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Evans, N. J., II

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

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

The energetics of molecular clouds. III - The S235 molecular cloud

The molecular cloud associated with the S235 H II region has been studied by means of molecular lines and near-infrared observations. The cloud consists of two components, one of which partly surrounds the S235 H II region. The other component contains a dense, hot region of active star formation, marked by self-reversed CO profiles, compact H II regions, masers, and infrared sources. From the molecular line data, the size and mass of the two components are estimated to be 6-8 pc and 3000-4000 solar masses. More detailed studies near the region of active star formation yield estimates of density (approximately 200,000-500,000/cu cm) and the abundances of H2CO, HCO(+), HCN, and (C-13)O. Analysis of the energetics suggests that the cloud is heated by the exciting star of S235 and by the exciting stars of the compact H II regions. Assuming that the gas is heated by collisions with warm dust grains, the far-infrared luminosity has been predicted. The observations of far-infrared emission are in reasonable agreement with predictions.

Evans, N. J., II↗
NASA NTRS1981

Infrared studies of the S235 molecular cloud

Infrared observations from 7.8 to 200 microns have been obtained for the S235 molecular cloud. Far-infrared maps were obtained for a region of active star formation, as marked by the presence of compact H II regions, water masers, and compact near-infrared sources. The primary heating source for the far-infrared emission appears to be the compact H II region, S235A. Detailed examination of the gas energetics in the region supports the plausibility of the picture in which the gas is heated by collisions with warm dust grains. The ratio of far-infrared optical depth to (C-13)O column density is somewhat lower in this source than is commonly found. This effect may be caused by the presence of substantial (C-13)O in regions where the dust is not warm enough to emit substantial 50-100 micron radiation.

Evans, N. J., II↗
NASA NTRS1981

Far-infrared observations of the Cepheus OB3 molecular cloud

The molecular cloud accompanying the Cepheus OB3 association is observed in the infrared at wavelengths of 10-400 microns. Far-infrared emission at 55 microns and 125 microns from the two CO hot spots, Cep A and Cep B, is mapped with a resolution of approximately one arcmin. Cep A is also mapped at 400 microns with a resolution of approximately one arcmin, and both hot spots are searched for 20-micron sources. It is noted that the Cep A hot spot appears to be heated by energy sources internal to the molecular cloud. The dust temperatures are considered sufficient to explain the gas temperatures provided the dust heats the gas by collisions. In marked contrast, the Cep B region appears to be heated from outside the cloud, with the strongest far-infrared emission arising near the interface between the molecular cloud and the S155 H II region. It is thought that the gas heating rate through collisions with dust may be insufficient to achieve the gas temperature observed in Cep B.

Evans, N. J., II↗
NASA NTRS1981

Infrared sources in dense molecular clouds

Recent accomplishments in the infrared study of dense molecular clouds are reviewed, and the outstanding problems in the area are discussed. Such questions as whether clouds are collapsing, how they can be classified, and whether star formation is occurring on the edges of clouds are analyzed. Problems of determining size, energy distribution and luminosity, spectral features, evolutionary state, and energetics of infrared sources are discussed.

Evans, N. J., II↗
NASA NTRS1980

Models of molecular clouds and the abundances of H2CO and HCO/+/

Observations of HCO(+) and H2CO in a sample of 13 molecular clouds have been analyzed by construction of uniform, spherical cloud models. The total densities and the abundance of HCO(+) and H2CO relative to H2 which result from these models fall into two domains: one group of clouds has a low temperature, moderate density, and high abundances; the other group has higher temperature and density, but lower abundances. The factor distinguishing these groups may be depletion onto grains in the denser sources.

Wootten, A.↗
NASA NTRS1980

Physical and dynamical conditions in interstellar clouds

Techniques for measuring the local and global parameters that describe molecular clouds are presented and illustrated by selected results. The local parameters which characterize a given location in a cloud are gas kinetic temperature, dust temperature, total density, magnetic field strength, abundance of species i, volume cooling and heating rates for gas and dust, and the thermal and turbulent velocities. Global properties which characterize the cloud as a whole include the size expressed as a length or area, the orientation and shape, the mass, and the integrated heating and cooling rates. If the cloud as a whole is collapsing or rotating, then the collapse velocity and the rotational velocity or angular momentum are global properties. An intermediate class of parameters which shares features of local and global properties includes the column density of species i, the total column density, and the average density.

Evans, N. J., II↗
NASA NTRS1978

The energetics of molecular clouds. II - The S140 molecular cloud

Techniques of analysis which allow estimates of kinetic temperature, gas density, gas cooling rate, and dust cooling rate are applied to the S140 molecular cloud. Maps of this cloud in various molecular lines are examined which reveal the existence of a strong temperature and density peak near the H-alpha rim and the presence of a strong near-IR source very near that peak. This source is found to have an energy distribution which is very similar to that of the Becklin-Neugebauer object, including the silicate feature at 9.5 microns. The heating and cooling rates for the gas and dust in S140 are estimated, and the predicted cooling rate for dust is shown to be much greater than that for gas. The molecular-line observations are compared with a spherical large-velocity-gradient trapping calculation. An optimum cloud model is developed which indicates that: (1) (C-13)O, HCO(+), and H2CO are about 10 times less abundant in S140 than in dark clouds; (2) the HCN abundance is approximately 4 times the H2CO abundance in S140, compared with roughly equal values in dark clouds; and (3) the (C-13)O/C(O-18) ratio is consistent with terrestrial isotope ratios.

Blair, G. N.↗
NASA NTRS1978

Carbon recombination-line mapping of the Orion Nebula

The distribution of the C75-alpha line in the Orion Nebula has been mapped with a spatial resolution of 1.3 arcmin over a field of view of 4 by 8 arcmin. It is found that ionized carbon is more extended than the radio continuum distribution, particularly to the northeast, and possibly also beyond the ionization front in the southeast. However, the emission falls off across the ionization front northwest of the nebula. The emission is generally consistent with a model in which the line is formed in a thin sheet in the molecular cloud immediately adjacent to the H II region. However, the spatial extent of the emission does suggest that some of the emitting gas may be cooler and less dense than has previously been assumed.

Kuiper, T. B. H.↗
NASA NTRS1978

Star formation in molecular clouds

A general overview is presented of the findings of molecular line and infrared astronomy regarding star formation. Beginning with the statement that stars, and therefore, planets, form from molecular clouds, it is argued that the growing body of data on conditions in molecular clouds should serve as input parameters for theoretical calculations of star and planet formation. The fact that the gravitational potential energy of molecular clouds far exceeds their thermal energy implies that the required rate of star formation is easily explained. Indeed the most important problem in this field is that the rate of star formation predicted from molecular cloud studies is too high. Until this problem is understood, additional modes of inducing star formation, while they may be occurring, only add to the existing difficulties.

Evans, N. J., II↗
NASA NTRS1977

New infrared objects associated with OH masers

Results of a search for IR radiation at 10 microns from a sample of OH maser sources are reported. An IR source is discovered near each of six OH masers which emit mainly at 1612 MHz in two distinct velocity ranges. It is shown that all the sources have varied with time, that two sources have exhibited significant variations in color temperature, and that the energy distributions of the six objects over the spectral range from 2 to 20 microns fall into two categories. Absorption in the 2.3-micron band of CO is detected in two sources, and four are found to display substantial silicate absorption features. The energy distributions of the sources indicate that pumping by 35-micron photons is feasible, but 2.8-micron pumping is very unlikely; the distributions are best explained as emission from very thick dust shells at temperatures of about 500 K. The data are taken as supporting the hypothesis that every double-peaked OH maser emitting principally at 1612 MHz is associated with an IR object. The nature of the objects inside the dust shells and their evolutionary state are briefly discussed.

Evans, N. J., II↗
NASA NTRS1977

The energetics of molecular clouds. I - Methods of analysis and application to the S255 molecular cloud

Observations of the S255 molecular cloud in the radio lines of CO, (C-13)O, and H2O as well as in the IR region from 12 to 20 microns are presented. These results show that an extended molecular cloud is associated with the optically visible H II regions S255 - S257, both the kinetic temperature and molecular density are enhanced in an area centered on a compact near-IR source, this source is also coincident with an OH and an H2O maser (within the errors), and the IR source exhibits the silicon absorption feature. First-order techniques which allow physical properties of molecular clouds to be estimated and the energetics of the gas and dust in the clouds to be analyzed are developed and applied to S255. It is concluded that: (1) the gas cooling rate is much less than the dust cooling rate; (2) the primary energy flow is in the IR through dust emission over a range of temperatures; (3) the near-IR source is apparently responsible for the local peak in gas kinetic temperature; and (4) the exciting stars of nearby H II regions appear to be the primary heat sources for the overall molecular cloud.

Evans, N. J., II↗
NASA NTRS1976

A dense molecular cloud in the OMC-1/OMC-2 region

H2CO emission at 2 mm is seen over a region 30 arcmin in extent which includes OMC-1 and OMC-2. The mass of this cloud, estimated from H2CO and CO observations, is about 7000 solar masses. The velocity pattern is one of rotation, with evidence for fragmentation into two or three distinct condensations. A sharp boundary to the molecular cloud is observed at the edge of the H II region in NGC 1977. It appears likely that NGC 1977 is a condensation at the northern end of the cloud, complementary to the Orion Nebula at the southern end.

Kutner, M. L.↗
NASA NTRS1976

Infrared observations of Monoceros R2

A cluster of compact infrared sources plus extended emission has been found at the peak of the molecular cloud in Mon R2. The extended emission is probably due to heated dust associated with PKS 0605-06, a compact H II region embedded in the molecular cloud. Several of the compact infrared sources have properties indicative of protostars. For one of these, the associated dust is apparently seen in projection against the H II region.

Beckwith, S.↗
NASA NTRS1976

H2CO emission at 2 millimeters in dark clouds

The paper reports the detection of the 2-mm emission line due to the 2(12)-1(11) transition of H2CO at two positions in the Taurus dark cloud and one position in the dark cloud L134 N. The profiles of the observed emissions are plotted, and peak radiation temperatures at 2 mm are determined for the three positions. These radiation temperatures are used along with data on the 2-mm H2CO absorption line and computed cross sections for H2-H2CO collisions to deduce the molecular hydrogen density and the ortho-H2CO column density in the two clouds. The results are shown to support previous theoretical estimates of the H2CO/H2 and the H2(C-12)O/H2(C-13)O ratios. It is concluded that if the abundances of CS and HCN in the clouds are equal to that of H2CO, emissions from these molecules should be easily detectable.

Evans, N. J., II↗
NASA NTRS1976

Radio continuum measurements of compact H II regions and other sources

Results are reported for measurements of the flux densities of 33 sources at wavelengths of 13, 4, and 2 cm, which were conducted using the NASA/JPL Deep Space Network 64-m antenna at Goldstone, Calif. Besides compact H II regions, the sources included five Uhuru X-ray sources, several IR sources, an extragalactic radio source, and a H2O maser. The Lyman continuum flux is derived from the measured radio flux and the assumed distance and temperature for 10 possible H II regions; the spectra of previously known sources are extended to higher frequencies, and upper limits are placed on the flux densities for the five X-ray sources. New compact H II regions are found in the bright nebula M78, the nebula K5, and the Ophiuchus dark cloud/IR region.

Matsakis, D. N.↗