700 research outputs found
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV observations of some massive globular clusters uncovered a significant
population of very hot stars below the hot end of the horizontal branch (HB),
the so-called blue hook stars. This feature might be explained either as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich
stars show strong carbon enrichment as predicted by the late hot flasher
scenario. We conclude that the helium-rich HB stars in omega Cen cannot be
explained solely by the helium-enrichment scenario invoked to explain the blue
main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
Incoherent dynamics in neutron-matter interaction
Coherent and incoherent neutron-matter interaction is studied inside a
recently introduced approach to subdynamics of a macrosystem. The equation
describing the interaction is of the Lindblad type and using the Fermi
pseudopotential we show that the commutator term is an optical potential
leading to well-known relations in neutron optics. The other terms, usually
ignored in optical descriptions and linked to the dynamic structure function of
the medium, give an incoherent contribution to the dynamics, which keeps
diffuse scattering and attenuation of the coherent beam into account, thus
warranting fulfilment of the optical theorem. The relevance of this analysis to
experiments in neutron interferometry is briefly discussed.Comment: 15 pages, revtex, no figures, to appear in Phys. Rev.
Completely Positive Quantum Dissipation
A completely positive master equation describing quantum dissipation for a
Brownian particle is derived starting from microphysical collisions, exploiting
a recently introduced approach to subdynamics of a macrosystem. The obtained
equation can be cast into Lindblad form with a single generator for each
Cartesian direction. Temperature dependent friction and diffusion coefficients
for both position and momentum are expressed in terms of the collision
cross-section.Comment: 8 pages, revtex, no figure
Dynamical Semigroup Description of Coherent and Incoherent Particle-Matter Interaction
The meaning of statistical experiments with single microsystems in quantum
mechanics is discussed and a general model in the framework of non-relativistic
quantum field theory is proposed, to describe both coherent and incoherent
interaction of a single microsystem with matter. Compactly developing the
calculations with superoperators, it is shown that the introduction of a time
scale, linked to irreversibility of the reduced dynamics, directly leads to a
dynamical semigroup expressed in terms of quantities typical of scattering
theory. Its generator consists of two terms, the first linked to a coherent
wavelike behaviour, the second related to an interaction having a measuring
character, possibly connected to events the microsystem produces propagating
inside matter. In case these events breed a measurement, an explicit
realization of some concepts of modern quantum mechanics ("effects" and
"operations") arises. The relevance of this description to a recent debate
questioning the validity of ordinary quantum mechanics to account for such
experimental situations as, e.g., neutron-interferometry, is briefly discussed.Comment: 22 pages, latex, no figure
Test Particle in a Quantum Gas
A master equation with a Lindblad structure is derived, which describes the
interaction of a test particle with a macroscopic system and is expressed in
terms of the operator valued dynamic structure factor of the system. In the
case of a free Fermi or Bose gas the result is evaluated in the Brownian limit,
thus obtaining a single generator master equation for the description of
quantum Brownian motion in which the correction due to quantum statistics is
explicitly calculated. The friction coefficients for Boltzmann and Bose or
Fermi statistics are compared.Comment: 9 pages, revtex, no figure
Studying the evolution of galaxies in compact groups over the past 3 Gyr - II. The importance of environment in the suppression of star formation
We present an in depth study on the evolution of galaxy properties in compact
groups over the past 3 Gyr. We are using the largest multi-wavelength sample
to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift
range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely
on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using
CIGALE. Our results suggest that during the 3 Gyr period covered by our sample,
the star formation activity of galaxies in our groups has been substantially
reduced (3-10 times). Moreover, their star formation histories as well as their
UV-optical and mid-infrared colors are significantly different from those of
field and cluster galaxies, indicating that compact group galaxies spend more
time transitioning through the green valley. The morphological transformation
from late-type spirals into early-type galaxies occurs in the mid-infrared
transition zone rather than in the UV-optical green valley. We find evidence of
shocks in the emission line ratios and gas velocity dispersions of the
late-type galaxies located below the star forming main sequence. Our results
suggest that in addition to gas stripping, turbulence and shocks might play an
important role in suppressing the star formation in compact group galaxies.Comment: (Accepted for publication in MNRAS, date of submission November 18,
2015
Source apportionment of submicron organic aerosols at an urban site by linear unmixing of aerosol mass spectra
International audienceSubmicron ambient aerosol was characterized in summer 2005 at an urban background site in Zurich, Switzerland, during a three-week measurement campaign. Highly time-resolved samples of non-refractory aerosol components were analyzed with an Aerodyne aerosol mass spectrometer (AMS). Positive matrix factorization (PMF) was used for the first time for AMS data to identify the main components of the total organic aerosol and their sources. The PMF retrieved factors were compared to measured reference mass spectra and were correlated with tracer species of the aerosol and gas phase measurements from collocated instruments. Six factors were found to explain virtually all variance in the data and could be assigned either to sources or to aerosol components such as oxygenated organic aerosol (OOA). Our analysis suggests that at the measurement site only a small (1 originates from freshly emitted fossil fuel combustion. Other primary sources identified to be of similar or even higher importance are charbroiling (10?15%) and wood burning (~10%), along with a minor source interpreted to be influenced by food cooking (6%). The fraction of all identified primary sources is considered as primary organic aerosol (POA). This interpretation is supported by calculated ratios of the modelled POA and measured primary pollutants such as elemental carbon (EC), NOx, and CO, which are in good agreement to literature values. A high fraction (60?69%) of the measured organic aerosol mass is OOA which is interpreted mostly as secondary organic aerosol (SOA). This oxygenated organic aerosol can be separated into a highly aged fraction, OOA I, (40?50%) with low volatility and a mass spectrum similar to fulvic acid, and a more volatile and probably less processed fraction, OOA II (on average 20%). This is the first publication of a multiple component analysis technique to AMS organic spectral data and also the first report of the OOA II component
The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
We present an examination of high resolution, ultraviolet spectroscopy from
Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the
massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based
optical spectra to determine the effective temperature and gravity of the O9.7
Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), line
blanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0
+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. The
optical spectrum is best fit with models that have enriched He and N
abundances. We fit the model spectral energy distribution for this temperature
and gravity to the UV, optical, and IR fluxes to determine the angular size of
and extinction towards the binary. The angular size then yields relations for
the stellar radius and luminosity as a function of distance. By assuming that
the supergiant rotates synchronously with the orbit, we can use the radius -
distance relation to find mass estimates for both the supergiant and black hole
as a function of the distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that limits the
solutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sun
for the supergiant and 11^{+5}_{-3} M_sun for the black hole.Comment: ApJ in pres
A hydrogen beam to characterize the ASACUSA antihydrogen hyperfine spectrometer
The antihydrogen programme of the ASACUSA collaboration at the antiproton
decelerator of CERN focuses on Rabi-type measurements of the ground-state
hyperfine splitting of antihydrogen for a test of the combined
Charge-Parity-Time symmetry. The spectroscopy apparatus consists of a microwave
cavity to drive hyperfine transitions and a superconducting sextupole magnet
for quantum state analysis via Stern-Gerlach separation. However, the small
production rates of antihydrogen forestall comprehensive performance studies on
the spectroscopy apparatus. For this purpose a hydrogen source and detector
have been developed which in conjunction with ASACUSA's hyperfine spectroscopy
equipment form a complete Rabi experiment. We report on the formation of a
cooled, polarized, and time modulated beam of atomic hydrogen and its detection
using a quadrupole mass spectrometer and a lock-in amplification scheme. In
addition key features of ASACUSA's hyperfine spectroscopy apparatus are
discussed.
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