Investigating the Low-Mass Stellar Initial Mass Function in Draco

Abstract

We present new analysis of the low-mass stellar initial mass function ($\sim0.4-0.8M_\odot$) in the Local Group dwarf spheroidal galaxy, Draco. Using archival HST/ACS and WFC3 optical imaging, we construct deep color-magnitude (CMD) diagrams in 3 different felds at 3 different galactocentric radii and measure the IMF by modeling the resolved lower main sequence. We model the optical color-magnitude diagrams of each feld assuming two different IMF models (power-law, log-normal), five different stellar evolution libraries (Padova, BaSTI, Dartmouth, Victoria, PARSEC), and a binary star model. For a single-sloped power-law IMF model, we find that the IMF slope steepens by up to 0.7 dex for radii between 150 and 300pc, while the binary fraction remains approximately constant. The absolute values of the IMF slopes at any radius depend strongly on the adopted stellar models, suggesting that current knowledge of the lower-main sequence stars is uncertain. In fact, utilizing different stellar models has resulted in up to 0.67 dex difference in the IMF slope. All fields show more consistent log-normal parameters, which are also in reasonable agreement with values for a standard Chabrier IMF. However, there are large degeneracies between the characteristic mass and dispersion of the log-normal, that can only be reduced with data that extends to lower stellar masses.