arXiv subjects

M. G. Burton

Publications and source records attributed to M. G. Burton.

At least 19 recordsLinked to original sources

arXiv2024

Disruption of a massive molecular cloud by a supernova in the Galactic Centre: Initial results from the ACES project

The Milky Way's Central Molecular Zone (CMZ) differs dramatically from our local solar neighbourhood, both in the extreme interstellar medium conditions it exhibits (e.g. high gas, stellar, and feedback density) and in the strong dynamics at play (e.g. due to shear and gas influx along the bar). Consequently, it is likely that there are large-scale physical structures within the CMZ that cannot form elsewhere in the Milky Way. In this paper, we present new results from the Atacama Large Millimeter/submillimeter Array (ALMA) large programme ACES (ALMA CMZ Exploration Survey) and conduct a multi-wavelength and kinematic analysis to determine the origin of the M0.8$-$0.2 ring, a molecular cloud with a distinct ring-like morphology. We estimate the projected inner and outer radii of the M0.8$-$0.2 ring to be 79" and 154", respectively (3.1 pc and 6.1 pc at an assumed Galactic Centre distance of 8.2 kpc) and calculate a mean gas density $> 10^{4}$ cm$^{-3}$, a mass of $\sim$ $10^6$ M$_\odot$, and an expansion speed of $\sim$ 20 km s$^{-1}$, resulting in a high estimated kinetic energy ($> 10^{51}$ erg) and momentum ($> 10^7$ M$_\odot$ km s$^{-1}$). We discuss several possible causes for the existence and expansion of the structure, including stellar feedback and large-scale dynamics. We propose that the most likely cause of the M0.8$-$0.2 ring is a single high-energy hypernova explosion. To viably explain the observed morphology and kinematics, such an explosion would need to have taken place inside a dense, very massive molecular cloud, the remnants of which we now see as the M0.8$-$0.2 ring. In this case, the structure provides an extreme example of how supernovae can affect molecular clouds.

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arXiv2022

Mapping the aliphatic hydrocarbon content of interstellar dust in the Galactic plane

We implement a new observational method for mapping the aliphatic hydrocarbon content in the solid phase in our Galaxy, based on spectrophotometric imaging of the 3.4 $μ$m absorption feature from interstellar dust. We previously demonstrated this method in a field including the Galactic Centre cluster. We applied the method to a new field in the Galactic centre where the 3.4 $μ$m absorption feature has not been previously measured and we extended the measurements to a field in the Galactic plane to sample the diffuse local interstellar medium, where the 3.4 $μ$m absorption feature has been previously measured. We have analysed 3.4 $μ$m optical depth and aliphatic hydrocarbon column density maps for these fields. Optical depths are found to be reasonably uniform in each field, without large source-to-source variations. There is, however, a weak trend towards increasing optical depth in a direction towards $b=0^{\circ}$ in the Galactic centre. The mean value of column densities and abundances for aliphatic hydrocarbon were found to be about several $\rm \times 10^{18} \, cm^{-2}$ and several tens $\times 10^{-6}$, respectively for the new sightlines in the Galactic plane. We conclude that at least 10-20% of the carbon in the Galactic plane lies in aliphatic form.

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arXiv2022

Modelling the Gamma-Ray Morphology of HESS J1804-216 from Two Supernova Remnants in a Hadronic Scenario

HESS J1804-216 is one of the brightest yet most mysterious TeV gamma-ray sources discovered to date. Previous arc-minute scale studies of the interstellar medium (ISM) surrounding this TeV gamma-ray source revealed HESS J1804-216 is likely powered by a mature supernova remnant (SNR) or pulsar, hence its origin remains uncertain. In this paper, we focus on the diffusive escape of cosmic-ray protons from potential SNR accelerators. These cosmic rays interact with the ISM to produce TeV gamma-rays. We utilise the isotropic diffusion equation solution for particles escaping from a shell, to model the energy-dependent escape and propagation of protons into the ISM. This work is the first attempt at modelling the spatial morphology of gamma-rays towards HESS J1804-216, using arc-minute ISM observations from both Mopra and the Southern Galactic Plane Survey. The spectral and spatial distributions of gamma-rays for the two nearby potential SNR counterparts, SNR G8.7-0.1 and the progenitor SNR of PSR J1803-2137, are presented here. We vary the diffusion parameters and particle spectrum and use a grid search approach to find the best combination of model parameters. We conclude that moderately slow diffusion is required for both candidates. The most promising candidate to be powering the TeV gamma-rays from HESS J1804-216 in a hadronic scenario is the progenitor SNR of PSR J1803-213.

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arXiv2021

Probing Dark Matter and Fundamental Physics with the Cherenkov Telescope Array

Astrophysical observations provide strong evidence that more than 80% of all matter in the Universe is in the form of dark matter (DM). Two leading candidates of particles beyond the Standard Model that could constitute all or a fraction of the DM content are the so-called Weakly Interacting Massive Particles (WIMPs) and Axion-Like Particles (ALPs). The upcoming Cherenkov Telescope Array, which will observe gamma rays between 20 GeV and 300 TeV with unprecedented sensitivity, will have unique capabilities to search for these DM candidates. A particularly promising target for WIMP searches is the Galactic Center. WIMPs with annihilation cross sections correctly producing the DM relic density will be detectable with CTA, assuming an Einasto-like density profile and WIMP masses between 200 GeV and 10 TeV. Regarding new physics beyond DM, CTA observations will also enable tests of fundamental symmetries of nature such as Lorentz invariance.

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arXiv2020

Arc-minute-scale studies of the interstellar gas towards HESS$\,$J1804$-$216: Still an unidentified TeV $γ$-ray source

The Galactic TeV $γ$-ray source HESS$\,$J1804$-$216 is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7 and 12$\,$mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap HESS$\,$J1804$-$216 at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV $γ$-ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV $γ$-ray emission from HESS$\,$J1804$-$216. For a hadronic scenario, SNR$\,$G8.7$-$0.1 and the progenitor SNR of PSR$\,$J1803$-$2137 require cosmic ray (CR) enhancement factors of $\mathord{\sim} 50$ times the solar neighbour CR flux value to produce the TeV $γ$-ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8$\,$kpc (the dispersion measure distance to PSR$\,$J1803$-$2137) tends to anti-correlate with features of the TeV emission from HESS$\,$J1804$-$216, making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.

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arXiv2020

A method for mapping the aliphatic hydrocarbon content of interstellar dust towards the Galactic Centre

In the interstellar medium, the cosmic elemental carbon abundance includes the total carbon in both gas and solid phases. The aim of the study was to trial a new method for measuring the amount and distribution of aliphatic carbon within interstellar dust over wide fields of view of our Galaxy. This method is based on measurement of the 3.4 $μ$m absorption feature from aliphatic carbonaceous matter. This can readily be achieved for single sources using IR spectrometers. However, making such measurements over wide fields requires an imaging IR camera, equipped with narrow-band filters that are able to sample the spectrum. While this cannot produce as good a determination of the spectra, the technique can be applied to potentially tens to hundreds of sources simultaneously, over the field of view of the camera. We examined this method for a field in the centre of the Galaxy, and produced a map showing the variation of 3.4 $μ$m optical depth across it.

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arXiv2020

Aliphatic hydrocarbon content of the interstellar dust

In the interstellar medium, carbon is distributed between the gas and solid phases. However, while about half of the expected carbon abundance can be accounted for in the gas phase, there is considerable uncertainty as to the amount incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 $μ$m absorption feature when viewed against a background radiation source. The optical depth of the 3.4 $μ$m absorption feature is related to the number of aliphatic carbon C-H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 $μ$m interstellar absorption feature providing that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We generated laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. The measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure which combined FTIR and $^{13}$C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement (4.76(8) $\times$ 10$^{-18}$ cm group$^{-1}$ and 4.69(14) $\times$ 10$^{-18}$ cm group$^{-1}$), less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium.

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arXiv2019

Connecting the ISM to TeV PWNe and PWNe candidates

We investigate the interstellar medium (ISM) towards seven TeV gamma-ray sources thought to be pulsar wind nebulae (PWNe) using Mopra molecular line observations at 7mm [CS(1-0), SiO(1-0,v=0)], Nanten CO(1-0) data and the SGPS/GASS HI survey. We have discovered several dense molecular clouds co-located to these TeV gamma-ray sources , which allows us to search for cosmic-rays (CRs) coming from progenitor SNRs or, potentially, from PWNe. We notably found SiO(1-0,v=0) emission towards HESS J1809-193, highlighting possible interaction between the adjacent supernova remnant SNR G011.0-0.0 and the molecular cloud at d $\sim$ 3.7 kpc. Using morphological features, and comparative studies of our column densities with those obtained from X-ray measurements, we claim a distance d $\sim$ 8.6 - 9.7 kpc for SNR G292.2-00.5, d $\sim$ 3.5 - 5.6 kpc for PSR J1418-6058 and d $\sim$ 1.5 kpc for the new SNR candidate found towards HESS J1303-631. From our mass and density estimates of selected molecular clouds, we discuss signatures of hadronic/leptonic components from PWNe and their progenitor SNRs. Interestingly, the molecular gas, which overlaps HESS J1026-582 at d $\sim$ 5 kpc, may support a hadronic origin. We find however that this scenario requires an undetected cosmic-ray accelerator to be located at d $\lt$ 10 pc from the molecular cloud. For HESS J1809-193, the cosmic-rays which have escaped SNR G011.0-0.0 could contribute to the TeV gamma-ray emission. Finally, from the hypothesis that at most 20% the pulsar spin down power could be converted into CRs, we find that, among the studied PWNe, only those from PSR J1809-1917 could potentially contribute to the TeV emission.

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arXiv2019

The Mopra Southern Galactic Plane CO Survey - Data Release 3

We present observations of fifty square degrees of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes $l = 300$-$350^\circ$ and latitudes $|b| \le 0.5^\circ$. These data have been taken at 0.6 arcminute spatial resolution and 0.1 km/s spectral resolution, providing an unprecedented view of the molecular clouds and gas of the Southern Galactic Plane in the 109-115 GHz $J = 1$-0 transitions of $^{12}$CO, $^{13}$CO, C$^{18}$O and C$^{17}$O. We present a series of velocity-integrated maps, spectra and position-velocity plots that illustrate Galactic arm structures and trace masses on the order of $\sim$10$^{6}$ M$_{\odot}$ per square degree; and include a preliminary catalogue of C$^{18}$O clumps located between $l=330$-$340^\circ$. Together with information about the noise statistics of the survey these data can be retrieved from the Mopra CO website, the PASA data store and the Harvard Dataverse (doi:10.7910/DVN/LH3BDN ).

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arXiv2018

Searching for an interstellar medium association for HESS J1534-571

The Galactic supernova remnant HESS J1534-571 (also known as G323.7-1.0) has a shell-like morphology in TeV gamma-ray emission and is a key object in the study of cosmic ray origin. Little is known about its distance and local environment. We examine Mopra $^{12}$CO/$^{13}$CO(1-0) data, Australian Telescope Compact Array HI, and Parkes HI data towards HESS J1534-571. We trace molecular clouds in at least five velocity ranges, including clumpy interstellar medium structures near a dip in HI emission at a kinematic velocity consistent with the Scutum-Crux arm at ~3.5 kpc. This feature may be a cavity blown-out by the progenitor star, a scenario that suggests HESS J1534-571 resulted from a core-collapse event. By employing parametrisations fitted to a sample of supernova remnants of known distance, we find that the radio continuum brightness of HESS J1534-571 is consistent with the ~3.5 kpc kinematic distance of the Scutum-Crux arm HI dip. Modelling of the supernova evolution suggests an ~8-24 kyr age for HESS J1534-571 at this distance.

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arXiv2017

ATLASGAL --- properties of a complete sample of Galactic clumps

Abridged: ATLASGAL is an unbiased 870 micron submillimetre survey of the inner Galactic plane. It provides a large and systematic inventory of all massive, dense clumps in the Galaxy (>1000 Msun) and includes representative samples of all embedded stages of high-mass star formation. Here we present the first detailed census of the properties (velocities, distances, luminosities and masses) and spatial distribution of a complete sample of ~8000 dense clumps located in the Galactic disk. We derive highly reliable velocities and distances to ~97% of the sample and use mid- and far-infrared survey data to develop an evolutionary classification scheme that we apply to the whole sample. Comparing the evolutionary subsamples reveals trends for increasing dust temperatures, luminosities and line-widths as a function of evolution indicating that the feedback from the embedded proto-clusters is having a significant impact on the structure and dynamics of their natal clumps. We find 88\,per\,cent are already associated with star formation at some level. We also find the clump mass to be independent of evolution suggesting that the clumps form with the majority of their mass in-situ. We estimate the statistical lifetime of the quiescent stage to be ~5 x 10^4 yr for clump masses ~1000 Msun decreasing to ~1 x 10^4 yr for clump masses >10000 Msun. We find a strong correlation between the fraction of clumps associated with massive stars and peak column density. The fraction is initially small at low column densities but reaching 100\,per\,cent for column densities above 10^{23} cm^{-2}; there are no clumps with column density clumps above this value that are not already associated with massive star formation. All of the evidence is consistent with a dynamic view of star formation wherein the clumps form rapidly and are initially very unstable so that star formation quickly ensues.

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arXiv2017

Towards a three-dimensional distribution of the molecular clouds in the Galactic Centre

We present a study of the three-dimensional structure of the molecular clouds in the Galactic Centre (GC) using CO emission and OH absorption lines. Two CO isotopologue lines, $^{12}$CO ($J$=1$\rightarrow$0) and $^{13}$CO ($J$=1$\rightarrow$0), and four OH ground-state transitions, surveyed by the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH), contribute to this study. We develop a novel method to calculate the OH column density, excitation temperature, and optical depth precisely using all four OH lines, and we employ it to derive a three-dimensional model for the distribution of molecular clouds in the GC for six slices in Galactic latitude. The angular resolution of the data is 15.5 arcmin, which at the distance of the GC (8.34 kpc) is equivalent to 38 pc. We find that the total mass of OH in the GC is in the range 2400-5100 Solar mass . The face-on view at a Galactic latitude of b = 0° displays a bar-like structure with an inclination angle of 67.5 $\pm$ 2.1° with respect to the line of sight. No ring-like structure in the GC is evident in our data, likely due to the low spatial resolution of the CO and OH maps.

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arXiv2017

H2O Southern Galactic Plane Survey (HOPS): Paper III - Properties of Dense Molecular Gas across the Inner Milky Way

The H2O Southern Galactic Plane Survey (HOPS) has mapped 100 square degrees of the Galactic plane for water masers and thermal molecular line emission using the 22-m Mopra telescope. We describe the automated spectral-line fitting pipelines used to determine the properties of emission detected in HOPS datacubes, and use these to derive the physical and kinematic properties of gas in the survey. A combination of the angular resolution, sensitivity, velocity resolution and high critical density of lines targeted make the HOPS data cubes ideally suited to finding precursor clouds to the most massive and dense stellar clusters in the Galaxy. We compile a list of the most massive HOPS ammonia regions and investigate whether any may be young massive cluster progenitor gas clouds. HOPS is also ideally suited to trace the flows of dense gas in the Galactic Centre. We find the kinematic structure of gas within the inner 500pc of the Galaxy is consistent with recent predictions for the dynamical evolution of gas flows in the centre of the Milky Way. We confirm a recent finding that the dense gas in the inner 100pc has an oscillatory kinematic structure with characteristic length scale of ~20pc, and also identify similar oscillatory kinematic structure in the gas at radii larger than 100pc. Finally, we make all of the above fits and the remaining HOPS data cubes across the 100 square degrees of the survey available to the community.

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arXiv2017

Very High Excitation Lines of H$_{2}$ in the Orion Molecular Cloud Outflow

Vibration-rotation lines of H$_{2}$ from highly excited levels approaching the dissociation limit have been detected at a number of locations in the shocked gas of the Orion Molecular Cloud (OMC-1), including in a Herbig-Haro object near the tip of one of the OMC-1 "fingers." Population diagrams show that while the excited H$_{2}$ is almost entirely at a kinetic temperature of $\sim$1,800 K, (typical for vibrationally shock-excited H$_{2}$), as in the previously reported case of Herbig-Haro object HH 7 up to a few percent of the H$_{2}$ is at a kinetic temperature of $\sim$5,000~K. The location with the largest fraction of hot H$_{2}$ is the Herbig-Haro object, where the outflowing material is moving at a higher speed than at the other locations. Although theoretical work is required for a better understanding of the 5,000 K H$_{2}$, (including how it cools), its existence and the apparent dependence of its abundance relative to that of the cooler component on the relative velocities of the outflow and the surrounding ambient gas appear broadly consistent with it having recently reformed. The existence of this high temperature H$_{2}$ appears to be a common characteristic of shock-excited molecular gas.

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arXiv2016

Interstellar gas towards the TeV gamma-ray sources HESS J1640-465 and HESS J1641-463

We present a detailed analysis of the interstellar medium towards the TeV $γ$-ray sources HESS J1640$-$465 and HESS J1641$-$463 using results from the Mopra Southern Galactic Plane CO Survey and from a Mopra 7 mm-wavelength study. The $γ$-ray sources are positionally coincident with two supernova remnants G338.3$-$0.0 and G338.5+0.1 respectively. A bright complex of HII regions connect the two SNRs and TeV objects. Observations in the CO(1-0) transition lines reveal substantial amounts of diffuse gas positionally coincident with the $γ$-ray sources at multiple velocities along the line of sight, while 7 mm observations in CS, SiO, HC$_{3}$N and CH$_{3}$OH transition lines reveal regions of dense, shocked gas. Archival HI data from the Southern Galactic Plane Survey was used to account for the diffuse atomic gas. Physical parameters of the gas towards the TeV sources were calculated from the data. We find that for a hadronic origin for the $γ$-ray emission, the cosmic-ray enhancement rates are $\sim 10^{3}$ and $10^{2}$ times the local solar value for HESS J1640$-$465 and HESS J1641$-$463 respectively.

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arXiv2016

Scaled up low-mass star formation in massive star-forming cores in the G333 giant molecular cloud

Three bright molecular line sources in G333 have recently been shown to exhibit signatures of infall. We describe a molecular line radiative transfer modelling process which is required to extract the infall signature from Mopra and Nanten2 data. The observed line profiles differ greatly between individual sources but are reproduced well by variations upon a common unified model where the outflow viewing angle is the most significant difference between the sources. The models and data together suggest that the observed properties of the high-mass star-forming regions such as infall, turbulence, and mass are consistent with scaled-up versions of the low-mass case with turbulent velocities that are supersonic and an order of magnitude larger than those found in low-mass star-forming regions. Using detailed radiative transfer modeling, we show that the G333 cores are essentially undergoing a scaled-up version of low mass star formation. This is an extension of earlier work in that the degree of infall and the chemical abundances are constrained by the RT modeling in a way that is not practical with a standard analysis of observational data. We also find high velocity infall and high infall mass rates, possibly suggesting accelerated collapse due to external pressure. Molecular depletion due to freeze-out onto dust grains in central regions of the cores is suggested by low molecular abundances of several species. Strong evidence for a local enhancement of 13C-bearing species towards the outflow cloud cores is discussed, consistent with the presence of shocks caused by the supersonic motions within them.

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arXiv2016

ISM gas studies towards the TeV PWN HESS J1825-137 and northern region

HESS J1825-137 is a pulsar wind nebula (PWN) whose TeV emission extends across ~1 deg. Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located in the north of the PWN as detected by previous CO Galactic survey (e.g Lemiere, Terrier & Djannati-Ataï 2006). Here we provide a detailed picture of the ISM towards the region north of HESS J1825-137, with the analysis of the dense molecular gas from our 7mm and 12mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1-0) and GRS $^{13}$CO(1-0) surveys. Our focus is the possible association between HESS J1825-137 and the unidentified TeV source to the north, HESS J1826-130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec)=(18.421h,-13.282$^{\circ}$) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud-cloud collision scenario. Furthermore, the presence of a H$α$ rim may be the first evidence of the progenitor SNR of the pulsar PSR J1826-1334 as the distance between the H$α$ rim and the TeV source matched with the predicted SNR radius R$_{\text{SNR}}$~120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826-130 emission, including the progenitor SNR of PSR J1826-1334 and the PWN G018.5-0.4 powered by PSR J1826-1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source.

astro-ph.HE↗

arXiv2016

Highly Excited H2 in Herbig-Haro 7: Formation Pumping in Shocked Molecular Gas?

We have obtained K-band spectra at R~5,000 and angular resolution 0.3" of a section of the Herbig-Haro 7 (HH7) bow shock, using the Near-Infrared Integral Field Spectrograph at Gemini North. Present in the portion of the data cube corresponding to the brightest part of the bow shock are emission lines of H2 with upper state energies ranging from ~6,000 K up to the dissociation energy of H2, ~50,000 K. Because of low signal-to-noise ratios, the highest excitation lines cannot be easily seen elsewhere in the observed region. However, excitation temperatures, measured throughout much of the observed region using lines from levels as high as 25,000 K, are a strong function of upper level energy, indicating that the very highest levels are populated throughout. The level populations in the brightest region are well fit by a two-temperature model, with 98.5% of the emitting gas at T=1800 K and 1.5% at T=5200 K. The bulk of the H2 line emission in HH7, from the 1,800 K gas, has previously been well modeled by a continuous shock, but the 5,200 K component is inconsistent with standalone standard continuous shock models. We discuss various possible origins for the hot component and suggest that this component is H2 newly reformed on dust grains and then ejected from them, presumably following dissociation of some of the H2 by the shock.

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