Interstellar Hydrides Gerin, Maryvonne; Neufeld, David A; Goicoechea, Javier R
Annual review of astronomy and astrophysics,
09/2016, Letnik:
54, Številka:
1
Journal Article
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Interstellar hydrides-that is, molecules containing a single heavy element atom with one or more hydrogen atoms-were among the first molecules detected outside the solar system. They lie at the root ...of interstellar chemistry, being among the first species to form in initially atomic gas, along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to FIR and submillimeter observations performed with the
Herschel Space Observatory
. In this review, we discuss observations of interstellar hydrides, along with the advanced modeling approaches that have been used to interpret them and the unique information that has thereby been obtained.
We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret ...previously published measurements of the abundance of four molecular ions: ArH+, OH+, , and . For diffuse atomic clouds at Galactocentric distances in the range , observations of ArH+, OH+, and imply a mean primary CRIR of per hydrogen atom, where . Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of and H2 imply a primary CRIR of per H atom, corresponding to a total ionization rate per H2 molecule of in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight line to Cas A. Here, our best-fit estimate for the primary CRIR is per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, , with a best-fit dependence for .
An infrared L- and M-band spectral survey, performed toward the young planetary nebula NGC 7027 with the iSHELL instrument on NASA’s Infrared Telescope Facility (IRTF), has revealed more than 20 ...vibrational lines of the molecules HeH+, H2, and CH+ and more than 50 spectral lines of atoms and atomic ions. The present paper focuses on the atomic line emissions, the molecular lines having been discussed in two previous publications. The atomic lines detected with high confidence in the 2.951–5.24 μm region covered (incompletely) by this survey comprise (1) six collisionally excited lines of metal ions that had previously been identified in astrophysical nebulae but for which the present observations provide the most accurate wavelength determinations obtained to date; (2) a spectral line at 4.6895 μm, not previously reported, for which the probable identification is the 4F7/2–4F9/2 fine-structure transition of Zn vi; (3) 39 recombination lines of H and He+, with upper states of principal quantum number up to 38 (H) or 24 (He+); (4) 10 recombination lines of the multielectron species He, C2+, and C3+.
Abstract
The unusual infrared emission patterns of CH
+
, recently detected in the planetary nebula NGC 7027, are examined theoretically with high-accuracy rovibrational wavefunctions and ab initio ...dipole moment curves. The calculated transition dipole moments quantitatively reproduce the observed
J
-dependent intensity variation, which is ascribed to underlying centrifugal distortion-induced interference effects. We discuss the implications of this anomalous behavior for astrochemical modeling of CH
+
production and excitation, and provide a simple expression to estimate the magnitude of this effect for other light diatomic molecules with small dipole derivatives.
Motivated by recent observations with Herschel/PACS, and the availability of new rate coefficients for the collisional excitation of CO, the excitation of warm astrophysical CO is revisited with the ...use of numerical and analytic methods. For the case of an isothermal medium, results have been obtained for a wide range of gas temperatures (100-5000 K) and H sub(2) densities (10 super(3)-10 super(9) cm super(-3)), and presented in the form of rotational diagrams, in which the logarithm of the column density per magnetic substate, log (N sub(J)/g sub(J)), is plotted for each state, as a function of its energy, E sub(J). For rotational transitions in the wavelength range accessible to Herschel/PACS, such diagrams are nearly linear when n(H sub(2)) > or =, slanted 10 super(8) cm super(-3). When n(H sub(2)) ~ 10 super(6.8)-10 super(8) cm super(-3), they exhibit significant negative curvature, whereas when n(H sub(2)) < or =, slant 10 super(4.8) cm super(-3), the curvature is uniformly positive throughout the PACS-accessible range. Thus, the observation of a positively curved CO rotational diagram does not necessarily require the presence of multiple temperature components. Indeed, for some sources observed with Herschel/PACS, the CO rotational diagrams show a modest positive curvature that can be explained by a single isothermal component. Typically, the required physical parameters are densities in the 10 super(4)-10 super(5) cm super(-3) range and temperatures close to the maximum at which CO can survive. Other sources exhibit rotational diagrams with more curvature than can be accounted for by a single temperature component. For the case of a medium with a power-law distribution of gas temperatures, dN/dT is proportional to T super(-)b, results have been obtained for H sub(2) densities 10 super(3)-10 super(9) cm super(-3) and power-law indices, b, in the range 1-5; such a medium can account for a CO rotational diagram that is more positively curved than any resulting from an isothermal medium.
For dark matter (DM) particles with masses in the 0.6-6mp range, we set stringent constraints on the interaction cross-sections for scattering with ordinary baryonic matter. These constraints follow ...from the recognition that such particles can be captured by-and thermalized within-the Earth, leading to a substantial accumulation and concentration of DM that interact with baryons. Here, we discuss the probability that DM intercepted by the Earth will be captured, the number of DM particles thereby accumulated over Earth's lifetime, the fraction of such particles retained in the face of evaporation, and the density distribution of such particles within the Earth. In the latter context, we note that a previous treatment of the density distribution of DM, presented by Gould and Raffelt and applied subsequently to DM in the Sun, is inconsistent with considerations of hydrostatic equilibrium. Our analysis provides an estimate of the DM particle density at Earth's surface, which may exceed 1014 cm−3, and leads to constraints on various scattering cross-sections, which are placed by (1) the lifetime of the relativistic proton beam at the Large Hadron Collider; (2) the orbital decay of spacecraft in low Earth orbit; (3) the vaporization rate of cryogenic liquids in well-insulated storage dewars; and (4) the thermal conductivity of Earth's crust. For the scattering cross-sections that were invoked recently in Barkana's original explanation for the anomalously deep 21 cm absorption reported by EDGES, DM particle masses in the 0.6-4mp range are excluded.
ABSTRACT We present a general parameter study in which the abundance of interstellar argonium (ArH+) is predicted using a model for the physics and chemistry of diffuse interstellar gas clouds. ...Results have been obtained as a function of UV radiation field, cosmic-ray ionization rate, and cloud extinction. No single set of cloud parameters provides an acceptable fit to the typical ArH+, OH+, and abundances observed in diffuse clouds within the Galactic disk. Instead, the observed abundances suggest that ArH+ resides primarily in a separate population of small clouds of total visual extinction of at most 0.02 mag per cloud, within which the column-averaged molecular fraction is in the range , while OH+ and reside primarily in somewhat larger clouds with a column-averaged molecular fraction ∼0.2. This analysis confirms our previous suggestion that the argonium molecular ion is a unique tracer of almost purely atomic gas.
To constrain the properties of dark matter (DM) that interacts with nucleons, we have conducted an experimental search for any anomalous heating of ordinary baryonic matter at 77 K. Our tabletop ...experiment is motivated by the possibility (discussed in a previous paper) that DM particles with masses in the ∼1-2 mp range could be captured by and concentrated within the Earth. For suitable parameters, this phenomenon could lead to a substantial density (∼1014 cm−3) of thermalized (300 K) DM particles at Earth's surface that would heat cooler baryonic matter. Our experiment involves precise differential measurements of the evaporation rate of liquid nitrogen in a storage dewar within which various materials are immersed. The results revealed no statistically significant detections of heating in the 27 elements with molar fractions 10−5 in Earth's crust. For material with the average composition of Earth's crust, our measurements imply a 3 upper limit of 1.32 × 10 − 27 n 14 − 1 ( m DM 2 m p ) − 1 2 cm 2 on the mean cross section for scattering with thermal HIDM at 300 K, where 1014 n14 cm−3 is the particle density at Earth's surface. In combination with a lower limit on the scattering cross section, obtained from a consideration of the heat flow through the Earth's crust, our experiment places an upper limit of 1.6 × 1013 cm−3 on the density of DM at the Earth's surface. This in turn significantly constrains the properties for any DM candidate that interacts with baryons.
Although they are only minor constituents of the interstellar medium, halogen-containing molecules are of special interest because of their unique thermochemistry. Here, we present a theoretical ...study of the chemistry of interstellar molecules containing the halogen elements chlorine and fluorine. We have modeled both diffuse and dense molecular clouds, making use of updated estimates for the rates of several key chemical processes. We present predictions for the abundances of the three halogen molecules that have been detected to date in the interstellar medium: HF, CF+, and HCl. As in our previous study of fluorine-bearing interstellar molecules, we predict HF to be the dominant gas-phase reservoir of fluorine within both diffuse and dense molecular clouds; we expect the Herschel Space Observatory to detect widespread absorption in the HF J = 1 - 0 transition. Our updated model now overpredicts the CF+ abundance by a factor 10 relative to observations of the Orion Bar; this discrepancy has widened because we now adopt a laboratory measurement of the CF+ dissociative recombination rate that is smaller than the estimate we adopted previously. This disagreement suggests that the reaction of C+ with HF proceeds more slowly than the capture rate assumed in our model; a laboratory measurement of this reaction rate would be very desirable. Our model predicts diffuse cloud HCl abundances that are similar to those predicted previously and detected tentatively toward Delta *z Oph. Two additional species are potentially detectable from photodissociation regions: the H2Cl+, and HCl+ molecular ions. Ortho-H2Cl+ has its lowest-lying transition in the millimeter spectral region observable from the ground, and the lowest rotational transition of HCl+ is observable with Herschel's HIFI instrument.
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