2,003 research outputs found
Long-Term Multiwavelength Studies of High-Redshift Blazar 0836+710
Aims. The observation of gamma -ray flares from blazar 0836+710 in 2011,
following a period of quiescence, offered an opportunity to study correlated
activity at different wavelengths for a high-redshift (z=2.218) active galactic
nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT gamma-ray
monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical,
ultraviolet, and X-ray information. Other contemporaneous observations were
used to construct a broad-band spectral energy distribution. Results. There is
evidence of correlation but not a measurable lag between the optical and
gamma-ray flaring emission. On the contrary, there is no clear correlation
between radio and gamma-ray activity, indicating radio emission regions that
are unrelated to the parts of the jet that produce the gamma-rays. The
gamma-ray energy spectrum is unusual in showing a change of shape from a power
law to a curved spectrum when going from the quiescent state to the active
state.Comment: 11 pages, 10 figures, Accepted for publication in A&
Signal processing in local neuronal circuits based on activity-dependent noise and competition
We study the characteristics of weak signal detection by a recurrent neuronal
network with plastic synaptic coupling. It is shown that in the presence of an
asynchronous component in synaptic transmission, the network acquires
selectivity with respect to the frequency of weak periodic stimuli. For
non-periodic frequency-modulated stimuli, the response is quantified by the
mutual information between input (signal) and output (network's activity), and
is optimized by synaptic depression. Introducing correlations in signal
structure resulted in the decrease of input-output mutual information. Our
results suggest that in neural systems with plastic connectivity, information
is not merely carried passively by the signal; rather, the information content
of the signal itself might determine the mode of its processing by a local
neuronal circuit.Comment: 15 pages, 4 pages, in press for "Chaos
On the Radii of Close-in Giant Planets
The recent discovery that the close-in extrasolar giant planet, HD209458b,
transits its star has provided a first-of-its-kind measurement of the planet's
radius and mass. In addition, there is a provocative detection of the light
reflected off of the giant planet, Boo b. Including the effects of
stellar irradiation, we estimate the general behavior of radius/age
trajectories for such planets and interpret the large measured radii of
HD209458b and Boo b in that context. We find that HD209458b must be a
hydrogen-rich gas giant. Furthermore, the large radius of close-in gas giant is
not due to the thermal expansion of its atmosphere, but to the high residual
entropy that remains throughout its bulk by dint of its early proximity to a
luminous primary. The large stellar flux does not inflate the planet, but
retards its otherwise inexorable contraction from a more extended configuration
at birth. This implies either that such a planet was formed near its current
orbital distance or that it migrated in from larger distances (0.5 A.U.),
no later than a few times years of birth.Comment: aasms4 LaTeX, 1 figure, accepted to Ap.J. Letter
Testing Spallation Processes With Beryllium and Boron
The nucleosynthesis of Be and B by spallation processes provides unique
insight into the origin of cosmic rays. Namely, different spallation schemes
predict sharply different trends for the growth of LiBeB abundances with
respect to oxygen. ``Primary'' mechanisms predict BeB O, and are well
motivated by the data if O/Fe is constant at low metallicity. In contrast,
``secondary'' mechanisms predict BeB O and are consistent with
the data if O/Fe increases towards low metallicity as some recent data suggest.
Clearly, any primary mechanism, if operative, will dominate early in the
history of the Galaxy. In this paper, we fit the BeB data to a two-component
scheme which includes both primary and secondary trends. In this way, the data
can be used to probe the period in which primary mechanisms are effective. We
analyze the data using consistent stellar atmospheric parameters based on
Balmer line data and the continuum infrared flux. Results depend sensitively on
Pop II O abundances and, unfortunately, on the choice of stellar parameters.
When using recent results which show O/Fe increasing toward lower metallicity,
a two-component Be-O fits indicates that primary and secondary components
contribute equally at [O/H] = -1.8 for Balmer line data; and
[O/H] = -1.4 to -1.8 for IRFM. We apply these constraints to recent
models for LiBeB origin. The Balmer line data does not show any evidence for
primary production. On the other hand, the IRFM data does indicate a preference
for a two-component model, such as a combination of standard GCR and
metal-enriched particles accelerated in superbubbles. These conclusions rely on
a detailed understanding of the abundance data including systematic effects
which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe
Sub-Saturn Planet Candidates to HD 16141 and HD 46375
Precision Doppler measurements from the Keck/HIRES spectrometer reveal
periodic Keplerian velocity variations in the stars HD 16141 and HD 46375. HD
16141 (G5 IV) has a period of 75.8 d and a velocity amplitude of 11 m/s,
yielding a companion having Msini = 0.22 Mjup and a semimajor axis, a = 0.35
AU. HD 46375 (K1 IV/V) has a period of 3.024 d and a velocity amplitude of 35
m/s, yielding a companion with Msini=0.25 Mjup, a semimajor axis of a = 0.041
AU, and an eccentricity of 0.04 (consistent with zero). These companions
contribute to the rising planet mass function toward lower masses.Comment: 4 Figure
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
Ten Low Mass Companions from the Keck Precision Velocity Survey
Ten new low mass companions have emerged from the Keck precision Doppler
velocity survey, with minimum (msini) masses ranging from 0.8 mjup to 0.34
msun. Five of these are planet candidates with msini < 12 mjup, two are brown
dwarf candidates with msini ~30 mjup, and three are low mass stellar
companions. Hipparcos astrometry reveals the orbital inclinations and masses
for three of the (more massive) companions, and it provides upper limits to the
masses for the rest. A new class of extrasolar planet is emerging,
characterized by nearly circular orbits and orbital radii greater than 1 AU.
The planet HD 4208b appears to be a member of this new class. The mass
distribution of extrasolar planets continues to exhibit a rapid rise from 10
mjup toward the lowest detectable masses near 1 msat.Comment: 26 pages, TeX, plus 13 postscript figure
Dynamical Stability and Habitability of Gamma Cephei Binary-Planetary System
It has been suggested that the long-lived residual radial velocity variations
observed in the precision radial velocity measurements of the primary of Gamma
Cephei (HR8974, HD222404, HIP116727) are likely due to a Jupiter-like planet
around this star (Hatzes et al, 2003). In this paper, the orbital dynamics of
this plant is studied and also the possibility of the existence of a
hypothetical Earth-like planet in the habitable zone of its central star is
discussed. Simulations, which have been carried out for different values of the
eccentricity and semimajor axis of the binary, as well as the orbital
inclination of its Jupiter-like planet, expand on previous studies of this
system and indicate that, for the values of the binary eccentricity smaller
than 0.5, and for all values of the orbital inclination of the Jupiter-like
planet ranging from 0 to 40 degrees, the orbit of this planet is stable. For
larger values of the binary eccentricity, the system becomes gradually
unstable. Integrations also indicate that, within this range of orbital
parameters, a hypothetical Earth-like planet can have a long-term stable orbit
only at distances of 0.3 to 0.8 AU from the primary star. The habitable zone of
the primary, at a range of approximately 3.1 to 3.8 AU, is, however, unstable.Comment: 25 pages, 7 figures, 3 tables, submitted for publicatio
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