# Gas-Phase Spectra of MgO Molecules: A Possible Connection from Gas-Phase Molecules to Planet Formation [EPA]

A more fine-tuned method for probing planet-forming regions, such as protoplanetary discs, could be rovibrational molecular spectroscopy observation of particular premineral molecules instead of more common but ultimately less related volatile organic compounds. Planets are created when grains aggregate, but how molecules form grains is an ongoing topic of discussion in astrophysics and planetary science. Using the spectroscopic data of molecules specifically involved in mineral formation could help to map regions where planet formation is believed to be occurring in order to examine the interplay between gas and dust. Four atoms are frequently associated with planetary formation: Fe, Si, Mg, and O. Magnesium, in particular, has been shown to be in higher relative abundance in planet-hosting stars. Magnesium oxide crystals comprise the mineral periclase making it the chemically simplest magnesium-bearing mineral and a natural choice for analysis. The monomer, dimer, and trimer forms of (MgO)_n with n = 1 – 3 are analyzed in this work using high-level quantum chemical computations known to produce accurate results. Strong vibrational transitions at 12.5 {\mu}m, 15.0 {\mu}m, and 16.5 {\mu}m are indicative of magnesium oxide monomer, dimer, and trimer making these wavelengths of particular interest for the observation of protoplanetary discs and even potentially planet-forming regions around stars. If such transitions are observed in emission from the accretion discs or absorptions from stellar spectra, the beginning stages of mineral and, subsequently, rocky body formation could be indicated.

K. Kloska and R. Fortenberry
Mon, 13 Nov 17
16/46

Comments: 10 pages, 2 figures, 6 tables, Accepted in MNRAS

# Gas-Phase Spectra of MgO Molecules: A Possible Connection from Gas-Phase Molecules to Planet Formation [EPA]

A more fine-tuned method for probing planet-forming regions, such as protoplanetary discs, could be rovibrational molecular spectroscopy observation of particular premineral molecules instead of more common but ultimately less related volatile organic compounds. Planets are created when grains aggregate, but how molecules form grains is an ongoing topic of discussion in astrophysics and planetary science. Using the spectroscopic data of molecules specifically involved in mineral formation could help to map regions where planet formation is believed to be occurring in order to examine the interplay between gas and dust. Four atoms are frequently associated with planetary formation: Fe, Si, Mg, and O. Magnesium, in particular, has been shown to be in higher relative abundance in planet-hosting stars. Magnesium oxide crystals comprise the mineral periclase making it the chemically simplest magnesium-bearing mineral and a natural choice for analysis. The monomer, dimer, and trimer forms of (MgO)_n with n = 1 – 3 are analyzed in this work using high-level quantum chemical computations known to produce accurate results. Strong vibrational transitions at 12.5 {\mu}m, 15.0 {\mu}m, and 16.5 {\mu}m are indicative of magnesium oxide monomer, dimer, and trimer making these wavelengths of particular interest for the observation of protoplanetary discs and even potentially planet-forming regions around stars. If such transitions are observed in emission from the accretion discs or absorptions from stellar spectra, the beginning stages of mineral and, subsequently, rocky body formation could be indicated.

K. Kloska and R. Fortenberry
Mon, 13 Nov 17
22/46

Comments: 10 pages, 2 figures, 6 tables, Accepted in MNRAS

# The formation of urea in space I. Ion-molecule, neutral-neutral, and radical gas-phase reactions [GA]

Many organic molecules have been observed in the interstellar medium thanks to advances in radioastronomy, and very recently the presence of urea was also suggested. While those molecules were observed, it is not clear what the mechanisms responsible to their formation are. In fact, if gas-phase reactions are responsible, they should occur through barrierless mechanisms (or with very low barriers). In the past, mechanisms for the formation of different organic molecules were studied, providing only in a few cases energetic conditions favorable to a synthesis at very low temperature. A particularly intriguing class of such molecules are those containing one N–C–O peptide bond, which could be a building block for the formation of biological molecules. Urea is a particular case because two nitrogen atoms are linked to the C–O moiety. Thus, motivated also by the recent tentative observation of urea, we have considered the synthetic pathways responsible to its formation.
We have studied the possibility of forming urea in the gas phase via different kinds of bi-molecular reactions: ion-molecule, neutral, and radical. In particular we have focused on the activation energy of these reactions in order to find possible reactants that could be responsible for to barrierless (or very low energy) pathways.
We have used very accurate, highly correlated quantum chemistry calculations to locate and characterize the reaction pathways in terms of minima and transition states connecting reactants to products.
Most of the reactions considered have an activation energy that is too high; but the ion-molecule reaction between NH$_2$OH$_2^+$ and formamide is not too high. These reactants could be responsible not only for the formation of urea but also of isocyanic acid, which is an organic molecule also observed in the interstellar medium.

F. Brigiano, Y. Jeanvoine, A. Largo, et. al.
Fri, 10 Nov 17
16/55

Comments: 9 pages + 8 pages of appendix

# ExoMol line lists XXIV: A new hot line list for silicon monohydride, SiH [SSA]

SiH has long been observed in the spectrum of our Sun and other cool stars. Computed line lists for the main isotopologues of silicon monohydride, $^{28}$SiH, $^{29}$SiH, $^{30}$SiH and $^{28}$SiD are presented. These line lists consider rotation-vibration transitions within the ground $X$ $^{2}\Pi$ electronic state as well as transitions to the low-lying $A$ $^{2}\Delta$ and $a$ $^{4}\Sigma^-$ states. Ab initio potential energy (PECs) and dipole moment curves (DMCs) along with spin-orbit and electronic-angular-momentum couplings between them are calculated using the MRCI level of theory with the MOLPRO package. The PEC for the ground X $^2\Pi$ state is refined to available experimental data with a typical accuracy of around 0.01 cm$^{-1}$ or better. The $^{28}$SiH line list includes 11,785 rovibronic states and 1,724,841 transitions with associated Einstein-A coefficients for angular momentum $J$ up to $82.5$ and covering wavenumbers up to 31340 cm$^{-1}$ ($\lambda$ $<$ 0.319 $\mu$m). Spectra are simulated using the new line list and comparisons made with various experimental spectra. These line lists are applicable up to temperatures of ~5000 K, making them relevant to astrophysical objects such as exoplanetary atmospheres and cool stars and opening up the possibility of detection in the interstellar medium. These line lists are available at the ExoMol www.exomol.com and CDS database websites.

S. Yurchenko, F. Sinden, L. Lodi, et. al.
Fri, 20 Oct 17
9/42

# Associative detachment (AD) paths for H and CN- in the gas-phase: astrophysical implications [GA]

The direct dynamical paths leading to Associative Detachment (AD) in the gas-phase, and specifically in the low-temperature regions of the Dark Molecular Clouds (DMC) in the ISM, or in cold trap laboratory experiments, are investigated with quantum chemical methods by using a high-level multi-reference Configuration Interaction (CI) approach that employs single and double excitations plus Davidson perturbative correction [MRSDCI(Q)] and the d-aug-cc-pV5Z basis set. The potential energy curves for H + CN- are constructed for different directions of the H partner approaching the CN- anion within the framework of the Born-Oppenheimer approximation. The present calculations found that the AD energetics at low temperature becomes favorable only along a selected range of approaching directions, thus showing that there is a preferred path of forming HCN at low temperatures, while that of forming its HNC isomer is found to be energetically forbidden. Given the existence in the ISM of different HCN/HNC ratios in different environments, we discuss the implications of our findings for selective formation of either isomer in the low-temperature conditions of the Molecular Cloud Cores.

S. Jerosimic, F. Gianturco and R. Wester
Wed, 18 Oct 2017
42/62

# The efficient photodesorption of nitric oxide (NO) ices A laboratory astrophysics study [GA]

The study and quantification of UV photon-induced desorption of frozen molecules furthers our understanding of the chemical evolution of cold interstellar regions. Nitric oxide (NO) is an important intermediate species in both gas-phase and solid-phase chemical networks. In this work, we present quantitative measurements of the photodesorption of a pure NO ice.We used the tunable monochromatic synchrotron light of the DESIRS beamline of the SOLEIL facility near Paris to irradiate NO ices in the 6 – 13.6 eV range and measured desorption by quadrupole mass spectrometry.We find that NO photodesorption is very efficient, its yield being around 1e-2 molecule per incident photon for UV fields relevant to the diffuse and dense interstellar medium. We discuss the extrapolation of our results to an astrophysical context and we compare photodesorption of NO to previously studied molecules.

R. Dupuy, G. Feraud, M. Bertin, et. al.
Wed, 4 Oct 17
17/73

Almost 20 % of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH$_3$CN and CH$_3$NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption (TPD) in an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory (DFT) to represent the organised solid support. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50\% as compared to adsorption on perfect graphene planes. The most stable isomer (CH$_3$CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH$_3$NC), however.