by heating of ices can have long-lasting influences on planetary composition. Today’s paper seeks to constrain abundances of oxygen-bearing COMs in Class 0 protostellar systems found in the Perseus and Serpens molecular clouds to allow for better modeling and understanding of ice mantle to gas-phase chemistry.

ALMA Observations and Analysis

The revolutionary capabilities of the radio telescope Atacama Large Millimeter/submillimeter Array (ALMA) allow for high sensitivity chemical studies called line surveys. Using ALMA, the authors specifically targeted the several COMs (e.g. methanol, ethanol, and methyl formate) for 7 different objects. COMs were detected in only 3 out of the 7 objects (B1-bS and B1-c in Perseus and S68N in Serpens). In Figure 2, intensity maps for the three objects show the spatial distribution of methanol.

Figure 2: Spatial distribution emission maps of methanol (CH3OH) for the 3 systems with COM detections, with continuum emission shown as the black contours. The white ellipse in the lower right depicts beam size to indicate resolution. Reproduction of Figure 1 in the paper.

The authors determine that B1-c is the most COM-rich source of the 3 objects, as seen by the colorbars in Figure 2—B1-c spans a much larger intensity scale compared to the other two sources. Abundance ratios for the other COMs are calculated relative to methanol and compared to literature values for protostars in the Ophiuchus and Orion Molecular Clouds, as discussed in Figure 3. 

Unexpected Result #1: No relation between COM emission and stellar luminosity.

Interestingly, though previous studies (e.g. Jørgensen et al. 2002, Young & Evans 2005, and Visser et al. 2009) of stellar systems have associated higher luminosity with more emission of COMs, there is no such correlation for the 7 young stellar systems observed in this paper. In fact, the source with the highest luminosity, SMM3, had no COM emission at all, indicating some other chemical effects are at work or that observational biases—such as high dust opacity—are hiding COM emission. However, the most remarkable result is depicted in Figure 3. 

Unexpected Result #2: Chemical abundance similarities across 4 star-forming clouds!

Abundances derived for the above targets in Perseus and Serpen Clouds were compared to literature values for protostars in Ophiuchus and Orion as shown in Figure 3.

Figure 3: Comparison between derived abundances for this paper’s targets (B1-bS, B1-c, S68N) with literature values for abundances for other sources (IRAS 16293B in Ophiuchus; HH 212 in Orion). Illustrates unlikely result: very different environments with surprisingly similar chemical/COM abundances. Reproduction of Figure 7 in the paper.

These 4 clouds have hugely varying properties (e.g. luminosity and temperature), and thus the similarity in abundances for several COMs (e.g. CH2DOH, CH3OCHO, CH3OCH3, and CH3COCH3) is unexpected. Similar abundance values across vastly different cloud regions suggest an (unlikely) universality governing complex molecular formation pathways in the (warm) protostellar phase and (cold) interstellar medium

 JWST and the Future of Astrochemistry

The sample discussed here is rather small, and little is known about the chemical evolution of young protostellar systems. These unexpected results show that we need more ice observations to constrain the molecular abundances to compare with laboratory measurements and improve our chemical reaction modeling pathways. The upcoming James Webb Space Telescope (JWST) will directly observe the COMs embedded in ices of stellar nurseries, thus paving the way for our understanding of gas-grain chemistry and its potential to form the most basic—yet chemically complex—building blocks of life.

Astrobite edited by Olivia R. Cooper and Jamie Wilson

Featured image credit: University of Hawaiʻi 88-inch Telescope (UH88), Nedachi et al., NASA

About Suchitra Narayanan

Aloha! I am a fourth-year PhD candidate, P.E.O. Scholar, and NSF Fellow at the Institute of Astronomy (UH Mānoa) jointly working at the Center for Astrophysics | Harvard & Smithsonian studying the “Organosulfur Chemistry in the Birthplaces of Planets.” I characterize the fundamental properties and formation pathways of complex sulfur organics through ultra-high vacuum chamber experiments. I am also part of the eDisk ALMA Large Program to understand the structure, dynamics, and chemistry of embedded disks (the earliest stages of stellar and planet formation). I focus on combining laboratory experiments, telescope observations, and theoretical modeling (both at the micro (computational chemistry) and macro (2D thermo-chemical models) scales) to reconcile what is known as the “missing sulfur” problem. I'm broadly interested in astrochemistry, with a focus in its role in planetary formation and eventual evolution of resulting (exo)planetary atmospheres. I am also part of the AAS Sustainability Committee, and am passionate about using our knowledge as astronomers to better our life here on the only planet we can call home. I originally am from Coimbatore but have spent most of my life in the Bay Area. I studied both chemical engineering and astrophysics at University of California, Berkeley. When I’m not in the laboratory, you can find me at the piano (I’ve been classically trained since I was 4!) or in the ocean (I’ve been a competitive swimmer/water polo player, and open water lifeguard for East Bay Regional Park District). Please reach out if you're interested in astro+climate work!

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