Disentangling Stellar and Airglow Emission Lines from Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) Spectra

H i Ly alpha (1215.67 angstrom) and the O i triplet (1302.17, 1304.86, and 1306.03 angstrom) are bright far-ultraviolet (FUV) emission lines that trace the stellar chromosphere. Observations of stellar Ly alpha and O i using the Hubble Space Telescope's (HST) most sensitive FUV spectrograph, the Cosmic Origins Spectrograph (COS), are contaminated with geocoronal emission, or airglow. This study demonstrates that airglow emission profiles as observed by COS are sufficiently stable to create airglow templates that can be reliably subtracted from the data, recovering the underlying stellar flux. We developed a graphical user interface to implement the airglow subtraction on a sample of 171 main-sequence F-, G-, K-, and M-type dwarfs from the COS data archive. Correlations between recovered stellar emission and measures of stellar activity were investigated. Several power-law relationships are presented for predicting the stellar Ly alpha and O i emission. The apparent brightness of the stellar emission relative to the airglow is a critical factor in the success or failure of an airglow subtraction. We developed a predictor for the success of an airglow subtraction using the signal-to-noise ratio of the nearby chromospheric emission line Si iii (1206.51 angstrom). The minimum attenuated Ly alpha flux that was successfully recovered is 1.39 x 10(-14) erg cm(-2) s(-1), and we recommend this as a minimum flux for COS Ly alpha recoveries.

Automated summary

This study demonstrates that the observations of stellar Ly alpha and O i using the COS are contaminated with geocoronal emission, but the airglow emission profiles are stable enough to create templates for subtraction. A graphical user interface was developed to subtract the airglow emission and recover the underlying stellar flux. Power-law relationships between the recovered stellar emission and measures of stellar activity were presented.