Rotational modulation in TESS B stars Balona, L A; Handler, G; Chowdhury, S ...
Monthly notices of the Royal Astronomical Society,
05/2019, Volume:
485, Issue:
3
Journal Article
Peer reviewed
Open access
Abstract
Light curves and periodograms of 160 B stars observed by the Transiting Exoplanet Survey Satellite (TESS) space mission and 29 main-sequence B stars from Kepler and K2 were used to classify ...the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 per cent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.
ABSTRACT
Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into centrifugal magnetospheres (CMs) by rotational support of the corotating plasma. ...The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in H α – the principal diagnostic of these structures – are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with H α emission originating in a CM are (1) rapid rotators, (2) strongly magnetic, and (3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.
Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement ...(which sets the Alfvén radius R
A) and stellar rotation (which sets the Kepler co-rotation radius R
K) provides a useful organization of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with T
eff 16 kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of R
K versus R
A. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with R
A > R
K, versus dynamical magnetospheres (DM), with R
K > R
A. We show how key observational diagnostics, like the presence and characteristics of Hα emission, depend on a star's position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with Hα emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall time-scale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown time-scales. Finally, we discuss how other observational diagnostics, e.g. variability of UV wind lines or X-ray emission, relate to the inferred magnetic properties of these stars, and summarize prospects for future developments in our understanding of massive-star magnetospheres.
Translating the unique characteristics of individual single-walled carbon nanotubes into macroscopic materials such as fibres and sheets has been hindered by ineffective assembly. Fluid-phase ...assembly is particularly attractive, but the ability to dissolve nanotubes in solvents has eluded researchers for over a decade. Here, we show that single-walled nanotubes form true thermodynamic solutions in superacids, and report the full phase diagram, allowing the rational design of fluid-phase assembly processes. Single-walled nanotubes dissolve spontaneously in chlorosulphonic acid at weight concentrations of up to 0.5 wt%, 1,000 times higher than previously reported in other acids. At higher concentrations, they form liquid-crystal phases that can be readily processed into fibres and sheets of controlled morphology. These results lay the foundation for bottom-up assembly of nanotubes and nanorods into functional materials.
ABSTRACT
The time evolution of angular momentum and surface rotation of massive stars are strongly influenced by fossil magnetic fields via magnetic braking. We present a new module containing a ...simple, comprehensive implementation of such a field at the surface of a massive star within the Modules for Experiments in Stellar Astrophysics (mesa) software instrument. We test two limiting scenarios for magnetic braking: distributing the angular momentum loss throughout the star in the first case, and restricting the angular momentum loss to a surface reservoir in the second case. We perform a systematic investigation of the rotational evolution using a grid of OB star models with surface magnetic fields (M⋆ = 5–60 M⊙, Ω/Ωcrit = 0.2–1.0, Bp = 1–20 kG). We then employ a representative grid of B-type star models (M⋆ = 5, 10, 15 M⊙, Ω/Ωcrit = 0.2, 0.5, 0.8, Bp = 1, 3, 10, 30 kG) to compare to the results of a recent self-consistent analysis of the sample of known magnetic B-type stars. We infer that magnetic massive stars arrive at the zero-age main sequence (ZAMS) with a range of rotation rates, rather than with one common value. In particular, some stars are required to have close-to-critical rotation at the ZAMS. However, magnetic braking yields surface rotation rates converging to a common low value, making it difficult to infer the initial rotation rates of evolved, slowly rotating stars.
In this paper we present an analysis of the first high-resolution full Stokes vector spectropolarimetric observations of the active M dwarf AD Leo. Based on observations collected in 2016 with the ...ESPaDOnS instrument at CFHT, we derived the least-squares deconvolved Stokes profiles and detected linear polarization signatures in spectral lines. At the same time, we discovered that the circular polarisation profiles corresponding to our data set are significantly weaker compared to all archival spectra of AD Leo, which exhibited approximately constant profiles over the time-scale of at least 6 yr until 2012. Magnetic maps obtained using Zeeman Doppler imaging confirm the sudden change in the surface magnetic field. Although the total magnetic field energy decreased by about 20 per cent between 2012 and 2016, the field component responsible for the observed circular polarization signatures corresponds to a stronger field occupying a smaller fraction of the stellar surface in the more recent map. These results represent the first evidence that active M dwarfs with dipole-dominated axisymmetric field topologies can undergo a long-term global magnetic variation.
Previous observations of the Ap star HD 32633 indicated that its magnetic field was unusually complex in nature and could not be characterized by a simple dipolar structure. Here we derive magnetic ...field maps and chemical abundance distributions for this star using full Stokes vector (Stokes IQUV) high-resolution observations obtained with the ESPaDOnS and Narval spectropolarimeters. Our maps, produced using the invers10 magnetic Doppler imaging (MDI) code, show that HD 32633 has a strong magnetic field which features two large regions of opposite polarity but deviates significantly from a pure dipole field. We use a spherical harmonic expansion to characterize the magnetic field and find that the harmonic energy is predominately in the ℓ = 1 and 2 poloidal modes with a small toroidal component. At the same time, we demonstrate that the observed Stokes parameter profiles of HD 32633 cannot be fully described by either a dipolar or dipolar plus quadrupolar field geometry. We compare the magnetic field topology of HD 32633 with other early-type stars for which MDI analyses have been performed, supporting a trend of increasing field complexity with stellar mass. We then compare the magnetic field topology of HD 32633 with derived chemical abundance maps for the elements Mg, Si, Ti, Cr, Fe, Ni and Nd. We find that the iron-peak elements show similar distributions, but we are unable to find a clear correlation between the location of local chemical enhancements or depletions and the magnetic field structure.
ABSTRACT
HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H α ...emission. We have analysed the Transiting Exoplanet Survery Satellite (TESS) light curve, and find that it is a β Cep pulsator with 11 detectable frequencies, 4 of which are independent p-modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of ∼years; significant differences in the frequencies determined from TESS and RV data sets are consistent with a light-time effect from orbital motion. Close examination of the star’s spectrum reveals the presence of a spectroscopic companion, however, as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of least-squares deconvolution profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with 〈Bz〉 about +1.5 kG as compared to 〈Bz〉 ∼−0.8 kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H α emission appears to originate around HD 156424B. Using H α, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.
ABSTRACT
In this paper we describe and evaluate new spectral line polarization observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete ...or partial Stokes IQUV sequences, corresponding to 297 individual polarized spectra, have been obtained for seven bright Ap stars using the Échelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) and NARVAL high‐resolution spectropolarimeters. The targets span a range of masses from approximately 1.8 to 3.4 M⊙, a range of rotation periods from 2.56 to 6.80 d and a range of maximum longitudinal magnetic field strengths from 0.3 to over 4 kG. For three of the seven stars, we have obtained dense phase coverage sampling the entire rotational cycle. These data sets are suitable for immediate magnetic and chemical abundance surface mapping using magnetic Doppler imaging. For the remaining four stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal‐to‐noise ratio of the reduced observations is over 700 per 1.8 km s−1 pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal‐to‐noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50–100σ level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. In addition to providing the foundation for the next generation of surface maps of Ap stars, this study establishes the performance and stability of the ESPaDOnS and NARVAL high‐resolution spectropolarimeters during the period 2006–2010.
We present our analysis of HD 35502 based on high- and medium-resolution spectropolarimetric observations. Our results indicate that the magnetic B5IVsnp star is the primary component of a ...spectroscopic triple system and that it has an effective temperature of 18.4 ± 0.6 kK, a mass of 5.7 ± 0.6 M⊙, and a polar radius of
$3.0^{+1.1}_{-0.5}\,{\rm R}_{\odot }$
. The two secondary components are found to be essentially identical A-type stars for which we derive effective temperatures (8.9 ± 0.3 kK), masses (2.1 ± 0.2 M⊙), and radii (2.1 ± 0.4 R⊙). We infer a hierarchical orbital configuration for the system in which the secondary components form a tight binary with an orbital period of 5.668 66(6) d that orbits the primary component with a period of over 40 yr. Least-Squares Deconvolution profiles reveal Zeeman signatures in Stokes V indicative of a longitudinal magnetic field produced by the B star ranging from approximately −4 to 0 kG with a median uncertainty of 0.4 kG. These measurements, along with the line variability produced by strong emission in Hα, are used to derive a rotational period of 0.853 807(3) d. We find that the measured v sin i = 75 ± 5 km s−1 of the B star then implies an inclination angle of the star's rotation axis to the line of sight of
$24^{+6}_{-10}{}^\circ$
. Assuming the Oblique Rotator Model, we derive the magnetic field strength of the B star's dipolar component (
$14^{+9}_{-3}\,{\rm kG}$
) and its obliquity (
$63\pm 13\deg$
). Furthermore, we demonstrate that the calculated Alfvén radius (
$41^{+17}_{-6}R_\ast$
) and Kepler radius (
$2.1^{+0.4}_{-0.7}R_\ast$
) place HD 35502's central B star well within the regime of centrifugal magnetosphere-hosting stars.
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