arXiv subjects

T. G. Phillips

Publications and source records attributed to T. G. Phillips.

59 records · Page 4Linked to original sources

arXiv1998

CO, CI and CII observations of NGC 7023

We present new data on the photodissociation regions associated with the reflection nebula NGC7023. 13CO(3-2) emission, delineates a molecular cloud containing a cavity largely devoid of molecular gas around this star. Neutral carbon is closely associated with the 13CO emission while ionized carbon is found inside and at the edges of the cavity. The ionized carbon appears to be, at least in part, associated with HI. We have mapped the northern and southern rims in 12CO(6-5) emission and found a good association with the H2 rovibrational emission, though the warm CO gas permeates a larger fraction of the molecular cloud than the vibrationally excited H2. The results are compared with PDR models. We suggest that a second PDR has been created at the surface of the molecular cloud by the scattered radiation from HD 200775. This second PDR produces a layer of atomic carbon at the surface of the sheet, which increases the predicted [C]/[CO] abundance ratio to 10%, close to the observed value.

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arXiv1997

Detection of the ^3P_2 - ^3P_1 Submillimeter Transition of 13CI in the Interstellar Medium: Implication for Chemical Fractionation

We report the first detection of the submm emission from the 13C isotope of atomic carbon in the ISM. The F = 5/2 - 3/2 component of the ^3P_2 - ^3P_1 transition was observed with the CSO in a region ~4' S of Orion IRc2, near the western end of the Orion Bar. The 12C to 13C isotopic abundance ratio is 58 +/- 12 corrected for opacity of the 12CI line and the fractional intensity of the 13CI hyperfine component (60%). This is in agreement with the value for the equivalent ratio in C+. In comparison, our measurement of the C18O to 13C18O ratio from observations of 2-1 and 3-2 lines toward the same position gives a value of 75 +/- 9. PDR models predict that the 12C to 13C abundance ratio is particularly sensitive to chemical fractionation effects. If 13C+ is preferentially incorporated into 13CO at cloud edges, there will be a dramatic reduction in the abundance of 13C. This is contrary to our observations, implying that the importance of chemical fractionation is small or is compensated for by isotopic-selective photo-dissociation of 13CO in this region with a large UV illumination.

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arXiv1997

Redshifted molecular absorption systems towards PKS1830-211 and B0218+357: submillimeter CO, CI and H2O data

We have detected the J=4-3 rotational transition of 12CO in absorption at z=0.89 towards the quasar PKS1830-211, but not the 12CO 5-4 or the 3P1-3P0 fine structure line of neutral carbon. The intervening molecular medium thus has a total 12CO column density of N(CO)~2x10^18 cm^-2, which corresponds to the large column density of molecular hydrogen of N(H2)=2.5x10^22 cm^-2 and a reddening of Av=25 magnitudes. The 12CO excitation temperature is low, below 15 K. Comparison with the molecular absorption results of Wiklind and Combes (1996) shows that the absorbing material has similar molecular abundances to Galactic dark clouds. We find an upper limit for atomic carbon of N(CI)<10^18 cm^-2, which again would be the case for most Galactic dark clouds. We also report new observations of the absorbing system towards B0218+357 at z=0.68. We have tentatively detected the 13CO 4-3 line, but for H2O, although a feature is seen at the correct velocity, due to the inadequate signal to noise ratio we report only an upper limit for the fundamental line of ortho water vapor. The tentative detection of the 13CO J=4-3 line implies that the 13CO excitation temperature is lower than 20K and the column density is fairly large, N(13CO)~10^17 cm^-2, giving rise to saturated absorption in the J=2-1 transition. The total column density of molecular gas is again large in this source, N(H2)>2x10^22 cm^-2, corresponding to a reddening >20 magnitudes.

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arXiv1995

TURBULENCE IN MOLECULAR CLOUDS

We generate random Gaussian turbulent velocity fields with a Kolmogorov spectrum and use these to obtain synthetic line-of-sight velocity profiles. The profiles are found to be similar to line profiles observed in molecular clouds. We suggest methods for analysing measured line profiles to test whether they might arise from Gaussian Kolmogorov turbulence.

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arXiv1994

Maps of the Molecular Emission Around 18 Evolved Stars

We present maps at $20''$ resolution of the molecular emission around 18 evolved stars (14~asymptotic giant branch stars, one supergiant, two proto-planetary nebulae and one planetary nebula), mostly in the $^{12}$CO(3-2) line. Almost all molecular envelopes appear to be at least marginally resolved at this resolution. A substantial fraction of the molecular envelopes show clear deviations from spherical symmetry in the form of elliptical or bipolar envelopes. This indicates that there is a need to implement non-spherical mass loss in current scenarios of the late stages of stellar evolution, in particular on the asymptotic giant branch.

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