2,674 research outputs found
Circulating Subbeam Systems and the Physics of Pulsar Emission
The purpose of this paper is to suggest how detailed single-pulse
observations of ``slow'' radio pulsars may be utilized to construct an
empirical model for their emission. It links the observational synthesis
developed in a series of papers by Rankin starting in the 1980s to the more
recent empirical feedback model of Wright (2003a) by regarding the entire
pulsar magnetosphere as a non-steady, non-linear interactive system with a
natural built-in delay. It is argued that the enhanced role of the outer gap in
such a system indicates an evolutionary link to younger pulsars, in which this
region is thought to be highly active, and that pulsar magnetospheres should no
longer be seen as being ``driven'' by events on the neutron star's polar cap,
but as having more in common with planetary magnetospheres and auroral
phenomena.Comment: 15 pages, 3 figure
The `Periodic Nulls' of Radio Pulsar J1819+1305
We present a single-pulse study of the four-component pulsar J1819+1305,
whose ``null'' pulses bunch at periodic intervals of around 57 times the
rotation period. The emission bursts between the null bunches exhibit
characteristic modulations at two shorter periodicities of approximately 6.2
and 3 times the rotation period, the former found largely in the two outer
components, and the latter only in the first component. Many bursts commence
with bright emission in second component, exhibit positive six-period drift
across the full profile width, and end with 3-period modulation in the leading
component. The 57-period cycle can be modelled geometrically as a sparsely
filled subbeam carousel with nulls appearing whenever our line of sight
intersects a circulating empty region. This interpretation is compatible with
other recent evidence for periodic, carousel-related nulling and appears to
support the physics of a polar-gap emission model for ``drifting'' subpulses,
but the subtle structure of the emission bursts defies an easy explanation.Comment: 8 pages, 9 figure
Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow
We report the discovery of variability in the linear polarization from the
Galactic Center black hole source, Sagittarius A*. New polarimetry obtained
with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm
shows a position angle that differs by 28 +/- 5 degrees from observations 6
months prior and then remains stable for 15 months. This difference may be due
to a change in the source emission region on a scale of 10 Schwarzschild radii
or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a
change in the source physics unlikely, however, since we see no corresponding
change in the total intensity or polarized intensity fraction. On the other
hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could
readily account for the magnitude and time scale of the position angle change.Comment: accepted for publication in ApJ
Fourteen New Companions from the Keck & Lick Radial Velocity Survey Including Five Brown Dwarf Candidates
We present radial velocities for 14 stars on the California & Carnegie Planet
Search target list that reveal new companions. One star, HD 167665, was fit
with a definitive Keplerian orbit leading to a minimum mass for the companion
of 50.3 Mjup at a separation from its host of ~5.5 AU. Incomplete or limited
phase coverage for the remaining 13 stars prevents us from assigning to them
unique orbital parameters. Instead, we fit their radial velocities with
Keplerian orbits across a grid of fixed values for Msini and period, P, and use
the resulting reduced chi-square surface to place constraints on Msini, P, and
semimajor axis, a. This technique allowed us to restrict Msini below the brown
dwarf -- stellar mass boundary for an additional 4 companions (HD 150554, HD
8765, HD 72780, HD 74014). If the combined 5 companions are confirmed as brown
dwarfs, these results would comprise the first major catch of such objects from
our survey beyond ~3 AU.Comment: 29 pages, 14 figures, accepted to Ap
Drifting, moding, and nulling: another look at pulsar B1918+19
Arecibo observations of the conal triple pulsar B1918+19 at 0.327- and
1.4-GHz are used to analyse its subpulse behaviour in detail. We confirm the
presence of three distinct drift modes (A,B,C) plus a disordered mode (N) and
show that they follow one another in specific cycles. Interpreting the pulsar's
profile as resulting from a sightline traverse which cuts across an outer cone
and tangentially grazes an inner cone, we demonstrate that the phase modulation
of the inner cone is locked to the amplitude modulation of the outer cone in
all the drift modes. The 9% nulls are found to be largely confined to the
dominant B and N modes, and, in the N mode, create alternating bunches of nulls
and emission in a quasi-periodic manner with an averaged fluctuation rate of
about 12 rotation periods (). We explore the assumption that the apparent
drift is the first alias of a faster drift of subbeams equally spaced around
the cones. This is shown to imply that all modes A, B and C have a common
circulation time of 12 and differ only in the number of subbeams. This
timescale is on the same order as predicted by the classic {\bf E}{\bf
B} drift and also coincides with the N-mode modulation. We therefore arrive at
a picture where the circulation speed remains roughly invariant while the
subbeams progressively diminish in number from modes A to B to C, and are then
re-established during the N mode. We suggest that aliasing combined with
subbeam loss may be responsible for apparently dramatic changes in drift rates
in other pulsars
The TRENDS High-Contrast Imaging Survey. V. Discovery of an Old and Cold Benchmark T-dwarf Orbiting the Nearby G-star HD 19467
The nearby Sun-like star HD 19467 shows a subtle radial velocity (RV)
acceleration of -1.37+/-0.09 m/s/yr over an 16.9 year time baseline (an RV
trend), hinting at the existence of a distant orbiting companion. We have
obtained high-contrast adaptive optics images of the star using NIRC2 at Keck
Observatory and report the direct detection of the body that causes the
acceleration. The companion, HD 19467 B, is dK=12.57+/-0.09 mag fainter than
its parent star (contrast ratio of 9.4e-6), has blue colors J-K_s=-0.36+/-0.14
(J-H=-0.29+/-0.15), and is separated by 1.653+/-0.004" (51.1+/-1.0 AU).
Follow-up astrometric measurements obtained over an 1.1 year time baseline
demonstrate physical association through common parallactic and proper motion.
We calculate a firm lower-limit of m>51.9^{+3.6}_{-4.3}Mjup for the companion
mass from orbital dynamics using a combination of Doppler observations and
imaging. We estimate a model-dependent mass of m=56.7^{+4.6}_{-7.2}Mjup from a
gyrochronological age of 4.3^{+1.0}_{-1.2} Gyr. Isochronal analysis suggests a
much older age of Gyr, which corresponds to a mass of
m=67.4^{+0.9}_{-1.5}Mjup. HD 19467 B's measured colors and absolute magnitude
are consistent with a late T-dwarf [~T5-T7]. We may infer a low metallicity of
[Fe/H]=-0.15+/-0.04 for the companion from its G3V parent star. HD 19467 B is
the first directly imaged benchmark T-dwarf found orbiting a Sun-like star with
a measured RV acceleration.Comment: Updated to reflect ApJ versio
Synthesis of (+)-Bovidic Acid
(+)-Bovidic acid is an 18-carbon hydroxyfuranoid acid isolated from the pelage of the gaur (Bosfrontalis) and displays potential as an insect repellant. Both a formal and total synthesis of (+)-bovidic acid was carried out with the core of the natural product being synthesized through Mukaiyama aerobic oxidative cyclization employing the second-generation Co(nmp)2 catalyst
Two Exoplanets Discovered at Keck Observatory
We present two exoplanets detected at Keck Observatory. HD 179079 is a G5
subgiant that hosts a hot Neptune planet with Msini = 27.5 M_earth in a 14.48
d, low-eccentricity orbit. The stellar reflex velocity induced by this planet
has a semiamplitude of K = 6.6 m/s. HD 73534 is a G5 subgiant with a
Jupiter-like planet of Msini = 1.1 M_jup and K = 16 m/s in a nearly circular
4.85 yr orbit. Both stars are chromospherically inactive and metal-rich. We
discuss a known, classical bias in measuring eccentricities for orbits with
velocity semiamplitudes, K, comparable to the radial velocity uncertainties.
For exoplanets with periods longer than 10 days, the observed exoplanet
eccentricity distribution is nearly flat for large amplitude systems (K > 80
m/s), but rises linearly toward low eccentricity for lower amplitude systems (K
> 20 m/s).Comment: 8 figures, 6 tables, accepted, Ap
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