485 research outputs found
Baby Boomers vs Their Parents: Economic Well-being and Health Status
In this chapter, we use the Social Security Administration’s simulation model known as MINT (Modeling Income in the Near Term) to examine the projected health and economic status of Baby Boomers and their parents during retirement. Our projections indicate that boomers will enjoy higher levels of economic well-being and health than their parents, yet the distribution of income and wealth is more unequal among Boomers. For example, the ratio of income to poverty-level income grows three times faster at the 90th percentile than at the 10th percentile. Health problems are concentrated among persons of lower economic status in both generations, but the degree of concentration does not increase across generations
Polarimetric Properties of the Crab Pulsar between 1.4 and 8.4 GHz
New polarimetric observations of the Crab pulsar at frequencies between 1.4
and 8.4 GHz are presented. Additional pulse components discovered in earlier
observations (Moffett & Hankins 1996, astro-ph/9604163) are found to have high
levels of linear polarization, even at 8.4 GHz. No abrupt sweeps in position
angle are found within pulse components; however, the position angle and
rotational phase of the interpulse do change dramatically between 1.4 and 4.9
GHz. The multi-frequency profile morphology and polarization properties
indicate a non-standard origin of the emission. Several emission geometries are
discussed, but the one favored locates sites of emission both near the pulsar
surface and in the outer magnetosphere.Comment: 20 pages, 7 postscript figures, uses aaspp4 Latex style. To appear in
Volume 522 of The Astrophysical Journa
New Prospects for Research on Manipulation of Insect Vectors by Pathogens
International audienc
Double and single recycled pulsars: an evolutionary puzzle?
We investigate the statistics of isolated recycled pulsars and double neutron
star binaries in the Galactic disk. Since recycled pulsars are believed to form
through accretion and spinup in close binaries, the isolated objects presumably
originate from disrupted progenitors of double neutron stars. There are a
comparable number of double neutron star systems compared to isolated recycled
pulsars. We find that standard evolutionary models cannot explain this fact,
predicting several times the number of isolated recycled pulsars than those in
double neutron star systems. We demonstrate, through population synthesis
calculations, that the velocity distribution of isolated recycled pulsars is
broader than for binary systems. When this is accounted for in a model for
radio pulsar survey selection effects, which include the effects of Doppler
smearing for the double neutron star binaries, we find that there is a small
(25%) bias towards the detection of double neutron star systems. This bias,
however, is not significant enough to explain the observational discrepancy if
standard (sigma = 265 km/s) neutron star natal kick velocities are invoked in
binary population syntheses. Population syntheses in which the 1D Maxwellian
velocity dispersion of the natal kick is sigma=170 km/s are consistent with the
observations. These conclusions further support earlier findings the neutron
stars formed in close interacting binaries receive significantly smaller natal
kicks than the velocities of Galactic single pulsars would seem to indicate.Comment: 12 pages, MNRAS (accepted
Variable Radio Sources in the Galactic Plane
Using three epochs of VLA observations of the Galactic Plane in the first
quadrant taken ~15 years apart, we have conducted a search for a population of
variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm.
We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting
for various selection effects and for the extragalactic variable population of
active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg
which vary by more than 50% on a time scale of years (or shorter). We show that
these sources are much more highly variable than extragalactic objects; more
than 50% show variability by a factor >2 compared to <10% for extragalactic
objects in the same flux density range. We also show that the fraction of
variable sources increases toward the Galactic center (another indication that
this is a Galactic population), and that the spectral indices of many of these
sources are flat or inverted. A small number of the variables are coincident
with mid-IR sources and two are coincident with X-ray emitters, but most have
no known counterparts at other wavelengths. Intriguingly, one lies at the
center of a supernova remnant, while another appears to be a very compact
planetary nebula; several are likely to represent activity associated with star
formation regions. We discuss the possible source classes which could
contribute to the variable cohort and followup observations which could clarify
the nature of these sources.Comment: 11 pages, 7 figures; to be published in the Astronomical Journal;
data available on MAGPIS website at http://third.ucllnl.org/gps
X-ray Evolution of SNR 1987A: The Radial Expansion
We present the evolution of the radial expansion of SNR 1987A as measured
using Chandra X-ray observations taken over the last 10 years. To characterize
the complex structure of the remnant and isolate the expansion measurement, we
fit the images to several empirical models including: a simple circular torus,
a torus with bilateral lobes, and a torus with four tangentially extended
lobes. We discuss the results of this measure in the context of the overall
evolution of the supernova remnant, for which we believe we have measured the
end of the free expansion phase and its transition to the adiabatic phase (at
least along the equatorial ring). The timing of this event is in agreement with
early predictions of the remnant evolution.Comment: Accepted for publication in ApJ, 21 pages, 4 figures, 3 table
Is it Round? Spectropolarimetry of the Type II-P Supernova 1999em
We present the first multi-epoch spectropolarimetry of a type II plateau
supernova (SN II-P), with optical observations of SN 1999em on days 7, 40, 49,
159, and 163 after discovery. These data are used to probe the geometry of the
electron-scattering atmosphere before, during, and after the plateau phase,
which ended roughly 90 days after discovery. Weak continuum polarization with
an unchanging polarization angle (theta ~ 160 deg) is detected at all epochs,
with p ~ 0.2% on day 7, p ~ 0.3% on days 40 and 49, and p ~ 0.5% in the final
observations. Distinct polarization modulations across strong line features are
present on days 40, 49, 159, and 163. Uncorrected for interstellar polarization
(which is believed to be quite small), polarization peaks are associated with
strong P Cygni absorption troughs and nearly complete depolarization is seen
across the H-alpha emission profile. The temporal evolution of the continuum
polarization and sharp changes across lines indicate polarization intrinsic to
SN 1999em. When modeled in terms of the oblate, electron-scattering atmospheres
of Hoeflich, the observed polarization implies anasphericity of at least 7%
during the period studied. The temporal polarization increase may indicate
greater asphericity deeper into the ejecta. We discuss the implications of
asphericity on the use of type II-P supernovae as primary extragalactic
distance indicators through the expanding photosphere method (EPM). If
asphericity produces directionally dependant flux and peculiar galaxy motions
are characterized by sigma_v_rec = 300 km/s, it is shown that the agreement
between previous EPM measurements of SNe II and distances to the host galaxies
predicted by a linear Hubble law restrict mean SN II asphericity to values less
than 30% (3-sigma) during the photospheric phase.Comment: 65 pages (29 Figures, 4 Tables), Accepted for publication in the June
1, 2001 edition of ApJ. Revised statistical analysis of scatter in Hubble
diagram of previous EPM distances and the implications for mean SN II
asphericit
Evolution of the Chandra CCD Spectra of SNR 1987A: Probing the Reflected-Shock Picture
We continue to explore the validity of the reflected shock structure (RSS)
picture in SNR 1987A that was proposed in our previous analyses of the X-ray
emission from this object. We used an improved version of our RSS model in a
global analysis of 14 CCD spectra from the monitoring program with Chandra. In
the framework of the RSS picture, we are able to match both the expansion
velocity curve deduced from the analysis of the X-ray images and light curve.
Using a simplified analysis, we also show that the X-rays and the non-thermal
radio emission may originate from the same shock structure (the blast wave). We
believe that using the RSS model in the analysis of grating data from the
Chandra monitoring program of SNR 1987A that cover a long enough time interval,
will allow us to build a more realistic physical picture and model of SNR
1987A.Comment: 14 pages, 1 Table, 8 figures, accepted for publication in MNRA
Modeling FETCH Observations of 2005 May 13 CME
This paper evaluates the quality of CME analysis that has been undertaken
with the rare Faraday rotation observation of an eruption. Exploring the
capability of the FETCH instrument hosted on the MOST mission, a four-satellite
Faraday rotation radio sounding instrument deployed between the Earth and the
Sun, we discuss the opportunities and challenges to improving the current
analysis approaches.Comment: 33 pages, 24 figure
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