H<sub>2</sub>absorption in a dense interstellar filament in the Milky Way halo

Richter, Philipp; Sembach, Kenneth R.; Howk, J. C.

doi:10.1051/0004-6361:20030722

Cited by 35 publications

(41 citation statements)

References 40 publications

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“…De Boer et al (1980 found UV absorption at 20 km s −1 (Galactic) and at 220, 250 and 290 km s −1 towards the LMC and proposed that the 220 km s −1 component might originate in coronal gas associated with the LMC. Intermediateand high-velocity clouds floating in the Galactic Halo have also been seen in front of the LMC at 60 and 120 km s −1 , interestingly also in H 2 absorption (Richter et al 1999;Richter et al 2003;Lehner et al 2009). We only, tentatively, identify one kinematic component between 30 and 230 km s −1 , viz.…”

Section: An Expanding Bubble Around R 136?mentioning

confidence: 90%

The VLT-FLAMES Tarantula Survey

Loon

Bailey

Tatton

et al. 2013

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Context. The Tarantula Nebula (a.k.a. 30 Dor) is a spectacular star-forming region in the Large Magellanic Cloud (LMC), seen through gas in the Galactic disc and halo. Diffuse interstellar bands (DIBs) offer a unique probe of the diffuse, cool-warm gas in these regions. Aims. The aim is to use DIBs as diagnostics of the local interstellar conditions, whilst at the same time deriving properties of the yet-unknown carriers of these enigmatic spectral features. Methods. Spectra of over 800 early-type stars from the Very Large Telescope Flames Tarantula Survey (VFTS) were analysed. Maps were created, separately, for the Galactic and LMC absorption in the DIBs at 4428 and 6614 Å and -in a smaller region near the central cluster R 136 -neutral sodium (the Na i D doublet); we also measured the DIBs at 5780 and 5797 Å.Results. The maps show strong 4428 and 6614 Å DIBs in the quiescent cloud complex to the south of 30 Dor but weak absorption in the harsher environments to the north (bubbles) and near the OB associations. The Na maps show at least five kinematic components in the LMC and a shell-like structure surrounding R 136, and small-scale structure in the Milky Way. The strengths of the 4428, 5780, 5797 and 6614 Å DIBs are correlated, also with Na absorption and visual extinction. The strong 4428 Å DIB is present already at low Na column density but the 6614, 5780 and 5797 Å DIBs start to be detectable at subsequently larger Na column densities. Conclusions. The carriers of the 4428, 6614, 5780 and 5797 Å DIBs are increasingly prone to removal from irradiated gas. The relative strength of the 5780 and 5797 Å DIBs clearly confirm the Tarantula Nebula as well as Galactic high-latitude gas to represent a harsh radiation environment. The resilience of the 4428 Å DIB suggests its carrier is large, compact and neutral. Structure is detected in the distribution of cool-warm gas on scales between one and >100 pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds from the central cluster R 136 have created an expanding shell; some infalling gas is also detected, reminiscent of a galactic "fountain".

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Section: An Expanding Bubble Around R 136?mentioning

confidence: 90%

The VLT-FLAMES Tarantula Survey

Loon

Bailey

Tatton

et al. 2013

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“…Methods for accurately deriving the density from multi-line observations have been successfully developed and applied e.g. by Schreyer et al (1997); Richter et al (2003); Sonnentrucker et al (2003). They are based on a combination of information from different species tracing a wide range of critical densities, but the accurate determination of ρ 0 still remains a challenging task.…”

Section: Derivation Of the Velocity Structure From Density-contaminatmentioning

confidence: 99%

Interstellar cloud structure: the statistics of centroid velocities

Ossenkopf

Esquivel

Lazarian

et al. 2006

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Context. The statistical properties of maps of line centroids have been used for almost 50 years, but there is still no general agreement on their interpretation. Aims. We have tried to quantify which properties of underlying turbulent velocity fields can be derived from centroid velocity maps, and we tested conditions under which the scaling behaviour of the centroid velocities matches the scaling of the three-dimensional velocity field. Methods. Using fractal cloud models we systematically studied the relation between three-dimensional density and velocity fields and the statistical properties of the resulting line centroid maps. We paid special attention to cases with large density fluctuations resembling supersonic interstellar turbulence. Starting from the ∆-variance analysis, we derived a new tool to compute the scaling behaviour of the three-dimensional velocity field from observed intensity and centroid velocity maps. Results. We provide two criteria to decide whether the information from the centroid velocities directly reflects the properties of the underlying velocity field. Applying these criteria allows us to understand the different results found so far in the literature for interpreting the statistics of velocity centroids. The new iteration scheme can be used to derive the three-dimensional velocity scaling from centroid velocity maps for arbitrary density and velocity fields, but it requires accurate knowledge of the average density of the interstellar cloud under consideration.

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“…From fine structure levels of C i, Welty (2007) discovered that the observed variations in Na i and Ca ii lines towards HD 219188 reflect the variation in the density and ionisation in the gas, and do not refer to high-density clumps. A different approach was used by Richter et al (2003a,b) who found, using the many absorption lines of molecular hydrogen in the Lyman and Werner bands, that the molecular gas in their studied lines of sight resides in compact filaments, which possibly correspond to the tiny-scale atomic structures in the diffuse ISM (Richter et al 2003a).…”

Section: Introductionmentioning

confidence: 99%

Interstellar absorptions towards the LMC:

Nasoudi-Shoar¹,

Richter

Boer³

et al. 2010

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Observations show that the interstellar medium (ISM) contains sub-structure on scales less than 1 pc, detected in the form of spatial and temporal variations in column densities or optical depth. Despite the number of detections, the nature and ubiquity of the smallscale structure in the ISM is not yet fully understood. We use UV absorption data mainly from the Far Ultraviolet Spectroscopic Explorer (FUSE) and partly from the Space Telescope Imaging Spectrograph (STIS) of six Large Magellanic Cloud (LMC) stars (Sk -67 • 111, LH 54-425, Sk -67 • 107, Sk -67 • 106, Sk -67 • 104, and Sk -67 • 101) that are all located within 5 of each other, and analyse the physical properties of the Galactic disc gas in front of the LMC on sub-pc scales. We analyse absorption lines of a number of ions within the UV spectral range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and fine-structure levels of neutral carbon have been used in order to study changes in the density and the physical properties of the Galactic disc gas over small angular scales. At an assumed distance of 1 kpc, the 5 separation between Sk -67 • 111 and Sk -67 • 101 implies a linear extent of 1.5 pc.We report on column densities of H 2 , C i, N i, O i, Al ii, Si ii, P ii, S iii, Ar i, and Fe ii in our six lines of sight, as well as C i*, C i**, Mg ii, Si iv, S ii, Mn ii, and Ni ii for four of them. While most species do not show any significant variation in their column densities, we find an enhancement of almost 2 dex for H 2 from Sk -67 • 111 to Sk -67 • 101, accompanied by only a small variation in the O i column density. Based on the formation-dissociation equilibrium, we trace these variations to the actual density variations in the molecular gas. On the smallest spatial scale of <0.08 pc, between Sk -67 • 107 and LH 54-425, we find a gas density variation of a factor of 1.8. The line of sight towards LH 54-425 does not follow the relatively smooth change seen from Sk -67 • 101 to Sk -67 • 111, suggesting that sub-structure might exist on a smaller spatial scale than the linear extent of our sight-lines. The results show that we sample a mix of both neutral and ionised gas in our six lines of sight. Towards Sk -67 • 101 to Sk -67 • 107, we derive the temperature T exc 70 K for the inner self-shielded part of the gas based on the rotational excitation levels of H 2 , and an average density of n H 60 cm −3 , typical of that for CNM. The gas towards LH 54-425 and Sk -67 • 111 shows different properties, and T exc 200 K. Our observations suggest that the detected H 2 in these six lines of sight (with the extent of <1.5 pc) is not necessarily physically connected, but that we are sampling molecular cloudlets with pathlengths <0.1−1.8 pc and possibly different densities.

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H₂absorption in a dense interstellar filament in the Milky Way halo

Cited by 35 publications

References 40 publications

The VLT-FLAMES Tarantula Survey

The VLT-FLAMES Tarantula Survey

Interstellar cloud structure: the statistics of centroid velocities

Interstellar absorptions towards the LMC:

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H2absorption in a dense interstellar filament in the Milky Way halo

Cited by 35 publications

References 40 publications

The VLT-FLAMES Tarantula Survey

The VLT-FLAMES Tarantula Survey

Interstellar cloud structure: the statistics of centroid velocities

Interstellar absorptions towards the LMC:

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Product

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H₂absorption in a dense interstellar filament in the Milky Way halo