Using High-Resolution Spectroscopy To Improve The Determination Of Effective Temperatures OF Pre-Main Sequence Stars

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Grunhut, Jason
stars , high-resolution spectroscopy , pre-main sequence stars , Herbig stars , HAeBe stars , effective temperature
Herbig Ae/Be (HAeBe) stars are the pre-main sequence progenitors of main sequence A and B stars, and are characterized observationally by strong emission in spectral lines and significant infra-red excess that results from their presence in dust-obscured regions. These stars are usually surrounded by a complex environment composed of gas and dust and often a significant stellar wind and a circumstellar disc. This complex circumstellar environment can have a significant affect on their spectral energy distributions, leading to large systematic uncertainties in determinations of their effective temperatures from photometric methods. In an attempt to improve temperature determinations for HAeBe stars, we have conducted an experiment to evaluate the potential of high-resolution spectra to constrain their atmospheric parameters. To this end, high-resolution (R~68 000) and low-resolution (R~1500) spectra obtained using the ESPaDOnS spectropolarimeter (at the Canada-France-Hawaii telescope) and the FORS1 spectropolarimeter (at the Very Large Telescope) have been used with an automatic spectrum fitting procedure. This procedure compares spectroscopic data to a grid of synthetic LTE, solar abundance spectra, spanning a range in effective temperature, surface gravity, and micro-turbulence. This analysis was applied to the spectra of a sample of twelve previously well-studied HAeBe stars. Our temperatures were found to be consistent with previously published values, while providing much lower uncertainties - in some cases about 5 times smaller. Numerous methods were investigated to obtain these quantitative uncertainties (chi-squared statistics, Bayesian analysis, Monte Carlo bootstrap method, individual temperature sensitive line region analysis). We conclude that our method can be used to efficiently and effectively obtain temperatures of HAeBe stars in addition to providing us with a characterization of the degree of departure of the spectrum from solar abundance, LTE photospheric models.
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