Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC$_3$N, CH$_3$CN, and $c$-C$_3$H$_2$ (2109.06319v2)

Abstract: The precursors to larger, biologically-relevant molecules are detected throughout interstellar space, but determining the presence and properties of these molecules during planet formation requires observations of protoplanetary disks at high angular resolution and sensitivity. Here we present 0.3" observations of HC$_3$N, CH$_3$CN, and $c$-C$_3$H$_2$ in five protoplanetary disks observed as part of the Molecules with ALMA at Planet-forming Scales (MAPS) Large Program. We robustly detect all molecules in four of the disks (GM Aur, AS 209, HD 163296 and MWC 480) with tentative detections of $c$-C$_3$H$_2$ and CH$_3$CN in IM Lup. We observe a range of morphologies -- central peaks, single or double rings -- with no clear correlation in morphology between molecule nor disk. Emission is generally compact and on scales comparable with the millimetre dust continuum. We perform both disk-integrated and radially-resolved rotational diagram analysis to derive column densities and rotational temperatures. The latter reveals 5-10 times more column density in the inner 50-100 au of the disks when compared with the disk-integrated analysis. We demonstrate that CH$_3$CN originates from lower relative heights in the disks when compared with HC$_3$N, in some cases directly tracing the disk midplane. Finally, we find good agreement between the ratio of small to large nitriles in the outer disks and comets. Our results indicate that the protoplanetary disks studied here are host to significant reservoirs of large organic molecules, and that this planet- and comet-building material can be chemically similar to that in our own Solar System. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement Series.

• The paper demonstrates that ALMA’s high-resolution imaging reveals diverse spatial distributions of HC3N, CH3CN, and c-C3H2, including central peaks and rings.
• The study employs disk-integrated and radially-resolved rotational diagram analyses to derive molecular column densities and temperatures below sublimation thresholds.
• These findings suggest chemical similarities between protoplanetary disks and comets, refining models of planet formation and the origins of life.

Analysis of Organic Molecules in Protoplanetary Disks

The paper by Ilee et al., titled "Molecules with ALMA at Planet-forming Scales (MAPS) IX: Distribution and properties of the large organic molecules \ce{HC3N}, \ce{CH3CN}, and $c$-\ce{C3H2}," presents a comprehensive paper on the presence and distribution of complex organic molecules in several protoplanetary disks. Utilizing the high-angular resolution and sensitivity capabilities of ALMA, the paper focuses on five specific protoplanetary disks: GM Aur, AS 209, HD 163296, MWC 480, and IM Lup. The primary aims include investigating molecular distribution across these disks and deciphering the connections between these molecules and the overall chemistry of planet formation.

Molecular Detection and Spatial Distribution

The research robustly identifies the presence of \ce{HC3N}, \ce{CH3CN}, and $c$-\ce{C3H2} in GM Aur, AS 209, HD 163296, and MWC 480, with tentative detections in IM Lup. Each molecule exhibited various morphologies across the disks, including central peaks and ring formations. The paper found no consistent correlation between the morphology of different molecules within the same disk, highlighting the complexity of the chemical environment in these planetary-forming systems.

Analytical Techniques and Findings

Ilee et al. employed disk-integrated and radially-resolved rotational diagram analyses to derive molecular column densities and rotational temperatures. This approach disclosed that the molecules are concentrated primarily within about 50-100 au from the central star, suggesting significant reservoirs of complex organics in the regions where planet formation is most active. The detection of molecules at these scales indicates that material available for planet formation in these disks may share chemical similarities with that of our Solar System.

Optical Depth and Temperature Insights

The rotational temperatures for \ce{HC3N} and \ce{CH3CN} were found to be below their expected sublimation temperatures, indicating that direct thermal desorption from icy grains was unlikely to be the main source of these molecules. The authors found that emission from these complex organics is generally optically thick in some regions, which suggests the possibility of yet-undetected reservoirs within these disks.

Comparisons and Implications

The results extend prior detections of large hydrocarbons and nitriles in protoplanetary disks, aligning well with the chemical inventories of comets. The paper proposes that protoplanetary disks are chemically complex and rich in materials that could potentially foster the development of life-supporting planets.

Future Work and Scientific Impact

The paper's outcomes reinforce the hypothesis of a shared chemical heritage between comets and protoplanetary disks, driven by the advanced capabilities of ALMA. Future studies should aim to deepen our understanding of organic molecule formation pathways and their connection to exoplanetary system chemistry. Expanding upon these findings could also refine our models of ice and gas-phase chemistry in disk environments, ultimately contributing to theories about the origin of life and solar system formation.

In conclusion, Ilee et al.'s work provides significant insights into the molecular complexity present in protoplanetary disks, offering a window into the building blocks available for planet formation and their potential link to the conditions necessary for life as we understand it. This paper establishes a foundation for future research aimed at exploring the chemical evolution of forming planetary systems.