4,285 research outputs found
The light variability of the helium strong star HD 37776 as a result of its inhomogeneous elemental surface distribution
We simulate light curves of the helium strong chemically peculiar star HD
37776 assuming that the observed periodic light variations originate as a
result of inhomogeneous horizontal distribution of chemical elements on the
surface of a rotating star. We show that chemical peculiarity influences the
monochromatic radiative flux, mainly due to bound-free processes. Using the
model of the distribution of silicon and helium on HD 37776 surface, derived
from spectroscopy, we calculate a photometric map of the surface and
consequently the uvby light curves of this star. Basically, the predicted light
curves agree in shape and amplitude with the observed ones. We conclude that
the basic properties of variability of this helium strong chemically peculiar
star can be understood in terms of the model of spots with peculiar chemical
composition.Comment: 11 pages, accepted for the publication in Astronomy & Astrophysic
Outliers from the Mass--Metallicity Relation II: A Sample of Massive Metal-Poor Galaxies from SDSS
We present a sample of 42 high-mass low-metallicity outliers from the
mass--metallicity relation of star-forming galaxies. These galaxies have
stellar masses that span log(M_*/M_sun) ~9.4 to 11.1 and are offset from the
mass--metallicity relation by -0.3 to -0.85 dex in 12+log(O/H). In general,
they are extremely blue, have high star formation rates for their masses, and
are morphologically disturbed. Tidal interactions are expected to induce
large-scale gas inflow to the galaxies' central regions, and we find that these
galaxies' gas-phase oxygen abundances are consistent with large quantities of
low-metallicity gas from large galactocentric radii diluting the central
metal-rich gas. We conclude with implications for deducing gas-phase
metallicities of individual galaxies based solely on their luminosities,
specifically in the case of long gamma-ray burst host galaxies.Comment: Accepted for publication in ApJ; 11 pages, 11 figure
Estudio de las abundancias químicas de dos estrellas CP magnéticas: HD 43819 y HD 147550
Utilizando material espectroscópico de alta dispersión, y modelos de atmósferas calculados con el programa ATLAS 9, cuyas predicciones se ajustan a la espectro-fotometría y a los perfiles de Hγ, se realiza un fine analysis de las estrellas CP magnéticas HD 43819 y HD 147550. Se discute especialmente la influencia del uso de las nuevas funciones de distribución que incluyen metales reforzados, en la determinación de parámetros atmosféricos y en la estimación de las abundancias químicas.Asociación Argentina de Astronomí
Air quality and radiative forcing impacts of anthropogenic volatile organic compound emissions from ten world regions
Non-methane volatile organic compounds (NMVOCs) influence air quality and global climate change through their effects on secondary air pollutants and climate forcers. Here we simulate the air quality and radiative forcing (RF) impacts of changes in ozone, methane, and sulfate from halving anthropogenic NMVOC emissions globally and from 10 regions individually, using a global chemical transport model and a standalone radiative transfer model. Halving global NMVOC emissions decreases global annual average tropospheric methane and ozone by 36.6 ppbv and 3.3 Tg, respectively, and surface ozone by 0.67 ppbv. All regional reductions slow the production of peroxyacetyl nitrate (PAN), resulting in regional to intercontinental PAN decreases and regional NOx increases. These NOx increases drive tropospheric ozone increases nearby or downwind of source regions in the Southern Hemisphere (South America, Southeast Asia, Africa, and Australia). Some regions' NMVOC emissions contribute importantly to air pollution in other regions, such as East Asia, the Middle East, and Europe, whose impact on US surface ozone is 43%, 34%, and 34% of North America's impact. Global and regional NMVOC reductions produce widespread negative net RFs (cooling) across both hemispheres from tropospheric ozone and methane decreases, and regional warming and cooling from changes in tropospheric ozone and sulfate (via several oxidation pathways). The 100 yr and 20 yr global warming potentials (GWP100, GWP20) are 2.36 and 5.83 for the global reduction, and 0.079 to 6.05 and −1.13 to 18.9 among the 10 regions. The NMVOC RF and GWP estimates are generally lower than previously modeled estimates, due to the greater NMVOC/NOx emissions ratios simulated, which result in less sensitivity to NMVOC emissions changes and smaller global O3 burden responses, in addition to differences in the representation of NMVOCs and oxidation chemistry among models. Accounting for a fuller set of RF contributions may change the relative magnitude of each region's impacts. The large variability in the RF and GWP of NMVOCs among regions suggest that regionally specific metrics may be necessary to include NMVOCs in multi-gas climate trading schemes
Pinning of a solid--liquid--vapour interface by stripes of obstacles
We use a macroscopic Hamiltonian approach to study the pinning of a
solid--liquid--vapour contact line on an array of equidistant stripes of
obstacles perpendicular to the liquid. We propose an estimate of the density of
pinning stripes for which collective pinning of the contact line happens. This
estimate is shown to be in good agreement with Langevin equation simulation of
the macroscopic Hamiltonian. Finally we introduce a 2--dimensional mean field
theory which for small strength of the pinning stripes and for small capillary
length gives an excellent description of the averaged height of the contact
line.Comment: Plain tex, 12 pages, 3 figures available upon reques
Mergers of luminous early-type galaxies in the local universe and gravitational wave background
Supermassive black hole (SMBH) coalescence in galaxy mergers is believed to
be one of the primary sources of very low frequency gravitational waves (GWs).
Significant contribution of the GWs comes from mergers of massive galaxies with
redshifts z<2. Very few previous studies gave the merger rate of massive
galaxies. % We selected a large sample (1209) of close pairs of galaxies with
projected separations 7<r_p<50 kpc from 87,889 luminous early-type galaxies
(M_r<-21.5) from the Sloan Digital Sky Survey Data Release 6. These pairs
constitute a complete volume-limited sample in the local universe (z<0.12).
Using our newly developed technique, 249 mergers have been identified by
searching for interaction features. From them, we found that the merger
fraction of luminous early-type galaxies is 0.8%, and the merger rate in the
local universe is % R_g=(1.0+/-0.4)*10^{-5} Mpc^{-3} Gyr^{-1}} % with an
uncertainty mainly depending on the merging timescale. % We estimated the
masses of SMBHs in the centers of merging galaxies based on their luminosities.
We found that the chirp mass distribution of the SMBH binaries follows a power
law with an index of -3.0+/-0.5 in the range 5*10^8--5*10^{9} M_{\odot}. %
Using the SMBH population in the mergers and assuming that the SMBHs can be
efficiently driven into the GW regime, we investigated the stochastic GW
background in the frequency range 10^{-9}--10^{-7} Hz. We obtained the spectrum
of the GW background of h_c(f)=10^{-15}(f/yr^{-1})^{-2/3}, which is one
magnitude higher than that obtained by Jaffe & Backer in 2003, but consistent
with those calculated from galaxy-formation models.Comment: 27 pages, 9 figures, Corrected typos and reference
Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis
CU Vir and V901 Ori belong among these few magnetic chemically peculiar stars
whose rotation periods vary on timescales of decades. We aim to study the
stability of the periods in CU Vir and V901 Ori using all accessible
observational data containing phase information. We collected all available
relevant archived observations supplemented with our new measurements of these
stars and analysed the period variations of the stars using a novel method that
allows for the combination of data of diverse sorts. We found that the shapes
of their phase curves were constant, while the periods were changing. Both
stars exhibit alternating intervals of rotational braking and acceleration. The
rotation period of CU Vir was gradually shortening until the year 1968, when it
reached its local minimum of 0.52067198 d. The period then started increasing,
reaching its local maximum of 0.5207163 d in the year 2005. Since that time the
rotation has begun to accelerate again. We also found much smaller period
changes in CU Vir on a timescale of several years. The rotation period of V901
Ori was increasing for the past quarter-century, reaching a maximum of 1.538771
d in the year 2003, when the rotation period began to decrease. A theoretically
unexpected alternating variability of rotation periods in these stars would
remove the spin-down time paradox and brings a new insight into structure and
evolution of magnetic upper-main-sequence stars.Comment: 5 pages, 3 figure
Induced measures in the space of mixed quantum states
We analyze several product measures in the space of mixed quantum states. In
particular we study measures induced by the operation of partial tracing. The
natural, rotationally invariant measure on the set of all pure states of a N x
K composite system, induces a unique measure in the space of N x N mixed states
(or in the space of K x K mixed states, if the reduction takes place with
respect to the first subsystem). For K=N the induced measure is equal to the
Hilbert-Schmidt measure, which is shown to coincide with the measure induced by
singular values of non-Hermitian random Gaussian matrices pertaining to the
Ginibre ensemble. We compute several averages with respect to this measure and
show that the mean entanglement of pure states behaves as lnN-1/2.Comment: 12 latex pages, 2 figures in epsf, submited to J. Phys. A. ver.3,
some improvements and a few references adde
The complex universe: recent observations and theoretical challenges
The large scale distribution of galaxies in the universe displays a complex
pattern of clusters, super-clusters, filaments and voids with sizes limited
only by the boundaries of the available samples. A quantitative statistical
characterization of these structures shows that galaxy distribution is
inhomogeneous in these samples, being characterized by large-amplitude
fluctuations of large spatial extension. Over a large range of scales, both the
average conditional density and its variance show a nontrivial scaling
behavior: at small scales, r<20 Mpc/h, the average (conditional) density scales
as 1/r. At larger scales, the density depends only weakly (logarithmically) on
the system size and density fluctuations follow the Gumbel distribution of
extreme value statistics. These complex behaviors are different from what is
expected in a homogeneous distribution with Gaussian fluctuations. The observed
density inhomogeneities pose a fundamental challenge to the standard picture of
cosmology but it also represent an important opportunity which points to new
directions with respect to many cosmological puzzles. Indeed, the fact that
matter distribution is not uniform, in the limited range of scales sampled by
observations, rises the question of understanding how inhomogeneities affect
the large-scale dynamics of the universe. We discuss several attempts which try
to model inhomogeneities in cosmology, considering their effects with respect
to the role and abundance of dark energy and dark matter.Comment: 30 pages, 10 figure
Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Vir
The spectral energy distribution (SED) in chemically peculiar stars may be
significantly affected by their abundance anomalies. The observed SED
variations are usually assumed to be a result of inhomogeneous surface
distribution of chemical elements, flux redistribution and stellar rotation.
However, the direct evidence for this is still only scarce. We aim to identify
the processes that determine the SED and its variability in the UV and visual
spectral domains of the helium-weak star CU Vir. We used the model atmospheres
to obtain the emergent flux and predict the rotationally modulated flux
variability of the star. We show that most of the light variations in the vby
filters of the Stromgren photometric system are a result of the uneven surface
distribution of silicon, chromium, and iron. Our models are only able to
explain a part of the variability in the u filter, however. The observed UV
flux distribution is very well reproduced, and the models are able to explain
most of the observed features in the UV light curve. The variability observed
in the visible is merely a faint gleam of that in the UV. While the amplitude
of the light curves reaches only several hundredths of magnitude in the visual
domain, it reaches about 1 mag in the UV. The visual and UV light variability
of CU Vir is caused by the flux redistribution from the far UV to near UV and
visible regions, inhomogeneous distribution of the elements and stellar
rotation. Bound-free transitions of silicon and bound-bound transitions of iron
and chromium contribute the most to the flux redistribution. This mechanism can
explain most of the rotationally modulated light variations in the filters
centred on the Paschen continuum and on the UV continuum of the star CU Vir.
However, another mechanism(s) has to be invoked to fully explain the observed
light variations in the u filter and in the region 2000-2500 A.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
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