Molecule survival in magnetized protostellar disk winds. II. Predicted H2O line profiles versus Herschel/HIFI observations

We investigate whether the broad wings of H2O emission identified with Herschel towards low-mass Class 0 and Class 1 protostars may be consistent with an origin in a dusty MHD disk wind, and the constraints it would set on the underlying disk properties. We present synthetic H2O line profiles predictions for a typical MHD disk wind solution with various values of disk accretion rate, stellar mass, extension of the launching area, and view angle. We compare them in terms of line shapes and intensities with the HIFI profiles observed by the WISH Key Program. We find that a dusty MHD disk wind launched from 0.2--0.6 AU AU to 3--25 AU can reproduce to a remarkable degree the observed shapes and intensities of the broad H2O component, both in the fundamental 557 GHz line and in more excited lines. Such a model also readily reproduces the observed correlation of 557 GHz line luminosity with envelope density, if the infall rate at 1000 AU is 1--3 times the disk accretion rate in the wind ejection region. It is also compatible with the typical disk size and bolometric luminosity in the observed targets. However, the narrower line profiles in Class 1 sources suggest that MHD disk winds in these sources, if present, would have to be slower and/or less water rich than in Class 0 sources. In conclusion, MHD disk winds appear as a valid (though not unique) option to consider for the origin of the broad H2O component in low-mass protostars. ALMA appears ideally suited to further test this model by searching for resolved signatures of the warm and slow wide-angle molecular wind that would be predicted.Comment: accepted for publication in A&

H2 formation and excitation in the Stephan's Quintet galaxy-wide collision

Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg s-1) mid-infrared H2 emission towards the galaxy-wide collision in the Stephan's Quintet (SQ) galaxy group. This discovery was followed by the detection of more distant H2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations set molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the collision velocity. These observations raise three questions we propose to answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume filling intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is smaller in clouds. We show that gas heated to temperatures less than 10^6 K cools, keeps its dust content and becomes H2 within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic energy of the H2 gas is the dominant energy reservoir, we consider that the H2 emission is powered by the dissipation of kinetic turbulent energy. (Abridged)Comment: 19 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics Minor editing and typo

ISOCAM spectro-imaging of the H2 rotational lines in the supernova remnant IC443

We report spectro-imaging observations of the bright western ridge of the supernova remnant IC 443 obtained with the ISOCAM circular variable filter (CVF) on board the Infrared Space Observatory (ISO). This ridge corresponds to a location where the interaction between the blast wave of the supernova and ambient molecular gas is amongst the strongest. The CVF data show that the 5 to 14 micron spectrum is dominated by the pure rotational lines of molecular hydrogen (v = 0--0, S(2) to S(8) transitions). At all positions along the ridge, the H2 rotational lines are very strong with typical line fluxes of 10^{-4} to 10^{-3} erg/sec/cm2/sr. We compare the data to a new time-dependent shock model; the rotational line fluxes in IC 443 are reproduced within factors of 2 for evolutionary times between 1,000 and 2,000 years with a shock velocity of 30 km/sec and a pre-shock density of 10^4 /cm3.Comment: To appear in Astronomy and Astrophysic

Shock excitation of H2_2 in the James Webb Space Telescope era

(Abridged) H2 is the most abundant molecule in the Universe. Thanks to its widely spaced energy levels, it predominantly lights up in warm gas, T > 100 K, such as shocked regions, and it is one of the key targets of JWST observations. These include shocks from protostellar outflows, all the way up to starburst galaxies and AGN. Shock models are able to simulate H2 emission. We aim to explore H2 excitation using such models, and to test over which parameter space distinct signatures are produced in H2 emission. We present simulated H2 emission using the Paris-Durham shock code over an extensive grid of 14,000 plane-parallel stationary shock models, a large subset of which are exposed to an external UV radiation field. The grid samples 6 input parameters: preshock density, shock velocity, transverse magnetic field strength, UV radiation field strength, cosmic-ray-ionization rate, and PAH abundance. Physical quantities, such as temperature, density, and width, have been extracted along with H2 integrated line intensities. The strength of the transverse magnetic field, set by the scaling factor, b, plays a key role in the excitation of H2. At low values of b (<~ 0.3, J-type shocks), H2 excitation is dominated by vibrationally excited lines; at higher values (b >~ 1, C-type shocks), rotational lines dominate the spectrum for shocks with an external radiation field comparable to (or lower than) the solar neighborhood. Shocks with b >= 1 can be spatially resolved with JWST for nearby objects. When the input kinetic energy flux increases, the excitation and integrated intensity of H2 increases similarly. An external UV field mainly serves to increase the excitation, particularly for shocks where the input radiation energy is comparable to the input kinetic energy flux. These results provide an overview of the energetic reprocessing of input kinetic energy flux and the resulting H2 line emission.Comment: Published in A&

Nitrogen superfractionation in dense cloud cores

We report new calculations of interstellar 15N fractionation. Previously, we have shown that large enhancements of 15N/14N can occur in cold, dense gas where CO is frozen out, but that the existence of an NH + N channel in the dissociative recombination of N2H+ severely curtails the fractionation. In the light of recent experimental evidence that this channel is in fact negligible, we have reassessed the 15N chemistry in dense cloud cores. We consider the effects of temperatures below 10 K, and of the presence of large amounts of atomic nitrogen. We also show how the temporal evolution of gas-phase isotope ratios is preserved as spatial heterogeneity in ammonia ice mantles, as monolayers deposited at different times have different isotopic compositions. We demonstrate that the upper layers of this ice may have 15N/14N ratios an order of magnitude larger than the underlying elemental value. Converting our ratios to delta-values, we obtain delta(15N) > 3,000 per mil in the uppermost layer, with values as high as 10,000 per mil in some models. We suggest that this material is the precursor to the 15N `hotspots' recently discovered in meteorites and IDPsComment: accepted by MNRA

Energetics of the molecular gas in the H_2 luminous radio galaxy 3C 326: Evidence for negative AGN feedback

We present a detailed analysis of the gas conditions in the H_2 luminous radio galaxy 3C 326 N at z ~ 0.1, which has a low star-formation rate (SFR ~ 0.07 M_⊙ yr^(−1)) in spite of a gas surface density similar to those in starburst galaxies. Its star-formation efficiency is likely a factor ~ 10−50 lower than those of ordinary star-forming galaxies. Combining new IRAM CO emission-line interferometry with existing Spitzer mid-infrared spectroscopy, we find that the luminosity ratio of CO and pure rotational H_2 line emission is factors 10−100 lower than what is usually found. This suggests that most of the molecular gas is warm. The Na D absorption-line profile of 3C 326 N in the optical suggests an outflow with a terminal velocity of ~−1800 km s^(−1) and a mass outflow rate of 30−40 M_⊙ yr^(−1), which cannot be explained by star formation. The mechanical power implied by the wind, of order 10^(43) erg s^(−1), is comparable to the bolometric luminosity of the emission lines of ionized and molecular gas. To explain these observations, we propose a scenario where a small fraction of the mechanical energy of the radio jet is deposited in the interstellar medium of 3C 326 N, which powers the outflow, and the line emission through a mass, momentum and energy exchange between the different gas phases of the ISM. Dissipation times are of order 10^(7−8) yrs, similar or greater than the typical jet lifetime. Small ratios of CO and PAH surface brightnesses in another 7 H_2 luminous radio galaxies suggest that a similar form of AGN feedback could be lowering star-formation efficiencies in these galaxies in a similar way. The local demographics of radio-loud AGN suggests that secular gas cooling in massive early-type galaxies of ≥ 10^(11) M_⊙ could generally be regulated through a fundamentally similar form of “maintenance-phase” AGN feedback

Atomic Carbon Emission from Individual Molecular Clouds in M33

We present observations of the 492 GHz [CI] emission for four individual giant molecular clouds in the Local Group galaxy M33 obtained with the James Clerk Maxwell Telescope. The average [CI] to CO J=1-0 integrated intensity ratio of 0.10+/-0.03 is similar to what is observed in Galactic molecular clouds but smaller than what is seen in starburst galaxies. Similarly, the column density ratio N(C)/N(CO) is similar to that observed in the Orion Bar, but smaller than values obtained for starburst galaxies. The [CI] line is found to be a more important coolant than the lowest three rotational transitions of CO for all the clouds in the sample. The [CI] luminosity does not appear to be enhanced significantly in two low-metallicity clouds, which may be due to the unusual ionization environment of the clouds.Comment: 12 pages, aastex, 1 postscript figure; accepted for publication in ApJ Letters; also available at http://www.physics.mcmaster.ca/Wilson_Preprints

Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds

Aim: To determine whether or not gas-phase chemical models with homogeneous and time-independent physical conditions explain the many observed molecular abundances in astrophysical sources, it is crucial to estimate the uncertainties in the calculated abundances and compare them with the observed abundances and their uncertainties. Non linear amplification of the error and bifurcation may limit the applicability of chemical models. Here we study such effects on dense cloud chemistry. Method: Using a previously studied approach to uncertainties based on the representation of rate coefficient errors as log normal distributions, we attempted to apply our approach using as input a variety of different elemental abundances from those studied previously. In this approach, all rate coefficients are varied randomly within their log normal (Gaussian) distribution, and the time-dependent chemistry calculated anew many times so as to obtain good statistics for the uncertainties in the calculated abundances. Results: Starting with so-called ``high-metal'' elemental abundances, we found bimodal rather than Gaussian like distributions for the abundances of many species and traced these strange distributions to an extreme sensitivity of the system to changes in the ratio of the cosmic ray ionization rate zeta\_He for He and that for molecular hydrogen zeta\_H2. The sensitivity can be so extreme as to cause a region of bistability, which was subsequently found to be more extensive for another choice of elemental abundances. To the best of our knowledge, the bistable solutions found in this way are the same as found previously by other authors, but it is best to think of the ratio zeta\_He/zeta\_H2 as a control parameter perpendicular to the ''standard'' control parameter zeta/n\_H.Comment: Accepted for publicatio

Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons

With the high-resolution spectroscopy now available in the optical and satellite UV, it is possible to determine the neutral/ionized column density ratios for several different elements in a single cloud. Assuming ionization equilibrium for each element, one can make several independent determinations of the electron density. For the clouds for which such an analysis has been carried out, these different estimates disagree by large factors, suggesting that some process (or processes) besides photoionization and radiative recombination might play an important role in the ionization balance. One candidate process is collisions of ions with dust grains. Making use of recent work quantifying the abundances of polycyclic aromatic hydrocarbon molecules and other grains in the interstellar medium, as well as recent models for grain charging, we estimate the grain-assisted ion recombination rates for several astrophysically important elements. We find that these rates are comparable to the rates for radiative recombination for conditions typical of the cold neutral medium. Including grain-assisted ion recombination in the ionization equilibrium analysis leads to increased consistency in the various electron density estimates for the gas along the line of sight to 23 Orionis. However, not all of the discrepancies can be eliminated in this way; we speculate on some other processes that might play a role. We also note that grain-assisted recombination of H+ and He+ leads to significantly lower electron fractions than usually assumed for the cold neutral medium.Comment: LaTeX(12 pages, 8 figures, uses emulateapj5.sty, apjfonts.sty); submitted to ApJ; corrected typo