arXiv subjects

Shmuel Bialy

Publications and source records attributed to Shmuel Bialy.

At least 37 records · Page 2Linked to original sources

arXiv2022

Constraining the cosmic-ray ionization rate and their spectrum with NIR spectroscopy of dense clouds -- A test-bed for JWST

Low-energy cosmic-rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds, the sites of star-formation. However, current estimates of the low-energy CR spectrum ($E \lesssim 1$ GeV) and the associated CR ionization rate are highly uncertain. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H$_2$ rovibrational lines from cold molecular clouds. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores, G150, G157, G163, G198, with column densities $N_{\rm H_2} \approx 10^{22}$ cm$^{-2}$. We derive 3$σ$ upper limits on the H$_2$ $(1-0)$S(0) line (2.22 $μ$m) brightness in the range $I = 5.9 \times 10^{-8}$ to $1.2 \times 10^{-7}$ erg cm$^{-2}$ s$^{-1}$ sr$^{-1}$ for the different targets. Using both an analytic model and a numerical model of CR propagation, we convert these into upper limits on the CR ionization rate in the clouds' interior, $ζ= 1.5$ to $3.6 \times 10^{-16}$ s$^{-1}$, and lower limits on the low-energy spectral slope of interstellar CR protons, $α= -0.97$ to $-0.79$. We show that while MMT was unable to detect the H$_2$ lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H$_2$ lines, making it the ideal method for constraining the otherwise elusive low-energy CRs, shedding light on the sources and propagation modes of CRs.

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arXiv2022

Star formation near the Sun is driven by expansion of the Local Bubble

For decades we have known that the Sun lies within the Local Bubble, a cavity of low-density, high-temperature plasma surrounded by a shell of cold, neutral gas and dust. However, the precise shape and extent of this shell, the impetus and timescale for its formation, and its relationship to nearby star formation have remained uncertain, largely due to low-resolution models of the local interstellar medium. Leveraging new spatial and dynamical constraints from the Gaia space mission, here we report an analysis of the 3D positions, shapes, and motions of dense gas and young stars within 200 pc of the Sun. We find that nearly all the star-forming complexes in the solar vicinity lie on the surface of the Local Bubble and that their young stars show outward expansion mainly perpendicular to the bubble's surface. Tracebacks of these young stars' motions support a scenario where the origin of the Local Bubble was a burst of stellar birth and then death (supernovae) taking place near the bubble's center beginning 14 Myr ago. The expansion of the Local Bubble created by the supernovae swept up the ambient interstellar medium into an extended shell that has now fragmented and collapsed into the most prominent nearby molecular clouds, in turn providing robust observational support for the theory of supernova-driven star formation.

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arXiv2022

The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds

Observations of the emission of the carbon cycle species (C, C+ CO) are commonly used to diagnose gas properties in the interstellar medium but are significantly sensitive to the cosmic-ray ionization rate. The carbon-cycle chemistry is known to be quite sensitive to the cosmic-ray ionization rate, $ζ$, controlled by the flux of low-energy cosmic rays which get attenuated through molecular clouds. However, astrochemical models commonly assume a constant cosmic-ray ionization rate in the clouds. We investigate the effect of cosmic-ray attenuation on the emission of carbon cycle species from molecular clouds, in particular the [CII] 158 $μ$m, [CI] 609 $μ$m and CO (J = 1 - 0) 115.27 GHz lines. We use a post-processed chemical model of diffuse and dense simulated molecular clouds and quantify the variation in both column densities and velocity integrated line emission of the carbon cycle with different cosmic-ray ionization rate models. We find that the abundances and column densities of carbon cycle species is significantly impacted by the chosen cosmic-ray ionization rate model: no single constant ionization rate can reproduce the abundances modelled with an attenuated cosmic-ray model. Further, we show that constant ionization rate models fail to simultaneously reproduce the integrated emission of the lines we consider, and their deviations from a physically derived cosmic-ray attenuation model is too complex to be simply corrected. We demonstrate that the two clouds we model exhibit a similar average $A_{\rm V, eff}$ -- $n_{\rm H}$ relationship, resulting in an average relation between the cosmic-ray ionization rate and density $ζ(n_{\rm H})$. We conclude by providing a number of implementation recommendations for CRs in astrochemical models, but emphasize the necessity for column-dependent cosmic-ray ionization rate prescriptions.

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arXiv2021

On the Three-Dimensional Structure of Local Molecular Clouds

We leverage the 1 pc spatial resolution of the Leike et al. 2020 3D dust map to characterize the three-dimensional structure of nearby molecular clouds ($d \lesssim 400$ pc). We start by "skeletonizing" the clouds in 3D volume density space to determine their "spines," which we project on the sky to constrain cloud distances with $\approx 1\%$ uncertainty. For each cloud, we determine an average radial volume density profile around its 3D spine and fit the profiles using Gaussian and Plummer functions. The radial volume density profiles are well-described by a two-component Gaussian function, consistent with clouds having broad, lower-density outer envelopes and narrow, higher-density inner layers. The ratio of the outer to inner envelope widths is $\approx 3:1$. We hypothesize that these two components may be tracing a transition between atomic and diffuse molecular gas or between the unstable and cold neutral medium. Plummer-like models can also provide a good fit, with molecular clouds exhibiting shallow power-law wings with density, $n$, falling off like $n^{-2}$ at large radii. Using Bayesian model selection, we find that parameterizing the clouds' profiles using a single Gaussian is disfavored. We compare our results with 2D dust extinction maps, finding that the 3D dust recovers the total cloud mass from integrated approaches with fidelity, deviating only at higher levels of extinction ($A_V \gtrsim 2 - 3$ mag). The 3D cloud structure described here will enable comparisons with synthetic clouds generated in simulations, offering unprecedented insight into the origins and fates of molecular clouds in the interstellar medium.

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arXiv2021

The Per-Tau Shell: A Giant Star-Forming Spherical Shell Revealed by 3D Dust Observations

A major question in the field of star formation is how molecular clouds form out of the diffuse Interstellar Medium (ISM). Recent advances in 3D dust mapping are revolutionizing our view of the structure of the ISM. Using the highest-resolution 3D dust map to date, we explore the structure of a nearby star-forming region, which includes the well-known Perseus and Taurus molecular clouds. We reveal an extended near-spherical shell, 156 pc in diameter, hereafter the "Per-Tau Shell", in which the Perseus and Taurus clouds are embedded. We also find a large ring structure at the location of Taurus, hereafter, the "Tau Ring". We discuss a formation scenario for the Per-Tau Shell, in which previous stellar and supernova (SN) feedback events formed a large expanding shell, where the swept-up ISM has condensed to form both the shell and the Perseus and Taurus molecular clouds within it. We present auxiliary observations of HI, H$α$, $^{26}$Al, and X-rays that further support this scenario, and estimate Per-Tau Shell's age to be $\approx 6-22$ Myrs. The Per-Tau shell offers the first three-dimensional observational view of a phenomenon long-hypothesized theoretically, molecular cloud formation and star formation triggered by previous stellar and SN feedback.

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arXiv2021

HI-to-H$_2$ Transitions in Dust-Free Interstellar Gas

We present numerical computations and analysis of atomic to molecular (HI-to-H$_2$) transitions in cool ($\sim$100 K) low-metallicity dust-free (primordial) gas, in which molecule formation occurs via cosmic-ray driven negative ion chemistry, and removal is by a combination of far-UV photodissociation and cosmic-ray ionization and dissociation. For any gas temperature, the behavior depends on the ratio of the Lyman-Werner (LW) band FUV intensity to gas density, $I_{\rm LW}/n$, and the ratio of the cosmic-ray ionization rate to the gas density, $ζ/n$. We present sets of HI-to-H$_2$ abundance profiles for a wide range of $ζ/n$ and $I_{\rm LW}/n$, for dust-free gas. We determine the conditions for which H$_2$ absorption line self-shielding in optically thick clouds enables a transition from atomic to molecular form for ionization-driven chemistry. We also examine the effects of cosmic-ray energy losses on the atomic and molecular density profiles and transition points. For a unit Galactic interstellar FUV field intensity ($I_{\rm LW}=1$) with LW flux $2.07\times 10^7$ photons cm$^{-2}$ s$^{-1}$, and a uniform cosmic-ray ionization rate $ζ=10^{-16}$ s$^{-1}$, an HI-to-H$_2$ transition occurs at a total hydrogen gas column density of $4\times 10^{21}$ cm$^{-2}$, within $3\times 10^7$ yr, for a gas volume density of $n=10^6$ cm$^{-3}$ at 100 K. For these parameters, the dust-free limit obtains for a dust-to-gas ratio Z$^\prime_d \lesssim 10^{-5}$, which may be reached for overall metallicities $Z^\prime\lesssim 0.01$ relative to Galactic solar values.

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arXiv2020

A Prospective ISRO-CfA Himalayan Sub-millimeter-wave Observatory Initiative

The Smithsonian Astrophysical Observatory (SAO), a member of the Center for Astrophysics | Harvard and Smithsonian, is in discussions with the Space Applications Centre (SAC) of the Indian Space Research Organization (ISRO) and its partners in the newly formed Indian Sub-millimetre-wave Astronomy Alliance (ISAA), to collaborate in the construction of a sub-millimeter-wave astronomy observatory in the high altitude deserts of the Himalayas, initially at the 4500 m Indian Astronomical Observatory, Hanle. Two primary science goals are targeted. One is the mapping of the distribution of neutral atomic carbon, and the carbon monoxide (CO) molecule in higher energy states, in large parts of the Milky Way, and in selected external galaxies. Such studies would advance our understanding of molecular hydrogen present in the interstellar medium, but partly missed by existing observations; and characterize Galaxy-wide molecular cloud excitation conditions, through multi-level CO observations. Stars form in interstellar clouds of molecular gas and dust, and these observations would allow research into the formation and destruction processes of such molecular clouds and the life cycle of galaxies. As the second goal, the observatory would add a new location to the global Event Horizon Telescope (EHT) network, which lacks a station in the Himalayan longitudes. This addition would enhance the quality of the images synthesized by the EHT, support observations in higher sub-millimeter wave bands, sharpening its resolving ability, improve its dynamic imaging capability and add weather resilience to observing campaigns. In the broader context, this collaboration can be a starting point for a wider, mutually beneficial scientific exchange between the Indian and US astronomy communities, including a potential future EHT space component.

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arXiv2020

Study Turbulence and Probe Magnetic Field Using Gradients Technique: Application to HI-to-H2 Transition Regions

The atomic-to-molecular (HI-to-H$_2$) transition in photodissociation regions (PDRs) has been investigated over the last several decades through analytic and numerical modeling. However, classical PDR models typically assume uniform density gas, ignoring the turbulent nature of the interstellar medium. Recently, Bialy et al. (2017b, 2019) have presented a theoretical framework for studying the HI-to-H$_2$ in a realistic turbulent medium with a non-homogeneous density structure. Here we extend these turbulent-chemical models to explore the possibility of tracing the magnetic field direction in turbulent PDRs using the Gradients Technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for chemical HI/H$_2$ balance calculations. We confirm that the density fluctuations induced by turbulence can disperse the distribution of H$_2$ and HI fraction. We find that the energy spectrum of moment maps gets shallower when the sonic Mach number MS increases. We explore the ability in magnetic field tracing of gradients of higher-order velocity centroids and compare their performance with that of traditional velocity centroid gradients (VCGs) and with intensity gradients (IGs). We find that the velocity gradients of the second-order centroids (VC$_2$Gs) are more accurate than VCGs and IGs in probing the magnetic field orientation.

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arXiv2020

The far-UV Interstellar Radiation Field in Galactic Disks: Numerical and Analytic Models

The intensity of the far-ultraviolet (FUV; 6-13.6 eV) interstellar radiation field (ISRF) in galaxies determines the thermal and chemical evolution of the neutral interstellar gas and is key for interpreting extragalactic observations and for theories of star-formation. We run a series of galactic disk models and derive the FUV ISRF intensity as a function of the dust-to-gas ratio, star-formation rate density, gas density, scale radius, and observer position. We develop an analytic formula for the median FUV ISRF flux. We identify two dimensionless parameters in the problem: (1) the dimensionless galactic radius, $X$, which measures the radial extent of FUV sources (OB stellar associations) in the disk; (2) the opacity over the inter-source distance, $τ_{\star}$, which measures the importance of dust absorption. These parameters encapsulate the dependence on all of the physical parameters. We find that there exists a critical $τ_{\star}$, or equivalently a critical dust-to-gas ratio, $Z_{d,{\rm crit}}' \approx 0.01-0.1$ the Milky Way value, at which the ISRF changes behavior. For $Z'_d>Z_{d,{\rm crit}}'$ the ISRF is limited by dust absorption. With decreasing $Z'_d$, the ISRF intensity increases as more sources contribute to the flux. For $Z'_d < Z_{d,{\rm crit}}'$ the ISRF saturates as the disk becomes optically thin. We find that the ISRF per star-formation rate density in low metallicity systems, such as dwarf and high redshift galaxies, is higher by up to a factor of 3-6 compared to their Milky-Way counterparts. We discuss implications to the potential mechanisms that regulate star-formation in low metallicity galaxies.

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arXiv2020

The driving scale -- density decorrelation scale relation in a turbulent medium

Density fluctuations produced by supersonic turbulence are of great importance to astrophysical chemical models. A property of these density fluctuations is that the two point correlation function decreases with increasing scale separation. The relation between the density decorrelation length scale ($L_{\rm dec}$) and the turbulence driving scale ($L_{\rm drive}$) determines how turbulence affects the density and chemical structures in the interstellar medium (ISM), and is a key component for using observations of atomic and molecular tracers to constrain turbulence properties. We run a set of numerical simulations of supersonic magnetohydrodynamic turbulence, with different sonic Mach numbers ($\mathcal{M}_s=4.5, 7$) , and driven on varying scales (1/2.5, 1/5, 1/7) the box length. We derive the $L_{\rm dec}-L_{\rm drive}$ relation as a function of Mach number, driving scale, and the orientation of the line-of-sight (LOS) in respect to the magnetic-field. We find that the mean ratio $L_{\rm dec}/L_{\rm drive} = 0.19 \pm 0.10$, when averaged over snapshots, Mach numbers, driving lengths, and the three LOSs. For LOS parallel to the magnetic field the density structures are statistically smaller and the $L_{\rm dec}-L_{\rm drive}$ relation is tighter, with $L_{\rm dec}/L_{\rm drive} = 0.112 \pm 0.024$. We discuss our results in the context of using observations of chemical tracers to constrain the dominant turbulence driving scale in the ISM.

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arXiv2020

Cold Clouds as Cosmic-Ray Detectors

Low energy cosmic-rays (CRs) are responsible for gas heating and ionization of interstellar clouds, which in turn introduces coupling to Galactic magnetic fields. So far the CR ionization rate (CRIR) has been estimated using indirect methods, such as its effect on the abundances of various rare molecular species. Here we show that the CRIR may be constrained from line emission of H$_2$ rovibrational transitions, excited by CRs. We derive the required conditions for CRs to dominate line excitation, and show that CR-excited lines may be detected with the Very Large Telescope (VLT) over 8 hours integration. Our method, if successfully applied to a variety of clouds at different Galactic locations will provide improved constraints on the spectrum of low energy CRs and their origins.

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arXiv2019

Chemical Abundances in a Turbulent Medium -- H$_2$, OH$^+$, H$_2$O$^+$, ArH$^+$

Supersonic turbulence results in strong density fluctuations in the interstellar medium (ISM), which have a profound effect on the chemical structure. Particularly useful probes of the diffuse ISM are the ArH$^+$, OH$^+$, H$_2$O$^+$ molecular ions, which are highly sensitive to fluctuations in the density and the H$_2$ abundance. We use isothermal magnetohydrodynamic (MHD) simulations of various sonic Mach numbers, $\mathcal{M}_s$, and density decorrelation scales, $y_{\rm dec}$, to model the turbulent density field. We post-process the simulations with chemical models and obtain the probability density functions (PDFs) for the H$_2$, ArH$^+$, OH$^+$ and H$_2$O$^+$ abundances. We find that the PDF dispersions increases with increasing $\mathcal{M}_s$ and $y_{\rm dec}$, as the magnitude of the density fluctuations increases, and as they become more coherent. Turbulence also affects the median abundances: when $\mathcal{M}_s$ and $y_{\rm dec}$ are high, low-density regions with low H$_2$ abundance become prevalent, resulting in an enhancement of ArH$^+$ compared to OH$^+$ and H$_2$O$^+$. We compare our models with Herschel observations. The large scatter in the observed abundances, as well as the high observed ArH$^+$/OH$^+$ and ArH$^+$/H$_2$O$^+$ ratios are naturally reproduced by our supersonic $(\mathcal{M}_s=4.5)$, large decorrelation scale $(y_{\rm dec}=0.8)$ model, supporting a scenario of a large-scale turbulence driving. The abundances also depend on the UV intensity, CR ionization rate, and the cloud column density, and the observed scatter may be influenced by fluctuations in these parameters.

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arXiv2019

Thermal Phases of the Neutral Atomic Interstellar Medium -- from Solar Metallicity to Primordial Gas

We study the thermal structure of the neutral atomic (H {\small I}) interstellar medium across a wide range of metallicities, from supersolar down to vanishing metallicity, and for varying UV intensities and cosmic-ray ionization rates. We calculate self-consistently the gas temperature and species abundances (with a special focus on the residual H$_2$), assuming thermal and chemical steady-state. For solar metallicity, $Z' \equiv 1$, we recover the known result that there exists a pressure range over which the gas is multiphased, with the warm ($\sim 10^4$ K, WNM) and cold ($\sim 100$ K, CNM) phases coexisting at the same pressure. At a metallicity $Z' \approx 0.1$, the CNM is colder (compared to $Z'=1$) due to the reduced efficiency of photoelectric heating. For $Z' \lesssim 0.1$, cosmic-ray ionization becomes the dominant heating mechanism and the WNM-to-CNM transition shifts to ever increasing pressure/density as the metallicity is reduced. For metallicities $Z' \lesssim 0.01$, H$_2$ cooling becomes important, lowering the temperature of the WNM (down to $\approx 600$ K), and smoothing out the multiphase phenomenon. At vanishing metallicities, H$_2$ heating becomes effective and the multiphase phenomenon disappears entirely. We derive analytic expressions for the critical densities for the warm-to-cold phase transition in the different regimes, and the critical metallicities for H$_2$ cooling and heating. We discuss potential implications on the star-formation rates of galaxies and self-regulation theories.

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arXiv2019

Active Galactic Nuclei: Boon or Bane for Biota?

Active Galactic Nuclei (AGNs) emit substantial fluxes of high-energy electromagnetic radiation, and have therefore attracted some recent attention for their negative impact on galactic habitability. In this paper, we propose that AGNs may also engender the following beneficial effects: (i) prebiotic synthesis of biomolecular building blocks mediated by ultraviolet (UV) radiation, and (ii) powering photosynthesis on certain free-floating planets and moons. We also reassess the harmful biological impact of UV radiation originating from AGNs, and find that their significance could have been overestimated. Our calculations suggest that neither the positive nor negative ramifications stemming from a hypothetical AGN in the Milky Way are likely to affect putative biospheres in most of our Galaxy. On the other hand, we find that a sizable fraction of all planetary systems in galaxies with either disproportionately massive black holes ($\sim 10^{9-10}\,M_\odot$) or high stellar densities (e.g., compact dwarf galaxies) might be susceptible to both the beneficial and detrimental consequences of AGNs, with the former potentially encompassing a greater spatial extent than the latter.

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arXiv2018

Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration?

`Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, \citet{Micheli2018} reported that `Oumuamua showed deviations from a Keplerian orbit at a high statistical significance. The observed trajectory is best explained by an excess radial acceleration $Δa \propto r^{-2}$, where $r$ is the distance of `Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that `Oumuamua is not an active comet. We explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is $(m/A)\approx 0.1$ g cm$^{-2}$. For a thin sheet this requires a thickness of $\approx 0.3-0.9$ mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of $\sim 5$ kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. We discuss the possible origins of such an object. Our general results apply to any light probes designed for interstellar travel.

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arXiv2018

The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies

We investigate the metallicity dependence of HI surface densities in star-forming regions along many lines of sight within 70 nearby galaxies, probing kpc to 50 pc scales. We employ HI, SFR, stellar mass, and metallicity (gradient) measurements from the literature, spanning a wide range (5 dex) in stellar and gas mass and (1.6 dex) in metallicity. We consider metallicities as observed, or rescaled to match the mass-metallicity relation determined for SDSS galaxies. At intermediate to high metallicities (0.3-2 times solar), we find that the HI surface densities saturate at sufficiently large total gas surface density. The maximal HI columns vary approximately inversely with metallicity, and show little variation with spatial resolution, galactocentric radius, or among galaxies. In the central parts of massive spiral galaxies the HI gas is depressed by factors of 2. The observed behavior is naturally reproduced by metallicity dependent shielding theories for the HI-to-H2 transitions in star-forming galaxies. We show that the inverse scaling of the maximal HI columns with metallicity suggests that the area filling fraction of atomic-molecular complexes in galaxies is of order unity, and weakly dependent on metallicity.

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arXiv2017

The HI-to-H2 Transition in a Turbulent Medium

We study the effect of density fluctuations induced by turbulence on the HI/H$_2$ structure in photodissociation regions (PDRs) both analytically and numerically. We perform magnetohydrodynamic numerical simulations for both subsonic and supersonic turbulent gas, and chemical HI/H$_2$ balance calculations. We derive atomic-to-molecular density profiles and the HI column density probability density function (PDF) assuming chemical equilibrium. We find that while the HI/H$_2$ density profiles are strongly perturbed in turbulent gas, the mean HI column density is well approximated by the uniform-density analytic formula of Sternberg et al. (2014). The PDF width depends on (a) the radiation intensity to mean density ratio, (b) the sonic Mach number and (c) the turbulence decorrelation scale, or driving scale. We derive an analytic model for the HI PDF and demonstrate how our model, combined with 21 cm observations, can be used to constrain the Mach number and driving scale of turbulent gas. As an example, we apply our model to observations of HI in the Perseus molecular cloud. We show that a narrow observed HI PDF may imply small scale decorrelation, pointing to the potential importance of subcloud-scale turbulence driving.

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arXiv2017

Cosmic-ray induced destruction of CO in star-forming galaxies

We explore the effects of the expected higher cosmic ray (CR) ionization rates $ζ_{\rm CR}$ on the abundances of carbon monoxide (CO), atomic carbon (C), and ionized carbon (C$^+$) in the H$_2$ clouds of star-forming galaxies. The study of Bisbas et al. (2015) is expanded by: a) using realistic inhomogeneous Giant Molecular Cloud (GMC) structures, b) a detailed chemical analysis behind the CR-induced destruction of CO, and c) exploring the thermal state of CR-irradiated molecular gas. CRs permeating the interstellar medium with $ζ_{\rm CR}$$\gtrsim 10\times$(Galactic) are found to significantly reduce the [CO]/[H$_2$] abundance ratios throughout the mass of a GMC. CO rotational line imaging will then show much clumpier structures than the actual ones. For $ζ_{\rm CR}$$\gtrsim 100\times$(Galactic) this bias becomes severe, limiting the utility of CO lines for recovering structural and dynamical characteristics of H$_2$-rich galaxies throughout the Universe, including many of the so-called Main Sequence (MS) galaxies where the bulk of cosmic star formation occurs. Both C$^+$ and C abundances increase with rising $ζ_{\rm CR}$, with C remaining the most abundant of the two throughout H$_2$ clouds, when $ζ_{\rm CR}\sim (1-100)\times$(Galactic). C$^+$ starts to dominate for $ζ_{\rm CR}$$\gtrsim 10^3\times$(Galactic). The thermal state of the gas in the inner and denser regions of GMCs is invariant with $T_{\rm gas}\sim 10\,{\rm K}$ for $ζ_{\rm CR}\sim (1-10)\times$(Galactic). For $ζ_{\rm CR}$$\sim 10^3\times$(Galactic) this is no longer the case and $T_{\rm gas}\sim 30-50\,{\rm K}$ are reached. Finally we identify OH as the key species whose $T_{\rm gas}-$sensitive abundance could mitigate the destruction of CO at high temperatures.

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