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Data from the Cosmic Origins Spectrograph (COS) are presented for the first highly reddened target (Cyg OB2 8A) under the COS Science Team's guaranteed time allocation. Column densities of ionic, atomic, and molecular species are reported and implications are discussed. Data from Cyg OB2 8A demonstrate the ability to analyze highly reddened interstellar sight lines with the COS that were unavailable to previous UV instruments. Measured column densities indicate that the Cyg OB2 8A line of sight contains multiple diffuse clouds rather than a dominant translucent cloud.

Using archival, high-resolution far-ultraviolet HST/STIS spectra of 34 Galactic O and B stars, we measure CI column densities and compare them with measurements from the literature of CO and H_2 with regard to understanding the presence of translucent clouds along the line-of-sight. We find that the CO/H_2 and CO/CI ratios provide good discriminators for the presence of translucent material, and both increase as a function of molecular fraction, f = 2N(H_2)/N(H). We suggest that sightlines with values below CO/H_2 ~ 1E-6 and CO/CI ~ 1 contain mostly diffuse molecular clouds, while those with values above sample clouds in the transition region between diffuse and dark. These discriminating values are also consistent with the change in slope of the CO v. H_2 correlation near the column density at which CO shielding becomes important, as evidenced by the change in photochemistry regime studied by Sheffer et al. (2008). Based on the lack of correlation of the presence of translucent material with traditional measures of extinction we recommend defining 'translucent clouds' based on the molecular content rather than line-of-sight extinction properties., Comment: 9 pages, accepted for publication in the Astrophysical Journal; new version corrects minor typographical errors

High resolution spectroscopy with FUSE and STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models for the abundance kinematics in planetary nebulae. The simple picture of a very high speed (~ 1000 km/s), high ionization, radiation driven stellar wind surrounded by a slower (~ 10 km/s) mostly molecular outflow, with low ionization and neutral atomic species residing at the wind interaction interface, is not supported... We find ...there is a fast (33 -- 65 km/s) low ionization zone, surrounding a slower ( H_2^+ + e -> H(1s) + H (nl)), which ultimately produces fluorescence of Ha and Lya. In the optically thin limit at the inferred radius of the velocity transition we find dissociation of H_2 by stellar Lyc photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. We suggest that the importance of this H_2 destruction process in HII regions has been overlooked., Comment: In emulateapj 2 column, 28 pages total, 18 figures. Accepted for publication in the Astrophysical Journal on 5 January 2007. Abstract abridged

Observations of comet C/2001 A2 (LINEAR) were made with the Far Ultraviolet Spectroscopic Explorer beginning 2001 July 12.58 and coinciding with a photometric increase of ~1.5 mag. Spectra were obtained in the 905-1180 A range at 0.25 A spectral resolution using the 30'' × 30'' aperture. Several new cometary emissions were identified, particularly the (0, 0) bands of the CO Hopfield-Birge systems C-X and B-X at 1088 and 1151 A, respectively, O I (1D-1D) at 1152 A, and three lines of the H2 Lyman system at 1071.6, 1118.6, and 1166.8 A, pumped by solar Lyβ fluorescence. Also detected were O I multiplets at 989, 1027, and 1040 A and several lines of the H I Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both "cold" and "hot" components, the cold component accounting for 70% of the flux and with a rotational temperature of 55 ± 5 K. The hot component may be indicative of a CO2 source. The CO bands, H2 lines, and O I λ1152 all decreased by a factor of 2 over the 7.5 hr observation. The derived time-averaged production rates are Q(CO) = 1.3 × 1027 molecules s-1 and Q(H2O) = 2.1 × 1029 molecules s-1. These values may be uncertain by as much as a factor of 2 because of uncertainties in the solar flux and the electron impact contribution to the excitation.

Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are dominated by emission of the CO Fourth Positive band system and strong OI and CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and C - X (0,0) at 1088 A, are detected for the first time, and there is a weak indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B - X band is blended with emission from OI 1152. Modeling the relative intensities of these bands suggests that resonance fluorescence of CO is the dominant source of the emission, as it is for the Fourth Positive system. Shortward of Lyman-alpha, other emission features detected include OII 834, OI lambda 989, HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses of the OI, OII, and HI features are about one-half of those reported by Hord et al. (1991) from Galileo EUV measurements made in February 1990. This result is consistent with the expected variation from solar maximum to solar minimum. The ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived mixing ratio of Ar is of the order of 2%, consistent with previous determinations., Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 2000

A sounding rocket observation of comet Hale-Bopp was conducted on 1997 April 6 at 3:51 UT when the comet was at heliocentric and geocentric distances of 0.92 and 1.39 AU, respectively. The instrument was a telescope and long-slit (65 × 260''), ultraviolet (1280-1880 A) spectrograph that sampled the coma parallel to the Sun-comet line from ~105 km sunward of the nucleus to ~2 × 105 km tailward with ~6'' (6000 km) spatial resolution. Two spectral images were obtained with the slit offset 20'' and 40'' from the nucleus in the direction perpendicular to the long axis of the slit. Spatial profiles and production rates for C, O, CO, and S are presented. Modeling of the spatial profiles of CO and C emissions indicate that photodissociation of the detected CO can account for all of the C I emissions observed. The brightnesses of the strong bands of the CO Fourth Positive system and the S I λ1814 multiplet along near-nuclear lines of sight were diminished by self-absorption. A CO production rate of ~3 × 1030 molecules s-1 is derived.

As part of a program to study translucent interstellar clouds, NGC 2024 IRS 1 was observed by the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope. IRS 1 is a heavily reddened B0.5 V star (E(B – V) = 1.69, RV = 4.5, and AV = 7.61) lying outside the core of the NGC 2024 cluster and not considered to be the ionizing source of the nebula. At wavelengths below about 1300 A, the observed spectrum deviates from that expected of a reddened B star, showing an increase in flux to shorter wavelengths and the presence of P-Cygni profiles in both the C III] λ1176 and N V λ1240 lines. Because of the presence of these lines, the absence of any P-Cygni signature in the Si IV λ1394/1403 lines, and the measurement of extended emission on the detector focal plane, we believe this portion of the spectrum to be scattered light from an O6 to B0 supergiant. The COS entrance aperture is 2.5 arcsec in diameter and can let in substantial scattered light from the nebula. NGC 2024 IRS 2b is an O8 star and believed to be the ionizing source of the nebula. It is currently identified as a main-sequence star, but should it be a supergiant the presence of such an evolved star implies an age for the cluster of the order of 3 Myr, in conflict with age determinations based on infrared spectroscopy, and may have implications for the energetics and density structure of the nebula.

Carbon monoxide is a commonly used IR/submillimeter tracer of gas in protoplanetary disks. We present an analysis of ultraviolet CO emission in Hubble Space Telescope Cosmic Origins Spectrograph spectra for 12 Classical T Tauri stars (CTTSs). Several ro-vibrational bands of the CO A 1Π–X 1Σ+ (Fourth Positive) electronic transition system are spectrally resolved from emission of other atoms and H2. The CO A 1Π v′ = 14 state is populated by absorption of Lyα photons, created at the accretion column on the stellar surface. For targets with strong CO emission, we model the Lyα radiation field as an input for a simple fluorescence model to estimate CO rotational excitation temperatures and column densities. Typical column densities range from N CO = 1018 to 1019 cm−2. Our measured excitation temperatures are mostly below T CO = 600 K, cooler than typical M-band CO emission. These temperatures and the emission line widths imply that the UV emission originates in a different population of CO than that which is IR-emitting. We also find a significant correlation between CO emission and the disk accretion rate and age. Our analysis shows that ultraviolet CO emission can be a useful diagnostic of CTTS disk gas.

We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm−2 and T rot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A–X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H i Lyα. All three objects show emission from CO bands at λ > 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a “continuum” whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ∼ 1018–1019 cm−2, T rot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (≡ N(CO)/N(H2)) in the inner disk is ∼1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.