The Astrophysical Journal, 168:L53-L58, 1971 September 1 © 1971. The University of Chicago. All rights reserved. Printed in U.S.A. 197lApJ...168L..53P INTERSTELLAR CARBON MONOSULFIDE A. A. Penzias Bell Telephone Laboratories, Inc., Holmdel, New Jersey P. M. Solomon Columbia University R. W. Wilson Bell Telephone Laboratories, Inc., Holmdel, New Jersey AND K. B. Jefeerts Bell Telephone Laboratories, Inc., Murray Hill, New Jersey Received 1971 May 25 ABSTRACT We have observed line emission from the 146969.16-MHz / = 3 to / = 2 transition in CS in four sources. Typical column densities are near 1014 molecules cm-2. The excitation rates required to produce the observed line intensities are used to derive densities for the central regions of the sources. I. INTRODUCTION We have observed line emission from the 146969.16-MHz / = 3 to / = 2 transition in carbon monosulfide, CS, in the directions of four sources from which we previously observed carbon monoxide emission at corresponding velocities. Observations were made with the 36-foot antenna of the National Radio Astronomy Observatory1 at Kitt Peak. We used a line receiver of forty channels, each 2 MHz (4.08 km s“1) wide separated by 1 Mz (2.04 km s_1). Carbon monosulfide is the first molecule containing sulfur that has been detected in interstellar space. The observation of this line, originating from a level of high excitation, makes CS a useful vehicle for the study of physical conditions in the interstellar medium. The molecule is a simple rotor with the / = 1, / = 2, and / = 3 states 49,147, and 294 GHz, respectively, above the ground state. The decay rate of the observed transition is , 647tVm2 = 6.5 X 10"6 s-1. An Ihâ II. OBSERVATIONS We investigated four sources known to contain molecules and found CS in all of them. The galactic center was, unfortunately, not observable during the time of these observations. The sources in which we have found CS are Orion A, W51, IRC+10216, and DR 21. The results are summarized in Table 1. In addition, measurements of the corresponding transition in 13C32S at 138,738 MHz were made in the two brightest sources, Orion A and W51, in order to obtain opacity information from the line intensity ratios. Unfortunately, an equipment failure prevented observation of the same transition in ^C^S which ought to be more intense than for 13C32S owing to the greater expected relative abundance of the sulfur isotope. The terrestrial 12C/13C ratio is 89 and the 1 The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation. L53 © American Astronomical Society • Provided by the NASA Astrophysics Data System L54 A. A. PENZIAS ET AL. Vol. 168 TABLE 1 CS Sources 197lApJ...168L..53P Ta v Av Source R.A. Decl. (° K) (km s-A) (km s_1) Orion 5h32r l55* -5° 24'16" 0.4 +12 5 51 23 1.5 + 10 5 51 24 0.9 + 10 5 51 25 0.5 + 10 5 51 26 0.4 + 9 6 47 19 0.4 + 10 4 47 20 0.8 + 10 4 47 21 1.7 +10 5 47 22 2.1 + 10 5 47 23 2.2 +10 5 47 24 3.0 +10 5 47 25 2.4 + 9 5 47 26 2.6 + 8 7 47 27 1.6 + 9 5 47 28 1.8 +10 5 47 29 0.9 + 9 5 43 23 1.4 + 9 5 43 24 1.8 + 9 5 43 25 2.1 + 9 5 43 26 2.8 + 8 6 39 24 0.8 + 9 5 35 24 1.0 + 9 5 31 24 0.7 +10 6 W51 19 21 31 14 24 30 0.9 +57 8 27 25 1.1 +56 6 27 24 2.3 +57 12 27 23 1.2 +57 7 23 24 1.8 +60 9 DR 21 20 37 13. 42 08 59.9 1.1 - 2 6 DR 21 (OH). 20 37 13. 42 11 59.9 1.4 - 4 6 IRC+10216. 9 45 14.8 13 30 40 0.6 -22 24 interstellar ratio is thought to be close to this value (Penzias, Jefferts, and Wilson 1971) while the terrestrial ^S/^S ratio is 25. A determination of the interstellar values for this latter ratio should be an important by-product of future CS studies. a) Orion A The Orion Nebula was studied most extensively (Table 1). In contrast to 115-GHz CO emission (Wilson, Jefferts, and Penzias 1970), the CS emission is sharply peaked with the half-intensity contour some 3'(decl.) X 9'(R.A.). The most intense feature occurs in the scan taken at R.A. 5h33m47s, decl. —5°24/16// (Fig. \a). From the intensi- ties obtained in adjacent directions, we find the source of maximum intensity to be approximately centered on this position. This is the OH maser position (Raimond and Eliasson 1967) coincident with the infrared point source of Becklin and Neugebauer. It is difficult, however, to exclude unambiguously the infrared nebula of Kleinmann and Low (1967) which is located some 17 seconds of arc to the south. No emission peak was found from the direction of the infrared nebula associated with the Trapezium stars (Ney and Allen 1969). However, Ney and Allen reported a southward extension of the Kleinmann and Low object somewhat to the east of the Trapezium, and near the position of our secondary maximum. © American Astronomical Society • Provided by the NASA Astrophysics Data System CU 00LO co - No. 2, 1971 INTERSTELLAR CARBON MONOSULFIDE L55 \—II b) W51 ^ W51 (Fig. 1¿>) was measured at five locations spaced 1 arc min apart in the form of a cross. The central spectrum was peaked at 58 km s_]—in good agreement with our previous CO studies of the source. The weaker 70 km s-1 CO feature (Penzias et al. 1971, Table la) is completely absent in CS. However, the asymmetry of the CS spectrum suggests a component corresponding to a lower-velocity (~53 km s-1) feature present in the CO data, which has also been detected in 6-cm H2CO absorption (Scoville and Solomon 1971). The angular extent of the CS emission also corresponds closely to 2-mm H2CO measurements for the source. c) IRC+10216 This radio source has a CS line with the same velocity, width, and shape as the CO emission (Solomon et al. 1971) with about one-third the intensity. We have detected emission in this object from four molecular lines (12CO, 13CO, CN, CS), and a discussion of the results including the CS observations will be presented in a later paper. d) DR 21 In DR 21 we note that the emission is stronger at the position of the OH emission than at the continuum peak. This association with OH sources was illustrated by failure to obtain data from W3. We had planned to investigate the OH source in W3 for CS emission, but we inadvertently pointed the antenna to the W3 continuum position, which is separated from the OH position by some 20 arc min, and found no emission. Measurements of 13C32S were made at 138,738 MHz for Orion and W51. In both cases, residuals between 0.1° and 0.2° of antenna temperatute were obtained. We expect to pursue this investigation in the near future and for the present regard these values as merely representing upper limits. m. ABUNDANCES Unless the emitting gas is known to be optically thin, one cannot obtain unambiguous column densities from emission spectra without additional opacity information. We may, however, deduce useful limits to the density from the data at hand. Fig. la.—Orion peak. R.A. 5h33m478, decl. — 5"24'16" (1950). The feature is broadened by the 2 MHz (4 km s-1) width of the individual channels. Fig. 1&.—W51. R.A. 19h21m278, decl. 14°24'30" (1950). © American Astronomical Society • Provided by the NASA Astrophysics Data System L56 A. A. PENZIAS ET AL. Vol. 168 We may obtain a lower limit to the column density of molecules in the / = 2 state by using the relation between column density and antenna temperature valid for the 197lApJ...168L..53P optically thin case when the rotational temperature T2z> hv/K. We then have N > STgivkTAàv 2 2 1 “ hc gzAz2V where rj is the beam efficiency of the antenna (0.6), TA is the antenna temperature, and Av is the line width. We can maximize the fraction of molecules in / = 2 and obtain a lower limit to the total column density of CS by assuming the / = 0, 1, and 2 states to be populated according to their statistical weights, the 7 = 3 state to be populated at the minimum excitation temperature consistent with the observed antenna temperature, and all states with 7 > 3 to be unpopulated. This gives N > 2N2. Combining this result with the above, we have N> 3.IX 10«AvTa . In the two sources where we have measured upper limits to corresponding 13C32S emission we may obtain upper limits to the opacity by assuming a real isotope ratio 12C/13C = 89. Under this assumption, r^C^S) < 6 for Orion, and less than 10 for W51. These values would correspond to a 13C32S antenna temperature of 0.2° K. In this case, 2 N = Swg2v tAv 2 c*g3Av [1 - exp (-W¿r23)] ’ where r is the opacity and TW is the excitation temperature of the transition. In this case of high opacity is a minimum and is computed from the relation T _ \ hv hv ) A ^ I ¿[exp (hu/kTn) - 1] ¿[exp (hi>/kTbg) - l]f ’ where Tbg is the temperature of the microwave background.