Zooming in on the Chemistry of Star and Planet Formation

Abstract

The dust- and gas-rich environments of protoplanetary disks provide the raw materials needed for forming planets. The molecular gas and associated chemistry are directly tied to the resulting exoplanet architectures, including what types of planets form, their atmospheres, and potential habitability. While molecular line observations often provide the best probes of disk characteristics relevant to planet formation, such as gas surface density, ionization, temperature, and C/N/O ratios, most line observations have been limited to coarse angular resolutions. Thus, the detailed structure of the gas component in disks remains largely unexplored, especially toward the inner, planet-forming regions ( au). To remedy this, we performed a survey of over 50 molecular lines toward five protoplanetary disks at 10 au scales as part of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. MAPS represents the most comprehensive disk chemistry survey conducted at these scales to date. Using these observations, I showed that chemical substructures in the form of rings and gaps are ubiquitous and extremely varied in their radial locations, widths, and contrasts. This suggests that planets form in diverse chemical settings across disks and at different radii within the same disk. The favorable inclinations of the MAPS disks also provided a direct view of their vertical gas distributions. In each disk, I mapped this vertical structure, from midplane to disk atmosphere, by extracting emitting heights of several CO isotopologues and used these to derive 2D gas temperatures, which are critical inputs for disk models. I then applied these techniques to extract emitting surfaces from a large sample of disks with ALMA archival data. I showed that disks exhibit a wide range of CO gas heights and since the vertical distribution of gas influences the chemical reservoirs available to nascent planets, this implies further diversity in local planet-forming environments. The planet formation process is also expected to alter the physical and chemical structure of the disk itself through local gas heating or shocks that sputter heavy atoms from dust grains, which should result in detectable chemical asymmetries. Using ALMA archival data, I also identified several chemical signatures related to ongoing planet formation in the giant-planet-hosting HD 169142 disk, including compact SO and SiS emission as well as localized 12CO and 13CO emission counterparts coincident with the location of a proposed giant planet. This is the first tentative detection of SiS emission in a protoplanetary disk and suggests that the planet is driving sufficiently strong shocks to produce gas-phase SiS. In addition to studying planet-forming disks, the high angular resolution and sensitivity of ALMA also allows us to detect the molecular gas around massive protostars. Such sources are excellent interstellar laboratories to study complex organic molecules (COMs), which are likely present, but too faint to directly detect, in disks. In the high-mass star-forming region G10.6-0.4, I found bright and highly-structured COM emission and pinpointed the location of two hot molecular cores, which are signposts of the formation of young massive stars. I derived spatially-resolved maps of rotational temperature and column density for a large sample of COMs, which revealed several intriguing spatial correlations that suggest that our current understanding of COM chemistry is far from complete.