119 research outputs found
HST Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution
The optical expansion parallax of NGC 6543 has been detected and measured
using two epochs of HST images separated by a time baseline of only three
years. We have utilized three separate methods of deriving the angular
expansion of bright fiducials, the results of which are in excellent agreement.
We combine our angular expansion estimates with spectroscopically obtained
expansion velocities to derive a distance to NGC 6543 of 1001269 pc. The
deduced kinematic age of the inner bright core of the nebula is 1039259
years; however, the kinematic age of the polar caps that surround the core is
larger - perhaps the result of deceleration or earlier mass ejection. The
morphology and expansion patterns of NGC 6543 provide insight into a complex
history of axisymmetric, interacting stellar mass ejections.Comment: Accepted for publication in AJ. 18 pages. 6 figure
Confirmation of SBS 1150+599A As An Extremely Metal-Poor Planetary Nebula
SBS 1150+599A is a blue stellar object at high galactic latitude discovered
in the Second Byurakan Survey. New high-resolution images of SBS 1150+599A are
presented, demonstrating that it is very likely to be an old planetary nebula
in the galactic halo, as suggested by Tovmassian et al (2001). An H-alpha image
taken with the WIYN 3.5-m telescope and its "tip/tilt" module reveals the
diameter of the nebula to be 9.2", comparable to that estimated from spectra by
Tovmassian et al. Lower limits to the central star temperature were derived
using the Zanstra hydrogen and helium methods to determine that the star's
effective temperature must be > 68,000K and that the nebula is optically thin.
New spectra from the MMT and FLWO telescopes are presented, revealing the
presence of strong [Ne V] lambda 3425, indicating that the central star
temperature must be > 100,000K. With the revised diameter, new central star
temperature, and an improved central star luminosity, we can constrain
photoionization models for the nebula significantly better than before. Because
the emission-line data set is sparse, the models are still not conclusive.
Nevertheless, we confirm that this nebula is an extremely metal-poor planetary
nebula, having a value for O/H that is less than 1/100 solar, and possibly as
low as 1/500 solar.Comment: 19 pages, 6 figures. Accepted for publication in the Astronomical
Journa
Ergs: The Evolution of Shell Supernova Remnants
This paper reports on a workshop hosted by the University of Minnesota, March
23-26, 1997. It addressed fundamental dynamical issues associated with the
evolution of shell supernova remnants and the relationships between supernova
remnants and their environments. The workshop considered, in addition to
classical shell SNRs, dynamical issues involving X-ray filled composite
remnants and pulsar driven shells, such as that in the Crab Nebula.
Approximately 75 participants with wide ranging interests attended the
workshop. An even larger community helped through extensive on-line debates
prior to the meeting. Each of the several sessions, organized mostly around
chronological labels, also addressed some underlying, general physical themes:
How are SNR dynamics and structures modified by the character of the CSM and
the ISM and vice versa? How are magnetic fields generated in SNRs and how do
magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and
ions) produced in SNRs and how does their presence influence or reveal SNR
dynamics? How does SNR blast energy partition into various components over time
and what controls conversion between components? In lieu of a proceedings
volume, we present here a synopsis of the workshop in the form of brief
summaries of the workshop sessions. The sharpest impressions from the workshop
were the crucial and under-appreciated roles that environments have on SNR
appearance and dynamics and the critical need for broad-based studies to
understand these beautiful, but enigmatic objects. \\Comment: 54 pages text, no figures, Latex (aasms4.sty). submitted to the PAS
Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the `Knee' in the Primary Cosmic Ray Spectrum
The Monogem ring is a bright, diffuse, 25-degree-diameter supernova remnant
easily visible in soft X-ray images of the sky. Projected within the ring is a
young radio pulsar, PSR B0656+14. An association between the remnant and pulsar
has been considered, but was seemingly ruled out by the direction and magnitude
of the pulsar proper motion and by a distance estimate that placed the pulsar
twice as far from Earth as the remnant. Here we show that in fact the pulsar
was born very close to the center of the expanding remnant, both in distance
and projection. The inferred pulsar and remnant ages are in good agreement. The
conclusion that the pulsar and remnant were born in the same supernova
explosion is nearly inescapable. The remnant distance and age are in remarkable
concordance with the predictions of a model for the primary cosmic ray energy
spectrum in which the `knee' feature is produced by a single dominant source.Comment: 4 pages, to appear in the Astrophys. J. Lett. Full size color figure
can be found at http://www.thorsett.org/researc
Many-worlds interpretation of quantum theory and mesoscopic anthropic principle
We suggest to combine the Anthropic Principle with Many-Worlds Interpretation
of Quantum Theory. Realizing the multiplicity of worlds it provides an
opportunity of explanation of some important events which are assumed to be
extremely improbable. The Mesoscopic Anthropic Principle suggested here is
aimed to explain appearance of such events which are necessary for emergence of
Life and Mind. It is complementary to Cosmological Anthropic Principle
explaining the fine tuning of fundamental constants. We briefly discuss various
possible applications of Mesoscopic Anthropic Principle including the Solar
Eclipses and assembling of complex molecules. Besides, we address the problem
of Time's Arrow in the framework of Many-World Interpretation. We suggest the
recipe for disentangling of quantities defined by fundamental physical laws and
by an anthropic selection.Comment: 11 page
Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of z~6 Quasars
We study the process of cosmic reionization and estimate the ionizing
background in the IGM using the Lyman series absorption in the spectra of the
four quasars at 5.7<z<6.3 discovered by the SDSS. We derive the evolution of
the ionizing background at high redshifts, using both semi-analytic techniques
and cosmological simulations to model the density fluctuations in the IGM. The
existence of the complete Ly alpha Gunn-Peterson trough in the spectrum of the
z=6.28 quasar SDSS 1030+0524 indicates a photoionization rate Gamma_{-12} at
z~6 lower than 0.08, at least a factor of 6 smaller than the value at z~3. The
Ly beta and Ly gamma Gunn-Peterson troughs give an even stronger limit
Gamma_{-12}<0.02 due to their smaller oscillator strengths, indicating that the
ionizing background in the IGM at z~6 is more than 20 times lower than that at
z~3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from
10^{-5} at z~3 to >10^{-3} at z~6. At this redshift, the mass-averaged neutral
hydrogen fraction is larger than 1%; the mildly overdense regions (delta > 3)
are still mostly neutral and the comoving mean free path of ionizing photons is
shorter than 8 Mpc. Comparison with simulations of cosmological reionization
shows that the observed properties of the IGM at z~6 are typical of those in
the era at the end of the overlap stage of reionization when the individual HII
regions merge. Thus, z~6 marks the end of the reionization epoch. The redshift
of reionization constrains the small scale power of the mass density
fluctuations and the star forming efficiency of the first generation of
objects.Comment: AJ accepted, 27 pages; minor change
Hubble Space Telescope imaging of a peculiar stellar complex in NGC 6946
The stellar populations in a stellar complex in NGC 6946 are analyzed on
images taken with the Wide Field Planetary Camera 2 on board the Hubble Space
Telescope. The complex is peculiar by its very high density of stars and
clusters and semicircular shape. Its physical dimensions are about the same as
for the local Gould Belt, but the stellar density is 1 - 2 orders of magnitude
higher. In addition to an extremely luminous, 15 Myr old cluster discussed in
an earlier paper, accounting for about 17% of the integrated V-band light, we
identify 18 stellar clusters within the complex with luminosities similar to
the brightest open clusters in the Milky Way. The color-magnitude diagram of
individual stars in the complex shows a paucity of red supergiants compared to
model predictions in the 10-20 Myr age range for a uniform star formation rate.
We thus find tentative evidence for a gap in the dispersed star formation
history, with a concentration of star formation into a young globular cluster
during this gap. Confirmation of this result must, however, await a better
understanding of the late evolution of stars in the corresponding mass range (>
12 Msun). A reddening map based on individual reddenings for 373 early-type
stars is presented, showing significant variations in the absorption across the
complex. These may be responsible for some of the arc-like structures
previously identified on ground-based images. We finally discuss various
formation scenarios for the complex and the star clusters within it.Comment: 51 pages, including 19 figures and 4 tables. Accepted for publication
in Ap
Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar
We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99
and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the
Ly Alpha absorption in the spectra of these quasars evolves strongly with
redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an
extrapolation from lower redshifts. However, in the highest redshift object,
SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is
0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A
(5.95<z(abs)<6.16), consistent with zero flux. Thus the flux level drops by a
factor of >150, and is consistent with zero flux in the Ly Alpha forest region
immediately blueward of the Ly Alpha emission line, compared with a drop by a
factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the
decreased oscillator strength of this transition, this allows us to put a
considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha
absorption at z=6.
This is a clear detection of a complete Gunn-Peterson trough, caused by
neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen
fraction in the intergalactic medium would result in an undetectable flux in
the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson
trough by itself does not indicate that the quasar is observed prior to the
reionization epoch. However, the fast evolution of the mean absorption in these
high-redshift quasars suggests that the mean ionizing background along the line
of sight to this quasar has declined significantly from z~5 to 6, and the
universe is approaching the reionization epoch at z~6.Comment: Revised version (2001 Sep 4) accepted by the Astronomical Journal
(minor changes
Tight Cosmological Constraints from the Angular-Size/Redshift Relation for Ultra-Compact Radio Sources
Some years ago (Jackson and Dodgson 1997) analysis of the
angular-size/redshift relationship for ultra-compact radio sources indicted
that for spatially flat universes the best choice of cosmological parameters
was Omega_m=0.2 and Omega_Lambda=0.8. Here I present an astrophysical model of
these sources, based upon the idea that for those with redshift z>0.5 each
measured angular size corresponds to a single compact component which is moving
relativistically towards the observer; this model gives a reasonable account of
their behaviour as standard measuring rods. A new analysis of the original data
set (Gurvits 1994), taking into account possible selection effects which bias
against large objects, gives Omega_m=0.24+0.09/-0.07 for flat universes. The
data points match the corresponding theoretical curve very accurately out to
z~3, and there is clear and sustained indication of the switch from
acceleration to deceleration, which occurs at z=0.85.Comment: 21 pages, 7 figures, published versio
A High-Resolution Study of Nonthermal Radio and X-Ray Emission from SNR G347.3-0.5
G347.3-0.5 is one of three shell-type supernova remnants in the Galaxy whose
X-ray spectrum is dominated by nonthermal emission. This puts G347.3-0.5 in the
small, but growing class of SNRs for which the X-ray emission reveals directly
the presence of extremely energetic electrons accelerated by the SNR shock. We
have obtained new high-resolution X-ray and radio data on G347.3-0.5 using the
Chandra X-ray Observatory and the Australia Telescope Compact Array (ATCA)
respectively. The bright northwestern peak of the SNR seen in ROSAT and ASCA
images is resolved with Chandra into bright filaments and fainter diffuse
emission. These features show good correspondence with the radio morphological
structure, providing strong evidence that the same population of electrons is
responsible for the synchrotron emission in both bands in this part of the
remnant. Spectral index information from both observations is presented. We
found significant difference in photon index value between bright and faint
regions of the SNR shell. Spectral properties of these regions support the
notion that efficient particle acceleration is occurring in the bright SNR
filaments. We report the detection of linear radio polarization towards the
SNR, which is most ordered at the northwestern shell where particle
acceleration is presumably occurring. Using our new Chandra and ATCA data we
model the broad-band emission from G347.3-0.5 with the synchrotron and inverse
Compton mechanisms and discuss the conditions under which this is a plausible
scenario.Comment: 24 pages, 10 figures, accepted for publication in Ap
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