811 research outputs found
Search for TeV Gamma-Rays from Shell-Type Supernova Remnants
If cosmic rays with energies <100 TeV originate in the galaxy and are
accelerated in shock waves in shell-type supernova remnants (SNRs), gamma-rays
will be produced as the result of proton and electron interactions with the
local interstellar medium, and by inverse Compton emission from electrons
scattering soft photon fields. We report on observations of two supernova
remnants with the Whipple Observatory's 10 m gamma-ray telescope. No
significant detections have been made and upper limits on the >500 GeV flux are
reported. Non-thermal X-ray emission detected from one of these remnants
(Cassiopeia A) has been interpreted as synchrotron emission from electrons in
the ambient magnetic fields. Gamma-ray emission detected from the
Monoceros/Rosette Nebula region has been interpreted as evidence of cosmic-ray
acceleration. We interpret our results in the context of these observations.Comment: 4 pages, 2 figures, to appear in the proceedings of 26th
International Cosmic Ray Conference (Salt Lake City, 1999
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The Distance To The Hyades Cluster Based On Hubble Space Telescope Fine Guidance Sensor Parallaxes
Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m - M = 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.NASA HST GTO, HF-1042.01-93A, HF-1046.01-93A, NAS526555Astronom
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
A novel approach to fireball modeling: The observable and the calculated
Estimating the mass of a meteoroid passing through the Earth's atmosphere is essential to determining potential meteorite fall positions. High-resolution fireball images from dedicated camera networks provide the position and timing for fireball bright flight trajectories. There are two established mass determination methods: the photometric and the dynamic. A new approach is proposed, based on the dynamic method. A dynamic optimization initially constrains unknown meteoroid characteristics which are then used in a parametric model for an extended Kalman filter. The extended Kalman filter estimates the position, velocity, and mass of the meteoroid body throughout its flight, and quantitatively models uncertainties. Uncertainties have not previously been modeled so explicitly and are essential for determining fall distributions for potential meteorites. This two-step method aims to automate the process of mass determination for application to any trajectory data set and has been applied to observations of the Bunburra Rockhole fireball. The new method naturally handles noisy raw data. Initial and terminal bright flight mass results are consistent with other works based on the established photometric method and cosmic ray analysis. A full analysis of fragmentation and the variability in the heat-transfer coefficient will be explored in future versions of the model
THE SPIRAL WAVE INSTABILITY INDUCED BY A GIANT PLANET. I. PARTICLE STIRRING IN THE INNER REGIONS OF PROTOPLANETARY DISKS
We have recently shown that spiral density waves propagating in accretion
disks can undergo a parametric instability by resonantly coupling with and
transferring energy into pairs of inertial waves (or inertial-gravity waves
when buoyancy is important). In this paper, we perform inviscid
three-dimensional global hydrodynamic simulations to examine the growth and
consequence of this instability operating on the spiral waves driven by a
Jupiter-mass planet in a protoplanetary disk. We find that the spiral waves are
destabilized via the spiral wave instability (SWI), generating hydrodynamic
turbulence and sustained radially-alternating vertical flows that appear to be
associated with long wavelength inertial modes. In the interval , where denotes the semi-major axis of the planetary orbit
(assumed to be 5~au), the estimated vertical diffusion rate associated with the
turbulence is characterized by . For the disk model considered here, the diffusion rate is such that
particles with sizes up to several centimeters are vertically mixed within the
first pressure scale height. This suggests that the instability of spiral waves
launched by a giant planet can significantly disperse solid particles and trace
chemical species from the midplane. In planet formation models where the
continuous local production of chondrules/pebbles occurs over Myr time scales
to provide a feedstock for pebble accretion onto these bodies, this stirring of
solid particles may add a time constraint: planetary embryos and large
asteroids have to form before a gas giant forms in the outer disk, otherwise
the SWI will significantly decrease the chondrule/pebble accretion efficiency.Comment: Accepted for publication in the The Astrophysical Journal, 19 pages,
12 figures, 1 tabl
Astrometry with The \u3cem\u3eHubble Space Telescope\u3c/em\u3e: A Parallax of the Central Star of the Planetary Nebula NGC 6853
We present an absolute parallax and relative proper motion for the central star of the planetary nebula NGC 6853 (the Dumbbell). We obtain these with astrometric data from the Fine Guidance Sensor 3, a white-light interferometer on the Hubble Space Telescope. Spectral classifications and VRIJHKT2M and DDO51 photometry of the stars making up the astrometric reference frame provide spectrophotometric estimates of their absolute parallaxes. Introducing these into our model as observations with error, we find πabs = 2.10 ± 0.48 mas for the DAO central star of NGC 6853. A weighted average with a previous ground-based USNO determination yields πabs = 2.40 ± 0.32. We assume that the extinction suffered by the reference stars nearest (in angular separation and distance) to the central star is the same as for the central star. Correcting for color differences, we find AV = 0.30 ± 0.06 for the central star, hence, an absolute magnitude MV = 5.48. A recent determination of the central star effective temperature aided in estimating the central star radius, R* = 0.055 ± 0.02 R⊙, a star that may be descending to the white dwarf cooling track
Gravitational bending of light by planetary multipoles and its measurement with microarcsecond astronomical interferometers
General relativistic deflection of light by mass, dipole, and quadrupole
moments of gravitational field of a moving massive planet in the Solar system
is derived. All terms of order 1 microarcsecond are taken into account,
parametrized, and classified in accordance with their physical origin. We
calculate the instantaneous patterns of the light-ray deflections caused by the
monopole, the dipole and the quadrupole moments, and derive equations
describing apparent motion of the deflected position of the star in the sky
plane as the impact parameter of the light ray with respect to the planet
changes due to its orbital motion. The present paper gives the physical
interpretation of the observed light-ray deflections and discusses the
observational capabilities of the near-future optical (SIM) and radio (SKA)
interferometers for detecting the Doppler modulation of the radial deflection,
and the dipolar and quadrupolar light-ray bendings by the Jupiter and the
Saturn.Comment: 33 pages, 10 figures, accepted to Phys. Rev.
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