Engineering topics

Rehill, Una S.

Publications and source records attributed to Rehill, Una S..

5 records

DOE OSTI2024

Ultraviolet H 2 luminescence in molecular clouds induced by cosmic rays

Context. Galactic cosmic rays (CRs) play a crucial role in ionisation, dissociation, and excitation processes within dense cloud regions where UV radiation is absorbed by dust grains and gas species. CRs regulate the abundance of ions and radicals, leading to the formation of more and more complex molecular species, and determine the charge distribution on dust grains. A quantitative analysis of these effects is essential for understanding the dynamical and chemical evolution of star-forming regions. Aims. The CR-induced photon flux has a significant impact on the evolution of the dense molecular medium in its gas and dust components. This study evaluates the flux of UV photons generated by CRs to calculate the photon-induced dissociation and ionisation rates of a vast number of atomic and molecular species, as well as the integrated UV photon flux. Methods. To achieve these goals, we took advantage of recent developments in the determination of the spectra of secondary electrons, in the calculation of state-resolved excitation cross sections of H 2 by electron impact, and of photodissociation and photoionisation cross sections. Results. We calculated the H 2 level population of each rovibrational level of the X, B, C, B′, D, B″, D′, and a states. We then computed the UV photon spectrum of H 2 in its line and continuum components between 72 and 700 nm, with unprecedented accuracy, as a function of the CR spectrum incident on a molecular cloud, the H 2 column density, the isomeric H 2 composition, and the dust properties. The resulting photodissociation and photoionisation rates are, on average, lower than previous determinations by a factor of about 2, with deviations of up to a factor of 5 for the photodissociation of species such as AlH, C 2 H 2 , C 2 H 3 , C 3 H 3 , LiH, N 2 , NaCl, NaH, O 2 + , S2, SiH, l-C 4 , and l-C 5 H. A special focus is given to the photoionisation rates of H 2 , HF, and N 2 , as well as to the photodissociation of H 2 , which we find to be orders of magnitude higher than previous estimates. We give parameterisations for both the photorates and the integrated UV photon flux as a function of the CR ionisation rate, which implicitly depends on the H 2 column density, as well as the dust properties.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Ultraviolet H 2 luminescence in molecular clouds induced by cosmic rays

Context. Galactic cosmic rays (CRs) play a crucial role in ionisation, dissociation, and excitation processes within dense cloud regions where UV radiation is absorbed by dust grains and gas species. CRs regulate the abundance of ions and radicals, leading to the formation of more and more complex molecular species, and determine the charge distribution on dust grains. A quantitative analysis of these effects is essential for understanding the dynamical and chemical evolution of star-forming regions. Aims. The CR-induced photon flux has a significant impact on the evolution of the dense molecular medium in its gas and dust components. This study evaluates the flux of UV photons generated by CRs to calculate the photon-induced dissociation and ionisation rates of a vast number of atomic and molecular species, as well as the integrated UV photon flux. Methods. To achieve these goals, we took advantage of recent developments in the determination of the spectra of secondary electrons, in the calculation of state-resolved excitation cross sections of H 2 by electron impact, and of photodissociation and photoionisation cross sections. Results. We calculated the H 2 level population of each rovibrational level of the X, B, C, B', D, B", D', and a states. We then computed the UV photon spectrum of H 2 in its line and continuum components between 72 and 700 nm, with unprecedented accuracy, as a function of the CR spectrum incident on a molecular cloud, the H 2 column density, the isomeric H 2 composition, and the dust properties. The resulting photodissociation and photoionisation rates are, on average, lower than previous determinations by a factor of about 2, with deviations of up to a factor of 5 for the photodissociation of species such as AlH, C 2 H 2 , C 2 H 3 , C 3 H 3 , LiH, N 2 , NaCl, NaH, O$^+_2$, S 2 , SiH, l-C 4 , and l-C 5 H. A special focus is given to the photoionisation rates of H 2 , HF, and N 2 , as well as to the photodissociation of H 2 , which we find to be orders of magnitude higher than previous estimates. We give parameterisations for both the photorates and the integrated UV photon flux as a function of the CR ionisation rate, which implicitly depends on the H 2 column density, as well as the dust properties.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Elastic scattering and rotational excitation of H 2 by electron impact: Convergent close-coupling calculations

We apply the adiabatic-nuclei molecular convergent close-coupling (MCCC) method to the study of elastic scattering and rotational excitation of H 2 by 0.01–20-eV electrons. Integral cross sections are presented for all rotational transitions with |Δ⁢N| ≤ 2 and N i = 0–31 within the v = 0 vibrational level and differential cross sections for a selection of transitions. Agreement with the available measurements and previous calculations is mostly excellent, depending on the transition and incident energy. Finally, we suggest possible reasons, and argue for the accuracy of the MCCC data.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2022

Cosmic rays in molecular clouds probed by H 2 rovibrational lines

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

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2021

Linear polarization fractions of Fulcher- α fluorescence in electron collisions with H 2

We apply the molecular convergent close-coupling method to the calculation of linear polarization fractions for Fulcher-band fluorescence following electron-impact excitation of the H 2 d 3 Π u state. Furthermore, the results exhibit the opposite threshold behavior compared to the only previous calculations, but are in agreement with the most recent measurements for the Q(1), R(1), and Q(3) transitions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗