2002ApJ...572L..95S

In an echelle spectrum of X Per acquired with the Space Telescope Imaging Spectrograph, we have identified individual rotational lines of 11 triplet-singlet (intersystem) absorption bands of ¹²CO. Four bands provide first detections for interstellar clouds. From a comparison with the ζ Oph sight line, we find that X Per is obscured by a higher ¹²CO column density of 1.4x10¹⁶ cm^–2. Together with the high spectral resolution of 1.3 km.s^–1, this allows (1) an improved measurement of previously published interstellar f-values for seven bands and (2) an extraction of the first astrophysical oscillator strengths for d-X (8-0), (9-0), and (10-0), as well as for e-X (12-0). The ¹³CO d-X (12-0) band, previously suspected to exist toward ζ Oph, is now readily resolved and modeled. Our derived intersystem f-values for ¹²CO include a few mild (≤34%) disagreements with recent predictions from a perturbation analysis calculated for the interstellar excitation temperature. Overall, the comparison confirms the superiority of employing multiple singlet levels in the calculations of mixing coefficients over previous single-level predictions.