3,651 research outputs found
Physics Results from COMPASS
The COMPASS Experiment at the CERN SPS has a broad physics program focused on
the nucleon spin structure and on hadron spectroscopy, using muon and hadron
beams. Main objectives for the spin program with the muon beam are the direct
measurement of the gluon contribution to the spin of the nucleon,
semi-inclusive measurements, and the measurement of the transverse spin
distribution . The COMPASS apparatus consists of a two-stage large
acceptance spectrometer designed for high data rates and equipped with
high-resolution tracking, particle identification and electromagnetic and
hadronic calorimetry. The data taking is ongoing since 2002 and till now was
mainly devoted to the spin programme using a 160 GeV naturally polarized,
beam and a polarized \mrf{^6LiD} target. First physics results from the
2002 and 2003 runs are presented.Comment: 9 pages, 4 figures, Plenary talk at the "16th International Spin
Physics Symposium", October 10-16, 2004, Trieste, Italy; to be published in
the Conference Proceedings, World Scientifi
Sum-factorization techniques in Isogeometric Analysis
The fast assembling of stiffness and mass matrices is a key issue in
isogeometric analysis, particularly if the spline degree is increased. We
present two algorithms based on the idea of sum factorization, one for matrix
assembling and one for matrix-free methods, and study the behavior of their
computational complexity in terms of the spline order . Opposed to the
standard approach, these algorithms do not apply the idea element-wise, but
globally or on macro-elements. If this approach is applied to Gauss quadrature,
the computational complexity grows as instead of as
previously achieved.Comment: 34 pages, 8 figure
The impact of metallicity and dynamics on the evolution of young star clusters
The early evolution of a dense young star cluster (YSC) depends on the
intricate connection between stellar evolution and dynamical processes. Thus,
N-body simulations of YSCs must account for both aspects. We discuss N-body
simulations of YSCs with three different metallicities (Z=0.01, 0.1 and 1
Zsun), including metallicity-dependent stellar evolution recipes and
metallicity-dependent prescriptions for stellar winds and remnant formation. We
show that mass-loss by stellar winds influences the reversal of core collapse.
In particular, the post-collapse expansion of the core is faster in metal-rich
YSCs than in metal-poor YSCs, because the former lose more mass (through
stellar winds) than the latter. As a consequence, the half-mass radius expands
more in metal-poor YSCs. We also discuss how these findings depend on the total
mass and on the virial radius of the YSC. These results give us a clue to
understand the early evolution of YSCs with different metallicity.Comment: to appear in "Massive Young Star Clusters Near and Far: From the
Milky Way to Reionization", 2013 Guillermo Haro Conference, Eds. Y. D. Mayya,
D. Rosa-Gonzalez & E. Terlevich, INAOE and AMC. 4 pages, 2 figure
SPITZER IRS spectra of Virgo early type galaxies: detection of stellar silicate emission
We present high signal to noise ratio Spitzer Infrared Spectrograph
observations of 17 Virgo early-type galaxies. The galaxies were selected from
those that define the colour-magnitude relation of the cluster, with the aim of
detecting the silicate emission of their dusty, mass-losing evolved stars. To
flux calibrate these extended sources we have devised a new procedure that
allows us to obtain the intrinsic spectral energy distribution and to
disentangle resolved and unresolved emission within the same object. We have
found that thirteen objects of the sample (76%) are passively evolving galaxies
with a pronounced broad silicate feature which is spatially extended and likely
of stellar origin, in agreement with model predictions. The other 4 objects
(24%) are characterized by different levels of activity. In NGC 4486 (M 87) the
line emission and the broad silicate emission are evidently unresolved and,
given also the typical shape of the continuum, they likely originate in the
nuclear torus. NGC 4636 shows emission lines superimposed on extended (i.e.
stellar) silicate emission, thus pushing the percentage of galaxies with
silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by
polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central
unresolved region. A more detailed analysis of our sample, with updated models,
will be presented in a forthcoming paper.Comment: 6 pages; ApJ Letters, accepte
Modelling Intermediate Age and Old Stellar Populations in the Infrared
We have investigated the spectro-photometric properties of the Asymptotic
Giant Branch (AGB) stars and their contribution to the integrated infrared
emission in simple stellar populations (SSP). Adopting analytical relations
describing the evolution of these stars in the HR diagram and empirical
relations for the mass-loss rate and the wind terminal velocity, we were able
to model the effects of the dusty envelope around these stars, with a minimal
number of parameters. We computed isochrones at different age and initial metal
content. We compare our models with existing infrared colors of M giants and
Mira stars and with IRAS PSC data. Contrary to previous models, in the new
isochrones the mass-loss rate, which establishes the duration of the AGB phase,
also determines the spectral properties of the stars. The contribution of these
stars to the integrated light of the population is thus obtained in a
consistent way. We find that the emission in the mid infrared is about one
order of magnitude larger when dust is taken into account in an intermediate
age population, irrespective of the particular mixture adopted. The dependence
of the integrated colors on the metallicity and age is discussed, with
particular emphasis on the problem of age-metallicity degeneracy. We show that,
contrary to the case of optical or near infrared colors, the adoption of a
suitable pass-band in the mid infrared allows a fair separation of the two
effects. We suggest intermediate redshift elliptical galaxies as possible
targets of this method of solving the age-metallicity dilemma. The new SSP
models constitute a first step in a more extended study aimed at modelling the
spectral properties of the galaxies from the ultraviolet to the far infrared.Comment: 16 pages, 10 figures, to appear in A&
On Core Collapse Supernovae in Normal and in Seyfert Galaxies
This paper estimates the relative frequency of different types of
core-collapse supernovae, in terms of the ratio f between the number of type
Ib--Ic and of type II supernovae. We estimate f independently for all normal
and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at
least one supernova event recorded in the Asiago catalogue from January 1986 to
August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal
galaxies. This value is consistent with constant star formation rate and with a
Salpeter Initial Mass Function and average binary rate approx. 50 %. On the
contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor
approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for
Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as
yet). Assumed real, this excess of type Ib and Ic with respect to type II
supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a
high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert
galaxies, or a top-loaded mass function.Comment: Accepted for Publication in MNRA
Star Formation History and Extinction in the central kpc of M82-like Starbursts
We report on the star formation histories and extinction in the central kpc
region of a sample of starburst galaxies that have similar far infrared (FIR),
10 micron and K-band luminosities as those of the archetype starburst M82. Our
study is based on new optical spectra and previously published K-band
photometric data, both sampling the same area around the nucleus. Model
starburst spectra were synthesized as a combination of stellar populations of
distinct ages formed over the Hubble time, and were fitted to the observed
optical spectra and K-band flux. The model is able to reproduce simultaneously
the equivalent widths of emission and absorption lines, the continuum fluxes
between 3500-7000 Ang, the K-band and the FIR flux. We require a minimum of 3
populations -- (1) a young population of age < 8 Myr, with its corresponding
nebular emission, (2) an intermediate-age population (age < 500 Myr), and (3)
an old population that forms part of the underlying disk or/and bulge
population. The contribution of the old population to the K-band luminosity
depends on the birthrate parameter and remains above 60% in the majority of the
sample galaxies. Even in the blue band, the intermediate age and old
populations contribute more than 40% of the total flux in all the cases. A
relatively high contribution from the old stars to the K-band nuclear flux is
also apparent from the strength of the 4000 Ang break and the CaII K line. The
extinction of the old population is found to be around half of that of the
young population. The contribution to the continuum from the relatively old
stars has the effect of diluting the emission equivalent widths below the
values expected for young bursts. The mean dilution factors are found to be 5
and 3 for the Halpha and Hbeta lines respectively.Comment: 20 pages, uses emulateapj.cls. Scheduled to appear in ApJ Jan 1, 200
The role of the synchrotron component in the mid infrared spectrum of M 87
We study in detail the mid-infrared Spitzer-IRS spectrum of M 87 in the range
5 to 20 micron. Thanks to the high sensitivity of our Spitzer-IRS spectra we
can disentangle the stellar and nuclear components of this active galaxy. To
this end we have properly subtracted from the M 87 spectrum, the contribution
of the underlying stellar continuum, derived from passive Virgo galaxies in our
sample. The residual is a clear power-law, without any additional thermal
component, with a zero point consistent with that obtained by high spatial
resolution, ground based observations. The residual is independent of the
adopted passive template. This indicates that the 10 micron silicate emission
shown in spectra of M 87 can be entirely accounted for by the underlying old
stellar population, leaving little room for a possible torus contribution. The
MIR power-law has a slope alpha ~ 0.77-0.82 (S),
consistent with optically thin synchrotron emission.Comment: 5 pages, 4 figures, accepted for publication in ApJ main journa
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