Integrated properties
• Integrated colors and spectra
• Especially interesting for distant and
extragalactic star clusters
• “Think small”
• Pleiades: 2
o
• No new analysis
with Gaia data,
for example
1’
Aperture Slit (spectrograph)
Integrated Colors
As for distant Galaxies, we are able to observe
integrated colors I(m) of star clusters. We are
mainly able to estimate the age and total mass.
Techniques:
• “Aperture photometry” for distant star
clusters
• Sum up colors of members for resolved star
clusters
Aperture
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
because of their
brightness.
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: older clusters (age > 100 Myr)
will not have any very hot (O and B) type
stars any more as members because they
have evolved (e.g. Supernova)
 Technique: slit spectrum over cluster =>
integrated spectrum of all members
 Assumption: slit covers a representative
sample for the cluster
Integrated spectra of Star Clusters
Slit of spectrograph
This will not work This will work
Integrated spectra of Star Clusters
 How to get a standard library?
1. Use isochrones together with the Initial Mass
Function (IMF)
2. Let the cluster evolve
3. Calculate an integrated spectrum of „what‘s left“ in
the cluster taking into account the luminosities of
stars
4. Do this for a wide variety of ages and metallicities
(Z)
 Library for Globular clusters:
https://www.noao.edu/ggclib/
Ahumada et al., 2000, A&AS, 141, 79
Here the assumption of the
metallicity (Z) is solar
Bica & Alloin, 1986, A&A, 162, 21
Age
Integrated spectra
Z
Same age
Santos & Piatti, 2004, A&A, 428, 79
Measured equivalent widths of lines versus age and metallicity
No correlations
for GCLs
No correlation
for OCLs