Shocks in the interstellar medium

Shocks are ubiquitous in the interstellar medium (ISM), occurring whenever large pressure gradients lead to fluid-dynamical disturbances that move at a velocity that exceeds the local sound speed. As shocks dissipate kinetic energy into heat, they give rise to strong cooling radiation that constitutes excellent diagnostics for the study of the conditions in the shocked gas. The interpretation of this radiation requires the application of detailed numerical shock models. Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high-density environments. However, an analysis of the effects of shattering and vaporization on molecular line emission had remained open. I have developed a new method for implementing grain-grain processing into a 2-fluid magnetohydrodynamic (MHD) shock model, which includes a self-consistent treatment of the molecular line transfer. Using this combined model, it was shown that shattering has a strong influence on continuous MHD shocks ("C-type shocks") for a broad range of shock parameters: the shocks become hotter and thinner. Predictions were made for the emission of H2, CO, OH and H2O. The main focus of the study lay on SiO, which is a prominent indicator of shock processing in dense clouds and is released into the gas-phase by the vaporization of grain cores. The release by vaporization already early in the shock changes the excitation characteristics of the SiO line radiation, although it does not change the width of SiO rotational lines. This study has significantly improved our understanding of shock emission in high-density environments. The method that was developed will make it possible to easily implement the effect of grain-grain processing in other numerical shock models. MHD shock models were applied in the interpretation of observations of supernova remnants (SNRs) interacting with molecular clouds. New CO rotational line observations with the APEX telescope from shocked regions in two of these SNRs, W28 and W44, were presented. Towards W28, data was also taken with the SOFIA telescope. The integrated CO intensities observed towards positions of shock interaction were compared with a large grid of MHD shock models. Towards W28, it was found that only stationary C-type shock models were compatible with the observed emission. These shocks could satisfactorily account for the pure rotational H2 emission as observed with Spitzer. In W44, however, only models of much younger, non-stationary shocks could reproduce the observations. The preshock densities found in both SNRs were too low for grain-grain processing to be significant. Based on our modelling, we were able to constrain the physical and chemical conditions in the shocked regions, give predictions for H2O and the full ladder of CO rotational transitions, and quantify the momentum and energy injection of the SNR into the ISM. The results are important for a proper understanding of the local characteristics of SNR-cloud interactions, as well as for the study of the global energetics and dynamics of the ISM and the study of cosmic rays. The developed method enables a systematic comparison of a large grid of detailed MHD shock models with observations of shocked molecular gas and will be further applied in future studies. I conclude with a critical reflection of research on astrophysical shocks within the framework of recent discussions in the philosophy of science

Impacts of pure shocks in the BHR71 bipolar outflow

During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow regulates star formation by e.g. inducing turbulence and heating the surrounding gas. Understanding the associated shocks is therefore essential to the study of star formation. We present comparisons of shock models with CO, H2, and SiO observations in a 'pure' shock position in the BHR71 bipolar outflow. These comparisons provide an insight into the shock and pre-shock characteristics, and allow us to understand the energetic and chemical feedback of star formation on Galactic scales. New CO (Jup = 16, 11, 7, 6, 4, 3) observations from the shocked regions with the SOFIA and APEX telescopes are presented and combined with earlier H2 and SiO data (from the Spitzer and APEX telescopes). The integrated intensities are compared to a grid of models that were obtained from a magneto-hydrodynamical shock code which calculates the dynamical and chemical structure of these regions combined with a radiative transfer module based on the 'large velocity gradient' approximation. The CO emission leads us to update the conclusions of our previous shock analysis: pre-shock densities of 1e4 cm-3 and shock velocities around 20-25 km s-1 are still constrained, but older ages are inferred ( 4000 years). We evaluate the contribution of shocks to the excitation of CO around forming stars. The SiO observations are compatible with a scenario where less than 4% of the pre-shock SiO belongs to the grain mantles. We infer outflow parameters: a mass of 1.8x1e-2 Msun was measured in our beam, in which a momentum of 0.4 Msun km s-1 is dissipated, for an energy of 4.2x1e43erg. We analyse the energetics of the outflow species by species. Comparing our results with previous studies highlights their dependence on the method: H2 observations only are not sufficient to evaluate the mass of outflows.Comment: 14 pages, 10 figures, 4 Tables, accepted in A&

Gas and dust cooling along the major axis of M 33 (HerM33es). Herschel/PACS [C II] and [O I] observations

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Context. M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular clouds. Aims. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. Methods. We mapped the emission of gas and dust in M 33 using the far-infrared lines of [CII] and [OI](63 mu m) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the Herschel Space Observatory. These maps have 50 pc resolution and form a similar to 370 pc wide stripe along its major axis covering the sites of bright HII regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24 mu m from Spitzer/MIPS, and at 70 mu m, 100 mu m, and 160 mu m from Herschel/PACS were combined to obtain a map of the TIR. Results. TIR and [CII] intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G(0, obs)similar to 2 to 200 in units of the average Galactic radiation field. The binned [CII]/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the [CII] emission, as estimated from VLA HI data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the [OI] data before comparing it with models of photon dominated regions. Most of the ratios of [CII]/[OI] and ([CII]+[OI])/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n similar to 2x10(2) cm(-3), G(0)similar to 60, and a second low-FUV solution at n similar to 10(4) cm(-3), G(0)similar to 1.5. Conclusions. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors similar to 1. A fraction of the gas may, however, be represented by the second solution. © C. Kramer et al. 2020M.R. and S.V. acknowledge support by the research projects AYA2014-53506-P and AYA2017-84897-P from the Spanish Ministerio de Economia y Competitividad, from the European Regional Development Funds (FEDER) and the Junta de Andalucia (Spain) grants FQM108. This study has been partially financed by the Consejeria de Conocimiento, Investigacion y Universidad, Junta de Andalucia and European Regional Development Fund (ERDF), ref. SOMM17/6105/UGR. FST thanks the Spanish Ministry of Economy and Competitiveness (MINECO) for support under grant number AYA2016-76219-P.Peer reviewe

A plea for useless curiosity

International audienc

IR continuum, [CII] and [OI] maps of M33

VizieR online Data Catalogue associated with article published in journal Astronomy & Astrophysics with title 'Gas and dust cooling along the major axis of M 33 (HerM33es). Herschel/PACS [CII] and [OI] observations.' (bibcode: 2020A&A...639A..61K