Engineering topics

Evans, Neal J., II

Publications and source records attributed to Evans, Neal J., II.

24 records · Page 2

NASA NTRS1990

High-resolution, far-infrared observations of NGC 2071

The far-IR emission of the visible reflection nebula NGC 2071 has been resolved at both 50 and 100 microns along several directions. The observations reveal an extended, roughly spherical source with an average source diameter of about 12 arcsec or 4700 AU at 50 microns and about 16 arcsec or 6200 AU at 100 microns. The source is modeled using a radiative transport code to match scans of the source and previous photometry. The luminosity of the source is 520 solar at a distance of 390 pc. The optical depth at 100 microns is 0.20, implying a mass of 1.2-10 solar within a radius of 5900 AU. The density gradient is in good agreement with theoretical models for infalling envelopes around protostars and in reasonable agreement with other observational determinations.

Butner, Harold M.↗

NASA NTRS1990

Molecular clouds in the outer Galaxy. IV - Studies of star formation

Star formation has been studied in 17 outer Galaxy molecular clouds using the VLA to perform 6 and 20 cm continuum observations to search for H II regions as evidence of massive star formation. IRAS data are used to measure the far-IR luminosity as an indicator of the total star formation rate. H II regions are found to be associated with each of the clouds. The number of ionizing photons required by radio luminosity ranges from 6 x 10 to the 45th/s to 5.5 x 10 to the 49th/s. Far-IR emission was detected from each of the clouds with the range of luminosities being 500 to one million solar. From the data, it is concluded that there is no strong correlation between star formation activity and cloud mass.

Mead, Kathryn N.↗

NASA NTRS1989

Physical conditions in molecular clouds

Recent developments have complicated the picture of the physical conditions in molecular clouds. The discoveries of widespread emission from high-J lines of CD and 12-micron IRAS emission have revealed the presence of considerably hotter gas and dust near the surfaces of molecular clouds. These components can complicate interpretation of the bulk of the cloud gas. Commonly assumed relations between column density or mean density and cloud size are called into question by conflicting results and by consideration of selection effects. Analysis of density and density structure through molecular excitation has shown that very high densities exist in star formation regions, but unresolved structure and possible chemical effects complicate the interpretation. High resolution far-IR and submillimeter observations offer a complementary approach and are beginning to test theoretical predictions of density gradients in clouds.

Evans, Neal J., II↗

NASA NTRS1988

CO observations of L1582

In this paper, the bright-rimmed cloud L1582 is mapped in (C-12)O and (C-13)O, and the embedded NH3 core is mapped in the CS(3 - 2) line. The optical bright rim coincides with the cloud edge seen in CO. Both the kinetic temperature and column density maps show elongated features along the bright rim, and the NH3 core is located right near the edge of the cloud inside a column density plateau region corresponding to an optical extinction patch. The density enhancement in the CS core is about a factor of three over the ambient cloud and the core mass is estimated at eight solar. The dust luminosity can be supplied by radiation from the Gamma Ori OB association. The most plausible heating source for the cloud gas is the photoelectric ejection of energetic electrons from grains induced by the far-UV flux from the Gamma Ori OB association.

Zhou, Shudong↗

NASA NTRS1987

The interstellar clouds toward 3C 154 and 3C 353

Molecular observations of the interstellar clouds toward the radio sources 3C 154 and 3C 353 were obtained in order to elucidate the physical conditions within the clouds. Maps of (C-12)O emission in the J = 1-0 and J = 2-1 lines were compared with observations of the (C-13)O, CH, and OH molecules. The peak emission in the (C-12)O transitions does not occur in the direction of the continuum sources, and thus, an incomplete picture arises when only one line of sight in the two clouds is analyzed. The cloud toward 3C 154 appears to have a low extinction, but a relatively high CO abundance, suggesting that it is similar to high-latitude clouds and CO-rich diffuse clouds. The cloud toward 3C 353 is considerably denser than that toward 3C 154 and may be more like a dark cloud.

Federman, S. R.↗

NASA NTRS1987

Molecular clouds in the outer galaxy. I - Far-infrared observations

From the Kuiper Airborne Observatory, far infrared emission were detected from 4 molecular clouds in the outer galaxy. Eleven areas in six clouds were observed at 50 and 100 microns. The far-infrared luminosities indicate the presence of stars with spectral types ranging from B3 to O7. Therefore, despite the fact that these clouds have lower CO line temperatures than comparable clouds in the inner galaxy, they are giving birth to massive stars.

Mead, Kathryn N.↗