Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Until relatively recently, the only known planets were those in our own solar system (Mayor & Queloz, 1995). However modern instruments and techniques have revealed that planets are ubiquitous around main sequence stars over the last three decades. From this we have obtained a multitude of insights into planetary populations. For instance, while the terrestrial and gas giant planet types are both well accounted for in extra-solar systems, the arrangement of our own system with inner terrestrial planets and outer gas giants is not clearly the typical organization (van der Marel & Mulders, 2021). In fact we see that in many cases of super-Jovian mass planets at very small separations from their host stars. In order to understand how extra-solar systems come to their final arrangements we must understand how planets form in the first place. To that end we study protoplanetary disks, regions of gas and dust surrounding young stars. These young stars can be broadly categorized into T Tauri and Herbig Ae/Be stars.
The former is comprised of roughly solar-type stars, while the latter are higher mass from roughly 2 M ⊙ upwards (e.g. Herbig, 1960; Hillenbrand et al., 1992). In contrast to their main sequence counterparts, these classes of young stellar objects (YSOs) have very few detections of planets “caught in the act” of formation owing to a variety of observational difficulties. One of the few exceptions is the case of PDS 70 (e.g. Haffert et al., 2019; Isella et al., 2019; Zurlo et al., 2020; Benisty et al., 2021)
My area of research focuses on (near-) infrared spectroscopy of disks rather than imaging. This allows for potential indirect detection of planets, as well as for the extraction of disk parameters which may be useful in the testing and refining of models. In particular my research primarily uses ro-vibrational emission of the fairly abundant CO molecule, including some of its isotopologues.
In the near-infrared, there are dozens of potentially excited ro-vibrational lines to be investigated in the various constituent vibrational bands. This is increased when there are multiple isotopologues available. The various lines can be analyzed to determine, for instance, the temperature, extent, and shape of the emitting region of gas.
This spectroscopic data can also complement direct imaging due to the different regions of the disk that they probe. Millimeter imaging tends to be sensitive to the outer dozens or hundreds of au of a disk where the material is cooler, while the type of ro-vibrational spectroscopy presented here is useful for investigating the warmer gas from a few tens of au and inward.
As part of my research, a variety of Herbig Ae/Be objects have been observed using the iSHELL spectrograph at the Infrared Telescope Facility (IRTF) in order to analyze their CO emission. Possibly the most interesting of these is HD 141569, an old and extended disk. The iSHELL data shows a rich variety of ro-vibrational emission, including lines from vibrational bands as high as v=7-6 from the most common isotopologue 13 12 CO, as well as the first two vibrational bands from CO while C 18 O emission was not detected.
Spectroscopy provides a complementary venue of investigation to direct imaging. However, it is possible to extract some spatial details from spectroscopic data. The first method used here is simply that inferred through the shape of the line profile. This can be done directly, or more robustly through modeling. The second method employed is spectro-astrometry, which is accomplished by measuring the center of light of the emission using of the 2D spectrum. This method is used throughout the work presented here. It is used as a test of asymmetry as well as a means of investigating any non-Keplerian features in disks. Additionally these methods can be used to detect or constrain the existence of planets in protoplanetary disks.
Jensen, Stanley, "CO Molecular Spectroscopy and Spectro-astrometry of Protoplanetary Disks" (2022). All Dissertations. 3161.
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