1,366 research outputs found
RR Lyrae variables in the globular cluster M3 (NGC5272). I. BVI CCD photometry
New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular
cluster M3. Light curves have been constructed and ephemerides have been
(re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209),
although recognized as variables, had no previous period determinations. Also,
the period derived for V129 is significantly different from the one published
by Sawyer-Hogg (1973). Light curve parameters, i.e. mean magnitudes, amplitudes
and rise-times, have been derived. The discussion of these results in the
framework of the stellar evolution and pulsation theories will be presented in
a forthcoming paper.Comment: 19 pages, latex, uses mn.sty, 12 encapsulated figures, to be
published in MNRAS, text and figures also available at
http://www.bo.astro.it/bap/BAPhome.html or via anonymous ftp at
ftp://boas3.bo.astro.it/bap/files (bap98-12-textfig.ps
Theoretical predictions for charm and bottom production at the LHC
We present predictions for a variety of single-inclusive observables that
stem from the production of charm and bottom quark pairs at the 7 TeV LHC. They
are obtained within the FONLL semi-analytical framework, and with two "Monte
Carlo + NLO" approaches, MC@NLO and POWHEG. Results are given for final states
and acceptance cuts that are as close as possible to those used by experimental
collaborations and, where feasible, are compared to LHC data.Comment: 22 pages, 10 figure
D^* production from e^+e^- to ep collisions in NLO QCD
Fragmentation functions for D mesons, based on the convolution of a
perturbative part, related to the heavy quark perturbative showering, and a
non-perturbative model for its hadronization into the meson, are used to
describe D^* production in e^+e^- and ep collisions. The non-perturbative part
is determined by fitting the e^+e^- data taken by ARGUS and OPAL at 10.6 and
91.2 GeV respectively. When fitting with a non perturbative Peterson
fragmentation function and using next-to-leading evolution for the perturbative
part, we find an epsilon parameter sensibly different from the one commonly
used, which is instead found with a leading order fit. The use of this new
value is shown to increase considerably the cross section for D^* production at
HERA, suggesting a possible reconciliation between the next-to-leading order
theoretical predictions and the experimental data.Comment: 20 pages, LaTeX2e, 8 Postscript figure
Charmonium Production via Fragmentation at DESY HERA
The cross section for the photoproduction of large-p_T J/psi mesons at HERA
is calculated at next-to-leading order, adopting a perturbative approach to
describe the fragmentation of charm quarks and gluons into J/psi mesons. We
treat the charm quark according to the massless factorization scheme, where it
is assumed to be one of the active flavours inside the proton and the resolved
photon. We present inclusive distributions in transverse momentum and rapidity,
including the contributions due to direct and resolved photons. The importance
of the colour-octet components of the J/psi wave function, which contribute to
the fragmentation process, is emphasized. In addition to prompt J/psi
production, we consider also the production of chi_{cJ} states followed by
radiative decays to J/psi mesons, both in the colour-singlet and colour-octet
channels.Comment: 32 pages (Latex), 12 figures (Postscript
New Metallicities of RR Lyrae Stars in omega Centauri: Evidence for a Non He-Enhanced Metal-Intermediate Population
We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega
Cen obtained with FLAMES. The well-known metallicity spread is visible among
the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~
-1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~
-1.7), in good agreement with the corresponding zero-age horizontal branch
models with cosmological helium abundance Y = 0.246. This result conflicts with
the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to
the anomalous blue main-sequence stars, which share a similar metallicity but
whose properties are currently explained by assuming for them a large helium
enhancement. Therefore, in this scenario, the coexistence within the cluster of
two different populations with similar metallicities ([Fe/H] ~ -1.2) and
different helium abundances has to be considered.Comment: 9 pages, 4 figures, accepted for publication by ApJ
Testing Quarkonium Production with Photoproduced
I compute the leading color-octet contributions to the process within the non-relativistic QCD (NRQCD)
factorization formalism. In the color-singlet model, can only
be produced when the photon interacts through its structure function, while the
color-octet mechanism allows for production of via direct
photon-gluon fusion. Resolved photon processes can be easily be distinguished
from direct photon processes by examining the fraction of the incident photon
energy carried away by the in the event. Therefore, this process
provides a conclusive test of the color-octet mechanism.
production is particularly sensitive to the NRQCD matrix element which figures
prominently in the fragmentation production of at large in
hadron colliders. I also examine the predictions of the color evaporation model
(CEM) of quarkonium production and find that this process can easily
discriminate between the NRQCD factorization formalism and the CEM.Comment: uses Revtex, 12 pages, 4 EPS figures embedded using epsf.sty. Some
references have been added. Version accepted for publication in Phys. Rev.
Correlations of Globular Cluster Properties: Their Interpretations and Uses
Correlations among the independently measured physical properties of globular
clusters (GCs) can provide powerful tests for theoretical models and new
insights into their dynamics, formation, and evolution. We review briefly some
of the previous work, and present preliminary results from a comparative study
of GC correlations in the Local Group galaxies. The results so far indicate
that these diverse GC systems follow the same fundamental correlations,
suggesting a commonality of formative and evolutionary processes which produce
them.Comment: An invited review, to appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, S.G. Djorgovski, and M. Riello, ASPCS,
in press (2003). Latex file, 8 pages, 5 eps figures, style files include
Pulsation Period Changes as a Tool to Identify Pre-Zero Age Horizontal Branch Stars
One of the most dramatic events in the life of a low-mass star is the He
flash, which takes place at the tip of the red giant branch (RGB) and is
followed by a series of secondary flashes before the star settles into the
zero-age horizontal branch (ZAHB). Yet, no stars have been positively
identified in this key evolutionary phase, mainly for two reasons: first, this
pre-ZAHB phase is very short compared to other major evolutionary phases in the
life of a star; and second, these pre-ZAHB stars are expected to overlap the
loci occupied by asymptotic giant branch (AGB), HB and RGB stars observed in
the color-magnitude diagram (CMD). We investigate the possibility of detecting
these stars through stellar pulsations, since some of them are expected to
rapidly cross the Cepheid/RR Lyrae instability strip in their route from the
RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a
consequence of their very high evolutionary speed, some of these stars may
present anomalously large period change rates. We constructed an extensive grid
of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for
the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR
Lyrae stars with high period change rates are found. Our results suggest that
some -- but certainly not all -- of the RR Lyrae stars in M3 with large period
change rates are in fact pre-ZAHB pulsators.Comment: Conference Proceedings HELAS Workshop on 'Synergies between solar and
stellar modelling', Rome, June 2009, Astrophys. Space Sci., in the pres
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