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B. E. Turner

Bio: B. E. Turner is an academic researcher from National Radio Astronomy Observatory. The author has contributed to research in topics: Interstellar medium & Interstellar cloud. The author has an hindex of 27, co-authored 63 publications receiving 2282 citations. Previous affiliations of B. E. Turner include University of California, Berkeley.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors have observed 10 molecular species of intermediate complexity (four to seven atoms) in three translucent clouds and in TMC-1 and L183, and they have been able to find a single, unique set of parameters including a specific epoch for each of the translucent clouds that explains eight of the 10 species.
Abstract: We have observed 10 molecular species of "intermediate" complexity (four to seven atoms) in three translucent clouds and in TMC-1 and L183. Of these species HNCO, HOCO+, H2CCO, CCH, and CH3CCH are detected in all five objects, while HCOOH, CH3CHO, CH3CN, CH2NH, and VyCN are detected in fewer objects. These species are chosen because they are expected to mark the transition between simpler species that are formed in the gas phase and complex hydrogenated species [NH2CHO, EtOH, EtCN, (CH3)2O, and CH3OCHO] that are believed to form by grain chemistry. The C/O ratio and the metal abundances are determining factors in the abundances of most of the species. The gas-phase chemistry of HNCO is set out for the first time. HNCO, H2CCO, CCH, and CH3CCH are successfully modeled for a wide range of C/O, metals, and cloud conditions. HOCO+, HCOOH, CH3CN, and VyCN are well modeled under fewer conditions, while CH3CHO and CH2NH are fitted in only a small range of conditions. For most species, the models tend to underestimate the observed abundances. By introducing or modifying a number of gas-phase reactions in the New Standard Model we are able to explain the abundances of most species for specific sets of physical conditions (density, temperature, extinction), abundances (metals, C/O), and time epoch. The parameter sets for the different species are largely non overlapping. However, we have been able to find a single, unique set of parameters including a specific epoch for each of TMC-1, L183, and the translucent clouds that explains eight of the 10 species. The exceptions are CH2NH, which requires higher C/O than other species, and CH3CHO, which is possibly modeled in dark clouds but not in translucent objects. For the eight well-fitted species, C/O=0.4 is favored, and the time epoch is 5(5)±4(5) yr for each type of object. In this exercise we cannot distinguish between high and low metals. Previously studied species (HCN, C3H2, CCS, and HC3N) fit the same set of conditions. It is possible that all 10 species may be explained by gas-phase chemistry, perhaps when suitable neutral-neutral reactions are found for species such as CH3CHO and CH2NH. Until then, grain processes cannot be ruled out for these and other species such as HOCO+ (formed from desorbed CO2) or VyCN (highly saturated). Ortho/para and E/A ratios are found to lie within the thermal limits for all cases except O/P (H2CCO), whose value of 5.9 in TMC-1 exceeds the limit of 3.0, which confirms an earlier result by Ohishi. This result cannot be explained by grain interactions. Nevertheless, in the species of intermediate complexity, we may be encountering the boundary between gas and grain chemistry for partly hydrogen-saturated species. The transition to grain chemistry seems clearly to occur at the next level of complexity, which includes NH2CHO, EtOH, EtCN, (CH3)2O, and CH3OCHO, species we have failed to detect after exhaustive searches in translucent and dark clouds.

200 citations

Journal ArticleDOI
TL;DR: In this article, the authors present observations of 10 deuterated molecular species in the dark clouds TMC-1, L183, and the translucent object CB 17, as well as a subset of species in and other objects.
Abstract: We present observations of 10 deuterated molecular species in the dark clouds TMC-1, L183, and the translucent object CB 17, as well as a subset of species in and other objects. With sensitive TMCNH 3 observations of the J \ 1¨0 and 2¨1 transitions of DCN, and DNC, we have been able to derive N 2 D‘, molecular constants that include the complex nuclear quadrupole hyper—ne splitting in these species, which is essential to determine accurate abundances. The spectroscopic results have required, in turn, new radiative transport techniques to handle the hyper—ne eUects. Our abundance determinations also utilize sensitive observations of secondary isotopomers involving 13C, 18O, and 15N. Compared with earlier molecular D/H ratios in the literature, these innovations have resulted in radically diUerent values in some cases in TMC-1 and in TMC-1; DCN/HCN in CB 17), (N 2 D‘/N 2 H‘ TMCNH 3 ;N H 2 D/NH 3 and important modi—cations in others in TMC-1). The new techniques usually produce (C 3 HD/C 3 H 2 deuteration ratios lower than those obtained earlier by simpler methods. Thus, in addition to the special cases noted above, our results are generally lower than previous ones by factors of typically 2. We also —nd that deuteration occurs only in regions of high density, while nondeuterated species generally reside at lower densities. A recently proposed model of translucent clouds as low-density objects containing embedded small, high-density fragments explains the observations. To study the chemistry of deuterated species, we have used the New Standard Model, modi—ed to include all monodeuterated species, and now containing 9930 reactions and 610 species. Our models explore the dependence of the molecular D/H ratios upon temperature, density, ionization rate, extinction, epoch, and elemental abundances. Within the uncertainties, we —nd agreement between observed and modeled ratios for nearly all species in nearly all sources. Our results generally agree with those of Roberts & Millar in a recent, similar study. We —nd signi—cantly higher ratios in L183 than in TMC, and intermediate values in CB 17. With our lower values in general, however, we believe that L183 is ii normal ˇˇ for a cold dark cloud, CB 17 is typical of a slightly warmer translucent object, and the TMC region is perhaps underdeuterated in general, certainly strongly so in the case of and These N 2 H‘ NH 3 . ii anomalous ˇˇ cases have no plausible single explanation in terms of gas-phase chemistry at this time. Grain processes are implicated. . .. . .. . ..

158 citations

Journal ArticleDOI
TL;DR: The role of catalysis on grains remains unclear as mentioned in this paper, but it has been suggested that evaporation from grains of copious amounts of less complex species such as CH3OH and H2CO, followed by gas-phase reactions among the evaporated species, explain the high abundance of four of the species, while isomerization (on grains) of the less stable glycol aldehyde and vinyl alcohol species accounts for their lower abundances.
Abstract: Vinyl alcohol (CH2=CHOH) has been detected in emission toward Sagittarius B2N by means of its millimeter-wave rotational transitions. The simplest enol, vinyl alcohol is an important intermediate in many organic chemistry reactions. All three stable isomers of the C2H4O family, vinyl alcohol, ethylene oxide (c-C2H4O), and acetaldehyde (CH3–CHO) have now been identified in the interstellar medium, as have the three members of the C2H4O2 isomeric group (Hollis, Lovas, and Jewell). These complex species cannot be produced in detected amounts by quiescent gas-phase chemistry models, and grain processes have long been envisioned. Our analysis of the abundances of the six species suggests that evaporation from grains of copious amounts of less complex species such as CH3OH and H2CO, followed by gas-phase reactions among the evaporated species, explain the high abundance of four of the species, while isomerization (on grains) of the less stable glycol aldehyde and vinyl alcohol species accounts for their lower abundances. The role of catalysis on grains remains unclear.

139 citations

Journal ArticleDOI
TL;DR: In this article, the New Standard Chemistry model has been modified to accommodate several cyclical and chain-type hydrocarbon species, including C2H, C3H2, C4H2 and C6H2.
Abstract: We have observed 10 hydrocarbon species at 3 mm wavelength in three translucent clouds and in TMC-1 and L183. The 10 species are C2H, c-C3H, l-C3H, c-C3H2, l-C3H2, CH3CCH, C4H, C4H2, C5H, and C6H, where c- and l- designate the cyclical and linear isomers. Abundances for these, as well as the previously observed species CH3C4H and C6H2, have been derived from statistical equilibrium analyses. These species are key members of the hydrocarbon network—the "backbone" of gas-phase interstellar chemistry, from which most species containing N, O, and C atoms stem directly. All of the observed hydrocarbon species except c-C3H and c-C3H2 are "chain"-type molecules or have a linear-chain isomer. The latter species are known to have stable cyclic isomers as well. We observe large c-/l-abundance ratios for C3H2, especially in the dark clouds, that can be explained under steady state conditions by existing knowledge of the chemistry. We also observe ortho/para ratios for c- and l-C3H2, and for C4H2, that exceed the high-temperature limit in nearly every case, often by large factors. O/P ratios as large as ~5 can be explained by protonation via H. By contrast, E/A ratios are observed to be less than the high-T limit for all three C3v species (CH3C2H, CH3C4H, CH3CN), because H protonation is not possible. The New Standard Chemistry model (Lee et al.) has been modified to accommodate several cyclical and chain-type hydrocarbon species. In addition to the large c-/l- and O/P ratios, we find very good agreement between model predictions and observed total abundances for all 12 species under steady state conditions. Early-time chemistry is not required but cannot be ruled out. It easily produces the very large c-C3H2/l-C3H2 ratio observed in the dark clouds, consistent with the possibility that they are chemically young, while translucent clouds are in chemical and dynamical steady state. For a consistent set of C/O ratios, over 80% of all model abundances investigated are within a factor of 5 of the observed abundances. Low-metal abundances and C/O ratios of 0.7 or higher are required to explain all 12 species. Distinctions between translucent and dark clouds are at most nominal. The success of the New Standard Model in explaining hydrocarbons exceeds its success in explaining all other species studied to date. In particular, hydrocarbons are better explained than the equally complex nonhydrocarbon species studied in the twelfth paper in this series.

139 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated eight molecular species in the hot dense clump IC 443G, believed to be impacted by the shock wave from the SNR IC 443, and found that fractional abundances fit ND shock models if L is about 6.6 x 10 exp 15 cm.
Abstract: Eight molecular species, in the hot dense clump IC 443G, believed to be impacted by the shock wave from the SNR IC 443, are investigated. The clump consists of two distinct regions, one relatively cool, and one hotter and denser. Region 1 contains CO, HCO(+), HCN, and CN, whose abundances may be explained either by ion-molecule chemistry, or by a D shock of 60-90 km/s, passing through a clump of about 100,000/cu cm. Region 2 gives rise to SiO, CS, SO, and H2CO, and requires an ND shock of 5-15 km/s passing through a region of about 1,000,000/cu cm. Observed fractional abundances fit ND shock models if L is about 6.6 x 10 exp 15 cm. In general, observed line widths vary inversely with derived excitation density, while centroid velocities of all species are essentially identical.

129 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, the authors review the theoretical underpinning, techniques, and results of efforts to estimate the CO-to-H2 conversion factor in different environments, and recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty.
Abstract: CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ≈ 1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

2,004 citations

Journal ArticleDOI
TL;DR: The Cologne Database for Molecular Spectroscopy (CDMS) as discussed by the authors contains a catalog of transition frequencies from the radio-frequency to the far-infrared region covering atomic and molecular species that (may) occur in the interstellar or circumstellar medium or in planetary atmospheres.

1,842 citations

Journal ArticleDOI
TL;DR: In this paper, atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, in-cluding energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients.
Abstract: Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, in- cluding energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to E/k ∼ 1000 K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/∼moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calcu- lations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

1,542 citations

Journal ArticleDOI
TL;DR: In this paper, the authors discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way and discuss both their spectra and chemistry, and conclude that complex molecules are excellent probes of the physical conditions and history of the sources where they reside.
Abstract: Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

1,470 citations

Journal ArticleDOI
TL;DR: In this paper, the authors discuss recent progress in their study, including the newly discovered IR dark clouds that are likely precursors to stellar clusters, and provide a unique glimpse of the conditions prior to stellar birth.
Abstract: Cold dark clouds are nearby members of the densest and coldest phase in the Galactic interstellar medium, and represent the most accessible sites where stars like our Sun are currently being born. In this review we discuss recent progress in their study, including the newly discovered IR dark clouds that are likely precursors to stellar clusters. At large scales, dark clouds present filamentary mass distributions with motions dominated by supersonic turbulence. At small, subparsec scales, a population of subsonic starless cores provides a unique glimpse of the conditions prior to stellar birth. Recent studies of starless cores reveal a combination of simple physical properties together with a complex chemical structure dominated by the freeze-out of molecules onto cold dust grains. Elucidating this combined structure is both an observational and theoretical challenge whose solution will bring us closer to understanding how molecular gas condenses to form stars.

986 citations