Ultraviolet Imaging Telescope (UIT) Studies of the Star Formation History of a Field in the SMC
Session 108 -- Dwarf and Irregular Galaxies
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [108.07] Ultraviolet Imaging Telescope (UIT) Studies of the Star Formation History of a Field in the SMC


UIT 1620\AA, and 2490\AA~ photometry of stars in a 40 arcmin field near the \ha~ emission object Henize 19 is used to study the history of star formation and the nature and distribution of dust in the southwest SMC. Magnitudes \mbfive~ and \maone~ are measured for 1309 objects in common between the 1620\AA~ and 2490\AA~ UIT images of the field.

Reddenings and masses for the field stars are derived using a new technique using a database of stellar atmosphere and evolutionary models. The isochrone age for the field is determined by choosing the isochrone which gives equal mean reddenings for supergiants and main sequence stars. Because the field contains a range of stellar ages, an aged isochrone'' is used, such that each mass bin on the model main sequence is represented by a star of the mean main sequence lifetime for that mass bin. This results in a main sequence which is progressively older with decreasing mass, which is appropriate for a field containing a range of stellar ages.

The SMC reddening law is used to place the observed stars on the aged'' isochrone, yielding reddenings and masses for each star. The superiority of the SMC law, compared with Galactic and LMC laws, is confirmed for this field. Derived mean E(B$-$V) values show a well-defined dependence on position in the field, with larger values in the southwest quadrant. The best-fit age for the field has a supergiant age of approximately 10 Myr. Aging'' of the isochrone results in better mean fits of observed (UV $-$ optical) colors, a slightly older best-fit isochrone age, and a flatter mass function. We derive a present-day, zero-age mass function (PDZAMF) for the field, equivalent to an Initial Mass Function for a coeval cluster, and discuss its implications and uncertainties.