The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

https://zenodo.org/record/1233981/files/article.pdf

Photodissociation regions in the interstellar medium of galaxies

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fν(60 μm)/Fν(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fν(60 μm)/Fν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fν(60 μm)/Fν(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ nα, where α ≈ 1.4 . We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

https://iopscience.iop.org/article/10.1086/323046/pdf

Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium

▪ Abstract Observations of interstellar gas-phase and solid-state species in the 2.4–200 μm range obtained with the spectrometers on board the Infrared Space Observatory (ISO) are reviewed. Lines and bands caused by ices, polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms and molecules (in particular H2, CO, H2O, OH, and CO2) are summarized and their diagnostic capabilities illustrated. The results are discussed in the context of the physical and chemical evolution of star-forming regions, including photon-dominated regions, shocks, protostellar envelopes, and disks around young stars.

/pdf/iso-spectroscopy-of-gas-and-dust-from-molecular-clouds-to-3o3prmfaiq.pdf

ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks

Before the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favorable environments on early Earth. At this time, meteorites and interplanetary dust particles delivered organics such as nucleobases (the characteristic molecules of nucleotides) to warm little ponds whose wet–dry cycles promoted rapid polymerization. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth’s early evolution with complex prebiotic chemistry in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (less than a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean, but this increasing availability of “targets” for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissociation during dry periods, mean that the synthesis of nucleotides and their polymerization into RNA occurred in just one to a few wet–dry cycles. Under these conditions, RNA polymers likely appeared before 4.17 billion years ago.

https://www.pnas.org/content/pnas/114/43/11327.full.pdf

Origin of the RNA world: The fate of nucleobases in warm little ponds

A new hot line list for $^{14}$NH$_3$ is presented. The line list CoYuTe was constructed using an accurate, empirically refined potential energy surface and a CCSD(T)/aug-cc-pVQZ ab initio dipole moment surface of ammonia, previously reported. The line list is an improvement of the ammonia line list BYTe [Yurchenko et al., Mon. Not. R. Astron. Soc., 413, 1828 (2011)]. The CoYuTe line list covers wavenumbers up to 20000 cm$^{-1}$, i.e. wavelengths beyond 0.5 $\mu$m for temperatures up to 1500 K. Comparisons with the high temperature experimental data from the literature show excellent agrement for wavenumbers below 6000 cm$^{-1}$. The CoYuTe line list contains 16.9 billion transitions and is available from the ExoMol website (this http URL) and the CDS database.

ExoMol molecular line lists - XXXV. A rotation-vibration line list for hot ammonia

Context. Nitrogen chemistry in protoplanetary disks and the freeze-out on dust particles is key to understand the formation of nitrogen bearing species in early solar system analogs. In dense cores, 10% to 20% of the nitrogen reservoir is locked up in ices like NH3, NH + and OCN . So far, ammonia has not been detected beyond the snowline in protoplanetary disks. Aims. We aim to find gas-phase ammonia in a protoplanetary disk and characterize its abundance with respect to water vapor. Methods. Using HIFI on the Herschel Space Observatory we detect, for the first time, the ground-state rotational emission of orthoNH3 in a protoplanetary disk, around TW Hya. We use detailed models of the disk’s physical structure and the chemistry of ammonia and water to infer the amounts of gas-phase molecules of these species. We explore two radial distributions (extended across the disk and confined to <60 au like the millimeter-sized grains) and two vertical distributions (near the midplane and at intermediate heights above the midplane, where water is expected to photodesorb o icy grains) to describe the (unknown) location of the molecules. These distributions capture the e ects of radial drift and vertical settling of ice-covered grains. Results. The NH3 10‐00 line is detected simultaneously with H2O 110‐101 at an antenna temperature of 15.3 mK in the Herschel beam; the same spectrum also contains the N2H + 6‐5 line with a strength of 18.1 mK. We use physical-chemical models to reproduce the fluxes with assuming that water and ammonia are co-spatial. We infer ammonia gas-phase masses of 0.7-11.0 10 21 g, depending on the adopted spatial distribution, in line with previous literature estimates. For water, we infer gas-phase masses of 0.2-16.0 10 22 g, improving upon earlier literature estimates This corresponds to NH3/H2O abundance ratios of 7%-84%, assuming that water and ammonia are co-located. The inferred N2H + gas mass of 4.9 10 21 g agrees well with earlier literature estimates based on lower excitation transitions. This masses correspond to a disk-averaged abundances of 0.2‐17.0 10 11 , 0.1‐9.0 10 10 and 7.6 10 11 for NH3, H2O and N2H + respectively. Conclusions. Only in the most compact and settled adopted configuration is the inferred NH3/H2O consistent with interstellar ices and solar system bodies of 5%‐10%; all other spatial distributions require addition gas-phase NH3 production mechanisms. Volatile release in the midplane may occur via collisions between icy bodies if the available surface for subsequent freeze-out is significantly reduced, e.g., through growth of small grains into pebbles or larger.

First detection of gas-phase ammonia in a planet-forming disk NH3, N2H + and H2O in the disk around TW Hya

The first observation of a fine-structure transition in NGC 7027 at wavelengths greater than 30 microns is reported. The flux in the 63.2-micron line is found to be approximately 10 to the -16th W/sq cm. It is noted that if NGC 7027 lies at 1 kpc, this corresponds to a luminosity, in the line, approximately 30 times greater than the solar luminosity. The flux of the 88.35-micron line is less than 10 to the -17th W/sq cm, and the upper limit to the line flux at 51.8 microns is found to be 10 to the -16th W/sq cm. These upper limits imply an electron density that is less than 1.6 x 10 to the 5th/cu cm. It is noted that the continuum measurements obtained support a dust temperature of 90 K with an emissivity approximately equal to the reciprocal of the square of the wavelength. The data exclude grain materials whose wavelength-dependent emissivity (the reciprocal of the wavelength raised to nth power) requires n to be outside the range between 1.6 and 2, unless the temperature varies drastically across the dust cloud

Far-infrared emission line and continuum observations of NGC 7027

/pdf/orientation-of-planetary-nebulae-within-the-galaxy-wlh4gnnrn7.pdf

Orientation of Planetary Nebulae Within the Galaxy

The 51.8 micron fine structure transition P2:3P2 3P1 for doubly ionized oxygen was observed in the Orion nebula. The observed line strength is of 5 plus or minus 3 times 10 to the minus 15th power watt/sq cm is in good agreement with theoretical predictions. Observations are consistent with the newly predicted 51.8 micron line position. The line lies close to an atmospheric water vapor feature at 51.7 micron, but is sufficiently distant so that corrections for this feature are straightforward. Observations of the 51.8 (O III) line are particularly important since the previously discovered 88 micron line from the same ion also is strong. This pair of lines should, therefore, yield new data about densities in observed H II regions; or else, if density data already are available from radio or other observations, the lines can be used to determine the differential dust absorption between 52 and 88 micron in front of heavily obscured regions.

/pdf/observations-of-the-51-8-micron-o-iii-emission-line-in-orion-37j0eu948y.pdf

Observations of the 51.8 micron (O III) emission line in Orion

The 51.8-micron forbidden O III line has been detected in four H II regions: M42, M17, W51, and NGC 6357A. The respective line strengths are 7 x 10 to the -15th, 1.0 x 10 to the -14th, 2.1 x 10 to the -15th, and 2.6 x 10 to the -15th W/sq cm. The observations are consistent with a previously reported line position and place the line at 51.80 + or - 0.05-micron. When combined with the 88.35-micron forbidden O III observations reported earlier, clumpiness is found to be an important factor in NGC 6357A and M42 and nonnegligible in W51 and M17. The combined data also suggest an O III abundance of about 0.0003 times the electron density, which is a factor of 2 greater than a number of investigators have reported.

51.8 micron forbidden O III line emission observed in four galatic H II regions

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