4,772 research outputs found
Bifunctional dithiocarbamates: a bridge between coordination chemistry and nanoscale materials
Just how hot are the Centauri extreme horizontal branch pulsators?
Past studies based on optical spectroscopy suggest that the five Cen
pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars
with effective temperatures of around 50 000 K. This places the stars below the
red edge of the theoretical instability strip in the log Teff diagram,
where no pulsation modes are predicted to be excited. Our goal is to determine
whether this temperature discrepancy is real, or whether the stars' effective
temperatures were simply underestimated. We present a spectral analysis of two
rapidly pulsating extreme horizontal branch (EHB) stars found in Cen.
We obtained Hubble Space Telescope/COS UV spectra of two Cen
pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines
to better constrain their effective temperatures. As a by-product we also
obtained FUV lightcurves of the two pulsators. Using the relative strength of
the N IV and N V lines as a temperature indicator yields Teff values close to
60 000 K, significantly hotter than the temperatures previously derived. From
the FUV light curves we were able to confirm the main pulsation periods known
from optical data. With the UV spectra indicating higher effective temperatures
than previously assumed, the sdO stars would now be found within the predicted
instability strip. Such higher temperatures also provide consistent
spectroscopic masses for both the cool and hot EHB stars of our previously
studied sample.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic
The rise of an ionized wind in the Narrow Line Seyfert 1 Galaxy Mrk 335 observed by XMM-Newton and HST
We present the discovery of an outflowing ionized wind in the Seyfert 1
Galaxy Mrk 335. Despite having been extensively observed by most of the largest
X-ray observatories in the last decade, this bright source was not known to
host warm absorber gas until recent XMM-Newton observations in combination with
a long-term Swift monitoring program have shown extreme flux and spectral
variability. High resolution spectra obtained by the XMM-Newton RGS detector
reveal that the wind consists of three distinct ionization components, all
outflowing at a velocity of 5000 km/s. This wind is clearly revealed when the
source is observed at an intermediate flux state (2-5e-12 ergs cm^-2 s^-1). The
analysis of multi-epoch RGS spectra allowed us to compare the absorber
properties at three very different flux states of the source. No correlation
between the warm absorber variability and the X-ray flux has been determined.
The two higher ionization components of the gas may be consistent with
photoionization equilibrium, but we can exclude this for the only ionization
component that is consistently present in all flux states (log(xi)~1.8). We
have included archival, non-simultaneous UV data from HST (FOS, STIS, COS) with
the aim of searching for any signature of absorption in this source that so far
was known for being absorption-free in the UV band. In the COS spectra obtained
a few months after the X-ray observations we found broad absorption in CIV
lines intrinsic to the AGN and blueshifted by a velocity roughly comparable to
the X-ray outflow. The global behavior of the gas in both bands can be
explained by variation of the covering factor and/or column density, possibly
due to transverse motion of absorbing clouds moving out of the line of sight at
Broad Line Region scale.Comment: 24 pages, 12 figures, ApJ accepte
Did LIGO detect dark matter?
We consider the possibility that the black-hole (BH) binary detected by LIGO
may be a signature of dark matter. Interestingly enough, there remains a window
for masses where
primordial black holes (PBHs) may constitute the dark matter. If two BHs in a
galactic halo pass sufficiently close, they radiate enough energy in
gravitational waves to become gravitationally bound. The bound BHs will rapidly
spiral inward due to emission of gravitational radiation and ultimately merge.
Uncertainties in the rate for such events arise from our imprecise knowledge of
the phase-space structure of galactic halos on the smallest scales. Still,
reasonable estimates span a range that overlaps the Gpc yr
rate estimated from GW150914, thus raising the possibility that LIGO has
detected PBH dark matter. PBH mergers are likely to be distributed spatially
more like dark matter than luminous matter and have no optical nor neutrino
counterparts. They may be distinguished from mergers of BHs from more
traditional astrophysical sources through the observed mass spectrum, their
high ellipticities, or their stochastic gravitational wave background. Next
generation experiments will be invaluable in performing these tests.Comment: 5 pages, 2 figures, updated to match version published in PR
Effect of Two Breeds and Two Concentrate: Roughage Ratios on the Carcass Composition of Drylot Lambs
Animal Scienc
Procedures of Leibnizian infinitesimal calculus: An account in three modern frameworks
Recent Leibniz scholarship has sought to gauge which foundational framework
provides the most successful account of the procedures of the Leibnizian
calculus (LC). While many scholars (e.g., Ishiguro, Levey) opt for a default
Weierstrassian framework, Arthur compares LC to a non-Archimedean framework SIA
(Smooth Infinitesimal Analysis) of Lawvere-Kock-Bell. We analyze Arthur's
comparison and find it rife with equivocations and misunderstandings on issues
including the non-punctiform nature of the continuum, infinite-sided polygons,
and the fictionality of infinitesimals. Rabouin and Arthur claim that Leibniz
considers infinities as contradictory, and that Leibniz' definition of
incomparables should be understood as nominal rather than as semantic. However,
such claims hinge upon a conflation of Leibnizian notions of bounded infinity
and unbounded infinity, a distinction emphasized by early Knobloch.
The most faithful account of LC is arguably provided by Robinson's framework.
We exploit an axiomatic framework for infinitesimal analysis called SPOT
(conservative over ZF) to provide a formalisation of LC, including the
bounded/unbounded dichotomy, the assignable/inassignable dichotomy, the
generalized relation of equality up to negligible terms, and the law of
continuity.Comment: 52 pages, to appear in British Journal for the History of Mathematic
Cauchy, infinitesimals and ghosts of departed quantifiers
Procedures relying on infinitesimals in Leibniz, Euler and Cauchy have been
interpreted in both a Weierstrassian and Robinson's frameworks. The latter
provides closer proxies for the procedures of the classical masters. Thus,
Leibniz's distinction between assignable and inassignable numbers finds a proxy
in the distinction between standard and nonstandard numbers in Robinson's
framework, while Leibniz's law of homogeneity with the implied notion of
equality up to negligible terms finds a mathematical formalisation in terms of
standard part. It is hard to provide parallel formalisations in a
Weierstrassian framework but scholars since Ishiguro have engaged in a quest
for ghosts of departed quantifiers to provide a Weierstrassian account for
Leibniz's infinitesimals. Euler similarly had notions of equality up to
negligible terms, of which he distinguished two types: geometric and
arithmetic. Euler routinely used product decompositions into a specific
infinite number of factors, and used the binomial formula with an infinite
exponent. Such procedures have immediate hyperfinite analogues in Robinson's
framework, while in a Weierstrassian framework they can only be reinterpreted
by means of paraphrases departing significantly from Euler's own presentation.
Cauchy gives lucid definitions of continuity in terms of infinitesimals that
find ready formalisations in Robinson's framework but scholars working in a
Weierstrassian framework bend over backwards either to claim that Cauchy was
vague or to engage in a quest for ghosts of departed quantifiers in his work.
Cauchy's procedures in the context of his 1853 sum theorem (for series of
continuous functions) are more readily understood from the viewpoint of
Robinson's framework, where one can exploit tools such as the pointwise
definition of the concept of uniform convergence.
Keywords: historiography; infinitesimal; Latin model; butterfly modelComment: 45 pages, published in Mat. Stu
Low energy electron interactions with resveratrol and resorcinol: anion states and likely dissociation pathways
We report a computational study of the anion states of the resveratrol (RV)
and resorcinol (RS) molecules, also investigating dissociative electron
attachment (DEA) pathways. RV has well known beneficial effects in human
health, and its antioxidant activity was previously associated with DEA
reactions producing H. Our calculations indicate a valence bound state
() and four resonances ( to ) for that system. While
the computed thermodynamical thresholds are compatible with DEA reactions
producing H at 0~eV, the well known mechanism involving vibrational
Feshbach resonances built on a dipole bound state should not be present in RV.
Our results suggest that the shallow valence bound state is expected
to account for H elimination, probably involving
/ couplings along the vibration dynamics. The RS
molecule is also an oxidant and a subunit of RV. Since two close-lying hydroxyl
groups are found in the RS moiety, the H-elimination reaction in RV should
take place at the RS site. Our calculations point out a correspondence between
the anion states of RV and RS, and even between the thresholds. Nevertheless,
the absence of bound anion states in RS, indicated by our calculations, is
expected to suppress the H-formation channel at 0~eV. One is lead to
conclude that the ethene and phenol subunits in RV stabilize the
state, thus switching on the DEA mechanism producing H.Comment: 29 pages, 13 figure
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