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Excitation mechanisms in newly discovered H_2-bearing damped Lyman-α clouds: systems with low molecular fractions
Aims. We probe the physical conditions in high-redshift dampedLyman-α systems (DLAs) using the observed molecular fraction andthe rotational excitation of molecular hydrogen. Methods: We searchfor Lyman- and Werner-band absorption lines of molecular hydrogen in theVLT/UVES spectra of background QSOs at the redshift of known DLAs. Results: We report two new detections of molecular hydrogen in thesystems at z_abs=2.402 and 1.989 toward, respectively, HE 0027-1836 andHE 2318-1107, discovered in the course of the Hamburg-ESO DLA survey. Wealso present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f ≤ -4,with f=2N(H2)/(2N(H2)+N(H i)). Only one such H2 system was knownpreviously. Two of them (toward Q 2343+125 and HE 2318-1107) havehigh-metallicities, [ X/H]>-1, whereas the DLA toward HE 0027-1836 isthe system with the lowest metallicity ([ Zn/H]=-1.63) among knownH2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Niin the three systems are found to be very similar to what is observed indiffuse gas of the Galactic halo. Molecular hydrogen absorption fromrotational levels up to J=5 is observed in a single well-definedcomponent toward HE 0027-1836. We show that the width (Dopplerparameter) of the H2 lines increases with increasing J and that thekinetic energy derived from the Doppler parameter is linearly dependenton the relative energy of the rotational levels. There is however novelocity shift between lines from different rotational levels. Theexcitation temperature is found to be 90 K for J=0 to J=2 and ~500 K forhigher J levels. Single isothermal PDR models fail to reproduce theobserved rotational excitations. A two-component model is needed: onecomponent of low density (~50 cm-3) with weak illumination(χ = 1) to explain the J ≤ 2 rotational levels and another ofhigh density (~500 cm-3) with strong illumination (χ =30) for J ≥ 3 levels. However, the juxtaposition of these two PDRcomponents may be ad-hoc and the multicomponent structure could resulteither from turbulent dissipation or C-shocks.Based on observations carried out at the European Southern Observatory(ESO) under progs. ID 67.A-0022, 69.A-0204, 072.A-0346, 072.A-0442,073.A-0071 and 074.A-0201 with the UVES spectrograph installed at theVery Large Telescope (VLT) Kueyen UT2 on Cerro Paranal, Chile. Figures19-25 are only available in electronic form at http://www.aanda.org
| H2 excitation in turbulent interstellar clouds
We discuss the observational differences between lines of sight thatintercept a group of turbulent dissipative structures and lines of sightthat cross less-active regions. Using time-dependent calculations weshow that the energy level distribution of the hydrogen molecule evolvesin time in response to the local thermal phase. We find that relativelysimple models can explain the observed properties of molecular hydrogenin diffuse interstellar clouds in terms of time evolution induced bycollisional excitation in a low-density, high-temperature gas.
| A far UV study of interstellar gas towards HD 34078: High excitation H2 and small scale structure
To investigate the presence of small scale structure in the spatialdistribution of H2 molecules we have undertaken repeated FUSEUV observations of the runaway O9.5V star, HD 34078. In this paper wepresent five spectra obtained between January 2000 and October 2002.These observations reveal an unexpectedly large amount of highly excitedH2>. Column densities for H2 levels from (v =0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levelsare determined. These results are interpreted in the frame of a modelinvolving essentially two components: i) a foreground cloud (unaffectedby HD 34078) responsible for the H2 (J = 0, 1), CI, CH,CH+ and CO absorptions; ii) a dense layer of gas (n ≃104 cm-3) close to the O star and stronglyilluminated by its UV flux which accounts for the presence of highlyexcited H2. Our model successfully reproduces theH2 excitation, the CI fine-structure level populations aswell as the CH, CH+ and CO column densities. We also examinethe time variability of H2 absorption lines tracing each ofthese two components. From the stability of the J = 0, 1 and 2 dampedH2 profiles we infer a 3σ upper limit on column densityvariations Δ N(H2)/N(H2) of 5% over scalesranging from 5 to 50 AU. This result clearly rules out any pronouncedubiquitous small scale density structure of the kind apparently seen inHI. The lines from highly excited gas are also quite stable (equivalentto Δ N/N ≤30%) indicating i) that the ambient gas throughwhich HD 34078 is moving is relatively uniform and ii) that the gas flowalong the shocked layer is not subject to marked instabilities.Based on observations performed by the FUSE mission and at the CFHTtelescope.
| Local interstellar medium kinematics towards the Southern Coalsack and Chamaeleon-Musca dark clouds
We present the results of a spectroscopic programme aiming toinvestigate the kinematics of the local interstellar medium componentstowards the Southern Coalsack and Chamaeleon-Musca dark clouds. Theanalysis is based upon high-resolution (R~ 60000) spectra of theinterstellar Na I D absorption lines towards 63 B-type stars (d<= 500pc) selected to cover these clouds and the connecting area defined bythe Galactic coordinates: 308°>=l>= 294° and-22°<=b<= 5°. The radial velocities, column densities,velocity dispersions, colour excess and photometric distances to thestars are used to understand the kinematics and distribution of theinterstellar cloud components. The analysis indicates that theinterstellar gas is distributed in two extended sheet-like structurespermeating the whole area, one at d<= 60 pc and another around120-150 pc from the Sun. The nearby feature is approaching the localstandard of rest with an average radial velocity of -7 kms-1, has low average column density logNNaI~ 11.2cm-2 and velocity dispersion b~ 5 km s-1. The moredistant feature has column densities between 12.3 <=logNNaI<= 13.2, average velocity dispersion b~ 2.5 kms-1 and seems associated with the dust sheet observed towardsthe Coalsack, Musca and Chamaeleon direction. Its velocity is centredaround 0 km s-1, but there is a trend for increasing from -3km s-1 near b= 1° to 3 km s-1 near b=-18°.The nearby low column density feature indicates a general outflow fromthe Sco-Cen association, in agreement with several independent lines ofdata in the general searched direction. The dust and gas feature around120-150 pc seem to be part of an extended large-scale feature of similarkinematic properties, supposedly identified with the interaction zone ofthe Local and Loop I Bubbles. Assuming that the interface and thering-like volume of dense neutral matter that would have been formedduring the collision of the two bubbles have similar properties, ourresults suggest that the interaction zone between the bubbles is twistedand folded.
| Inferring Physical Conditions in Interstellar Clouds of H2
We have developed a code that models the formation, destruction,radiative transfer, and vibrational/rotational excitation ofH2 in a detailed fashion. We discuss generally how suchcodes, together with Far Ultraviolet Spectrographic Explorer (FUSE)observations of H2 in diffuse and translucent lines of sight,may be used to infer various physical parameters. We illustrate theeffects of changes in the major physical parameters (UV radiation field,gas density, metallicity), and we point out the extent to which changesin one parameter may be mirrored by changes in another. We provide ananalytic formula for the molecular fraction, fH2,as a function of cloud column density, radiation fields, and grainformation rate of H2. Some diffuse and translucent lines ofsight 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 physicalparameters with a line of sight. Finally, we illustrate the applicationof our code to an ensemble of data, such as our FUSE survey ofH2 in the Large and Small Magellanic Clouds, in order toconstrain the elevated UV radiation field intensity and reduced grainformation rate of H2 in those low-metallicity environments.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Ultra-high-resolution observations of CH in Southern Molecular Cloud envelopes
We present a mini-survey of ultrahigh-resolution spectroscopy (UHRS) ofCH towards three southern molecular cloud envelopes. The sightlines areselected to probe physically similar gas in different Galacticenvironments. With a velocity resolution of ~0.5kms-1(R=575000) these observations resolve most kinematic components of theabsorption lines. We do, however, detect one line component in the Lupusregion, which is not resolved and for which an upper limit ofb<0.3kms-1 is found. We find a correlation betweendistance of the absorbing gas from the Galactic mid-plane and thefractional abundance of CH. We show that this correlation can beexplained as being a result of a fall-off in the ultraviolet radiationfield intensity and propose that CH observations in carefully selectedsightlines might allow a mapping of the variations in the interstellarradiation field.
| H2 formation and excitation in the diffuse interstellar medium
We use far-UV absorption spectra obtained with FUSE towards three late Bstars to study the formation and excitation of H2 in thediffuse ISM. The data interpretation relies on a model of the chemicaland thermal balance in photon-illuminated gas. The data constrain wellthe n R product between gas density and H2 formation rate ondust grains: n R = 1 to 2.2 x 10-15 s-1. For eachline of sight the mean effective H2 density n, assumeduniform, is obtained by the best fit of the model to the observedN(J=1)/N(J=0) ratio, since the radiation field is known. Combining nwith the n R values, we find similar H2 formation rates forthe three stars of about R = 4 x 10-17 cm3s-1. Because the target stars do not interact with theabsorbing matter we can show that the H2 excitation in theJ> 2 levels cannot be accounted for by the UV pumping of the coldH2 but implies collisional excitation in regions where thegas is much warmer. The existence of warm H2 is corroboratedby the fact that the star with the largest column density ofCH+ has the largest amount of warm H2.
| On the distance to the Chamaeleon I and II associations
Constraints on the distances to the dark clouds Chamaeleon I and II areinvestigated in detail. A compilation of photometric data, spectraltypes and absolute magnitudes for field stars towards each cloud ispresented, and results are used to examine the distribution of reddeningwith distance along each line of sight. The distances to starsassociated with reflection nebulae in each cloud are examined in detail.On the basis of these results, we deduce the most probable distance ofCha I to be 160+/-15pc, and that of Cha II to be 178+/-18pc. Anexamination of the mean fluxes of T Tauri stars in each cloud providesindependent evidence to suggest that Cha II is significantly moredistant than Cha I. Both clouds appear to be embedded in a macroscopicsheet-like structure extending over much of the Chamaeleon-Musca-Cruxregion. The Chamaeleon III and DC\ts 300.2--16.9 clouds are probablypart of the same structure, with probable distances ~ 140--160pc.
| Dust Metamorphosis in the Galaxy
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Observation and Astrometry data
| Constellation: | Caméléon |
| Right ascension: | 12h32m19.73s |
| Declination: | -78°11'38.8" |
| Apparent magnitude: | 7.766 |
| Distance: | 332.226 parsecs |
| Proper motion RA: | -17.7 |
| Proper motion Dec: | -3.3 |
| B-T magnitude: | 7.855 |
| V-T magnitude: | 7.774 |
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