4. Reddening and the () and () diagrams
The OH/IR stars are shown in the (H-K,K-L) plane in Fig. 6 together with all the isolated, non-variable stars obtained from the photometry of the images. Also shown in Fig. 6 are the position of unreddened, non-variable red giants in the Baade's window (Frogel & Whitford 1987) and the position of the LPVs in Baade's Window (including IRAS sources) from Glass et al. (1995) and Wood & Bessell (1983). The arrow shows a reddening vector corresponding to 30 magnitudes of visual extinction (see below for the reddening law used).
It is clear from Fig. 6 that the vast majority of objects seen in the infrared are highly reddened with 12 to 30 magnitudes. An examination of individual () fields showed that the reddening varied distinctly between fields. A median E(H-K) was derived for each field as follows. Firstly, the median H-K of all isolated stars with 14 and H- 0.7 was computed for each field (stars with H- 0.7 are clearly foreground objects). The extinction was derived from H-K rather than K-L because of the large number of stars detected in both H and K. It was then assumed that the majority of the stars detected in each field were red giants with intrinsic H-K=0.25. The difference between the median H-K and 0.25 gave the median E(H-K) for each field.
The points shown in Fig. 6 were dereddened using the following procedure. From Fig. 6 we find that in our photometric system E(K-L) = 0.72 E(H-K). This relation was used to calculate a median E(K-L) for each field from the E(H-K) derived above. Then we used the relation = 2.07 E(K-L) from Rieke & Lebofsky (1985) to calculate the extinction. The dereddened points are shown replotted in Fig. 7. The scatter of the non-variable stars along the reddening vector is now greatly reduced although a residual scatter in reddening exists due to reddening variations on spatial scales less than one arcminute.
The OH/IR stars are well separated from the non-variable stars in Fig. 7. The majority of the OH/IR stars discovered by LWHN lie beyond the sequence occupied by the LPVs in the Baade's Window while the Sjouwerman sources and the newly-discovered variables mostly lie near the bulge LPV sequence. Those LWHN variables lying beyond the bulge sequence presumably have thicker and/or cooler circumstellar shells than the variables (including IRAS sources) in Baade's window. However, very red objects similar to the LWHN sources are known to exist in the direction of the bulge - about a dozen of them were found by Whitelock et al. (1991) who monitored IRAS sources in a 60 deg2 area of the bulge between = to . Massive, variable OH/IR stars which are even redder are known to occur along the Galactic plane (eg. Engels et al. 1983)
A similar separation of the non-variable stars and the variable stars is seen in the (K, K-L) diagram, Fig. 8. The non-variable red giant branch occurs in the position expected for a Baade's window giant branch reddened by 20-25 mag. of visual extinction. Once again, the large circumstellar reddening and/or cool shells of the OH sources of LWHN is apparent. The Sjouwerman OH sources and the newly discovered variables are mostly less extreme in their K-L colours than the OH/IR stars discovered by LWHN and they bridge the gap between the non-variable and the OH/IR sequences.
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998