Multi-wavelength observations of a nearby multi-phase interstellar cloud

Nehmé, Cyrine; Gry, C.; Boulanger, F.; Bourlot, J. Le; Forêts, G. Pineau des; Falgarone, É.

doi:10.1051/0004-6361:20078373

Cited by 23 publications

(22 citation statements)

References 41 publications

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“…We also note that strong Si III (1206 Å) interstellar absorption has been reported for several sight-lines towards nearby stars (Holberg et al 1999;Nehme et al 2008). This ion has a relatively high rate coefficient for charge exchange with neutral hydrogen and its presence within the local interstellar medium is unlikely to be linked to the remnant of some past ionization event such as a supernova shock (Lyu and Bruhweiler 1996; Fig.…”

Section: 1supporting

confidence: 51%

The trouble with the Local Bubble

Welsh

Shelton

2009

Astrophys Space Sci

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The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence of the soft X-ray background diffuse emission. We summarize the current knowledge on this local interstellar region, paying particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere, has led to a reassessment of the generally accepted model that requires a hot local plasma.In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane, has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and cooler gas remains to be found. The second possible explanation is that of the 'Hot Top' model, in which the Local Cavity is an old su-B.Y. Welsh ( ) Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu R.L. Shelton Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA pernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity is now thought to be filled with partially ionized cloudlets of temperature ∼ 7000 K that are surrounded by lower density envelopes of photo-ionized gas of temperature ∼ 20,000 K. Although this new scenario provides a natural explanation for many of the observations that were in conflict with the Local Hot Bubble model, we cannot (as yet) provide a satisfactory explanation or the emission levels observed in the B and Be ultra-soft X-ray bands.

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Section: 1supporting

confidence: 51%

The trouble with the Local Bubble

Welsh

Shelton

2009

Astrophys Space Sci

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“…This drag corresponds to an exchange of momentum and energy with the background plasma. This transfer of energy between clouds and hot plasma has been discussed in diverse astrophysical contexts, in particular infalling clouds in cooling flows of clusters of galaxies (Pope et al 2008) and galactic halos (Murray & Lin 2004), as well as cometary clouds in the local interstellar medium (Nehmé et al 2008) and the Helix planetary nebula tails (Dyson et al 2006).…”

Section: Introductionmentioning

confidence: 99%

H₂formation and excitation in the Stephan's Quintet galaxy-wide collision

Guillard¹,

Boulanger²,

Forêts

et al. 2009

A&A

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Context. The Spitzer Space Telescope has detected a powerful (L H 2 ∼ 10 41 erg s −1 ) mid-infrared H 2 emission towards the galaxywide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H 2 -luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H 2 gas coexisting with a hot (∼5 × 10 6 K), X-ray emitting plasma. The surface brightness of H 2 lines exceeds that of the X-rays and the 0−0 S(1) H 2 linewidth is ∼900 km s −1 , of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H 2 present in the postshock gas? (ii) How can we account for the H 2 excitation? (iii) Why is H 2 a dominant coolant? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H 2 formation and excitation in the postshock medium. Results. (i)The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a ∼5 × 10 6 K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than 10 6 K cools, keeps its dust content and becomes H 2 within the SQ collision age (∼5 × 10 6 years).(ii) Since the bulk kinetic energy of the H 2 gas is the dominant energy reservoir, we consider that the H 2 emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H 2 excitation can be reproduced by a combination of low velocities shocks (5−20 km s −1 ) within dense (n H > 10 3 cm −3 ) H 2 gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H 2 a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H 2 emission lines. Conclusions. This study provides a physical framework to interpret H 2 emission from H 2 -luminous galaxies. It highlights the role that H 2 formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H 2 emission, of mechanical energy dissipation in multiphase gas.

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“…In a companion paper (Nehmé et al 2008), we presented a multi-wavelength study combining spectroscopic UV, optical, IR and radio observations of the interstellar matter, along the line of sight to the nearby (170 pc) moderately reddened (E(B − V) = 0.17) star HD 102065. Absorption observations provide the column densities of H 2 in the J = 0 to 5 states, C i in its three fine structure states, CO in the J = 0 to 2 states, CH and CH + .…”

Section: Introductionmentioning

confidence: 99%

Modeling of diffuse molecular gas applied to HD 102065 observations

Nehmé¹,

Bourlot²,

Boulanger

et al. 2008

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Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H 2 , C i, CO, CH, and CH + column densities and excitation. These data are complemented by observations of C + , CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH + and H 2 in its excited (J ≥ 2) levels, are consistent with a cloud model implying a Galactic radiation field (G ∼ 0.4 in Draine's unit), a density of 80 cm −3 and a temperature (60−80 K) set by the equilibrium between heating and cooling processes. To account for excited (J ≥ 2) H 2 levels column densities, an additional component of warm (∼250 K) and dense (n H ≥ 10 4 cm −3 ) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H 2 ratio at formation is ∼1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H 2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H 2 is required to account for the CH + column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.

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Multi-wavelength observations of a nearby multi-phase interstellar cloud

Cited by 23 publications

References 41 publications

The trouble with the Local Bubble

The trouble with the Local Bubble

H₂formation and excitation in the Stephan's Quintet galaxy-wide collision

Modeling of diffuse molecular gas applied to HD 102065 observations

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Multi-wavelength observations of a nearby multi-phase interstellar cloud

Cited by 23 publications

References 41 publications

The trouble with the Local Bubble

The trouble with the Local Bubble

H2formation and excitation in the Stephan's Quintet galaxy-wide collision

Modeling of diffuse molecular gas applied to HD 102065 observations

Contact Info

Product

Resources

About

H₂formation and excitation in the Stephan's Quintet galaxy-wide collision