Blue stragglers in dwarf spheroidal galaxies

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Blue stragglers in dwarf spheroidal galaxies
L.Rizzi Osservatorio Astronomico di Padova
In collaboration with Enrico V. Held, Giampaolo
Bertelli, Ivo Saviane
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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
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Another example
The Draco dwarf spheroidal from Aparicio et al.
2002
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Our sample
Wide field observations collected at the 2.2m
telescope Reduction performed using the WFPRED
package
Sextans
Sculptor
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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
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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

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Spatial distribution
If the blue-plume stars are actually
blue-stragglers, their spatial distribution
should closely follow the distribution of old
stars.
Sextans
Sculptor
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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 !
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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.
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The helium-burning descendants
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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.
Sextans
Sculptor
BHB
RC
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The helium-burning descendants
These star formation histories are perfectly able
to reproduce the observed main sequence
luminosity functions.
Correspondingly, we should be able to reproduce
the observed number of stars in the blue-loop
region
Nobs 26 ? 5
Nsim 35 ? 8
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Conclusions
What are these stars?