McGuire Research Group

New Molecular Discovery

The first step in understanding interstellar chemistry is to establish which molecules are present. We compare accurate laboratory spectra with radio telescope observations, using the full pattern of expected transitions to test an identification. Through the GOTHAM collaboration, our work has helped move interstellar aromatic chemistry from evidence for a broad molecular population to measurements of individual species.

The two-ring cyanonaphthalenes, four-ring cyanopyrene, and seven-ring cyanocoronene provide a growing set of molecules whose structures and abundances can be compared directly. Many parent PAHs have no permanent electric dipole moment and cannot be observed through ordinary pure rotational spectroscopy. Their cyano-substituted relatives provide useful probes, although converting a nitrile abundance into that of its parent hydrocarbon requires chemical assumptions. See our molecule gallery for selected discoveries.

A historical selection of molecular structures identified in space by group members and collaborators

A selection of earlier molecular discoveries. The molecule gallery includes more recent aromatic species and links to the underlying papers.

Chemistry in Dark Molecular Clouds

Cold, dark clouds let us study chemistry before a protostar heats its surroundings. TMC-1 is especially useful because its narrow spectral lines make weak molecular patterns easier to separate. Our GOTHAM survey with the Green Bank Telescope provides both new detections and a common set of abundance measurements against which chemical models can be tested.

The published GOTHAM inventory analyzes 102 detected molecular species, including isotopically substituted species, across observations from 3.9 to 36.4 GHz. Among the aromatic molecules, increasing size does not produce the steep fall in abundance familiar from many carbon-chain families. Cyanocoronene is present at a column density comparable to those of several smaller aromatic nitriles. Explaining that distribution is now one of the central questions in our work.

We are testing whether these molecules grow in the cold gas, survive from earlier stages of the interstellar carbon cycle, or reflect a combination of both processes. Which ring arrangements are detected, which remain absent at meaningful sensitivity, and how their isotope ratios differ can help separate these possibilities. We are extending these studies to other cold clouds and more strongly irradiated environments to learn which features of TMC-1 chemistry are widespread.

Photograph of the Taurus Molecular Cloud complex taken by McGuire from Charlottesville, VA. The dark lanes in the top of the image aren't regions without stars - they are deeply obscured clouds of gas and dust that block out all starlight from behind them.

Chemistry in Star-Forming Regions

As stars form, heating, radiation, and the exchange of material between gas and dust change the molecular inventory. With ALMA and other radio facilities, we study complex organic molecules in these environments and ask how their abundances respond to physical conditions. The comparison matters: the molecules in a cold cloud are a starting inventory, but the material entering a planetary system has continued to react.

Our detection of 2-methoxyethanol in NGC 6334I, together with its non-detection in the low-mass source IRAS 16293-2422B, provides a test of how different environments affect related oxygen-bearing chemistry. Through collaborations including the COMPASS ALMA Large Program, we are also comparing molecular inventories across protostellar sources, with the aim of separating environmental effects from changes associated with evolution.

Aromatic chemistry can now be followed beyond starless clouds as well. The detection of the three-ring PAH phenalene in MC27/L1521F, which contains a very low-luminosity protostar, extends these measurements to an early stage of star formation. Its abundance relative to benzonitrile differs from that in TMC-1. This gives us a new chemical comparison, while leaving open whether the observed population survived, was inherited, or was replenished.

Photograph of the Galactic Center taken by McGuire from the summit of Mauna Kea. Very near the central supermassive black hole Sgr A^* is a massive star-forming region called Sgr B2. It is here that a huge fraction of the new complex organic molecules are discovered outside our solar system.

Extracting More Chemistry from Spectral Surveys

Many molecules contribute signals too weak to interpret one transition at a time. Spectral stacking combines their expected lines, and matched filtering tests the resulting pattern against a model. We use these methods together with Bayesian fitting to evaluate detections and constrain molecular abundances, taking account of line blending and the physical conditions of the source.

Our work on aggregate isotope signals tests how these techniques can probe carbon-isotope ratios when individual rare isotopic species are too faint to detect. The published proof of concept establishes how these searches can test aromatic formation histories. We are pairing this approach with measurements of the required laboratory spectra. In parallel, our automated survey-assignment methods combine spectroscopic matches with chemical context to analyze dense line surveys and suggest targets for follow-up.