Anthony J. Remijan

The Catholic University of America, Washington, Washington, D.C., United States

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Publications (99)285.11 Total impact

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    ABSTRACT: An extensive search has been conducted to confirm transitions of \textit{trans}-ethyl methyl ether (tEME, C$_2$H$_5$OCH$_3$), toward the high mass star forming region W51 e1/e2 using the 12 m Telescope of the Arizona Radio Observatory (ARO) at wavelengths from 2 mm and 3 mm. In short, we cannot confirm the detection of tEME toward W51 e1/e2 and our results call into question the initial identification of this species by \citet{FuchsSpace}. Additionally, reevaluation of the data from the original detection indicates that tEME is not present toward W51 e1/e2 in the abundance reported by Fuchs and colleagues. Typical peak-to-peak noise levels for the present observations of W51 e1/e2 were between 10 - 30 mK, yielding an upper limit of the tEME column density of $\leq$ 1.5 $\times$ 10$^{15}$ cm$^{-2}$. This would make tEME at least a factor 2 times less abundant than dimethyl ether (CH$_3$OCH$_3$) toward W51 e1/e2. We also performed an extensive search for this species toward the high mass star forming region Sgr B2(N-LMH) with the NRAO 100 m Green Bank Telescope (GBT). No transitions of tEME were detected and we were able to set an upper limit to the tEME column density of $\leq$ 4 $\times$ 10$^{14}$ cm$^{-2}$ toward this source. Thus, we are able to show that tEME is not a new molecular component of the interstellar medium and that an exacting assessment must be carried out when assigning transitions of new molecular species to astronomical spectra to support the identification of large organic interstellar molecules.
    11/2014;
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    ABSTRACT: 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.
    10/2014;
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    ABSTRACT: We present spectrally and spatially resolved maps of HNC and HC3N emission from Titan's atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titan's northern and southern hemispheres. The HC3N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titan's photochemistry and atmospheric circulation. Differences between the spectrally integrated flux distributions of HNC and HC3N show that these species are not co-spatial. The observed spectral line shapes are consistent with HNC being concentrated predominantly in the mesosphere and above (at altitudes z 400 km), whereas HC3N is abundant at a broader range of altitudes (z 70-600 km). From spatial variations in the HC3N line profile, the locations of the HC3N emission peaks are shown to be variable as a function of altitude. The peaks in the integrated emission from HNC and the line core (upper atmosphere) component of HC3N (at z 300 km) are found to be asymmetric with respect to Titan's polar axis, indicating that the mesosphere may be more longitudinally variable than previously thought. The spatially integrated HNC and HC3N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio profiles are found to be in reasonable agreement with previous measurements for these species. Vertical column densities of the best-fitting gradient models for HNC and HC3N are 1.9 × 1013 cm–2 and 2.3 × 1014 cm–2, respectively.
    The Astrophysical Journal Letters 10/2014; 795(2):L30. · 5.60 Impact Factor
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    ABSTRACT: We present spectrally and spatially-resolved maps of HNC and HC$_3$N emission from Titan's atmosphere, obtained with the Atacama Large Millimeter Array (ALMA) on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titan's northern and southern hemispheres. The HC$_3$N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titan's photochemistry and atmospheric circulation. Differences between the integrated flux distributions of HNC and HC$_3$N show that these species are not co-spatial. The observed spectral line shapes are consistent with HNC residing predominantly in the mesosphere and above (at altitudes $z$ greater than about 400 km), whereas HC$_3$N is abundant at a broader range of altitudes ($z\approx70-500$ km). From spatial variations in the HC$_3$N line profile, the locations of the HC$_3$N emission peaks are shown to be variable as a function of altitude. The integrated emission peaks for HNC and the upper-atmosphere HC$_3$N component (at $z\gtrsim300$ km) are found to be asymmetric with respect to Titan's polar axis, indicating that the mesosphere may be more longitudinally-variable than previously thought. Disk-averaged HNC and HC$_3$N spectra are modeled using the RADTRANS planetary atmosphere code and the resulting best-fitting vertical mixing ratio (VMR) profiles are found to be in reasonable agreement with previous measurements for these species.
    08/2014;
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    ABSTRACT: Results are presented from the first cometary observations using the Atacama Large Millimeter/Submillimeter Array (ALMA), including measurements of the spatially-resolved distributions of HCN, HNC, H$_2$CO and dust within the comae of two comets: C/2012 F6 (Lemmon) and C/2012 S1 (ISON), observed at heliocentric distances of 1.5 AU and 0.54 AU, respectively. These observations (with angular resolution $\approx0.5''$), reveal an unprecedented level of detail in the distributions of these fundamental cometary molecules, and demonstrate the power of ALMA for quantitative measurements of the distributions of molecules and dust in the inner comae of typical bright comets. In both comets, HCN is found to originate from (or within a few hundred km of) the nucleus, with a spatial distribution largely consistent with spherically-symmetric, uniform outflow. By contrast, the HNC distributions are clumpy and asymmetrical, with peaks at cometocentric radii $\sim$500-1000~km, consistent with release of HNC in collimated outflow(s). Compared to HCN, the H$_2$CO distribution in comet Lemmon is very extended. The interferometric visibility amplitudes are consistent with coma production of H$_2$CO and HNC from unidentified precursor material(s) in both comets. Adopting a Haser model, the H$_2$CO parent scale-length is found to be a few thousand km in Lemmon and only a few hundred km in ISON, consistent with destruction of the precursor by photolysis or thermal degradation at a rate which scales in proportion to the Solar radiation flux.
    The Astrophysical Journal 08/2014; 792(1):L2. · 6.28 Impact Factor
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    ABSTRACT: The Jupiter-family comet 103P/Hartley 2 (103P) was the target of the NASA EPOXI mission. In support of this mission, we conducted observations from radio to submillimeter wavelengths of comet 103P in the three weeks preceding the spacecraft rendezvous on UT 2010 November 4.58. This time period included the passage at perihelion and the closest approach of the comet to the Earth. Here we report detections of HCN, H2CO, CS, and OH and upper limits for HNC and DCN towards 103P, using the Arizona Radio Observatory Kitt Peak 12m telescope (ARO 12m) and submillimeter telescope (SMT), the James Clerk Maxwell Telescope (JCMT) and the Greenbank Telescope (GBT). The water production rate, QH2O = (0.67 - 1.07) x 10^28 s^-1, was determined from the GBT OH data. From the average abundance ratios of HCN and H2CO relative to water (0.13 +/- 0.03 % and 0.14 +/- 0.03 %, respectively), we conclude that H2CO is depleted and HCN is normal with respect to typically-observed cometary mixing ratios. However, the abundance ratio of HCN with water shows a large diversity with time. Using the JCMT data we measured an upper limit for the DCN/HCN ratio <0.01. Consecutive observations of ortho- H2CO and para-H2CO on November 2 (from data obtained at the JCMT), allowed us to derive an ortho : para ratio (OPR) ~ 2.12 +/- 0.59 (1sigma), corresponding to Tspin > 8 K (2sigma).
    The Astrophysical Journal 07/2014; 794(1). · 6.28 Impact Factor
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    ABSTRACT: 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.
    05/2014;
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    ABSTRACT: Pety et al. have reported the detection of eight transitions of a closed-shell, linear molecule (B11244) in observations toward the Horsehead photodissociation region (PDR), which they attribute to the l-C3H+ cation. Recent high-level ab initio calculations have called this assignment into question; the anionic C3H─ molecule has been suggested as a more likely candidate. Here, we examine observations of the Horsehead PDR, Sgr B2(N), TMC-1, and IRC+10216 in the context of both l-C3H+ and C3H─. We find no observational evidence of Ka = 1 lines, which should be present were the carrier indeed C3H─. Additionally, we find a strong anticorrelation between the presence of known molecular anions and B11244 in these regions. Finally, we discuss the formation and destruction chemistry of C3H─ in the context of the physical conditions in the regions. Based on these results, we conclude there is little evidence to support the claim that the carrier is C3H─.
    The Astrophysical Journal 03/2014; 783(1):36. · 6.28 Impact Factor
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    ABSTRACT: We will present molecular and continuum submillimeter observerations of Comet C/2012 S1 (ISON) pre-perihelion, during outbursts, and the final break-up event.
    02/2014;
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    ABSTRACT: Multiwavelength observations toward Comets C/2009 P1 and 103P/Hartley 2 to determine their taxonomy and some cosmogonic quantities from 12 m, SMT, JCMT, and GBT.
    02/2014;
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    ABSTRACT: In this paper, we present the results of an observational search for gas phase urea [(NH2)2CO] observed towards the Sgr B2(N-LMH) region. We show data covering urea transitions from 100 GHz to 250 GHz from five different observational facilities: BIMA, CARMA, the NRAO 12 m telescope, the IRAM 30 m telescope, and SEST. The results show that the features ascribed to urea can be reproduced across the entire observed bandwidth and all facilities by best fit column density, temperature, and source size parameters which vary by less than a factor of 2 between observations merely by adjusting for telescope-specific parameters. Interferometric observations show that the emission arising from these transitions is cospatial and compact, consistent with the derived source sizes and emission from a single species. Despite this evidence, the spectral complexity, both of (NH2)2CO and of Sgr B2(N), makes the definitive identification of this molecule challenging. We present observational spectra, laboratory data, and models, and discuss our results in the context of a possible molecular detection of urea.
    The Astrophysical Journal 01/2014; 783(2). · 6.28 Impact Factor
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    ABSTRACT: A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3) rotational lines is presented for the first time using a set of theoretical collisional rate coefficients. These coefficients have been computed in the temperature range 5-30 K by combining coupled-channel scattering calculations with a high accuracy potential energy surface for HCOOCH3-He. The results are compared to observations toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. A total of 49 low-lying transitions of methyl formate, with upper levels below 25 K, are identified. These lines are found to probe a presumably cold (~30 K), moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the column density of the trans conformer in the same source. Provided that the two conformers have the same spatial distribution, this result suggests that strongly non-equilibrium processes must be involved in their synthesis. Finally, our calculations show that all detected emission lines with a frequency below 30 GHz are (collisionally pumped) weak masers amplifying the continuum of Sgr B2(N). This result demonstrates the importance and generality of non-LTE effects in the rotational spectra of complex organic molecules at centimetre wavelengths.
    The Astrophysical Journal 01/2014; 783(2). · 6.28 Impact Factor
  • Ryan A. Loomis, C. H. Johnson, A. J. Remijan
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    ABSTRACT: Molecular isomers provide a unique tool for probing interstellar regions, as molecular abundance ratios contain information about the history and conditions of the region, acting as a chemical fingerprint. Comparing abundances of complex molecules in an attempt to extract this information proves to be difficult, however, for many sources. Due to complex source structure and vast networks of molecular interactions, it is exceedingly difficult to create accurate models (Quan & Herbst 2007, A&A 474, 521). However, molecular isomer abundance ratios may provide a way to slowly disentangle the puzzle (Lovas et al. 2010, JMS 264, 10). As isomers are the same size and present a similar level of chemical complexity, comparing their abundances will provide information about both their formation chemistry and the energetics of the formation region. Toward this end, we have fully characterized the spectra of the three isomers of C3H2O toward the well known molecular region Sgr B2. Evidence for the detection of the isomers cyclopropenone (c-C3H2O) and propynal (HCCCHO) is presented, along with evidence for the non-detection of the the lowest energy isomer, propadienone (CH2CCO). We interpret this to show 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 (Lattelais et al. 2009, ApJ 696, L133). A brief discussion of possible formation routes and analysis of their implications is also presented.
    01/2014;
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    ABSTRACT: We report detections of several molecular gas tracers in clouds along the line of sight to the Galactic center. Spectra were obtained from the Robert C. Byrd Green Bank Telescope as part of the PRIMOS survey, using Sagittarius B2 as the bright background continuum source to see these diffuse clouds in absorption. Clouds along the line of sight to Sgr B2 are distinguished by their distinct line-of-sight velocities. This study identified four previously known clouds and detected four new clouds. Detected molecules include cC3H2, CCS, HCS+, SiO, 29SiO, H2CO, H213CO, HC3N, CH3CN, NH2CHO, CH3CHO, CH2CHCN, and CH3OH. Clouds along the line of sight to Sgr B2 are distinguished by their distinct line-of-sight velocities. This study identified four previously known clouds and detected four new clouds. The absorption line technique is shown to be an effective means of determining the small scale structure of the inner Galaxy.
    01/2014;
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    ABSTRACT: CBET 3693 available at Central Bureau for Astronomical Telegrams.
    Central Bureau Electronic Telegrams. 11/2013;
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    ABSTRACT: Cometary ices contain material left over from the birth of the solar system, and studying their composition provides an important source of information regarding the physical and chemical conditions of the early Solar Nebula. Previous observations have been unable to ascertain the precise origin of fundamental coma species H2CO, HCN and HNC, and details regarding their possible formation in the coma are currently not well understood. In order to ascertain the chemical origin of these molecules and to place constraints on their coma release mechanisms, spatially and spectrally-resolved molecular emission maps of comets at mm and sub-mm wavelengths are required. In 2013, as part of our Director's Discretionary Time program, observations of the unusually-bright, gas-rich comet F6 (Lemmon) were executed using ALMA in the frequency range 339-362 GHz, covering emission lines from CH3OH, H2CO, HCN and HNC. We will present full details of these unique observations, and an analysis of the observed spectra.
    10/2013;
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    ABSTRACT: 103P/Hartley 2 (103P) is a Jupiter-family comet which has a short orbital period (6.5 years) and perihelion at 1.06 AU. 103P was discovered on 4 June 1984 by Malcolm Hartley at the Siding Spring Observatory and has been frequently observed over the 20 years following its discovery, both by ground-based and space telescopes. Observation from 1991 and 1997 indicated a maximum water production rate of 3 x 10^28 molecules/s. 103P passed perihelion on 28 October 2010 at q = 1.059 AU. It made an exceptional close approach to the Earth just before perihelion on 21 October 2010 at Δ = 0.12 AU. On UT 2010 November 4.58, the comet was visited by NASA’s EPOXI spacecraft with a flyby at 700 km. As part of an ongoing investigation to establish the contribution of the natal molecular cloud from which the solar system was formed to primitive materials and comets, we have been conducting observations toward the comet 103P to determine taxonomy and cosmogonic quantities, such as the ortho:para ratio and isotope ratios. Here we report detections of HCN, H2CO, CS, and OH and upper limits on HNC and DCN toward comet 103P, using the Arizona Radio Observatory Kitt Peak 12m (12m) and submillimeter telescopes (SMT), the James Clerk Maxwell Telescope (JCMT) and the Greenbank Telescope (GBT). From these data physical parameters such as temperature, column densities, and production rates have been determined toward comet 103P. The ortho:para ratio has been derived from H2CO. We used the JCMT data to compute the D/H ratio from DCN and HCN. Here we present our analysis and discuss the origin of volatiles in cometary material.
    10/2013;
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    ABSTRACT: Comets are comprised of molecular ices that may be pristine interstellar remnants of Solar System formation, along with high-temperature crystalline silicate dust that is indicative of a more thermally varied history in the protosolar nebula [1]. Comparing abundances of cometary parent volatiles and isotopic fractionation ratios to those found in the interstellar medium, in disks around young stars, and between cometary families, is vital to understanding planetary system formation and the processing history experienced by organic matter in the so-called interstellar-comet connection. Recently, there have been complimentary observations from multiple facilities to try to unravel the chemical complexity of comets. We report spectral observations of C/2012 F6 (Lemmon) from the Arizona Radio Observatory’s Submillimeter Telescope and the Atacama Pathfinder Experiment. Multiple parent volatiles (e.g. HCN, CH3OH, CO) as well as a number of daughter products (e.g. CS) have been detected at multiple transitions from these facilities. Previous work has revealed a range of abundances of parent species (from “organics-poor” to “organics-rich”) with respect to water among comets [2,3,4], however the statistics are still poorly constrained and interpretations of the observed compositional diversity are uncertain. We will compare the molecular abundances of F6 to a broad range of comets from diverse dynamical families and chemical compositions, supporting our long-term effort of building a comet taxonomy based on composition. [1] Wooden 2008, SpSciRev., 138, 75. [2] DiSanti, M. A., & Mumma, M. J. (2008), Space Sci. Rev., 138, 127. [3] Mumma, M. J. and Charnley, S.B. (2011), ARA&A, 49, 471. [4] Crovisier, J. et al. (2009) Earth, Moon Planet, 105, 267.
    10/2013;
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    ABSTRACT: Pety et al. (2012) recently reported the detection of several transitions of an unknown carrier in the Horsehead PDR and attribute them to l-C3H+. Here, we have tested the predictive power of their fit by searching for, and identifying, the previously unobserved J=1-0 and J=2-1 transitions of the unknown carrier (B11244) towards Sgr B2(N) in data from the publicly available PRIMOS project. Also presented here are observations of the J=6-5 and J=7-6 transitions towards Sgr B2(N) and Sgr B2(OH) using the Barry E. Turner Legacy Survey and results from the Kaifu et al. (2004) survey of TMC-1. We calculate an excitation temperature and column density of B11244 of ~10 K and ~10^13 cm-2 in Sgr B2(N) and ~79 K with an upper limit of < 1.5 x 10^13 cm-2 in Sgr B2(OH) and find trace evidence for the cation's presence in TMC-1. Finally, we present spectra of the neutral species in both Sgr B2(N) and TMC-1, and comment on the robustness of the assignment of the detected signals to l-C3H+.
    The Astrophysical Journal 09/2013; 774:56. · 6.28 Impact Factor

Publication Stats

795 Citations
285.11 Total Impact Points

Institutions

  • 2014
    • The Catholic University of America
      • Department of Physics
      Washington, Washington, D.C., United States
  • 2006–2014
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2009–2011
    • University of Virginia
      • • Department of Chemistry
      • • Center for Chemistry of the Universe (CCU)
      Charlottesville, VA, United States
    • National Institute of Standards and Technology
      Maryland, United States
  • 2004–2010
    • University of Illinois, Urbana-Champaign
      • Department of Astronomy
      Urbana, Illinois, United States