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At least 217 records · Page 12Linked to original sources
PubMed1987

Comet Sugano-Saigusa-Fujikawa (1983V)--a small, puzzling comet.

Spectroscopic and infrared observations of Comet Sugano-Saigusa-Fujikawa (1983V) were obtained during its close approach to the Earth on 11-14 June 1983. The [O I] production rates of 1.8 +/- 0.9 x 10(26) atoms/s observed on 12.3 June and 7 +/- 3.5 x 10(26) atoms/s on 13.4 June lead to derived water-production rates of 3 x 10(27) mol/s on 12 June and 1.1 x 10(28) mol/s on 13 June. The abundances of the minor species NH2, CN, C2, and C3 are unusually low relative to [O I]. The upper limit to the average nuclear radius from our infrared and visual photometry on 12-13 June (assuming that the entire signal came from the nucleus) is approximately 370 m. The dust/gas mass ratio was <0.01 on June 12 and <0.005 on June 13.

Astronomical Phenomena↗
PubMed1985

Search for molecular oxygen in dense interstellar clouds.

We have carried out a search for the 234 GHz N = 2 --> 0, J = 1 --> 1 transition of 16O18O using the 13.7 m FCRAO radio telescope. No emission was detected toward six giant molecular clouds. Observations of the 220 GHz J = 2 --> 1 transition of C18O yield column densities for this species 1-3 x 10(16) cm-2; the resulting limits on the [O2]/[CO] ratio lie between <0.5 and <4. According to various chemical models, the ratio of molecular oxygen to carbon monoxide is primarily sensitive to the age of a cloud and to its carbon to oxygen ratio. For ages exceeding 3 x 10(6) yr and total carbon-to-oxygen ratio < 1, [O2]/[CO] can approach unity. Our best limits can be interpreted as indicating that the observed clouds are not chemically "mature" or that [carbon]/[oxygen] > 1. However, significant exploitation of molecular oxygen as a tracer of cloud structure and evolution will require more sensitive observations, which may be best carried out from above Earth's atmosphere.

Astronomical Phenomena↗
PubMed1986

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds.

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.

Astronomical Phenomena↗
PubMed1992

SO and SO2 in mass-loss envelopes of red giants: probes of nonequilibrium circumstellar chemistry and mass-loss rates.

We have made a search for SO in the mass-loss envelopes of 24 mostly oxygen-rich red giants, using the 8(8)-7(7) (344.3 GHz), 6(5)-5(4) (219.9 GHz), 4(3)-3(2) (138.2 GHz), and 3(2)-2(1) (99.3 GHz) transitions. One or more of these transitions were detected toward 14 envelopes, including those of the supergiants VY CMa and IRC +10420. The SO envelope of IK Tau was measurably extended, and that of IRC +10420 marginally so, allowing us to determine the envelope sizes directly. The SiO (v = 0) J = 8-7, J = 5-4, and J = 3-2 lines at 347.3, 217.1, and 130.3 GHz were detected simultaneously with the 344.3, 219.9, and 138.2 GHz SO lines, respectively. From our SO data, as well as from SO2 data published elsewhere (for a few sources), we derive the circumstellar SO and SO2 abundances and compare them with theoretical predictions. We find that the photospheric abundances of SO and SO2 are insignificant, and that SO and SO2 must be produced in the circumstellar envelopes. However, the (average) SO abundances are larger than even the peak values predicted by nonequilibrium circumstellar chemistry models. The large SO abundances imply that S is not heavily depleted into grains in these envelopes, and further suggest that (i) the stellar mass-loss rates in these objects are substantially larger than accepted values, (ii) the circumstellar H/H2 abundance ratio is quite small, and (iii) the reaction of O and SH dominates over the reaction of S and OH in the production of SO. In the peculiar bipolar nebula OH 231.8+4.2, the combined SO and SO2 abundance exceeds the cosmic S abundance, if the mass-loss rate in this object is smaller than or equal to l0(-4) M solar yr-1. In IK Tau the data can only be fitted with models in which the SO is distributed in a hollow shell, as predicted by the nonequilibrium chemistry models. In another three, which have no measurable OH, the discovery of SO is difficult to explain by such models.

Astronomical Phenomena↗
PubMed1993

Interferometric observations for oxygen-containing organic molecules toward Orion-KL.

High spatial resolution observations (approximately 5") were made for the 3 mm transitions of methanol (CH3OH), methyl formate (HCOOCH3), and dimethyl ether [(CH3)2O] toward Orion-KL using the Nobeyama Millimeter Array. The 15(3)-14(4) A- CH3OH emission appears to be elongated along the line connecting IRc2 and "the southern condensation (SC)", which may suggest a relation between methanol and the outflow from IRc2. The HCOOCH3 (7(1,6)-6(1,5)) and (CH3)2O (15(2,13)-15(1,14)) emissions appear to be well concentrated toward SC with an angular size of approximately 6".5 (at the 2 sigma level). There also exists another oxygen-rich condensation to the west of IRc2 (angular size approximately 4".5) having column densities of HCOOCH3 and (CH3)2O comparable to those of SC. We derive the total column densities 6.8 x 10(16) cm-2, 1.4 x 10(16) cm-2 and 2.7 x 10(16) cm-2 for CH3OH, HCOOCH3, and (CH3)2O, respectively, at the core of SC.

Astronomical Phenomena↗
PubMed1994

The circumstellar environment of IRAS 05338-0624.

Millimeter continuum and spectral line observations with 10", 30", and 60" resolution are used to characterize the structure and chemistry of the gas around the young, embedded star, IRAS 05338-0624. On arcminute scales, emission from dense gas tracers outline an isolated condensation centered on the IRAS source position. The condensation is characterized by a size of approximately 60", a density of 2 x 10(5) cm-3, and a virial mass of 40 M solar. Interferometric CS J = 2-1 observations show two peaks, one toward the continuum peak and the other toward a position 14" west and 8" south. Single-dish maps of SO, CH3OH, and SiO show pronounced wing emission to the west of the IRAS source, which interferometer observations reveal to be a compact region of outflow activity. CS emission as redshifted and blueshifted velocities reveals a bipolar outflow oriented with a position angle of 45 degrees, while SiO emission appears to be tracing a fast shock interaction region at the CS red-lobe peak, 14" west and 8" south of the IRAS source. Finally, H13CO+ emission traces clumps of quiescent gas toward the IRAS source and adjacent to the blue lobe of the outflow. Column densities and molecular fractional abundances are derived to explore the interaction between the surrounding condensation and the young stellar object. We find evidence for gas phase depletions within the overall condensation in several gas tracers (CO, CS, HCN, SO) but not in the region immediately around the young stellar object. Enhanced abundances of SO, CH3OH, and SiO (by factors of 4, >100, >1000, respectively) are observed in the shocked gas; these enhancements may be explained in terms of a nondissociative shock liberating mantle materials that contain some amount of refractory materials, a moderate velocity dissociative shock in which only minor sputtering of Si occurs, or a shock that impacts surrounding material with a range of speeds.

Astronomy↗
PubMed1987

Detection of interstellar PN: the first phosphorus-bearing species observed in molecular clouds.

Phosphorus nitride (PN) has been detected in the interstellar medium. The J = 2-1, 3-2, 5-4, and 6-5 rotational lines of this species have been observed toward Orion-KL, and the J = 2-1 transition in Sgr B2 and W51. The PN line profiles in Orion indicate that the molecule's emission arises from the "plateau" or "doughnut" region associated with the outflow from IRc2. The species is thus primarily present in hot, dense gas. Column densities derived for PN toward Orion-KL are 3-4 X 10(13) cm-2, but may be as high as 10(14) cm-2, if the species is located in a 10" region. These column densities imply a fractional abundance for PN in the Orion "plateau" of approximately 1-4 X 10(-10). Such a large abundance for phosphorus nitride is not predicted by quiescent cloud ion-molecule chemistry and suggests that high-temperature processes are responsible for the synthesis of PN in the KL outflow.

Astronomical Phenomena↗
PubMed1998

H2O ice in the envelopes of OH/IR stars.

In an attempt to better understand the conditions under which molecules condense onto grains in the envelopes of evolved stars, we have searched for the presence of H2O ice in the circumstellar envelopes of several evolved (OH/IR) stars. The sample of stars observed was selected on the basis of mass-loss rates, luminosities, and outflow velocities in order to cover a range of physical conditions that might affect the amount of ice present in stellar envelopes. Despite the clear presence of H2O ice around other, previously observed, evolved stars, our search in six OH/IR stars has resulted in only one clear detection, in OH 26.5 + 0.6, and the tentative detection in one other, OH 26.4-1.9. We provide column densities or upper limits for the amount of ice that is present on the grains around these stars and explore the possibility that there could be a relationship between M* or M*/L* and the H2O ice column density to explain the observations.

Astronomical Phenomena↗
PubMed1997

Inference of a 7.75 eV lower limit in the ultraviolet pumping of interstellar polycyclic aromatic hydrocarbon cations with resulting unidentified infrared emissions.

The discrete infrared features known as the unidentified infrared (UIR) bands originating in starburst regions of other galaxies, and in H II regions and planetary nebulae within the Milky Way, are widely thought to be the result of ultraviolet pumped infrared fluorescence of polycyclic aromatic hydrocarbon (PAH) molecules and ions. These UIR emissions are estimated to account for 10%-30% of the total energy emitted by galaxies. Laboratory absorption spectra including the vacuum ultraviolet region, as described in this paper, show a weakening of the intensity of absorption features as the population of cations increases, suggesting that strong pi* <-- pi transitions are absent in the spectra of PAH cations. This implies a lower energy bound for ultraviolet photons that pump infrared emissions from such ions at 7.75 eV, an amount greater than previously thought. The implications include size and structure limitations on the PAH molecules and ions which are apparent constituents of the interstellar medium. Also, this might affect estimations of the population of early-type stars in regions of rapid star formation.

Astronomy↗
PubMed1997

The L1457 molecular/atomic cloud complex: H I and CO maps.

L1457 is the closest known molecular cloud (65 pc), and it lies near the edge of the local hot bubble and well out of the Galactic plane (b approximately -34 degrees). We have mapped an 8 degrees x 8 degrees region at 35' resolution and a 3 degrees x 5 degrees region at approximately 2' resolution in H I 21 cm emission. We have also mapped a 2 degrees x 4 degrees region at 2' resolution in 12CO J = 1-0. We find that there is an extended component of atomic gas, clearly associated with the molecular complex and comparable to it in total mass. The H I structure at small scales in the vicinity of the molecular clouds is remarkable, consisting largely of long, narrow filaments less than 20' (0.2 pc) in width and 1 degree-4 degrees in length. A thin (<10') limb-brightened atomic halo is seen to surround the CO at some velocities, but it is ill-defined at other velocities. The halo may be disturbed by external pressure, perhaps from the hot gas in the local bubble. The molecular clouds are part of a large structure approximately 5 degrees x 3 degrees in extent with a small "funnel-shaped" extension to the south. The structure, which we call the L1457 atomic/molecular complex, is dominated by H I in the north and H2 in the south extension. Roughly one-half the mass of the complex is molecular. The structure of this complex at both large- and small-scale suggests that the south end has been recently compressed.

Astronomical Phenomena↗
PubMed1997

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.

Astronomical Phenomena↗
PubMed1997

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.

Alkynes↗
PubMed1998

Processing of icy mantles in protostellar envelopes.

We have obtained CO absorption profiles of several young stellar objects (YSOs), spanning a range of mass and luminosity, in order to investigate their ice mantle composition. We present the first detection of CO toward the class I YSO L1489 IRS in the Taurus dark cloud. In general, the CO profiles for YSOs show evidence for both processed and pristine ices in the same line of sight, strong indirect evidence for CO, is suggested in R CrA IRS 7, L1489 IRS, Elias 18, and GL 961E. Toward other sources (R CrA IRS 1, IRS 2, W33A, NGC 7538 IRS 9, Mon R2 IRS 2) CO is present in (nearly) pure form. We propose an evolutionary scenario to explain the chemical diversity of the icy mantles toward these objects.

Astronomical Phenomena↗
PubMed1999

The interstellar 4.62 micron band.

We present new 4.5-5.1 micron (2210-1970 cm-1) spectra of embedded protostars, W33 A, AFGL 961 E, AFGL 2136, NGC 7538 IRS 9, and Mon R2 IRS 2, which contain a broad absorption feature located near 4.62 micron (2165 cm-1), commonly referred to in the literature as the "X-C triple bond N" band. The observed peak positions and widths of the interstellar band agree to within 2.5 cm-1 and 5 cm-1, respectively. The strengths of the interstellar 4.62 micrometers band and the ice absorption features in these spectra are not correlated, which suggests a diversity of environmental conditions for the ices we are observing. We explore several possible carriers of the interstellar band and review possible production pathways through far-ultraviolet photolysis (FUV), ion bombardment of interstellar ice analog mixtures, and acid-base reactions. Good fits to the interstellar spectra are obtained with an organic residue produced through ion bombardment of nitrogen-containing ices or with the OCN- ion produced either through acid-base reactions or FUV photolysis of NH3-containing ices.

Ammonia↗
PubMed1999

Polycyclic aromatic hydrocarbons and the diffuse interstellar bands: a survey.

We discuss the proposal relating the origin of some of the diffuse interstellar bands (DIBs) to neutral and ionized polycyclic aromatic hydrocarbons (PAHs) present in interstellar clouds. Laboratory spectra of several PAHs, isolated at low temperature in inert gas matrices, are compared with the spectra of five reddened early-type stars selected from an extensive set of astronomical spectra. From this comparison, it is concluded that PAH ions are good candidates to explain some of the DIBS. Unambiguous assignments are difficult, however, because of the shift in wavelengths and the band broadening induced in the laboratory spectra by the solid matrix. This situation is illustrated by a comparison with the gas-phase spectra made available recently for two PAH ions. Definitive band assignments and, ultimately, the test of the proposal that PAH ions carry some of the DIBs must await the availability of a larger set of gas-phase measurements in the laboratory. The present assessment offers a guideline for future laboratory experiments by allowing the preselection of promising PAH molecules to be studied in jet expansions.

Astronomical Phenomena↗
PubMed1996

Detection of a new interstellar molecular ion, H2COH+ (protonated formaldehyde).

A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+.

Astronomical Phenomena↗
PubMed1999

Direct spectroscopic evidence for ionized polycyclic aromatic hydrocarbons in the interstellar medium.

Long-slit 8-13 micrometers spectroscopy of the nebula around NGC 1333 SVS 3 reveals spatial variations in the strength and shape of emission features that are probably produced by polycyclic aromatic hydrocarbons (PAHs). Close to SVS 3, the 11.2 micrometers feature develops an excess at approximately 10.8-11.0 micrometers and a feature appears at approximately 10 micrometers. These features disappear with increasing distance from the central source, and they show striking similarities to recent laboratory data of PAH cations, providing the first identification of emission features arising specifically from ionized PAHs in the interstellar medium.

Astronomical Phenomena↗
PubMed2000

Chemistry of the organic-rich hot core G327.3-0.6.

We present gas-phase abundances of species found in the organic-rich hot core G327.3-0.6. The data were taken with the Swedish-ESO Submillimetre Telescope (SEST). The 1-3 mm spectrum of this source is dominated by emission features of nitrile species and saturated organics, with abundances greater than those found in many other hot cores, including Sgr B2 and OMC-1. Population diagram analysis indicates that many species (CH3CN, C2H3CN, C2H5CN, CH3OH, etc.) have hot components that originate in a compact (~2") region. Gas-phase chemical models cannot reproduce the high abundances of these molecules found in hot cores, and we suggest that they originate from processing and evaporation of icy grain mantle material. In addition, we report the first detection of vibrationally excited ethyl cyanide and the first detection of methyl mercaptan (CH3SH) outside the Galactic center.

Astronomical Phenomena↗