OH Emission Lines Research Articles

OH Observations of the Open Cluster NGC 2264.

view Abstract Citations (14) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS OH Observations of the Open Cluster NGC 2264. Riegel, Kurt W. ; Crutcher, Richard M. Abstract The young open cluster NGC 2264 has been observed as a source of 1667- and 1665-MHz OH line emission. Further observations of the cluster and several nearby points in the sky, in the main lines and in the 1612- and 1720-MHz satellite lines, show that the OH emission is from interstellar dust clouds. Our data support the interpretation of thermal OH emission from dust clouds toward and near NGC 2264, and do not support Class lIb IR-OH or other discrete sources as the origin of the 18-cm OH emission. Publication: The Astrophysical Journal Pub Date: March 1972 DOI: 10.1086/180902 Bibcode: 1972ApJ...172L.107R full text sources ADS |

Microwave Emission from Stars

During the past several years, the radio astronomical detection of a large number of complex organic and inorganic molecules has led to an entirely new concept of the chemical composition, kinematics and excitation of the interstellar medium. The microwave molecular transitions detected thus far fall into two natural categories: 1. „normal“ (i.e. nearly LTE) emission and absorption lines, and 2. anomalous or „masered“ emission lines. The first category includes molecules found in both high and low excitation regions whose intensity, linewidth and other characteristics may be described in terms of a physically realistic excitation temperature. Nearly all of the known microwave lines fall into this category. Typically, molecular lines of this type are found in HII regions, dark nebulae and, occasionally, extended circum-stellar shells. Only two molecules populate the second category: OH and H20. Emission from OH lambda doublet transitions in several rotational states and from the 618—523 rotational transition of H20 exhibits many strange characteristics: tremendous intensity, narrow line-width, non LTE distribution of hyperfine intensities, linear and circular polarization, and rapid variability. OH and H20 emission originates from extremely localized regions, often as small as a few A.U. in diameter, in or near some HII regions and some cool, late type stars. The behavior of OH and H20 emission lines is best explained by assuming that the microwave radiation is amplified by some narrow band non-linear process in the sources.

The Galactic Radio Source DR 21 AT 3.3 MM

view Abstract Citations (13) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Galactic Radio Source DR 21 AT 3.3 MM Riegel, Kurt W. ; Epstein, Eugene E. Abstract The flux of DR 21 at 3.3 mm is (21 ± 4) X 1O~ W m~ Hz'; this value is consistent with a model of DR 21 consisting of a single high-density thermal component. Ryle and Downes (1967) pointed out the high-intensity, small-diameter nature of the radio source DR 21 in the Cygnus X complex of thermal radio sources. This source and DR 23 form W75. There exist contour maps of the continuum distribution of Cygnus X at 21 cm (Pike and Drake 1964) and at 6 cm (Downes and Rinehart 1966). The very high observed emission measure of DR 21, about 1O~ cm6 pc, places in it the class of very dense H ii regions discussed by Mezger, Schraml, and Terzian (1967). They found that the 2-cm continuum flux of DR 21 agrees well with the continuum spectrum derived by Downes and Rinehart. This spectrum, together with our point at 3.3 mm (91 GHz), appears in Figure 1. Mezger et al. (1967) have also found 158a hydrogen recombination line emission at a velocity of +7.8 km/sec with respect to the local standard of rest. Weaver, Dieter, and Williams (1968) report 1665-MHz OH line emission at a position 4~ 7 north of DR 21 at +1, +6, and +12 km/sec with respect to the local standard of rest. The +1 km/sec component is 42 per cent linearly polarized. To continue our study of very dense H ii regions, which began with a study of W49 (Riegel and Epstein 1967), we have observed DR 21 to extend its spectrum to higher frequencies and to test for the existence of very high-density structure. The instrument and observing and reduction procedures were identical to those used for W49. We ob- served the source position R.A. = 20h37m14 ~2, deci. = +42°09'07", for 30 hours with the 15-foot antenna of the Space Radio Systems Facility of the Aerospace Corporation. The antenna temperature of the source, corrected for atmospheric attenuation by the use of radiosonde measurements of precipitable water vapor (Epstein 1968), was (71 ± 6) X 10~ ° K. The main spatial component of the source has a half-power width of 20" at 21 cm, an amount small compared with our antenna half-power beam width of 2 8. Therefore, if we assume that the 3-mm and the 21-cm brightness distributions are similar, the effective antenna pattern f' defined in equation (2) of Riegel and Epstein (1967), under the assumption that the source was at the beam center, is 0.986, nearly unity. Using equation (1) of the same paper, we find that the total flux is S = (21 ± 4) X 1O_26 W m2 Hz-'. (1) The uncertainty includes an estimate of the systematic errors, plus the rms noise of 2 flux units. We have also assumed that there is no difference between the shapes of the background brightness distributions at 6 cm and at 3.3 mm. From the contour maps of the continuum brightness distribution produced by Mezger et al. (1967) at 6 cm, we conclude that no significant error is introduced because of our switched-beam mode of * Supported by the United States Air Force under contract F04695-67-C-0158. K. W. R. acknowledges the support of NSF and NASA Publication: The Astrophysical Journal Pub Date: January 1968 DOI: 10.1086/180134 Bibcode: 1968ApJ...151L..33R full text sources ADS |

Nature of the Radio Source W 49.

The galactic radio source W 49 is an association of a giant H II region, a nonthermal source, and OH emission clouds. The nature of this source has been investigated using radio astronomical observaLions of both its continuum and associated line radiation. W 49 consists of the thermal component A (G 43.2+0.0) and the nonthermal component B(G 43.30.2). Both components are at about the same distance, 14.1 kpc, from the sun and a distance of about 10 kpc from the galactic center. The projected distance between the centers of components A and B is 12.5 min of arc or 51 pc. If the nonthermal component is a supernova remnant it must be a considerably older and originally much more powerful supernova remnant than Cassiopeia A. Both the analysis of the spectrum and high angular resolution observations of the brightness temperature distribution of the radio continuum of the thermal component A suggests that it consists of one or more high-density components A2 of small angular diameters (Os< 14"; N0> 1.2 X 10~ cm-3) which are embedded in a component Al of lower density and larger angular diameter (Os = 3.5'; N0 =271 cm-3). The high-density component A2- although different from any type of emission nebula known to date could be ionized by a star with Mv= -7.0 and T5=5.1040K. It is, however, doubtful if stars with this characteristics exist. It is more probable that component A2 consists of several high-density condensations, which can not be resolved with an angular resolution of 2 min of arc. The total thermal component A seems to be surrounded by a shell of neutral hydrnge~ which may contain as much as 3.3 X 10~ solar masses. Shell and H II region together from a gravitationally stable system, and could be a young 0 and B star association. Xonthermal emission of the 18 cm OH lines close to W 49 comes from two emission centers. There is a close correlation in the radial velocities of the OH emission lines and the lO9a recombination line emitted by the thermal component A of W 49. Our observations show that the projected positions of the two OH emission centers lie within the 30% contours of a contour map of component A observed at N= 1.95 cm with an angular resolution of 2'. The OH emission clouds seem to be located inside the low-density H II region Al and close to the high- density H II regions A2. The Xational Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the Xational Science Foundation. The Arecibo Ionospheric Observatory is operated by Cornell University with the support of the Advanced Research Projects Agency under a research contract with the Air Force Office of Scientific Research OAR.

Theory of interstellar molecules

Various estimates are described for the rate of formation of hydrogen molecules on the surface of interstellar grains in H 1 regions. The average abundance of molecular hydrogen relative to atomic hydrogen remains uncertain by a few orders of magnitude.Possibilities are presented for the formation of OH molecules (and, more briefly, for other diatomic molecules), either on the surface of dust grains or by exchange reactions in the interstellar gas after cloud-cloud collisions. The rate for photodissociation of OH in H 1 regions is uncertain.A brief discussion is given of various possible mechanisms for maser action in the OH molecule, such as optical pumping of microwave levels by UV radiation at the edge of an H 11 region. It appears that the narrow OH emission lines must originate in cool gas bordering an H 11 region.

Collision broadening of OH emission lines, measured in the infrared

The OH emission spectrum from an H2/O2 flame was recorded in the infrared, for the (1, 0) vibrational transition of the ground 2IIi state. The spectral region covered was 3.18 to 3.65 μ [four lines each for P(10) to P(18)]. Use of a high-resolution spectrometer with a slit width (full width at half maximum) of 0.06 to 0.04 cm−1 allowed the direct estimation of linewidths, which were found to range from 0.11 cm−1 for the P(10) lines to 0.08 cm−1 for the P(18) lines. The deduced collision linewidths for the fundamental vibrational band in the ground state are larger than expected on the basis of comparable data for HCl, and are about five times greater than those reported for the (0, 0) band of the 2Σ+→2IIi transition, measured in the ultraviolet. A theoretical estimate of the dipole-induced dipole, dispersion forces and hardcore contributions to the OH linewidth, under flame conditions, gave a value of ≈0.03 cm−1. It is suggested that resonance and near-resonance effects between energy-level spacings for OH and H2O (in the infrared) are the cause of the large linewidths.

Search for Line Emission from Interstellar OH Radicals.

view Abstract Citations (8) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Search for Line Emission from Interstellar OH Radicals. Penzias, A. A. Abstract A search for 1667-Mc OH line emission from the galactic plane was carried out using a fixed horn-reflector antenna and a multichannel radiometer. The antenna was oriented so that its beam would drift through the galactic plane at longitude 580 and at the anticenter. From a plot of the average of twelve 24-h scans no emission was detected, with sensitivities of 0.05 0K and 0.15 0K for the two longitudes respectively. (The measurements at the anticenter were less sensitive because of the proximity of the sun to this direction during the period of observations.) These measurements allow us to place an upper limit of 10-~ on non/nn for the regions investigated, provided that the excitation temperature of the transition is much larger than the background temperature. The principal excitation of this OH transition comes from an interaction between the fields of passing ions and the permanent electric dipole moment of the radical. The cross section for this process, computed from a Fourier analysis of the interaction, is 3 X lO-~~ cm2 for a particle velocity of 10~ cm/sec. We obtain an excitation temperature of approximately 500K from this process, for an ion density of 5X10-3 per cm3. A comparison of our result with recent successful OH absorption measurements indicates that the typical value of the OH excitation temperature is probably closer to 100K, suggesting that the actual ion density in interstellar clouds is substantially smaller than the generally accepted value employed above. Publication: The Astronomical Journal Pub Date: 1964 DOI: 10.1086/109430 Bibcode: 1964AJ.....69R.146P full text sources ADS |

THE NIGHT-SKY SPECTRUM λλ 5000-6500 A.

view Abstract Citations (29) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS THE NIGHT-SKY SPECTRUM λλ 5000-6500 A. Blackwell, D. E. ; Ingham, M. F. ; Rundle, H. N. Abstract The paper describes a spectrum of the night sky obtained at the cosmic-ray station of Chacaltaya (latitude - 16 , height 17100 feet) at zenith distance 840, using a dispersion of 78 A/mm The spectrum is unusually intense, and in the region XX 5577-6500 fifty-one OH emission lines (some of which are multiple) are identified; the spin doubling of many lines is clearly resolved. A comparison between observed frequencies and frequencies calculated by means of newly derived band origins and rotational constants is given Rotational temperatures are determined, and the absolute intensities of OH lines and the N I emission at X 5199 have been measured The fine structure of the OH emission in the region of Ha is calculated Publication: The Astrophysical Journal Pub Date: January 1960 DOI: 10.1086/146801 Bibcode: 1960ApJ...131...15B full text sources ADS |