3,316 research outputs found
Invisible pushdown languages
Context free languages allow one to express data with hierarchical structure,
at the cost of losing some of the useful properties of languages recognized by
finite automata on words. However, it is possible to restore some of these
properties by making the structure of the tree visible, such as is done by
visibly pushdown languages, or finite automata on trees. In this paper, we show
that the structure given by such approaches remains invisible when it is read
by a finite automaton (on word). In particular, we show that separability with
a regular language is undecidable for visibly pushdown languages, just as it is
undecidable for general context free languages
OGLE-2002-BLG-360: from a gravitational microlensing candidate to an overlooked red transient
OGLE-2002-BLG-360 was discovered as a microlensing candidate by the OGLE-III
project. The subsequent light curve however clearly showed that the brightening
of the object could not have resulted from the gravitational microlensing
phenomenon. We aim to explain the nature of OGLE-2002-BLG-360 and its eruption
observed in 2002--2006. The observational data primarily come from the archives
of the OGLE project, which monitored the object in 2001--2009. The archives of
the MACHO and MOA projects also provided us with additional data obtained in
1995--99 and 2000--2005, respectively. These data allowed us to analyse the
light curve of the object during its eruption, as well as the potential
variability of its progenitor. In the archives of several infrared surveys,
namely 2MASS, MSX, Spitzer, AKARI, WISE, and VVV, we found measurements of the
object, which allowed us to study the spectral energy distribution (SED) of the
object. We constructed a simple model of a star surrounded by a dusty envelope,
which was used to interpret the observed SED. Our analysis of the data clearly
shows that OGLE-2002-BLG-360 was most probably a red transient, i.e. an object
similar in nature to V838 Mon, whose eruption was observed in 2002. The SED in
all phases, i.e. progenitor, eruption, and remnant, was dominated by infrared
emission, which we interpret as evidence of dust formation in an intense mass
outflow. Since 2009 the object has been completely embedded in dust. We suggest
that the progenitor of OGLE-2002-BLG-360 was a binary, which had entered the
common-envelope phase a long time (at least decades) before the observed
eruption, and that the eruption resulted from the final merger of the binary
components. We point out similarities between OGLE-2002-BLG-360 and CK Vul,
whose eruption was observed in 1670--72, and this strengthens the hypothesis
that CK Vul was also a red transient.Comment: accepted in A&
Discovery of a high state AM CVn binary in the Galactic Bulge Survey
We report on the discovery of a hydrogen-deficient compact binary (CXOGBS
J175107.6-294037) belonging to the AM CVn class in the Galactic Bulge Survey.
Deep archival X-ray observations constrain the X-ray positional uncertainty of
the source to 0.57 arcsec, and allow us to uniquely identify the optical and UV
counterpart. Optical spectroscopic observations reveal the presence of broad,
shallow He i absorption lines while no sign of hydrogen is present, consistent
with a high state system. We present the optical lightcurve from Optical
Gravitational Lensing Experiment monitoring, spanning 15 years. It shows no
evidence for outbursts; variability is present at the 0.2 mag level on
timescales ranging from hours to weeks. A modulation on a timescale of years is
also observed. A Lomb-Scargle analysis of the optical lightcurves shows two
significant periodicities at 22.90 and 23.22 min. Although the physical
interpretation is uncertain, such timescales are in line with expectations for
the orbital and superhump periods. We estimate the distance to the source to be
between 0.5 - 1.1 kpc. Spectroscopic follow-up observations are required to
establish the orbital period, and to determine whether this source can serve as
a verification binary for the eLISA gravitational wave mission.Comment: Accepted for publication in MNRAS Letter
Deciphering the physiological response of Escherichia coli under high ATP demand
One long‐standing question in microbiology is how microbes buffer perturbations in energy metabolism. In this study, we systematically analyzed the impact of different levels of ATP demand in Escherichia coli under various conditions (aerobic and anaerobic, with and without cell growth). One key finding is that, under all conditions tested, the glucose uptake increases with rising ATP demand, but only to a critical level beyond which it drops markedly, even below wild‐type levels. Focusing on anaerobic growth and using metabolomics and proteomics data in combination with a kinetic model, we show that this biphasic behavior is induced by the dual dependency of the phosphofructokinase on ATP (substrate) and ADP (allosteric activator). This mechanism buffers increased ATP demands by a higher glycolytic flux but, as shown herein, it collapses under very low ATP concentrations. Model analysis also revealed two major rate‐controlling steps in the glycolysis under high ATP demand, which could be confirmed experimentally. Our results provide new insights on fundamental mechanisms of bacterial energy metabolism and guide the rational engineering of highly productive cell factories
Revenue divergence and competitive balance in a divisional sports league
The North American model of resource allocation in professional sports leagues is adapted for English (association) football. The theoretical relationship between revenue and competitive balance is shown to be robust with respect to changes in teams’ objectives and labour market conditions. Empirical revenue functions are reported for 1926-1999. These indicate a shift in the composition of demand favouring big-city teams and an increase in the sensitivity of revenue to performance. An analysis of match results in the FA Cup competition suggests an increase in competitive imbalance between teams at different levels of the league’s divisional hierarchy, as the theory suggests
Stability of Ge-related point defects and complexes in Ge-doped SiO_2
We analyze Ge-related defects in Ge-doped SiO_2 using first-principles
density functional techniques. Ge is incorporated at the level of ~ 1 mol % and
above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency,
with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and
O vacancies binding strongly to Ge impurities. All the centers considered
exhibit potentially EPR-active states, candidates for the identification of the
Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its
extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure
Density-functional studies of tungsten trioxide, tungsten bronzes, and related systems
Tungsten trioxide adopts a variety of structures which can be intercalated
with charged species to alter the electronic properties, thus forming `tungsten
bronzes'. Similar optical effects are observed upon removing oxygen from WO_3,
although the electronic properties are slightly different. Here we present a
computational study of cubic and hexagonal alkali bronzes and examine the
effects on cell size and band structure as the size of the intercalated ion is
increased. With the exception of hydrogen (which is predicted to be unstable as
an intercalate), the behaviour of the bronzes are relatively consistent. NaWO_3
is the most stable of the cubic systems, although in the hexagonal system the
larger ions are more stable. The band structures are identical, with the
intercalated atom donating its single electron to the tungsten 5d valence band.
Next, this was extended to a study of fractional doping in the Na_xWO_3 system
(0 < x < 1). A linear variation in cell parameter, and a systematic change in
the position of the Fermi level up into the valence band was observed with
increasing x. In the underdoped WO_3-x system however, the Fermi level
undergoes a sudden jump into the conduction band at around x = 0.2. Lastly,
three compounds of a layered WO_4×a,wdiaminoalkane hybrid series were
studied and found to be insulating, with features in the band structure similar
to those of the parent WO_3 compound which relate well to experimental
UV-visible spectroscopy results.Comment: 12 pages, 16 figure
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph
We have observed an evolved star with a rare combination of spectral
features, MSX SMC 029, in the Small Magellanic Cloud (SMC) using the
low-resolution modules of the Infrared Spectrograph on the Spitzer Space
Telescope. A cool dust continuum dominates the spectrum of MSX SMC 029. The
spectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs)
and absorption at 13.7 micron from C2H2, a juxtaposition seen in only two other
sources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giant branch (AGB)
objects. As in these sources, the PAH spectrum has the unusual trait that the
peak emission in the 7-9 micron complex lies beyond 8.0 micron. In addition,
the 8.6 micron feature has an intensity as strong as the C-C modes which
normally peak between 7.7 and 7.9 micron. The relative flux of the feature at
11.3 micron to that at 8 micron suggests that the PAHs in MSX SMC 029 either
have a low ionization fraction or are largely unprocessed. The 13-16 micron
wavelength region shows strong absorption features similar to those observed in
the post-AGB objects AFGL 618 and SMP LMC 11. This broad absorption may arise
from the same molecules which have been identified in those sources: C2H2,
C4H2, HC3N, and C6H6. The similarities between MSX SMC 029, AFGL 2688, and AFGL
618 lead us to conclude that MSX SMC 029 has evolved off the AGB in only the
past few hundred years, making it the third post-AGB object identified in the
SMC.Comment: 4 figures, Fig. 4 color; to appear in the 20 November 2006
Astrophysical Journal Letter
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