Astron. Astrophys. 334, 646-658 (1998)

3. Results

The 33 sources we study in detail are selected as distinct peaks in the CS survey of Bally et al. (1987). They are, therefore, cores that should have large amounts of dense gas, which are embedded in a smooth, lower intensity molecular intercloud medium. Since we wish to derive the properties of typical clouds in the Galactic center environment, the non-typical molecular peaks associated with Sgr A and Sgr B2 were not included in the sample.

The () and () transitions of and the () transition of ²⁸ SiO were observed toward all sources of our sample. Both the and (more notably) the SiO lines were easily seen in all sources. We observed the 10 strongest sources in the () submillimeter transition of , again detecting all. The rare isotopomers ²⁹ SiO and ³⁰ SiO were observed and detected toward 12 clouds showing strong emission in ²⁸ SiO. Of 8 sources measured in the ²⁸ SiO() line, 7 showed emission. In Fig. 1 and Table 2, we give an overview of our results. Sample spectra for eight sources are displayed in Fig. 1. We mark the position of all sources observed on a survey map of the () transition of (Papers I and II). Galactic coordinates are used for source names. For the corresponding equatorial coordinates, refer to Hüttemeister et al. (1993b). Most of the clouds we have studied are located within the molecular bulge region, while three lie within the `Clump 2' complex (Bania 1977, Stark & Bania 1986) at . `Clump 1' is located south of the Galactic center.

Fig. 1. Sample spectra of eight sources with very complete observations. In the top panels, the lines are displayed: The () transition is drawn as a dotted line, the () transition is drawn as a thin solid line and the () transition is displayed as a thick solid line. The bottom panels show the SiO spectra. 28 SiO() is denoted as a thin solid line, 28 SiO() is given by a thick solid line; the 29 SiO() spectrum is drawn as a thick dotted line and the 30 SiO() is shown as a thin dotted line. The intensities of both rare isotopomers were multiplied by 5 to allow clearer comparisons. Black squares overlayed on the map of the overall distribution of the line of (taken from Paper I) mark all 33 sources we observed except Clump 1 at .

Integrated line intensities for the () and ²⁸ SiO() transitions and line intensity ratios derived from integrated intensities are presented in Table 2. The errors are determined from the (usually small) formal error of a Gaussian fit to the lines or, in case of non-Gaussian lineshapes, from the rms noise in the spectrum. In addition, a calibration uncertainty of 10% for data taken with the same telescope or 20% for data obtained with different instruments was assumed. For Gaussian lines, the formal error of the fit agrees closely with the error obtained from the rms noise in the spectrum.

Table 2. Integrated intensities of the () and SiO() transitions and line intensity ratios of and SiO.

For a given species, the center velocities and lineshapes of the different transitions always agree, to within the noise. Between SiO and , however, there can be significant differences. The sources M+0.50+0.00 and M+0.24+0.02 are good examples: In both cases, the lines show two distinct peaks, while the SiO transitions are single-peaked. In M+0.50+0.00, the central velocity of the SiO is close to the weaker line; in M+0.24+0.02 SiO and the stronger peak agree. It is also noteworthy that narrow lines close to a of 0 , likely of local origin, never have a counterpart in SiO. This is illustrated by the source M-0.51-0.16 in Fig. 1 and demonstrates the unusual nature of the Galactic center sources as compared to Galactic disk clouds.

Since the lines of ²⁹ SiO and ³⁰ SiO are very likely optically thin, we can directly check whether the ratio of ²⁹ Si/³⁰ Si in the Galactic center region agrees with the terrestrial value. We find a line intensity ratio of in our sample, in excellent agreement with the terrestrial isotope ratio of 1.5. This confirms that this ratio does not depend on the galactocentric distance (Wilson & Rood 1994, Penzias 1981), which is an expected result if both isotopes are synthezised in the s-process in stars of the same type.

For ²⁸ SiO/²⁹ SiO, we take the ratio to be the terrestrial value of 20, as suggested by Penzias (1981). Since the ²⁸ SiO() transition is optically thick, we cannot check this assumption, but it is supported by the largest line ratio (toward the exceptional source M+1.31-0.13, see Sect. 4.3), which is indeed close to 20.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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