The Galactic Radio Source DR 21 AT 3.3 MM

Abstract

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 |