Systematic studies of dense molecular cores in regions of high mass star formation (HMSF) are of great importance for our general understanding of star formation. In comparison with low mass star formation regions, so far only a few rather arbitrarily selected cores associated with HMSF have been investigated in some detail.
In recent years we performed extensive surveys of dense cores in regions of high mass star formation, mainly in CS (Zinchenko et al. 1995, 1998). We used water masers as signposts of high mass star formation. Both outer and inner Galaxy were covered by these surveys ( and ). The innermost part of the Galaxy () was observed in a similar way by Juvela (1996). In addition, sources associated with water masers were surveyed in thermal SiO (Harju et al. 1998) which is supposed to be a good indicator of shocks in molecular clouds. From these observations we derived basic physical parameters of the cores and constructed their statistical distributions (Zinchenko 1995; Zinchenko et al. 1998). In order to investigate a range of core densities, observations of lines with different excitation conditions are needed. One of the interesting candidates is the HNCO (isocyanic acid) molecule.
HNCO was first detected by Snyder & Buhl (1972) in Sgr B2. Subsequent studies have concentrated mostly on the Galactic center region where the HNCO emission was found to be particularly strong (e.g., Churchwell et al. 1986; Wilson et al. 1996; Lindqvist et al. 1995; Kuan & Snyder 1996; Dahmen et al. 1997; Sato et al. 1997). A survey of HNCO emission throughout the Galaxy was made by Jackson et al. (1984) in the and transitions with the 11 m NRAO telescope. Seven (from 18) clouds including Orion KL were detected at rather low levels of intensity (typically K on a scale). Churchwell et al. (1986) obtained strict upper limits on HNCO and emission towards about 20 galactic sources with the 36.6 m Haystack antenna.
HNCO is a slightly asymmetric rotor. Its levels may be designated as where J is the total angular momentum and , are quantum numbers corresponding to the projection of J on the symmetry axis for the limiting cases of prolate and oblate symmetric top, respectively (e.g. Townes & Schawlow 1975). The structure of the HNCO energy levels can be represented as a set of "ladders" with different values, like for a symmetric top. However, due to the asymmetry of the molecule radiative transitions between different ladders (b-type transitions) are allowed and, moreover, they are very fast. The corresponding component of the dipole moment is similar to its component for transitions inside the ladders (a-type transitions). Churchwell et al. (1986) found that as a result the HNCO excitation is governed mostly by radiative rather than collisional processes (at least in Sgr B2).
On the basis of their estimates of source parameters Jackson et al. (1984) concluded that HNCO is a potentially valuable probe of the densest regions ( cm-3) of molecular clouds. It was shown also that HNCO is rather sensitive to far infrared (FIR) radiation fields due to the fact that the lowest levels of the 1 and 2 ladders are separated by energies corresponding to FIR wavelengths (330 µm and 110 µm, respectively).
From this consideration it is clear that multitransitional data are needed to understand HNCO excitation and to derive the source properties. Bearing this in mind we undertook a survey of HNCO emission in various rotational lines, also trying to detect emission from higher excited ladders (). Five cores were mapped in HNCO to estimate the extent of the emission.
Several other species were observed simultaneously with HNCO. The most prominent are C18O and SO. In the following we thus also compare HNCO with C18O.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000