215 research outputs found
Forward modelling of brightness variations in Sun-like stars I. Emergence and surface transport of magnetic flux
The latitudinal distribution of starspots deviates from the solar pattern
with increasing rotation rate. Numerical simulations of magnetic flux emergence
and transport can help model the observed stellar activity patterns and the
associated brightness variations. We set up a composite model for the processes
of flux emergence and transport on Sun-like stars, to simulate stellar
brightness variations for various levels of magnetic activity and rotation
rates. Assuming that the distribution of magnetic flux at the base of the
convection zone follows solar scaling relations, we calculate the emergence
latitudes and tilt angles of bipolar regions at the surface for various
rotation rates, using thin-flux-tube simulations. Taking these two quantities
as input to a surface flux transport SFT model, we simulate the
diffusive-advective evolution of the radial field at the stellar surface,
including effects of active region nesting. As the rotation rate increases, (1)
magnetic flux emerges at higher latitudes and an inactive gap opens around the
equator, reaching a half-width of for , (2) the tilt
angles of freshly emerged bipolar regions show stronger variations with
latitude. Polar spots can form at by accumulation of
follower-polarity flux from decaying bipolar regions. From to
, the maximum spot coverage changes from 3 to 20%, respectively,
compared to 0.4% for the solar model. Nesting of activity can lead to strongly
non-axisymmetric spot distributions. On Sun-like stars rotating at
( days), polar spots can form, owing to
higher levels of flux emergence rate and tilt angles. Defining spots by a
threshold field strength yields global spot coverages that are roughly
consistent with stellar observations.Comment: 16 pages, 13 figures. Astron. & Astrophys. (in press); minor language
corrections mad
Generalized four-point characterization method for resistive and capacitive contacts
In this paper, a four-point characterization method is developed for
resistive samples connected to either resistive or capacitive contacts.
Provided the circuit equivalent of the complete measurement system is known
including coaxial cable and connector capacitances as well as source output and
amplifier input impedances, a frequency range and capacitive scaling factor can
be determined, whereby four-point characterization can be performed. The
technique is demonstrated with a discrete element test sample over a wide
frequency range using lock-in measurement techniques from 1 Hz - 100 kHz. The
data fit well with a circuit simulation of the entire measurement system. A
high impedance preamplifier input stage gives best results, since lock-in input
impedances may differ from manufacturer specifications. The analysis presented
here establishes the utility of capacitive contacts for four-point
characterizations at low frequency.Comment: 21 pages, 10 figure
Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-Region Nesting
Ultra-precise astrometry from the Gaia mission is expected to lead to
astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the
factors that could hamper such detections is the astrometric jitter caused by
the magnetic activity of the planet host stars. In our previous study, we
modeled astrometric jitter for the Sun observed equator-on. In this work, we
generalize our model and calculate the photocenter jitter as it would be
measured by the Gaia and Small-JASMINE missions for stars with solar rotation
rate and effective temperature, but with various values of the inclination
angle of the stellar rotation axis. In addition, we consider the effect of
metallicity and of nesting of active regions (i.e. the tendency of active
regions to emerge in the vicinity of each other). We find that, while the
jitter of stars observed equator-on does not have any long-term trends and can
be easily filtered out, the photocenters of stars observed out of their
equatorial planes experience systematic shifts over the course of the activity
cycle. Such trends allow the jitter to be detected with continuous
measurements, in which case it can interfere with planet detectability. An
increase in the metallicity is found to increase the jitter caused by stellar
activity. Active-region nesting can further enhance the peak-to-peak amplitude
of the photocenter jitter to a level that could be detected by Gaia.Comment: 24 pages, 20 figures in the main body and 4 in the appendix, accepted
for publication in Ap
Observing and modelling the young solar analogue EK Draconis: starspot distribution, elemental abundances, and evolutionary status
Observations and modelling of stars with near-solar masses in their early
phases of evolution is critical for a better understanding of how dynamos of
solar-type stars evolve. We examine the chemical composition and the spot
distribution of the pre-main-sequence solar analogue EK Dra. Using spectra from
the HERMES Spectrograph (La Palma), we obtain the abundances of 23 elements
with respect to the solar ones, which lead to a , with
significant overabundance of Li and Ba. The s-process elements Sr, Y, and Ce
are marginally overabundant, while Co, Ni, Cu, Zn are marginally deficient
compared to solar abundances. The overabundance of Ba is most likely due to the
assumption of depth-independent microturbulent velocity. Li abundance is
consistent with the age and the other abundances may indicate distinct initial
conditions of the pre-stellar nebula. We estimate a mass of 1.04 and
an age of \,Myr using various spectroscopic and photometric
indicators. We study the surface distribution of dark spots, using 17 spectra
collected during 15 nights using the CAFE Spectrograph (Calar Alto). We also
conduct flux emergence and transport (FEAT) simulations for EK Dra's parameters
and produce 15-day-averaged synoptic maps of the likely starspot distributions.
Using Doppler imaging, we reconstruct the surface brightness distributions for
the observed spectra and FEAT simulations, which show overall agreement for
polar and mid-latitude spots, while in the simulations there is a lack of
low-latitude spots compared to the observed image. We find indications that
cross-equatorial extensions of mid-latitude spots can be artefacts of the less
visible southern-hemisphere activity.Comment: 15 pages, 16 figures, accepted for publication in MNRAS; corrections
in Abstract and Introductio
3-Benzyl-6-(2-chlorobenzoyl)-1,3-benzoxazol-2(3H)-one
In the title compound, C21H14ClNO3, the benzoxazolone ring system is planar (r.m.s. deviation = 0.022 Å) and forms dihedral angles of 75.38 (10) and 65.92 (13)° with the mean planes of the chlorobenzoyl (r.m.s. deviation = 0.045 Å, excluding O atom) and benzyl (r.m.s. deviation = 0.023 Å) groups. The observed structure is stabilized by weak C—H⋯O hydrogen bonds and weak intermolecular C—H⋯π interactions
Plot by plot: plotting urbanism as an ordinary process of urbanization
With this paper, we analyse an ordinary urban process, which has received little attention so far and propose a new concept to take account of it: plotting urbanism. It is usually subsumed
under terms like “urban informality” or “incremental urbanism” and not studied as a distinct process. In comparing Lagos, Istanbul and Shenzhen we captured four defining features of
plotting urbanism: first, it unfolds in a piecemeal fashion with limited comprehensive planning. Second, it emerges from conflicts between multiple overlapping modes of territorial regulation, land tenure and property rights, which result in specific territorial compromises. Third, plotting is based on commodification of housing and land, which might accentuate socio-economic differentiations between property-owners, who often live in the same area, and their tenants. The term “plotting” highlights the key role of the plot in the process. It also alludes to strategic acts of collaboration for individual and collaborative benefit
Governance and assessment insights in Information Technology: the Val IT Model
The purpose of this paper is the investigation of the relationship between Information Technology and value creation. In doing so, the paper presents a conceptual framework based on the Val IT model. This framework can assist companies in the selection, implementation and optimization of IT investments to create long-term value. The paper is based on a qualitative approach with the inclusion of a single-method approach. Research is developed according to the study of national and international literature. Following an initial analysis of existing literature on the evaluation of company investments, research has been conducted through the Val IT method to assess these intangible assets. The sources of research are secondary in nature (documents, reports, newspaper articles, papers and scientific books). This methodology emphasises the connection between the modern services created in the knowledge economy and the application of new technologies, especially in the field of Information Technology. Val IT is a governance framework for creating business value from IT investments. It allows companies to increase the possibility of selecting investments with a high potential of value creation, as well as with a greater possibility of success in executing, creating, strengthening and using these services
Operational Research: methods and applications
This is the final version. Available on open access from Taylor & Francis via the DOI in this recordThroughout its history, Operational Research has evolved to include methods, models and algorithms that have been applied to a wide range of contexts. This encyclopedic article consists of two main sections: methods and applications. The first summarises the up-to-date knowledge and provides an overview of the state-of-the-art methods and key developments in the various subdomains of the field. The second offers a wide-ranging list of areas where Operational Research has been applied. The article is meant to be read in a nonlinear fashion and used as a point of reference by a diverse pool of readers: academics, researchers, students, and practitioners. The entries within the methods and applications sections are presented in alphabetical order. The authors dedicate this paper to the 2023 Turkey/Syria earthquake victims. We sincerely hope that advances in OR will play a role towards minimising the pain and suffering caused by this and future catastrophes
"The Location of URU Tuspa and A New Inscribed Column Base Belonging to Ispuini, King of Urartu"
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