89 research outputs found
Flow instabilities of magnetic flux tubes IV. Flux storage in the solar overshoot region
We consider the effects of material flows on the dynamics of toroidal
magnetic flux tubes located close to the base of the solar convection zone,
initially within the overshoot region. The problem is to find the physical
conditions in which magnetic flux can be stored for periods comparable to the
dynamo amplification time, which is of the order of a few years. We carry out
nonlinear numerical simulations to investigate the stability and dynamics of
thin flux tubes subject to perpendicular and longitudinal flows. We compare the
simulations with the results of simplified analytical approximations. We
determine ranges of the flow parameters for which a linearly Parker-stable
magnetic flux tube is stored in the middle of the overshoot region for a period
comparable to the dynamo amplification time. The residence time for magnetic
flux tubes with fluxes of 2x10^{21} Mx in the convective overshoot layer is
comparable to the dynamo amplification time, provided that the average speed
and the duration of the downflow do not exceed about 50 m/s and 100 days,
respectively, and that the lateral extension of the flow is smaller than about
10 degrees.Comment: Accepted to be published in Astronomy and Astrophysics. 16 pages, 16
figures. To access GIF animations, use
http://www.mps.mpg.de/homes/ishik/flute/frict_inst.gif,
http://www.mps.mpg.de/homes/ishik/flute/TF60.gif and
http://www.mps.mpg.de/homes/ishik/flute/TF180.gi
First Chromospheric Activity and Doppler Imaging Study of PW And Using a New Doppler Imaging Code: SpotDIPy
Measuring coverage of dark spots on cool stars is important in understanding
how stellar magnetic activity scales with the rotation rate and convection zone
depth. In this respect, it is crucial to infer surface magnetic patterns on G
and K stars, to reveal solar-like stellar dynamos in action. Molecular bands
serve as invaluable indicators of cool spots on the surfaces of stars, as they
play a crucial role in enabling accurate assessments of the extent of spot
coverage across the stellar surface. Therefore, more reliable surface images
can be obtained considering the inversion of atomic lines with molecular bands.
In this context, we simultaneously carry out Doppler imaging (DI) using atomic
lines as well as Titanium Oxide (TiO) band profiles of PW And (K2 V) and also
investigate chromospheric activity indicators for the first time in the
literature, using the high-resolution spectra. The surface spot distribution
obtained from the inversion process represents both atomic line and TiO-band
profiles quite accurately. The chromospheric emission is also correlated with
photospheric spot coverage, except during a possible flare event during the
observations. We detect frequent flare activity, using TESS photometry. We also
introduce a new open-source, Python-based DI code SpotDIPy that allows
performing surface reconstructions of single stars using the maximum entropy
method. We test the code by comparing surface reconstruction simulations with
the extensively used DoTS code. We show that the surface brightness
distribution maps reconstructed via both codes using the same simulated data
are consistent with each other.Comment: Accepted for publication by the Astrophysical Journa
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
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
Gas and seismicity within the Istanbul seismic gap
Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain
High-K volcanism in the Afyon region, western Turkey: from Si-oversaturated to Si-undersaturated volcanism
Volcanic rocks of the Afyon province (eastern
part of western Anatolia) make up a multistage potassic and ultrapotassic alkaline series dated from 14 to 12 Ma. The early-stage Si-oversaturated volcanic rocks around the Afyon city and further southward are trachyandesitic volcanic activity (14.23 ± 0.09 Ma). Late-stage Si-undersaturated volcanism in the southernmost part of the Afyon volcanic province took place in three episodes inferred from their stratigraphic relationships and ages. Melilite–
leucitites (11.50 ± 0.03 Ma), spotted rachyandesites, tephryphonolites and lamproites (11.91 ± 0.13 Ma) formed in the first episode; trachyandesites in the second episode and finally phonotephrites, phonolite, basaltic trachyandesites and nosean-bearing trachyandesites during the last episode.
The parameter Q [normative q-(ne + lc + kls + ol)] of western Anatolia volcanism clearly decreased southward with time becoming zero in the time interval 10–15 Ma.
The magmatism experienced a sudden change in the extent of Si saturation after 14 Ma, during late-stage volcanic activity of Afyon volcanic province at around 12 Ma, though there was some coexistence of Si-oversaturated and Si-undersaturated magmas during the whole life of Afyon volcanic province
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
Presacral and intrathoracic extramedullary hematopoiesis: a case report.
Extramedullary hematopoiesis (EMH) refers to the location of hematopoietic elements in locations other than the bone marrow and peripheral blood. It may be seen as a compensatory condition in many hematological conditions including thalassemia, or it may occur as an incidental finding. Intrathoracic EMH commonly develops in the posteroinferior mediastinum. Presacral EMH is an extremely rare condition, and there are a limited number of case reports published in the literature. We are reporting a case of EMH that involves the posterior mediastinum and presacral region on computed tomography and magnetic resonance imaging in a patient with thalassemia intermedia
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