25,848 research outputs found
Dense molecular clouds in the SN2008fp host galaxy
(abridged) We use observations of interstellar absorption features, such as
atomic and molecular lines as well as diffuse interstellar bands (DIBs),
towards SN2008fp to study the physical properties of extra-galactic diffuse
interstellar clouds in the host galaxy, ESO428-G14. The properties of the
intervening dust are investigated via spectropolarimetry. The spectra of
SN2008fp reveal a complex of diffuse atomic clouds at radial velocities in line
with the systematic velocities of the host galaxy (~1700 km/s). A translucent
(A_V ~ 1.5 mag) cloud is detected at a heliocentric velocity of 1770 km/s This
cold dense cloud is rich in dense atomic gas tracers, molecules, as well as
diffuse interstellar bands. We have detected both C2 and C3 for the first time
in a galaxy beyond the Local Group. The CN (0,0) band line ratios are used to
derive an in-situ measurement of the cosmic background radiation temperature in
an external galaxy; this gives an excitation temperature of T = 2.9 +- 0.3 K.
The interstellar polarization law deviates significantly from what is observed
in the Galaxy, indicating substantial differences in the composition or size
distribution of dust grains in the SN2008fp host galaxy. C2 is used to probe
the cold diffuse ISM density and temperature. The lack of variability in the
extra-galactic absorption line profiles over a period of one month implies that
the absorbing material is not circumstellar and thus not affected directly by
the SN event. Also it shows that there are no significant density variation in
the small-scale structure of the molecular cloud down to 100 AU.Comment: 10 pages. Accepted for publication in A&A. Revisions include several
small correction
Studying the small scale ISM structure with supernovae
AIMS. In this work we explore the possibility of using the fast expansion of
a Type Ia supernova photosphere to detect extra-galactic ISM column density
variations on spatial scales of ~100 AU on time scales of a few months.
METHODS. We constructed a simple model which describes the expansion of the
photodisk and the effects of a patchy interstellar cloud on the observed
equivalent width of Na I D lines. Using this model we derived the behavior of
the equivalent width as a function of time, spatial scale and amplitude of the
column density fluctuations.
RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na
I column densities exceeding a few 10^11 cm^-2 would be easily detected. In
contrast, the effects of a more realistic, patchy ISM become measurable in a
fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on
a scale of 1000 AU.
CONCLUSIONS. The proposed technique provides a unique way to probe the
extra-galactic small scale structure, which is out of reach for any of the
methods used so far. The same tool can also be applied to study the sub-AU
Galactic ISM structure.Comment: 6 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
The incidence of mid-infrared excesses in G and K giants
Using photometric data from the 2MASS and GLIMPSE catalogues, I investigate
the incidence of mid-infrared excesses (~10 microns) of G and K stars of
luminosity class III. In order to obtain a large sample size, stars are
selected using a near-IR colour-magnitude diagram. Sources which are candidates
for showing mid-IR excess are carefully examined and modelled to determined
whether they are likely to be G/K giants. It is found that mid-IR excesses are
present at a level of (1.8 +/- 0.4) x 10^-3. While the origin of these excesses
remains uncertain, it is plausible that they arise from debris discs around
these stars. I note that the measured incidence is consistent with a scenario
in which dust lifetimes in debris discs are determined by Poynting-Robertson
drag rather than by collisions.Comment: Accepted for publication in MNRAS. 13 pages, 5 figures, 2 tables (1
landscape table
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Hyperopic Cops and Robbers
We introduce a new variant of the game of Cops and Robbers played on graphs,
where the robber is invisible unless outside the neighbor set of a cop. The
hyperopic cop number is the corresponding analogue of the cop number, and we
investigate bounds and other properties of this parameter. We characterize the
cop-win graphs for this variant, along with graphs with the largest possible
hyperopic cop number. We analyze the cases of graphs with diameter 2 or at
least 3, focusing on when the hyperopic cop number is at most one greater than
the cop number. We show that for planar graphs, as with the usual cop number,
the hyperopic cop number is at most 3. The hyperopic cop number is considered
for countable graphs, and it is shown that for connected chains of graphs, the
hyperopic cop density can be any real number in $[0,1/2].
Blind Normalization of Speech From Different Channels
We show how to construct a channel-independent representation of speech that
has propagated through a noisy reverberant channel. This is done by blindly
rescaling the cepstral time series by a non-linear function, with the form of
this scale function being determined by previously encountered cepstra from
that channel. The rescaled form of the time series is an invariant property of
it in the following sense: it is unaffected if the time series is transformed
by any time-independent invertible distortion. Because a linear channel with
stationary noise and impulse response transforms cepstra in this way, the new
technique can be used to remove the channel dependence of a cepstral time
series. In experiments, the method achieved greater channel-independence than
cepstral mean normalization, and it was comparable to the combination of
cepstral mean normalization and spectral subtraction, despite the fact that no
measurements of channel noise or reverberations were required (unlike spectral
subtraction).Comment: 25 pages, 7 figure
On the Complexity of Solving Zero-Dimensional Polynomial Systems via Projection
Given a zero-dimensional polynomial system consisting of n integer
polynomials in n variables, we propose a certified and complete method to
compute all complex solutions of the system as well as a corresponding
separating linear form l with coefficients of small bit size. For computing l,
we need to project the solutions into one dimension along O(n) distinct
directions but no further algebraic manipulations. The solutions are then
directly reconstructed from the considered projections. The first step is
deterministic, whereas the second step uses randomization, thus being
Las-Vegas.
The theoretical analysis of our approach shows that the overall cost for the
two problems considered above is dominated by the cost of carrying out the
projections. We also give bounds on the bit complexity of our algorithms that
are exclusively stated in terms of the number of variables, the total degree
and the bitsize of the input polynomials
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
Hardy type spaces on certain noncompact manifolds and applications
In this paper we consider a complete connected noncompact Riemannian manifold
M with Ricci curvature bounded from below, positive injectivity radius and
spectral gap b. We introduce a sequence X^1(M), X^2(M), ... of new Hardy spaces
on M, the sequence Y^1(M/, Y^2(M), ... of their dual spaces, and show that
these spaces may be used to obtain endpoint estimates for purely imaginary
powers of the Laplace-Beltrami operator and for more general spectral
multipliers associated to the Laplace--Beltrami operator L on M. Under the
additional condition that the volume of the geodesic balls of radius r is
controlled by C r^a e^{2\sqrt{b} r} for some real number a and for all large r,
we prove also an endpoint result for first order Riesz transforms D L^{-1/2}.
In particular, these results apply to Riemannian symmetric spaces of the
noncompact type.Comment: 27 pages, v2: the first version has been revised and rearranged, with
additions, in two papers, of which this new version is the first. The second
paper is posted as arXiv:1002.1161v
Accurate and Automated High-Coverage Identification of Chemically Cross-Linked Peptides with MaxLynx
Cross-linking combined with mass spectrometry (XL-MS) provides a wealth of information about the three-dimensional (3D) structure of proteins and their interactions. We introduce MaxLynx, a novel computational proteomics workflow for XL-MS integrated into the MaxQuant environment. It is applicable to noncleavable and MS-cleavable cross-linkers. For both, we have generalized the Andromeda peptide database search engine to efficiently identify cross-linked peptides. For noncleavable peptides, we implemented a novel dipeptide Andromeda score, which is the basis for a computationally efficient N-squared search engine. Additionally, partial scores summarize the evidence for the two constituents of the dipeptide individually. A posterior error probability (PEP) based on total and partial scores is used to control false discovery rates (FDRs). For MS-cleavable cross-linkers, a score of signature peaks is combined with the conventional Andromeda score on the cleavage products. The MaxQuant 3D peak detection was improved to ensure more accurate determination of the monoisotopic peak of isotope patterns for heavy molecules, which cross-linked peptides typically are. A wide selection of filtering parameters can replace the manual filtering of identifications, which is often necessary when using other pipelines. On benchmark data sets of synthetic peptides, MaxLynx outperforms all other tested software on data for both types of cross-linkers and on a proteome-wide data set of cross-linked Drosophila melanogaster cell lysate. The workflow also supports ion mobility-enhanced MS data. MaxLynx runs on Windows and Linux, contains an interactive viewer for displaying annotated cross-linked spectra, and is freely available at https://www.maxquant.org/
- …