With its relatively low ionization potential, C⁺ can be found throughout the interstellar medium (ISM) and provides one of the main cooling channels of the ISM via the [C ii] 157 μm emission. While the strength of the [C ii] line correlates with the star formation rate, the contributions of the various gas phases to the [C ii] emission on galactic scales are not well established. In this study we establish an empirical multi-component model of the ISM, including dense H ii regions, dense photon dissociation regions (PDRs), the warm ionized medium (WIM), low density and surfaces of molecular clouds (SfMCs), and the cold neutral medium (CNM). We test our model on ten luminous regions within the two nearby galaxies NGC 3184 and NGC 628 on angular scales of 500-600 pc. Both galaxies are part of the Herschel key program KINGFISH, and are complemented by a large set of ancillary ground- and space-based data. The five modeled phases together reproduce the observed [C ii] emission quite well, overpredicting the total flux slightly (about 45%) averaged over all regions. We find that dense PDRs are the dominating component, contributing 68% of the [C ii] flux on average, followed by the WIM and the SfMCs, with mean contributions of about half of the contribution from dense PDRs, each. CNM and dense H ii regions are only minor contributors with less than 5% each. These estimates are averaged over the selected regions, but the relative contributions of the various phases to the [C ii] flux vary significantly between these regions.

@en

The level of porosity of interstellar ices, largely comprised of amorphous solid water (ASW), contains clues about the trapping capacity of other volatile species and determines the surface accessibility that is needed for solid-state reactions to take place. Aims. Our goal is to simulate the growth of amorphous water ice at low temperature (10 K) and to characterise the evolution of the porosity (and the specific surface area) as a function of temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are used to mimic the formation and the thermal evolution of pores in amorphous water ice. We follow the accretion of gas-phase water molecules as well as their migration on surfaces with different grid sizes, both at the top growing layer and within the bulk. Results. We show that the porosity characteristics change substantially in water ice as the temperature increases. The total surface of the pores decreases to a great extend while the total volume decreases only slightly for higher temperatures. This will decrease the overall reaction efficiency, but in parallel, small pores connect and merge, which allows trapped molecules to meet and react within the pores network and provides a pathway to increase the reaction efficiency. We introduce pore coalescence as a new solid-state process that may boost the solid-state formation of new molecules in space, and which has not been considered so far.

@en

Aims. This article aims to provide an alternative method of measuring the porosity of multi-phase composite ices from their refractive indices and of characterising how the abundance of a premixed contaminant (e.g., CO ₂) affects the porosity of water-rich ice mixtures during omni-directional deposition. Methods. We combine optical laser interference and extended effective medium approximations (EMAs) to measure the porosity of three astrophysically relevant ice mixtures: H₂O:CO₂ = 10:1, 4:1, and 2:1. Infrared spectroscopy is used as a benchmarking test of this new laboratory-based method. Results. By independently monitoring the O-H dangling modes of the different water-rich ice mixtures, we confirm the porosities predicted by the extended EMAs. We also demonstrate that CO₂ premixed with water in the gas phase does not significantly affect the ice morphology during omni-directional deposition, as long as the physical conditions favourable to segregation are not reached. We propose a mechanism in which CO₂ molecules diffuse on the surface of the growing ice sample prior to being incorporated into the bulk and then fill the pores partly or completely, depending on the relative abundance and the growth temperature.

@en

We present IRAM 30 m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli et al. (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance x_in ^HDO = 1 × 10^-7 and an outer abundance x_out ^HDO ≤ 1 × 10^-9 (3σ). This result is in favor of HDO enhancement due to ice evaporation from the grains in the inner envelope. The deuteration ratio HDO/H₂O is found to be f_in = 3% and f_out ≤ 0.2% (3σ) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H₂O ices consistently derive a H₂O ice abundance of several 10^-5 to 10^-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

@en

We present the first results of a systematic (sub)millimeter unbiased spectral line survey performed with the IRAM-30m and JCMT radiotele-scopes towards the low-mass protostar IRAS 16293-2422 (IRAS16293). When completed this survey will cover most of the frequency band attainable with the 30m (80-280 GHz) and with the JCMT in the 350 GHz band (335-365 GHz), i.e. 201 GHz.

@en

Molecular hydrogen is the most abundant molecule in the Universe and dominates the mass budget of the gas, particularly in regions of star formation. H2 is also an important chemical intermediate in the formation of larger species and can be an important gas coolant when the medium lacks metals. Because of the inefficiency of gas-phase reactions to form H2, this molecule is generally thought to form on grain surfaces. Observations of H 2 in a wide variety of objects showed that this molecule could form efficiently over a wide range of physical conditions. To understand the mechanism responsible for such an efficient formation, we developed a model for molecular hydrogen formation on grain surfaces. This model considers the interaction between atom and surface as beeing either weak (Van der Waals interaction - physisorption) or strong (covalent bound - chemisorption), as well as the mobility of the atom on a surface due to quantum mechanical diffusion and thermal hopping. This model solves the time-dependent kinetic rate equation for the formation of molecular hydrogen and its deuterated forms. Our results have been benchmarked with laboratory experiments on silicates, carbonaceous and graphitic surfaces. This comparison allowed us to derive some characteristics of the considered surfaces. An extension of our model to astrophysical conditions gives an estimate of H2 formation efficiency for a wide range of physical conditions. One of our main results is the efficient formation of molecular hydrogen for gas and grain temperatures up to several hundreds of kelvins. We also compared our predictions to observations in astrophysical objects such as photodissociation regions (PDRs). The addition of deuterium in our model for the formation of HD and D2 molecules is also discussed.@en

The most abundant interstellar molecule, H₂, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H₂ molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (∼500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures (T ≤ 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

@en

We report the detection of complex molecules (HCOOCH₃, HCOOH, and CH₃CN), signposts of a hot core-like region, toward the low-mass Class 0 source NGC 1333 IRAS 4A. This is the second low-mass protostar in which such complex molecules have been searched for and reported, the other source being IRAS 16293-2422. It is therefore likely that compact (a few tens of AU) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles and where complex molecules are found, are common in low-mass Class 0 sources. Given that the chemical formation timescale is much shorter than the gas hot-core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first-generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second-generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar-type protostars, suggesting perhaps a common origin.

@en

Complex organic molecules have previously been discovered in solar-type protostars, raising the questions of where and how they form in the envelope. Possible formation mechanisms include grain mantle evaporation, the interaction of the outflow with its surroundings, and/or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH₃CN and HCOOCH₃ toward the solar-type protostar IRAS 16293-2422. The images show that the emission originates from two compact regions centered on the two components of the binary system. We discuss how these results favor the grain mantle evaporation scenario, and we investigate the implications of these observations for the chemical composition and physical and dynamical state of the two components.

@en

While warm dense gas is prevalent around low-mass protostars, the presence of complex saturated molecules - the chemical inventory characteristic of hot cores - has remained elusive in such environments. Here we report the results of an IRAM 30 m study of the molecular composition associated with the low-mass protostar IRAS 16293-2422. Our observations highlight an extremely rich organic inventory in this source with abundant amounts of complex O- and N-bearing molecules such as formic acid, HCOOH, acetaldehyde, CH₃CHO, methyl formate, CH₃OCHO, dimethyl ether, CH₃OCH₃, acetic acid, CH₃COOH, methyl cyanide, CH₃CN, ethyl cyanide, C₂H₅CN, and propyne, CH₃CCH. We compare the composition of the hot core around this low-mass young stellar object with those around massive protostars and address the chemical processes involved in molecular complexity in regions of star formation.

@en

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H₂ formation on silicate surfaces. The results show that H₂ formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H₂ formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H ₂ formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H₂ formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

@en