Blue stragglers in dwarf spheroidal galaxies L.Rizzi Osservatorio Astronomico di Padova In collaboration with: Enrico V. Held, Giampaolo Bertelli, Ivo Saviane

Blue stragglers… Large populations of blue straggler stars are commonly observed in a number of Local Group galaxies. The Ursa Minor wide field diagram from Carrera et al. 2002

Another example The Draco dwarf spheroidal from Aparicio et al. 2002

Our sample SextansSculptor Wide field observations collected at the 2.2m telescope Reduction performed using the WFPRED package

What are these stars? The nature of these stars in dwarf galaxies is unclear. They could be either blue- stragglers stars like those observed in Galactic globular clusters, or a populations of young/ intermediate-age stars. 1.5 Gyr 5 Gyr

What are these stars? There are a number of possible criteria to distinguish between the two alternatives. (number of stars relative to HB stars) spatial distribution maximum turn-off mass descendants in the helium-burning phase

Spatial distribution If the “blue-plume” stars are actually blue-stragglers, their spatial distribution should closely follow the distribution of old stars. Sextans Sculptor

Maximum turn-off mass Star resulting from the coalescence of two main sequence stars, cannot have a mass larger than 1.8/2 solar masses. Sextans: brightest star is V ~ 21, corresponding to a turn off mass of 2 solar masses Sculptor: brightest star is V ~ 21, corresponding to a turn off mass of 2 solar masses Both values are too near to the critical value !

The helium-burning descendants Stars younger than 10 Gyr burn helium above the HB, near the RGB, in a region called Red Clump. As age decreases, stars becomes bluer and brighter, and a vertical extension of the RC appears. At even younger ages, well defined blue-loops are observed. These stars are ONLY observed in dwarf galaxies, and not in globular clusters, even if in principle, the star resulting from the merging of two main sequence stars MIGHT evolve as a normal higher mass star.

The helium-burning descendants

For the two galaxies in our sample, we derived the star formation history using simulated CMDs and in the hypothesis that blue-plume stars are actually young stars. BHB RC Sculptor Sextans

The helium-burning descendants These star formation histories are perfectly able to reproduce the observed main sequence luminosity functions. N obs = 26  5 N sim = 35  8 Correspondingly, we should be able to reproduce the observed number of stars in the blue-loop region

Conclusions SextansSculptor Spatial distribution Young starsBlue stragglers Maximum turn- off mass Uncertain Helium-burning descendants Young stars What are these stars?