792 research outputs found
Metal-rich or misclassified? The case of four RR Lyrae stars
We analysed the light curve of four, apparently extremely metal-rich
fundamental-mode RR Lyrae stars. We identified two stars, MT Tel and ASAS
J091803-3022.6 as RRc (first-overtone) pulsators that were misclassified as
RRab ones in the ASAS survey. In the case of the other two stars, V397 Gem and
ASAS J075127-4136.3, we could not decide conclusively, as they are outliers in
the period-Fourier-coefficient space from the loci of both classes, but their
photometric metallicities also favour the RRc classification.Comment: 5 pages, 2 figures, published in IBVS:
http://ibvs.konkoly.hu/cgi-bin/IBVS?617
Precursors of an upcoming solar cycle at high latitudes from coronal green line data
After reviewing potential early indicators of an upcoming solar cycle at high
latitudes, we focus attention on the rush-to-the-poles (RTTP) phenomenon in
coronal green line emission. Considering various correlations between
properties of the RTTP with the upcoming solar cycle we find a correlation
between the rate of the RTTP and the time delay until the maximum of the next
solar cycle. On the basis of this correlation and the known internal
regularities of the sunspot number series we predict that, following a minimum
in 2019, cycle 25 will peak in late 2024 at an amplitude of about 130 (in terms
of smoothed monthly revised sunspot numbers). This slightly exceeds the
amplitude of cycle 24 but it would still make cycle 25 a fairly weak cycle.Comment: 17 pages, 5 figures J. Atm. Sol.-Terr. Phys., in pres
A Bayesian Approach to Identify Bitcoin Users
Bitcoin is a digital currency and electronic payment system operating over a
peer-to-peer network on the Internet. One of its most important properties is
the high level of anonymity it provides for its users. The users are identified
by their Bitcoin addresses, which are random strings in the public records of
transactions, the blockchain. When a user initiates a Bitcoin-transaction, his
Bitcoin client program relays messages to other clients through the Bitcoin
network. Monitoring the propagation of these messages and analyzing them
carefully reveal hidden relations. In this paper, we develop a mathematical
model using a probabilistic approach to link Bitcoin addresses and transactions
to the originator IP address. To utilize our model, we carried out experiments
by installing more than a hundred modified Bitcoin clients distributed in the
network to observe as many messages as possible. During a two month observation
period we were able to identify several thousand Bitcoin clients and bind their
transactions to geographical locations
Gaia Data Release 2: Validation of the classification of RR Lyrae and Cepheid variables with the Kepler and K2 missions
The second data release of the Gaia mission includes an advance catalog of
variable stars. The classification of these stars are based on sparse
photometry from the first 22 months of the mission. We set out to investigate
the purity and completeness of the all-sky Gaia classification results with the
help of the continuous light curves of the observed targets from the Kepler and
K2 missions, focusing specifically on RR Lyrae and Cepheid pulsators, outside
the Galactic Bulge region. We crossmatched the Gaia identifications with the
observations collected by the Kepler space telescope. We inspected the light
curves visually, then calculated the relative Fourier coefficients and period
ratios for the single- and double-mode K2 RR Lyrae stars to further classify
them. We identified 1443 and 41 stars classified as RR Lyrae or Cepheid
variables in Gaia DR2 in the targeted observations of the two missions and 263
more RR Lyre targets in the Full-Frame Images (FFI) of the original mission. We
provide the crossmatch of these sources. We conclude that the RR Lyrae catalog
has a completeness between 70-78%, and provide a purity estimate between 92-98%
(targeted observations) with lower limits of 75% (FFI stars) and 51% (K2
worst-case scenario). The low number of Cepheids prevents us from drawing
detailed conclusions but the purity of the DR2 sample is estimated to be around
66%.Comment: 15 pages, 12 figures, 10 tables, accepted into the Gaia DR2 Special
Issue in A&A. V1: submitted version, v2: accepted version. Lists available at
http://konkoly.hu/~lmolnar/gaiakepler_datafiles_R1.zi
Evidence for fresh frost layer on the bare nucleus of comet Hale--Bopp at 32 AU distance
Here we report that the activity of comet Hale--Bopp ceased between late 2007
and March, 2009, at about 28 AU distance from the Sun. At that time the comet
resided at a distance from the Sun that exceeded the freeze-out distance of
regular comets by an order of magnitude. A Herschel Space Observatory PACS scan
was taken in mid-2010, in the already inactive state of the nucleus. The albedo
has been found to be surprisingly large (8.10.9%{}), which exceeds the
value known for any other comets. With re-reduction of archive HST images from
1995 and 1996, we confirm that the pre-perihelion albedo resembled that of an
ordinary comet, and was smaller by a factor of two than the post-activity
albedo. Our further observations with the Very Large Telescope (VLT) also
confirmed that the albedo increased significantly by the end of the activity.
We explain these observations by proposing gravitational redeposition of icy
grains towards the end of the activity. This is plausible for such a massive
body in a cold environment, where gas velocity is lowered to the range of the
escape velocity. These observations also show that giant comets are not just
the upscaled versions of the comets we know but can be affected by processes
that are yet to be fully identified.Comment: 7 pages, 6 figures, accepted for publication in Ap
A Constitutive Model of Shape Memory Alloys Based on Viscoplastic like Evolution Equations
The paper presents a new model that allows the
simulation of the macroscopic thermomechanical behavior of shape
memory alloys (SMA).
In this paper we present a description of the austenite-martensite
phase transition, which takes into account the two types of martensite
by introducing suitable internal variables: the volume fractions of
the self-accommodating product phase (pure thermal effect) and the
oriented (stress induced) product phase. The evolution equations of
the internal variables are similar to the evolution equation in
viscoplasticity. Differing from previous models, the presented one is
able to model the behaviour of the SMA under combined thermal and
mechanical loads. A numerical example is given that illustrates the
ability of the model to capture the thermomechanical behavior of shape
memory alloys under temperature change and proportional loadings
The additional-mode garden of RR Lyrae stars
Space-based photometric missions revealed a surprising abundance of
millimagnitude-level additional modes in RR Lyrae stars. The modes that appear
in the modulated fundamental-mode (RRab) stars can be ordered into four major
categories. Here we present the distribution of these groups in the Petersen
diagram, and discuss their characteristics and connections to additional modes
observed in other RR Lyrae stars.Comment: 4 pages, 4 figures, proceedings of the Joint
TASC2-KASC9-SPACEINN-HELAS8 Conference "Seismology of the Sun and the Distant
Stars 2016", to be published in EPJ Wo
Origin of sediments transported from differnt directions into the lake Pannon during the late neogene, based on mineralogical composition of sands and sandstones in the Hungarian Plain
A tall space with a small bottom
We introduce a general method of constructing locally compact scattered spaces from certain families of sets and then, with the help of this method, we prove that if kappa(<kappa) = kappa then there is such a space of height kappa(+) with only kappa many isolated points. This implies that there is a locally compact scattered space of height omega(2) with omega(1) isolated points in ZFC, solving an old problem of the first author
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