520 research outputs found
The Initial Helium Abundance of the Galactic Globular Cluster System
We estimate the initial He content in about 30% of the Galactic globular
clusters (GGCs) from new star counts we have performed on the recently
published HST snapshot database of Colour Magnitude Diagrams (Piotto et al.
2002). More in detail, we use the so-called -parameter and estimate the He
content from a calibration based on a recently updated set of stellar models.
We performed an accurate statistical analysis in order to assess whether GGCs
show a statistically significant spread in their initial He abundances, and
whether there is a correlation with the metallicity. We do not find any
significant dependence of the He abundance on the GC metallicity; this provides
an important constraint for models of Galaxy formation and evolution. Apart
from GGCs with the bluest HB morphology, the observed spread in the individual
He abundances is statistically compatible with the individual errors. This
means that either there is no intrinsic He spread among the GGCs, or that this
is masked by the errors. In the latter case we have estimated a firm 1
upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with
the bluest HB morphology we detect a significant spread towards higher
abundances inconsistent with the individual errors. In the hypothesis that the
intrinsic dispersion on the individual He abundances is zero, taking into
account the errors on the individual R-parameter estimates, as well as the
uncertainties on the GGC [M/H] scale and theoretical calibration, we have
determined an initial He abundance Y(GGC)=0.250\pm0.006 a value in perfect
agreement with current estimates based on CMB radiation analyses and
cosmological nucleosynthesis computations.Comment: 10 pages, 4 figures, in press on Astronomy & Astrophysic
Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
Prominent approaches to quantum gravity struggle when it comes to
incorporating a positive cosmological constant in their models. Using
quantization of a complex Chern-Simons theory we
include a cosmological constant, of either sign, into a model of quantum
gravity.Comment: 5 pages and 2 figure
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
We study the expectation value of a nonplanar Wilson graph operator in
SL(2,C) Chern-Simons theory on . In particular we analyze its asymptotic
behaviour in the double-scaling limit in which both the representation labels
and the Chern-Simons coupling are taken to be large, but with fixed ratio. When
the Wilson graph operator has a specific form, motivated by loop quantum
gravity, the critical point equations obtained in this double-scaling limit
describe a very specific class of flat connection on the graph complement
manifold. We find that flat connections in this class are in correspondence
with the geometries of constant curvature 4-simplices. The result is fully
non-perturbative from the perspective of the reconstructed geometry. We also
show that the asymptotic behavior of the amplitude contains at the leading
order an oscillatory part proportional to the Regge action for the single
4-simplex in the presence of a cosmological constant. In particular, the
cosmological term contains the full-fledged curved volume of the 4-simplex.
Interestingly, the volume term stems from the asymptotics of the Chern-Simons
action. This can be understood as arising from the relation between
Chern-Simons theory on the boundary of a region, and a theory defined by an
action in the bulk. Another peculiarity of our approach is that the sign
of the curvature of the reconstructed geometry, and hence of the cosmological
constant in the Regge action, is not fixed a priori, but rather emerges
semiclassically and dynamically from the solution of the equations of motion.
In other words, this work suggests a relation between 4-dimensional loop
quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in
3-dimensions with knotted graph defects.Comment: 54+11 pages, 9 figure
Spacetime thermodynamics without hidden degrees of freedom
A celebrated result by Jacobson is the derivation of Einstein's equations
from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius
relation. This has been repeatedly taken as evidence for an interpretation of
Einstein's equations as equations of state for unknown degrees of freedom
underlying the metric. We show that a different interpretation of Jacobson
result is possible, which does not imply the existence of additional degrees of
freedom, and follows only from the quantum properties of gravity. We introduce
the notion of quantum gravitational Hadamard states, which give rise to the
full local thermodynamics of gravity.Comment: 12 pages, 1 figur
Accurate Internal Proper Motions of Globular Clusters
We have undertaken a long term program to measure high precision proper
motions of nearby Galactic globular cluster (GC) stars using multi-epoch
observations with the WFPC2 and the ACS cameras on-board the Hubble Space
Telescope. The proper motions are used to study the internal cluster
kinematics, and to obtain accurate cluster distances. In this paper, we also
show how the proper motions of the field stars projected in the direction of
the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
The Red Giant Branch Bump
We present a comparison between theoretical models and the observed magnitude
difference between the horizontal branch and the red giant branch bump for a
sample of 53 clusters. We find a general agreement, though some discrepancy is
still present at the two extremes of the metallicity range of globular
clusters.Comment: 2 pages, 1 figure, A.S.P. Conf. Ser., in press in Vol. 296, 200
Explosion of a massive, He-rich star at z=0.16
We present spectroscopic and photometric data of the peculiar SN 2001gh,
discovered by the 'Southern inTermediate Redshift ESO Supernova Search'
(STRESS) at a redshift z=0.16. SN 2001gh has relatively high luminosity at
maximum (M_B = -18.55 mag), while the light curve shows a broad peak. An
early-time spectrum shows an almost featureless, blue continuum with a few weak
and shallow P-Cygni lines that we attribute to HeI. HeI lines remain the only
spectral features visible in a subsequent spectrum, obtained one month later. A
remarkable property of SN 2001gh is the lack of significant spectral evolution
over the temporal window of nearly one month separating the two spectra. In
order to explain the properties of SN 2001gh, three powering mechanism are
explored, including radioactive decays of a moderately large amount of 56Ni,
magnetar spin-down, and interaction of SN ejecta with circumstellar medium. We
favour the latter scenario, with a SN Ib wrapped in a dense, circumstellar
shell. The fact that no models provide an excellent fit with observations,
confirms the troublesome interpretation of the nature of SN 2001gh. A rate
estimate for SN 2001gh-like event is also provided, confirming the intrinsic
rarity of these objects.Comment: 11 pages, 8 figures, 3 tables. Accepted by MNRA
HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitude diagrams
for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the
F439W and F555W bands. A detailed discussion of the various reduction steps is
also presented, and of the procedures to transform instrumental magnitudes into
both the HST F439W and F555W flight system and the standard Johnson B and V
systems. We also describe the artificial star experiments which have been
performed to derive the star count completeness in all the relevant branches of
the color magnitude diagram. The entire photometric database and the
completeness function will be made available on the Web immediately after the
publication of the present paper.Comment: 21 pages, 77 figures. High resolution version of this paper can be
retrived at http://dipastro.pd.astro.it/globular
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
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