623,784 research outputs found
Angular power spectrum analysis on current and future high-energy neutrino data
Astrophysical neutrino events have been measured in the last couple of years,
which show an isotropic distribution, and the current discussion is their
astrophysical origin. We use both isotropic and anisotropic components of the
diffuse neutrino data to constrain the contribution of a broad number of
extra-galactic source populations to the observed neutrino sky. We simulate
up-going muon neutrino events by applying statistical distributions for the
flux of extragalactic sources, and by Monte Carlo method we exploit the
simulation for current and future IceCube, IceCube-Gen2 and KM3NeT exposures.
We aim at constraining source populations by studying their angular patterns,
for which we assess the angular power spectrum. We leave the characteristic
number of sources () as a free parameter, which is roughly the
number of neutrino sources over which the measured intensity is divided. With
existing two-year IceCube data, we can already constrain very rare, bright
sources with 100. This can be improved to - with IceCube-Gen2 and KM3NeT with ten-year exposure, constraining
the contribution of BL Lacs (). On the other hand, we
can constrain weak sources with large number densities, like starburst galaxies
(), if we measure an anisotropic neutrino sky with future
observations.Comment: 19 pages, 19 figures. Accepted by JCA
The fine line between total and partial tidal disruption events
Flares from tidal disruption events are unique tracers of quiescent black
holes at the centre of galaxies. The appearance of these flares is very
sensitive to whether the star is totally or partially disrupted, and in this
paper we seek to identify the critical distance of the star from the black hole
(r_d) that enables us to distinguish between these two outcomes. We perform
here Mesh-free Finite Mass, traditional, and modern Smoothed Particle
Hydrodynamical simulations of star-black hole close encounters, with the aim of
checking if the value of r_d depends on the simulation technique. We find that
the critical distance (or the so-called critical disruption parameter beta_d)
depends only weakly on the adopted simulation method, being beta_d=0.92\pm 0.02
for a gamma=5/3 polytrope and beta_d=2.01\pm 0.01 for a gamma=4/3 polytrope.Comment: 8 pages, 8 figures, 2 tables; accepted to A&
Hunting for millimeter flares from magnetic reconnection in pre-main sequence spectroscopic binaries
Recent observations of the low-mass pre-main sequence, eccentric
spectroscopic binaries DQ Tau and V773 Tau A reveal that their millimeter
spectrum is occasionally dominated by flares from non-thermal emission
processes. The transient activity is believed to be synchrotron in nature,
resulting from powerful magnetic reconnection events when the separate magnetic
structures of the binary components are capable of interacting and forced to
reorganize, typically near periastron. We conducted the first systematic study
of the millimeter variability toward a sample of 12 PMS spectroscopic binaries
with the aim to characterize the proliferation of flares amongst sources likely
to experience similar interbinary reconnection events. The source sample
consists of short-period, close-separation binaries that possess either a high
orbital eccentricity or a circular orbit. Using the MAMBO2 array on the IRAM
30m telescope, we carried out continuous monitoring at 1.25 mm over a 4-night
period during which all of the high-eccentricity binaries approached
periastron. We also obtained simultaneous optical VRI measurements, since a
strong link is often observed between stellar reconnection events and optical
brightenings. UZ Tau E is the only source to be detected at millimeter
wavelengths: it exhibited significant variation; it is also the only source to
undergo strong simultaneous optical variability. The binary possesses the
largest orbital eccentricity in the current sample, a predicted factor in
star-star magnetic interaction events. With orbital parameters and variable
accretion activity similar to DQ Tau, the millimeter behavior of UZ Tau E draws
many parallels to the DQ Tau model for colliding magnetospheres. However, on
the basis of our observations alone, we cannot determine whether the
variability is repetitive, or if it could also be due to variable free-free
emission in an ionized wind.Comment: 19 pages in referee format, 3 figures, 1 table, 3 on-line tables,
accepted for publication in Astronomy and Astrophysic
KOI-54: The Kepler Discovery of Tidally Excited Pulsations and Brightenings in a Highly Eccentric Binary
Kepler observations of the star HD 187091 (KIC 8112039, hereafter KOI-54) revealed a remarkable light curve exhibiting sharp periodic brightening events every 41.8 days with a superimposed set of oscillations forming a beating pattern in phase with the brightenings. Spectroscopic observations revealed that this is a binary star with a highly eccentric orbit, e = 0.83. We are able to match the Kepler light curve and radial velocities with a nearly face-on (i = 5 degrees.5) binary star model in which the brightening events are caused by tidal distortion and irradiation of nearly identical A stars during their close periastron passage. The two dominant oscillations in the light curve, responsible for the beating pattern, have frequencies that are the 91st and 90th harmonic of the orbital frequency. The power spectrum of the light curve, after removing the binary star brightening component, reveals a large number of pulsations, 30 of which have a signal-to-noise ratio greater than or similar to 7. Nearly all of these pulsations have frequencies that are either integer multiples of the orbital frequency or are tidally split multiples of the orbital frequency. This pattern of frequencies unambiguously establishes the pulsations as resonances between the dynamic tides at periastron and the free oscillation modes of one or both of the stars. KOI-54 is only the fourth star to show such a phenomenon and is by far the richest in terms of excited modes.NASA, Science Mission DirectorateNASA NNX08AR14GEuropean Research Council under the European Community 227224W.M. Keck FoundationMcDonald Observator
The Stellar Populations and Evolution of Lyman Break Galaxies
Using deep near-IR and optical observations of the HDF-N from the HST NICMOS
and WFPC2 and from the ground, we examine the spectral energy distributions
(SEDs) of Lyman break galaxies (LBGs) at 2.0 < z < 3.5. The UV-to-optical
rest-frame SEDs of the galaxies are much bluer than those of present-day spiral
and elliptical galaxies, and are generally similar to those of local starburst
galaxies with modest amounts of reddening. We use stellar population synthesis
models to study the properties of the stars that dominate the light from LBGs.
Under the assumption that the star-formation rate is continuous or decreasing
with time, the best-fitting models provide a lower bound on the LBG mass
estimates. LBGs with ``L*'' UV luminosities are estimated to have minimum
stellar masses ~ 10^10 solar masses, or roughly 1/10th that of a present-day L*
galaxy. By considering the effects of a second component of maximally-old
stars, we set an upper bound on the stellar masses that is ~ 3-8 times the
minimum estimate. We find only loose constraints on the individual galaxy ages,
extinction, metallicities, initial mass functions, and prior star-formation
histories. We find no galaxies whose SEDs are consistent with young (< 10^8
yr), dust-free objects, which suggests that LBGs are not dominated by ``first
generation'' stars, and that such objects are rare at these redshifts. We also
find that the typical ages for the observed star-formation events are
significantly younger than the time interval covered by this redshift range (~
1.5 Gyr). From this, and from the relative absence of candidates for quiescent,
non-star-forming galaxies at these redshifts in the NICMOS data, we suggest
that star formation in LBGs may be recurrent, with short duty cycles and a
timescale between star-formation events of < 1 Gyr. [Abridged]Comment: LaTeX, 37 pages, 21 figures. Accepted for publication in the
Astrophysical Journa
Hot Jupiters and stellar magnetic activity
Recent observations suggest that stellar magnetic activity may be influenced
by the presence of a close-by giant planet. Specifically, chromospheric hot
spots rotating in phase with the planet orbital motion have been observed
during some seasons in a few stars harbouring hot Jupiters. The spot leads the
subplanetary point by a typical amount of about 60-70 degrees, with the extreme
case of upsilon And where the angle is about 170 degrees. The interaction
between the star and the planet is described considering the reconnection
between the stellar coronal field and the magnetic field of the planet.
Reconnection events produce energetic particles that moving along magnetic
field lines impact onto the stellar chromosphere giving rise to a localized hot
spot. A simple magnetohydrostatic model is introduced to describe the coronal
magnetic field of the star connecting its surface to the orbiting planet. The
field is assumed to be axisymmetric around the rotation axis of the star and
its configuration is more general than a linear force-free field. With a
suitable choice of the free parameters, the model can explain the phase
differences between the hot spots and the planets observed in HD 179949,
upsilon And, HD 189733, and tau Bootis, as well as their visibility modulation
on the orbital period and seasonal time scales. The possible presence of cool
spots associated with the planets in tau Boo and HD 192263 cannot be explained
by the present model. However, we speculate about the possibility that
reconnection events in the corona may influence subphotospheric dynamo action
in those stars producing localized photospheric (and chromospheric) activity
migrating in phase with their planets.Comment: 9 pages, 5 figures, 2 tables, 2 appendixes, accepted by Astronomy &
Astrophysic
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