arXiv subjects

Anthony J. Remijan

Publications and source records attributed to Anthony J. Remijan.

At least 55 records · Page 3Linked to original sources

arXiv2019

ALMA Detection of Vibrationally Excited ($v\mathrm{_t} = 1,2$) Acetic Acid toward NGC 6334I

Vibrationally excited states of detected interstellar molecules have been shown to account for a large portion of unidentified spectral lines in observed interstellar spectra toward chemically rich sources. Here, we present the first interstellar detection of the first and second vibrationally excited torsional states of acetic acid ($v_\mathrm{t} = 1, 2$) toward the high-mass star-forming region NGC 6334I. The observations presented were taken with the Atacama Large Millimeter/submillimeter Array in bands 4, 6, and 7 covering a frequency range of 130 - 352 GHz. By comparing a single excitation temperature model to the observations, the best-fit excitation temperature and column density are obtained to be 142(25) K and $1.12(7) \times 10^{17} \mathrm{cm^{-2}}$ respectively. Based on the intensity maps of the vibrationally excited CH$_3$COOH transitions, we found that the CH$_3$COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2 arcsec. After locating the emission from different CH$_3$COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH$_3$COOH emission peak within the MM2 core by invoking continuum absorption or outflows.

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arXiv2019

Modeling C-Shock Chemistry in Isolated Molecular Outflows

Shocks are a crucial probe for understanding the ongoing chemistry within ices on interstellar dust grains where many complex organic molecules (COMs) are believed to be formed. However, previous work has been limited to the initial liberation into the gas phase through non-thermal desorption processes such as sputtering. Here, we present results from the adapted three-phase gas-grain chemical network code NAUTILUS, with the inclusion of additional high-temperature reactions, non-thermal desorption, collisional dust heating, and shock-physics parameters. This enhanced model is capable of reproducing many of the molecular distributions and abundance ratios seen in our prior observations of the prototypical shocked-outflow L1157. In addition, we find that, among others, NH$_2$CHO, HCOOCH$_3$, and CH$_3$CHO have significant post-shock chemistry formation routes that differ from those of many other COMs observed in shocks. Finally, a number of selected species and phenomena are studied here with respect to their usefulness as shock tracers in various astrophysical sources.

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arXiv2018

ALMA Observations of the Spatial Distribution of three C$_2$H$_4$O$_2$ Isomers towards Sgr B2(N)

The C$_2$H$_4$O$_2$ isomers have been previously investigated primarily via disparate sets of observations involving single dish and array measurements. The only attempt at using a uniform set of observations was performed with the IRAM 30 m observation in 2013 (Belloche et al. 2013). In this study, we present an intensive and rigorous spectral and morphological analysis of the C$_2$H$_4$O$_2$ isomers towards Sgr B2(N) with interferometers, ALMA Band 3 observations. We propose a quantitative selection method, which automates the determination of the most uncontaminated transitions and allows us to report the discovery of previously undetected transitions of the three isomers. With the least contaminated transitions, the high spatial-resolution millimeter maps of the C$_2$H$_4$O$_2$ isomers reveal that HCOOCH$_3$ and CH$_2$OHCHO each display two different velocity components, while only one velocity component of CH$_3$COOH is resolved. Moreover, the distribution of HCOOCH$_3$ is extended and offset from the continuum emission, unlike CH$_2$OHCHO and CH$_3$COOH, for which the low-velocity component is found to be compact and concentrated toward the continuum emission peak of Sgr B2(N). The distinct morphologies of these C$_2$H$_4$O$_2$ isomeric species indicate that HCOOCH$_3$ have significant differences in chemical processes than CH$_2$OHCHO and CH$_3$COOH, which display similar spatial distributions.

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arXiv2018

Science with an ngVLA: Observing the Effects of Chemistry on Exoplanets and Planet Formation

One of the primary mechanisms for inferring the dynamical history of planets in our Solar System and in exoplanetary systems is through observation of elemental ratios (i.e. C/O). The ability to effectively use these observations relies critically on a robust understanding of the chemistry and evolutionary history of the observed abundances. Significant efforts have been devoted to this area from within astrochemistry circles, and these efforts should be supported going forward by the larger exoplanetary science community. In addition, the construction of a next-generation radio interferometer will be required to test many of these predictive models in situ, while simultaneously providing the resolution necessary to pinpoint the location of planets in formation.

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arXiv2018

First Results of an ALMA Band 10 Spectral Line Survey of NGC 6334I: Detections of Glycolaldehyde (HC(O)CH$_2$OH) and a New Compact Bipolar Outflow in HDO and CS

We present the first results of a pilot program to conduct an ALMA Band 10 spectral line survey of the high-mass star-forming region NGC 6334I. The observations were taken in exceptional weather conditions (0.19 mm precipitable water) with typical system temperatures $T_{\rm{sys}}$ $<$950 K at $\sim$890 GHz. A bright, bipolar north-south outflow is seen in HDO and CS emission, driven by the embedded massive protostar MM1B. This has allowed, for the first time, a direct comparison of the thermal water in this outflow to the location of water maser emission from prior 22 GHz VLA observations. The maser locations are shown to correspond to the sites along the outflow cavity walls where high velocity gas impacts the surrounding material. We also compare our new observations to prior Herschel HIFI spectral line survey data of this field, detecting an order of magnitude more spectral lines (695 vs 65) in the ALMA data. We focus on the strong detections of the complex organic molecule glycolaldehyde (HC(O)CH$_2$OH) in the ALMA data that is not detected in the heavily beam-diluted HIFI spectra. Finally, we stress the need for dedicated THz laboratory spectroscopy to support and exploit future high-frequency molecular line observations with ALMA.

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arXiv2018

Collisional Excitation and Weak Maser Action of Interstellar Methanimine

The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH$_2$NH) by cold H$_2$ have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root mean square error of $\sim$ 1-2 cm$^{-1}$ in the region of the potential well whose depth is -374.0 cm$^{-1}$. The rotationally inelastic rate coefficients have been combined with spectroscopic data to generate the non-equilibrium spectrum of CH$_2$NH towards the interstellar molecular cloud Sgr~B2(N). The transition $1_{10}\to 1_{11}$ at 5.29 GHz is found to be inverted and the predicted weak maser emission spectrum is in excellent agreement with new observations performed with the 100-meter Green Bank Telescope.

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arXiv2018

Detection of the Aromatic Molecule Benzonitrile ($c$-C$_6$H$_5$CN) in the Interstellar Medium

Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are thought to be widespread throughout the Universe, because these classes of molecules are probably responsible for the unidentified infrared bands, a set of emission features seen in numerous Galactic and extragalactic sources. Despite their expected ubiquity, astronomical identification of specific aromatic molecules has proven elusive. We present the discovery of benzonitrile ($c$-C$_6$H$_5$CN), one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium. We observed hyperfine-resolved transitions of benzonitrile in emission from the molecular cloud TMC-1. Simple aromatic molecules such as benzonitrile may be precursors for polycyclic aromatic hydrocarbon formation, providing a chemical link to the carriers of the unidentified infrared bands.

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arXiv2017

ALMA Detection of Interstellar Methoxymethanol (CH$_3$OCH$_2$OH)

We report the detection of interstellar methoxymethanol (CH$_3$OCH$_2$OH) in ALMA Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ~0.1" - 1" spatial resolution. A column density of 4(2) x $10^{18}$ cm$^{-2}$ at $T_{ex}$ = 200 K is derived toward MM1, ~34 times less abundant than methanol (CH$_3$OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH$_3$OH photodissociation products, the methoxy (CH$_3$O) and hydroxymethyl (CH$_2$OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase vs grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH$_3$OH rather than photodissociation.

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arXiv2017

Detection of HC$_5$N and HC$_7$N Isotopologues in TMC-1 with the Green Bank Telescope

We report the first interstellar detection of DC$_7$N and six $^{13}$C-bearing isotopologues of HC$_7$N toward the dark cloud TMC-1 through observations with the Green Bank Telescope, and confirm the recent detection of HC$_5$$^{15}$N. For the average of the $^{13}$C isotopomers, DC$_7$N, and HC$_5$$^{15}$N, we derive column densities of 1.9(2)$\times$10$^{11}$, 2.5(9)$\times$10$^{11}$, and 1.5(4)$\times$10$^{11}$ cm$^{-2}$, respectively. The resulting isotopic ratios are consistent with previous values derived from similar species in the source, and we discuss the implications for the formation chemistry of the observed cyanopolyynes. Within our uncertainties, no significant $^{13}$C isotopomer variation is found for HC$_7$N, limiting the significance CN could have in its production. The results further show that, for all observed isotopes, HC$_5$N may be isotopically depleted relative to HC$_3$N and HC$_7$N, suggesting that reactions starting from smaller cyanopolyynes may not be efficient to form HC$_{n}$N. This leads to the conclusion that the dominant production route may be the reaction between hydrocarbon ions and nitrogen atoms.

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arXiv2017

Detection of Interstellar HC$_5$O in TMC-1 with the Green Bank Telescope

We report the detection of the carbon-chain radical HC$_5$O for the first time in the interstellar medium toward the dark cloud TMC-1 using the 100 m Green Bank Telescope. We observe four hyperfine components of this radical in the $J = 17/2 \rightarrow 15/2$ rotational transition that originates from the $^2Π_{1/2}$ fine structure level of its ground state, and calculate an abundance of $n/n_{H_2}$ = $1.7\times 10^{-10}$, assuming an excitation temperature of $T_{ex} = 7$~K. No indication of HC$_3$O, HC$_4$O, HC$_6$O, is found in these or archival observations of the source, while we report tentative evidence for HC$_7$O. We compare calculated upper limits, and the abundance of HC$_5$O to predictions based on (1) the abundance trend of the analogous HC$_n$N family in TMC-1 and (2) a gas-grain chemical model. We find that the gas-grain chemical model well reproduces the observed abundance of HC$_5$O, as well as the upper limits of HC$_3$O, HC$_6$O, and HC$_7$O, but HC$_4$O is over produced. The prospects for astronomical detection of both shorter and longer HC$_n$O chains are discussed.

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arXiv2016

Non-Detection of HC$_{11}$N toward TMC-1: Constraining the Chemistry of Large Carbon-Chain Molecules

Bell et al. (1997) reported the first detection of the cyanopolyyne HC$_{11}$N toward the cold dark cloud TMC-1; no subsequent detections have been reported toward any source. Additional observations of cyanopolyynes and other carbon-chain molecules toward TMC-1 have shown a log-linear trend between molecule size and column density, and in an effort to further explore the underlying chemical processes driving this trend, we have analyzed GBT observations of HC$_9$N and HC$_{11}$N toward TMC-1. Although we find an HC$_9$N column density consistent with previous values, HC$_{11}$N is not detected and we derive an upper limit column density significantly below that reported in Bell et al. Using a state-of-the-art chemical model, we have investigated possible explanations of non-linearity in the column density trend. Despite updating the chemical model to better account for ion-dipole interactions, we are not able to explain the non-detection of HC$_{11}$N, and we interpret this as evidence of previously unknown carbon-chain chemistry. We propose that cyclization reactions may be responsible for the depleted HC$_{11}$N abundance, and that products of these cyclization reactions should be investigated as candidate interstellar molecules.

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arXiv2016

Molecular Polymorphism: Microwave Spectra, Equilibrium Structures, and an Astronomical Investigation of the HNCS Isomeric Family

The rotational spectra of thioisocyanic acid (HNCS), and its three energetic isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution by a combination of chirped-pulse and Fabry-Pérot Fourier-transform microwave spectroscopy between 6 and 40~GHz in a pulsed-jet discharge expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid (HSNC), calculated here to be 35-37~kcal/mol less stable than the ground state isomer HNCS, have been detected for the first time. Precise rotational, centrifugal distortion, and nitrogen hyperfine coupling constants have been determined for the normal and rare isotopic species of both molecules; all are in good agreement with theoretical predictions obtained at the coupled cluster level of theory. On the basis of isotopic spectroscopy, precise molecular structures have been derived for all four isomers by correcting experimental rotational constants for the effects of rotation-vibration calculated theoretically. Formation and isomerization pathways have also been investigated; the high abundance of HSCN relative to ground state HNCS, and the detection of strong lines of SH using CH$_3$CN and H$_2$S, suggest that HSCN is preferentially produced by the radical-radical reaction HS + CN. A radio astronomical search for HSCN and its isomers has been undertaken toward the high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m Green Bank Telescope. While we find clear evidence for HSCN, only a tentative detection of HNCS is proposed, and there is no indication of HCNS or HSNC at the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations from a single-excitation temperature model, suggesting weak masing may be occurring in some transitions in this source.

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arXiv2016

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH$_3$CHCH$_2$O)

Life on Earth relies on chiral molecules, that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH$_3$CHCH$_2$O), in absorption toward the Galactic Center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found.

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arXiv2016

CSO and CARMA Observations of L1157. II. Chemical Complexity in the Shocked Outflow

L1157, a molecular dark cloud with an embedded Class 0 protostar possessing a bipolar outflow, is an excellent source for studying shock chemistry, including grain-surface chemistry prior to shocks, and post-shock, gas-phase processing. The L1157-B1 and B2 positions experienced shocks at an estimated ~2000 and 4000 years ago, respectively. Prior to these shock events, temperatures were too low for most complex organic molecules to undergo thermal desorption. Thus, the shocks should have liberated these molecules from the ice grain-surfaces en masse, evidenced by prior observations of SiO and multiple grain mantle species commonly associated with shocks. Grain species, such as OCS, CH3OH, and HNCO, all peak at different positions relative to species that are preferably formed in higher velocity shocks or repeatedly-shocked material, such as SiO and HCN. Here, we present high spatial resolution (~3") maps of CH3OH, HNCO, HCN, and HCO+ in the southern portion of the outflow containing B1 and B2, as observed with CARMA. The HNCO maps are the first interferometric observations of this species in L1157. The maps show distinct differences in the chemistry within the various shocked regions in L1157B. This is further supported through constraints of the molecular abundances using the non-LTE code RADEX (Van der Tak et al. 2007). We find the east/west chemical differentiation in C2 may be explained by the contrast of the shock's interaction with either cold, pristine material or warm, previously-shocked gas, as seen in enhanced HCN abundances. In addition, the enhancement of the HNCO abundance toward the the older shock, B2, suggests the importance of high-temperature O-chemistry in shocked regions.

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arXiv2015

CSO and CARMA Observations of L1157. I. A Deep Search for Hydroxylamine (NH$_2$OH)

A deep search for the potential glycine precursor hydroxylamine (NH$_2$OH) using the Caltech Submillimeter Observatory (CSO) at $λ= 1.3$ mm and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at $λ= 3$ mm is presented toward the molecular outflow L1157, targeting the B1 and B2 shocked regions. We report non-detections of NH$_2$OH in both sources. We a perform non-LTE analysis of CH$_3$OH observed in our CSO spectra to derive kinetic temperatures and densities in the shocked regions. Using these parameters, we derive upper limit column densities of NH$_2$OH of $\leq1.4 \times 10^{13}$~cm$^{-2}$ and $\leq1.5 \times 10^{13}$~cm$^{-2}$ toward the B1 and B2 shocks, respectively, and upper limit relative abundances of $N_{NH_2OH}/N_{H_2} \leq1.4 \times 10^{-8}$ and $\leq1.5 \times 10^{-8}$, respectively.

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arXiv2015

An ATCA Survey of Sagittarius~B2 at 7~mm: Chemical Complexity Meets Broadband Interferometry

We present a 30 - 50 GHz survey of Sagittarius B2(N) conducted with the Australia Telescope Compact Array (ATCA) with 5 - 10 arcsec resolution. This work releases the survey data and demonstrates the utility of scripts that perform automated spectral line fitting on broadband line data. We describe the line-fitting procedure, evaluate the performance of the method, and provide access to all data and scripts. The scripts are used to characterize the spectra at the positions of three HII regions, each with recombination line emission and molecular line absorption. Towards the most line-dense of the three regions characterised in this work, we detect ~500 spectral line components of which ~90 per cent are confidently assigned to H and He recombination lines and to 53 molecular species and their isotopologues. The data reveal extremely subthermally excited molecular gas absorbing against the continuum background at two primary velocity components. Based on the line radiation over the full spectra, the molecular abundances and line excitation in the absorbing components appear to vary substantially towards the different positions, possibly indicating that the two gas clouds are located proximate to the star forming cores instead of within the envelope of Sgr B2. Furthermore, the spatial distributions of species including CS, OCS, SiO, and HNCO indicate that the absorbing gas components likely have high UV-flux. Finally, the data contain line-of-sight absorption by $\sim$15 molecules observed in translucent gas in the Galactic Center, bar, and intervening spiral arm clouds, revealing the complex chemistry and clumpy structure of this gas. Formamide (NH$_2$CHO) is detected for the first time in a translucent cloud.

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arXiv2014

Investigating the Minimum Energy Principle in Searches for New Molecular Species - the Case of H$_{2}$C$_{3}$O Isomers

Recently, Lattelais et al. (2009) have interpreted aggregated observations of molecular isomers to suggest that there exists a "minimum energy principle'', such that molecular formation will favor more stable molecular isomers for thermodynamic reasons. To test the predictive power of this principle, we have fully characterized the spectra of the three isomers of C$_{3}$H$_{2}$O toward the well known molecular region Sgr B2(N). Evidence for the detection of the isomers cyclopropenone (c-C$_{3}$H$_{2}$O) and propynal (HCCCHO) is presented, along with evidence for the non-detection of the lowest zero-point energy isomer, propadienone (CH$_2$CCO). We interpret these observations as evidence that chemical formation pathways, which may be under kinetic control, have a more pronounced effect on final isomer abundances than thermodynamic effects such as the minimum energy principle.

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arXiv2014

A CSO Search for $l$-C$_3$H$^+$: Detection in the Orion Bar PDR

The results of a Caltech Submillimeter Observatory (CSO) search for $l$-C$_3$H$^+$, first detected by Pety et al. (2012) in observations toward the Horsehead photodissociation region (PDR), are presented. A total of 39 sources were observed in the 1 mm window. Evidence of emission from $l$-C$_3$H$^+$ is found in only a single source - the Orion Bar PDR region, which shows a rotational temperature of 178(13) K and a column density of 7(2) x $10^{11}$ cm$^{-2}$. In the remaining sources, upper limits of ~10$^{11} - 10^{13}$ cm$^{-2}$ are found. These results are discussed in the context of guiding future observational searches for this species.

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