An investigation of the physical parameters of young stellar objects

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Deen, Casey Patrick

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Studies of the temporal evolution of young stars and their associated properties rely upon the ability of astronomers to determine ages and masses of objects in different evolutionary states. The best method for determining the age and mass of a young stellar object is to place the object on the Hertzsprung-Russell (HR) diagram and to compare to theoretical evolutionary tracks. Accurate ages allow the investigation of the temporal evolution of properties associated with stellar youth (accretion rates, X-ray activity, circumstellar excess, etc...). One property intimately linked with stellar youth is the presence (or absence) of an optically thick primordial circumstellar disk. Objects in "young" star forming regions are more likely to show evidence for a disk than objects in "older" clusters. Within a single cluster, the picture is not as clear. There exist objects in very young clusters (~1 Myr) which show no evidence for circumstellar disks, and there exist objects in very old clusters (~10 Myr), which show evidence for robust disks, suggesting a variable other than stellar age is driving the evolution of the disks. To investigate whether these outliers are due to age spreads, initial conditions, or simply appear anomalous due to erroneous age determinations, we must determine better placements in the HR diagram by carefully transforming observable quantities (spectral type and apparent magnitude) into the quantities necessary for comparison evolutionary models (effective temperature and luminosity). In the Ophiuchus star forming region, I investigate whether or not objects with disks are younger than disk-less objects. I find no difference in the ages of the two populations, but the systematic and random uncertainties are large enough to mask all but the largest age differences. In the hope of better determining the physical parameters of young stellar objects, I embark on a spectral synthesis campaign to produce comparison synthetic spectra which account for the effects of magnetic fields. This requires the modification of the MOOG spectral synthesis program to handle the full Stokes vector treatment for polarized radiation through a magnetized medium. I create a grid of synthetic spectra covering ranges in effective temperature, surface gravity, and average magnetic field strength relevant for studies of young stellar objects, and develop a Chi-squared minimization routine to determine the best fit synthetic spectrum for a given observed spectrum at an arbitrary resolving power. This grid of synthetic spectra will be an invaluable complement to future near infrared, large band-pass, high-resolving power spectrographs (i.e. IGRINS). In addition to these observational and theoretical attempts to reduce systematic errors, I also helped to develop a suite of silicon and KRS-5 grisms for use in the FORCAST instrument, a mid infrared camera on the SOFIA telescope. These grisms will afford the imaging instrument a mid infrared spectroscopic capability at wavelengths normally inaccessible from the ground. I also report on my work to help write FG Widget, the quick-look reduction software package developed to support grism observations.




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