bluuebunny/crossref_metadata_2025_split huggingface dataset:
Do Magnetic Fields Actually Inflate Low-Mass Stars?
Authors: Feiden Gregory A. , Chaboyer Brian 
Published: August 2014

AbstractMagnetic fields have been hypothesized to inflate the radii of low-mass stars—defined as less than 0.8 M⊙–in detached eclipsing binaries (DEBs). We evaluate this hypothesis using the magnetic Dartmouth stellar evolution code. Results suggest that magnetic suppression of thermal convection can inflate low-mass stars that possess a radiative core and convective outer envelope. A scaling relation between X-ray luminosity and surface magnetic flux indicates that model surface magnetic field strength predictions are consistent with observations. This supports the notion that magnetic fields may be inflating these stars. However, magnetic models are unable to reproduce radii of fully convective stars in DEBs. Instead, we propose that model discrepancies below the fully convective boundary are related to metallicity.
A new spectropolarimeter for the San Pedro Martir National Observatory
Authors: Iñiguez-Garín E. , Hiriart D. , Ramirez-Velez J. , Núñez-Alfonso J. M. , Herrera J. , Castro-Chacón J. 
Published: August 2014

AbstractWe present the design of a stellar spectropolarimeter to measure the magnetic field of point sources. The polarization module interfaces the Boller and Chivens (B&Ch) intermediate-low resolution (R ~ 500 − 4000) spectrograph to the 2.1-m telescope of the San Pedro Martir National Astronomical Observatory in Mexico. The module uses a Savart plate to split the beam into two orthogonal states of polarization and a quarter (half) waveplate to measure circular (linear) polarization. The module is mounted to the telescope and it feeds the spectrograph through a set of four fibers, two for the polarized star images and two for the spectrograph calibration lamp. The instrument will be capable of measuring polarization in spectral lines to determine the longitudinal and transversal fields in magnetic stars.
Age, Activity and Rotation in Mid and Late-Type M Dwarfs from MEarth
Authors: West Andrew A. , Weisenburger Kolby L. , Irwin Jonathan , Charbonneau David , Dittmann Jason , Berta-Thompson Zachory K. 
Published: August 2014

AbstractUsing spectroscopic observations and photometric light curves of 280 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by Hα emission), rotation period, and stellar age (derived from three-dimensional space velocities). Although we have known for decades that a large fraction of mid-late-type M dwarfs are magnetically active, it was not clear what role rotation played in the magnetic field generation (and subsequent chromospheric heating). Previous attempts to investigate the relationship between magnetic activity and rotation in mid-late-type M dwarfs were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that vsini measurements only probe rapid rotation). However, the photometric data from the MEarth survey allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from less than one to over 100 days). Over all M spectral types we find that magnetic activity decreases with longer rotation periods, including late-type, fully convective M dwarfs. We find that the most magnetically active (and hence, most rapidly rotating) stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.
On the reliability of measuring differential rotation of spotted stars
Authors: Kővári Zsolt , Bartus János , Kriskovics Levente , Vida Krisztián , Oláh Katalin 
Published: August 2014

AbstractCross-correlation of consecutive Doppler images is one of the most common techniques used to detect surface differential rotation (hereafter DR) on spotted stars. The disadvantage of a single cross-correlation is, however, that the expected DR pattern can be overwhelmed by sudden changes in the apparent spot configuration. Another way to reconstruct the image shear using Doppler imaging is to include a predefined latitude-dependent rotation law in the inversion code (‘sheared image method’). However, special but not unusual spot distributions, such like a large polar cap or an equatorial belt (e.g., small random spots evenly distributed along the equator), can distort the rotation profile similarly as the DR does, consequently, yielding incorrect measure of the DR from the sheared image method. To avoid these problems, the technique of measuring DR from averaged cross-correlations using time-series Doppler images (‘ACCORD’) is introduced and the reliability of this tool is demonstrated on artificial data.
Stellar Magnetism in the Era of Space-Based Precision Photometry
Authors: Walkowicz Lucianne M. 
Published: August 2014

AbstractThe advent of precision space-based photometric missions such as MOST, CoRoT and Kepler has revealed stellar magnetic activity in unprecedented detail. These observations enable new investigations into the fundamental nature of stellar magnetism by furthering our understanding of the stellar rotation and differential rotation that generate the field, and the photometric variability caused by the surface manifestations of the field. In the case of stars with planetary candidates, these data also offer synergy between studies of stars and planets. Here, I review the possibilities and challenges for deepening our understanding of magnetism in solar-like stars in the era of space-based precision photometry.
The new age of spotted star research using Kepler and CHARA
Authors: Roettenbacher Rachael M. , Monnier John D. , Harmon Robert O. , Korhonen Heidi H. 
Published: August 2014

AbstractWith the precise, nearly-continuous photometry from the Kepler satellite and the sub-milliarcsecond resolving capabilities of the CHARA Array, astronomy is entering a new age for the imaging and understanding of stellar magnetic activity. We present first results from our Guest Observer Program, where 180 single-epoch surface image reconstructions of KIC 5110407 have revealed differential rotation and hints of magnetic activity cycles based on both spot and flare variations. Analysis of our larger, full dataset will establish in unprecedented detail how surface magnetic activity correlates with stellar age and spectral type. In addition to Kepler work, we have harnessed the power of the world's largest infrared interferometer to “directly” image the spotted surfaces of a few of the closest RS CVn systems, allowing a comparison of contemporaneous Doppler and light-curve inversion imaging techniques.
Detecting activity cycles of late-type dwarfs in Kepler data
Authors: Vida Krisztián , Oláh Katalin 
Published: August 2014

AbstractUsing data of fast-rotating active dwarf stars in the Kepler database, we perform time-frequency analysis of the light curves in order to search for signs of activity cycles. We use the phenomenon that the active region latitudes vary with the cycle (like the solar butterfly diagram), which causes the observed rotation period to change as a consequence of differential rotation. We find cycles in 8 cases of the 39 promising targets with periods between of 300–900 days.
Planetary protection in the extreme environments of low-mass stars
Authors: Vidotto A. A. , Jardine M. , Morin J. , Donati J.-F. , Lang P. , Russell A. J. B. 
Published: August 2014

AbstractRecent results showed that the magnetic field of M-dwarf (dM) stars, currently the main targets in searches for terrestrial planets, is very different from the solar one, both in topology as well as in intensity. In particular, the magnetised environment surrounding a planet orbiting in the habitable zone (HZ) of dM stars can differ substantially to the one encountered around the Earth. These extreme magnetic fields can compress planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. Using observed surface magnetic maps for a sample of 15 dM stars, we investigate the minimum degree of planetary magnetospheric compression caused by the intense stellar magnetic fields. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (~1 G) orbiting at the inner (outer) edge of the HZ of these stars would present magnetospheres that extend at most up to 6.1 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, the terrestrial planet would either need to orbit significantly farther out than the traditional limits of the HZ; or else, if it were orbiting within the life-bearing region, it would require a minimum magnetic field ranging from a few G to up to a few thousand G.
Planets spinning up their host stars: a twist on the age-activity relationship
Authors: Poppenhaeger K. , Wolk S. J. 
Published: August 2014

AbstractIt is a long-standing question in exoplanet research if Hot Jupiters can influence the magnetic activity of their host stars. While cool stars usually spin down with age and become inactive, an input of angular momentum through tidal interaction, as seen for example in close binaries, can preserve high activity levels over time. This may also be the case for cool stars hosting a Hot Jupiter. However, selection effects from planet detection methods often dominate the activity levels seen in samples of exoplanet host stars, and planet-induced, systematically enhanced stellar activity has not been detected unambiguously so far. We have developed an approach to identify planet-induced stellar spin-up avoiding the selection biases from planet detection, by using visual proper motion binaries in which only one of the stars possesses a Hot Jupiter. This approach immediately rids one of the ambiguities of detection biases: with two co-eval stars, the second star acts as a negative control. We present results from our ongoing observational campaign at X-ray wavelengths and in the optical, and present several outstanding systems which display significant age/activity discrepancies presumably caused by their Hot Jupiters.
Bow shocks and winds around HD 189733b
Authors: Llama J. , Vidotto A. A. , Jardine M. , Wood K. , Fares R. 
Published: August 2014

AbstractAsymmetries in exoplanet transits are a useful tool for developing our understanding of magnetic activity on both stars and planets outside our Solar System. Near-UV observations of the WASP-12 system have revealed asymmetries in the timing of the transit when compared with the optical light curve. In this proceedings we review a number of reported asymmetries and present work simulating near-UV transits for the hot-Jupiter hosting star HD 189733.
Bp stars in Orion OB1 association
Authors: Romanyuk Iosif I. , Yakunin Ilya A. 
Published: August 2014

AbstractA total of 85 CP stars of various types are identified among 814 members of the Orion OB1 association. We selected 59 Bp stars, which account for 13.4% of the total number of B type stars in the association. The fraction of peculiar B type stars in the association is found to be twice higher than that of peculiar A type stars.Magnetic field are found in 22 stars, 17 of them are objects with anomalous helium lines. No significant differences are found between the field strengths in the Bp type stars of the association and Bp type field stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied which is the standard ratio for CP stars.
Partial Paschen-Back splitting of Si ii and Si iii lines in magnetic CP stars
Authors: Khalack Viktor , Landstreet John 
Published: August 2014

AbstractA number of prominent spectral lines in the spectra of magnetic A and B main sequence stars are produced by closely spaced doublets or triplets. Depending on the strength and orientation of magnetic field, the PPB magnetic splitting can result in the Stokes I profiles of a spectral line that differ significantly from those predicted by the theory of Zeeman effect. Such lines should be treated using the theory of the partial Paschen-Back (PPB) effect. To estimate the error introduced by the use of the Zeeman approximation, numerical simulations have been performed for Si ii and Si iii lines assuming an oblique rotator model. The analysis indicates that for high precision studies of some spectral lines the PPB approach should be used if the field strength at the magnetic poles is Bp > 6-10 kG and V sin i < 15 km s−1. In the case of the Si ii line 5041 Å, the difference between the simulated PPB and Zeeman profiles is caused by a significant contribution from a so called “ghost” line. The Stokes I and V profiles of this particular line simulated in the PPB regime provide a significantly better fit to the observed profiles in the spectrum of the magnetic Ap star HD 318107 than the profiles calculated assuming the Zeeman effect.
Modeling surface magnetic fields in stars with radiative envelopes
Authors: Kochukhov Oleg 
Published: August 2014

AbstractStars with radiative envelopes, specifically the upper main sequence chemically peculiar (Ap) stars, were among the first objects outside our solar system for which surface magnetic fields have been detected. Currently magnetic Ap stars remains the only class of stars for which high-resolution measurements of both linear and circular polarization in individual spectral lines are feasible. Consequently, these stars provide unique opportunities to study the physics of polarized radiative transfer in stellar atmospheres, to analyze in detail stellar magnetic field topologies and their relation to starspots, and to test different methodologies of stellar magnetic field mapping. Here I present an overview of different approaches to modeling the surface fields in magnetic A- and B-type stars. In particular, I summarize the ongoing efforts to interpret high-resolution full Stokes vector spectra of these stars using magnetic Doppler imaging. These studies reveal an unexpected complexity of the magnetic field geometries in some Ap stars.
New Experiments with Zeeman Doppler Mapping
Authors: Martin Alex J. , Bagnulo Stefano , Stift Martin J. 
Published: August 2014

AbstractWe present recent experiments using a Levenberg-Marquardt algorithm and the polarised radiative transfer code COSSAM to produce a new ZDM code. Currently the code is able to recover the magnetic parameters of model stars with either a decentred dipole morphology or a morphology consisting of a centred dipole and a quadrupole, while simultaneously calculating multiple chemical abundances (including a basic stratification model). The ZDM code has been tested using both synthetic spectra and real, well studied stars. Additional features are currently being added such as a multipole morphology of arbitrary order and more sophisticated chemical stratification models.
Candidate Ap stars in close binary systems
Authors: Folsom C. P. , Wade G. A. , Likuski K. , Kochukhov O. , Alecian E. , Shulyak D. , Johnson N. M. 
Published: August 2014

AbstractShort period binary systems containing magnetic Ap stars are anomalously rare. This apparent anomaly may provide insight into the origin of the magnetic fields in theses stars. As an early investigation of this, we observed three close binary systems that have been proposed to host Ap stars. Two of these systems (HD 22128 and HD 56495) we find contain Am stars, but not Ap stars. However, for one system (HD 98088) we find the primary is indeed an Ap star, while the secondary is an Am star. Additionally, the Ap star is tidally locked to the secondary, and the predominately dipolar magnetic field of the Ap star is roughly aligned with the secondary. Further investigations of HD 98088 are planned by the BinaMIcS collaboration.
Pollux: a stable weak dipolar magnetic field but no planet?
Authors: Aurière Michel , Konstantinova-Antova Renada , Espagnet Olivier , Petit Pascal , Roudier Thierry , Charbonnel Corinne , Donati Jean-François , Wade Gregg A. 
Published: August 2014

AbstractPollux is considered as an archetype of a giant star hosting a planet: its radial velocity (RV) presents sinusoidal variations with a period of about 590 d, which have been stable for more than 25 years. Using ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic field at the surface of Pollux and followed up its variations with Narval during 4.25 years, i.e. more than for two periods of the RV variations. The longitudinal magnetic field is found to vary with a sinusoidal behaviour with a period close to that of the RV variations and with a small shift in phase. We then performed a Zeeman Doppler imaging (ZDI) investigation from the Stokes V and Stokes I least-squares deconvolution (LSD) profiles. A rotational period is determined, which is consistent with the period of variations of the RV. The magnetic topology is found to be mainly poloidal and this component almost purely dipolar. The mean strength of the surface magnetic field is about 0.7 G. As an alternative to the scenario in which Pollux hosts a close-in exoplanet, we suggest that the magnetic dipole of Pollux can be associated with two temperature and macroturbulent velocity spots which could be sufficient to produce the RV variations. We finally investigate the scenarii of the origin of the magnetic field which could explain the observed properties of Pollux.
Magnetic Field Structure and Activity of the He-burning Giant 37 Comae
Authors: Tsvetkova S. , Petit P. , Konstantinova-Antova R. , Aurière M. , Wade G. A. , Charbonnel C. , Drake N. A. 
Published: August 2014

AbstractWe present the first magnetic map of the late-type giant 37 Com. The Least Squares Deconvolution (LSD) method and Zeeman Doppler Imaging (ZDI) inversion technique were applied. The chromospheric activity indicators Hα, S-index, Ca ii IRT and the radial velocity were also measured. The evolutionary status of the star has been studied on the basis of state-of-the-art stellar evolutionary models and chemical abundance analysis. 37 Com appears to be in the core Helium-burning phase.
Magnetic field of the classical Cepheid η Aql: new results
Authors: Butkovskaya V. , Plachinda S. , Baklanova D. , Butkovskyi V. 
Published: August 2014

AbstractWe present the results of the spectropolarimetric study of the classical Cepheid η Aql in 2002, 2004, 2010, and 2012. The longitudinal magnetic field of η Aql was found to be variable with the pulsation cycle of 7.176726 day. The amplitude, phase, and mean value of the field vary from year to year presumably due to stellar rotation or dynamo mechanisms.
Magnetohydrodynamic equilibria in barotropic stars
Authors: Armaza C. , Reisenegger A. , Valdivia J. A. , Marchant P. 
Published: August 2014

AbstractAlthough barotropic matter does not constitute a realistic model for magnetic stars on short timescales, it would be interesting to confirm a recent conjecture that states that magnetized stars with a barotropic equation of state would be dynamically unstable (Reisenegger 2009). In this work we construct a set of barotropic equilibria, which can eventually be tested using a stability criterion. A general description of the ideal MHD equations governing these equilibria is summarized, allowing for both poloidal and toroidal magnetic field components. A new finite-difference numerical code is developed in order to solve the so-called Grad-Shafranov equation describing the equilibrium of these configurations, and some properties of the equilibria obtained are briefly discussed.
Magnetic field structures inside magnetars with strong toroidal field
Authors: Fujisawa Kotaro 
Published: August 2014

AbstractWe have analyzed the magnetized equilibrium studies with strong toroidal magnetic fields and found that the negative toroidal current density inside the star is very important for the strong toroidal magnetic fields. The strong toroidal magnetic fields require the strong poloidal current, but the strong poloidal current results in the localized strong toroidal current density in the axisymmetric system. This localized toroidal current changes the magnetic field configuration and makes the size of the toroidal magnetic field region smaller. As a result, the toroidal magnetic field energy can not become large. We need to cancel out the localized toroidal current density in order to obtain the large toroidal fields solutions. We have found and showed that the negative toroidal current cancels out the localized toroidal current density and sustain the large toroidal magnetic field energy inside the star. We can explain the magnetized equilibrium studies with strong toroidal magnetic fields systematically using the negative current density. Physical meaning of the negative current is key to the magnetar interior magnetic fields.
Effects of strong magnetic fields in dense stellar matter
Authors: Lavagno A. , Lingua F. 
Published: August 2014

AbstractWe study the effects of strong magnetic fields in dense stellar matter within an effective relativistic equation of state with the inclusion of hyperons and Δ(1232)-isobar degrees of freedom. The effects of high magnetic field interactions significantly affect the nuclear equation of state and the macroscopic properties of the star. In this framework we investigate the role of the presence of the Δ-isobars degrees of freedom in structure and in the bulk properties of the compact star.
Electron-impact resonant vibrational excitation and dissociation processes involving vibrationally excited N2 molecules
Authors: Laporta V , Little D A , Celiberto R , Tennyson J 
Published: August 2014

Abstract
Resonant vibrational excitation cross sections and the corresponding rate coefficients for electron–N2 collisions occurring through the

resonant state are reviewed. New calculations are performed using accurate potential energy curves for the N2 electronic ground state, taken from the literature, and for the

resonant state, obtained from R-matrix calculations. The calculations are extended to resonant excitation processes involving the N2 ground state vibrational continuum, leading to dissociation. Electron-impact dissociation is found to be significant from higher vibrational levels. Accurate analytical fits for the complete set of the rate coefficients are provided. The behavior of the dissociative cross sections is investigated for rotationally excited N2 molecules, with J = 50, 100 and 150, and for different vibrational levels.
Does Participation in 4‐H Improve Schooling Outcomes? Evidence from Florida
Authors: Flores‐Lagunes Alfonso , Timko Troy 
Published: August 2014

AbstractWe examine the effect that participation in 4‐H, the largest youth development program in the United States, has on standardized test scores. We do this by utilizing grade‐level longitudinal data on Florida's school districts from the Florida Department of Education, combined with 4‐H participation statistics from the Florida 4‐H. Specifically, we analyze the effect of the extent of 4‐H participation for third‐ through tenth grade on the mathematics and reading subtests of the Florida Comprehensive Assessment Test (FCAT). We use a difference‐in‐difference‐in‐differences approach to control for potential confounders of the causal relationship at the level of school districts, grades, and years. Our results indicate that the extent of 4‐H participation at the district‐grade‐year level is positively and significantly related to several measures of performance on the FCAT test. Furthermore, we find suggestive evidence that these positive effects of 4‐H participation accumulate over time.