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We report the detection of a rich spectrum of more than one hundred optical emission lines of vibrationally hot molecular hydrogen (H2) from the photodissociation region (PDR) around the mini-starburst cluster NGC 346 in the Small Magellanic Cloud. The lines are concentrated in the spectral range |$6000 \, {{\mathring{\rm A}}}$| to |$9300 \, {{\mathring{\rm A}}}$| and have observed brightnesses ranging from 0.01 per cent to 0.4 per cent times that of the H β λ4861 hydrogen recombination line. Analysis of the spatial distribution of the H2 lines shows that they originate from a range of depths in the PDR, intermediate between the shallow layers probed by fluorescent lines of neutral nitrogen and oxygen, and the more shielded layers probed by neutral carbon recombination lines. Comparison with other PDRs shows that the relative strength of the H2 lines with respect to the [ |$\mathrm{C\, \scriptstyle I}$| ] λ8727 line increases rapidly with decreasing metallicity, being at least 40 times larger in NGC 346 than in the prototypical PDR of the Orion Bar. The internal PDR dust extinction is also found to be anomalously low in NGC 346. A separate result is the discovery of a high-ionization bow shock around the O2 star Walborn 3. [ABSTRACT FROM AUTHOR]

Astrochemistry has been widely developed as a power tool to probe the physical properties of the interstellar medium (ISM) in various conditions of the Milky Way (MW) Galaxy, and in near and distant galaxies. Most current studies conventionally apply linear scaling to all elemental abundances based on the gas-phase metallicity. However, these elements, including carbon and oxygen, are enriched differentially by stellar nucleosynthesis and the overall galactic chemical evolution, evident from α-enhancement in multiple galactic observations such as starbursts, high-redshift star-forming galaxies, and low-metallicity dwarfs. We perform astrochemical modelling to simulate the impact of an α-enhanced ISM gas cloud on the abundances of the three phases of carbon (C+, C, CO) dubbed as 'the carbon cycle'. The ISM environmental parameters considered include two cosmic-ray ionization rates (ζCR = 10−17 and |$10^{-15}\, {\rm s}^{-1}$|⁠), two isotropic FUV radiation field strengths (χ/χ0 = 1 and 102), and (sub-)linear dust-to-gas relations against metallicity, mimicking the ISM conditions of different galaxy types. In galaxies with [C/O] < 0, CO, C, and C+, all decrease in both abundances and emission, though with differential biases. The low- J CO emission is found to be the most stable tracer for the molecular gas, while C and C+ trace H2 gas only under limited conditions, in line with recent discoveries of [C  i ]-dark galaxies. We call for caution when using [C  ii ] |$158\, \mu$| m and [C  i ](1–0) as alternative H2-gas tracers for both diffuse and dense gas with non-zero [C/O] ratios. [ABSTRACT FROM AUTHOR]

Protostellar jets and outflows are signposts of active star formation. In H  ii regions, molecular tracers like CO only reveal embedded portions of the outflow. Outside the natal cloud, outflows are dissociated, ionized, and eventually completely ablated, leaving behind only the high-density jet core. Before this process is complete, there should be a phase where the outflow is partially molecular and partially ionized. In this paper, we capture the HH 900 outflow while this process is in action. New observations from the Enhanced Resolution Imager and Spectrograph/SPIFFIER near-infrared (IR) integral field unit spectrograph using the K-middle filter (λ = 2.06–2.34 μ m) reveal H2 emission from the dissociating outflow and Br-γ tracing its ionized skin. Both lines trace the wide-angle outflow morphology but H2 only extends ∼5000 au into the H  ii region while Br-γ extends the full length of the outflow (∼12 650 au), indicating rapid dissociation of the molecules. H2 has higher velocities further from the driving source, consistent with a jet-driven outflow. Diagnostic line ratios indicate that photoexcitation, not just shocks, contributes to the excitation in the outflow. We argue that HH 900 is the first clear example of an evaporating molecular outflow and predict that a large column of neutral material that may be detectable with Atacama Large Millimeter Array accompanies the dissociating molecules. Results from this study will help guide the interpretation of near-IR images of externally irradiated jets and outflows such as those obtained with the JWST in high-mass star-forming regions where these conditions may be common. [ABSTRACT FROM AUTHOR]

We present a new fried grid of mass-loss rates for externally far-ultraviolet (FUV) irradiated protoplanetary discs. As a precursor to the new grid, we also explore the microphysics of external photoevaporation, determining the impact of polycyclic aromatic hydrocarbon (PAH) abundance, metallicity, coolant depletion (via freeze out and radial drift), and grain growth (depletion of small dust in the outer disc) on disc mass-loss rates. We find that metallicity variations typically have a small effect on the mass-loss rate, since the impact of changes in heating, cooling and optical depth to the disc approximately cancel out. The new fried grid therefore focuses on (i) expanding the basic physical parameter space (disc mass, radius, UV field, stellar mass), (ii) on enabling variation of the the PAH abundance, and (iii) including an option for grain growth to have occurred or not in the disc. What we suggest is the fiducial model is comparable to the original fried grid. When the PAH-to-dust ratio is lower, or the dust in the wind more abundant, the mass-loss rate can be substantially lower. We demonstrate with a small set of illustrative disc evolutionary calculations that this in turn can have a significant impact on the disc mass/radius/ evolution and lifetime. [ABSTRACT FROM AUTHOR]

The presence of short-lived radioisotopes (SLRs) 26Al and 60Fe in the Solar system places constraints on the initial conditions of our planetary system. Most theories posit that the origin of 26Al and 60Fe is in the interiors of massive stars, and they are either delivered directly to the protosolar disc from the winds and supernovae of the massive stars, or indirectly via a sequential star formation event. However, massive stars that produce SLRs also emit photoionizing far and extreme ultraviolet radiation, which can destroy the gas component of protoplanetary discs, possibly precluding the formation of gas giant planets like Jupiter and Saturn. Here, we perfom N -body simulations of star-forming regions and determine whether discs that are enriched in SLRs can retain enough gas to form Jovian planets. We find that discs are enriched and survive the photoionizing radiation only when the dust radius of the disc is fixed and not allowed to move inwards due to the photoevaporation, or outwards due to viscous spreading. Even in this optimal scenario, not enough discs survive until the supernovae of the massive stars and so have zero or very little enrichment in 60Fe. We therefore suggest that the delivery of SLRs to the Solar system may not come from the winds and supernovae of massive stars. [ABSTRACT FROM AUTHOR]

The anion C5N− is one of the largest linear (C,N)-bearing chains detected in the interstellar medium. Here we present and discuss the general features of new ab initio potential energy surfaces describing the interaction of this linear anion with He and H2. We employ a Legendre Polynomials expansion representation for the former and an artificial neural network fit for the latter. We then carry out quantum scattering calculations to yield rotationally inelastic cross-sections for collisions with He and H2, using relative translational energy values in the range of 0.1–300 cm−1. We then obtained the corresponding inelastic rate coefficients as a function of temperature covering the range from 1 to 100 K. The results for these two systems are compared with each other, as well as with the earlier results on the C3N− colliding with the same partners. We found that the final inelastic rate coefficients for this anion are all fairly large, those from collisions with H2 being the largest. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are discussed in our conclusions. [ABSTRACT FROM AUTHOR]

The anion C5N− is one of the largest linear (C,N)-bearing chains detected in the interstellar medium. Here we present and discuss the general features of new ab initio potential energy surfaces describing the interaction of this linear anion with He and H2. We employ a Legendre Polynomials expansion representation for the former and an artificial neural network fit for the latter. We then carry out quantum scattering calculations to yield rotationally inelastic cross-sections for collisions with He and H2, using relative translational energy values in the range of 0.1–300 cm−1. We then obtained the corresponding inelastic rate coefficients as a function of temperature covering the range from 1 to 100 K. The results for these two systems are compared with each other, as well as with the earlier results on the C3N− colliding with the same partners. We found that the final inelastic rate coefficients for this anion are all fairly large, those from collisions with H2 being the largest. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are discussed in our conclusions. [ABSTRACT FROM AUTHOR]

Whether ionization feedback triggers the formation of massive stars is highly debated. Using ALMA 3-mm observations with a spatial resolution of ∼0.05 pc and a mass sensitivity of 1.1  |$\rm M_\odot$|  per beam at 20 K, we investigate the star formation and gas flow structures within the ionizing feedback-driven structure, a clump-scale massive (≳ 1500  |$\rm M_\odot$|⁠) bright-rimmed cloud (BRC) associated with IRAS 18290–0924. This BRC is bound only if external compression from ionized gas is considered. A small-scale (≲ 1 pc) age sequence along the direction of ionizing radiation is revealed for the embedded cores and protostars, which suggests triggered star formation via radiation-driven implosion (RDI). Furthermore, filamentary gas structures converge towards the cores located in the BRC's centre, indicating that these filaments are fueling mass towards cores. The local core-scale mass infall rate derived from H13CO+  J  = 1 − 0 blue profile is of the same order of magnitude as the filamentary mass inflow rate, approximately 1  |$\rm M_\odot$|  kyr−1. A photodissociation region (PDR) covering the irradiated clump surface is detected in several molecules, such as CCH, HCO+, and CS whereas the spatial distribution stratification of these molecules is indistinct. CCH spectra of the PDR possibly indicate a photoevaporation flow leaving the clump surface with a projected velocity of ∼2 km s−1. Our new observations show that RDI accompanied by a clump-fed process is operating in this massive BRC. Whether this combined process works in other massive BRCs is worth exploring with dedicated surveys. [ABSTRACT FROM AUTHOR]

Expansion of wind-blown bubbles or H  ii regions lead to formation of shocks in the interstellar medium, which compress surrounding gas into dense layers. We made spatially and velocity-resolved observations of the RCW 120 photo-dissociation region (PDR) and nearby molecular gas with CO(6–5) and 13CO(6–5) lines and distinguished a bright CO-emitting layer, which we related with the dense shocked molecular gas moving away from the ionizing star due to expansion of H  ii region. Simulating gas density and temperature, as well as brightness of several CO and C+ emission lines from the PDR, we found reasonable agreement with the observed values. Analysing gas kinematics, we revealed the large-scale shocked PDR and also several dense environments of embedded protostars and outflows. We observe the shocked layer as the most regular structure in the CO(6–5) map and in the velocity space, when the gas around Young stellar objects (YSOs) is dispersed by the outflows. [ABSTRACT FROM AUTHOR]

The QUIJOTE-MFI Northern Hemisphere Wide Survey has provided maps of the sky above declinations −30° at 11, 13, 17, and 19 GHz. These data are combined with ancillary data to produce Spectral Energy Distributions in intensity in the frequency range 0.4–3 000 GHz on a sample of 52 candidate compact sources harbouring anomalous microwave emission (AME). We apply a component separation analysis at 1° scale on the full sample from which we identify 44 sources with high AME significance. We explore correlations between different fitted parameters on this last sample. QUIJOTE-MFI data contribute to notably improve the characterization of the AME spectrum, and its separation from the other components. In particular, ignoring the 10–20 GHz data produces on average an underestimation of the AME amplitude, and an overestimation of the free–free component. We find an average AME peak frequency of 23.6 ± 3.6 GHz, about 4 GHz lower than the value reported in previous studies. The strongest correlation is found between the peak flux density of the thermal dust and of the AME component. A mild correlation is found between the AME emissivity (A AME/τ250) and the interstellar radiation field. On the other hand no correlation is found between the AME emissivity and the free–free radiation Emission Measure. Our statistical results suggest that the interstellar radiation field could still be the main driver of the intensity of the AME as regards spinning dust excitation mechanisms. On the other hand, it is not clear whether spinning dust would be most likely associated with cold phases of the interstellar medium rather than with hot phases dominated by free–free radiation. [ABSTRACT FROM AUTHOR]

Protoplanetary discs are crucial to understanding how planets form and evolve, but these objects are subject to the vagaries of the birth environments of their host stars. In particular, photoionizing radiation from massive stars has been shown to be an effective agent in disrupting protoplanetary discs. External photoevaporation leads to the inward evolution of the radii of discs, whereas the internal viscous evolution of the disc causes the radii to evolve outwards. We couple N -body simulations of star-forming regions with a post-processing analysis of disc evolution to determine how the radius and mass distributions of protoplanetary discs evolve in young star-forming regions. To be consistent with observations, we find that the initial disc radii must be of the order of 100 au, even though these discs are readily destroyed by photoevaporation from massive stars. Furthermore, the observed disc radius distribution in the Orion Nebula Cluster (ONC) is more consistent with moderate initial stellar densities (100 M⊙ pc−3), in tension with dynamical models that posit much higher initial densities for the ONC. Furthermore, we cannot reproduce the observed disc radius distribution in the Lupus star-forming region if its discs are subject to external photoevaporation. A more detailed comparison is not possible due to the well-documented uncertainties in determining the ages of pre-main-sequence (disc-hosting) stars. [ABSTRACT FROM AUTHOR]

We present a charge distribution based model that computes the infrared spectrum of polycyclic aromatic hydrocarbon (PAH) molecules using recent measurements or quantum chemical calculations of specific PAHs. The model is applied to a sample of well-studied photodissociation regions (PDRs) with well-determined physical conditions (the radiation field strength, G 0, electron density ne , and the gas temperature, T gas). Specifically, we modelled the emission of five PAHs ranging in size from 18 to 96 carbon atoms, over a range of physical conditions characterized by the ionization parameter |$\gamma = G_{0}\times T_{\rm gas}^{1/2}/n_{e}$|⁠. The anions emerge as the dominant charge carriers in low γ (<2 × 102) environments, neutrals in the intermediate γ (103 – 104) environments, and cations in the high γ (>105) environments. Furthermore, the PAH anions and cations exhibit similar spectral characteristics. The similarity in the cationic and anionic spectra translates into the interpretation of the 6.2/(11.0+11.2) band ratio, with high values of this ratio associated with large contributions from either cations or anions. The model's predicted values of 6.2/(11.0+11.2) and 3.3/6.2 compared well to the observations in the PDRs NGC 7023, NGC 2023, the horsehead nebula, the Orion bar, and the diffuse interstellar medium, demonstrating that changes in the charge state can account for the variations in the observed PAH emission. We also reassess the diagnostic potential of the 6.2/(11.0+11.2) versus 3.3/(11.0+11.2) ratios and show that without any prior knowledge about γ, the 3.3/(11.0+11.2) can predict the PAH size, but the 6.2/(11.0 + 11.2) cannot predict the γ of the astrophysical environment. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the X-ray, infrared, and carbon monoxide (CO) emission for a sample of 35 local (z ≤ 0.15), active (L X ≥ 1042 erg s−1) galaxies. Our goal is to infer the contribution of far-ultraviolet (FUV) radiation from star formation (SF), and X-ray radiation from the active galactic nuclei (AGNs), respectively, producing photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), to the molecular gas heating. To this aim, we exploit the CO spectral line energy distribution (CO SLED) as traced by Herschel, complemented with data from single-dish telescopes for the low- J lines, and high-resolution ALMA images of the mid- J CO emitting region. By comparing our results to the Schmidt–Kennicutt relation, we find no evidence for AGN influence on the cold and low-density gas on kpc-scales. On nuclear (r  = 250 pc) scales, we find weak correlations between the CO line ratios and either the FUV or X-ray fluxes: this may indicate that neither SF nor AGN radiation dominates the gas excitation, at least at r  = 250 pc. From a comparison of the CO line ratios with PDR and XDR models, we find that PDRs can reproduce observations only in presence of extremely high gas densities (n > 105 cm−3). In the XDR case, instead, the models suggest moderate densities (n ≈ 102−4 cm−3). We conclude that a mix of the two mechanisms (PDR for the mid- J , XDR, or possibly shocks for the high- J) is necessary to explain the observed CO excitation in active galaxies. [ABSTRACT FROM AUTHOR]

We study the formation of star clusters in molecular clouds by performing three-dimensional radiation hydrodynamics simulations with far-ultraviolet (FUV; 6 eV≦ h ν≦13.6 eV) and extreme ultraviolet (EUV; h ν≧13.6 eV) radiative feedback. We find that the FUV feedback significantly suppresses the star formation in diffuse clouds with the initial surface densities of |$\Sigma _{\rm cl} \lesssim \rm 50~M_{\odot } \,\, pc^{-2}$|⁠. In the cases of clouds with |$\Sigma _{\rm cl} \sim \rm 100-200~M_{\odot } \,\, pc^{-2}$|⁠ , the EUV feedback plays a main role and decrease the star formation efficiencies less than 0.3. We show that thermal pressure from photodissociation regions or H  ii regions disrupts the clouds and makes the size of the star clusters larger. Consequently, the clouds with the mass |$M_{\rm cl} \lesssim 10^{5}~\rm M_{\odot }$| and the surface density |$\Sigma _{\rm cl} \lesssim 200~\rm M_{\odot }\,\, pc^{-2}$| remain the star clusters with the stellar densities of |$\sim 100~\rm M_{\odot }\,\, pc^{-3}$| that nicely match the observed open clusters in the Milky Way. If the molecular clouds are massive (⁠|$M_{\rm cl} \gtrsim 10^{5}~\rm M_{\odot }$|⁠) and compact (⁠|$\Sigma \gtrsim 400~\rm M_{\odot }\,\, pc^{-2}$|⁠), the radiative feedback is not effective and they form massive dense cluster with the stellar densities of |$\sim 10^{4}~\rm M_{\odot }\,\, pc^{-3}$| like observed globular clusters or young massive star clusters. Thus, we suggest that the radiative feedback and the initial conditions of molecular clouds are key factors inducing the variety of the observed star clusters. [ABSTRACT FROM AUTHOR]

Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 µm, attributed to polycyclic aromatic hydrocarbons (PAHs), show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer onboard the Stratospheric Observatory for Infrared Astronomy of the gas cooling lines [O  i ] 63, 146 µm and [C  ii ] 158 µm in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) onboard Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer onboard Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7–9 µm emission are due to multiple PAH subpopulations and that the plateaus are distinct from the features perched on top. The 6–9 µm PAH emission exhibits a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH subpopulations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the far-ultraviolet radiation field strength but no clear correlations with the PAH ionization parameter. [ABSTRACT FROM AUTHOR]

We present three-dimensional astrochemical simulations and synthetic observations of magnetized, turbulent, self-gravitating molecular clouds. We explore various galactic interstellar medium environments, including cosmic ray ionization rates in the range of ζ CR = 10−17– |$10^{-14}\, {\rm s}^{-1}$|⁠ , far-UV intensities in the range of G 0 = 1–103 and metallicities in the range of Z = 0.1– |$2\, {\rm Z}_{\odot }$|⁠. The simulations also probe a range of densities and levels of turbulence, including cases where the gas has undergone recent compression due to cloud–cloud collisions. We examine: (i) the column densities of carbon species across the cycle of C  ii , C  i , and CO, along with O  i , in relation to the H  i -to-H2 transition; (ii) the velocity-integrated emission of [C  ii ] 158 μm, [13C  ii ] 158 μm, [C  i ] 609 μm and 370 μm, [O  i ] 63 μm and 146 μm, and of the first ten 12CO rotational transitions; (iii) the corresponding Spectral Line Energy Distributions; (iv) the usage of [C  ii ] and [O  i ] 63 μm to describe the dynamical state of the clouds; (v) the behaviour of the most commonly used ratios between transitions of CO and [C  i ]; and (vi) the conversion factors for using CO and C  i as H2-gas tracers. We find that enhanced cosmic ray energy densities enhance all aforementioned line intensities. At low metallicities, the emission of [C  ii ] is well connected with the H2 column, making it a promising new H2 tracer in metal-poor environments. The conversion factors of X CO and X C  i depend on metallicity and the cosmic ray ionization rate, but not on FUV intensity. In the era of ALMA, SOFIA, and the forthcoming CCAT-prime telescope, our results can be used to understand better the behaviour of systems in a wide range of galactic and extragalactic environments. [ABSTRACT FROM AUTHOR]

The first stages of planet formation usually occur when the host star is still in a (relatively) dense star-forming region, where the effects of the external environment may be important for understanding the outcome of the planet formation process. In particular, star-forming regions that contain massive stars have strong far-ultraviolet (FUV) and extreme ultraviolet (EUV) radiation fields, which can induce mass-loss from protoplanetary discs due to photoevaporation. In this paper, we present a parameter-space study of the expected FUV and EUV fields in N -body simulations of star-forming regions with a range of initial conditions. We then use recently published models to determine the mass-loss due to photoevaporation from protoplanetary discs. In particular, we focus on the effects of changing the initial degree of spatial structure and initial virial ratio in the star-forming regions, as well as the initial stellar density. We find that the FUV fields in star-forming regions are much higher than in the interstellar medium, even when the regions have stellar densities as low as in the Galactic field, due to the presence of intermediate-mass, and massive, stars (>5 M⊙). These strong radiation fields lead to the destruction of the gas component in protoplanetary discs within 1 Myr, implying that gas giant planets must either form extremely rapidly (<1 Myr), or that they exclusively form in star-forming regions like Taurus, which contain no intermediate-mass or massive stars. The latter scenario is in direct tension with meteoritic evidence from the Solar system that suggests the Sun and its protoplanetary disc was born in close proximity to massive stars. [ABSTRACT FROM AUTHOR]

We use the measured fluxes of polycyclic aromatic hydrocarbon (PAH) emission features at 6.2, 7.7, 8.6, 11.0, and 11.2 μ m in the reflection nebula NGC 2023 to carry out a principal component analysis (PCA) as a means to study previously reported variations in the PAH emission. We find that almost all of the variations (99 per cent) can be explained with just two parameters – the first two principal components (PCs). We explore the characteristics of these PCs and show that the first PC (PC 1), which is the primary driver of the variation, represents the amount of emission of a mixture of PAHs with ionized species dominating over neutral species. The second PC (PC 2) traces variations in the ionization state of the PAHs across the nebula. Correlations of the PCs with various PAH ratios show that the 6.2 and 7.7 μ m bands behave differently than the 8.6 and 11.0 μ m bands, thereby forming two distinct groups of ionized bands. We compare the spatial distribution of the PCs to the physical conditions, in particular to the strength of the radiation field, G 0, and the G 0/ n H ratio and find that the variations in PC 2, i.e. the ionization state of PAHs are strongly affected by G 0 whereas the amount of PAH emission (as traced by PC 1) does not depend on G 0. [ABSTRACT FROM AUTHOR]

Biphenyl (C12H10), or phenylbenzene, is an important building block of polycyclic aromatic hydrocarbons (PAHs), whose infrared spectral features are present in a variety of galactic and extragalactic sources. In this work, we use synchrotron radiation coupled with time-of-flight spectrometry to study the photoionization and photodissociation processes of biphenyl upon its interaction with soft X-ray photons at energies around the inner-shell C1s resonance. These results are compared with our previous studies with benzene (C6H6) and naphthalene (C10H8), and discussed in the context of four planetary nebulae featuring PAH infrared emission: BD+30○3639, NGC 7027, NGC 5315, and NGC 40. We show that the mass spectrum of biphenyl before the C1s resonance energy is dominated by single photoionization processes leading to C6H |$_{5}^+$|⁠ , C6H |$_{4}\, ^{+\cdot}$|⁠ , and C12H |$_{10}\, ^{+\cdot}$|⁠ , while after the resonance dissociation following multiple photoionization processes is dominant. The release of neutral C6H6 and C6H |$_{5}\, ^\cdot$| species accounts for one of the most relevant dissociation processes starting from the doubly ionized biphenyl, indicating that heterolytic charge separation of the two phenyl rings is also achieved. By using quantum chemical calculations, we show that the biphenylic structure is a high-lying isomer of the singly and doubly ionized C12H10 species, whose minimum energy geometries are related to the acenaphthene molecule, composed of a C2-bridged naphthalene. Furthermore, we estimate the lifetime of biphenyl for 275 and 310 eV in photon-dominated regions of planetary nebulae. We discuss distinct processes that may enhance its lifetime and those of other small-sized PAHs in such astrophysical environments. [ABSTRACT FROM AUTHOR]

We combine Multi-Unit Spectroscopic Explorer and Atacama Large Millimeter/sub-millimeter Array observations with theoretical models to evaluate how a tadpole-shaped globule located in the Carina Nebula has been influenced by its environment. This globule is now relatively small (radius ∼2500 au), hosts a protostellar jet+outflow (HH 900), and, with a blueshifted velocity of ∼10 km s−1, is travelling faster than it should be if its kinematics were set by the turbulent velocity dispersion of the precursor cloud. Its outer layers are currently still subject to heating, but comparing the internal and external pressures implies that the globule is in a post-collapse phase. Intriguingly the outflow is bent, implying that the Young Stellar Object (YSO) responsible for launching it is comoving with the globule, which requires that the star formed after the globule was up to speed since otherwise it would have been left behind. We conclude that the most likely scenario is one in which the cloud was much larger before being subject to radiatively driven implosion, which accelerated the globule to the high observed speeds under the photoevaporative rocket effect and triggered the formation of the star responsible for the outflow. The globule may now be in a quasi-steady state following collapse. Finally, the HH 900 YSO is likely ≳1 M⊙ and may be the only star forming in the globule. It may be that this process of triggered star formation has prevented the globule from fragmenting to form multiple stars (e.g. due to heating) and has produced a single higher mass star. [ABSTRACT FROM AUTHOR]

The aim of this work is to study structure and gas kinematics in the photodissociation regions (PDRs) around the compact H  ii regions S235 A and S235 C. We observe the [C  ii ], [13C  ii ], and [O  i ] line emission, using SOFIA/upGREAT, and complement them by data of HCO+ and CO. We use the [13C  ii ] line to measure the optical depth of the [C  ii ] emission, and find that the [C  ii ] line profiles are influenced by self-absorption, while the [13C  ii ] line remains unaffected by these effects. Hence, for dense PDRs, [13C  ii ] emission is a better tracer of gas kinematics. The optical depth of the [C  ii ] line is up to 10 in S235 A. We find an expanding motion of the [C  ii ]-emitting layer of the PDRs into the front molecular layer in both regions. Comparison of the gas and dust columns shows that gas components visible neither in the [C  ii ] nor in low- J CO lines may contribute to the total column across S235 A. We test whether the observed properties of the PDRs match the predictions of spherical models of expanding H  ii region + PDR + molecular cloud. Integrated intensities of the [13C  ii ], [C  ii ], and [O  i ] lines are well represented by the model, but the models do not reproduce the double-peaked [C  ii ] line profiles due to an insufficient column density of C+. The model predicts that the [O  i ] line could be a more reliable tracer of gas kinematics, but the foreground self-absorbing material does not allow using it in the considered regions. [ABSTRACT FROM AUTHOR]

We present new Atacama Large Millimeter/submillimeter Array observations of the tadpole, a small globule in the Carina Nebula that hosts the HH 900 jet+outflow system. Our data include 12CO, 13CO, C18O J =2–1, 13CO, C18O J =3–2, and serendipitous detections of DCN J =3–2 and CS J =7–6. With angular resolution comparable to the Hubble Space Telescope , our data reveal for the first time the bipolar molecular outflow in CO, seen only inside the globule, that is launched from the previously unseen jet-driving protostar (the HH 900 YSO). The biconical morphology joins smoothly with the externally irradiated outflow seen in ionized gas tracers outside the globule, tracing the overall morphology of a jet-driven molecular outflow. Continuum emission at the location of the HH 900 YSO appears to be slightly flattened perpendicular to outflow axis. Model fits to the continuum have a best-fitting spectral index of ∼2, suggesting cold dust and the onset of grain growth. In position–velocity space, 13CO and C18O gas kinematics trace a C-shaped morphology, similar to infall profiles seen in other sources, although the global dynamical behaviour of the gas remains unclear. Line profiles of the CO isotopologues display features consistent with externally heated gas. We estimate a globule mass of ∼1.9 M⊙, indicating a remaining lifetime of ∼4 Myr, assuming a constant photoevaporation rate. This long globule lifetime will shield the disc from external irradiation perhaps prolonging its life and enabling planet formation in regions where discs are typically rapidly destroyed. [ABSTRACT FROM AUTHOR]

We present the warpfield emission predictor, warpfield-emp , which couples the 1D stellar feedback code warpfield with the cloudy H  ii region/PDR code and the polaris radiative transfer code, in order to make detailed predictions for the time-dependent line and continuum emission arising from the H  ii region and PDR surrounding an evolving star cluster. warpfield-emp accounts for a wide range of physical processes (photoionization, stellar winds, supernovae, radiation pressure, gravity, thermal conduction, radiative cooling, dust extinction etc.) and yet runs quickly enough to allow us to explore broad ranges of different model parameters. We compare the results of an extensive set of models with SITELLE observations of a large sample of H  ii regions in NGC 628 and find very good agreement, particularly for the highest signal-to-noise observations. We show that our approach of modelling individual clouds from first principles (instead of in terms of dimensionless quantities such as the ionization parameter) allows us to avoid long-standing degeneracies in the interpretation of H  ii region diagnostics and enables us to relate these diagnostics to important physical parameters such as cloud mass or cluster age. Finally, we explore the implications of our models regarding the reliability of simple metallicity diagnostics, the properties of long-lived embedded clusters, and the role played by winds and supernovae in regulating H  ii region and PDR line emission. [ABSTRACT FROM AUTHOR]

We study the anomalous microwave emission (AME) in the Lynds Dark Nebula (LDN) 1780 on two angular scales. With publicly available data at an angular resolution of 1°, we studied the spectral energy distribution of the cloud in the 0.408–2997 GHz frequency range. The cloud presents a significant (>20 σ) amount of AME, making it one of the clearest examples of AME on 1 ° scales, and its spectrum can be well fitted with a spinning dust (SD) model. We also find at these angular scales that the location of the peak of the emission at lower frequencies (23–70 GHz) differs from the location at the higher frequencies (90–3000 GHz) maps. In addition to the analysis on 1° angular scales, we present data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at 31 GHz with an angular resolution of 2 arcmin, in order to study the origin of the AME in LDN 1780. We studied morphological correlations between the CARMA map and different infrared tracers of dust emission. We found that the best correlation is with the 70-  μ m template, which traces warm dust (T ∼ 50 K). Finally, we study the difference in radio emissivity between two locations within the cloud. We measured a factor of ≈6 difference in 31-GHz emissivity. We show that this variation can be explained, using the SD model, by a variation on the dust grain size distribution across the cloud, particularly changing the fraction of polycyclic aromatic hydrocarbon for a fixed total amount of carbon. [ABSTRACT FROM AUTHOR]

Recent stacked ALMA observations have revealed that normal, star-forming galaxies at z ≈ 6 are surrounded by extended (≈10 kpc) [C  ii ]-emitting haloes, which are not predicted by the most advanced, zoom-in simulations. We present a model in which these haloes are the result of supernova-driven cooling outflows. Our model contains two free parameters, the outflow mass loading factor, η , and the parent galaxy dark matter halo circular velocity, v c. The outflow model successfully matches the observed [C  ii ] surface brightness profile if η = 3.20 ± 0.10 and |$v_{\rm c} = 170 \pm 10 \, \rm km\, s^{-1}$|⁠ , corresponding to a dynamical mass of |${\approx }10^{11}\, {\rm M}_{\odot }$|⁠. The predicted outflow rate and velocity range are |$128 \pm 5\, {\rm M}_{\odot }\, {\rm yr}^{-1}$| and 300–500  |$\, \rm km\, s^{-1}$|⁠ , respectively. We conclude that (a) extended haloes can be produced by cooling outflows; (b) the large η value is marginally consistent with starburst-driven outflows, but it might indicate additional energy input from active galactic nuclei; and (c) the presence of [C  ii ] haloes requires an ionizing photon escape fraction from galaxies f esc ≪ 1. The model can be readily applied also to individual high- z galaxies, as those observed, e.g. by the ALMA ALPINE survey now becoming available. [ABSTRACT FROM AUTHOR]

We present a novel method to simultaneously characterize the star formation law and the interstellar medium properties of galaxies in the epoch of reionization (EoR) through the combination of [C  ii ] 158  μ m (and its known relation with star formation rate) and C  iii ] λ1909 Å emission line data. The method, based on a Markov chain Monte Carlo algorithm, allows us to determine the target galaxy average density, n , gas metallicity, Z , and 'burstiness' parameter, κs, quantifying deviations from the Kennicutt–Schmidt relation. As an application, we consider COS-3018 (z  = 6.854), the only EoR Lyman Break Galaxy so far detected in both [C  ii ] and C  iii ]. We show that COS-3018 is a moderate starburst (κs ≈ 3), with |$Z \approx 0.4 \, \mathrm{Z}_{\odot }$|⁠ , and |$n \approx 500\, {\rm cm^{-3}}$|⁠. Our method will be optimally applied to joint ALMA and James Webb Space Telescope targets. [ABSTRACT FROM AUTHOR]

Despite the many studies in the last decades, the low-ionization structures (LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imaging surveys have demonstrated that LISs are composed of molecular gas. Here, we report H2 emission in the LISs of NGC 7009 and NGC 6543 by means of very deep narrow-band H2 images taken with NIRI@Gemini. The surface brightness of the H2 1-0 S(1) line is estimated to be (0.46–2.9)× 10−4 erg s−1 cm−2 sr−1 in NGC 7009 and (0.29–0.48)× 10−4 erg s−1 cm−2 sr−1 in NGC 6543, with signal-to-noise ratios of 10–42 and 3–4, respectively. These findings provide further confirmation of hidden H2 gas in LISs. The emission is discussed in terms of the recent proposed diagnostic diagram R (H2) = H2 1-0 S(1)/H2 2-1 S(1) versus R (Brγ) = H2 1-0 S(1)/Brγ, which was suggested to trace the mechanism responsible for the H2 excitation. Comparing our observations to shock and ultraviolet (UV) molecular excitation models, as well as a number of observations compiled from the literature showed that we cannot conclude for either UV or shocks as the mechanism behind the molecular emission. [ABSTRACT FROM AUTHOR]

We report the first spectroscopic observations of a background star seen through the region between the ionization front and the dissociation front of the nebula IC 63. This photodissociation region (PDR) exhibits intense extended red emission (ERE) attributed to fluorescence by large molecules/ions. We detected strong diffuse interstellar bands (DIB) in the stellar spectrum, including an exceptionally strong and broad DIB at λ4428. The detection of strong DIBs in association with ERE could be consistent with the suggestion that the carriers of DIBs and ERE are identical. The likely ERE process is recurrent fluorescence, enabled by inverse internal conversions from highly excited vibrational levels of the ground state to low-lying electronic states with subsequent transitions to ground. This provides a path to rapid radiative cooling for molecules/molecular ions, greatly enhancing their ability to survive in a strongly irradiated environment. The ratio of the equivalent widths (EWs) of DIBs λ5797 and λ5780 in IC 63 is the same as that observed in the low-density interstellar medium with ultraviolet (UV) interstellar radiation fields (ISRF) weaker by at least two orders of magnitude. This falsifies suggestions that the ratio of these two DIBs can serve as a measure of the UV strength of the ISRF. Observations of the nebular spectrum of the PDR of IC 63 at locations immediately adjacent to where DIBs were detected failed to reveal any presence of sharp emission features seen in the spectrum of the Red Rectangle nebula. This casts doubts upon proposals that the carriers of these features are the same as those of DIBs seen at slightly shorter wavelengths. [ABSTRACT FROM AUTHOR]

We demonstrate the utility of C  i as a tracer of photoevaporative winds that are being driven from discs by their ambient UV environment. Commonly observed CO lines only trace these winds in relatively weak UV environments and are otherwise dissociated in the wind at the intermediate to high UV fields that most young stars experience. However, C  i traces unsubtle kinematic signatures of a wind in intermediate UV environments (∼1000 G0) and can be used to place constraints on the kinematics and temperature of the wind. In C  i position–velocity (PV) diagrams external photoevaporation results in velocities that are faster than those from Keplerian rotation alone, as well as emission from quadrants of PV space in which there would be no Keplerian emission. This is independent of viewing angle because the wind has components that are perpendicular to the azimuthal rotation of the disc. At intermediate viewing angles (∼30–60°) moment 1 maps also exhibit a twisted morphology over large scales (unlike other processes that result in twists, which are typically towards the inner disc). C  i is readily observable with ALMA, which means that it is now possible to identify and characterize the effect of external photoevaporation on planet-forming discs in intermediate UV environments. [ABSTRACT FROM AUTHOR]

A tight relation between the [C  ii ] 158  |$\mu$| m line luminosity and star formation rate is measured in local galaxies. At high redshift (z > 5), though, a much larger scatter is observed, with a considerable (15–20 per cent) fraction of the outliers being [C  ii ]-deficient. Moreover, the [C  ii ] surface brightness (⁠|$\Sigma_{\rm [C\, \small {II}]}$|⁠) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [C  ii ]-deficiency, we have developed an analytical model that fits local [C  ii ] data and has been validated against radiative transfer simulations performed with cloudy. The model predicts an overall increase of |$\Sigma_{\rm [C\, \small {II}]}$|  with ΣSFR. However, for ΣSFR  |${\gtrsim} 1 \, \mathrm{M}_\odot \,{\rm yr}^{-1}\,{\rm kpc}^{-2}$|⁠ , |$\Sigma_{\rm [C\, \small {II}]}$|  saturates. We conclude that underluminous [C  ii ] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt–Schmidt relation (κs ≫ 1), parametrized by the 'burstiness' parameter κs; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [C  ii ] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [O  iii ]88  |$\mu$| m, C  iii ]1909 Å, CO lines). Simple formulae are given to interpret available data for low- and high- z galaxies. [ABSTRACT FROM AUTHOR]

We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV (⁠|$6\, {\rm eV} \lt {\rm h}\nu \lt 13.6\, {\rm eV}$|⁠) radiation, through 3D radiative transfer hydrodynamical simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H2. We run a set of simulations considering different clump masses (⁠|$M=10\!-\!200\, {\rm M}_{\odot }$|⁠) and impinging fluxes (G 0 = 2 × 103 to 8 × 104 in Habing units). In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H2 mass by a factor |$40\!-\!90{{\ \rm per\ cent}}$|⁠. Then, a weak (⁠|$\mathcal {M}\simeq 2$|⁠) shock develops and travels towards the centre of the clump, which collapses while losing mass from its surface. All considered clumps remain gravitationally unstable even if radiation rips off most of the clump mass, showing that external FUV radiation is not able to stop clump collapse. However, the FUV intensity regulates the final H2 mass available for star formation: for example, for G 0 < 104 more than 10 per cent of the initial clump mass survives. Finally, for massive clumps (⁠|${\gtrsim } 100\, {\rm M}_{\odot }$|⁠) the H2 mass increases by |$25\!-\!50{{\ \rm per\ cent}}$| during the collapse, mostly because of the rapid density growth that implies a more efficient H2 self-shielding. [ABSTRACT FROM AUTHOR]

High-resolution ALMA images towards the Orion Bar show no discernible offset between the peak of H2 emission in the photodissociation region (PDR) and the 13CO(3–2) and HCO+(4–3) emission in the molecular region. This implies that positions of H2 and CO dissociation fronts are indistinguishable in the limit of ALMA resolution. We use the chemo-dynamical model MARION to show that the ALMA view of the Orion Bar, namely, no appreciable offset between the 13CO(3–2) and HCO+(4–3) peaks, merged H2 and CO dissociation fronts, and high intensity of HCO+(4–3) emission, can only be explained by the ongoing propagation of the dissociation fronts through the molecular cloud, coupled to the dust motion driven by the stellar radiation pressure, and are not reproduced in the model where the dissociation fronts are assumed to be stationary. Modelling line intensities, we demonstrate that after the fronts have merged, the angular separation of the 13CO(3–2) and HCO+(4–3) peaks is indeed unresolvable with the ALMA observations. Our model predictions are consistent with the results of the ALMA observations about the relation of the bright HCO+(4–3) emission to the compressed gas at the border of the PDR in the sense that the theoretical HCO+(4–3) peak does correspond to the gas density enhancement, which naturally appears in the dynamical simulation, and is located near the H2 dissociation front at the illuminated side of the CO dissociation front. [ABSTRACT FROM AUTHOR]

In this work, we perform molecular dynamic (MD) simulations to investigate the stability and fragmentation processes of vibrationally excited linear polycyclic aromatic hydrocarbons (PAHs). The program of CP2K in combination with the semi-empirical method PM3 is utilized for the MD simulations. The simulations show that the formation of molecular hydrogens (H2) is different than previous studies, in particular, different than compact PAHs. At high temperatures, linear PAHs tend to open aromatic rings and convert the sp3 C–C or sp2 C=C bonds to sp C≡C bonds by removing H2; i.e. polyynes are formed in such process. Besides polyynes, PAHs attached with sp-bonded polyyne chains are commonly observed at high temperatures. We notice that due to the addition of flexible tails (polyynes), the ring-polyyne molecules do not dissociate for a long period of time at high temperatures. Such structures facilitate the molecules to survive in the harsh environment of the interstellar medium. In addition, the ring-polyyne structures induce dipole moments that could, in principle, be detected by radio astronomy. [ABSTRACT FROM AUTHOR]

When studying chemistry of photodissociation regions (PDRs), time dependence becomes important as visual extinction increases, since certain chemical time-scales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high G 0 PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O2), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long time-scales promote the destruction of oxygen-bearing molecules in the inner parts of low G 0 PDRs, while favouring their formation and that of carbon-bearing molecules in high G 0 PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO2 is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H2O in lower layers than CO2. Regarding steady state, the PDR edge reaches chemical equilibrium at early times (≲105 yr). This time is even shorter (<104 yr) for high G 0 PDRs. By contrast, inner regions reach equilibrium much later, especially low G 0 PDRs, where steady state is reached at ∼106–107 yr. [ABSTRACT FROM AUTHOR]

We analyse N -body simulations of star-forming regions to investigate the effects of external far- and extreme-ultraviolet photoevaporation from massive stars on protoplanetary discs. By varying the initial conditions of simulated star-forming regions, such as the spatial distribution, net bulk motion (virial ratio), and density, we investigate which parameters most affect the rate at which discs are dispersed due to external photoevaporation. We find that disc dispersal due to external photoevaporation is faster in highly substructured star-forming regions than in smooth and centrally concentrated regions. Subvirial star-forming regions undergoing collapse also show higher rates of disc dispersal than regions that are in virial equilibrium or are expanding. In moderately dense (∼100 M⊙ pc−3) regions, half of all protoplanetary discs with radii ≥100 au are photoevaporated within 1 Myr, three times faster than is currently suggested by observational studies. Discs in lower density star-forming regions (∼10 M⊙ pc−3) survive for longer, but half are still dispersed on short time-scales (∼2 Myr). This demonstrates that the initial conditions of the star-forming regions will greatly impact the evolution and lifetime of protoplanetary discs. These results also imply that either gas giant planet formation is extremely rapid and occurs before the gas component of discs is  evaporated, or gas giants only form in low-density star-forming regions where no massive stars are present to photoevaporate gas from protoplanetary discs. [ABSTRACT FROM AUTHOR]

Modern observations of the interstellar medium (ISM) in galaxies detect a variety of atomic and molecular species. The goal is to connect these observations to the astrochemical properties of the ISM. 3D hydro-chemical simulations attempt this but due to extreme computational cost, they have to rely on simplified chemical networks and are bound to individual case studies. We present an alternative approach which models the ISM at larger scales by an ensemble of pre-calculated 1D thermo-chemical photodissociation region (PDR) calculations that determine the abundance and excitation of atomic and molecular species. We adopt lognormal distributions of column density (A V-PDFs) for which each column density is linked to a volume density as derived by hydrodynamical simulations. We consider two lognormal A V-PDFs: a diffuse, low-density medium with average visual extinction of |$\overline{{\rm A}_\mathrm{ V}}=0.75\, {\rm mag}$| and dispersion of σ = 0.5 and a denser giant molecular cloud with |$\overline{{\rm A}_\mathrm{ V}}=4\, {\rm mag}$| and σ = 0.8. We treat the UV radiation field, cosmic ray ionization rate, and metallicity as free parameters. We find that the low-density medium remains fully H  i - and C  ii -dominated under all explored conditions. The denser cloud remains almost always molecular (i.e. H2-dominated) while its carbon phase (CO, C  i, and C  ii) is sensitive to the above free parameters, implying that existing methods of tracing H2-rich gas may require adjustments depending on environment. Our numerical framework can be used to estimate the PDR properties of large ISM regions and quantify trends with different environmental parameters as it is fast, covers wide parameter space, and is flexible for extensions. [ABSTRACT FROM AUTHOR]

Computing the flow from externally far-ultraviolet (FUV) irradiated protoplanetary discs requires solving complicated and expensive photodissociation physics iteratively in conjunction with hydrodynamics. Previous studies have therefore been limited to 1D models of this process. In this paper, we compare 2D axisymmetric models of externally photoevaporating discs with their 1D analogues, finding that mass-loss rates are consistent to within a factor of 4. The mass-loss rates in 2D are higher, in part because half of the mass loss comes from the disc surface (which 1D models neglect). 1D mass-loss rates used as the basis for disc viscous evolutionary calculations are hence expected to be conservative. We study the anatomy of externally driven winds including the streamline morphology, kinematic, thermal, and chemical structure. A key difference between the 1D and 2D models is in the chemical abundances. For instance in the 2D models CO can be dissociated at smaller radial distances from the disc outer edge than in 1D calculations because gas is photodissociated by radiation along trajectories that are assumed infinitely optically thick in 1D models. Multidimensional models will hence be critical for predicting observable signatures of environmentally photoevaporating protoplanetary discs. [ABSTRACT FROM AUTHOR]

We present an open access grid of 3930 calculations of externally evaporating protoplanetary discs. This spans a range of disc sizes (1–400 au), disc masses, UV field strengths (10–104 G0), and stellar masses (0.05–1.9 M⊙). The grid is publicly available for download, and offers a means of cheaply including external photoevaporation in disc evolutionary calculations. It can also be queried using an online tool for quick estimates of instantaneous mass-loss rates (e.g for convenient evaluation of real observed systems). The '' fried'' grid itself illustrates that for discs around stars ≤0.3 M⊙, external photoevaporation is effective down to small radii (<50 au) down to UV fields at least as weak as 10 G0. At the other end of the scale, in a 104 G0 environment photoevaporation is effective down to 1 au even for stellar masses at least as high as 1.9 M⊙. We also illustrate in which regimes CO survives in the photoevaporative outflow for significant mass-loss rates; marking a system a good candidate to detect external photoevaporation in weak–intermediate UV environments through sub–Keplerian rotation. Finally, we make illustrative mass-loss rate estimates for discs in Taurus based on the Guilloteau et al. (2011) star–disc parameters, finding that around half are expected to have both significant mass-loss and retain CO in the photoevaporative outflow. [ABSTRACT FROM AUTHOR]

During the evolution of protoplanetary disc, photoevaporations of both central and external stars play important roles. Considering the complicated radiation surroundings in the clusters, where the star formed, the protoplanetary discs survive in different lifetimes due to flyby events. In this paper, we mainly focus on the disc around a T Tauri star, which encounters with another main-sequence star with different temperatures in hyperbolic orbits with different pericentre distances, eccentricities, and inclinations. We find the criterion for gap opening due to photoevaporation of central star after the flyby event. A gap is opened in the late stage of gas disc, and induces that the gap only influence the planet formation and migration limitedly. If the flyby orbit has a moderate value of pericentre distance, which weakly depends on the eccentricity and inclination, the external photoevaporation lead to a maximum mass loss during the flyby event. Flyby stars in orbits with smaller eccentricities or larger inclinations induce larger mass loss. Adopting a simple multiple flyby models, we conclude that in open clusters, gas discs usually survive in typical lifetimes between 1 and 10 Myr, except there are many massive stars in dense open clusters. In globular clusters, discs disperse very quickly and hardly produce the gas giant planets. The fast-depleted discs are probably responsible for the null detection of giant planets in globular clusters. [ABSTRACT FROM AUTHOR]

Rate constants for the formation of carbon monosulfide (CS) by radiative association are calculated using accurate molecular data. The rate constants include both direct and indirect formation processes. The indirect processes (inverse rotational and electronic predissociation) are evaluated for conditions of local thermodynamic equilibrium (LTE) and also in the non-LTE limit of zero radiation temperature and atomic density. Phenomenological rate constants for CS formation in realistic astrophysical environments are expected to lie in-between these limiting cases. An analytic formula is used to fit the rate constants for convenient use in astrophysical models. The impact of the results on various astrophysical environments is briefly discussed. [ABSTRACT FROM AUTHOR]

We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1-like system under the action of accretion and external photoevaporation in different radiation environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed 4M⊕ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams distribution of UV fields, typical initial disc masses have to be >30 per cent the stellar (which are still Toomre Q stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor of 2 higher than that which would be lost by accretion on to the star during 10Myr of evolution. Consequently, even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive (>10 per cent of the stellar mass). [ABSTRACT FROM AUTHOR]

Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellarmedium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated threedimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. [ABSTRACT FROM AUTHOR]

We conducted high-sensitivity, high-velocity resolution CO(1-0) observations in a region containing a portion of the diffuse molecular cloud MBM 53 to determine whether weak CO emission was present. The results of our observations increase the amount of CO-detectable molecular gas in the region by a factor of 2. The increased molecular mass for the cloud, if applicable to the molecular clouds in the entire Pegasus-Pisces region, decreases the dark molecular gas content from 58 per cent of the total H2 mass to ~30 per cent. If the results for MBM53 are applicable to other diffuse clouds, then the fraction of dark gas directly detectable via sensitive CO(1-0) observations in diffuse molecular clouds is similar to that predicted by models for giant molecular clouds. [ABSTRACT FROM AUTHOR]

We study the photoevaporation of molecular clumps exposed to a UV radiation field including hydrogen-ionizing photons (hv > 13.6 eV) produced by massive stars or quasars. We follow the propagation and collision of shockwaves inside clumps and take into account self-shielding effects, determining the evolution of clump size and density with time. The structure of the ionization-photodissociation region is obtained for different initial clump masses (M = 0.01-104M⊙) and impinging fluxes (G0 = 102-105 in units of the Habing flux). The cases of molecular clumps engulfed in the HII region of an OB star and clumps carried within quasar outflows are treated separately. We find that the clump undergoes in both cases an initial shock-contraction phase and a following expansion phase, which lets the radiation penetrate in until the clump is completely evaporated. Typical evaporation time-scales are ≃0.01 Myr in the stellar case and 0.1 Myr in the quasar case, where the clump mass is 0.1 M⊙ and 10³M⊙, respectively. We find that clump lifetimes in quasar outflows are compatible with their observed extension, suggesting that photoevaporation is the main mechanism regulating the size of molecular outflows. [ABSTRACT FROM AUTHOR]

At the interface of HII regions and molecular gas, peculiar structures appear, some of them with pillar-like shapes. Understanding their origin is important for characterizing triggered star formation and the impact of massive stars on the interstellar medium. In order to study the molecular environment and influence of radiation on two pillar-like features related to the HII region G46.5-0.2, we performed molecular line observations with the Atacama Submillimeter Telescope Experiment and spectroscopic optical observations with the Isaac Newton Telescope. From the optical observations, we identified the star that is exciting the HII region as spectral type O4-6. The molecular data allowed us to study the structure of the pillars and an HCO+ cloud lying between them. In this HCO+ cloud, which has no well-defined 12CO counterpart, we found direct evidence of star formation: two molecular outflows and two associated near-IR nebulosities. The outflow axis orientation is perpendicular to the direction of the radiation flow from the HII region. Several Class I sources are also embedded in this HCO+ cloud, showing that it is usual that young stellar objects (YSOs) form large associations occupying a cavity bounded by pillars. On the other hand, it was confirmed that the radiation-driven implosion (RDI) process is not occurring in one of the pillar tips. [ABSTRACT FROM AUTHOR]

We analysed new wide-field, wide- and narrow-band optical images of IC 59 and IC 63, two nebulae that are externally illuminated by the early B-star β Cas, with the objective of mapping the extended red emission (ERE), a dust-related photoluminescence process that is still poorly understood, in these two clouds. The spatial distribution of the ERE relative to the direction of the incident radiation and relative to other emission processes, whose carriers and excitation requirements are known, provides important constraints on the excitation of the ERE. In both nebulae, we find the ERE intensity to peak spatially well before the more extended distribution of mid-infrared emission in the unidentified infrared bands, supporting earlier findings that point towards far-ultraviolet (11 < Ephoton < 13.6 eV) photons as the source of ERE excitation. The band-integrated absolute intensities of the ERE in IC 59 and IC 63 measured relative to the number density of photons available for ERE excitation are lower by about two orders of magnitude compared to ERE intensities observed in the high-latitude diffuse interstellar medium. This suggests that the lifetime of the ERE carriers is significantly reduced in the more intense radiation field prevailing in IC 59 and IC 63, pointing towards potential carriers that are only marginally stable against photoprocessing under interstellar conditions. A model involving isolated molecules or molecular ions, capable of inverse internal conversion and recurrent fluorescence, appears to provide the most likely explanation for our observational results. [ABSTRACT FROM AUTHOR]

We consider the evolution of an isentropic thermal instability in the atomic zone of a photodissociation region (PDR). In this zone, gas heating and cooling are associated mainly with photoelectric emission from dust grains and fine-structure lines ([C II] 158, [O I] 63 and [O I] 146 μm), respectively. The instability criterion has a multi-parametric dependence on the conditions of the interstellar medium. We found that instability occurs when the intensity of the incident far-ultraviolet field G0 and gas density n are high. For example, we have 3 × 10 3 < G0 < 106 and 4.5 × 104 < n < 106 cm-3 at temperatures 360 < T < 104 K for optical thin fine-structure lines and for typical carbon and oxygen abundances ξC =1.4×10-4 and ξO = 3.2 × 10-4 If we take line opacities into account, these ranges of G0, n and T are expanded. Moreover, the instability criterion depends significantly on the relation between ξC and ξO abundances. We also give examples of observed PDRs where instability could occur. For these PDRs, the characteristic perturbation growth time is t..KWDinst ~ 103-104 yr and the distance characterizing the formation of secondary waves is L ~ 10.-3-5 × 10.-2 pc. For objects that are older than tinst and have sizes of the atomic zone larger than L, we expect that instability influences the PDR structure significantly. The presence of multiple shock waves, turbulent velocities of several kilometres per second and inhomogeneities with higher density and temperature than the surrounding medium can characterize isentropic thermal instability in PDRs. [ABSTRACT FROM AUTHOR]

We model the radiatively driven flow from IM Lup - a large protoplanetary disc expected to be irradiated by only a weak external radiation field (at least 104 times lower than the ultraviolet field irradiating the Orion Nebula Cluster proplyds). We find that material at large radii (>400 au) in this disc is sufficiently weakly gravitationally bound that significant massloss can be induced. Given the estimated values of the disc mass and accretion rate, the viscous time-scale is long (~10 Myr) so the main evolutionary behaviour for the first Myr of the disc's lifetime is truncation of the disc by photoevaporation, with only modest changes effected by viscosity. We also produce approximate synthetic observations of our models, finding substantial emission from the flow that can explain the CO halo observed about IM Lup out to ≥ 1000 au. Solutions that are consistent with the extent of the observed CO emission generally imply that IM Lup is still in the process of having its disc outer radius truncated.We conclude that IM Lup is subject to substantial external photoevaporation, which raises the more general possibility that external irradiation of the largest discs can be of significant importance even in low mass star forming regions. [ABSTRACT FROM AUTHOR]