Minimizing Strong Telluric Absorption in Near Infra-red Stellar Spectra

We have obtained high resolution spectra (R ∼ 25000) of an A star over varying airmass to determine the effectiveness of telluric removal in the limit of high signal to noise. The near infra-red line He I at 2.058μm, which is a sensitive indicator of physical conditions in massive stars, supergiants, H II regions and YSOs, resides among pressure broadened telluric absorption from CO2 and water vapor that varies both in time and with observed airmass. Our study shows that in the limit of bright stars at high resolution, accuracies of 5% are typical for high airmass observations (greater than 1.9), improving to a photonlimited accuracy of 2% at smaller airmasses (less than 1.15). We find that by using the continuum between telluric absorption lines of a ro-vibrational fan a photon-limited 1% accuracy is achievable. Subject headings: techniques: spectroscopic — methods: data analysis