THE EXCITATION, ABUNDANCE, AND DISTRIBUTION OF MgC₂ and CaC₂ IN IRC+10216

The laboratory and astronomical discovery of the mental dicarbides, MgC₂ and CaC₂, discussed in the preceding talk represents a key advance in the study of metal carbides and fills a longstanding gap in the molecular inventory of evolved carbon stars. In this talk we will discuss how characterizing the distribution of the two species in IRC+10216, together with a careful analysis of their excitation and abundance might help elucidate the role of metals in the chemistry of IRC+10216, and the state of refractory elements in carbon-rich circumstellar environments more generally. We will also discuss the utility of dicarbides as physico-chemical probesᵃ of circumstellar regions, as well as the prospects of detecting larger metal-carbon compounds there

Protonated Ethyl Cyanide: Quantum Chemistry And Rotational Spectroscopy

Protonated ethyl cyanide, \chem{CH_3CH_2CNH^+}, a likely intermediate in interstellar clouds and in the planetary atmosphere of Titan, has been detected at high spectral resolution by means of Fourier transform microwave spectroscopy at centimeter wavelengths. From 13 $a$-type rotational transitions between 8 and 44 GHz, the three rotational constants have been determined to better than $0.05\%$, and two of the leading centrifugal distortion terms to a few percent. Since nitrogen hyperfine structure in the lower rotational transitions is highly compact, only the quadrupole coupling tensor element along the $a$-inertial axis $\chi_{aa}$(N) could be determined. The agreement between the experimental rotational constants and those calculated theoretically is very good, of order $0.2\%$, a clear indication that the CCSD(T) level of theory provides an accurate treatment of the electronic structure. By scaling to isoelectronic butyne, even better agreement between the two is achieved ($\ll 0.1\%$). The similarity of the $eQq$(N) values derived along the C--N bond axis for both protonated vinyl cyanide and protonated ethyl cyanide along with the very small magnitudes of these constants implies a quadruply-bound nitrogen atom and an \ce{H-N^{+}#C-R} type structure that is affected little by protonation. Closely spaced torsional doublets in one $K_{a}=0$ line and three $K_{a}=+1$ lines allow an estimate of the threefold barrier to internal rotation of $V_{3}=2.50\pm0.09$ kcal~mol$^{-1}$, which is within $4\%$ of that calculated theoretically. Ethyl cyanide has a high proton affinity and is abundant in rich astronomical molecular sources, implying its protonated variant is a good candidate for astronomical detection, particularly since this species is calculated to possess a sizable dipole moment along the $a$-inertial axis (2.91 D)

Herschel/HIFI Spectral Mapping of C+^+, CH+^+, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH+^+ Formation

The CH$^+$ ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigation CH$^+$ formation with the first velocity-resolved spectral mapping of the CH$^+$ $J=1-0, 2-1$ rotational transitions, three sets of CH $\Lambda$-doubled triplet lines, $^{12}$C$^+$ and $^{13}$C$^+$, and CH$_3$OH 835~GHz E-symmetry Q branch transitions, obtained with Herschel/HIFI over $\approx$12 arcmin$^2$ centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and $^{12}$C$^+$ optical depths. Emission from C$^+$, CH$^+$, and CH is indicated to arise in the diluted gas, outside of the explosive, dense BN/KL outflow. Our models show that UV-irradiation provides favorable conditions for steady-state production of CH$^+$ in this environment. Surprisingly, no spatial or kinematic correspondences of these species are found with H$_2$ S(1) emission tracing shocked gas in the outflow. We propose that C$^+$ is being consumed by rapid production of CO to explain the lack of C$^+$ and CH$^+$ in the outflow, and that fluorescence provides the reservoir of H$_2$ excited to higher ro-vibrational and rotational levels. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of CH$^+$ formation and excitation conditions is incomplete without including the important --- possibly dominant --- role of UV irradiation.Comment: Accepted for publication in The Astrophysical Journa

THE DISTRIBUTION, EXCITATION, AND ABUNDANCE OF C+, CH+, AND CH IN ORION KL

The CH$^+$ ion was one of the first molecules identified in the interstellar gas over 75 years ago, and is postulated to be a key species in the initial steps of interstellar carbon chemistry. The high observed abundances of CH$^+$ in the interstellar gas remain a puzzle, because the main production pathway of CH$^+$, {it viz.}, $mathrm{C^{+} + H_{2} rightarrow CH^{+} + H}$, is so endothermic (4640~K), that it is unlikely to proceed at the typical temperatures of molecular clouds. One way in which the high endothermicity may be overcome, is if a significant fraction of the H$_2$ is vibrationally excited, as is the case in molecular gas exposed to intense far-ultraviolet radiation fields. Elucidating the formation of CH$^+$ in molecular clouds requires characterization of its spatial distribution, as well as that of the key participants in the chemical pathways yielding CH$^+$. Here we present high-resolution spectral maps of the two lowest rotational transitions of CH$^+$, the fine structure transition of C$^+$, and the hyperfine-split fine structure transitions of CH in a $sim 3^{prime} times 3^{prime}$ region around the Orion Kleinmann-Low (KL) nebula, obtained with the {em Herschel Space Observatory's} Heterodyne Instrument for the Far-Infrared (HIFI).footnote{These observations were done as part of the Herschel observations of EXtraordinary sources: the Orion and Sagittarius star-forming regions (HEXOS) Key Programme, led by E. A. Bergin at the University of Michigan, Ann Arbor, MI.} We compare these maps to those of CH$^+$ and C$^+$ in the Orion Bar photodissociation region (PDR), and discuss the excitation and abundance of CH$^+$ toward Orion KL in the context of chemical and radiative transfer models, which have recently been successfully applied to the Orion Bar PDR.footnote{Nagy, Z. et al. 2013, A&A 550, A96

THE ROTATIONAL SPECTRUM OF PROTONATED ETHYL CYANIDE

Ethyl cyanide ($\mathrm{CH_{3}CH_{2}CN}$) is a well-known constituent of interstellar clouds and has recently been detected in the atmosphere of Titan.\footnote{Cordiner, M. A., Palmer, M. Y., Nixon, C. A. et al. 2015, ApJL, 800, L14.} It is so abundant in some interstellar clouds that its doubly substituted carbon-13 isotopologues, as well as highly excited vibrational satellites have been detected there.\footnote{Margul{\`e}s, L., Belloche, A., M{\"u}ller, H. S. P. et al. 2016, A\&A, 590, A93 and references therein.} Because of the high abundance and high proton affinity of $\mathrm{CH_{3}CH_{2}CN}$, protonated ethyl cyanide ($\mathrm{CH_{3}CH_{2}CNH^{+}}$) is a plausible intermediate in the chemistry of interstellar clouds and planetary atmospheres. Here we report the detection of $\mathrm{CH_{3}CH_{2}CNH^{+}}$ by Fourier transform microwave spectroscopy of a supersonic molecular beam. Thirteen $a$-type rotational transitions have been observed between 8 and 44 GHz, some with partially resolved nitrogen hyperfine structure. This data set allows determination of all three rotational constants, as well as several of the leading centrifugal distortion constants to high accuracy. The derived rotational constants and those calculated at the CCSD(T) level of theory agree to better than 0.2\%. Nitrogen hyperfine structure in the lower rotational transitions is so compact that only the quadrupole coupling tensor element along the $a$-inertial axis $(\chi_{aa})$ could be determined. With accurate laboratory data in hand, a radio astronomical search for $\mathrm{CH_{3}CH_{2}CNH^{+}}$ in publicly available spectral line surveys as well as through dedicated observations can now be undertaken with high confidence

VIBRATIONALLY EXCITED c-C3H2 RE-VISITED: NEW LABORATORY MEASUREMENTS AND THEORETICAL CALCULATIONS

Cyclopropenylidene, $c$-C$_3$H$_2$, is one of the more abundant organic molecules in the interstellar medium, as evidenced from astronomical detection of its single $^{13}$C and both its singly- and doubly-deuterated isotopic species. For this reason, vibrational satellites are of considerable astronomical interest, and were the primary motivation for the earlier laboratory work by Mollaaghababa and co-workers [1].\_x000d_ indent The recent detection of intense unidentified lines near 18,GHz in a hydrocarbon discharge by FT microwave spectroscopy has spurred a renewed search for the vibrational satellite transitions of $c$-C$_3$H$_2$. Several strong lines have been definitively assigned to the $v_6$ progression on the basis of follow-up measurements at 3,mm, double resonance and millimeter-wave absorption spectroscopy, and new theoretical calculations using a rovibrational VMP2 method [2] and a high-quality ab initio potential energy surface. The treatment was applied to several excited states as well as the ground state, and included deperturbation of Coriolis interactions._x000d_ _x000d_ [1] R. Mollaaghababa, C.A. Gottlieb, J. M. Vrtilek, and P. Thaddeus, textit{J. Chem. Phys.}, textbf{99}, 890-896 (1992)._x000d_ _x000d_ [2] P.~B. Changala and J.~H. Baraban. textit{J. Chem. Phys.}, textbf{145}, 174106 (2016)._x000d_ _x000d

DETECTION OF C3H+, C4H, and CH3CHO TOWARD W49N: ELUCIDATING THE MOLECULAR COMPLEXITY OF THE DIFFUSE INTERSTELLAR GAS

The growth of molecular complexity in diffuse molecular clouds has remained a long standing problem in astrophysics. On the one hand, we have simple hydrides discovered with the \it{Herschel} space telescope and small hydrocarbon species such as \chem{C2H}, while on the other we have monolithic fullerenes including \chem{C60} and \ce{C60+}. The chemical and physical relationship between the two ends of this dichotomy remains elusive, and the species that connect the small with the large --- simple, medium-sized organic molecules possessing $>3$ carbon atoms --- have not yet been found. Recently, we have begun a detailed observing campaign with the 100\,m Green Bank Telescope (Abstract 4275) that aims to systematically explore the growth of chemical complexity in diffuse clouds. Some of our key findings include strong absorption by \chem{C3H+}, and perhaps more excitingly, \chem{C4H}, which to our knowledge is the largest carbon chain radical detected so far in the diffuse gas. In order to fully understand the behavior of these species in what was thought to be highly hostile environments for complex chemistry, we require accurate theoretical predictions of structures and thermochemical properties pertaining to molecules that are thought to play important roles in these regions. In this talk, I will discuss our latest efforts to perform high accuracy quantum chemical calculations of several small hydrocarbon species, both as to constrain the relative energetics of their isomers, as well as high precision predictions of their spectroscopic constants to guide their searches

Herschel observations of interstellar chloronium

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.Comment: Accepted for publication in the Astrophysical Journa

Herschel Observations of Extraordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey toward Orion KL. I. Methods

We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced χ^2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>10^6 cm^(–3)) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H_2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T_(rot), for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T_(rot) distributions, indicating the hot core has the most complex thermal structure

Herschel/HIFI Spectral Mapping of C^+, CH^+, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH^+ Formation

The CH^+ ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH^+ J = 1−0, 2−1 rotational transitions, three sets of CH Λ-doubled triplet lines, ^(12)C^+ and ^(13)C^+ ^(2)P_(3/2) - ^(2)P_(1/2), and CH_(3)OH 835 GHz E-symmetry Q-branch transitions, obtained with Herschel/HIFI over a region of ≈ 12 arcmin^2 centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and ^(12)C^+ optical depths. Emission from all of C^+, CH^+, and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH^+ in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H_2 S(1) emission tracing shocked gas in the outflow. We propose that C^+ is being consumed by rapid production of CO to explain the lack of both C^+ and CH^+ in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH^+ formation and excitation is incomplete without including the important—possibly dominant—role of UV irradiation