397 research outputs found
Investigation of the Progenitors of the Type Ia Supernovae Associated With the LMC Supernova Remnants 0505-67.9 and 0509-68.7
Although Type Ia supernovae have been heavily scrutinized due to their use in
making cosmological distance estimates, we are still unable to definitively
identify the progenitors for the entire population. While answers have been
presented for certain specific systems, a complete solution remains elusive. We
present observations of two supernova remnants (SNRs) in the Large Magellanic
Cloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center
of the remnant and the 99.73% containment central region in which any companion
star left over after the supernova must be located. Both remnants have a number
of potential ex-companion stars near their centers; all possible single and
double degenerate progenitor models remain viable for these two supernovae.
Future observations may be able to identify the true ex-companions for both
remnants.Comment: 8 pages, 4 figures, 4 tables, ApJ In Press; Table 2 truncated, full
version available in published paper or directly from author
Identifying and Quantifying Recurrent Novae Masquerading as Classical Novae
Recurrent novae (RNe) are cataclysmic variables with two or more nova
eruptions within a century. Classical novae (CNe) are similar systems with only
one such eruption. Many of the so-called 'CNe' are actually RNe for which only
one eruption has been discovered. Since RNe are candidate Type Ia supernova
progenitors, it is important to know whether there are enough in our galaxy to
provide the supernova rate, and therefore to know how many RNe are masquerading
as CNe. To quantify this, we collected all available information on the light
curves and spectra of a Galactic, time-limited sample of 237 CNe and the 10
known RNe, as well as exhaustive discovery efficiency records. We recognize RNe
as having (a) outburst amplitude smaller than 14.5 - 4.5 * log(t_3), (b)
orbital period >0.6 days, (c) infrared colors of J-H > 0.7 mag and H-K > 0.1
mag, (d) FWHM of H-alpha > 2000 km/s, (e) high excitation lines, such as Fe X
or He II near peak, (f) eruption light curves with a plateau, and (g) white
dwarf mass greater than 1.2 M_solar. Using these criteria, we identify V1721
Aql, DE Cir, CP Cru, KT Eri, V838 Her, V2672 Oph, V4160 Sgr, V4643 Sgr, V4739
Sgr, and V477 Sct as strong RN candidates. We evaluate the RN fraction amongst
the known CNe using three methods to get 24% +/- 4%, 12% +/- 3%, and 35% +/-
3%. With roughly a quarter of the 394 known Galactic novae actually being RNe,
there should be approximately a hundred such systems masquerading as CNe.Comment: 3 figures, 7 tables, accepted for publication in Ap
The Diffuse Source at the Center of LMC SNR 0509-67.5 is a Background Galaxy at z = 0.031
Type Ia supernovae (SNe Ia) are well-known for their use in the measurement
of cosmological distances, but our continuing lack of concrete knowledge about
their progenitor stars is both a matter of debate and a source of systematic
error. In our attempts to answer this question, we presented unambiguous
evidence that LMC SNR 0509-67.5, the remnant of an SN Ia that exploded in the
Large Magellanic Cloud 400 +/- 50 years ago, did not have any point sources
(stars) near the site of the original supernova explosion, from which we
concluded that this particular supernova must have had a progenitor system
consisting of two white dwarfs (Schaefer & Pagnotta 2012). There is, however,
evidence of nebulosity near the center of the remnant, which could have been
left over detritus from the less massive WD, or could have been a background
galaxy unrelated to the supernova explosion. We obtained long-slit spectra of
the central nebulous region using GMOS on Gemini South to determine which of
these two possibilities is correct. The spectra show H-alpha emission at a
redshift of z = 0.031, which implies that the nebulosity in the center of LMC
SNR 0509-67.5 is a background galaxy, unrelated to the supernova.Comment: 2 figures, accepted for publication in Ap
More than one dynamic crossover in protein hydration water
Studies of liquid water in its supercooled region have led to many insights
into the structure and behavior of water. While bulk water freezes at its
homogeneous nucleation temperature of approximately 235 K, for protein
hydration water, the binding of water molecules to the protein avoids
crystallization. Here we study the dynamics of the hydrogen bond (HB) network
of a percolating layer of water molecules, comparing measurements of a hydrated
globular protein with the results of a coarse-grained model that has been shown
to successfully reproduce the properties of hydration water. With dielectric
spectroscopy we measure the temperature dependence of the relaxation time of
protons charge fluctuations. These fluctuations are associated to the dynamics
of the HB network of water molecules adsorbed on the protein surface. With
Monte Carlo (MC) simulations and mean--field (MF) calculations we study the
dynamics and thermodynamics of the model. In both experimental and model
analyses we find two dynamic crossovers: (i) one at about 252 K, and (ii) one
at about 181 K. The agreement of the experiments with the model allows us to
relate the two crossovers to the presence of two specific heat maxima at
ambient pressure. The first is due to fluctuations in the HB formation, and the
second, at lower temperature, is due to the cooperative reordering of the HB
network
On the progenitor of SNR 0509-67.5
We have used three independent methods to determine an accurate and precise geometric center of SNR 0509-67.5, at RA = 05:09:31.208, DEC = -67:31:17.48 (J2000). This supernova, which occurred approximately 400 years ago in the Large Magellanic Cloud, was confirmed to be a Type Ia by Rest et al. (2005), Rest et al. (2008) based on spectra of a light echo from the eruption. If this supernova had a single-degenerate progenitor system, we would see the leftover companion star within a certain distance of the remnant\u27s center. Accounting for an offset due to enhanced ISM in the west-southwest quadrant of the remnant, we find the eruption position to be at RA = 05:09:30.976, DEC = -67:31:17.90; the error circle which should contain any possible ex-companion star has a radius of 1.60″ for 99.73% (3-sigma) containment. This accounts for the proper motion of the stars, the possibility of kicks from the supernova, and asymmetries in the explosion and remnant expansion. We find no possible ex-companion stars within this ellipse, to a limiting magnitude of V = 26.9: there are no red giants, which precludes symbiotic progenitors, no subgiants, which when combined with the lack of red giants precludes recurrent nova progenitors, and no main sequence stars with mass greater than 1.16 solar masses (V brighter than 22.7 mag), which precludes persistent supersoft X-ray source progenitors. Indeed, all published SD models are eliminated, so we conclude that this particular Type Ia supernova had a double-degenerate progenitor. Copyright © International Astronomical Union 2013
Decentralizing money: bitcoin prices and blockchain security
We address the determination of bitcoin prices and decentralized security. Users forecast the transactional and resale value of holdings, pricing the risk of malicious systemic attacks. Miners contribute resources to protect against attackers, competing for block rewards. Bitcoin’s design leads to multiple equilibria: the same technology and fundamentals are consistent with sharply different price and security levels. Bitcoin’s monetary policy can lead to welfare losses and deviations from quantity theory. Price–security feedback amplifies fundamental shocks’ volatility impact and leads to boom–busts not driven by fundamentals. We show how Bitcoin’s viability versus fiat currency depends on relative acceptability and inflation protection
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