Interstellar Hydrogen and the Local System by David S

Proceedings of the NATIONAL ACADEMY OF SCIENCES

Volume 40 Number 12 December 15, 1954

INTERSTELLAR HYDROGEN AND THE LOCAL SYSTEM BY DAVID S. HEESCHEN AND A. EDWARD LILLEY

HARVARD COLLEGE OBSERVATORY Communicated by D. H. Menzel, August 11, 1954 In two regional studies now under way at the George R. Agassiz Station of Har- vard Observatory, we have concentrated on observations of the 21-cm. line of hydrogen for the sections of the galactic center and anticenter.i 2 The general pro- gram of 21-cm.-line research and the equipment in use have been described by Bok and Ewen.3 An interesting feature of galactic structure near the sun is revealed by the com- bined observations of the center and anticenter surveys. The radio measures for the galactic center and anticenter taken, respectively, at galactic longitudes 3270 and 147° lie on a circle which is normal to the galactic plane. The observations are presented here in polar form, with galactic latitude the angular co-ordinate and hydrogen-line peak brightness temperature the radial co-ordinate. All observations were made with the antenna tracking a fixed position in the sky. The receiver, as it is tuned in frequency, continuously records the brightness temperature in the hydrogen line as a function of frequency. The maximum value recorded for the brightness temperature is referred to as the "peak brightness temperature," and its value is shown in the diagram. Four or more frequency scans were made at each position. The probable error of a peak brightness temperature, from the mean of four scans, is about 4 5' K. i The diagram shows two distinct features of brightness distribution. The brightness temperature reaches its highest values in the plane and drops off rapidly with latitude, as is to be expected. In addition, however, there is a second concentra- tion of hydrogen along a line inclined at about 200 to the galactic plane. It was first pointed out by John Herschel4 that the plane of symmetry for the brightest stars was inclined 20° to the galactic equator. This phenomenon, known as Gould's Belt, has been studied by many investigators; Bok has summarized their results5 and lists bright B, Be, and A stars and extended dark nebulae as be- longing to Gould's Belt. Recently, Nassau and Morgan have shown6 that the OB stars within 400 parsecs of the sun show a similar tendency. It now appears that the near-by clouds of neutral hydrogen are also concentrated to Gould's Belt. This is not surprising, in view of the close association of gas and dust recently pointed out by one of us.2 The regions of high intensity of the 21- cm. line of neutral hydrogen at latitudes +200 and -20°, shown in Figure 1, coincide with regions of heavy obscuration. These regions of high peak brightness 1095 Downloaded by guest on September 27, 2021 1096 ASTRONOMY: G. P. KUIPER PROC. N. A. S.

~~147O 0 GALATI PANE ~=3270 10 20 ° SUSICI -

~~~~~~ L.....1 10°K

FIG. 1.-Distribution of the peak brightness temperature with galactic latitude for a section perpendicular to the galactic plane at longitudes 327° and 1470. temperature correspond well to the position of Gould's Belt at these longitudes and lend additional support to the hypothesis of a "local system" or give evidence for some definite feature of local structure in the neighborhood of the sun. It appears that the neutral hydrogen gas, as well as the dust and early-type stars, is a constit- uent of the local system, which means that a good fraction of all matter near the sun is concentrated toward Gould's Belt. This work was supported in part by grants from the National Science Foundation and by a gift from a friend of the George R. Agassiz Station. 1 D. S. Heeschen, "Investigations of the 21 cm. Line in the Section of the Galactic Center" (paper presented at the meeting of the American Astronomical Society, Ann Arbor, Michigan, 1954), Astron. J. (in press [abstr.]). 2 A. E. Lilley, "21 cm. Analysis of the Taurus Dark Nebulae Complex" (paper presented at the meeting of the American Astronomical Society, Ann Arbor, Michigan, 1954), Astron. J. (in press [abstr.]). 3 B. J. Bok and H. I. Ewen, "21 cm. Research at Agassiz Station" (paper presented at the meeting of the American Astronomical Society, Ann Arbor, Michigan, 1954), Astron. J. (in press [abstr.1). 4 J. F. W. Herschel, Results of Astronomical Observations at the Cape of Good Hope (London, 1847), p. 385. 6 B. J. Bok, The Distribution of Stars in Space (Chicago: University of Chicago Press, 1937), p. 97. 6 J. J. Nassau and W. W. Morgan, Pub. Observatory Univ. Mich., 10, 49, 1951.

ON THE ORIGIN OF THE LUNAR SURFACE FEATURES BY GERARD P. KUIPER YERKES OBSERVATORY, UNIVERSITY OF CHICAGO Communicated October 29, 1954 1. Introduction.-Much has been written on the surface features of the moon and their possible interpretations. Two main types of explanation have been advanced: volcanic origin or origin by impact of meteorites. Recent advocates of the former hypothesis are Goodacre,1 Spurr,2 and Escher;3 of the latter, Daly,4 Baldwin,5 and Urey.s* An examination of the arguments used in these studies shows that the same empirical facts are sometimes used to derive opposite con- *Jeffreys (The Earth, p. 353) appears to favor the volcanic hypothesis. Downloaded by guest on September 27, 2021