1,079 research outputs found
Accretion and activity on the post-common-envelope binary RR~Cae
Current scenarios for the evolution of interacting close binaries - such as
cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star
angular momentum loss (AML) mechanisms. The coupling of stellar wind with its
magnetic field, i.e., magnetic braking, is the most promising mechanism to
drive AML in these stars. There are basically two properties driving magnetic
braking: the stellar magnetic field and the stellar wind. Understanding the
mechanisms that drive AML therefore requires a comprehensive understanding of
these two properties. RRCae is a well-known nearby (d=20pc) eclipsing DA+M
binary with an orbital period of P=7.29h. The system harbors a metal-rich cool
white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation.
The metallicity of the WD suggests that wind accretion is taking place, which
provides a good opportunity to obtain the mass-loss rate of the M-dwarf
component. We analyzed multi-epoch time-resolved high-resolution spectra of
RRCae in search for traces of magnetic activity and accretion. We selected a
number of well-known activity indicators and studied their short and long-term
behavior. Indirect-imaging tomographic techniques were also applied to provide
the surface brightness distribution of the magnetically active M-dwarf, and
reveals a polar feature similar to those observed in fast-rotating solar-type
stars. The blue part of the spectrum was modeled using a atmosphere model to
constrain the WD properties and its metal enrichment. The latter was used to
improve the determination of the mass-accretion rate from the M-dwarf wind. The
presence of metals in the WD spectrum suggests that this component arises from
accretion of the M-dwarf wind. A model fit to the WD gives Teff=(7260+/-250)K
and logg=(7.8+/-0.1) dex with a metallicity of =(-2.8+/-0.1)dex,
and a mass-accretion rate of dotMacc=(7+/-2)x1e-16Msun/yr.Comment: 14 pages, 7 Figures, 6 Table
Orbit-resolved photometry and echelle spectroscopy of the cataclysmic variable ST LMi during a 2007 high state
We present high-resolution echelle spectra and contemporaneous photometry of the polar ST LMi during a high state in 2007 March. Emission lines at Hα, He I λ5876, and He I λ7065 show similar line profiles over orbital phase and have narrow and broad components. These profile changes with phase are very similar to those reported in earlier high-state studies of ST LMi. The radial velocity curves from double Gaussian fits to the line profiles are interpreted as two crossing curves, neither of which is coincident with the orbital motion of the secondary star. We attribute one component to infall motions near the white dwarf and the other to a gas streaming along magnetic field lines connecting the two stars
Recent atmospheric neutrino results from Soudan 2
An updated measurement of the atmospheric nu_mu/nu_e ratio-of-ratios,
0.68+-0.11+-0.06, has been obtained using a 4.6-kty exposure of the Soudan-2
iron tracking calorimeter. The L/E distributions have been analyzed for effects
of nu_mu -> nu_x oscillations, and an allowed region in the Delta m^2 vs. sin^2
2 theta plane has been determined.Comment: 3 pages, 4 figures; presented at TAUP99, the 6th Int. Workshop on
Topics in Astroparticle and Underground Physics, Sept. 6-10, 1999, College de
France, Paris, Franc
VV Pup in a low state: secondary-star irradiation or stellar activity?
Aims. Emission lines in polars show complex profiles with multiple components
that are typically ascribed to the accretion stream, threading region,
accretion spot, and the irradiated secondary-star. In low-state polars the
fractional contribution by the accretion stream, and the accretion spot is
greatly reduced offering an opportunity to study the effect of the
secondary-star irradiation or stellar activity. We observed VV Pup during an
exceptional low-state to study and constrain the properties of the line-forming
regions and to search for evidence of chromospheric activity and/or
irradiation. Methods. We obtained phase-resolved optical spectra at the ESO
VLT+FORS1 with the aim of analyzing the emission line profile and radial
velocity as a function of the orbital period. We also tailored irradiated
secondary-star models to compare the predicted and the observed emission lines
and to establish the nature of the line-forming regions. Results. Our
observations and data analysis, when combined with models of the irradiated
secondary-star, show that, while the weak low ionization metal lines (FeI and
MgI) may be consistent with irradiation processes, the dominant Balmer H
emission lines, as well as NaI and HeI, cannot be reproduced by the irradiated
secondary-star models. We favor the secondary-star chromospheric activity as
the main forming region and cause of the observed H, NaI, and He emission
lines, though a threading region very close to the L1 point cannot be excluded.Comment: 10 pages, 9 figures, in press on A&
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
p_T-fluctuations in high-energy p-p and A-A collisions
The event-by-event p_T-fluctuations in proton-proton and central Pb-Pb
collisions, which have been experimentally studied by means of the so-called
Phi-measure, are analyzed. The contribution due to the correlation which
couples the average p_T to the event multiplicity is computed. The correlation
appears to be far too weak to explain the preliminary experimental value of Phi
(p_T) in p-p interactions. The significance of the result is discussed.Comment: 5 pages, 2 figures, minor improvement
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