2,964 research outputs found
Magnetic Structure of Rapidly Rotating FK Comae-Type Coronae
We present a three-dimensional simulation of the corona of an FK Com-type
rapidly rotating G giant using a magnetohydrodynamic model that was originally
developed for the solar corona in order to capture the more realistic,
non-potential coronal structure. We drive the simulation with surface maps for
the radial magnetic field obtained from a stellar dynamo model of the FK Com
system. This enables us to obtain the coronal structure for different field
topologies representing different periods of time. We find that the corona of
such an FK Com-like star, including the large scale coronal loops, is dominated
by a strong toroidal component of the magnetic field. This is a result of part
of the field being dragged by the radial outflow, while the other part remains
attached to the rapidly rotating stellar surface. This tangling of the magnetic
field,in addition to a reduction in the radial flow component, leads to a
flattening of the gas density profile with distance in the inner part of the
corona. The three-dimensional simulation provides a global view of the coronal
structure. Some aspects of the results, such as the toroidal wrapping of the
magnetic field, should also be applicable to coronae on fast rotators in
general, which our study shows can be considerably different from the
well-studied and well-observed solar corona. Studying the global structure of
such coronae should also lead to a better understanding of their related
stellar processes, such as flares and coronal mass ejections, and in
particular, should lead to an improved understanding of mass and angular
momentum loss from such systems.Comment: Accepted to ApJ, 10 pages, 6 figure
Measuring differential rotation of the K-giant \,And
We investigate the temporal spot evolution of the K-giant component in the RS
CVn-type binary system \,Andromedae to establish its surface
differential rotation. Doppler imaging is used to study three slightly
overlapping spectroscopic datasets, obtained independently at three different
observing sites. Each dataset covers one full stellar rotation with good phase
coverage, and in total, results in a continuous coverage of almost three
stellar rotations (17.8\,d). Therefore, these data are well
suited for reconstructing surface temperature maps and studying temporal
evolution in spot configurations. Surface differential rotation is measured by
the means of cross-correlation of all the possible image pairs. The individual
Doppler reconstructions well agree in the revealed spot pattern, recovering
numerous low latitude spots with temperature contrasts of up to
1000\,K with respect to the unspotted photosphere, and also an
asymmetric polar cap which is diminishing with time. Our detailed
cross-correlation study consistently indicate solar-type differential rotation
with an average surface shear , in agreement with former
results.Comment: accepted for publication in A&A, 4 pages, 3 figure
A search for flares and mass ejections on young late-type stars in the open cluster Blanco-1
We present a search for stellar activity (flares and mass ejections) in a
sample of 28 stars in the young open cluster Blanco-1. We use optical spectra
obtained with ESO's VIMOS multi-object spectrograph installed on the VLT. From
the total observing time of 5 hours, we find four H flares but
no distinct indication of coronal mass ejections (CMEs) on the investigated
dK-dM stars. Two flares show "dips" in their light-curves right before their
impulsive phases which are similar to previous discoveries in photometric
light-curves of active dMe stars. We estimate an upper limit of 4 CMEs per
day per star and discuss this result with respect to a semi- empirical
estimation of the CME rate of main-sequence stars. We find that we should have
detected at least one CME per star with a mass of 1-15 g
depending on the star's X-ray luminosity, but the estimated H fluxes
associated with these masses are below the detection limit of our observations.
We conclude that the parameter which mainly influences the detection of stellar
CMEs using the method of Doppler-shifted emission caused by moving plasma is
not the spectral resolution or velocity but the flux or mass of the CME.Comment: Accepted for publication in MNRAS, accepted 2014 June 10, received
2014 June 5, in original form 2014 March 24, 14 pages, 5 figure
Investigating magnetic activity in very stable stellar magnetic fields: long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Peg
The ultrafast-rotating () fully convective
single M4 dwarf V374 Peg is a well-known laboratory for studying intense
stellar activity in a stable magnetic topology. As an observable proxy for the
stellar magnetic field, we study the stability of the light curve, and thus the
spot configuration. We also measure the occurrence rate of flares and coronal
mass ejections (CMEs). We analyse spectroscopic observations,
photometry covering 5 years, and additional photometry that expands the
temporal base over 16 years. The light curve suggests an almost rigid-body
rotation, and a spot configuration that is stable over about 16 years,
confirming the previous indications of a very stable magnetic field. We
observed small changes on a nightly timescale, and frequent flaring, including
a possible sympathetic flare. The strongest flares seem to be more concentrated
around the phase where the light curve indicates a smaller active region.
Spectral data suggest a complex CME with falling-back and re-ejected material,
with a maximal projected velocity of 675km/s. We observed a CME rate
much lower than expected from extrapolations of the solar flare-CME relation to
active stars.Comment: 15 figures, 4 tables, accepted for publication in A&
MurtodifferentiaaliyhtÀlöiden numeerinen ratkaiseminen
TiivistelmÀ. MurtodifferentiaaliyhtÀlö on differentiaaliyhtÀlö, jossa esiintyy derivaattoja, joiden kertaluku ei ole kokonaisluku. TÀmÀn kaltaisilla yhtÀlöillÀ voidaan mallintaa ilmiöitÀ, joissa esiintyy muistia. TÀllöin ilmiön tuleva kÀyttÀytyminen riippuu sen hetkisen tilanteen lisÀksi myös aikaisemmasta kÀyttÀytymisestÀ. IlmiöitÀ, joissa esiintyy muistia, on mahdollista löytÀÀ esimerkiksi sÀhkömagnetismin, kvanttifysiikan ja lÀÀketieteen aloilta.
TÀssÀ työssÀ tutustutaan murtodifferentiaaliyhtÀlöiden perusteisiin niin teorian kuin kÀytÀnnönkin osalta. Teorian osalta esitellÀÀn tunnetuimmat mÀÀritelmÀt ja työkalut, jotka ovat myös vÀlttÀmÀttömiÀ numeeristen laskenta-algoritmien ymmÀrtÀmisessÀ. MurtodifferentiaaliyhtÀlöitÀ kÀsiteltÀessÀ murtoderivaatan ja -integraalin mÀÀritelmÀt ovat keskeisiÀ. NÀitÀ mÀÀritelmiÀ on useita erilaisia ja tÀssÀ tutkielmassa lÀhdetÀÀn liikkeelle Riemann-Liouvillen esitystavoista. KÀytÀnnön sovellusten kannalta, ja sen vuoksi myös tÀmÀn työn keskeisin kÀsitelty mÀÀritelmÀ, on kuitenkin Caputon derivaatta, joka mahdollistaa luonnollisen tavan kÀsitellÀ alkuarvotehtÀviÀ.
KÀytÀnnön osuus tÀssÀ työssÀ alustetaan tutustumalla muutamiin eri tapoihin ratkaista tavallisten differentiaaliyhtÀlöiden alkuarvotehtÀviÀ. NÀmÀ menetelmÀt toimivat johdantona ja vertailukohtana työssÀ esiteltÀviin murtodifferentiaaliyhtÀlöiden numeerisiin ratkaisumenetelmiin, jotka johdetaan ja perustellaan yksityiskohtaisesti. RatkaisumenetelmÀt tÀhÀn työhön on valikoitu siten, ettÀ niiden teoreettinen perusteleminen ei vaadi kohtuutonta mÀÀrÀÀ taustatietoja, mutta kuitenkin niin, ettÀ ne ovat kÀyttökelpoisia ja perustuvat kohtuullisen tuoreeseen tutkimukseen. Kannattaa kuitenkin huomata, ettÀ numeerisia ratkaisutekniikoita murtodifferentiaaliyhtÀlöille on paljon muitakin kuin tÀssÀ työssÀ esitetyt tavat. Vertailtaessa numeerisia ratkaisutekniikoita tavallisille differentiaaliyhtÀlöille ja murtodifferentiaaliyhtÀlöille havaittiin, ettÀ muisti-ilmiö on lÀsnÀ myös siellÀ ja kasvattaa tarvittavien laskutoimitusten mÀÀrÀÀ laskennan edetessÀ kauemmaksi lÀhtötilanteesta.
TÀssÀ tutkielmassa esiteltyjen menetelmien suoriutumista testataan muutamalla eri malliongelmalla ja suoriutumista mitataan tarkkuuden lisÀksi myös nopeudella. Laskenta-algoritmit on ohjelmoitu osana tÀtÀ työtÀ ja ne ovat mukana työn liitteinÀ. NÀiden algoritmien avulla tÀssÀ työssÀ tutkitaan menetelmien suorituskykyÀ niin tarkkuuden kuin nopeudenkin osalta. Menetelmien vertailu ja tulosten analysointi muodostavat keskeisen osan tÀtÀ työtÀ. Vertailussa kÀytetÀÀn neljÀÀ erilaista menetelmÀÀ eri askelpituuksilla ja menetelmiÀ sovelletaan useisiin erityyppisiin testiongelmiin. Saatujen tulosten perusteella voitiin tehdÀ muutamia erityisen tÀrkeitÀ havaintoja. EnsimmÀinen merkittÀvÀ havainto oli ja on, ettÀ murtodifferentiaaliyhtÀlön kertaluvun kasvattaminen parantaa ratkaisevasti menetelmien tarkkuutta. Toinen tÀrkeÀ havainto tulosten perusteella oli, ettÀ pienillÀ muutoksilla algoritmeissa voidaan saavuttaa merkittÀviÀ eroja nopeudessa. Kolmantena keskeisenÀ havaintona voitaneen pitÀÀ sitÀ, ettÀ menetelmien suppenemisnopeudet noudattivat varsin hyvin teoreettista mallia.Numerical methods for solving fractional differential equations. Abstract. A fractional differential equation is a differential equation involving derivatives with a non-integer order. These kind of equations can be useful when modelling phenomena with memory. If the system has a memory effect, not only the present situation affects to the later behaviour, but the history of the system as well.
We familiarize ourselves with the basics of the theory and practise of the fractional differential equations in this work. The most known definitions and tools, that are also crucial for understanding the numerical algorithms, are presented. The most important definition presented, for this work and when dealing with the real life applications, is the Caputo derivative. The Caputo derivative allows to use natural initial conditions which is necessary when solving initial value problems with fractional derivatives.
As an introduction to practise and numerical analysis of fractional differential equations, a couple of numerical methods for initial value problems of ordinary differential equations are given. These methods can be compared with and lead to the numerical methods for solving fractional differential equations presented in this work. These methods are justified with a detailed manner and they are applied to few model problems. The speed and precision of the methods for each of the problems were measured and the results are published in this work. The algorithms for the methods have been programmed as a part of this work and the source codes of these routines can be found as attachments
Modeling affirmative and negated action processing in the brain with lexical and compositional semantic models
Recent work shows that distributional semantic models can be used to decode patterns of brain activity associated with individual words and sentence meanings. However, it is yet unclear to what extent such models can be used to study and ecode fMRI patterns associated with specific aspects of semantic composition such as the negation function. In this paper, we apply lexical and compositional semantic models to decode fMRI patterns associated with negated and affirmative sentences containing hand-action verbs. Our results show reduced decoding (correlation) of sentences where the verb is in the negated context, as compared to the affirmative one, within brain regions implicated in action-semantic processing. This supports behavioral and brain imaging studies, suggesting that negation involves reduced access to aspects of the affirmative mental representation. The results pave the way for testing alternate semantic models of negation against human semantic processing in the brain
Comparing apples and oranges: assessment of the relative video quality in the presence of different types of distortions
<p>Abstract</p> <p>Video quality assessment is essential for the performance analysis of visual communication applications. Objective metrics can be used for estimating the relative quality differences, but they typically give reliable results only if the compared videos contain similar types of quality distortion. However, video compression typically produces different kinds of visual artifacts than transmission errors. In this article, we focus on a novel subjective quality assessment method that is suitable for comparing different types of quality distortions. The proposed method has been used to evaluate how well different objective quality metrics estimate the relative subjective quality levels for content with different types of quality distortions. Our conclusion is that none of the studied objective metrics works reliably for assessing the co-impact of compression artifacts and transmission errors on the subjective quality. Nevertheless, we have observed that the objective metrics' tendency to either over- or underestimate the perceived impact of transmission errors has a high correlation with the spatial and temporal activity levels of the content. Therefore, our results can be useful for improving the performance of objective metrics in the presence of both source and channel distortions.</p
Program planning decisions in an external degree program: A case study
Some adult educators pursue a "grand" program planning model (Pennington & Green, 1976; Sork & Caffarella, 1990). Others question the applicability of any standard set of principles and procedures to all learning contexts (Cervero & Wilson, 1991; Brookfield, 1988). The purpose of this study was to explore this issue through a qualitative case study in which "planning in action" is documented in a single program and compared to models to determine which, if any of the models account for the processes and actions recorded.The target population of the study was university administrators, governmental administrators, continuing educators, faculty, and students. The context of the study is a university sponsored, external graduate program involving an institution of higher education, a governmental agency, and their respective subgroups.Data were collected through a qualitative process including interviews, participant observation, and document analysis. A modified dialectic hermeneutic process was followed to clarify issues and themes. Data were analyzed using a constant comparative method to develop a rich description of the case.Analysis led to a rich description of the case in both an historical and a present context. Four main themes emerged from the data, appropriateness of the use of institutional resources in the program, issues of program ownership and control, issues of quality, and elements directly related to program planning decisions.The study found that no single model nor any classification of models accounts for the myriad program planning activities and issues in this case. For example, individuals were proactive developers in contrast to the responsive role depicted in models. Developers used relationships and networks to build support and gather resources. Each of the stakeholders shared considerable decision making power. The process included the design of a reward structure for instructors and the sponsoring institution. The learner emphasis espoused by many models was not evident in this case.These findings suggest that program planning is an art rather than a science. Therefore, attempts to define a single, ideal process are not advised. Rather, the field should seek expanded understanding of the variety of approaches and processes that lead to effective program development
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