230 research outputs found
Towards the absolute planes: a new calibration of the Bolometric Corrections and Temperature scales for Population II Giants
We present new determinations of bolometric corrections and effective
temperature scales as a function of infrared and optical colors, using a large
database of photometric observations of about 6500 Population II giants in
Galactic Globular Clusters (GGCs), covering a wide range in metallicity
(-2.0<[Fe/H]<0.0). New relations for BC_K vs (V-K), (J-K) and BC_V vs (B-V),
(V-I), (V-J), and new calibrations for T_eff, using both an empirical relation
and model atmospheres, are provided. Moreover, an empirical relation to derive
the R parameter of the Infrared Flux Method as a function of the stellar
temperature is also presented.Comment: 10 pages, 12 .ps figures, MN Latex, accepted by MNRA
The surface density profile of NGC 6388: a good candidate for harboring an intermediate-mass black hole
We have used a combination of high resolution (HST ACS-HRC, ACS-WFC, and
WFPC2) and wide-field (ESO-WFI) observations of the galactic globular cluster
NGC 6388 to derive its center of gravity, projected density profile, and
central surface brightness profile. While the overall projected profiles are
well fit by a King model with intermediate concentration (c=1.8) and sizable
core radius (rc=7"), a significant power law (with slope \alpha=-0.2) deviation
from a flat core behavior has been detected within the inner 1 arcsecond. These
properties suggest the presence of a central intermediate mass black hole. The
observed profiles are well reproduced by a multi-mass isotropic, spherical
model including a black hole with a mass of ~5.7x10^3 Msol.Comment: ApJ Letter in pres
Blue Straggler Stars: The Spectacular Population in M80
Using HST-WFPC2 observations in two ultraviolet (UV) filters (F225W and
F336W) of the central region of the high density Galactic Globular cluster
(GGC) M80 we have identified 305 Blue Straggler Stars (BSS) which represents
the largest and most concentrated population of BSS ever observed in a GGC. We
also identify the largest, clean sample of evolved BSS yet found. The high
stellar density alone cannot explain the BSS, and we suggest that in M80 we are
witnessing a transient dynamical state, during which stellar interactions are
delaying the core-collapse process leading to an exceptionally large population
of collisional-BSS.Comment: 15 pages, 5 figures, Astrophysical Journal Letters, in pres
Discovery of another peculiar radial distribution of Blue Stragglers in Globular Clusters: The case of 47 Tuc
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) in the
globular cluster 47 Tuc spanning the entire radial extent of the cluster.
The BSS distribution is highly peaked in the cluster center, rapidly
decreases at intermediate radii, and finally rises again at larger radii. The
observed distribution closely resembles that discovered in M3 by Ferraro et al
(1993,1997). To date, complete BSS surveys covering the full radial extent (HST
in the center and wide field CCD ground based observations of the exterior)
have been performed for only these two clusters. Both show a bimodal radial
distribution, despite their different dynamical properties. BSS surveys
covering the full spatial extent of more globular clusters are clearly required
to determine how common bimodality is and what its consequence is for theories
of BSS formation and cluster dynamics.Comment: 21 pages, 6 figures, accepted for pubblication in Ap
Photometric metallicity for 694233 Galactic giant stars from Gaia DR3 synthetic Stromgren photometry. Metallicity distribution functions of halo sub-structures
We use the calibrations by Calamida et al. and by Hilker et al., and the
standardised synthetic photometry in the v, b, and y Stromgren passbands from
Gaia DR3 BP/RP spectra, to obtain photometric metallicities for a selected
sample of 694233 old Galactic giant stars having |b|>20.0 and parallax
uncertainties lower than 10%. The zero point of both sets of photometric
metallicities has been shifted to to ensure optimal match with the
spectroscopic [Fe/H] values for 44785 stars in common with APOGEE DR17,
focusing on the metallicity range where they provide the highest accuracy. The
metallicities derived in this way from the Calamida et al. calibration display
a typical accuracy of ~0.1 dex and 1 sigma precision ~0.2 dex in the range -2.2
<=[Fe/H]<= -0.4, while they show a systematic trend with [Fe/H] at higher
metallicity, beyond the applicability range of the relation. Those derived from
the Hilker et al. calibration display, in general, worse precision, and lower
accuracy in the metal-poor regime, but have a median accuracy < 0.05 dex for
[Fe/H]>= -0.8. These results are confirmed and, consequently, the metallicities
validated, by comparison with large sets of spectroscopic metallicities from
various surveys. The newly obtained metallicities are used to derive
metallicity distributions for several previously identified sub-structures in
the Galactic halo with an unprecedented number of stars. The catalogue
including both sets of metallicities and the associated uncertainties is made
publicly available.Comment: Accepted for publication by Astronomy & Astrophysics. Latex. 20
pages, 21 color figures. The catalogue will be publicly available at CDS. In
the meanwhile it is available upon request to the first autho
Another Non-segregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419
We have used a combination of ACS-HST high-resolution and wide-field SUBARU
data in order to study the Blue Straggler Star (BSS) population over the entire
extension of the remote Galactic globular cluster NGC 2419. The BSS population
presented here is among the largest ever observed in any stellar system, with
more than 230 BSS in the brightest portion of the sequence. The radial
distribution of the selected BSS is essentially the same as that of the other
cluster stars. In this sense the BSS radial distribution is similar to that of
omega Centauri and unlike that of all Galactic globular clusters studied to
date, which have highly centrally segregated distributions and, in most cases,
a pronounced upturn in the external regions. As in the case of omega Centauri,
this evidence indicates that NGC 2419 is not yet relaxed even in the central
regions. This observational fact is in agreement with estimated half-mass
relaxation time, which is of the order of the cluster age.Comment: in press in the Ap
The Dearth of UV-Bright Stars in M32: Implications for Stellar Evolution Theory
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope,
we have obtained deep far-ultraviolet images of the compact elliptical galaxy
M32. When combined with earlier near-ultraviolet images of the same field,
these data enable the construction of an ultraviolet color-magnitude diagram of
the hot horizontal branch (HB) population and other hot stars in late phases of
stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the
galaxy, implying that these stars either cross the HR diagram more rapidly than
expected, and/or that they spend a significant fraction of their time
enshrouded in circumstellar material. The predicted luminosity gap between the
hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected,
especially when compared to evolutionary tracks with enhanced helium
abundances, implying that the presence of hot HB stars in this metal-rich
population is not due to (Delta Y)/(Delta Z) > 4. Only a small fraction (~2%)
of the HB population is hot enough to produce significant UV emission, yet most
of the UV emission in this galaxy comes from the hot HB and AGBM stars,
implying that PAGB stars are not a significant source of UV emission even in
those elliptical galaxies with a weak UV excess.Comment: Accepted for publication in The Astrophysical Journal. Latex, 18
pages, 18 black & white figures, in emulate-ApJ format. Figures 11 & 16 have
been degraded due to size constraints; the high-quality version of the paper
is at http://www.stsci.edu/~tbrown/research/m32fuv.pd
DDO Photometry of M71: Carbon and Nitrogen Patterns Among Evolving Giants
We present V, B-V, and DDO C(41-42) and C(42-45) photometry for a sample of
75 red giants down to M_V = +2 in the relatively metal-rich Galactic globular
cluster M71. The C(41-42) colors reveal a bimodal distribution of CN band
strengths generally anticorrelated with CH band strength as measured by the
C(42-45) color. Both DDO colors agree well with those found in 47 Tucanae -- a
nearby globular cluster of similar metallicity -- and suggest nearly identical
C and N abundance patterns among the giants of both clusters. A comparison with
synthetic DDO colors demonstrates that little change in surface C or N
abundance is required to match the colors of the M71 giants over the entire
luminosity range observed. Apparently like 47 Tuc (a cluster of much greater
mass and central concentration), M71 exhibits an abundance pattern which cannot
be solely the result of internal mixing.Comment: To appear in the Astronomical Journal, November 2001. 17 Pages, 5
Figure
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