scholarly journals Rotational modulation and single g-mode pulsation in the B9pSi star HD 174356?

2020 ◽  
Vol 498 (1) ◽  
pp. 548-564
Author(s):  
Z Mikulášek ◽  
E Paunzen ◽  
S Hümmerich ◽  
E Niemczura ◽  
P Walczak ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Light Curve ◽  
Main Sequence ◽  
Chemical Elements ◽  
Single Star ◽  
Rotational Modulation ◽  
Global Magnetic Field ◽  
Periodic Signals ◽  
Rotating Star ◽  
Light Variability

ABSTRACT Chemically peculiar (CP) stars of the upper main sequence are characterized by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve variable in both amplitude and shape, which is not expected in a CP2 star. Employing archival and new photometric and spectroscopic observations, we carry out a detailed abundance analysis of HD 174356 and discuss its photometric and astrophysical properties in detail. We employ phenomenological modelling to decompose the light curve and the observed radial velocity variability. Our abundance analysis confirms that HD 174356 is a silicon-type CP2 star. No magnetic field stronger than 110 G was found. The star’s light curve can be interpreted as the sum of two independent strictly periodic signals with $P_1=4{_{.}^{\rm d}}043\, 55(5)$ and $P_2=2{_{.}^{\rm d}}111\, 69(3)$. The periods have remained stable over 17 yr of observations. In all spectra, HD 174356 appears to be single-lined. From the simulation of the variability characteristics and investigation of stars in the close angular vicinity, we put forth the hypothesis that the peculiar light variability of HD 174356 arises in a single star and is caused by rotational modulation due to surface abundance patches (P1) and g-mode pulsation (P2).

scholarly journals MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’

2019 ◽  
Vol 490 (3) ◽  
pp. 4154-4165 ◽  
Author(s):  
M E Shultz ◽  
C Johnston ◽  
J Labadie-Bartz ◽  
V Petit ◽  
A David-Uraz ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Light Curve ◽  
Line Profile ◽  
Longitudinal Magnetic Field ◽  
Stellar Structure ◽  
Line Profiles ◽  
Dipole Strength ◽  
Rotational Modulation ◽  
Brightness Modulation

ABSTRACT HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M⊙. The primary’s longitudinal magnetic field 〈Bz〉 varies between about +100 and −250 G, suggesting a surface magnetic dipole strength Bd = 850 G. Bayesian analysis of the Stokes V profiles indicates Bd = 650 G for the primary and Bd < 110 G for the secondary. The primary’s line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that star’s chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent with the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete.

scholarly journals The NGC 346 massive star census

2020 ◽  
Vol 634 ◽  
pp. A6 ◽  
Author(s):  
P. L. Dufton ◽  
C. J. Evans ◽  
D. J. Lennon ◽  
I. Hunter
Keyword(s):  
Magnetic Field ◽  
Massive Star ◽  
Main Sequence ◽  
Large Magellanic Cloud ◽  
Magellanic Cloud ◽  
Evolutionary Models ◽  
Qualitative Comparison ◽  
Single Star ◽  
Early Type Stars ◽  
The Magnetic Field

Previous analyses of two large spectroscopic surveys of early-type stars in the Large Magellanic Cloud (LMC) have found an excess of nitrogen enriched B-type targets with a ve sin i ≤ 40 km s−1 compared with the predictions of single star evolutionary models that incorporate rotational mixing. By contrast, the number of such targets with 40 <  ve sin i ≤ 80 km s−1 was consistent with such models. We have undertaken a similar analysis for 61 B-type targets which lie towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). These again have projected rotational velocities, ve sin i ≤ 80 km s−1, are not classified as supergiants, and are apparently single. Approximately 65% of these SMC targets could have nitrogen enhancements of less than 0.3 dex, which is consistent with them having experienced only small amounts of mixing due to their low rotational velocities. However, as with the previous LMC surveys, an excess of stars with low projected rotational velocities, ve sin i ≤ 40 km s−1, and significant nitrogen enrichments is found. This is estimated to be approximately 5% of the total population of apparently single B-type stars or 40% of all stars with current rotational velocities of less than 40 km s−1; these percentages are similar to those found previously for the two LMC samples. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and that it is indeed possible for all the single stars with current rotational velocities of less than 40 km s−1 to be nitrogen enriched. Two possible explanations incorporate the effects of the magnetic field, via either a stellar merger followed by magnetic braking or the evolution of a single star with a large magnetic field. Both mechanisms would appear to be compatible with the observed frequency of nitrogen-enriched stars in the Magellanic Clouds. Differences in the properties of the nitrogen-enriched stars compared with the remainder of the sample would be consistent with the former mechanism. For the latter, a qualitative comparison with Galactic evolutionary models that incorporate magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the zero-age main sequence.

scholarly journals Using Chandra/LETG Spectra to Probe Stellar Coronae

2004 ◽  
Vol 219 ◽  
pp. 301-305
Author(s):  
Gaitee A. J. Hussain ◽  
Nancy Brickhouse ◽  
A. K. Dupree ◽  
Adriaan A. van Ballegooijen ◽  
Andrew Collier Cameron ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Light Curves ◽  
High Latitudes ◽  
Intrinsic Variability ◽  
Period Analysis ◽  
X Ray ◽  
Rotational Modulation ◽  
Rotating Star ◽  
The Relationship ◽  
Coronal Field

We probe the relationship between surface magnetic fields and the X-ray emitting corona in the rapidly rotating star, AB Dor. Circularly polarised spectra have been inverted to produce a surface (photospheric) magnetic field map. This surface map has been extrapolated to model AB Dor's coronal field topology and X-ray light curve. Chandra/LETG light curves of AB Dor from the same epoch show intrinsic variability at the 30% level. Period analysis indicates a fraction of this is due to rotational modulation. We measure velocity shifts in emission line centroids as a function of Prot and find evidence of rotational modulation (max. vel. ∼ 40 ± 13km s—1). This modulation may indicate the presence of a localised X-ray emitting region at mid-to-high latitudes.

The interaction of rotation and magnetic field in the Solar System

1984 ◽  
Vol 313 (1524) ◽  
pp. 19-25 ◽  
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Main Sequence ◽  
Young Star ◽  
Convective Envelope ◽  
T Tauri ◽  
Rotating Star ◽  
Braking Process ◽  
Young Star Clusters ◽  
Early Phases

During the early phases of star formation, torsional Alfven waves propagating along the locally distorted galactic magnetic field can transport away the bulk of the initial angular momentum of a condensation, so enabling it to contract to solar nebula dimensions. Enough primeval magnetic flux may be retained for magnetic redistribution of angular momentum to continue until the proto-Sun reaches the pre-Main Sequence Hayashi phase. A rotating star with a convective envelope has a dynamomaintained field which brakes the star through coupling to the stellar wind. Observational evidence from young star clusters and from T Tauri stars suggests that the zero-age Main Sequence Sun had about ten times its present angular momentum. The same braking process, scaled up because of the much more powerful T Tauri winds, can explain why the zero-age Sun had lost at least nine-tenths of the centrifugal upper limit, and is more acceptable than the suggestion that the ‘missing’ solar angular momentum has been magnetically fed into the planetary system.

scholarly journals The complex fossil magnetic field of the δ Scuti star HD 41641

2020 ◽  
Vol 500 (2) ◽  
pp. 1992-1999
Author(s):  
K Thomson-Paressant ◽  
C Neiner ◽  
K Zwintz ◽  
A Escorza
Keyword(s):  
Magnetic Field ◽  
Light Curve ◽  
Field Structure ◽  
Complex Field ◽  
Rotational Modulation ◽  
Scuti Star ◽  
Scuti Stars ◽  
Dipolar Structure ◽  
Δ Scuti Stars

ABSTRACT Only three magnetic δ Scuti stars are known as of today. HD 41641 is a δ Scuti star showing chemical peculiarities and rotational modulation of its light curve, making it a good magnetic candidate. We acquired spectropolarimetric observations of this star with NARVAL at Télescope Bernard Lyot (TBL) to search for the presence of a magnetic field and characterize it. We indeed clearly detect a magnetic field in HD 41641, making it the fourth known magnetic δ Scuti star. Our analysis shows that the field is of fossil origin, like magnetic OBA stars, but with a complex field structure rather than the much more usual dipolar structure.

scholarly journals The effects of surface fossil magnetic fields on massive star evolution: III. The case of τ Sco

2021 ◽  
Author(s):  
Z Keszthelyi ◽  
G Meynet ◽  
F Martins ◽  
A de Koter ◽  
A David-Uraz
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Massive Star ◽  
Slow Rotation ◽  
Single Star ◽  
Surface Magnetic Field ◽  
Fundamental Parameters ◽  
Chemical Enrichment ◽  
Star Models ◽  
Nitrogen Excess

Abstract τ Sco, a well-studied magnetic B-type star in the Uτer Sco association, has a number of surprising characteristics. It rotates very slowly and shows nitrogen excess. Its surface magnetic field is much more complex than a purely dipolar configuration which is unusual for a magnetic massive star. We employ the cmfgen radiative transfer code to determine the fundamental parameters and surface CNO and helium abundances. Then, we employ mesa and genec stellar evolution models accounting for the effects of surface magnetic fields. To reconcile τ Sco’s properties with single-star models, an increase is necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in the efficiency of magnetic braking by a factor of 10. The spin down could be explained by assuming a magnetic field decay scenario. However, the simultaneous chemical enrichment challenges the single-star scenario. Previous works indeed suggested a stellar merger origin for τ Sco. However, the merger scenario also faces similar challenges as our magnetic single-star models to explain τ Sco’s simultaneous slow rotation and nitrogen excess. In conclusion, the single-star channel seems less likely and versatile to explain these discrepancies, while the merger scenario and other potential binary-evolution channels still require further assessment as to whether they may self-consistently explain the observables of τ Sco.

scholarly journals Broadband Linear Polarization in Ap Stars

1993 ◽  
Vol 138 ◽  
pp. 305-309
Author(s):  
Marco Landolfi ◽  
Egidio Landi Degl’Innocenti ◽  
Maurizio Landi Degl’Innocenti ◽  
Jean-Louis Leroy ◽  
Stefano Bagnulo
Keyword(s):  
Magnetic Field ◽  
Circular Polarization ◽  
Linear Polarization ◽  
Rotation Axis ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Long Time ◽  
Rotating Star ◽  
Ap Stars ◽  
The Magnetic Field

AbstractBroadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

scholarly journals On the origin of variable structures in the winds of hot luminous stars

2013 ◽  
Vol 440 (1) ◽  
pp. 2-9 ◽  
Author(s):  
Yannick J. L. Michaux ◽  
Anthony F. J. Moffat ◽  
André-Nicolas Chené ◽  
Nicole St-Louis
Keyword(s):  
Magnetic Field ◽  
Empirical Evidence ◽  
Temporal Variability ◽  
Large Scale ◽  
Small Scale ◽  
Ionization Zone ◽  
Corotating Interaction Regions ◽  
Wind Variability ◽  
Global Magnetic Field ◽  
Interaction Regions

Abstract Examination of the temporal variability properties of several strong optical recombination lines in a large sample of Galactic Wolf–Rayet (WR) stars reveals possible trends, especially in the more homogeneous WC than the diverse WN subtypes, of increasing wind variability with cooler subtypes. This could imply that a serious contender for the driver of the variations is stochastic, magnetic subsurface convection associated with the 170 kK partial-ionization zone of iron, which should occupy a deeper and larger zone of greater mass in cooler WR subtypes. This empirical evidence suggests that the heretofore proposed ubiquitous driver of wind variability, radiative instabilities, may not be the only mechanism playing a role in the stochastic multiple small-scaled structures seen in the winds of hot luminous stars. In addition to small-scale stochastic behaviour, subsurface convection guided by a global magnetic field with localized emerging loops may also be at the origin of the large-scale corotating interaction regions as seen frequently in O stars and occasionally in the winds of their descendant WR stars.

scholarly journals Advanced test of the model stellar atmospheres: the nature of the light variability of magnetic chemically peculiar stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 347-348
Author(s):  
J. Krtička ◽  
Z. Mikulášek ◽  
J. Zverko ◽  
J. Žižňovský ◽  
P. Zvěřina
Keyword(s):  
Chemical Elements ◽  
Horizontal Distribution ◽  
Stellar Atmospheres ◽  
Silicon Iron ◽  
Surface Distribution ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Distribution Of Elements ◽  
Chemically Peculiar Stars ◽  
Light Variability

AbstractThe magnetic chemically peculiar stars exhibit both inhomogeneous horizontal distribution of chemical elements on their surfaces and the light variability. We show that the observed light variability of these stars can be successfully simulated using models of their stellar atmospheres and adopting the observed surface distribution of elements. The most important elements that influence the light variability are silicon, iron, and helium.

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