Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userNitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs
222
0
0.0
(
0
)
Authors
T. Morel
Ask ChatGPT about the research
No Arabic abstract
Evolutionary models for massive stars, accounting for rotational mixing effects, do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. Contrary to theoretical expectations, we present a detailed and fully-homogeneous, NLTE abundance analysis of 20 early B-type dwarfs and (sub)giants that reveals the existence of a population of nitrogen-rich and boron-depleted, yet intrinsically slowly-rotating objects. The low-rotation rate of several of these stars is firmly established, either from the occurrence of phase-locked UV wind line-profile variations, which can be ascribed to rotational modulation, or from theoretical modelling in the pulsating variables. The observational data presently available suggest a higher incidence of chemical peculiarities in stars with a (weak) detected magnetic field. This opens the possibility that magnetic phenomena are important in altering the photospheric abundances of early B dwarfs, even for surface field strengths at the one hundred Gauss level. However, further spectropolarimetric observations are needed to assess the validity of this hypothesis.
rate research
Read More
We present results of a search for identification of modes responsible for the six most significant frequency peaks detected in the rapidly rotating SPB star $mu$ Eridani. All published and some unpublished photometric data are used in our new analysis. The mode identification is carried out with the method developed by Daszynska-Daszkiewicz et al. employing the phases and amplitudes from multi-band photometric data and relying on the traditional approximation for the treatment of oscillations in rotating stars. Models consistent with the observed mean parameters are considered. For the five frequency peaks, the candidates for the identifications are searched amongst unstable modes. In the case of the third frequency, which is an exact multiple of the orbital frequency, this condition is relaxed. The systematic search is continued up to a harmonic degree $ell =6$. Determination of the angular numbers, $(ell,m)$, is done simultaneously with the rotation rate, $V_{rm rot}$, and the inclination angle, $i$, constrained by the spectroscopic data on the projected rotational velocity, $V_{rm rot}sin i$, which is assumed constant. All the peaks may be accounted for with g-modes of high radial orders and the degrees $ellle 6$. There are differences in some identifications between the models. For the two lowest--amplitude peaks the identifications are not unique. Nonetheless, the equatorial velocity is constrained to a narrow range of (135, 140) km/s. Our work presents the first application of the photometric method of mode identification in the framework of the traditional approximation and we believe that it opens a new promising direction in studies of SPB stars.
We report the latest set of spectropolarimetric observations of the magnetic $beta$ Cep star $xi^1$ CMa. The new observations confirm the long-period model of Shultz et al. (2017), who proposed a rotational period of about 30 years and predicted that in 2018 the star should pass through a magnetic null. In perfect agreement with this projection, all longitudinal magnetic field $langle B_z rangle$ measurements are close to 0 G. Remarkably, individual Stokes $V$ profiles all display a crossover signature, which is consistent with $langle B_z rangle sim 0$ but is {em not} expected when $vsin{i} sim 0$. The crossover signatures furthermore exhibit pulsationally modulated amplitude and sign variations. We show that these unexpected phenomena can all be explained by a `radial crossover effect related to the stars radial pulsations, together with an important deviation of the global field topology from a purely dipolar structure, which we explore via a dipole+quadrupole configuration as the simplest non-dipolar field.
It is thought that magnetic fields must be present in the interiors of stars to resolve certain discrepancies between theory and observation (e.g. angular momentum transport), but such fields are difficult to detect and characterise. Asteroseismology is a powerful technique for inferring the internal structures of stars by measuring their oscillation frequencies, and succeeds particularly with evolved stars, owing to their mixed modes, which are sensitive to the deep interior. The goal of this work is to present a phenomenological study of the combined effects of rotation and magnetism in evolved stars, where both are assumed weak enough that first-order perturbation theory applies, and we focus on the regime where Coriolis and Lorentz forces are comparable. Axisymmetric twisted-torus field configurations are used, which are confined to the core and allowed to be misaligned with respect to the rotation axis. Factors such as the field radius, topology and obliquity are examined. We observe that fields with finer-scale radial structure and/or smaller radial extent produce smaller contributions to the frequency shift. The interplay of rotation and magnetism is shown to be complex: we demonstrate that it is possible for nearly symmetric multiplets of apparently low multiplicity to arise even under a substantial field, which might falsely appear to rule out its presence. Our results suggest that proper modelling of rotation and magnetism, in a simultaneous fashion, may be required to draw robust conclusions about the existence/non-existence of a core magnetic field in any given object.
We study the evidence for a diversity of formation processes in early-type galaxies by presenting the first complete volume-limited sample of slow rotators with both integral-field kinematics from the ATLAS3D Project and high spatial resolution photometry from the Hubble Space Telescope. Analysing the nuclear surface brightness profiles of 12 newly imaged slow rotators, we classify their light profiles as core-less, and place an upper limit to the core size of ~10 pc. Considering the full magnitude and volume-limited ATLAS3D sample, we correlate the presence or lack of cores with stellar kinematics, including the proxy for the stellar angular momentum and the velocity dispersion within one half-light radius, stellar mass, stellar age, $alpha$-element abundance, and age and metallicity gradients. More than half of the slow rotators have core-less light profiles, and they are all less massive than $10^{11}$ Msun. Core-less slow rotators show evidence for counter-rotating flattened structures, have steeper metallicity gradients, and a larger dispersion of gradient values than core slow rotators. Our results suggest that core and core-less slow rotators have different assembly processes, where the former are the relics of massive dissipation-less merging in the presence of central supermassive black holes. Formation processes of core-less slow rotators are consistent with accretion of counter-rotating gas or gas-rich mergers of special orbital configurations, which lower the final net angular momentum of stars, but support star formation. We also highlight core fast rotators as galaxies that share properties of core slow rotators and core-less slow rotators. Formation processes similar to those for core-less slow rotators can be invoked to explain the assembly of core fast rotators, with the distinction that these processes form or preserve cores.[Abridged]
The powerful radiative winds of hot stars with strong magnetic fields are magnetically confined into large, corotating magnetospheres, which exert important influences on stellar evolution via rotational spindown and mass-loss quenching. They are detectable via diagnostics across the electromagnetic spectrum. Since the fossil magnetic fields of early-type stars are stable over long timescales, and the ion source is internal and isotropic, hot star magnetospheres are also remarkably stable. This stability, the relative ease with which they can be studied at multiple wavelengths, and the growing population of such objects, makes them powerful laboratories for plasma astrophysics. The magnetospheres of the magnetic early B-type stars stand out for being detectable in every one of the available diagnostics. In this contribution I review the basic methods by which surface magnetic fields are constrained; the theoretical tools that have been developed in order to reveal the key physical processes governing hot star magnetospheres; and some important recent results and open-ended questions regarding the properties of surface magnetic fields and the behaviour of magnetospheric plasma.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
