Evolution Of Molecular Clouds Research Articles

Multiscale accretion in dense cloud cores and the delayed formation of massive stars

ABSTRACT The formation mechanism of massive stars remains one of the main open problems in astrophysics, in particular the relationship between the mass of the most massive stars and that of the cores in which they form. Numerical simulations of the formation and evolution of molecular clouds, within which dense cores and stars form self-consistently, show that the core mass increases in time, and also that the most massive stars tend to appear later than lower mass stars. We present an idealized model that incorporates accretion onto the cores as well as onto the stars, in which the core mass growth is regulated by a ‘gravitational choking’ mechanism that does not involve any form of support. This process is of purely gravitational origin, and causes some of the mass accreted onto cores to stagnate there, rather than being transferred to the stars. In addition, we estimate the mass of the most massive allowed star before its photoionizing radiation is capable of overcoming the accretion flow. This model constitutes a proof of concept for the simultaneous growth of the gas reservoir and the stellar mass, the delay in the formation of massive stars observed in numerical simulations, the need for massive, dense cores in order to form massive stars, and the observed correlation between the mass of the most massive star and the mass of the cluster it resides in. Also, our model implies that by the time massive stars begin to form in a core, a number of low-mass stars are expected to have already formed.

CO Mapping of Cygnus-X—Volume Density Distribution

We present CO(2-1) and 13CO(2-1) maps of the Cygnus-X molecular cloud complex using the 10 m Heinrich Hertz Submillimeter Telescope. The maps cover the southern portion of the complex, which is strongly impacted by the feedback from the Cygnus OB2 association. Combining CO(1-0) and 13CO(1-0) maps from the Nobeyama 45 m Cygnus-X CO Survey, we carry out a multitransition molecular line analysis with RADEX and derive the volume density of velocity-coherent gas components. We select those components with a column density in the power-law tail part of the column density probability distribution function (N-PDF) and assemble their volume density into a volume density PDF (ρ-PDF). The ρ-PDF exhibits a power-law shape in the range of 104.5 cm−3 105.5 cm−3 with a fitted slope of α = −1.12 ± 0.05. The slope is shallower than what is predicted by simulations of rotationally supported structures or those undergoing gravitational collapse. Applying the same analysis to synthetic observations with feedback may help identify the cause of the shallow slope. The ρ-PDF provides another useful benchmark for testing models of molecular cloud formation and evolution.

Cloud–cloud collisions triggering star formation in galaxy simulations

ABSTRACT Cloud–cloud collisions (CCCs) are expected to compress gas and trigger star formation. However, it is not well understood how the collisions and the induced star formation affect galactic-scale properties. By developing an on-the-fly algorithm to identify CCCs at each time-step in a galaxy simulation and a model that relates CCC-triggered star formation to collision speeds, we perform simulations of isolated galaxies to study the evolution of galaxies and giant molecular clouds (GMCs) with prescriptions of self-consistent CCC-driven star formation and stellar feedback. We find that the simulation with the CCC-triggered star formation produces slightly higher star formation rates and a steeper Kennicutt–Schmidt relation than that with a more standard star formation recipe, although collision speeds and frequencies are insensitive to the star formation models. In the simulation with the CCC model, about $70{{\ \rm per\ cent}}$ of the stars are born via CCCs, and colliding GMCs with masses of $\approx 10^{5.5}\, \mbox{$\rm M_{\odot}$}$ are the main drivers of CCC-driven star formation. In the simulation with the standard star formation recipe, about 50 per cent of stars are born in colliding GMCs even without the CCC-triggered star formation model. These results suggest that CCCs may be one of the most important star formation processes in galaxy evolution. Furthermore, we find that a post-processing analysis of CCCs, as used in previous studies in galaxy simulations, may lead to slightly greater collision speeds and significantly lower collision frequencies than the on-the-fly analysis.

Clouds of Theseus: long-lived molecular clouds are composed of short-lived H2 molecules

ABSTRACT We use passive gas tracer particles in an Arepo simulation of a dwarf spiral galaxy to relate the Lagrangian evolution of star-forming gas parcels and their H2 molecules to the evolution of their host giant molecular clouds. We find that the median chemical lifetime of H2 is 4 Myr, with an interquartile range between 2 and 9 Myr. This chemical lifetime is independent of the lifetime of the host molecular cloud, which may extend up to 90 Myr, with around 50 per cent of star formation occurring in longer lived clouds (>25 Myr). The rapid ejection of gas from around young massive stars by early stellar feedback is responsible for the short H2 survival time, driving down the density of the surrounding gas, so that its H2 molecules are dissociated by the interstellar radiation field. This ejection of gas from the H2-dominated state is balanced by the constant accretion of new gas from the galactic environment, constituting a ‘competition model’ for molecular cloud evolution. Gas ejection occurs at a rate that is proportional to the molecular cloud mass, so that the cloud lifetime is determined by the accretion rate, which may be as high as 4 × 104 M⊙ Myr−1 in the longest lived clouds. Our findings therefore resolve the conflict between observations of rapid gas ejection around young massive stars and observations of long-lived molecular clouds in galaxies. We show that the fastest-accreting, longest lived, highest mass clouds drive supernova clustering on sub-cloud scales, which in turn is a key driver of galactic outflows.

2a Results: galaxy to cloud scales

Simulations from the scales of isolated galaxies to clouds have been instrumental in informing us about molecular cloud formation and evolution. Simulations are able to investigate the roles of gravity, feedback, turbulence, heating and cooling, and magnetic fields on the physics of the interstellar medium, and star formation. Compared to simulations of individual clouds, galactic and sub-galactic scale simulations can include larger galactic scale processes such as spiral arms, bars, and larger supernovae bubbles, which may influence star formation. Simulations show cloud properties and lifetimes in broad agreement with observations. Gravity and spiral arms are required to produce more massive GMCs, whilst stellar feedback, likely photoionisation, leads to relatively short cloud lifetimes. On larger scales, supernovae may be more dominant in driving the structure and dynamics, but photoionisation may still have a role. In terms of the dynamics, feedback is probably the main driver of velocity dispersions, but large scale processes such as gravity and spiral arms may also be significant. Magnetic fields are generally found to decrease star formation on galaxy or cloud scales, and simulations are ongoing to study whether clouds are sub or supercritical on different scales in galaxy scale simulations. Simulations on subgalactic scales, or zoom in simulations, allow better resolution of feedback processes, filamentary structure within clouds, and the study of stellar clusters.

Predicting the Radiation Field of Molecular Clouds Using Denoising Diffusion Probabilistic Models

Accurately quantifying the impact of radiation feedback in star formation is challenging. To address this complex problem, we employ deep-learning techniques known as denoising diffusion probabilistic models (DDPMs) to predict the interstellar radiation field (ISRF) strength based on three-band dust emission at 4.5, 24, and 250 μm. We adopt magnetohydrodynamic simulations from the STARFORGE project that model star formation and giant molecular cloud (GMC) evolution. We generate synthetic dust emission maps matching observed spectral energy distributions in the Monoceros R2 (MonR2) GMC. We train DDPMs to estimate the ISRF using synthetic three-band dust emission. The dispersion between the predictions and true values is within a factor of 0.1 for the test set. We extended our assessment of the diffusion model to include new simulations with varying physical parameters. While there is a consistent offset observed in these out-of-distribution simulations, the model effectively constrains the relative intensity to within a factor of 2. Meanwhile, our analysis reveals a weak correlation between the ISRF solely derived from dust temperature and the actual ISRF. We apply our trained model to predict the ISRF in MonR2, revealing a correspondence between intense ISRF, bright sources, and high dust emission, confirming the model’s ability to capture ISRF variations. Our model robustly predicts radiation feedback distribution, even in complex, poorly constrained ISRF environments like those influenced by nearby star clusters. However, precise ISRF predictions require an accurate training data set mirroring the target molecular cloud’s unique physical conditions.

A Theoretical Approach to the Complex Chemical Evolution of Phosphorus in the Interstellar Medium

The study of phosphorus chemistry in the interstellar medium has become a topic of growing interest in astrobiology because it is plausible that a wide range of P-bearing molecules were introduced in the early Earth by the impact of asteroids and comets on its surface, enriching prebiotic chemistry. Thanks to extensive searches in recent years, it has become clear that P mainly appears in the form of PO and PN in molecular clouds and star-forming regions. Interestingly, PO is systematically more abundant than PN by factors typically of ∼1.4–3, independently of the physical properties of the observed source. In order to unveil the formation routes of PO and PN, in this work we introduce a mathematical model for the time evolution of the chemistry of P in an interstellar molecular cloud and analyze its associated chemical network as a complex dynamical system. By making reasonable assumptions, we reduce the network to obtain explicit mathematical expressions that describe the abundance evolution of P-bearing species and study the dependences of the abundance of PO and PN on the system’s kinetic parameters with much faster computation times than available numerical methods. As a result, our model reveals that the formation of PO and PN is governed by just a few critical reactions, and fully explains the relationship between PO and PN abundances throughout the evolution of molecular clouds. Finally, the application of Bayesian methods constrains the real values of the most influential reaction rate coefficients making use of available observational data.

The impact of H II regions on giant molecular cloud properties in nearby galaxies sampled by PHANGS ALMA and MUSE

Context. The final stages of molecular cloud evolution involve cloud disruption due to feedback by massive stars, with recent literature suggesting the importance of early (i.e., pre-supernova) feedback mechanisms. Aims. We aim to determine whether feedback from massive stars in H II regions has a measurable impact on the physical properties of molecular clouds at a characteristic scale of ~ 100 pc, and whether the imprint of feedback on the molecular gas depends on the local galactic environment. Methods. We identified giant molecular clouds (GMCs) associated with H II regions for a sample of 19 nearby galaxies from catalogs of GMCs and H II regions released by the PHANGS-ALMA and PHANGS-MUSE surveys, using the overlap of the CO and Hα emission as the key criterion for physical association. We compared the distributions of GMC and H II region properties for paired and non-paired objects. We investigated correlations between GMC and H II region properties among galaxies and across different galactic environments to determine whether GMCs that are associated with H II regions have significantly distinct physical properties compared to the parent GMC population. Results. We identify trends between the Hα luminosity of an H II region and the CO peak brightness and molecular mass of GMCs that we tentatively attribute to a direct physical connection between the matched objects, and which arise independently of the underlying environmental variations of GMC and H II region properties within galaxies. The study of the full sample nevertheless hides a large galaxy-to-galaxy variability. Conclusions. At the ~100 pc scales accessed by the PHANGS-ALMA and PHANGS-MUSE data, pre-supernova feedback mechanisms in H II regions have a subtle but measurable impact on the properties of the surrounding molecular gas, as inferred from CO observations.

The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

Massive stars can determine the evolution of molecular clouds by eroding and photo-evaporating their surfaces with strong ultraviolet (UV) radiation fields. Moreover, UV radiation is relevant in setting the thermal gas pressure in star-forming clouds, whose influence can extend across various spatial scales, from the rims of molecular clouds to entire star-forming galaxies. Probing the fundamental structure of nearby molecular clouds is therefore crucial to understand how massive stars shape their surrounding medium and how fast molecular clouds are destroyed, specifically at their UV-illuminated edges, where models predict an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized gas whose size is directly related to the exposed physical conditions. We present the highest angular resolution (~0.″5, corresponding to 207 au) and velocity-resolved images of the molecular gas emission in the Horsehead nebula, using CO J = 3–2 and HCO+ J = 4−3 observations with the Atacama Large Millimeter/submillimeter Array (ALMA). We find that CO and HCO+ are present at the edge of the cloud, very close to the ionization (H+/H) and dissociation fronts (H/H2), suggesting a very thin layer of neutral atomic gas (<650 au) and a small amount of CO-dark gas (AV = 0.006–0.26 mag) for stellar UV illumination conditions typical of molecular clouds in the Milky Way. The new ALMA observations reveal a web of molecular gas filaments with an estimated thermal gas pressure of Pth = (2.3 – 4.0) × 106 K cm−3, and the presence of a steep density gradient at the cloud edge that can be well explained by stationary isobaric photo-dissociation region (PDR) models with pressures consistent with our estimations. However, in the H II region and PDR interface, we find Pth,PDR > Pth,H II suggesting the gas is slightly compressed. Therefore, dynamical effects cannot be completely ruled out and even higher angular observations will be needed to unveil their role.

Astrochemical models of interstellar ices: History matters

Context. Ice is ubiquitous in the interstellar medium. As soon as it becomes slightly opaque in the visible, it can be seen for visual extinctions (AV) above ~1.5. The James Webb Space Telescope (JWST) will observe the ice composition toward hundreds of lines of sight, covering a broad range of physical conditions in these extinct regions. Aims. We model the formation of the main constituents of interstellar ices, including H2O, CO2, CO, and CH3OH. We strive to understand what physical or chemical parameters influence the final composition of the ice and how they benchmark to what has already been observed, with the aim of applying these models to the preparation and analysis of JWST observations. Methods. We used the Nautilus gas-grain model, which computes the gas and ice composition as a function of time for a set of physical conditions, starting from an initial gas phase composition. All important processes (gas-phase reactions, gas-grain interactions, and grain surface processes) are included and solved with the rate equation approximation. Results. We first ran an astrochemical code for fixed conditions of temperature and density mapped in the cold core L429-C to benchmark the chemistry. One key parameter was revealed to be the dust temperature. When the dust temperature is higher than 12 K, CO2 will form efficiently at the expense of H2O, while at temperatures below 12 K, it will not form. Whatever hypothesis we assumed for the chemistry (within realistic conditions), the static simulations failed to reproduce the observed trends of interstellar ices in our target core. In a second step, we simulated the chemical evolution of parcels of gas undergoing different physical and chemical situations throughout the molecular cloud evolution and starting a few 107 yr prior to the core formation (dynamical simulations). We obtained a large sample of possible ice compositions. The ratio of the different ice components seems to be approximately constant for AV > 5, and in good agreement with the observations. Interestingly, we find that grain temperature and low AV conditions significantly affect the production of ice, especially for CO2, which shows the highest variability. Conclusions. Our dynamical simulations satisfactorily reproduce the main trends already observed for interstellar ices. Moreover, we predict that the apparent constant ratio of CO2/H2O observed to date is probably not true for regions of low AV, and that the history of the evolution of clouds plays an essential role, even prior to their formation.

Three-dimensional magnetic fields of molecular clouds

To investigate the role of magnetic fields in the evolution of the interstellar medium, formation and evolution of molecular clouds, and ultimately the formation of stars, their three-dimensional (3D) magnetic fields must be probed. Observing only one component of magnetic fields (along the line of sight or parallel to the plane of the sky) is insufficient to identify these 3D vectors. In recent years, novel techniques for probing each of these two components and integrating them with additional data (from observations or models), such as Galactic magnetic fields or magnetic field inclination angles, have been developed, in order to infer 3D magnetic fields. We review and discuss these advancements, their applications, and their future direction.

Molecular Cloud Populations in the Context of Their Host Galaxy Environments: A Multiwavelength Perspective

We present a rich, multiwavelength, multiscale database built around the PHANGS–ALMA CO (2 − 1) survey and ancillary data. We use this database to present the distributions of molecular cloud populations and subgalactic environments in 80 PHANGS galaxies, to characterize the relationship between population-averaged cloud properties and host galaxy properties, and to assess key timescales relevant to molecular cloud evolution and star formation. We show that PHANGS probes a wide range of kpc-scale gas, stellar, and star formation rate (SFR) surface densities, as well as orbital velocities and shear. The population-averaged cloud properties in each aperture correlate strongly with both local environmental properties and host galaxy global properties. Leveraging a variable selection analysis, we find that the kpc-scale surface densities of molecular gas and SFR tend to possess the most predictive power for the population-averaged cloud properties. Once their variations are controlled for, galaxy global properties contain little additional information, which implies that the apparent galaxy-to-galaxy variations in cloud populations are likely mediated by kpc-scale environmental conditions. We further estimate a suite of important timescales from our multiwavelength measurements. The cloud-scale freefall time and turbulence crossing time are ∼5–20 Myr, comparable to previous cloud lifetime estimates. The timescales for orbital motion, shearing, and cloud–cloud collisions are longer, ∼100 Myr. The molecular gas depletion time is 1–3 Gyr and shows weak to no correlations with the other timescales in our data. We publish our measurements online, and expect them to have broad utility to future studies of molecular clouds and star formation.

Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size–Linewidth Relationship

Stars form within molecular clouds, so characterizing the physical states of molecular clouds is key to understanding the process of star formation. Cloud structure and stability are frequently assessed using metrics including the virial parameter and Larson scaling relationships between cloud radius, velocity dispersion, and surface density. Departures from the typical Galactic relationships between these quantities have been observed in low-metallicity environments. The amount of H2 gas in cloud envelopes without corresponding CO emission is expected to be high under these conditions; therefore, this CO-dark gas could plausibly be responsible for the observed variations in cloud properties. We derive simple corrections that can be applied to empirical clump properties (mass, radius, velocity dispersion, surface density, and virial parameter) to account for CO-dark gas in clumps following power-law and Plummer mass density profiles. We find that CO-dark gas is not likely to be the cause of departures from Larson’s relationships in low-metallicity regions, but that virial parameters may be systematically overestimated. We demonstrate that correcting for CO-dark gas is critical for accurately comparing the dynamical state and evolution of molecular clouds across diverse environments.

Magnetic Field of Molecular Gas Measured with the Velocity Gradient Technique I. Orion A

Magnetic fields play an important role in the evolution of molecular clouds and star formation. Using the velocity gradient technique (VGT) model, we measured the magnetic field in Orion A using the 12CO, 13CO, and C18O(1-0) emission lines at a scale of ∼0.07 pc. The measured B field shows an east–west orientation that is perpendicular to the integral shaped filament of Orion A at large scale. The VGT magnetic fields obtained from 13CO and C18O are in agreement with the B field that is measured from the Planck 353 GHz dust polarization at a scale of ∼0.55 pc. Removal of density effects by using a velocity decomposition algorithm can significantly improve the accuracy of the VGT in tracing magnetic fields with the 12CO(1-0) line. The magnetic field strengths of seven subclouds, OMC-1, OMC-2, OMC-3, OMC-4, OMC-5, L 1641-N, and NGC 1999, have also been estimated with the Davis–Chandrasekhar–Fermi and the Two Mach Numbers technique, and these are found to be in agreement with previous results obtained from dust polarization at far-infrared and submillimeter wavelengths. At smaller scales, the VGT prove a good method to measure magnetic fields.

Penetration of Nonenergetic Hydrogen Atoms into Amorphous Solid Water and their Reaction with Embedded Benzene and Naphthalene

Chemical processes on the surface of icy grains play an important role in the chemical evolution in molecular clouds. In particular, reactions involving nonenergetic hydrogen atoms accreted from the gaseous phase have been extensively studied. These reactions are believed to effectively proceed only on the surface of the icy grains; thus, molecules embedded in the ice mantle are not considered to react with hydrogen atoms. Recently, Tsuge et al. suggested that nonenergetic hydrogen atoms can react with CO molecules even in ice mantles via diffusive hydrogenation. This investigation was extended to benzene and naphthalene molecules embedded in amorphous solid water (ASW) in the present study, which revealed that a portion of these molecules could be fully hydrogenated in astrophysical environments. The penetration depths of nonenergetic hydrogen atoms into porous and nonporous ASW were determined using benzene molecules to be >50 and ∼10 monolayers, respectively (1 monolayer ≈ 0.3 nm).

Density exponent analysis: gravity-driven steepening of the density profiles of star-forming regions

ABSTRACT The evolution of molecular interstellar clouds is a complex, multiscale process. The power-law density exponent describes the steepness of density profiles, and it has been used to characterize the density structures of the clouds; yet its usage is usually limited to spherically symmetric systems. Importing the Level-Set Method, we develop a new formalism that generates robust maps of a generalized density exponent kρ at every location for complex density distributions. By applying it to high fidelity, high dynamical range map of the Perseus molecular cloud constructed using data from the Herschel and Planck satellites, we find that the density exponent exhibits a surprisingly wide range of variation (−3.5 ≲ kρ ≲ −0.5). Regions at later stages of gravitational collapse are associated with steeper density profiles. Inside a region, gas located in the vicinities of dense structures has very steep density profiles with kρ ≈ −3, which forms because of depletion. This density exponent analysis reveals diverse density structures, forming a coherent picture that gravitational collapse leads to a continued steepening of the density profile. We expect our method to be effective in studying other power law-like density structures, including granular materials and the large-scale structure of the Universe.

Nature of Supersonic Turbulence and Density Distribution Function in the Multiphase Interstellar Medium

Supersonic flows in the interstellar medium (ISM) are believed to be a key driver of the molecular cloud formation and evolution. Among molecular clouds’ properties, the ratio between the solenoidal and compressive modes of turbulence plays important roles in determining the star formation efficiency. We use numerical simulations of supersonic converging flows of the warm neutral medium (WNM) resolving the thermal instability to calculate the early phase of molecular cloud formation, and we investigate the turbulence structure and the density probability distribution function (density PDF) of the multiphase ISM. We find that both the solenoidal and compressive modes have their power spectrum similar to the Kolmogorov spectrum. The solenoidal (compressive) modes account for ≳80% (≲20%) of the total turbulence power. When we consider both the cold neutral medium (CNM) and the thermally unstable neutral medium (UNM) up to T ≲ 400 K, the density PDF follows the lognormal distribution, whose width is well explained by the known relation from the isothermal turbulence as (where b is the parameter representing the turbulence mode ratio and is the turbulent Mach number). The density PDF of the CNM component alone (T ≤ 50 K), however, exhibits a narrower by a factor of ∼2. These results suggest that observational estimations of b based on the CNM density PDF requires the internal turbulence within each CNM clump but not the interclump relative velocity, the latter of which is instead powered by the WNM/UNM turbulence.

Velocity structure of the 50 pc long NGC 6334 filamentary cloud

Context. The interstellar medium is observed to be organized in filamentary structures, and in neutral (H I) and ionized (H II) bubbles. The expanding nature of these bubbles shapes the surrounding medium and possibly plays a role in the formation and evolution of the interstellar filaments. The impact of the expansion of these bubbles on the interstellar medium is not well understood. Aims. Our aim is to describe the kinematics of a filamentary molecular cloud forming high-mass stars and hosting multiple H II regions in order to study the possible environmental impact on the properties of molecular filaments. Methods. We present APEX 13CO and C18O(2–1) mapping observations of the 10 × 50 pc NGC 6334 molecular cloud complex. We investigated the gas velocity structure along and across the 50 pc long cloud and toward velocity-coherent filaments (VCFs). Results. The NGC 6334 complex is observed to have a coherent velocity structure smoothly varying by ~5 km s−1 over its 50 pc elongation parallel to the Galactic plane. We identify a sample of 75 VCFs in the C18O(2–1) position-position-velocity cube and present the properties of 47 VCFs with a length ≳1 pc (five beams). We measure a large number of velocity gradients along the VCFs. The amplitudes of these velocity gradients and the velocity dispersion measured along the crests increase with the column density of the VCFs. We derive the column density and velocity power spectra of the VCFs. These power spectra are well represented with power laws showing similar slopes for the two quantities (with a mean of about −2), although some differ by up to a factor of 2. The position velocity diagrams perpendicular to three VCFs (selected from different physical environments) show the V-shaped velocity pattern corresponding to a bent structure in velocity space with the filament at the tip of the V surrounded by an extended structure connected to it with a velocity gradient. This velocity structure is qualitatively similar to that resulting from numerical simulations of filament formation from large-scale compression from propagating shock fronts. In addition, the radial profiles perpendicular to these VCFs hint to small-scale internal impacts from neighboring H II bubbles on two of them, while the third is mostly unaffected. Conclusions. The observed opposite curvature in velocity space (V- and A-shaped) toward the VCFs points to various origins of large-scale external compressions from propagating H I bubbles. This suggests the plausible importance of multiple H I compressions, separated in space and time, in the formation and evolution of molecular clouds and their star formation history. These atomic compressions due to past and distant star formation events are complemented by the impact of H II bubbles from present time and local star formation activity.

Cosmic rays in molecular clouds probed by H2 rovibrational lines

Context. Low-energy cosmic rays (<1 TeV) play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H2. Their characterisation is therefore important both for the interpretation of observations and for the development of theoretical models. However, the methods used so far for estimating the cosmic-ray ionisation rate in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. Aims. We refine and extend a previously proposed method to estimate the cosmic-ray ionisation rate in molecular clouds by observing rovibrational transitions of H2 at near-infrared wavelengths, which are mainly excited by secondary cosmic-ray electrons. Methods. Combining models of interstellar cosmic-ray propagation and attenuation in molecular clouds with the rigorous calculation of the expected secondary electron spectrum and updated electron-H2 excitation cross sections, we derive the intensity of the four H2 rovibrational transitions observable in cold dense gas: (1−0)O(2), (1−0)Q(2), (1−0)S(0), and (1−0)O(4). Results. The proposed method allows the estimation of the cosmic-ray ionisation rate for a given observed line intensity and H2 column density. We are also able to deduce the shape of the low-energy cosmic-ray proton spectrum impinging upon the molecular cloud. In addition, we present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar cosmic-ray proton spectrum. We finally comment on the capability of the James Webb Space Telescope to detect these near-infrared H2 lines, which will make it possible to derive, for the first time, spatial variation in the cosmic-ray ionisation rate in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will finally be possible to test competing models of cosmic-ray propagation and attenuation in the interstellar medium, as well as compare cosmic-ray spectra in different Galactic regions.

Chiral recognition ability of alanine dipeptides studied by ultraviolet photodissociation spectroscopy and water adsorption of cold gas-phase hydrogen-bonded clusters

Enantiomer-selective reactions in cold gas-phase hydrogen-bonded clusters of l-alanine dipeptide (l-Ala2) and tryptophan (Trp) enantiomers were investigated as a model for chemical evolution in interstellar molecular clouds using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The ultraviolet-photoinduced NH2CHCOOH elimination and l-Ala2 detachment from the clusters showed the same wavelength dependence at 265–290 nm for heterochiral H+(d-Trp) (l-Ala2) and homochiral H+(l-Trp) (l-Ala2). The elimination of CO2 from the heterochiral clusters was the main reaction at 265–282 nm, while the elimination of CO2 from the homochiral hydrogen-bonded clusters was suppressed by intermolecular interactions. Water molecules preferentially adsorbed on the surface of the heterochiral hydrogen-bonded clusters, indicating a larger number of free X–H groups (X = O and N) on the surface of the heterochiral clusters compared to the surface of the homochiral clusters.