1. Investigating the Low- Mass Stellar Initial Mass Function in Draco Soroush Sotoudeh (University of Minnesota) Daniel Weisz, Andrew Dolphin, Evan Skillman 06/29/2015 – STScI Workshop on IMF

2. Outline Introduction Observations and data IMF analysis and results Radial variations Model dependencies Ongoing and future work

3. Introduction There’s no doubt that understanding the stellar IMF is one of the most important and sought after issues in astronomy, theoretically and observationally! The majority of previous investigations have calculated the distribution of masses from observations of stars near the Sun. IMF studies that are based on the local luminosity function offer both advantages and disadvantages over alternative methods. Dwarf galaxies in which the low mass IMF has been directly measured are the Milky Way satellites Ursa Minor (Wyse et al. 2002), SMC (Kalirai et al. 2013), Draco (Grillmair et al. 1998), Hercules and Leo IV (Geha et al. 2013). Deep and high-precision HST observations have made it possible to study the stellar IMF beyond the Galaxy and in the Local Group.

4. Observations and Data Archival data from HST Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) for 3 different fields in Draco Point-Spread Function (PSF) photometry using the DOLPHOT photometry package (Dolphin 2000) ~100k artificial star tests per field to quantify the completeness and photometric uncertainties

5. Observations and Data = Center of Draco

6. Observations and Data Draco dwarf spheroidal (dSph) galaxy (Wilson 1955) is among the very faint and low surface brightness known members of the Local Group of galaxies (Mateo 1998). It appears to be dominated by very old (age greater than 8-10 Gyr) and metal deficient stellar populations (Aparicio et al. 2001) Moreover, it is reported to be highly dark matter dominated (e.g. even up to M/L = 340 - 610: see Kleyna et al. (2002)) No corrections for dynamical evolution is required.

7. Observations and Data Green regions correspond to a mass range of ~0.5 – 0.8 M ☉

8. IMF Analysis and Results We fit powerlaw IMFs to our data using the CMD fitting program MATCH (Dolphin 2002). To test for systematics, we have used Dartmouth, BaSTI, Padova and Victoria stellar evolutionary tracks and models in our studies.

9. IMF Analysis and Results: LFs for Field 1

10. IMF Analysis and Results

11. IMF Analysis and Results: Confidence Contours 1)BF remains constant for a given library 2)Γ shows a slight radial gradient for all models

12. Radial Variations

14. Model Dependencies

16. Single-sloped Powerlaw IMF conclusions 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 and BFs at any radius depend strongly on the adopted stellar models, suggesting that current knowledge of the lower-main sequence stars is uncertain. We emphasize the importance of including variations in IMF slope measurements due to multiple stellar models as a means of estimating systematic uncertainties.

17. Ongoing and Future Work

18. Our lognormal analysis shows strong degeneracy in the Characteristic mass-Dispersion plane. We need to hold one of these two parameters fixed and then fit for the other one and the BF. We believe that this degeneracy can only be reduced with data that extends to lower stellar masses.

19. Single-sloped Powerlaw IMF conclusions 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 and BFs at any radius depend strongly on the adopted stellar models, suggesting that current knowledge of the lower-main sequence stars is uncertain. We emphasize the importance of including variations in IMF slope measurements due to multiple stellar models as a means of estimating systematic uncertainties.