2001A&A...380..384P

While conducting high signal-to-noise ratio (SNR) observations of multiple transitions and different isotopomers of CS, SO and CO towards L183 (also known as L134N), we found that the CS (J: 2->1) line frequency established by Kuiper et al. (1996ApJ...468..761K) during observations towards L1498 was in disagreement with our observations. We have consequently repeated their observations towards that object, but in a slightly different manner by observing simultaneously the CS (J: 2->1) line at 98 GHz and the CCS (J_N: 8₇->7₆) line at 94 GHz with the same telescope and the same receivers therefore eliminating the possibility of errors associated with either software or hardware. We found that our L183 data was best described with the standard frequency of 97980.95 MHz. We also found a real difference in the velocity position of the CS and CCS peaks in L1498, but one only half that reported by Kuiper et al. (1996ApJ...468..761K). Most importantly, we have established that the L1498 cloud is not well-suited to set the CS (J: 2->1) frequency despite the exceptional narrowness of the lines. The CS (J: 2->1) line shape is far from Gaussian and suffers from strong effects, due either to large-scale movements (infall and/or rotation), or to self-absorption, or both. These results and other works (Lemme et al., 1995A&A...302..509L; Lee et al. 1999ApJ...526..788L) have convinced us that these observations are consistent with the standard CS (J: 2->1) line frequency. We also checked the C³⁴S (J: 2->1) and the CS (J: 5->4) transitions and found a major discrepancy for the latter with the JPL catalogue. Finally, CS transitions have been recently re-measured in the laboratory by Gottlieb et al. (2001, in prep.) with high precision and are found to be consistent with our interpretation.