A Search for Argon and O [CSC]vi[/CSC] in Three Comets Using the [ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL]

We conducted a sensitive search for the resonance lines of Ar I (λλ1048.22, 1066.66) and O VI (λλ1031.93, 1037.62) in the spectra of three long-period comets observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Argon emission was not detected from C/1999 T1 (McNaught-Hartley), C/2001 A2 (LINEAR), or C/2000 WM1 (LINEAR). Compared with the solar value, the [Ar/O] ratio is depleted in C/A2 by at least a factor of 10 and in C/WM1 by at least a factor of 13. The [Ar/O] upper limit for C/T1 is essentially the solar value, as our measurement was much less sensitive for that case. We also detected CO, which has a volatility similar to that of argon, during the FUSE observations. C/T1 was CO-rich ([CO/H2O] ≈ 13%), while both C/A2 and C/WM1 were CO-poor ([CO/H2O] ≈ 0.7% for C/A2 and ≈0.4% for C/WM1). The argon and CO depletions in C/A2 and C/WM1 suggest formation temperatures 60 K for both of these comets. The high CO abundance and upper limit on the argon abundance in C/T1 suggest that its formation temperature was in the range of ~40-50 K. No O VI emission was detected from comets C/T1 or C/A2, but the stronger line was marginally detected in C/WM1.