Integrated Colors
As for distant Galaxies, we are able to observe
integrated colors I(m) of star clusters. We are able
to estimate the age and total mass.
Techniques:
1. “Aperture photometry” for distant star
clusters
2. Sum up colors of members for resolved star
clusters
Starting point are the dereddened colors and absolute
magnitudes. For the dereddening, here are the relations
from Lata et al. (2002, A&A, 388, 158) for the JohnsonCousins
UBVRI system:
E(U – B) = 0.72E(B – V) + 0.05E(B – V)2
E(U – V) = 1.72E(B – V)
E(V – R) = 0.60E(B - V)
E(V – I) = 1.25E(B – V)
For the integrated colors we get:
Most important is
the knowledge of
the membership
for giants
The
incompleteness
of the lower main
sequence is not
important
because of low
absolute
magnitudes
Integrale Farben 4/8
Lata et al., 2002, A&A, 388, 158
Clearly defined
upper and lower
mass limits
“Standard lines”
for total masses
from isochrones
and population
synthesis codes
González Delgado
et al., 2005,
MNRAS, 357,
945
Lata et al., 2002, A&A, 388, 158
Relations for
352 galactic open
clusters
The age and
reddening were
taken from the
literature
Errors given by Lata
et al. (2002):
sI(Mv) < 0.5 mag
sI(colors) < 0.2 mag
Results from Lata et al. (2002, A&A, 388, 158), important are
the errors for the determination of the uncertainties in log t:
Integrated spectra of Star Clusters
⚫ Idea: clusters of different ages have
different stellar content
⚫ Example: old clusters (log t > 100 Myr) will
not have any very hot (O and B) type stars
any more as members because they have
evolved
⚫ Technique: slit spectrum over cluster =>
integrated spectrum of all members
⚫ Assumption: slit covers a representative
sample for the cluster
Integrated spectra of Star Clusters
⚫ How to get a standard library?
1. Use isochrones together with IMF
2. Let the cluster evolve
3. Calculate an integrated spectrum of „what‘s left“
in the cluster taking into account the luminosity
of a star.
4. Do this for a wide variety of ages and metallicities
⚫ Library for Globular clusters:
https://www.noao.edu/ggclib/
Ahumada et al., 2000, A&AS, 141, 79
Bica & Alloin, 1986, A&A, 162, 21
Age
Integrated spectra
Z
Same age
Santos & Piatti, 2004, A&A, 428, 79