155 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
Negotiating the inhuman: Bakhtin, materiality and the instrumentalization of climate change
The article argues that the work of literary theorist Mikhail M. Bakhtin presents a starting point for thinking about the instrumentalization of climate change. Bakhtin’s conceptualization of human–world relationships, encapsulated in the concept of ‘cosmic terror’, places a strong focus on our perception of the ‘inhuman’. Suggesting a link between the perceived alienness and instability of the world and in the exploitation of the resulting fear of change by political and religious forces, Bakhtin asserts that the latter can only be resisted if our desire for a false stability in the world is overcome. The key to this overcoming of fear, for him, lies in recognizing and confronting the worldly relations of the human body. This consciousness represents the beginning of one’s ‘deautomatization’ from following established patterns of reactions to predicted or real changes. In the vein of several theorists and artists of his time who explored similar ‘deautomatization’ strategies – examples include Shklovsky’s ‘ostranenie’, Brecht’s ‘Verfremdung’, Artaud’s emotional ‘cruelty’ and Bataille’s ‘base materialism’ – Bakhtin proposes a more playful and widely accessible experimentation to deconstruct our ‘habitual picture of the world’. Experimentation is envisioned to take place across the material and the textual to increase possibilities for action. Through engaging with Bakhtin’s ideas, this article seeks to draw attention to relations between the imagination of the world and political agency, and the need to include these relations in our own experiments with creating climate change awareness
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
Search for Pairs of Isolated Radio Pulsars - Components in Disrupted Binary Systems
We have developed a method for analyzing the kinematic association of
isolated relativistic objects - possible remnants of disrupted close binary
systems. We investigate pairs of fairly young radio pulsars with known proper
motions and estimated distances (dispersion measures) that are spaced no more
than 2-3 kpc apart. Using a specified radial velocity distribution for these
objects, we have constructed 100-300 thousand trajectories of their possible
motion in the Galactic gravitational field on a time scale of several million
years. The probabilities of their close encounters at epochs consistent with
the age of the younger pulsar in the pair are analyzed. When these
probabilities exceed considerably their reference values obtained by assuming a
purely random encounter between the pulsars under consideration, we conclude
that the objects may have been gravitationally bound in the past. As a result,
we have detected six pulsar pairs (J0543+2329/J0528+2200,
J1453-6413/J1430-6623, J2354+6155/J2321+6024, J1915+1009/J1909+1102,
J1832-0827/J1836-1008, and J1917+1353/J1926+1648) that are companions in
disrupted binary systems with a high probability. Estimates of their kinematic
ages and velocities at binary disruption and at the present epoch are provided
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
CMB Anisotropy Induced by a Moving Straight Cosmic String
We showed that the part of strings could be detected by optical method is
only 20% from the total available amount of such objects, therefore the
gravitational lensing method has to be "completed" by CMB one. We found the
general structure of the CMB anisotropy generated by a cosmic string for simple
model of straight string moving with constant velocity. For strings with
deficit angle 1-2 arcsec the amplitude of generated anisotropy has to be 15-30
muK (the corresponding string linear density is (G mu) ~ 10^{-7} and energy is
GUT one, 10^{15} GeV). To use both radio and optical methods the deficit angle
has to be from 0.1 arcsec to 5-6 arcsec. If cosmic string can be detected by
optical method, the length of corresponding brightness spot of anisotropy has
to be no less than 100 degrees.Comment: 6 pages, 1 Postscript figure, will be published in proceedings of
QUARKS-2008, 15th International Seminar on High Energy Physics, Sergiev
Posad, Russia, 23-29 May, 200
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
Luminous Blue Variable eruptions and related transients: Diversity of progenitors and outburst properties
We present new light curves and spectra for a number of extragalactic optical
transients or "SN impostors" related to giant eruptions of LBVs, and we provide
a comparative discussion of LBV-like giant eruptions known to date. New data
include photometry and spectroscopy of SNe1999bw, 2000ch, 2001ac, 2002bu,
2006bv, and 2010dn. SN2010dn resembles SN2008S and NGC 300-OT, whereas SN2002bu
shows spectral evolution from a normal LBV at early times to a twin of these
cooler transients at late times. SN2008S, NGC300-OT, and SN2010dn appear to be
special cases of a broader eruptive phenomenon where the progenitor star was
enshrouded by dust. Examining the full sample, SN impostors have range of
timescales from a day to decades, potentially suffering multiple eruptions. The
upper end of the luminosity distribution overlaps with the least luminous SNe.
The low end of the luminosity distribution is poorly defined, and a distinction
between various eruptions is not entirely clear. We discuss observational clues
concerning winds or shocks as the relevant mass-loss mechanism, and we evaluate
possible ideas for physical mechanisms. Although examples of these eruptions
are sufficient to illustrate their diversity, their statistical distribution
will benefit greatly from upcoming transient surveys. Based on the distribution
of eruptions, we propose that SN1961V was not a member of this class of
impostors, but was instead a true core-collapse SNIIn preceded by a giant LBV
eruption. (abridged)Comment: 36(!) journal pages, 16 figures. submitted to MNRAS on october 12.
coments welcome. updated reference
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
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