Bibcode
DOI
Stephens, Andrew W.; Frogel, Jay A.; Freedman, Wendy; Gallart, Carme; Jablonka, Pascale; Ortolani, Sergio; Renzini, Alvio; Rich, R. Michael; Davies, Roger
Referencia bibliográfica
The Astronomical Journal, Volume 121, Issue 5, pp. 2597-2609.
Fecha de publicación:
5
2001
Número de citas
23
Número de citas referidas
21
Descripción
We have obtained HST-NICMOS observations of five of M31's most
metal-rich globular clusters: G1, G170, G174, G177, and G280. For the
two clusters farthest from the nucleus, G1 and G280, we statistically
subtract the field population and estimate metallicities using K-(J-K)
color-magnitude diagrams (CMDs). Based on the slopes of their infrared
giant branches we estimate [Fe/H]=-1.22+/-0.43 for G1 and -0.15+/-0.37
for G280. We combine our infrared observations of G1 with two epochs of
optical HST-WFPC2 V-band data and identify at least one long-period
variable (LPV) based on color and variability. The location of G1's
giant branch in the K-(V-K) CMD is very similar to that of M107,
indicating a somewhat higher metallicity than our purely infrared CMD;
[Fe/H]=-0.9+/-0.2. This latter value is more robust, and is supported by
the presence of LPVs. For the field surrounding G280, we estimate the
metallicity to be -0.3 from the slope of the giant branch, with a spread
of σ[Fe/H]~0.4 based on the width of the giant branch.
For the three central clusters, which are too compact for accurate
cluster star measurements, we present integrated cluster magnitudes and
CMDs for the surrounding fields. The K-band luminosity functions (LFs)
of the upper few magnitudes of G1 and G280, as well as for the fields
surrounding all of the clusters, are indistinguishable from the LF
measured in the bulge of our Galaxy. This indicates that these clusters
are very similar to Galactic globular clusters, and, at least in the
surrounding fields observed, there are no significant populations of
young luminous stars. Based on observations with the NASA/ESA Hubble
Space Telescope obtained at the Space Telescope Science Institute, which
is operated by the Association of Universities for Research in Astronomy
(AURA), Inc., for NASA under contract NAS 5-26555.