Deuterium fractionation processes in the interstellar medium (ISM) have been shown to be highly efficient in the family of nitrogen hydrides. To date, observations were limited to ammonia (NH$_2$D, NHD$_2$, ND$_3$) and imidogen radical (ND) isotopologues. We want to explore the high frequency windows offered by the \emph{Herschel Space Observatory} to search for deuterated forms of amidogen radical NH$_2$ and to compare the observations against the predictions of our comprehensive gas-grain chemical model. Making use of the new molecular spectroscopy data recently obtained at high frequencies for NHD and ND$_2$, both isotopologues have been searched for in the spectral survey towards the class 0 IRAS 16293-2422, a source in which NH$_3$, NH and their deuterated variants have been previously detected. We used the observations carried out with HIFI (Heterodyne Instrument for the Far Infrared) in the framework of the key program "Chemical Herschel surveys of star forming regions" (CHESS). We report the first detection of interstellar NHD and ND$_2$. Both species are observed in absorption against the continuum of the protostar. From the analysis of their hyperfine structure, accurate excitation temperature and column density values have been determined. The latter were combined with the column density of the parent species NH$_2$ to derive the deuterium fractionation in amidogen. The amidogen D/H ratio measured in the low-mass protostar IRAS 16293-2422 is comparable to the one derived for the related species imidogen and much higher than that observed for ammonia. Additional observations of these species will give more insights into the mechanism of ammonia formation and deuteration in the ISM. We finally indicate the current possibilities to further explore these species at submillimeter wavelengths.
Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In …
Observations of ammonia in interstellar environments have revealed high levels of deuteration, and all its D-containing variants, including ND$_3$, have been detected in cold prestellar cores and around young protostars. The observation of these deuterated isotopologues is very useful to elucidate the chemical and physical processes taking place during the very early stages of star formation, as the abundance of deuterated molecules is highly enhanced in dense and cold gas. Nitrogen hydride radicals are key species lying at the very beginning of the reaction pathway leading to the formation of…
We investigate the thermal condensation caused by a massive object that passes through the interstellar medium with high velocity, and propose a mechanism for creating a filamentary gaseous object, or interstellar contrail. Our main result shows that a long interstellar contrail can form with a certain parameter; a compact object more massive than $10^4\ {\rm M_\odot}$ can make a filament whose length is larger than $100\ {\rm pc}$. Observation of interstellar contrails may provide information on the number, masses, and velocities of fast-moving massive objects, and can be a new method for pro…
We present the first mid-IR detection of the linear polarization toward the star CygOB2-12, a luminous blue hypergiant that, with Av of ~10 mag of foreground extinction, is a benchmark in the study of the properties of dust in the diffuse interstellar medium. The 8-13 micrometer spectropolarimetry, obtained with the CanariCam multi-mode camera at the Gran Telescopio Canarias (GTC), shows clear trends with wavelength characteristic of silicate grains aligned in the interstellar magnetic field. The maximum polarization, detected with 7.8 statistical significance near 10.2 micrometers, is (1.24 +…
The interstellar medium (ISM) is a very complex medium which contains the matter needed to form stars and planets. The ISM is in permanent interaction with radiation, turbulence, magnetic and gravitational fields, and accelerated particles. Everything that happens in this medium has consequences on the dynamics and evolution of the Galaxy, resulting the link that relates the stellar scale with the galactic one. Thus, the study of the ISM is crucial to advance in the knowledge of stellar and galactic astrophysics. In this article I present a summary of what we know about the physics and chemist…
In this paper we discuss and confront recent results on metallicity variations in the local interstellar medium, obtained from observations of HII regions and neutral clouds of the Galactic thin disk, and compare them with recent high-quality metallicity determinations of other tracers of the chemical composition of the interstellar medium as B-type stars, classical Cepheids and young clusters. We find that the metallicity variations obtained for these last kinds of objects are consistent with each other and with that obtained for HII regions but significantly smaller than those obtained for n…
Dwarf galaxies are by far the most numerous galaxies in the Universe, showing properties that are quite different from those of their larger and more luminous cousins. This review focuses on the physical and chemical properties of the interstellar medium of those dwarfs that are known to host significant amounts of gas and dust. The neutral and ionized gas components and the impact of the dust will be discussed, as well as first indications for the existence of active nuclei in these sources. Cosmological implications are also addressed, considering the primordial helium abundance and the simi…
Since the developpment of astronephography, we know that the Sun is about to exit the Local Interstellar Cloud (LIC). To date, because of its rare absorption signatures and the paucity of suitable neighbour targets, the LIC interface has proved to be elusive to extensive investigations. Comparing the spatial distribution of Si III detections found in the litterature along with 3 new sigtlines, I show that most detections seem to arise from a cone whose axis is parallel to the LIC heliocentric velocity vector. I interpret this result as an evidence that the heliosphere is actually interacting w…
Magnetic fields permeate the entire Galaxy and are essential to, for example, the regulation of several stages of the star formation process and cosmic ray transportation. Unraveling its properties, such as intensity and topology, is an observational challenge that requires combining different and complementary techniques. The polarization of starlight due to the absorption by field-aligned non-spherical dust grains provides a unique source of information about the interstellar magnetic field in the optical band. This work introduces a first analysis of a new catalog of optical observations of…
We present a generic mechanism for the thermal damping of compressive waves in the interstellar medium (ISM), occurring due to radiative cooling. We solve for the dispersion relation of magnetosonic waves in a two-fluid (ion-neutral) system in which density- and temperature-dependent heating and cooling mechanisms are present. We use this dispersion relation, in addition to an analytic approximation for the nonlinear turbulent cascade, to model dissipation of weak magnetosonic turbulence. We show that in some ISM conditions, the cutoff wavelength for magnetosonic turbulence becomes tens to hun…
This is a brief review of our understanding of the properties of the interstellar medium (ISM) in dwarf galaxies in connection to their star formation activity. What are the dominant phases of the ISM in these objects? How do the properties of these phases depend on the galaxy properties? What do we know about their cold gas content and its link to star formation activity? Does star formation proceed differently in these galaxies? How does star formation feedback operate in dwarf galaxies? The availability of observations from space-based facilities such as FUSE, Spitzer, Herschel, and Fermi, …
We explore the effect of magnetic fields on the vertical distribution and multiphase structure of the supernova-driven interstellar medium (ISM) in simulations that admit dynamo action. As the magnetic field is amplified to become dynamically significant, gas becomes cooler and its distribution in the disc becomes more homogeneous. We attribute this to magnetic quenching of vertical velocity, which leads to a decrease in the cooling length of hot gas. A non-monotonic vertical distribution of the large-scale magnetic field strength, with the maximum at |z| $\approx$ 300 pc causes a downward pre…
This study is motivated by recent observations on ubiquitous interstellar density filaments and guided by modern theories of compressible magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structures. As the fundamental dynamics of compressible MHD turbulence, perpendicular turbulent mixing of density fluctuations entails elongated density structures aligned with the local magnetic field, accounting for low-density parallel filaments seen in diffuse atomic and molecular gas. The elongation of low-density parallel filaments depends on the turbulence an…
The detection of CF$^+$ in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF$^+$ abundance requires one to model its non-LTE excitation via both radiative and collisional processes. Here, we report quantum calculations of rate coefficients for the rotational excitation of CF$^+$ in collisions with para- and ortho-H2 (for temperatures up…
The interstellar medium is a key ingredient that governs star formation in galaxies. We present a detailed study of the infrared (~ 1-500 micron) spectral energy distributions of a large sample of 193 nearby (z ~ 0.088) luminous infrared galaxies (LIRGs) covering a wide range of evolutionary stages along the merger sequence. The entire sample has been observed uniformly by 2MASS, WISE, Spitzer, and Herschel. We perform multi-component decomposition of the spectra to derive physical parameters of the interstellar medium, including the intensity of the interstellar radiation field and the mass a…
Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are thought to be widespread throughout the Universe, because these classes of molecules are probably responsible for the unidentified infrared bands, a set of emission features seen in numerous Galactic and extragalactic sources. Despite their expected ubiquity, astronomical identification of specific aromatic molecules has proven elusive. We present the discovery of benzonitrile ($c$-C$_6$H$_5$CN), one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium. We observed hyperfine-resolved t…
Magnetism is one of the most important forces on the interstellar medium (ISM), anisotropically regulating the structure and star formation that drive galactic evolution. Recent high dynamic range observations of diffuse gas and molecular clouds have revealed new links between interstellar structures and the ambient magnetic field. ISM morphology encodes rich physical information, but deciphering it requires high-resolution measurements of the magnetic field: linear polarization of starlight and dust emission, and Zeeman splitting. These measure different components of the magnetic field, and …
The 2175 Å ultraviolet (UV) extinction bump in interstellar medium (ISM) of the Milky Way was discovered in 1965. After intensive exploration of more than a half century, however, its exact origin still remains a big conundrum that is being debated. Here we propose a mixture model by which the extinction bump in ISM is argued possibly relevant to the clusters of hydrogenated T-carbon (HTC) molecules (C40H16) that have intrinsically a sharp absorption peak at the wavelength 2175 Å. By linearly combining the calculated absorption spectra of HTC mixtures, graphite, MgSiO3 and Fe2SiO4, we show tha…
Whether ice in cold cosmic environments is physically separated from the silicate dust or mixed with individual silicate moieties is not known. However, different grain models give very different compositions and temperatures of grains. The aim of the present study is a comparison of the mid-IR spectra of laboratory silicate-grains/water-ice mixtures with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar and circumstellar environments. The laboratory data can explain the observations assuming reasonable mass-averaged temperatures for the protostellar envel…
The interstellar medium is a multi-phase, magnetized, and highly turbulent medium. In this paper, we address both theoretical and observational aspects of plasma turbulence in the interstellar medium. We successively consider radio wave propagation through a plasma and radio polarized emission. For each, we first provide a theoretical framework in the form of a few basic equations, which enable us to define useful diagnostic tools of plasma turbulence; we then show how these tools have been utilized to detect and interpret observational signatures of plasma turbulence, and what astronomers hav…
Radioactive components of the interstellar medium provide an entirely-different and new aspect to the studies of the interstellar medium. Injected from sources of nucleosynthesis, unstable nuclei decay along their trajectories. Measurements can occur through characteristic gamma rays that are emitted with the decay, or in cosmic material samples through abundances of parent and daughter isotopes as they change with decay. The dynamics and material flows within interstellar medium are thus accessible to measurement, making use of the intrinsic clock that radioactive decay provides. We describe …
This study is motivated by recent observations on the ubiquitous interstellar density filaments and guided by the modern theory of magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structure. The perpendicular turbulent mixing, as the fundamental dynamics of MHD turbulence, naturally entails parallel filaments presented in both the diffuse medium and molecular clouds (MCs). The minimum width is determined by the perpendicular neutral-ion decoupling scale in partially ionized media. Differently, dense perpendicular filaments arise in highly supersonic…
Synthetic observations are playing an increasingly important role across astrophysics, both for interpreting real observations and also for making meaningful predictions from models. In this review, we provide an overview of methods and tools used for generating, manipulating and analysing synthetic observations and their application to problems involving star formation and the interstellar medium. We also discuss some possible directions for future research using synthetic observations.
Massive stars drive strong winds that impact the surrounding interstellar medium, producing parsec-scale bubbles for isolated stars and superbubbles around young clusters. These bubbles can be observed across the electromagnetic spectrum, both the wind itself and the swept up interstellar gas. Runaway massive stars produce bow shocks that strongly compresses interstellar gas, producing bright infrared, optical and radio nebulae. With the detection of non-thermal radio emission from bow shocks, particle acceleration can now also be investigated. I review research on wind bubbles and bow shocks …
