Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[45.01] Diffuse Bands in Circumstellar Shells

C. Barnbaum (NRAO), S. Balm (UCLA/CfA)

The Diffuse Interstellar Bands (DIB's) are a set of over one hundred absorption features extending from 4430 Å\, in the blue to the near infrared and are observed in many different regions of the interstellar medium. Although they have been known for over 60 years, their origin is still a mystery. One of the most important questions is whether the carriers are formed locally in circumstellar envelopes or later in the Interstellar Medium. \par Le Bertre amp; Lequeux (1993) have reported observations of diffuse absorption bands in the spectra of a series of stars surrounded by circumstellar shells. They find evidence for absorption bands in a variety of sources, both carbon-rich (C/O > 1) and oxygen-rich (C/O < 1) and claim that diffuse band carriers are being produced in both types of objects. However, many of their sources are found in the Galactic plane where it is very difficult to distinguish between interstellar and circumstellar contributions. \par Any strategy to observe DIB's in circumstellar shells must target continuum stars with an IR excess that are located outside the Galactic plane. Although most stars having supergiant spectra are concentrated in the Galactic plane, there are some that are found at high galactic latitudes. These objects are thought to have either been ejected from the Galactic plane or are in fact true halo objects. Many of these stars are characterized by being both optically bright while also having an infra-red excess due to circumstellar dust. Most are thought to be post-AGB stars in which mass-loss has effectively stopped and evolution towards the planetary nebula phase has begun. Thus the observed IR-excess is due to the material expelled earlier, during the AGB-phase. These types of stars are ideal tools for investigating whether the diffuse absorption bands are of circumstellar or interstellar origin, since these warm, dusty stars are located at high galactic latitudes where contamination from interstellar material is less severe. \par We observed 11 of these stars with infrared excesses at a variety of galactic latitudes, with the Shane (3 m) telescope at Lick Observatory and the Kast spectrograph (resolution of 1.6 Å\). We find that the strength of the DIB's toward these stars is correlated with Galactic latitude rather than with the amount of circumstellar material. Thus, the circumstellar origin of the diffuse absorption bands has still not been demonstrated.