1,154 research outputs found
Fourteen candidate RR Lyrae star streams in the inner Galaxy
We apply the GC3 stream-finding method to RR Lyrae stars (RRLS) in the
Catalina survey. We find two RRLS stream candidates at confidence
and another 12 at confidence over the Galactocentric distance
range . Of these, only two are associated with known
globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan'
streams, consistent with the idea that globular cluster streams are most
visible close to dissolution. Our detections are likely a lower bound on the
total number of dissolving globulars in the inner galaxy, since many globulars
have few RRLS while only the brightest streams are visible over the Galactic
RRLS background, particularly given the current lack of kinematical
information. We make all of our candidate streams publicly available and
provide a new GALSTREAMS Python library for the footprints of all known streams
and overdensities in the Milky Way.Comment: 18 pages, 4 figures. Accepted for publication at MNRAS. GALSTREAMS
Milky Way Streams Footprint Library are available at
https://github.com/cmateu/galstreams . All RRL data and code used in the
paper are available at
https://cmateu.github.io/Cecilia_Mateu_WebPage/CatalinaGC3_Streams.htm
A High-Resolution Compton Scattering Study of the Electron Momentum Density in Al
We report high-resolution Compton profiles (CP's) of Al along the three
principal symmetry directions at a photon energy of 59.38 keV, together with
corresponding highly accurate theoretical profiles obtained within the
local-density approximation (LDA) based band-theory framework. A good accord
between theory and experiment is found with respect to the overall shapes of
the CP's, their first and second derivatives, as well as the anisotropies in
the CP's defined as differences between pairs of various CP's. There are
however discrepancies in that, in comparison to the LDA predictions, the
measured profiles are lower at low momenta, show a Fermi cutoff which is
broader, and display a tail which is higher at momenta above the Fermi
momentum. A number of simple model calculations are carried out in order to
gain insight into the nature of the underlying 3D momentum density in Al, and
the role of the Fermi surface in inducing fine structure in the CP's. The
present results when compared with those on Li show clearly that the size of
discrepancies between theoretical and experimental CP's is markedly smaller in
Al than in Li. This indicates that, with increasing electron density, the
conventional picture of the electron gas becomes more representative of the
momentum density and that shortcomings of the LDA framework in describing the
electron correlation effects become less important.Comment: 7 pages, 6 figures, regular articl
Novel approach to a perfect lens
Within the framework of an exact analytical solution of Maxwell equations in
a space domain, it is shown that optical scheme based on a slab with negative
refractive index () (Veselago lens or Pendry lens) does not possess
focusing properties in the usual sense . In fact, the energy in such systems
does not go from object to its "image", but from object and its "image" to an
intersection point inside a metamaterial layer, or vice versa. A possibility of
applying this phenomenon to a creation of entangled states of two atoms is
discussed.Comment: 4 pages, 6 figure
Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}
The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to
determine the magnitude of the electronic specific-heat coefficient \gamma, in
an attempt to gain an insight into the origin of the unusually large
thermoelectric power of this compound. It is found that \gamma is as large as
48 mJ/molK^2, which is an order of magnitude larger than \gamma of simple
metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated
electron system, where the strong correlation probably comes from the
low-dimensionality and the frustrated spin structure. We discuss how the large
thermopower and its dependence on Ca doping can be understood with the strong
electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
Constraining Ultra Light Dark Matter with the Galactic Nuclear Star Cluster
We use the Milky Way’s nuclear star cluster (NSC) to test the existence of a dark matter ‘soliton core’, as predicted in ultra-light dark matter (ULDM) models. Since the soliton core size is proportional to m−1DM, while the core density grows as m2DM, the NSC (dominant stellar component within ∼3 pc) is sensitive to a specific window in the dark matter particle mass, mDM. We apply a spherical isotropic Jeans model to fit the NSC line-of-sight velocity dispersion data, assuming priors on the precisely measured Milky Way’s supermassive black hole (SMBH) mass and the well-measured NSC density profile. We find that the current observational data reject the existence of a soliton core for a single ULDM particle with mass in the range 10−20.4 eV ≲ mDM ≲ 10−18.5 eV, assuming that the soliton core structure is not affected by the Milky Way’s SMBH. We test our methodology on mock data, confirming that we are sensitive to the same range in ULDM mass as for the real data. Dynamical modelling of a larger region of the Galactic centre, including the nuclear stellar disc, promises tighter constraints over a broader range of mDM. We will consider this in future work
Crossings as a side effect of dependency lengths
The syntactic structure of sentences exhibits a striking regularity:
dependencies tend to not cross when drawn above the sentence. We investigate
two competing explanations. The traditional hypothesis is that this trend
arises from an independent principle of syntax that reduces crossings
practically to zero. An alternative to this view is the hypothesis that
crossings are a side effect of dependency lengths, i.e. sentences with shorter
dependency lengths should tend to have fewer crossings. We are able to reject
the traditional view in the majority of languages considered. The alternative
hypothesis can lead to a more parsimonious theory of language.Comment: the discussion section has been expanded significantly; in press in
Complexity (Wiley
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