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Dickens, J. E.

Publications and source records attributed to Dickens, J. E..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Searches for new interstellar molecules, including a tentative detection of aziridine and a possible detection of propenal

Rotational spectroscopy at millimeter wavelengths is a powerful means of investigating the chemistry of dense interstellar clouds. These regions can exhibit an interesting complement of gas phase molecules, including relatively complex organics. Here we report the tentative first astronomical detection of aziridine (ethylenimine), the possible detection of propenal (acrolein), and upper limits on the abundances of cyclopropenone, furan, hydroxyethanal (glycolaldehyde), thiohydroxylamine (NH2SH), and ethenol (vinyl alcohol) in various interstellar clouds.

Acrolein/analysis↗
NASA NTRS1999

The formaldehyde ortho/para ratio as a probe of dark cloud chemistry and evolution

We present measurements of the H2CO ortho/para ratio toward four star-forming cores, L723, L1228, L1527, and L43, and one quiescent core, L1498. Combining these data with earlier results by Minh et al., three quiescent cores are found to have ortho/para ratios near 3, the ratio of statistical weights expected for gas-phase formation processes. In contrast, ortho/para ratios are 1.5-2.1 in five star-forming cores, suggesting thermalization at a kinetic temperature of 10 K. We attribute modification of the ortho/para ratio in the latter cores to formation and/or equilibration of H2CO on grains with sub-sequent release back into the gas phase due to the increased energy inputs from the forming star and outflow. We see accompanying enhancements in the H2CO abundance relative to H, to support this idea. The results suggest that the formaldehyde ortho/para ratio can differentiate between quiescent cores and those in which low-mass star formation has occurred.

Non-NASA Center↗
NASA NTRS1998

Abundances of ethylene oxide and acetaldehyde in hot molecular cloud cores

We have searched for millimetre-wave line emission from ethylene oxide (c-C2H4O) and its structural isomer acetaldehyde (CH3CHO) in 11 molecular clouds using SEST. Ethylene oxide and acetaldehyde were detected through multiple lines in the hot cores NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2. Acetaldehyde was also detected towards G10.47+0.03, G322.2+0.6, and Orion 3'N, and one ethylene oxide line was tentatively detected in G10.47+0.03. Column densities and rotational excitation temperatures were derived using a procedure which fits the observed line intensifies by finding the minimum chi 2-value. The resulting rotational excitation temperatures of ethylene oxide and acetaldehyde are in the range 16-38 K, indicating that these species are excited in the outer, cooler parts of the hot cores or that the excitation is significantly subthermal. For an assumed source size of 20", the deduced column densities are (0.6-1)x10(14) cm-2 for ethylene oxide and (2-5)x10(14) cm-2 for acetaldehyde. The fractional abundances with respect to H2 are X[c-C2H4O]=(2-6)xl0(-10), and X[CH3CHO]=(0.8-3)x10(-9). The ratio X[CH3CHO]/X[c-C2H4O] varies between 2.6 (NGC 6334F) and 8.5 (G327.3-0.6). We also detected and analysed multiple transitions of CH3OH, CH3OCH3, C2H5OH, and HCOOH. The chemical, and possibly evolutionary, states of NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2 seem to be very similar.

Non-NASA Center↗
NASA NTRS1998

HCO+ imaging of comet Hale-Bopp (C/1995 O1)

The HCO+ J = 1-0 rotational transition at 89.189 GHz has been mapped in comet Hale-Bopp (C/1995 O1) over a total of 38 individual days spanning the period 1997 March 10-June 20 with the Five College Radio Astronomy Observatory 14 m antenna. HCO+ is detectable over an extended region of the comet, with the peak emission commonly located 50,000-100,000 km in the antisolar direction. Maps made throughout the apparition show significant variability in the structure of the HCO+ coma, sometimes on timescales of several hours. The HCO+ brightness is usually depressed at the nucleus position, and on some occasions, the emission is spread into a ring around the position of the nucleus. Individual spectra within the maps display broad (approximately 4 km s-1) lines redshifted by 1-2 km s-1 or more from the nominal velocity of the nucleus, with the redshift typically increasing in the antisolar direction. The spectra and maps may be generally explained by models in which the ions are accelerated tailward at a rate on the order of 10 cm s-2, provided that HCO+ is destroyed within 50,000-100,000 km of the nucleus.

Non-NASA Center↗
NASA NTRS1998

Chemistry in cometary comae

Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale-Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion-molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.

Non-NASA Center↗
NASA NTRS1997

A study of the physics and chemistry of TMC-1

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

NASA Discipline Exobiology↗
NASA NTRS1997

Interstellar and cometary ices: Molecular emission from comet Hale-Bopp

Observations of rotational translations of neutral molecules, radicals and ions in the comet Hale-Bopp are reported on. Sample spectra and maps of the emission for the J = 1 to 0 transition of HCN, the J = 2 to 1 transition of CS and the J = 1 to 0 transition of HCO+ are presented. While the emission from HCN is typically centered on the position of the nucleus and is symmetric, the emission from HCO+ exhibits multiple peaks, together with evidence for acceleration away from the nucleus.

Irvine, W. M.↗
NASA NTRS1997

Hydrogenation of interstellar molecules: a survey for methylenimine (CH2NH)

Methylenimine (CH2NH) has been convincingly detected for the first time outside the Galactic center as part of a study of the hydrogenation of interstellar molecules. We have observed transitions from energy levels up to about 100 K above the ground state in the giant molecular clouds W51, Orion KL and G34.3 + 0.15. In addition, CH2NH was found at the " radical-ion peak" on the quiescent ridge of material in the Orion molecular cloud. The abundance ratio CH2NH/HCN at the radical-ion peak agrees with the predictions of recent gas-phase chemical models. This ratio is an order of magnitude higher in the warmer cloud cores, suggesting additional production pathways for CH2NH, probably on interstellar grains.

Non-NASA Center↗
NASA NTRS1997

HCO+ in the coma of comet Hale-Bopp

Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO+ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO+ emission require a low abundance of HCO+ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (Te). To set constraints on the formation of HCO+ in the coma, as well as the location and magnitude of the transition to higher Te, the data are compared with the results of ion-molecule chemistry models.

Non-NASA Center↗
NASA NTRS1997

The HNC/HCN ratio in comets

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.

NASA Discipline Exobiology↗
NASA NTRS1995

Measurements of the H2(13)CO ortho/para ratio in cold dark molecular clouds

H2(13)CO has been detected for the first time toward cold dark molecular clouds using the NRAO 12 m telescope. The H2(13)CO ortho/para abundance ratio R for B335, which we report as R approximately 1.7, suggests equilibrium at the local kinetic temperature and appears to be distinctly different from that for both TMC-1 and L134N, where R is close to or higher than the statistical value 3. Since only B335 among the observed positions includes an imbedded IR source, this difference may result from heating of the grain surfaces, providing the energy necessary for desorption of formaldehyde formed on the grains.

NASA Discipline Exobiology↗