3,910 research outputs found
Solar radius and luminosity variations induced by the internal dynamo magnetic fields
Although the occurrence of solar irradiance variations induced by magnetic
surface features (e.g., sunspots, faculae, magnetic network) is generally
accepted, the existence of intrinsic luminosity changes due to the internal
magnetic fields is still controversial. This additional contribution is
expected to be accompanied by radius variations, and to be potentially
significant for the climate of the Earth. We aim to constrain theoretically the
radius and luminosity variations of the Sun that are due to the effect of the
variable magnetic fields in its interior associated with the dynamo cycle. We
have extended a one-dimensional stellar evolution code to include several
effects of the magnetic fields on the interior structure. We investigate
different magnetic configurations, based on both observational constraints and
on the output of state-of-the-art mean field dynamo models. We explore both
step-like and simply periodic time dependences of the magnetic field peak
strength. We find that the luminosity and radius variations are in anti-phase
and in phase, respectively, with the magnetic field strength. For peak magnetic
field strengths of the order of tens of kilogauss, luminosity variations
ranging between 10^{-6} and 10^{-3} (in modulus) and radius variations between
10^{-6} and 10^{-5} are obtained. Modest but significant radius variations (up
to 10^{-5} in relative terms) are obtained for magnetic fields of realistic
strength and geometry, providing a potentially observable signature of the
intrinsic variations. Establishing their existence in addition to the accepted
surface effects would have very important implications for the understanding of
solar-induced long-term trends on climate.Comment: 18 pages, 7 figures; accepted for publication in Astronomische
Nachrichte
Power Density Spectra of Gamma-Ray Bursts in the Internal Shock Model
We simulate Gamma-Ray Bursts arising from internal shocks in relativistic
winds, calculate their power density spectrum (PDS), and identify the factors
to which the PDS is most sensitive: the wind ejection features, which determine
the wind dynamics and its optical thickness, and the energy release parameters,
which give the pulse 50-300 keV radiative efficiency. For certain combinations
of ejection features and wind parameters the resulting PDS exhibits the
features observed in real bursts. We found that the upper limit on the
efficiency of conversion of wind kinetic energy into 50-300 keV photons is
1%. Winds with a modulated Lorentz factor distribution of the ejecta
yield PDSs in accord with current observations and have efficiencies closer to
, while winds with a random, uniform Lorentz factor ejection must be
optically thick to the short duration pulses to produce correct PDSs, and have
an overall efficiency around .Comment: 6 pages, 4 figures, Latex, submitted to The Astrophysical Journal
(05/04/99
The angular momentum transport by unstable toroidal magnetic fields
We demonstrate with a nonlinear MHD code that angular momentum can be
transported due to the magnetic instability of toroidal fields under the
influence of differential rotation, and that the resulting effective viscosity
may be high enough to explain the almost rigid-body rotation observed in
radiative stellar cores. Only stationary current-free fields and only those
combinations of rotation rates and magnetic field amplitudes which provide
maximal numerical values of the viscosity are considered. We find that the
dimensionless ratio of the effective over molecular viscosity, ;,
linearly grows with the Reynolds number of the rotating fluid multiplied with
the square-root of the magnetic Prandtl number - which is of order unity for
the considered red sub-giant KIC 7341231.
For the considered interval of magnetic Reynolds numbers - which is
restricted by numerical constraints of the nonlinear MHD code - there is a
remarkable influence of the magnetic Prandtl number on the relative importance
of the contributions of the Reynolds stress and the Maxwell stress to the total
viscosity, which is magnetically dominated only for Pm 0.5. We also
find that the magnetized plasma behaves as a non-Newtonian fluid, i.e. the
resulting effective viscosity depends on the shear in the rotation law. The
decay time of the differential rotation thus depends on its shear and becomes
longer and longer during the spin-down of a stellar core.Comment: Revised version. 7 pages, 9 figures; accepted for publication in A&
Angular momentum transport efficiency in post-main sequence low-mass stars
Context. Using asteroseismic techniques, it has recently become possible to
probe the internal rotation profile of low-mass (~1.1-1.5 Msun) subgiant and
red giant stars. Under the assumption of local angular momentum conservation,
the core contraction and envelope expansion occurring at the end of the main
sequence would result in a much larger internal differential rotation than
observed. This suggests that angular momentum redistribution must be taking
place in the interior of these stars. Aims. We investigate the physical nature
of the angular momentum redistribution mechanisms operating in stellar
interiors by constraining the efficiency of post-main sequence rotational
coupling. Methods. We model the rotational evolution of a 1.25 Msun star using
the Yale Rotational stellar Evolution Code. Our models take into account the
magnetic wind braking occurring at the surface of the star and the angular
momentum transport in the interior, with an efficiency dependent on the degree
of internal differential rotation. Results. We find that models including a
dependence of the angular momentum transport efficiency on the radial
rotational shear reproduce very well the observations. The best fit of the data
is obtained with an angular momentum transport coefficient scaling with the
ratio of the rotation rate of the radiative interior over that of the
convective envelope of the star as a power law of exponent ~3. This scaling is
consistent with the predictions of recent numerical simulations of the
Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular
momentum transport process whose efficiency varies during the stellar evolution
through a dependence on the level of internal differential rotation is required
to explain the observed post-main sequence rotational evolution of low-mass
stars.Comment: 8 pages, 6 figures; accepted for publication in Astronomy &
Astrophysic
The metacognitions about smoking questionnaire : development and psychometric properties
The Metacognitions about Smoking Questionnaire was shown to possess good psychometric properties, as well as predictive and divergent validity within the populations that were tested. The metacognition factors explained incremental variance in smoking behaviour above smoking outcome expectancies
Development and Validation of the Metacognitions about Sex Scale: Exploring its Role as a Mediator between Negative Affect, Emotion Dysregulation Strategies, and Compulsive Sexual Behavior Disorder.
Metacognitions about sex are theorized to shape cognitive appraisal, coping mechanisms, and regulation or dysregulation before, during, and/or after exposure to sexual stimuli. In our study, we examined the construct structure and validity of the Metacognitions about Sex Scale (MSS) among a sample of adolescents. We estimated the convergent validity of the MSS by factors: negative affect, dysregulated thoughts, and impulsivity, and compulsive sexual behavior (CSB). We also ran a structural equation model in which we examined the possibility that metacognitions about sex would mediate the association between negative affect, dysregulated thoughts, and impulsivity on the one hand, and CSB on the other. The study population included 662 adolescents (252 boys and 410 girls, â=â16.70, SD = 1.32) between 13-18 years of age. The analyses indicated that the factorial structure of the MSS comprised the two expected factors. We also found that positive and negative metacognitions about sex significantly mediated the effect of negative affect, dysregulated thoughts, and impulsivity on CSB. The findings provide evidence that MSS among Israeli adolescents are psychometrically appropriate for use by researchers and practitioners in the prevention and treatment of CSB
Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
Current three-dimensional (3D) hydrodynamical model atmospheres together with
NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical
abundances from high-resolution stellar spectra. Not much is known about the
presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not
to mention its production in magnetically active targets like HD123351. From
fits of the observed CFHT spectrum with synthetic line profiles based on 1D and
3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region of a
high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are
computed with four different line lists, using in parallel 3D hydrodynamical
CO5BOLD and 1D LHD model atmospheres and treating the line formation of the
lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and
6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.
(2012), updated with new atomic data for V I, which results in the best fit of
the lithium line profile of HD123351. Two other line lists lead to similar
results but with inferior fit qualities. Our 2-sigma detection of the 6Li
isotope is the result of a careful statistical analysis and the visual
inspection of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected in the framework
of standard stellar evolution theory, non-standard, external lithium production
mechanisms, possibly related to stellar activity or a recent accretion of rocky
material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&
Intermittent Sea Level Acceleration
Using instrumental observations from the Permanent Service for Mean Sea Level (PSMSL), we provide a new assessment of the global sea{level acceleration for the last 2 centuries (1820-2010). Our results, obtained
by a stack of tide gauge time series, con firm the existence of a global sea level acceleration (GSLA) and, coherently with independent assessments so far, they point to a value close to 0:01 mm/yr2. However, di fferently from
previous studies, we discuss how change points or abrupt inflections in individual sea level time series have contributed to the GSLA. Our analysis,
based on methods borrowed from econometrics, suggests the existence of
two distinct driving mechanisms for the GSLA, both involving a minority
of tide gauges globally. The first effectively implies a gradual increase in the rate of sea level rise at individual tide gauges, while the second is manifest
through a sequence of catastrophic variations of the sea level trend. These occurred intermittently since the end of the 19th century and became more frequent during the last four decades
Ice melting and earthquake suppression in Greenland
It has been suggested that the Greenland ice sheet is the cause of earthquake
suppression in the region. With few exceptions, the observed seismicity
extends only along the continental margins of Greenland, which almost
coincide with the ice sheet margin. This pattern has been put forward as
further validation of the earthquake suppression hypothesis. In this review,
new evidence in terms of ice melting, post-glacial rebound and earthquake
occurrence is gathered and discussed to re-evaluate the connection between
ice mass unloading and earthquake suppression. In Greenland, the spatiotemporal
distribution of earthquakes indicates that seismicity is mainly con-
fined to regions where the thick layer of ice is absent and where significant ice
melting is presently occurring. A clear correlation between seismic activity
and ice melting in Greenland is not found. However, earthquake locations
and corresponding depth distributions suggest two distinct governing mechanisms:
post-glacial rebound promotes moderate-size crustal earthquakes at
Greenlandâs regional scale, while current ice melting promotes shallow low
magnitude seismicity locall
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