FORTIS is an innovative multi-object far-UV spectro/telescope featuring a number of new technologies developed with the support of the NASA/APRA and JWST programs.  Off-Axis FORTIS (OAxFORTIS) is a new version of the largely matured FORTIS instrument evolved into an Off-Axis telescope configuration. This new design will attempt to eliminate the problem with scattered geo-coronal Lyα that has prevented the FORTIS from making detector background limited observations.

This, the first flight of OAxFORTIS and the fifth of FORTIS, will acquire far-UV spectra of the globular cluster Messier 10 (M10). M10 is located in the equatorial constellation of Ophiuchus. The hot Horizontal Branch (HB) of globular clusters is populated by far-UV luminous stars at the end stage of evolution. HB stars lie along a roughly horizontal line in the Hertzsprung-Russell diagram. By the time stars reach the zero-age-horizontal-branch (ZAHB), nucleosynthetic energy generation has for the most part ceased. The temperature, mass, radius, and chemical abundance of the thin radiative atmospheres of stars on the ZAHB offer clues as to their progenitor mass and initial chemical abundance.

The goal is to identify the progenitor populations of the ZAHB in three globular clusters, starting with M10 (this flight) with similar ages and [Fe/H] abundances - as determined from near-UV and visible photometry - but exhibiting significant differences in the distribution of ZAHB temperatures. The objective is to determine the spread of temperature, mass, radius, and abundances of stars on the ZAHB sequence in a region extending from the rc out to ~ rt/2 and provide critical constraints to evolutionary models of stars in globular clusters.

The Principal Invetigator is Dr. McCandliss/Johns Hopkins University