VUV Survey of 12 CO/ 13 CO in the Solar Neighborhood with the Hubble Space Telescope Y. Sheffer, M. Rogers, S. R. Federman Department of Physics and Astronomy University of Toledo, Toledo, OH D. L. Lambert Astronomy Department University of Texas, Austin, TX R. Gredel Max Planck Institute for Astronomy Konigstuhl 17, Heidelberg, Germany

2006 JUNE 2161 st ISoMS, OSU VUV Survey of 12 CO/ 13 CO in the Solar Neighborhood with HST VUV = Vacuum Ultra-Violet. Survey = An archival sample of 23 lines of sight. 13 CO = 12 CO + n Isotopologues, not isotopomers… Solar Neighborhood = Stellar targets are typically less than 1-2 kpc from the Sun, a fraction of the Galactic Radius.

2006 JUNE 2161 st ISoMS, OSU Why? (1) 12 C and 13 C isotopes have different nucleosynthesis production avenues. Their ratio provides a check on the history of chemical enrichment in the Galaxy. Radio studies of mm-wave CO and H 2 CO emission have provided a mean of 12 C/ 13 C = 70 ± 7 based on high-density molecular clouds (Wilson and Rood 1994, Wilson 1999). Isotopologues are proxies for isotopes. VUV absorption probes diffuse clouds. This is a sub-sample from our larger CO & H 2 project.

2006 JUNE 2161 st ISoMS, OSU Why? (2) CO is susceptible to chemical fractionation via 12 CO + 13 C + → 13 CO + 12 C K. This makes 12/13 lower at lower T kin (Watson et al. 1976). Physical fractionation occurs via selective photodissociation (PD): 12 CO self-shields first, thus driving 12/13 to higher values (van Dishoeck & Black 1988). Fractionation provides information about physical and chemical processes along the line of sight.

2006 JUNE 2161 st ISoMS, OSU Observations Twenty-three lines of sight from HST archival data: 18 from STIS and 5 from GHRS. Instrumental resolution: λ/Δλ = 46,000 to 160,000 (STIS) and 18,500 (GHRS). Need to obtain exposures of both very strong and very weak absorption bands in order to derive the correct optical depth along sightline. STIS has full UV coverage in Echelle mode. Up to 14 simultaneous A-X (4 th positive) bands: τ range ~1000.

2006 JUNE 2161 st ISoMS, OSU Method ISMOD is a simplex code that models Voigt profiles of IS absorption lines, using equations from Black & van Dishoeck (1988). It can handle single atomic transitions or a multitude of lines from molecules (H 2, CO). Here, the input is f- values for A 1 Π - X 1 Σ bands (Chan et al. 1993, Eidelsberg et al. 1999) and λ (Morton & Noreau 1994). Spectrum synthesis returns radial velocity, column density, excitation temperature, and cloud structure (velocity differences, fractions, Doppler widths).

2006 JUNE 2161 st ISoMS, OSU P.S. Cloud structure The number of components, their velocities and widths affect the optical depth. For lo-res GHRS spectra, structure is imported from our hi-res optical data (McDonald, ESO) or from the literature. For hi-res STIS spectra, ISMOD can find the structure directly from the CO bands. There is a very good agreement b/w the hi-res, UV, CO-derived structure and the optical, CH- and/or CN-derived structures.

2006 JUNE 2161 st ISoMS, OSU N( 12 CO) ~ cm -2

2006 JUNE 2161 st ISoMS, OSU N( 13 CO) ~ cm -2

2006 JUNE 2161 st ISoMS, OSU Analysis We compute N tot ( 12 CO)/N tot ( 13 CO) and compare the result with N tot ( 12 C)/N tot ( 13 C). Determine the fraction of sight lines that significantly show fractionated ratios. Higher ratios = self-shielding of 12 CO. Lower ratios = CE with C + enhances 13 CO. Check for possible correlations of the CO isotopologic(?) ratio with other observables.

2006 JUNE 2161 st ISoMS, OSU Results 39% of sight lines show CO isotopologic ratios that are significantly fractionated (> 2 σ).

2006 JUNE 2161 st ISoMS, OSU

2006 JUNE 2161 st ISoMS, OSU Federman et al. (2003) Sheffer et al. (2002) Lambert et al. (1994)

2006 JUNE 2161 st ISoMS, OSU

2006 JUNE 2161 st ISoMS, OSU Results 39% of sight lines show CO isotopologic ratios that are significantly fractionated (> 2 σ). When plotted vs. N tot ( 12 CO), one sees 3 regimes of 12/13: selective photo-dissociation, charge exchange with C +, and no fractionation (≈ 70).

2006 JUNE 2161 st ISoMS, OSU Un-fractionated Selective PD 13 C +  13 CO

2006 JUNE 2161 st ISoMS, OSU Results 39% of sight lines show CO isotopologic ratios that are significantly fractionated (> 2 σ). When plotted vs. N tot ( 12 CO), one sees 3 regimes of 12/13: selective photodissociation, charge exchange with C +, and no fractionation (≈ 70). There are no obvious correlations with other observables, including I UV, n CN.

2006 JUNE 2161 st ISoMS, OSU

2006 JUNE 2161 st ISoMS, OSU

2006 JUNE 2161 st ISoMS, OSU To be continued… This is “work in progress”. The two primary CO isotopologues do show that fractionation is not uncommon in the diffuse ISM, by up to a factor of 2. Intrinsic vs. extrinsic effects need to be sorted out for each sight line. Additional diffuse avenues: 12 C 18 O (Lambert et al. 1994) and 12 C 17 O (Sheffer et al. 2002). The End