1,478,440 research outputs found
Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
We present Very Long Baseline Array phase-referenced measurements of the
parallax and proper motion of two pulsars, B0919+06 and B1857-26.
Sub-milliarcsecond positional accuracy was obtained by simultaneously observing
a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz.
We discuss the merits of using weak close calibrator sources for VLBI
observations at low frequencies, and outline a method of observation and data
reduction for these type of measurements. For the pulsar B1919+06 we measure a
parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is
approximately 0.5 mas, an order of magnitude improvement over most previous
determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa
The Metallicity of the Red Giant Branch in the Disk of NGC 6822
Deep J, H, and K' images obtained with the Canada-France-Hawaii Telescope
adaptive optics system are used to investigate the metallicity of red giant
branch (RGB) stars in three fields in the disk of the Local Group dwarf
irregular galaxy NGC 6822. The slope of the RGB on the (K, J-K) color-magnitude
diagrams indicates that = -1.0 +/- 0.3. The locus of the RGB is bluer
than that of globular clusters with the same RGB slope, by an amount that is
consistent with the majority of RGB stars in these fields having an age near 3
Gyr. It is demonstrated that if RGB stars in NGC 6822 are this young then the
metallicity computed from the RGB slope may be ~ 0.05 dex too low.Comment: 19 pages of text; 10 figures; to appear in the PAS
Fluid injection device for high-pressure systems
Screw activated device, consisting of a compressor, shielded replaceable ampules, a multiple-element rubber gland, and a specially constructed fluid line fitting, injects measured amounts of fluids into a pressurized system. It is sturdy and easily manipulated
Using a spatial overlap approach to estimate the risk of collisions between deep diving seabirds and tidal stream turbines : a review of potential methods and approaches
Peer reviewedPublisher PD
Spitzer IRAC Observations of White Dwarfs. I. Warm Dust at Metal-Rich Degenerates
This paper presents the results of a Spitzer IRAC 3-8 micron photometric
search for warm dust orbiting 17 nearby, metal-rich white dwarfs, 15 of which
apparently have hydrogen dominated atmospheres (type DAZ). G166-58, G29-38, and
GD 362 manifest excess emission in their IRAC fluxes and the latter two are
known to harbor dust grains warm enough to radiate detectable emission at
near-infrared wavelengths as short as 2 micron. Their IRAC fluxes display
differences compatible with a relatively larger amount of cooler dust at GD
362. G166-58 is presently unique in that it appears to exhibit excess flux only
at wavelengths longer than about 5 micron. Evidence is presented that this
mid-infrared emission is most likely associated with the white dwarf,
indicating that G166-58 bears circumstellar dust no warmer than T~400 K. The
remaining 14 targets reveal no reliable mid-infrared excess, indicating the
majority of DAZ stars do not have warm debris disks sufficiently opaque to be
detected by IRAC.Comment: Accepted to ApJ, 10 figures, 6 table
The euclidean propagator in a model with two non-equivalent instantons
We consider in detail how the quantum-mechanical tunneling phenomenon occurs
in a well-behaved octic potential. Our main tool will be the euclidean
propagator just evaluated between two minima of the potential at issue. For
such a purpose we resort to the standard semiclassical approximation which
takes into account the fluctuations over the instantons, i.e. the finite-action
solutions of the euclidean equation of motion. As regards the one-instanton
approach, the functional determinant associated with the so-called stability
equation is analyzed in terms of the asymptotic behaviour of the zero-mode. The
conventional ratio of determinants takes as reference the harmonic oscillator
whose frequency is the average of the two different frequencies derived from
the minima of the potential involved in the computation. The second instanton
of the model is studied in a similar way. The physical effects of the
multi-instanton configurations are included in this context by means of the
alternate dilute-gas approximation where the two instantons participate to
provide us with the final expression of the propagator.Comment: RevTex, 13 page
Spectroscopy of Stellar-Like Objects Contained in the Second Byurakan Survey. I
The results of spectroscopic observations of 363 star-like objects from the
Second Byurakan Survey (SBS) are reported. This SBS's subsample has proven to
be a rich source of newly identified quasars, Seyfert type galaxies, degenerate
stars and hot subdwarfs. In the subsample here studied, we identified 35 new
QSOs, 142 White Dwarfs (WDs) the majority of which, 114 are of DA type, 55
subdwarfs (29 of which are sdB-type stars), 10 HBB, 16 NHB, 54 G-type and 25
F-type stars, two objects with composite spectra, four Cataclismic Variables
(CV), two peculiar emission line stars, 17 objects with continuous spectra, as
well as one planetary nebula. Among the 35 QSOs we have found two Broad
Absorption Line (BAL) QSOs, namely SBS 1423+500 and SBS 1435+500A. Magnitudes,
redshifts, and slit spectra for all QSOs, also some typical spectra of the
peculiar stars are presented. We estimate the minimum surface density of bright
QSOs in redshift range 0.3<z<2.2 to be 0.05 per sq. deg. for B<17.0 and 0.10
per sq. deg. for B<17.5.Comment: 22 pages, 3 tables, 4 figures, PASP in pres
Shock wave focusing using geometrical shock dynamics
A finite-difference numerical method for geometrical shock dynamics has been developed based on the analogy between the nonlinear ray equations and the supersonic potential equation. The method has proven to be an efficient and inexpensive tool for approximately analyzing the focusing of weak shock waves, where complex nonlinear wave interactions occur over a large range of physical scales. The numerical results exhibit the qualitative behavior of strong, moderate, and weak shock focusing observed experimentally. The physical mechanisms that are influenced by aperture angle and shock strength are properly represented by geometrical shock dynamics. Comparison with experimental measurements of the location at which maximum shock pressure occurs shows good agreement, but the maximum pressure at focus is overestimated by about 60%. This error, though large, is acceptable when the speed and low cost of the method is taken into consideration. The error is primarily due to the under prediction of disturbance speed on weak shock fronts. Adequate resolution of the focal region proves to be particularly important to properly judge the validity of shock dynamics theory, under-resolution leading to overly optimistic conclusions
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