Protoplanetary Disk Chemistry across the Stellar Spectrum
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CitationPegues, Jamila. 2021. Protoplanetary Disk Chemistry across the Stellar Spectrum. Doctoral dissertation, Harvard University Graduate School of Arts and Sciences.
AbstractProtoplanetary disks are the birthplaces of Earth, its planetary neighbors, and the plethora of exoplanets that we now know orbit stars beyond our Sun. Statistical analyses and direct observations have shown that these planets, and therefore their ancestral protoplanetary disks, can form around a diverse range of young stars. The inherent differences in these stars, such as their mass, temperature, and radiation field, could lead to drastic chemical differences in the planet-forming environments of their disks. In this dissertation, we used observational surveys of millimeter-wavelength molecular line emission as tools to characterize protoplanetary disks, and occasionally their host stars, across the stellar spectrum.
We first used the Atacama Millimeter/submillimeter Array (ALMA) to observe line emission of the organic molecule formaldehyde (H2CO) toward a sample of 15 protoplanetary disks around mostly solar-type stars. In order to constrain the formation environment of H2CO in disks, we estimated excitation temperatures and column densities across the sample. From these values, and H2CO emission morphologies, we determined that the H2CO was likely produced significantly through both gas-phase and grain-surface formation pathways in our sample. These results indicated that protoplanetary disks likely have reservoirs of CO ice that support rich organic grain-surface chemistry.
We next used ALMA to survey a suite of small molecules toward a sample of five disks around M4-M5 stars. The target molecules in this survey included isotopologues of carbon monoxide (CO), the ethynyl radical (C2H), hydrogen cyanide (HCN), deuterium cyanide (DCN), and H2CO. Each of these molecules can be used as diagnostics of the underlying disk structure and chemistry, including the disk's gas morphology (CO), level of deuteration (DCN/HCN), and the relative prevalence of hydrocarbon chemistry (C2H/HCN). We compared emission morphologies and flux ratios across the sample, and we used fits to hyperfine structure to estimate excitation conditions and column densities. We found that C2H and HCN stood out among the other diagnostic molecules. C2H and HCN appeared correlated for the disks around M4-M5 stars, as has been noted for disks around solar-type stars. However, C2H/HCN column density ratios were higher for the disks around M4-M5 stars. This finding was consistent with previous infrared studies in the literature, which were sensitive to the inner AU of these low-mass M-star disks.
We then used our molecular line observations to characterize the M-stars at the centers of these disks. It is widely suspected in the literature that fiducial (i.e., not accounting for magnetic fields) stellar evolutionary models can give imprecise/inaccurate predictions of stellar mass for young, low-mass stars. Masses measured dynamically for stars can be used to benchmark and improve stellar evolutionary models in this uncertain regime. We used ALMA observations of CO emission, along with Markov Chain Monte Carlo (MCMC)-based software, to dynamically measure the masses for three of the five M4-M5 stars mentioned previously. These measurements were conducted using the Keplerian rotation of the CO emission in their protoplanetary disks. We combined our dynamical mass measurements with other dynamical masses presented in the literature for stars in the same mass regime. We used the combined sample to evaluate the accuracy of mass predictions for different stellar evolutionary models, and we found that while fiducial stellar evolutionary models yielded systematic underpredictions of the stellar masses, models with starspots proved capable of accurately predicting the dynamical masses.
On the opposite end of the young stellar spectrum, we used observations of the Submillimeter Array (SMA) to conduct a pilot investigation of millimeter-wavelength chemistry around five Herbig Ae/Be disks. When detected, we found intriguing asymmetries in the CO gas and/or dust emission for the sample. In particular, the disk HD 34282 showed an apparent orbital companion that was visible in the 12CO 2--1 emission and in multiple bands of dust continuum emission. We combined molecular line detections and upper limits from our sample with disks across the stellar spectrum from the literature. From our analysis across the combined sample, we found tentative evidence that Herbig Ae/Be/F disks may host relatively less cold chemistry and ionization than their lower-mass counterparts.
These new observational constraints can serve as valuable benchmarks for validating and improving our theoretical understanding of protoplanetary disk chemistry. We thus concluded this thesis with ongoing work on building and characterizing a set of representative astrochemical disk models around low-mass M-stars. So far, we have constructed an initial control group of four disk models, three around M-stars and one around a solar-type star, that radiate in the ultraviolet regime. From this initial control group, we found relatively large reservoirs of CO ice, nitrogen-bearing ices, and hydrocarbons in the disk models around M-stars relative to their solar-type counterpart. In the near future, we will expand these models to quantify the effects of X-rays, ultraviolet luminosity, and initial chemical composition on disk chemistry around low-mass M-stars.
We conclude by noting that interesting and exciting chemistry is detected in protoplanetary disks across the entire young stellar spectrum. We look forward to new and upcoming endeavors that will investigate protoplanetary disk chemistry across larger samples and on smaller spatial scales.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37368394
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