Stellar Feedback in the ISM Revealed by Wide-Field Far-Infrared Spectral-Imaging

The radiative and mechanical interaction of stars with their environment drives the evolution of the ISM and of galaxies as a whole. The far-IR emission (lambda ~30 to 350 microns) from atoms and molecules dominates the cooling of the warm gas in the neutral ISM, the material that ultimately forms stars. Far-IR lines are thus the most sensitive probes of stellar feedback processes, and allow us to quantify the deposition and cycling of energy in the ISM. While ALMA (in the (sub)mm) and JWST (in the IR) provide astonishing sub-arcsecond resolution images of point sources and their immediate environment, they cannot access the main interstellar gas coolants, nor are they designed to image entire star-forming regions (SFRs). Herschel far-IR photometric images of the interstellar dust thermal emission revealed the ubiquitous large-scale filamentary structure of SFRs, their mass content, and the location of thousands of prestellar cores and protostars. These images, however, provide a static view of the ISM: not only they dont constrain the cloud dynamics, moreover they cannot reveal the chemical composition and energy transfer within the cloud, thus giving little insight into the regulation process of star formation by stellar feedback. In this white paper we emphasize the need of a space telescope with wide-field spectral-imaging capabilities in the critical far-IR domain.Comment: White Paper submitted to the Astro 2020 Decadal Survey on Astronomy and Astrophysics (National Academies of Science, Engineering, and Medicine

HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds

%methods {We used the EMIR receiver and FTS spectrometer at the IRAM 30m to construct absorption spectra toward bright extra-galactic background sources at 195 kHz spectral resolution ($\approx$ 0.6 \kms). We used the IRAM Plateau de Bure interferometer to synthesize absorption spectra of \hthcop\ and HCO toward the galactic HII region W49.} %results {HCO, \cc3h\ and CF\p\ were detected toward the blazars \bll\ and 3C111 having \EBV\ = 0.32 and 1.65 mag. HCO was observed in absorption from ``spiral-arm'' clouds in the galactic plane occulting W49. The complement of detectable molecular species in the 85 - 110 GHz absorption spectrum of diffuse/translucent gas is now fully determined at rms noise level $\delta_\tau \approx 0.002$ at \EBV\ = 0.32 mag (\AV\ = 1 mag) and $\delta_\tau$/\EBV\ $\approx\ 0.003$ mag$^{-1}$ overall.} %conclusions {As with OH, \hcop\ and \cch, the relative abundance of \cc3h\ varies little between diffuse and dense molecular gas, with N(\cc3h)/N({\it o-c}-\c3h2) $\approx$ 0.1. We find N(CF\p)/N(H$^{13}$CO\p) $\approx 5$, N(CF\p)/N(\cch) $\approx$ 0.005-0.01 and because N(CF\p) increases with \EBV\ and with the column densities of other molecules we infer that fluorine remains in the gas phase as HF well beyond \AV\ = 1 mag. We find N(HCO)/N(H$^{13}$CO\p) = 16 toward \bll, 3C111 and the 40 km/s spiral arm cloud toward W49, implying X(HCO) $\approx 10^{-9}$, about 10 times higher than in dark clouds. The behaviour of HCO is consistent with previous suggestions that it forms from C\p\ and \HH, even when \AV\ is well above 1 mag. The survey can be used to place useful upper limits on some species, for instance N(\hhco)/N(\HH CS) $>$ 32 toward 3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase formation route to \hhco.}Comment: A\%A in pres

Interstellar Hydrides

Interstellar hydrides -- that is, molecules containing a single heavy element atom with one or more hydrogen atoms -- were among the first molecules detected outside the solar system. They lie at the root of interstellar chemistry, being among the first species to form in initially-atomic gas, along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to far-IR and submillimeter observations performed with the Herschel Space Observatory. In this review, we will discuss observations of interstellar hydrides, along with the advanced modeling approaches that have been used to interpret them, and the unique information that has thereby been obtained.Comment: Accepted for publication in Annual Review of Astronomy and Astrophysics 2016, Vol. 5

The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds

The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13C fractionation reactions and HCO+/HOC+ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. The detection of HOC+ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few times ~10(-9) in the core. PAH^- anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] >10(-8)).Comment: 13 pages, 7 figures, 6 tables. Accepted for publication in A&A (english not edited

Discovery of Water Vapor in the High-redshift Quasar APM 08279+5255 at z = 3.91

We report a detection of the excited 2_(20)-2_(11) rotational transition of para-H_2O in APM 08279+5255 using the IRAM Plateau de Bure Interferometer. At z = 3.91, this is the highest-redshift detection of interstellar water to date. From large velocity gradient modeling, we conclude that this transition is predominantly radiatively pumped and on its own does not provide a good estimate of the water abundance. However, additional water transitions are predicted to be detectable in this source, which would lead to an improved excitation model. We also present a sensitive upper limit for the hydrogen fluoride (HF) J = 1-0 absorption toward APM 08279+5255. While the face-on geometry of this source is not favorable for absorption studies, the lack of HF absorption is still puzzling and may be indicative of a lower fluorine abundance at z = 3.91 compared with the Galactic interstellar medium

Simulated CII observations for SPICA/SAFARI

We investigate the case of CII 158 micron observations for SPICA/SAFARI using a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in length. The interplay of thermal instability, magnetic field and self-gravity leads to the formation of cold, dense clumps within a warm, turbulent interclump medium. We sample several clumps along a line of sight through the simulated cube and use them as input density profiles in the Meudon PDR code. This allows us to derive intensity predictions for the CII 158 micron line and provide time estimates for the mapping of a given sky area.Comment: 4 pages, 5 figures, to appear in the proceedings of the workshop "The Space Infrared Telescope for Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies" (July 2009, Oxford, United Kingdom

The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas

Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively easily detected in our Galaxy and in other galaxies. We constrain their chemistry through observations of two positions in the Horsehead edge: the photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We derived column densities and abundances through Bayesian analysis using a large velocity gradient radiative transfer model. We report the first clear detection of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and 6 at the dense core position, and we resolved its hyperfine structure for 3 lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR position. We computed new electron collisional rate coefficients for CH3CN, and we found that including electron excitation reduces the derived column density by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR than in the dense core, HC3N has similar abundance at both positions. The isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of grains are destroyed through photo-desorption or thermal-evaporation in PDRs, and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic

High-velocity hot CO emission close to Sgr A*: Herschel/HIFI submillimeter spectral survey toward Sgr A*

The properties of molecular gas, the fuel that forms stars, inside the cavity of the circumnuclear disk (CND) are not well constrained. We present results of a velocity-resolved submillimeter scan (~480 to 1250 GHz}) and [CII]158um line observations carried out with Herschel/HIFI toward Sgr A*; these results are complemented by a ~2'x2' CO (J=3-2) map taken with the IRAM 30 m telescope at ~7'' resolution. We report the presence of high positive-velocity emission (up to about +300 km/s) detected in the wings of CO J=5-4 to 10-9 lines. This wing component is also seen in H2O (1_{1,0}-1_{0,1}) a tracer of hot molecular gas; in [CII]158um, an unambiguous tracer of UV radiation; but not in [CI]492,806 GHz. This first measurement of the high-velocity CO rotational ladder toward Sgr A* adds more evidence that hot molecular gas exists inside the cavity of the CND, relatively close to the supermassive black hole (< 1 pc). Observed by ALMA, this velocity range appears as a collection of CO (J=3-2) cloudlets lying in a very harsh environment that is pervaded by intense UV radiation fields, shocks, and affected by strong gravitational shears. We constrain the physical conditions of the high positive-velocity CO gas component by comparing with non-LTE excitation and radiative transfer models. We infer T_k~400 K to 2000 K for n_H~(0.2-1.0)x10^5 cm^-3. These results point toward the important role of stellar UV radiation, but we show that radiative heating alone cannot explain the excitation of this ~10-60 M_Sun component of hot molecular gas inside the central cavity. Instead, strongly irradiated shocks are promising candidates.Comment: Accepted for publication in A&A Letters ( this v2 includes corrections by language editor

The Horsehead nebula, a template source for interstellar physics and chemistry

We present a summary of our previous investigations of the physical and chemical structure of the Horsehead nebula, and discuss how these studies led to advances on the understanding of the impact of FUV radiation on the structure of dense interstellar clouds. Specific molecular tracers can be used to isolate different environments, that are more sensitive to changes in the FUV radiation or density than the classical tracers of molecular gas : the CO isotopologues or the dust (sub)millimeter continuum emission. They include the HCO or CCH radicals for the FUV illuminated interfaces, or the molecular ions H$^{13}$CO$^+$, DCO$^+$ and other deuterated species (DNC, DCN) for the cold dense core. We discuss future prospects in the context of Herschel and ALMA