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  • 1
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    Confirming Interstellar C60(+) Using the Hubble Space Telescope (2019)
    In:  Other Sources
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    Publication Date: 2019-05-21
    Description: Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C60(+)) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identication of interstellar C60(+) requires a match between the wavelengths and the expected strengths of all absorption features detectable in the laboratory and in space. Here we present Hubble Space Telescope (HST) spectra of the region covering the C60(+) 9348, 9365, 9428, and 9577 absorption bands toward seven heavily reddened stars. We focus in particular on searching for the weaker laboratory C60(+) bands, the very presence of which has been a matter for recent debate. Using the novel STIS-scanning technique to obtain ultra-high signal-to-noise spectra without contamination from telluric absorption that aficted previous ground-based observations, we obtained reliable detections of the (weak) 9365, 9428 and (strong) 9577 C60(+) bands. The band wavelengths and strength ratios are sufciently similar to those determined in the latest laboratory experiments that we consider this the rst robust identication of the 9428 band, and a conclusive conrmation of interstellar C60(+).
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN68405 , Astrophysical Journal Letters (ISSN 2041-8205) (e-ISSN 2041-8213); 875; 2; L28
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  • 2
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    The Volatile Fraction of Comets as Quantified at Infrared Wavelengths - An Emerging Taxonomy and Implications for Natal Heritage (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: It is relatively easy to identify the reservoir from which a given comet was ejected. But dynamical models demonstrate that the main cometary reservoirs (Kuiper Belt, Oort Cloud) each contain icy bodies that formed in a range of environments in the protoplanetary disk, and the Oort Cloud may even contain bodies that formed in disks of sibling stars in the Sun s birth cluster. The cometary nucleus contains clues to the formative region(s) of its individual components. The composition of ices and rocky grains reflect a range of processes experienced by material while on the journey from the natal interstellar cloud core to the cometary nucleus. For that reason, emphasis is placed on classifying comets according to their native ices and dust (rather than orbital dynamics). Mumma & Charnley [1] reviewed the current status of taxonomies for comets and relation to their natal heritage.
    Keywords: Astrophysics
    Type: GSFC.CPR.6307.2012 , Asteroids, Comets and Meteors 2012; May 16, 2012 - May 20, 2012; Niigata; Japan
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  • 3
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    On the Ubiquity of Molecular Anions in the Dense Interstellar Medium (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H() is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B, and TMC-1C. The C6H() /C6H column density ratio is approx. 〉 1.0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H()/C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud.We also report the first detection of C4H() in TMC-1 (at 4.8 sigma confidence), and derive an anion-to-neutral ratio C4H() /C4H = (1.2 +/- 0.4)10(exp 5)(= 0.0012% +/- 0.0004%). Such a low value compared with C6H() highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H() could be produced as a result of reactions of oxygen atoms with C5H() and C6H().
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN9490
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  • 4
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    Revised Models of Interstellar Nitrogen Isotopic Fractionation (2017)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15N enrichments fail to match observed values. We have investigated the effects of these reaction rate modifications on the chemical model of Wirstrom et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing 15N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14N/15N ratios of 〉2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14N/15N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN54008 , Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711) (e-ISSN 1365-2966); 474; 3; 3720-3726
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  • 5
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    EPOXI: Comet 103p/Hartley 2 Observations from a Worldwide Campaign (2011)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales. at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P (Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was approximately 16.4 hr. Starting in 2010 August the period changed from 16.6 hr to near 19 hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO2-driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production.
    Keywords: Astrophysics
    Type: GSFC.JA.6404.2012 , Astrophysical Journal Letters; 734; L1
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  • 6
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    Summary of Research (1999)
    In:  CASI
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    Publication Date: 2019-07-10
    Description: In protostellar cores where the dust temperature has been raised above 100K and subsequently allowed to fall below the condensation temperature of methanol, recondensation on to cooling grains removes methanol molecules from the gas at rates.faster (about 1000 times) than those of chemical reactions. Molecular recondensation can have a profound effect on the chemical composition of hot cores. The methanol chemistry of hot cores is solved analytically and the trend in molecule binding energies required is compared with theoretical and experimental values. It is demonstrated, through a model calculation incorporating recondensation, that it yields a consistent explanation of the similarity of the CH3OH , CH3OCH3 and HCOOCH3 abundances measured in G34.3 and W3(H2O). These observations suggest that the latter molecules could not be derived from CH3OH through gas phase reactions. The manner in which molecular recondensation could affect the interpretation of hot core chemistry in general, particularly of organic molecules, is briefly discussed.
    Keywords: Astrophysics
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  • 7
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    Chemistry of Star-Forming Cores (1994)
    In:  CASI
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    Publication Date: 2019-07-13
    Description: Chemical effects arising from the exchange of molecules between dust and gas in dense clumps are described. Selective desorption of CO and N2 from grains in cool cores can account for the presence of ammonia in several cores in the NGC 2024 cloud. Evaporation of ices containing methanol and ethanol can lead to detectable abundances of (C2H5)(sub 2)O and CH3OC2H5. Results are presented for the hot core chemistries of sulphur and phosphorus which are initiated by evaporated hydrogen sulphide and phosphine. The implications of these studies for understanding the nature of molecular mantles, the evolution of molecular complexity in the gas phase, and the presence of small-scale abundance gradients in star-forming regions, are briefly discussed.
    Keywords: Astrophysics
    Type: NASA-TM-112663 , NAS 1.15:112663 , Molecules and Grains in Space; 312; 155-159
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  • 8
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    Organic Molecules in Oxygen-Rich Circumstellar Envelopes: Methanol and Hydrocarbons (1995)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The existence of anomalously high abundances of gaseous CH4 has been invoked to explain the unexpectedly high abundances of the carbon-bearing molecules HCN and H2CO in the outflows from O-rich red giants. We have modelled the chemistry that proceeds in the outer envelope when CH4 is injected from the inner envelope. We find that photolysis by the interstellar radiation field drives an ion-neutral chemistry which produces several organic molecules. The calculated abundances of CH3OH, C2H and C2 can be comparable to those calculated for H2CO and HCN. Species such as C2H4, C2H2 and CH3CN can also be abundant. A search for CH3OH and C2H in several O-rich outflows known to exhibit strong HCN emission is needed. As it derives entirely from the CH4 photochain, is insensitive to the envelope temperature distribution, and has accessible transitions at millimetre wavelengths, the detection of the C2H radical would provide further indirect support for the presence of the hypothesized methane.
    Keywords: Astrophysics
    Type: NASA-TM-111838 , NAS 1.15:111838 , Mont. Not. R. Astron. Soc.; 274; L53-L57
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  • 9
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    Interstellar Alcohols (1995)
    In:  CASI
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    Publication Date: 2019-07-13
    Description: We have investigated the gas-phase chemistry in dense cores where ice mantles containing ethanol and other alcohols have been evaporated. Model calculations show that methanol, ethanol, propanol, and butanol drive a chemistry leading to the formation of several large ethers and esters. Of these molecules, methyl ethyl ether (CH3OC2H5) and diethyl ether (C2H5)2O attain the highest abundances and should be present in detectable quantities within cores rich in ethanol and methanol. Gas-phase reactions act to destroy evaporated ethanol and a low observed abundance of gas-phase C,H,OH does not rule out a high solid-phase abundance. Grain surface formation mechanisms and other possible gas-phase reactions driven by alcohols are discussed, as are observing strategies for the detection of these large interstellar molecules.
    Keywords: Astrophysics
    Type: NASA-CR-203222 , NAS 1.26:203222 , The Astrophysical Journal; 448; 232-239
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  • 10
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    On the Nature of the Enigmatic Object IRAS 19312+1950: A Rare Phase of Massive Star Formation? (2016)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: IRAS19312+1950 is a peculiar object that has eluded firm characterization since its discovery, with combined maser properties similar to an evolved star and a young stellar object (YSO). To help determine its true nature, we obtained infrared spectra of IRAS19312+1950 in the range 5-550 microns using the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a compact, slightly asymmetric continuum source at 170 microns, indicative of a large, dusty circumstellar envelope. The far-IR CO emission line spectrum reveals two gas temperature components: approx. = 0.22 Stellar Mass of material at 280+/-18 K, and 1.6 Me of material at 157+/-3 K. The OI 63 micron line is detected on-source but no significant emission from atomic ions was found. The HIFI observations display shocked, high-velocity gas with outflow speeds up to 90 km/s along the line of sight. From Spitzer spectroscopy, we identify ice absorption bands due to H2O at 5.8 microns and CO2 at 15 microns. The spectral energy distribution is consistent with a massive, luminous (approx. 2 10(exp 4) Stellar Luminosity) central source surrounded by a dense, warm circumstellar disk and envelope of total mass approx. 500-700 Stellar Mass with large bipolar outflow cavities. The combination of distinctive far-IR spectral features suggest that IRAS19312+1950 should be classified as an accreting, high-mass YSO rather than an evolved star. In light of this reclassification, IRAS19312+1950 becomes only the fifth high-mass protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH maser line ratios may not be reliable observational discriminators between evolved stars and YSOs.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN40387 , The Astrophysical Journal (ISSN 2041-8205) (e-ISSN 2041-8213); 828; 1; 51
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