About Me

My research interests are in protoplanetary disks and astrochemistry. I recently finished my PhD at the University of Michigan and am now a Kavli-Laukien Origins Fellow at the Center for Astrophysics at Harvard.  I seek to make connections between protoplanetary disks and exoplanets to better understand the origins of our solar system and life. 

I am an expert on thermo-chemical models and use these models to interpret observations. I use JWST and ALMA to characterize the thermal, chemical, and physical structure of the disk. 


UV-Chemistry as a Signpost for Late-Stage Planet Formation

Complex organic molecules like CH3CN and HC3N have been seen towards multiple Class II disks, these are disks that are move evolved (>1 Myr) yet still gas-rich. The detections of these COMs presents a bit of a chemical conundrum: They appear to be highly abundant in very cold gas, where they should be frozen-out onto grains, thus non-emissive. 

How could they be abundant in the gas in such cold temperatures? 

As the disk evolves, the small dust population in its gaseous atmosphere should diminish as it grows to larger sizes, and falls down towards the midplane and inward radially. The small dust is the main opacity source for the UV-flux coming from the central star...thus a decrease in the small dust opacity over time will lead to the UV flux penetrating deeper and deeper into the disk. The higher UV field in high gas densities acts as a non-thermal mechanisms  to pop COMs off of dust grains and back into the gas. This, paired with the already observed high C/O ratios triggers a carbon-rich chemistry in the gas, near the planet forming midplane. 

Using this solution, I was reproduced COM emission coming from the disks TW Hya and HD 163296. These results and hypothesis can be found in my Nature Astronomy paper, which has been accepted as of mid-October. 

Looking for water with JWST

JWST will be the instrument that can constrain the water abundance and distribution throughout disks. In order to do that, we need a theoretical exploration of signals that water will be giving us. Using the thermo-chemical code DALI, I predicted H2O and H2-18O spectra that will be seen with JWST. I identified H2-18O lines that will be prime to trace the entire water column, and characterized critical uncertainties in the known H2O/H2-18O ratio. This was published as a series of ApJ letters, including Calahan et al. 2022.

TW Hya 

TW Hya is a nearby T Tauri star with a face-on disk. It was the subject of Calahan et al. 2021a where I extracted its 2D thermal structure and mass using a 2D thermo-chemical code and ALMA observations

HD 163296

This is Herbig star located only 101 pc away from us, and was one of the targets in the MAPS-ALMA large program. In Calahan et al. 2021b I extracted its 2D thermal structure, motivated by a model that reproduced ALMA observations including the 2D emission heights! 

 Feel free to reach out to me at: jcalahan@umich.edu