37,666 research outputs found
Peak statistics on COBE maps
We perform the stastistics of temperature maxima and minima in COBE-DMR
2-year maps. For power-law spectra the surface distribution of peaks implies an
amplitude consistent with more conventional analyses of COBE data (for
instance, we get K for a spectral index but not with the measured quadrupole K. This
provides further support for the existence an infrared cutoff in the cosmic
spectrum.Comment: Latex file, Astronomy & Astrophysics L-aa style. Hardcopy figures
available separately, send requests to [email protected]
Revisiting the luminosity function of single halo white dwarfs
White dwarfs are the fossils left by the evolution of low-and
intermediate-mass stars, and have very long evolutionary timescales. This
allows us to use them to explore the properties of old populations, like the
Galactic halo. We present a population synthesis study of the luminosity
function of halo white dwarfs, aimed at investigating which information can be
derived from the currently available observed data. We employ an up-to-date
population synthesis code based on Monte Carlo techniques, that incorporates
the most recent and reliable cooling sequences for metal poor progenitors as
well as an accurate modeling of the observational biases. We find that because
the observed sample of halo white dwarfs is restricted to the brightest stars
only the hot branch of the white dwarf luminosity function can be used for such
purposes, and that its shape function is almost insensitive to the most
relevant inputs, like the adopted cooling sequences, the initial mass function,
the density profile of the stellar spheroid, or the adopted fraction of
unresolved binaries. Moreover, since the cut-off of the observed luminosity has
not been yet determined only lower limits to the age of the halo population can
be placed. We conclude that the current observed sample of the halo white dwarf
population is still too small to obtain definite conclusions about the
properties of the stellar halo, and the recently computed white dwarf cooling
sequences which incorporate residual hydrogen burning should be assessed using
metal-poor globular clusters.Comment: 9 pages, 9 figures, accepted for publication in A&
Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields
We have employed complete micromagnetic simulations to analyze dc current
driven self-oscillations of a vortex core in a spin-valve nanopillar in a
perpendicular field by including the coupled effect of the spin-torque and the
magnetostatic field computed self-consistently for the entire spin-valve. The
vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that
expands with applied current, resulting in blue-shifting of the frequency,
while the magnetization of the thinner nanomagnet is non-uniform due to the
bias current. The simulations explain the experimental magnetoresistance-field
hysteresis loop and yield good agreement with the measured frequency vs.
current behavior of this spin-torque vortex oscillator.Comment: 10 pages, 3 figures, to be appear on AP
Radio continuum and X-ray emission from the most extreme FIR-excess galaxy NGC 1377: An extremely obscured AGN revealed
Galaxies which strongly deviate from the radio-far IR correlation are of
great importance for studies of galaxy evolution as they may be tracing early,
short-lived stages of starbursts and active galactic nuclei (AGNs). The most
extreme FIR-excess galaxy NGC1377 has long been interpreted as a young dusty
starburst, but millimeter observations of CO lines revealed a powerful
collimated molecular outflow which cannot be explained by star formation alone.
We present new radio observations at 1.5 and 10 GHz obtained with the Jansky
Very Large Array (JVLA) and Chandra X-ray observations towards NGC1377. The
observations are compared to synthetic starburst models to constrain the
properties of the central energy source. We obtained the first detection of the
cm radio continuum and X-ray emission in NGC1377. We find that the radio
emission is distributed in two components, one on the nucleus and another
offset by 4.5 to the South-West. We confirm the extreme FIR-excess of the
galaxy, with a 4.2, which deviates by more than
7- from the radio-FIR correlation. Soft X-ray emission is detected on
the off-nucleus component. From the radio emission we estimate for a young
( Myr) starburst a star formation rate SFR0.1 M yr. Such
a SFR is not sufficient to power the observed IR luminosity and to drive the CO
outflow. We find that a young starburst cannot reproduce all the observed
properties of the nucleus of NGC1377. We suggest that the galaxy may be
harboring a radio-quiet, obscured AGN of 10M, accreting at
near-Eddington rates. We speculate that the off-nucleus component may be
tracing an hot-spot in the AGN jet.Comment: 14 pages, accepted for publication on Astronomy and Astrophysics on
08/07/201
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