New interstellar laboratories in the molecular ring.

Much of what is known about chemistry in star-forming regions comes from observations of nearby (d < 500 pc) low-mass protostars. For chemistry in high-mass star-forming regions, several more distant (d ∼ 2-8 kpc), exceptionally bright molecular sources have also been the subjects of repeated observations but with concomitantly poorer linear spatial resolution. Facilities such as ALMA and JWST, however, now provide the means for observing distant sources at dramatically higher spatial resolution and sensitivity. We used the modest resolving power of the Atacama Compact Array, a dedicated subset of ALMA antennas, to carry out a pilot survey of 11 giant molecular clouds selected from the Bolocam Galactic Plane Survey [Battisti & Heyer, Astrophys. J., 2014, 780, 173] within the so-called molecular ring between about 4 and 8 kpc from the galactic center. Within our observed sample, molecular emission regions-most of which correspond to at least one (candidate) young stellar object-exhibit a range of chemical complexity and diversity. Furthermore, nine target giant molecular clouds contain well-fit methanol emission, giving us a first look at the spatial chemical variability within the objects at relatively high (compared to past observations) resolutions of ∼5''. This work lays the foundation for future high angular resolution studies of gas-phase chemistry with the full ALMA.

Relationship between CH3OD Abundance and Temperature in the Orion KL Nebula.

The relative abundances of singly deuterated methanol isotopologues, [CH2DOH]/[CH3OD], in star-forming regions deviate from the statistically expected ratio of 3. In Orion KL, the nearest high-mass star-forming region to Earth, the singly deuterated methanol ratio is about 1, and the cause for this observation has been explored through theory for nearly three decades. We present high-angular resolution observations of Orion KL using the Atacama Large Millimeter/submillimeter Array to map small-scale changes in CH3OD column density across the nebula, which provide a new avenue to examine the deuterium chemistry during star and planet formation. By considering how CH3OD column densities vary with temperature, we find evidence of chemical processes that can significantly alter the observed gas-phase column densities. The astronomical data are compared with existing theoretical work and support D-H exchange between CH3OH and heavy water (i.e., HDO and D2O) at methanol's hydroxyl site in the icy mantles of dust grains. The enhanced CH3OD column densities are localized to the Hot Core-SW region, a pattern that may be linked to the coupled evolution of ice mantle chemistry and star formation in giant molecular clouds. This work provides new perspectives on deuterated methanol chemistry in Orion KL and informs considerations that may guide future theoretical, experimental, and observational work.