Engineering topics

Minh, Y. C.

Publications and source records attributed to Minh, Y. C..

18 records

NASA NTRS1998

Are clouds collapsing at the 2' north position of Sagittarius B2?

The 3 mm lines of HCO2+ and HNCO have been observed toward Sgr B2. Besides the well-known "principal cloud" and an extended envelope, we find another gas cloud 2' north of Sgr B2(M). This 2' north (2' N) cloud which may be located behind the principal cloud, has a total mass of approximately 10(5) Msolar and a diameter of approximately 7 pc. HCO2+ and HNCO exist mainly at 2' north, and their column densities are about 2.2 x 10(14) and 2.3 x 10(15) cm-2, respectively. The fractional abundances of these species relative to molecular hydrogen appear to be enhanced by at least a factor of 10 compared to the principal cloud. We have also identified redshifted and blueshifted high-velocity components which move toward the 2' N position with projected velocities of +/- 30 km s-1. These components are located symmetrically around 2' N, along the Galactic plane, and have diameters of about 4-5 pc and masses of approximately 1 x 10(4) Msolar. The flow energies are large enough to initiate new star formation in the 2' N region on the free-fall timescale of 10(5) yr. This large-scale collapsing motion may cause a strong shock in the 2' N cloud and result in the enhancement of HCO2+ and HNCO.

NASA Discipline Exobiology↗

NASA NTRS1996

The high-latitude cloud MBM 7. I. H I and CO observations

The high-latitude cloud (HLC) MBM 7 has been observed in the 21 cm H I line and the 12CO(1-0) and 13CO(1-0) lines with similar spatial resolutions. The data reveal a total mass approximately 30 M solar for MBM 7 and a complex morphology. The cloud consists of a cold dense core of 5 M solar surrounded by atomic and molecular gas with about 25 M solar, which is embedded in hotter and more diffuse H I gas. We derive a total column density N(H I + 2H2) of 1 x 10(21) cm-2 toward the center and 1 x 10(20) cm-3 toward the envelope of MBM 7. The CO line indicates the existence of dense cores [n(H2) > or = 2000 cm-3] of size (FWHM) approximately 0.5 pc. The morphology suggests shock compression from the southwest direction, which can form molecular cores along the direction perpendicular to the H I distribution. The H I cloud extends to the northeast, and the velocity gradient appears to be about 2.8 km s-1 pc-1 in this direction, which indicates a systematic outward motion which will disrupt the cloud in approximately 10(6) yr. The observed large line widths of approximately 2 km s-1 for CO suggest that turbulent motions exist in the cloud, and hydrodynamical turbulence may dominate the line broadening. Considering the energy and pressure of MBM 7, the dense cores appear not to be bound by gravity, and the whole cloud including the dense cores seem to be expanding. The distance to HLCs suggest that they belong to the galactic plane, since the scale height of the cloud is < or approximately equal to 100 pc. Compared to the more familiar dense dark clouds, HLCs may differ only in their small mass and low density, with their proximity reducing the filling factor and enhancing the contrast of the core and envelope structure.

Non-NASA Center↗

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↗

NASA NTRS1993

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

High spatial resolution observations 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 about 6.5 arcsec. There also exists another oxygen-rich condensation to the west of IRc2 having column densities of HCOOCH3 and (CH3)2O comparable to those of SC. We derive the total column densities 6.8 x 10 exp 16/sq cm, 1.4 x 10 exp 16/sq cm, and 2.7 x 10 exp 16/sq cm for CH3OH, HCOOCH3, and (CH3)2O, respectively, at the core of SC.

Minh, Y. C.↗

NASA NTRS1993

Measurement of the methyl cyanide E/A ratio in TMC-1

We have observed the methyl cyanide (CH3CN) J = 2-1 K = 0 and 1 transitions toward the cyanopolyyne peak of TMC-1 and have derived an E/A (ortho/para)abundance ratio N(E)/N(A) = 0.75 +/- 0.10. The total methyl cyanide column density is N(total) = 5 x 10 exp 12/sq cm toward TMC-1, in agreement with earlier results from the J = 1-0 lines.

Minh, Y. C.↗

NASA NTRS1993

Search for H2COH+ and H2(13)CO in dense interstellar molecular clouds

We have searched for the 2 mm transitions of H2COH+ (2(02) - 1(01)) and H2(13)CO (2(02) - 1(01), 2(12) - 1(11), and 2(11) - 1(10)) toward the dense interstellar molecular clouds Orion A, TMC-1 and L134N using the FCRAO 14m telescope. None of the transitions have been detected except the H2(13)CO transitions toward Orion-KL. We set upper limits for the abundances of the protonated formaldehyde ion (H2COH+), which are close to the abundances expected from ion-molecule chemistry.

NASA Discipline Exobiology↗

NASA NTRS1993

Observations of C3H2 (2(12) - 1(01)) toward the Sagittarius A molecular cloud

We have mapped the C3H2 2(12)-1(01) transition line toward the Sgr A molecular cloud on a 1' grid spacing and derived C3H2 column densities of 3 approximately 7 x 10(14) cm-2 for molecular clouds of Sgr A. The fractional abundances of C3H2 relative to H2 are obtained to be 3 approximately 6 x 10(-9), which are slightly lower than that for the cold dark cloud TMC-1 but are enhanced by factors of 5-60 compared to those for Sgr B2 and the Orion extended ridge. We also estimate from the C3H2 column densities total masses of approximately 10(6) M(solar) for two clouds (M - 0.13-0.08 and M - 0.02-0.07), which are thought to be close to the virial equilibrium. We suggest that the large abundance of C3H2 in Sgr A may be partly due to the activities of the Galactic center.

Non-NASA Center↗

NASA NTRS1992

Molecular abundances in the Sagittarius A molecular cloud

We have obtained column densities for HCO(+), HCO, HCS(+), C3H2, HC5N, SiO, OCS, HCOOH, CH3CH2OH, and CH3CCH toward Sgr A. The fractional abundance of SiO relative to molecular hydrogen in Sgr A is comparable to that for the Orion plateau, about 10 exp-7 to 10 exp -8, which may be a typical value for hot clouds. The abundances of HCO, CH3CH2OH, and CH3CCH all appear to be enhanced relative to other molecular clouds such as Sgr B2.

Minh, Y. C.↗

NASA NTRS1991

H2CS abundances and ortho-to-para ratios in interstellar clouds

Several H2CS ortho and para transitions have been observed toward interstellar molecular clouds, including cold, dark clouds and star-forming regions. H2CS fractional abundances f(H2CS) about 1-2 10 to the -9th relative to molecular hydrogen toward TMC-1, Orion A, and NGC 7538, and about 5 10 to the -10th for L134N are derived. The H2CS ortho-to-para ratios in TMC-1 are about 1.8 toward the cyanopolyyne peak and the ammonia peak, which may indicate the thermalization of H2CS on 10 K grains. A ratio of about 3, the statistical value, for Orion (3N, 1E) and NGC 7538 is derived, while a value of about 2 for Orion (KL) is found.

Minh, Y. C.↗

NASA NTRS1991

Abundance and chemistry of interstellar HOCO(+)

Column densities of 10 to the 15th/sq cm toward the Galactic center and not more than 10 to the 12th/sq cm for cold dark clouds are derived from observations using an LVG model, and the chemical implications are discussed. The HOCO(+) 4(04)-3(03) line toward Sgr A is mapped. The fractional abundance of HOCO(+) in the Galactic center region was found to be three orders of magnitude larger than predicted by quiescent ion-molecule chemistry and an order of magnitude larger than predicted by an MHD shock model. It is suggested that the possibly high CO2 abundance, and consequently the observed HOCO(+) abundance in the Galactic center, may result from UV photolysis of grain mantles.

Minh, Y. C.↗

NASA NTRS1991

Abundances of hydrogen sulfide in star-forming regions

Interstellar H2S and its isotopic variant H2(S-34) have been observed toward several star-forming regions via their 1(10)-1(01) transitions at 2 mm, using the FCRAO telescope. In sources where both isotopic species were observed, column densities of about 10 to the 16th/sq cm were measured. Column density lower limits of about 10 to the 14th/sq cm for H2S were found for other sources, where only the main isotopic line was observed. The fractional abundances of H2S relative to molecular hydrogen appear to be enhanced by at least an order of magnitude relative to quiescent cloud values (about 10 to the -9th) for many of the observed sources.

Minh, Y. C.↗

NASA NTRS1991

Upper limits for the ethyl-cyanide abundances in TMC-1 and L134N - Chemical implications

Interstellar ethyl-cyanide has been sought via its 2(02)-1(01) transition towards two cold, dark clouds, and upper limits of the total column densities of 3 x 10 to the 12th/sq cm and 2 x 10 to the 12th/sq cm for TMC-1 and L134N, respectively. The 2(02)-1(01) transition of vynil cyanide, previously identified in TMC-1 by Matthews and Sears (1983b), was also observed. The detection of vinyl cyanide and the nondetection of ethyl cyanide in TMC-1 are consistent with gas phase ion-molecule chemical models, and there is thus no necessity of invoking grain surface synthesis for vinyl cyanide in cold clouds.

Minh, Y. C.↗

NASA NTRS1990

Observations of the H2S toward OMC-1

Observations of the 1(10) - 1(01) transition of interstellar H2S and its isotopes toward OMC-1 are reported. The fractional abundance of H2S in the quiescent regions of OMC-1 seems difficult to explain by currently known ion-molecular reactions. The fractional abundance of H2S relative to H2 is enhanced by a factor of 1000 in the hot core and the plateau relative to the quiescent clouds. The (HDS)/(H2S) abundance ratio in the hot core is estimated at 0.02 or less.

Minh, Y. C.↗

NASA NTRS1990

Detection of nitric oxide in the dark cloud L134N

The first detection of interstellar nitric oxide (NO) in a cold dark cloud, L134N is reported. Nitric oxide was observed by means of its two 2 Pi 1/2, J = 3/2 - 1/2, rotational transitions at 150.2 and 150.5 GHz, which occur because of Lambda-doubling. The inferred column density for L134N is about 5 x 10 to the 14th/sq cm toward the SO peak in that cloud. This value corresponds to a fractional abundance relative to molecular hydrogen of about 6 x 10 to the -8th and is in good agreement with predictions of quiescent cloud ion-molecule chemistry. NO was not detected toward the dark cloud TMC-1 at an upper limit of 3 x 10 to the -8th or less.

Mcgonagle, D.↗

NASA NTRS1990

New observations of interstellar organic molecules

Discussed here are new observations of 3-carbon-containing interstellar molecules which play an important role in the chemistry of dense molecular clouds: protonated carbon dioxide, formic acid, and propynal. In 1984 a new oxide of carbon, C3O, was discovered in the interstellar medium (Matthews et al. 1984; Brown et al. 1985). Theoretical models suggest that C3O is produced by dissociative electron recombination with the ion H3C3O+. Although no laboratory data for the branching ratios of such a recombination exist, it seemed to us likely that additional products would include H2C3O. This molecule has more than one isomeric form, but one stable species is propynal (HC2CHO), which had been suggested as a possible interstellar molecule by Winnewisser (1973). In observations at the National Radio Astronomy Observatory 43 m telescope in Green Bank earlier this year, researchers detected a line in the cold cloud TMC-1 which they assign to the 2(0,2)-1(0,1) transition of propynal. The observed line agrees with the laboratory frequency to well within the experimental uncertainty a few parts in 10 to the 7th power. Researchers sought and failed to detect the corresponding 2(1,1)-1(1,0) line, which is understandable given the presence of both a and b components of the electric dipole moment in propynal. The b type transitions will drain population from energy levels with K(sub p) does not equal 0 into the K(sub p) equal 0 stack. If the researchers' assignment is correct, this is the first interstellar detection of propynal. Assuming typical rotational temperatures for TMC-1 and that the line is optically thin, the column density determined is about 5 times 10 to the 12th power cm to the -2nd power, or about three times that for C3O. Formic acid (HCOOH) was the first organic acid to be observed in the interstellar medium, in the Galactic center source Sgr B2. The only other interstellar detection has been recently made in the giant molecular cloud in Orion. As part of the same project that led to the detection of propynal, researchers sought formic acid in the nearby, cold dark clouds TMC-1 and L134N. It is seen only in L134N. This difference between the two clouds appears to be consistent with a model in which the oxygen/carbon ratio is higher in L134N. This would be consistent with the higher abundances for SO and SO2, observed in that cloud, and the higher abundances of many carbon-rich species in TMC-1 (Irvine et al. 1985).

Irvine, W. M.↗

NASA NTRS1990

Detection of formic acid in the cold, dark cloud L134N

The detection of formic acid (HCOOH) in a cold dark interstellar cloud (L134N) is reported. The observed abundance of 3 x 10 to the 10th relative to H2 is between one and two orders of magnitude lower than that calculated by published ion-molecule models of dark-cloud chemistry, but is quite consistent with recent model revisions based on new reaction rates. Formic acid was not detected in the archetypical dark cloud TMC-1, and was tentatively detected in the region of massive star formation W51.

Irvine, W. M.↗

NASA NTRS1989

Detection of interstellar hydrogen sulfide in cold, dark clouds

Interstellar H2S has been detected toward the cold, dark clouds L134N and TMC 1. Total column densities at the SO peak of L134N and the NH3 peak of TMC 1 are found to be about 2.6 X 10 to the 13th/sq cm and 7.0 X 10 to the 12th/sq cm, respectively. The results suggest that grain surface reactions may play a major role in the synthesis of H2S in cold, dark clouds.

Minh, Y. C.↗

NASA NTRS1988

Observations of interstellar HOCO(+) - Abundance enhancements toward the Galactic center

The 4(04)-3(03) and 1(01)-0(00) transitions of HOCO(+) toward several molecular clouds have been monitored. Of the molecular clouds considered, the HOCO(+) molecule has been detected only in Sgr B2 and Sgr A. The results suggest that gas phase CO2 is not a major carbon reservoir in typical molecular clouds. For Sgr B2, the column density of HOCO(+) at the northern peak is found to be about 3 x 10 to the 14th/sq cm, and a fractional abundance relative to H2 is obtained which is about 2 orders of magnitude greater than recent theoretical predictions.

Minh, Y. C.↗