198 research outputs found
Evolutionary and Physical Properties of Meteoroids
Astrophysical models for meteoroid formation and stellar and planetary evolutions are developed from simulation composition studies
Image-orthicon spectra of Geminids in 1969
The spectra of 25 meteors, recorded with an image orthicon technique in December 1969, are studied in relation to similar records made in August of the same year. Of 19 Geminid meteors in the absolute visual magnitude range 0 to +2, only one showed any evidence of the forbidden line of oxygen at 5577 A, while all Perseid meteors recorded in August exhibited the oxygen line, a result of the large difference in geocentric velocity between the two showers. Atoms identified in faint Geminid meteors include neutral iron, magnesium, calcium and sodium. The molecular bands of nitrogen are also observed
An unusual meteor spectrum
An extraordinary spectrum of a meteor at a velocity of about 18.5 + or - 1.0 km/s was observed with an image orthicon camera. The radiant of the meteor was at an altitude of about 49 deg. It was first seen showing a yellow red continuous spectrum alone at a height of 137 + or - 8 km which is ascribed to the first positive group of nitrogen bands. After the meteor had descended to 116 + or - 6 km above sea level it brightened rapidly from its previous threshold brightness into a uniform continuum, the D-line of neutral sodium appeared, and at height 105 + or - 5 km all the other lines of the spectrum also appeared. The continuum remained dominant to the end. Water of hydration and entrained carbon flakes of characteristic dimension about 0.2 micron or less are proposed as constituents of the meteoroid to explain these phenomena
Bostonia. Volume 16
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
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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
Atmospheric Density Uncertainty Quantification for Satellite Conjunction Assessment
Conjunction assessment requires knowledge of the uncertainty in the predicted
orbit. Errors in the atmospheric density are a major source of error in the
prediction of low Earth orbits. Therefore, accurate estimation of the density
and quantification of the uncertainty in the density is required. Most
atmospheric density models, however, do not provide an estimate of the
uncertainty in the density. In this work, we present a new approach to quantify
uncertainties in the density and to include these for calculating the
probability of collision Pc. For this, we employ a recently developed dynamic
reduced-order density model that enables efficient prediction of the
thermospheric density. First, the model is used to obtain accurate estimates of
the density and of the uncertainty in the estimates. Second, the density
uncertainties are propagated forward simultaneously with orbit propagation to
include the density uncertainties for Pc calculation. For this, we account for
the effect of cross-correlation in position uncertainties due to density errors
on the Pc. Finally, the effect of density uncertainties and cross-correlation
on the Pc is assessed. The presented approach provides the distinctive
capability to quantify the uncertainty in atmospheric density and to include
this uncertainty for conjunction assessment while taking into account the
dependence of the density errors on location and time. In addition, the results
show that it is important to consider the effect of cross-correlation on the
Pc, because ignoring this effect can result in severe underestimation of the
collision probability.Comment: 15 pages, 6 figures, 5 table
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
Nearly Optimal Robust Secret Sharing against Rushing Adversaries
Robust secret sharing is a strengthening of standard secret sharing that allows the shared secret to be recovered even if some of the shares being used in the reconstruction have been adversarially modified. In this work, we study the setting where out of all the shares, the adversary is allowed to adaptively corrupt and modify shares, where . Further, we deal with \textit{rushing} adversaries, meaning that the adversary is allowed to see the honest parties\u27 shares before modifying its own shares.
It is known that when , to share a secret of length bits and recover it with error less than , shares of size at least bits are needed. Recently, Bishop, Pastro, Rajaraman, and Wichs (EUROCRYPT 2016) constructed a robust secret sharing scheme with shares of size bits that is secure in this setting against non-rushing adversaries. Later, Fehr and Yuan (EUROCRYPT 2019) constructed a scheme that is secure against rushing adversaries, but has shares of size bits for an arbitrary constant . They also showed a variant of their construction with share size bits, but with super-polynomial reconstruction time.
We present a robust secret sharing scheme that is secure against rushing adversaries, has shares of size bits, and has polynomial-time sharing and reconstruction. Central to our construction is a polynomial-time algorithm for a problem on semi-random graphs that arises naturally in the paradigm of local authentication of shares used by us and in the aforementioned work
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