arXiv subjects

Jason Tumlinson

Publications and source records attributed to Jason Tumlinson.

At least 145 records · Page 8Linked to original sources

arXiv2005

The Galaxy Environment of O VI Absorption Systems

We combine a FUSE sample of OVI absorbers (z < 0.15) with a database of 1.07 million galaxy redshifts to explore the relationship between absorbers and galaxy environments. All 37 absorbers with N(OVI) > 10^{13.2} cm^-2 lie within 800 h_70^-1 kpc of the nearest galaxy, with no compelling evidence for OVI absorbers in voids. The OVI absorbers often appear to be associated with environments of individual galaxies. Gas with 10 +/- 5% of solar metallicity (OVI and CIII) has a median spread in distance of 350-500 kpc around L* galaxies and 200-270 kpc around 0.1 L* galaxies (ranges reflect uncertain metallicities of gas undetected in Lya absorption). In order to match the OVI line frequency, dN/dz = 20 for N(OVI) > 10^{13.2} cm^-2, galaxies with L < 0.1 L* must contribute to the cross section. The Lya absorbers with N(HI) > 10^{13.2} cm^-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show OVI and/or CIII absorption, corresponding to a 33-50% covering factor at 0.1 Z_sun and suggesting that metals are spread to a maximum distance of 800 kpc, within typical galaxy supercluster filaments. Approximately 50% of the OVI absorbers have associated Lya line pairs with separations Delta V = 50-200 km/s. These pairs could represent shocks at the speeds necessary to create copious OVI, located within 100 kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lya forest absorbers.

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arXiv2005

A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGN

We report results from a FUSE survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGN at high Galactic latitudes (|b| > 20 degrees). Most (39 of 45) of the sight lines show detectable Galactic H2 absorption from Lyman and Werner bands between 1000 and 1126 A, with column densities ranging from N(H2) = 10^(14.17-19.82) cm^-2. In the northern Galactic hemisphere, we identify many regions of low column, N(H2) < 10^15 cm^-2, between longitude l = 60-180 degrees and at b > 54 degrees. These `"H2 holes" provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the "Northern Chimney" (low Na I absorption) and "Lockman Hole" with low N(HI). A comparison of high-latitude H2 with 139 OB-star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H2 clouds may have different rates of heating, cooling, and UV excitation. For rotational states J = 0 and 1, the mean excitation temperature at high latitude, = 124 +/- 8 K, is somewhat above that in the Galactic disk, = 86 +/- 20 K. For J = 2-4, the = 498 +/- 28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with N(H2) > 10^18. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density, log N_H = 20.38 +/- 0.13, than in the Galactic disk, log N_H(disk) = 20.7. If the FUV radiation fields are similar in disk and low halo, this suggests an enhanced (dust-catalyzed) H2 formation rate in higher-density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.

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arXiv2005

Chemical Evolution in Hierarchical Models Of Cosmic Structure I: Constraints on the Early Stellar Initial Mass Function

I present a new Galactic chemical evolution model motivated by and grounded in the hierarchical theory of galaxy formation, as expressed by a halo merger history of the Galaxy. This model accurately reproduces the "metallicity distribution function" (MDF) for Population II stars residing today in the Galactic halo. The observed MDF and the apparent absence of true Population III stars from the halo strongly imply that there is some critical metallicity, Z_crit <~ 10^-4 Z_sun, below which low-mass star formation is inhibited, and perhaps impossible. The observed constraints from the halo MDF, relative metal abundances from Galactic halo stars, and the ionizing photon budget needed to reionize the IGM together imply a stellar IMF that is peaked in the range of massive stars that experience core-collapse supernovae, with mean mass = 8 - 42 Msun. This mass range is similar to the masses predicted by models of primordial star formation that account for formation feedback. The model also implies that metal-poor halo stars below [Fe/H] <~ -3 had only 1 - 10 metal-free stars as their supernova precursors, such that the relative abundances in these halo stars exhibit IMF-weighted averages over the intrinsic yields of the first supernovae. This paper is the first part of a long term project to connect the high-redshift in situ indicators of early star formation with the low-z, old remnants of the first stars.

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arXiv2005

Hot Baryons and the Distribution of Metals in the Intergalactic Medium

We use the observed number and column-density distributions of intergalactic O VI absorbers to constrain the distribution of metals in the low-redshift intergalactic medium (IGM). In this simple model the metals in the O VI absorbers are assumed to be produced in and propagated from low-redshift galaxies drawn from a real sample, in this case the Sloan Digital Sky Survey (SDSS). This model can explain the observed dN/dz of metals seen in O VI absorbers if these metals are dispersed out to ~ 200 kpc by galaxies down to L \~ 0.01 - 0.1 L_r^*. Massive galaxies (L ~ L_r^*) by themselves cannot provide the necessary enrichment unless they can enrich volumes out to $\gtrsim 0.5 - 1 Mpc. Alternatively, our model allows an estimate of the fraction of O VI absorbers directly caused by galaxies rather than hot IGM. With this assumption we explore the possible connections between the intergalactic O VI absorbers and the known populations of highly-ionized high-velocity clouds (HVCs) surrounding the Milky Way. Our model predicts that more sensitive, complete surveys optimized to uncover weaker O VI absorbers will find the tentative turnover below log N(O VI) ~ 13.5 to be a real effect resulting from the apparently limited volumes over which galaxies can enrich the IGM. If so, it would indicate that metals are not as widespread throughout the low-density IGM as they are assumed to be in cosmological simulations of the WHIM.

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arXiv2004

The Hot Intergalactic Medium - Galaxy Connection: Two Strong Absorbers in the Sightline Toward PG 1211+143

We present HST/STIS and FUSE spectra of the QSO PG 1211+143 (z_em = 0.081) and a galaxy survey of the surrounding field. This sightline shows two strong intergalactic absorption systems at cz ~ 15,300 and 19,300. This sightline addresses the nature and origin of the OVI absorbers, and their connection to galaxies. We explore the relationship of these absorbers to the nearby galaxies and compare them to other OVI-bearing absorbers in diverse environments. At 15,300 km s^-1, we find four distinct HI components and associated CII, CIII, CIV, SiII, SiIII, SiIV, NV, and OVI, lying near a spiral-dominated galaxy group with a bright member galaxy 137 h_70^-1 kpc from the sightline. The observed ions of C, Si, and N are likely to be photoionized, but the OVI is more consistent with collisional ionization. The ion ratios in this absorber resemble the highly-ionized Galactic HVCs; it may also trace the hot intragroup medium gas or the unbound wind of an undiscovered dwarf galaxy. At 19,300 km s^-1, we find five HI components and associated CIII, SiIII, and collisionally-ionized OVI lying 146 h_70^-1 kpc from an isolated galaxy. The properties of the OVI-bearing gas are consistent with an origin in strong shocks between low-metallicity gas (>= 2 - 6 % solar) and one or more of the warm photoionized components. It is likely that these absorbers are related to the nearby galaxies, perhaps by outflows or gas stripped from unseen satellite galaxies by interactions. However, we cannot reject completely the hypothesis that they reside in the same large-scale structure in which the galaxies are embedded but are otherwise not directly related.

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arXiv2004

FUSE's Five Years of Progress on the Interstellar Medium

I review the five years of progress by FUSE on current topics in the interstellar medium. FUSE's sensitivity and unique access to the far ultraviolet allow investigators to solve problems in all phases of the interstellar medium. I describe FUSE's contributions in four major areas: 1) the Local Interstellar Medium (LISM), 2) the hot phase (O VI), 3) the cold phase (H_2), and 4) interstellar gas abundances. I devote particular attention to the common themes of ISM phase interactions and changes with metallicity. As a whole, these results show that FUSE is the most powerful machine ever to address problems of the ISM, and that FUSE points vividly to the future of ISM studies in the Galaxy, Local Group, and beyond.

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arXiv2004

Absorption Line Study of Halo Gas in NGC 3067 Toward the Background Quasar 3C 232

We present new H I 21 cm absorption data and ultraviolet spectroscopy from HST/STIS of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sightline lies near the minor axis and 1.8 arcmin (11 kpc) above the plane of NGC 3067, a nearby luminous (cz = 1465 km/s, L = 0.5L*) starburst galaxy with a moderate star formation rate of 1.4 Solar masses per year. The UV spectra show that the Si IV and C IV doublets have the same three velocity components at cz = 1369, 1417, and 1530 km/s found in Ca II H & K, Na I D, Mg I, Mg II, and Fe II, implying that the low and high ionization gas are both found in three distinct absorbing clouds (only the strongest component at 1420 km/s is detected in H I 21 cm). The new Lyman alpha observation allows the first measurements of the spin and kinetic temperatures of halo gas: T_s = 435 +/- 140 K and T_k/T_s ~ 1. However, while a standard photoionization model can explain the low ions, the C IV and Si IV are explained more easily as collisionally-ionized boundary layers of the photoionized clouds. Due to their small inferred space velocity offsets (-260, -130, and +170 km/s) relative to the nucleus of NGC 3067 and the spatial coincidence of low and high ionization gas, we propose that these absorbers are analogous to Galactic high velocity clouds (HVCs). A comparison of the NGC 3067 clouds and Galactic HVCs finds similar H I column densities, kinematics, metallicities, spin temperatures, and inferred sizes. We find no compelling evidence that any halo gas along this sightline is escaping the gravitational potential of NGC 3067, despite its modest starburst.

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arXiv2004

Nucleosynthesis, Reionization, and the Mass Function of the First Stars

We critique the hypothesis that the first stars were very massive stars (VMS; M > 140 Msun). We review the two major lines of evidence for the existence of VMS: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment, and (2) that the high electron-scattering optical depth (tau_e) to the CMB found by WMAP requires VMS for reionization in a concordance LambdaCDM cosmology. The supernova yield patterns of VMS are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or ``hypernovae'' from metal-free progenitors with 8 - 40 Msun can better explain the observed trends. With a simple metal-transport model, we estimate that halo enrichment curtails metal-free star formation after ~10^8 yr at z ~ 20. Because the ionizing photon efficiency of metal-free stars peaks at ~120 Msun and declines at higher mass, an IMF with a lower bound at M ~ 10 - 20 Msun and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semi-analytic model for H I and He II reionization, we find that such an IMF can reproduce tau_e = 0.10 - 0.14, consistent with the range from WMAP. We conclude, on the basis of these results, that VMS are not necessary to meet the existing constraints commonly taken to motivate them.

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arXiv2003

The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers (soft radiation fields) with voids in the (H I) Ly-alpha distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Owing to photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to 460. Values of eta = 20-200 are consistent with models of photoionization by quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with eta < 30 may indicate the presence of local hard sources. We find that eta is higher in "void" regions, where H I is weak or undetected and 80% of the path length has eta > 100. These voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation softened by escape from the AGN cores or transfer through the "cosmic web". The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.

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arXiv2003

The Multiphase Intergalactic Medium towards PKS 2155-304

We study the cluster of H I and O VI absorption systems and the claimed detection of O VIII absorption from the intergalactic medium at z ~ 0.0567, associated with a group of galaxies toward the BL Lac object PKS 2155-304. As measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet Spectroscopic Explorer, and Chandra, this system appears to contain gas at a variety of temperatures. We analyze this multi-phase gas in a clumpy-infall model. From the absence of C IV and Si III absorption in the Ly-alpha clouds, we infer metallicities less than 2.5-10% of solar values. The only metals are detected in two O VI absorption components, offset by +/- 400 km/s from the group barycenter (cz ~ 16,600 km/s). The O VI components may signify "nearside" and "backside" infall into the group potential well, which coincides with the claimed O VIII absorption. If the claimed O VIII detection is real, our analysis suggests that clusters of strong Ly-alpha and O VI absorbers, associated with groups of galaxies, may be the "signposts" of shock-heated, metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI, O VII, and O VIII, one may be able to clarify the heating mechanism of this multiphase gas.

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arXiv2002

Cosmological Effects of the First Stars: Evolving Spectra of Population III

The first stars hold intrinsic interest for their uniqueness and for their potential importance to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). The metal-free composition of the first stars restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Pop III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Pop II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Pop II in the HeII ($E > 4$ Ryd) continuum. We suggest that large Ly$α$ equivalent widths ($W_{\rm Lyα} > 400$ Angstroms) may provide a means of detecting metal-free stellar populations at high redshift, and that HeII recombination lines ($λ$1640, $λ$4686) may confirm identifications of Population III. While Pop III clusters are intrinsically bluer than their Pop II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Pop III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Pop III for the reionization of the IGM in both H and He. (ABRIDGED)

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arXiv2002

Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize HI and HeII at redshifts, $z$ of about 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Pop III and Pop II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of HeIII reaches a maximum of about 60 percent at $z$ of about 5.6 if Pop III star formation lasts for $10^9$ yr. Future data on HI reionization can test the amount of small-scale power available to the formation of the first objects, and provide a constraint on values of $σ_8$ less than or about 0.7. Since current UV observations indicate an epoch of reionization for HeII at $z$ of about 3, HeII may reionize more than once. Measurements of the HeII Gunn-Peterson effect in the intergalactic medium at redshifts exceeding about 3 may reveal the significance of Pop III stars for HeII reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity.

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arXiv2002

Inferring physical conditions in interstellar clouds of H_2

We have developed a code that models the formation, destruction, radiative transfer, and vibrational/rotational excitation of H_2 in a detailed fashion. We discuss how such codes, together with FUSE observations of H_2 in diffuse and translucent lines of sight, may be used to infer various physical parameters. We illustrate the effects of changes in the major physical parameters (UV radiation field, gas density, metallicity), and we point out the extent to which changes in one parameter may be mirrored by changes in another. We provide an analytic formula for the molecular fraction, f_H2, as a function of cloud column density, radiation field, and grain formation rate of H_2. Some diffuse and translucent lines of sight may be concatenations of multiple distinct clouds viewed together. Such situations can give rise to observables that agree with the data, complicating the problem of uniquely identifying one set of physical parameters with a line of sight. Finally, we illustrate the application of our code to an ensemble of data, such as the FUSE survey of H_2 in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the elevated UV radiation field intensity and reduced grain formation rate of H_2 in those low- metallicity environments.

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arXiv2002

A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V $\gtrsim$ 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 $\gtrsim$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."

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arXiv2001

A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 (+0.005, -0.002) for the SMC and = 0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lya in pencil-beam sight lines as measures of N(HI). These results imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x 10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x 10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the Galactic mean value. We find that these high-radiation, low-formation-rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature = 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic gas. We discuss the implications of this work for theories of star formation in low-metallicity environments. [Abstract abridged]

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arXiv2001

High-Velocity Cloud Complex C: Galactic Fuel or galactic Waste?

We present HST GHRS and STIS observations of five QSOs that probe the prominent high-velocity cloud (HVC) Complex C, covering 10% of the northern sky. Based upon a single sightline measurement (Mrk 290), a metallicity [S/H]=-1.05+/-0.12 has been associated with Complex C by Wakker et al. (1999a,b). When coupled with its inferred distance (5<d<30 kpc) and line-of-sight velocity (v=-100 to -200 km/s), Complex C appeared to represent the first direct evidence for infalling low-metallicity gas onto the Milky Way, which could provide the bulk of the fuel for star formation in the Galaxy. We have extended the abundance analysis of Complex C to encompass five sightlines. We detect SII absorption in three targets (Mrk 290, Mrk 817, and Mrk 279); the resulting [SII/HI] values range from -0.36 (Mrk 279) to -0.48 (Mrk 817) to -1.10 (Mrk 290). Our preliminary OI FUSE analysis of the Mrk 817 sightline also supports the conclusion that metallicities as high as 0.3 times solar are encountered within Complex C. These results complicate an interpretation of Complex C as infalling low-metallicity Galactic fuel. Ionization corrections for HII and SIII cannot easily reconcile the higher apparent metallicities along the Mrk 817 and Mrk 279 sightlines with that seen toward Mrk 290, since H-alpha emission measures preclude the existence of sufficient HII. If gas along the other lines of sight has a similar pressure and temperature to that sampled toward Mrk 290, the predicted H-alpha emission measures would be 900 mR. It may be necessary to reclassify Complex C as mildly enriched Galactic waste from the Milky Way or processed gas torn from a disrupted neighboring dwarf, as opposed to low-metallicity Galactic fuel.

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arXiv2001

The Ionization and Metallicity of the Intervening O VI Absorber at z = 0.1212 in the Spectrum of H1821+643

We use high-resolution UV spectra of the radio-quiet QSO H1821+643 (z_em = 0.297), obtained with STIS and FUSE, to study the ionization and metallicity of an intervening O VI absorption line system at z(abs) = 0.1212. This absorber has the following notable properties: (1) Several galaxies are close to the sight line at the absorber redshift, including an actively star-forming galaxy at a projected distance of 144 kpc. (2) There is a complex cluster of H I lya absorption lines near the O VI redshift, including at least five components spread over a velocity range of ~700 km/s . (3) The strongest lya line in the cluster appears to be composed of a mildly saturated component with a typical b-value blended with a broad component with b = 85 km/s . (4) The O VI absorption is not aligned with the strongest H I absorption, but instead is well-aligned with the broad component. (5) The only detected species (>4sigma) are O VI and H I despite coverage of strong transitions of other abundant elements. Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with roughly 5.3 < log T < 5.6 and -1.8 < [O/H] < -0.6. However, we find that photoionization is also viable if the pathlength l through the absorbing gas is long enough; simple photoionization models require 85 < l < 1900 kpc and -1.1 < [O/H] < -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy.

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arXiv2001

FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: II. The Line of Sight Toward HD 110432

We report the second study from the FUSE survey of molecular hydrogen in translucent clouds, for the line of sight toward HD 110432. This star lies beyond the Coalsack dark nebula, and with E(B-V) = 0.40, and A_V = 1.32 this line of sight bridges the gap between less extinguished diffuse cloud lines of sight with A_V \sim 1, such as Zeta Oph, and the translucent clouds with A_V \gtrsim 2 such as HD 73882. Through profile fitting and a curve-of-growth analysis, we have derived rotational populations for H2 for J = 0--7. The line of sight has a total molecular hydrogen column density, log N(H2) = 20.68 \pm 0.05 cm^{-2}, nearly identical to that toward Zeta Oph, but a factor of three less than HD 73882. The ratio of N(J=1) to N(J=0) yields a kinetic temperature T_{kin} = 63 \pm 7 K, similar to other lines of sight with A_V \gtrsim 1. The high-J lines show considerable excitation above this temperature. The high-J excitation is similar to that toward Zeta Oph, but much smaller than that toward HD 73882. Chemical modeling indicates that the physical conditions in the cloud(s) are very similar to those in the cloud(s) toward Zeta Oph. An analysis of IUE spectra of the Lyman-alpha line gives log N(H I) = 20.85 \pm 0.15 cm^{-2}. Combined with N(H2), we derive a hydrogen molecular fraction, f_{H2} = 0.58 pm 0.12, statistically identical to that found for the lines of sight toward Zeta Oph and HD 73882. We also report column densities for the HD and CO molecules. From the combined measurements of hydrogen and carbon- containing molecules, the line of sight toward HD 110432 appears quite similar to the diffuse cloud line of sight toward Zeta Oph, and quite dissimilar to the translucent cloud line of sight toward HD 73882.

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