231 research outputs found
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
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Faraday Rotation Measure Gradients from a Helical Magnetic Field in 3C 273
Using high frequency (12-22 GHz) VLBA observations we confirm the existence
of a Faraday rotation measure gradient of ~ 500 rad/m^2/mas transverse to the
jet axis in the quasar 3C 273. The gradient is seen in two epochs spaced
roughly six months apart. This stable transverse rotation measure gradient is
expected if a helical magnetic field wraps around the jet. The overall order to
the magnetic field in the inner projected 40 parsecs is consistent with a
helical field. However, we find an unexpected increase in fractional
polarization along the edges of the source, contrary to expectations. This high
fractional polarization rules out internal Faraday rotation, but is not readily
explained by a helical field. After correcting for the rotation measure, the
intrinsic magnetic field direction in the jet of 3C 273 changes from parallel
to nearly perpendicular to the projected jet motion at two locations. If a
helical magnetic field causes the observed rotation measure gradient then the
synchrotron emitting electrons must be separate from the helical field region.
The presence or absence of transverse rotation measure gradients in other
sources is also discussed.Comment: Accepted to ApJ Letters; 11 pages, 4 figures (1 color figure
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
Review Article: Rethinking Holder
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68947/2/10.1177_096394709800700104.pd
2mm Wavelength VLBI of SiO Masers and AGN
In April 2002 an array of antennas operating at 129GHz successfully detected
VLBI fringes on both continuum AGN and SiO maser sources. The 129GHz fringes on
maser sources represent the highest frequency spectral line VLBI detections to
date. The AGN 3C279 was detected on long baselines at both 129GHz (and at
147GHz, see Krichbaum et al in these proceedings) yielding fringe spacings of
50-56 micro arc seconds, an angular resolution record. The array consisted of
the University of Arizona Kittpeak 12m antenna, the Heinrich Hertz 10m
Telescope (HHT), and the IRAM 30m dish on Pico Veleta.
At 129GHz, a number of evolved stars and several young stellar objects
exhibit strong SiO maser emission in the v=1 J=3-2 transition. Preliminary
cross power spectra of SiO masers around the red hypergiant VYCMa on the
HHT-KittPeak baseline ~190km are consistent with multiple spatially separate
maser spots associated with the star. Future observations will include
continuum observations of the radio source at the Galactic Center, SgrA*, and
higher frequency maser lines including HCN and methanol.Comment: 4 pages, 3 figures, appears in: Proceedings of the 6th European VLBI
Network Symposium held on June 25th-28th in Bonn, Germany. Edited by: E. Ros,
R.W. Porcas, A.P. Lobanov, and J.A. Zensus. New version includes all editors
in paper heade
Rotational velocities of nearby young stars
Stellar rotation is a crucial parameter driving stellar magnetism, activity
and mixing of chemical elements. Furthermore, the evolution of stellar rotation
is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are
believed to be responsible for rotational deceleration during the accretion
phase, and rotational spin-up during the contraction phase after decoupling
from the disk for fast rotators arriving at the ZAMS. We investigate the
projected rotational velocities vsini of a sample of young stars with respect
to the stellar mass and disk evolutionary state to search for possible
indications of disk-braking mechanisms. We analyse the stellar spectra of 220
nearby (mostly <100pc) young (2-600 Myr) stars for their vsini, stellar age,
Halpha emission, and accretion rates. The stars have been observed with FEROS
and HARPS in La Silla, Chile. The spectra have been cross-correlated with
appropriate theoretical templates. We build a new calibration to be able to
derive vsini values from the cross-correlated spectra. Stellar ages are
estimated from the LiI equivalent width at 6708 Ang. The equivalent width and
width at 10% height of the Halpha emission are measured to identify accretors
and used to estimate accretion rates. The vsini is then analysed with respect
to the evolutionary state of the circumstellar disks to search for indications
of disk-braking mechanisms in accretors. We find that the broad vsini
distribution of our targets extends to rotation velocities of up to more than
100 km/s and peaks at a value of 7.8+-1.2 km/s, and that ~70% of our stars show
vsini<30 km/s. Furthermore, we can find indications for disk-braking in
accretors and rotational spin-up of stars which are decoupled from their disks.
In addition, we show that a number of young stars are suitable for precise
radial-velocity measurements for planet-search surveys.Comment: 16 pages, 6 figures, accepted for publication in A&
Fermi Large Area Telescope Observations of Misaligned AGN
Analysis is presented on 15 months of data taken with the Large Area
Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar AGNs,
including 7 FRI radio galaxies and 4 FRII radio sources consisting of 2 FRII
radio galaxies and 2 steep spectrum radio quasars. The broad line FRI radio
galaxy 3C 120 is reported here as a gamma-ray source for the first time. The
analysis is based on directional associations of LAT sources with radio sources
in the 3CR, 3CRR and MS4 (collectively referred to as 3C-MS) catalogs. Seven of
the eleven LAT sources associated with 3C-MS radio sources have spectral
indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that
shows possible spectral curvature, are well described by a power law. No
evidence for time variability is found for any sources other than NGC 1275. The
gamma-ray luminosities of FRI radio galaxies are significantly smaller than
those of BL Lac objects detected by the LAT, whereas the gamma-ray luminosities
of FRII sources are quite similar to those of FSRQs, which could reflect
different beaming factors for the gamma-ray emission. A core dominance study of
the 3CRR sample indicate that sources closer to the jet axis are preferentially
detected with the Fermi-LAT, insofar as the gamma-ray--detected misaligned AGNs
have larger core dominance at a given average radio flux. The results are
discussed in view of the AGN unification scenario.Comment: 28 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
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