C. Gry

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with AV e1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J ¼ 0 and J ¼ 1, which contain e99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of � 10 21 cm � 2 , measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CNWe report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V

MOLECULAR HYDROGEN IN THE FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER TRANSLUCENT LINES OF SIGHT: THE FULL SAMPLE

gtrsim

H2 formation and excitation in the diffuse interstellar medium

1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain

Constraints on the Lyman continuum radiation from galaxies: First results with FUSE on Mrk 54

gtrsim

Interstellar C3 toward HD 210121

99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of 10^21 cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modification in terminology to distinguish between a translucent line of sight and a translucent cloud.

A far UV Study of Interstellar Gas Towards HD 34078: High Excitation H2 and Small Scale Structure

We report total abundances and related parameters for the full sample of the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the second half of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, RV = AV /E(B – V)—characteristic of larger than average dust grains—are associated with particularly low hydrogen molecular fractions (). In the lines of sight with large RV , there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H2 formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, N(Htot)/E(B – V), based on direct measurements of N(H2) and N(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low-to-moderate molecular fractions, we still find little evidence for single monolithic translucent clouds with .

Velocity Structure of the Local Interstellar Medium

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and ex- citation of H2 in the diuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon- illuminated gas. The data constrain well the nRproduct between gas density and H2 formation rate on dust grains: nR= 1t o 2:2 10 15 s 1 . For each line of sight the mean eective H2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J= 1)=N(J= 0) ratio, since the radiation field is known. Combining n with the nRvalues, we find similar H2 formation rates for the three stars of about R= 4 10 17 cm 3 s 1 . Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the J> 2 levels cannot be accounted for by the UV pumping of the cold H2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H2.

UVMag: stellar formation, evolution, structure and environment with space UV and visible spectropolarimetry

We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at z =0 :0448. The Lyman continuum radiation is not detected above the H i absorption edge in our Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 A, reasonably free of additional H i Lyman series line absorption, is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of 6:15 10 16 erg cm 2 s 1 A 1 is obtained for the flux at900 A in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the H flux, this limit translates into an upper limit of fesc < 0:062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z 3:4. The diculties and implications of a comparison are discussed.

Building galaxies, stars, planets and the ingredients for life between the stars. The science behind the European Ultraviolet-Visible Observatory

We report the detection of the 405 nm band of interstellar C3 in absorption toward HD 210121. The abundance of triatomic carbon is approximately 1/17 of that of diatomic carbon in the same diuse molecular cloud. Rotational levels of C3 up toJ = 14 are seen in this cloud. The rotational excitation of C3 in the interstellar medium may reflect a competition between inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. The abundance of C3 is compared with chemical models. Attention is called to molecular properties that need to be better determined.

Supernova Impact on a Molecular Cloud in the Local Bubble

To investigate the presence of small scale structure in the spatial distribution of H2 molecules we have undertaken repeated FUSE UV observations of the runaway O9.5V star, HD34078. In this paper we present five spectra obtained between January 2000 and October 2002. These observations reveal an unexpectedly large amount of highly excited H2. Column densities for H2 levels from (v = 0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levels are determined. These results are interpreted in the frame of a model involving essentially two components: i) a foreground cloud (unaffected by HD34078) responsible for the H2 (J = 0, 1), CI, CH, CH+ and CO absorptions; ii) a dense layer of gas (n = 10E4 cm-3) close to the O star and strongly illuminated by its UV flux which accounts for the presence of highly excited H2. Our model successfully reproduces the H2 excitation, the CI fine-structure level populations as well as the CH, CH+ and CO column densities. We also examine the time variability of H2 absorption lines tracing each of these two components. From the stability of the J = 0, 1 and 2 damped H2 profiles we infer a 3 sigma upper limit on column density variations Delta(N(H2))/N(H2) of 5% over scales ranging from 5 to 50 AU. This result clearly rules out any pronounced ubiquitous small scale density structure of the kind apparently seen in HI. The lines from highly excited gas are also quite stable (equivalent to Delta(N)/N <= 30%) indicating i) that the ambient gas through which HD34078 is moving is relatively uniform and ii) that the gas flow along the shocked layer is not subject to marked instabilitiesTo investigate the presence of small scale structure in the spatial distribution of H2 molecules we have undertaken repeated FUSE UV observations of the runaway O9.5V star, HD 34078. In this paper we present five spectra obtained between January 2000 and October 2002. These observations reveal an unexpectedly large amount of highly excited H2. Column densi- ties for H2 levels from (v = 0, J = 0) up to (v = 0, J = 11) and for several v = 1a ndv = 2 levels are determined. These results are interpreted in the frame of a model involving essentially two components: i) a foreground cloud (unaffected by HD 34078) responsible for the H2 (J = 0, 1), CI, CH, CH + and CO absorptions; ii) a dense layer of gas (n � 10 4 cm −3 ) close to the O star and strongly illuminated by its UV flux which accounts for the presence of highly excited H2. Our model successfully reproduces the H2 excitation, the CI fine-structure level populations as well as the CH, CH + and CO column densities. We also examine the time variability of H2 absorption lines tracing each of these two components. From the stability of the J = 0, 1 and 2 damped H2 profiles we infer a 3σ upper limit on column density variations ∆N(H2)/N(H2) of 5% over scales ranging from 5 to 50 AU. This result clearly rules out any pronounced ubiquitous small scale density structure of the kind apparently seen in HI. The lines from highly excited gas are also quite stable (equivalent to ∆N/N ≤ 30%) indicating i) that the ambient gas through which HD 34078 is moving is relatively uniform and ii) that the gas flow along the shocked layer is not subject to marked instabilities.