Rosetta-Alice calibration star reference spectra: Selection and correction

Calibration of the effective area (Aeff) for the Rosetta-Alice instrument requires observation of calibration stars with known FUV spectra. A number of calibration stars have been selected and observed for this purpose, and will continue to be observed in future mission phases. Calibrated spectra of the same stars from past and current FUV missions constitute the known spectra against which R-Alice instrumental spectra must be compared. This report describes the reference spectra used for each of the stars, including selection criteria, data source, corrections, uncertainties, and outstanding issues. The optimal calibration for a far-UV spectrograph can be derived by acquiring spectra of a number of primary flux standards: white dwarf stars with well-fit theoretical models, particularly sources and models already demonstrated in calibration campaigns for other instruments. Simultaneous fitting of multiple observed spectra to these well-understood models provides reliable Aeff calibration. Unfortunately, these white dwarf stars are relatively faint, and would result low signal-to-noise spectra for R-Alice, even with relatively long integration times. Further complicating matters, the effective area derived from the only white dwarf star observed with RAlice to date (WD 0501-289) is nearly a factor of two lower than Aeff derived using other sources. The cause of this discrepancy is unclear. Observations of the white dwarf stars G191-B2B and BD +28◦ 4211, which are several times brighter than WD 0501-289, are planned for observation during PC10 in the September 2009 timeframe.