Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC3N, CH3CN, and c-C3H2

John D Ilee,Kamber R Schwarz,Catherine Walsh,Charles J Law,Karin I Öberg,Yuri Aikawa,Ryan A Loomis,Jennifer B Bergner,L Ilsedore Cleeves,Gianni Cataldi,Arthur D Bosman,Viviana V Guzmán,Ke Zhang,Richard Teague,Alice S Booth,Takashi Tsukagoshi,Jane Huang,Ian Czekala,Romane Le Gal,Yoshihide Yamato,Sean M Andrews,Edwin A Bergin,Hideko Nomura,Chunhua Qi,David J Wilner,Jaehan Bae,F Ménard

doi:10.3847/1538-4365/ac1441

Abstract

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 HC3N, CH3CN, and c-C3H2 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-C3H2 and CH3CN in IM Lup. We observe a range of morphologies—central peaks, single or double rings—with no clear correlation in morphology between molecule or disk. Emission is generally compact and on scales comparable with the millimeter 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 CH3CN originates from lower relative heights in the disks when compared with HC3N, 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.

Highlights

Protoplanetary disks host the basic ingredients for planet formation
We present 0. 3 observations of HC3N, CH3CN, and c-C3H2 in five protoplanetary disks observed as part of the Molecules with Atacama Large Millimeter/submillimeter Array (ALMA) at Planet-forming Scales (MAPS) Large Program
We report high-angular resolution (0. 3) observations of multiple transitions of HC3N, c-C3H2, and CH3CN towards five nearby protoplanetary disks with ALMA (IM Lup, GM Aur, AS 209, HD 163296, and MWC 480)

Summary

Introduction

Protoplanetary disks host the basic ingredients for planet formation. The abundance and spatial distribution of organic molecules are of particular importance as the most complex of these — the so-called complex organic molecules (COMs) — are the precursors of larger, pre-biotic, molecules (see, e.g., Herbst & van Dishoeck 2009). The study of the chemical content of protoplanetary disks began during the advent of (sub)millimetre astronomy that enabled the detection of rotational transitions of small molecules such as CO, HCO+, CN, CS, C2H, HCN, HNC, and H2CO (e.g., Dutrey et al 1997; Kastner et al 1997; van Zadelhoff et al 2001; Aikawa et al 2003; Thi et al 2004; Oberg et al 2010) In these pioneering studies, it was realised that the gas-phase abundances of these species in protoplanetary disks were orders of magnitude lower than those in nearby dark clouds. The detection of emission from such species provides insight into the composition and distribution of the organic ice reservoir

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