A. Evans 1, S. P. S. Eyres 1, & P. H. Hauschildt 2
1 Physics Dept, Keele University Keele Staffordshire ST5 5BG United Kingdom
2 University of Georgia
XX Oph is a binary system containing of an early type star (B0V) and a late giant (M7III) (Evans et al. AA, 267, 161, and references therein). It is sometimes classed as a Be star and sometimes as a symbiotic.
It was long been known to be heavily reddened, with early estimates
giving
.
Polarimetric observations, however, show that the
polarization is consistent with only a modest degree of interstellar
reddening, corresponding to
:
there is clearly a major
circumstellar component of reddening. While this is not unusual, what
makes the extinction of XX Oph peculiar is that the extinction is
prominent ONLY IN THE BLUE: the reddening law is far steeper than normal.
Evans (AA, 288, L37) showed that the 8.6 and 11.25 micron UIR features are present in this object, although the usual 3.28 and 3.4 micron features are weak. With the discovery of UIR emission in XX Oph the reason for its anomalous reddening became clear: PAH molecules absorb strongly in the in the blue and (apart from the well-known UIR emission: Allamandola, Tielens & Barker, ApJS, 71, 733) give no infrared (IR) excess. We thus have strong CS extinction rising sharply in the blue with little or no IR excess, as observed.
We present ISO SWS spectroscopy and PHOT-P photometry of XX Oph which
throw further light on its circumstellar environment. The ISOPHOT data
show the presence of
of amorphous dust at 180 K. Fits of
model atmospheres to the SWS data further constrain the nature of the M
giant, while the UIR features and emission lines display evidence of
interaction between the hot and cool stars.