Multiband photometry of stellar populations in Omega Centauri

1
Multiband photometry of stellar populations in
Omega Centauri
Annalisa Calamida Osservatorio di Roma (INAF)
- Università di Roma Tor Vergata
P.B. Stetson (DAO, Victoria, BC) G. Bono, R.
Buonanno, C. E. Corsi, M. Monelli (Osservatorio
di Roma) L. M. Freyhammer (WE/OBSS Vrije
Universiteit Brussel) S. Cassisi, M. Del
Principe, A. Piersimoni, A. Pietrinferni
(Osservatorio di Teramo) F. Grundahl (Institute
of Physics and Astrophysics) M. Hilker
(University of Bonn, Germany)
Volterra, 13/10/2005
2
Omega Centauri (? Cen)

• Most luminous (MV -10) and most massive (M
  5?106 M?) Galactic globular cluster
• Retrograde orbit (zmax 1 Kpc Ra 6 Kpc)
• ? Metallicity dispersion -1.8
  lt Fe/H lt -0.5
• ? a-elements overabundance (O, Mg, Si,
  Ca)
• ? s-elements overabundance (Ba, Mo,
  La, Zi)

Evidence of primordial chemical self-enrichment
in short time scales
?
? Nucleus of a dwarf galaxy accreted on the
Milky Way
? Merging of two stellar systems
3
Introduction
- Freyhammer et al. (2005) best fit of the
anomalous red giant branch (?3) with 2
isochrones - ?µ 0.2 mag -
?E(B-V) 0.03 mag - -1.1 lt Fe/H lt
-0.8 - Coeval to the bulk of cluster stars

• ?3-branch might be a clump of stars located
  ?500pc beyond the bulk of cluster stars
• There could be a clumpy reddening distribution
  across the body of the cluster

4
Strömgren photometric Dataset

• Image acquisition with the 1.54m Danish
  Telescope (La Silla, ESO)
• Year 1999, 110 frames in u, v, b, y bands
• Hilker Strömgren dataset Danish Telescope,
  1993/1995 v, b, y (210)
• Grundahl Strömgren dataset Danish Telescope,
  1999 u, v, b, y (30)
• Seeing 1-1.5

Data reduction with DAOPHOTII/ALLFRAME
5
Differential reddening estimates
BVI (Stetson) uvby photometry (Grundahl)
M13 E(B-V) 0.02, Fe/H -1.54 (Harris
1996) NGC288 E(B-V) 0.03, Fe/H -1.24
c1 (u-v) - (v-b) is a surface gravity indicator
for cool stars and is affected by reddening
E(b-y) 0.74 E(B-V) (Crawford et al. 1976)
E(c1)/E(b-y) 0.2
c c1- 0.2(b-y) is a reddening and metallicity
independent index only for hot horizontal branch
stars (Te gt 8500K, Stetson 1991)
6
Differential reddening estimates
?E(u-y)
ltE(B-V)gt 0.130.03 (from the
NGC288 comparison) ltE(B-V)gt
0.100.03 (from M13)
E(B-V) 0.110.02 (Thompson et al. 2001)
Hot HB stars (Te gt 8500K) in ? Cen display, at
fixed c, larger color dispersions (s(u-y)
0.085) compared to M13 (s(u-y) 0.037) and
NGC288 (s(u-y) 0.016)

• Reddening estimates for individual HB stars
  present a star-to-star scatter larger than the
  error budget.
• This suggests variable reddening toward the
  cluster core with a dispersion of
  s E(B-V) 0.04
• Cannon Stobie (1973) s E(B-V)
  0.03-0.05

7
Spatial distributions

• Stars with color excess gt 1s and lt -1s (red or
  blu asterisks)
• Normal-reddened HB stars are plotted as diamonds

• More-reddened HB stars -gt more concentrated in
  the N-W region of the cluster
• The peak of more-reddened objects is shifted by
  1-1.5 along the right ascension (X-axis)
  compared to peaks of normal and less-reddened
  objects

(arcsec)
For more details see Calamida et al. 2005 (ApJl,
accepted)
8
Color excesses in optical and IR bands

• Histogram of the difference between the expected
  colors on the c vs color plane and the observed
  ones

• The fact that the Strömgren and broad-band
  colors show
• the same effect suggests that it is not an
  artifact of the photometry or the calibration.

9
(No Transcript)
10
Selection of BHB stars for NGC288 and ?Cen
?E (u-y)
11
Selection of BHB stars for M13 and ?Cen
12
u-jump (Grundahl et al. 1999)
Both the (u-y)0 jump color and the jump size are
essentially the same, irrespective of central
density, concentration, metallicity or mixing
history on the RGB.
?E (u-y) 0.25
?E (u-y) 0.22
Teffjump 11500K
13
Conclusions

• ?3-branch might be a clump of stars located
  500pc beyond the bulk of cluster stars
  difference in distance is 10, in agreement with
  distance of tidal tails detected in GC Pal5
  (Odenkirchen 2003)
• The color-c plane is a powerful means to check
  photometric calibration and to investigate the
  presence of differential reddening in a cluster
• We identified the u-jump in ? Cen CMD the jump
  color and size is essentially the same as in M13
  and NGC288. It does not depend on the cluster
  metallicity or central concentration

14
Future plans

• Observations of more standard stars to improve
  photometric Stroemgren calibration
• ALLFRAME over the entire Danish dataset it will
  allow us to detect variables (RRLyrae, LPV,
  SXPhe), and binaries with periods of order years,
  if any
• WFI telescope 2000 (85 frames U,V, I) and 2002
  (248 frames U, B, V, I), Fov 33 30, Seeing
  1 -gt Data reduction almost done with ALLFRAME

15
(No Transcript)