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Knapp, G. R.

Publications and source records attributed to Knapp, G. R..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Mass loss from evolved stars. II - Radio continuum emission and evolution to planetary nebulae

The radio continuum properties of cool evolved stars are investigated from the viewpoint of mass loss mechanisms and evolution through the planetary nebula stage. The VLA was used to search 32 high mass loss rate objects for 6-cm continuum emission, leading to the detection of NGC 7027, CRL 618, R Aqr, IRC + 10216, and Omicron Ceti. The evolution of such objects to the planetary nebula stage was studied by calculating the propagation of an ionization front into the remnant neutral envelope that is produced by continuous mass loss in the red giant stage. It is found that none of the stars has a hot, compact companion, except for the known cases of R Aqr and Mira, and that envelope fractional ionization is in most cases as small as less than about 0.001.

Spergel, D. N.↗
NASA NTRS1981

Temperatures of galactic molecular clouds showing CO self-absorption

The CO J = 2-1 line has been observed and, in most cases, mapped in 10 star-forming molecular clouds (W3, NGC 1333, NGC 2071, Mon R2, CRL 961, Rho Oph, W49N, W51A, DR 21, and Cep A). The CO J = 3-2 line has been observed in W3 and DR 21. The CO lines from all these sources are strongly self-absorbed. By comparing the present results with published CO(1-0) line profiles, it is found that large corrections to the temperatures of the cloud cores, as measured by the CO(1-0) lines, are required. The corrections for self-absorption bring the CO brightness temperatures into closer agreement with the grain temperatures inferred from far-IR photometry.

Phillips, T. G.↗
NASA NTRS1979

CO observations of mass outflow from the infrared star CIT 6

Results are presented for observations of CO emission from the molecular envelope surrounding the carbon star CIT 6 (IRC + 30219) made at five epochs spanning 17 months. There is no evidence for any variation in the CO line intensity on this time scale, but the intensity is much lower than that reported by previous authors. In addition, an earlier report of (C-13)O line emission cannot be confirmed, and the carbon isotope ratio C-12/C-13 is measured to be greater than 20. The shape of the CO emission line observed at all epochs can be fitted by a uniformly expanding cloud model in which the CO is optically thin except through the longest lines of sight.

Knapp, G. R.↗
NASA NTRS1978

Radio observations of interstellar CN toward diffuse clouds, dark clouds, black clouds, and circumstellar clouds

Emission at 113 GHz from the CN molecule has been searched for in a large number of interstellar regions, primarily dark dust clouds. Lines have been detected in four dark clouds, the first time CN has been observed in this type of object. Comparative CO observations were also performed. The CN/CO abundance ratio varies from cloud to cloud, even among objects which are apparently otherwise similar. This variation suggests that the chemistry of dark clouds may be time-dependent. A previously reported detection of CN emission from a diffuse cloud was not confirmed. Several black clouds and circumstellar clouds were reobserved to obtain better line parameters and to serve as comparative interstellar chemical systems.

Allen, M.↗
NASA NTRS1977

CO observations of galactic reflection nebulae

Carbon monoxide emission has been observed toward about 35 galactic reflection nebulae. The peaking of CO temperatures near the hotter stars indicates substantial local heating of the gas and dust by the embedded stars. Wide low-level emission wings are seen on several of the (C-12)O line profiles; these are most plausibly interpreted as due to cloud material accelerated by such processes as radiation pressure from the newborn stars.

Knapp, G. R.↗
NASA NTRS1976

CO observations of NGC 1579 /S222/ and S239

The 2.6-mm lines of (C-12)O and (C-13)O have been observed toward the small galactic diffuse nebulae NGC 1579 (S222) and S239. The H92-alpha recombination line has also been detected from NGC 1579. Toward NGC 1579, evidence was observed for self-absorption in the (C-12)O line. For both regions, high-velocity wings and line broadening are observable in the (C-12)O line; the features observed toward S239 suggest ordered free-fall collapse of a localized region of the cloud onto a newborn stellar cluster. Examination of the available data suggests that NGC 1579 is both a reflection nebula (illuminated by LKH-alpha 101) and an obscured H II region excited by a star of spectral type near B1, while S239 is a reflection nebula or a Herbig-Haro object. The present studies of these regions show that line widening in CO is at least as important an indicator of star-formation activity as enhanced line emission.

Knapp, G. R.↗
NASA NTRS1976

Observations of heavy-element recombination lines in the Rho Ophiuchi dark cloud at 13 centimeters wavelength

High-sensitivity observations at the carbon 140-alpha and 141-alpha recombination-line frequencies in the direction of the Rho Oph dust cloud show the presence of recombination lines of carbon and sulfur, but not of other heavy elements. These results require that the elements Si, Fe, and Mg are depleted by factors greater than 15, 10, and 2, respectively, most likely into interstellar grains, whereas sulfur is undepleted. They also require that only approximately one-sixth of the carbon is present in the gas phase in the cloud. From the observed size of the C II region together with the observed carbon depletion, it is inferred that the density in the carbon-line-emitting region is 25,000 per cu cm and that the size of the S II region is significantly larger than that of the C II region. The agreement between the widths of the recombination lines (which arise in a small region of the cloud) and those of the more widely distributed molecular species suggests that the lines in this cloud are broadened by microturbulence on a scale of much less than 0.1 pc.

Knapp, G. R.↗
NASA NTRS1976

Carbon recombination line observations of the sharpless 140 region

Carbon recombination-line emission has been detected at two frequencies from a dark cloud contiguous with the small H II region Sharpless 140. The observations show the dark cloud to be of unusually low temperature and to have a markedly inhomogeneous density distribution, with localized region of high density surrounding one or more embedded stars. The carbon is probably ionized by photons from both the exciting star of S140 and the embedded stars. The dark cloud and S140 apparently represent two stages of star formation which have occurred over a period of at least 500,000 years in adjacent regions of the same dark cloud.

Knapp, G. R.↗
NASA NTRS1976

CO observations of the expanding envelope of IRC + 10216

The paper reports high-sensitivity observations of (C-12)O and (C-13)O in the infrared source IRC + 10216, which were made with the NRAO 11-m telescope at frequencies of 115.2712 and 110.2014 GHz, respectively. The emission-line profiles are found to be in good qualitative agreement with model profiles predicted for a spherically symmetric and uniformly expanding envelope. Analysis of the profile shapes indicates that the (C-12)O line is optically thick, while the (C-13)O line is both optically thin and double-peaked. It is shown that the shape of the (C-12)O line is determined largely by the motion of the gas and that the expansion velocity at the edges of the CO cloud is 15 km/s, in contrast to 12 km/s for more centrally confined molecules. Based on the estimated (C-13)O content, the envelope mass is estimated to be 0.5 solar mass.

Kuiper, T. B. H.↗
NASA NTRS1975

Radio recombination line observations of the C II region NGC 2023

The reflection nebula NGC 2023 has been observed at four frequencies in a study of its carbon recombination line emission. The observations show the recombination line region to be about 2 pc in diameter, with an average electron density of about 0.2 per cu cm and electron temperature of about 20 K. The electron density increases exponentially near the B1.5 star HD 37903, the illuminating star of NGC 2023. The most reasonable interpretation of the observational results is that the carbon emission lines arise in a cold C II region surrounding HD 37903. The coexistence of the C II region with a region of enhanced CO emission suggests that the CO in NGC 2023 may be excited by electron collisions.

Knapp, G. R.↗
NASA NTRS1975

CO observations of the expanding envelope of IRC plus 10216

High-sensitivity emission profiles were observed for the transition of C12O16 and C13O16 towards IRC + or - 10216. It appears that the spherically symmetric uniform mass-outflow model proposed by Morris is necessary to describe the line profiles. The outflow appears to be slightly accelerated, having a velocity of 15 km/sec at the edges of the CO cloud, compared with 12 km/sec for the more centrally confined molecules.

Kuiper, T. B. H.↗
NASA NTRS1974

Observations of H I in dense interstellar dust clouds. I - A survey of 88 clouds. II - The cloud Khavtassi 3

An investigation was conducted concerning the presence or absence of atomic hydrogen self-absorption features in a large number of clouds. On-off observations of each cloud in the 21-cm line were made using high-velocity resolution. It was found that 52% of the clouds observed showed no sign of self-absorption. About a third of all clouds investigated showed fairly definite self-absorption. Questions of local dust cloud kinematics are considered.

Knapp, G. R.↗