172 research outputs found
Panchromatic fits to the Globular Cluster NGC 6366
We present panchromatic isochrone fits to the color magnitude data of the
globular cluster NGC 6366, based on HST ACS/WFC and SOAR photometric data.
Before performing the isochrone fits, we corrected the photometric data for
differential reddening and calculated the mean ridge line of the color
magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution
Database and PAdova and TRieste Stellar Evolution Code (with microscopic
diffusion starting on the main sequence). Based on previous determinations of
the metallicity of this cluster we test it from [Fe/H]=-1.00 to [Fe/H]= -0.50,
and the age from 9 to 13 Gyrs. The uncertainties do not decrease when we fit
simultaneous colors. We also find that the Dartmouth Stellar Evolution Database
isochrones have a better fit in the sub giant branch and low main sequence than
the PAdova and TRieste Stellar Evolution Code. Considering the most recent
spectroscopic determination of the metallicity ([Fe/H]= -0.67), we find
E(B-V)=0.69+/-0.02, (m-M)_V=15.02+/-0.07 and 11+/-2 Gyr for NGC 6366.Comment: 4 pages, 2 figures. Proceedings of the conference "Reading the book
of globular clusters with the lens of stellar evolution", to be published in
Memorie della Societ\'a Astronomica Italian
Mapping the differential reddening in globular clusters
We build differential-reddening maps for 66 Galactic globular clusters (GCs)
with archival HST WFC/ACS F606W and F814W photometry. Because of the different
GC sizes (characterised by the half-light radius ) and distances to the
Sun, the WFC/ACS field of view (200\arcsec\times200\arcsec) coverage
() lies in the range 1\la R_{obs}/R_h\la15 for about 85% of the
sample, with about 10% covering only the inner (R_{obs}\la R_h) parts. We
divide the WFC/ACS field of view across each cluster in a regular cell grid,
and extract the stellar-density Hess diagram from each cell, shifting it in
colour and magnitude along the reddening vector until matching the mean
diagram. Thus, the maps correspond to the internal dispersion of the reddening
around the mean. Depending on the number of available stars (i.e. probable
members with adequate photometric errors), the angular resolution of the maps
range from \approx7\arcsec\times7\arcsec to
\approx20\arcsec\times20\arcsec. We detect spatially-variable extinction in
the 66 globular clusters studied, with mean values ranging from
\mEBV\approx0.018 (NGC\,6981) up to \mEBV\approx0.16 (Palomar\,2).
Differential-reddening correction decreases the observed foreground reddening
and the apparent distance modulus but, since they are related to the same value
of \EBV, the distance to the Sun is conserved. Fits to the mean-ridge lines of
the highly-extincted and photometrically scattered globular cluster Palomar\,2
show that age and metallicity also remain unchanged after the
differential-reddening correction, but measurement uncertainties decrease
because of the reduced scatter. The lack of systematic variations of \mEBV\
with both the foreground reddening and the sampled cluster area indicates that
the main source of differential reddening is interstellar.Comment: Accepted by MNRA
Multichromatic colour-magnitude diagrams of the globular cluster NGC 6366
We present multichromatic isochrone fits to the colour-magnitude data of the
globular cluster NGC 6366, based on Hubble Space Telescope Advanced Camera for
Surveys/Wide Field Channel and Southern Astrophysical Research photometric
data. We corrected the photometric data for differential reddening and
calculated the mean ridge line of the colour-magnitude diagrams. We compared
the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste
Stellar Evolution Code both with microscopic diffusion starting on the main
sequence. Bracketing all previous determinations of this cluster, we tested
metallicities from [Fe/H]=-1.00 to [Fe/H]=-0.50, and ages from 9 to 13 Gyr.
After determining the total to selective extinction ratio only from stars
belonging to this cluster, R_V=3.06+/-0.14, we found the parameters for this
cluster to be E(B-V)=0.69+/-0.02(int)+/-0.04(ext),
(m-M)_V=15.02+/-0.07(int)+/-0.13(ext), Age=11+/-1.15 Gyr. Evolutionary models
fail to reproduce the low-Teff sequence in multiband colour-magnitude diagrams,
indicating that they still have an incomplete physics. We found that the
Dartmouth Stellar Evolution Database isochrones better fit the subgiant branch
and low main sequence than the PAdova and TRieste Stellar Evolution Code.Comment: 9 pages, 5 figure
Chemical analysis of eight giant stars of the globular cluster NGC 6366
The metal-rich Galactic globular cluster NGC 6366 is the fifth closest to the
Sun. Despite its interest, it has received scarce attention, and little is
known about its internal structure. Its kinematics suggests a link to the halo,
but its metallicity indicates otherwise. We present a detailed chemical
analysis of eight giant stars of NGC 6366, using high resolution and high
quality spectra (R > 40000, S/N > 60) obtained at the VLT (8.2 m) and CFHT (3.6
m) telescopes. We attempted to characterize its chemistry and to search for
evidence of multiple stellar populations. The atmospheric parameters were
derived using the method of excitation and ionization equilibrium of FeI and
FeII lines and from those atmospheric parameters we calculated the abundances
for other elements and found that none of the elements measured presents
star-to-star variation greater than the uncertainties. We compared the derived
abundances with those of other globular clusters and field stars available in
the literature. We determined a mean [Fe/H] = -0.60 +- 0.03 for NGC 6366 and
found some similarity of this object with M 71, another inner halo globular
cluster. The Na-O anticorrelation extension is short and no star-to-star
variation in Al is found. The presence of second generation stars is not
evident in NGC 6366.Comment: 17 pages, 14 figures. Accepted for publication in MNRAS; Corrected
typos and added reference
Chemical analysis of eight giant stars of the globular cluster NGC 6366
The metal-rich Galactic globular cluster NGC6366 is the fifth closest to the Sun. Despite its interest, it has received scarce attention, and little is known about its internal structure. Its kinematics suggests a link to the halo, but its metallicity indicates otherwise. We present a detailed chemical analysis of eight giant stars of NGC6366, using high-resolution and highquality spectra (R > 40 000, S/N > 60) obtained at the VLT (8.2 m) and CFHT (3.6 m) telescopes. We attempted to characterize its chemistry and to search for evidence of multiple stellar populations. The atmospheric parameters were derived using the method of excitation and ionization equilibrium of Fe I and Fe II lines and from those atmospheric parameters we calculated the abundances for other elements and found that none of the elements measured presents star-to-star variation greater than the uncertainties. We compared the derived abundances with those of other globular clusters and field stars available in the literature. We determined a mean [Fe/H] = −0.60 ± 0.03 for NGC6366 and found some similarity of this object with M71, another inner halo globular cluster. The Na–O anticorrelation extension is short and no star-to-star variation in Al is found. The presence of second generation stars is not evident in NGC6366
A comparative analysis of the observed white dwarf cooling sequence from globular clusters
We report our study of features at the observed red end of the white dwarf
cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae
and M\,4. We use deep colour-magnitude diagrams constructed from archival
Hubble Space Telescope (ACS) to systematically investigate the blue turn at
faint magnitudes and the age determinations for each cluster. We find that the
age difference between NGC\,6397 and 47\,Tuc is 1.98\,Gyr,
consistent with the picture that metal-rich halo clusters were formed later
than metal-poor halo clusters. We self-consistently include the effect of
metallicity on the progenitor age and the initial-to-final mass relation. In
contrast with previous investigations that invoked a single white dwarf mass
for each cluster, the data shows a spread of white dwarf masses that better
reproduce the shape and location of the blue turn. This effect alone, however,
does not completely reproduce the observational data - the blue turn retains
some mystery. In this context, we discuss several other potential problems in
the models. These include possible partial mixing of H and He in the atmosphere
of white dwarf stars, the lack of a good physical description of the
collision-induced absorption process and uncertainties in the opacities at low
temperatures. The latter are already known to be significant in the description
of the cool main sequence. Additionally, we find that the present day local
mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc
presents a bottom-heavy profile.Comment: Accepted for publication in MNRAS (16 pages, 19 figures
The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
We explore the physics of crystallization in the deep interiors of white
dwarf stars using the color-magnitude diagram and luminosity function
constructed from proper motion cleaned Hubble Space Telescope photometry of the
globular cluster NGC 6397. We demonstrate that the data are consistent with the
theory of crystallization of the ions in the interior of white dwarf stars and
provide the first empirical evidence that the phase transition is first order:
latent heat is released in the process of crystallization as predicted by van
Horn (1968). We outline how this data can be used to observationally constrain
the value of Gamma = E_{Coulomb}/E_{thermal} near the onset of crystallization,
the central carbon/oxygen abundance, and the importance of phase separation.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letter
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