Session 22 - YSOs' Variability.
Display session, Monday, January 15
North Banquet Hall, Convention Center

## [22.04] A Search of HST Images for Relative Variations of Proplyds

M. W. Castelaz, M. Bales (East Tenn St U)

Several dozen optical counterparts to highly ionized, compact (subarcsecond) emission features (Laques, amp; Vidal, 1979, Aamp;A, 73, 97; Churchwell, Felli, Wood, amp; Massi 1991, ApJ, 321, 516; Felli, Churchwell, Wilson, amp; Taylor 1992, Aamp;ASupp, 98, 137) have been resolved by the HST (O'Dell, Wen, amp; Hu 1993, ApJ, 410, 696; O'Dell, C. R. amp; Wen, Z. 1994, ApJ, 436, 194). These objects are sometimes referred to as proplyds" (proto-planetary disks"), EIDERS (externally ionized disks in the environs of radio sources"), or PIGs (partially ionized globules). Typically they appear as a faint central star with asymmetric highly ionized extended emission which is thought to be due to external ionization of circumstellar material by nearby OB stars in the Trapezium. The central stars of these objects are of spectral type F, G, and K. Their age has been estimated to be no more than a few hundred thousand years, so they are the youngest low- mass PMS objects known in which the central star is not optically obscured.

Because optical variability and sometimes ejection- driven morphological changes are seen in many higher-mass PMS stars, we compared proplyds in HST images of the Orion Nebula taken in August 1991 and January 1994 to search for morphological and flux variations. The 1991 and 1994 sets of images overlap in H\alpha and [OIII]. Twelve proplyds could be found in both sets of data.

We will present sets of images of several proplyds. No morphological changes are seen at H\alpha or [OIII]. We also did differential aperture photometry on each proplyd comparing the proplyd flux to several nearby comparison stars. We found no difference in fluxes at H\alpha or [OIII], which indicates that no FU Ori type flaring behavior was in progress while the images were being taken. Variations of less than about 0.2 mag, however, cannot be ruled out.

Support for this research is kindly provided by the STScI Archive Research Grant AR-04645.01-92A.