Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and ...circumstellar environment. Aims. One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. Methods. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e.g., surface abundance peculiarities, Hα emission), which are also of interest for future studies. Results. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Hα emission with respect to the theory of centrifugally-supported magnetospheres.
Recently, high-precision optical 2-min cadence light curves obtained with TESS for targets located in the mission's defined first four sectors have been released. The majority of these high-cadence ...and high-precision measurements currently span {˜ }28 d, thereby allowing periodic variability occurring on time-scales {≲ }14 d to potentially be detected. Magnetic chemically peculiar (mCP) A-type stars are well known to exhibit rotationally modulated photometric variability that is produced by inhomogeneous chemical abundance distributions in their atmospheres. While mCP stars typically exhibit rotation periods that are significantly longer than those of non-mCP stars, both populations exhibit typical periods {≲ }10 d; therefore, the early TESS releases are suitable for searching for rotational modulation of the light curves of both mCP and non-mCP stars. We present the results of our search for A-type stars that exhibit variability in their TESS light curves that is consistent with rotational modulation based on the first two data releases obtained from sectors 1 to 4. Our search yielded 134 high-probability candidate rotational variables - 60 of which have not been previously reported. Approximately half of these stars are identified in the literature as Ap (mCP) stars. Comparisons between the subsample of high-probability candidate rotationally variable Ap stars and the subsample of stars that are not identified as Ap reveal that the latter subsample exhibits statistically (i) shorter rotation periods and (ii) significantly lower photometric amplitudes.
Abstract
ι Ori is a well-studied massive binary consisting of an O9 III + B1 III/IV star. Due to its high eccentricity (e = 0.764) and short orbital period (Porb = 29.133 76 d), it has been ...considered to be a good candidate to show evidence of tidal effects; however, none have previously been identified. Using photometry from the BRIght Target Explorer (BRITE)-Constellation space photometry mission, we have confirmed the existence of tidal distortions through the presence of a heartbeat signal at periastron. We combine spectroscopic and light-curve analyses to measure the masses and radii of the components, revealing ι Ori to be the most massive heartbeat system known to date. In addition, using a thorough frequency analysis, we also report the unprecedented discovery of multiple tidally induced oscillations in an O star. The amplitudes of the pulsations allow us to empirically estimate the tidal circularization rate, yielding an effective tidal quality factor Q ∼ 4 × 104.
Context.
F-type stars are characterised by several physical processes such as different pulsation mechanisms, rotation, convection, diffusion, and magnetic fields. The rapidly rotating
δ
Scuti star
β
...Cas can be considered as a benchmark star to study the interaction of several of these effects.
Aims.
We investigate the pulsational and magnetic field properties of
β
Cas. We also determine the star’s apparent fundamental parameters and chemical abundances.
Methods.
Based on photometric time series obtained from three different space missions (BRITE-Constellation, SMEI, and TESS), we conduct a frequency analysis and investigate the stability of the pulsation amplitudes over four years of observations. We investigate the presence of a magnetic field and its properties using spectropolarimetric observations taken with the Narval instrument by applying the least-squares deconvolution and Zeeman-Doppler imaging techniques.
Results.
The star
β
Cas shows only three independent
p
-mode frequencies down to the few ppm-level; its highest amplitude frequency is suggested to be an
n
= 3,
ℓ
= 2,
m
= 0 mode. Its magnetic field structure is quite complex and almost certainly of a dynamo origin. The atmosphere of
β
Cas is slightly deficient in iron peak elements and slightly overabundant in C, O, and heavier elements.
Conclusions.
Atypically for
δ
Scuti stars, we can only detect three pulsation modes down to exceptionally low noise levels for
β
Cas. The star is also one of very few
δ
Scuti pulsators known to date to show a measurable magnetic field and the first
δ
Scuti star with a dynamo magnetic field. These characteristics make
β
Cas an interesting target for future studies of dynamo processes in the thin convective envelopes of F-type stars, the transition region between fossil and dynamo fields, and the interaction between pulsations and magnetic field.
ABSTRACT
HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period (Prot ∼ 1 d). Whilst it is clearly a rapid rotator, recent determinations of ...Prot are in formal disagreement. In this paper, we collect magnetic and photometric data with a combined 40-yr baseline in order to re-evaluate Prot and examine its stability. Both period analysis of individual data sets and O − C analysis of the photometric data demonstrate that Prot has decreased over the past 30 yr, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rate $\dot{P}$ of approximately −0.6 s yr−1, although there is some indication that $\dot{P}$ may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.
Context. The young, rapidly rotating Bp star HR 5624 (HD 133880) shows an unusually strong non-sinusoidal variability of its longitudinal magnetic field. This behaviour was previously interpreted as ...the signature of an exceptionally strong, quadrupole-dominated surface magnetic field geometry. Aims. We studied the magnetic field structure and chemical abundance distributions of HR 5624 with the aim to verify the unusual quadrupolar nature of its magnetic field and to investigate correlations between the field topology and chemical spots. Methods. We analysed high-resolution, time series Stokes parameter spectra of HR 5624 with the help of a magnetic Doppler imaging inversion code based on detailed polarised radiative transfer modelling of the line profiles. Results. We refined the stellar parameters, revised the rotational period, and obtained new longitudinal magnetic field measurements. Our magnetic Doppler inversions reveal that the field structure of HR 5624 is considerably simpler and the field strength is much lower than proposed by previous studies. We find a maximum local field strength of 12 kG and a mean field strength of 4 kG, which is about a factor of three weaker than predicted by quadrupolar field models. Our model implies that overall large-scale field topology of HR 5624 is better described as a distorted, asymmetric dipole rather than an axisymmetric quadrupole. The chemical abundance maps of Mg, Si, Ti, Cr, Fe, and Nd obtained in our study are characterised by large-scale, high-contrast abundance patterns. These structures correlate weakly with the magnetic field geometry and, in particular, show no distinct element concentrations in the horizontal field regions predicted by theoretical atomic diffusion calculations. Conclusions. We conclude that the surface magnetic field topology of HR 5624 is not as unusual as previously proposed. Considering these results together with other recent magnetic mapping analyses of early-type stars suggests that predominantly quadrupolar magnetic field topologies, invoked to be present in a significant number of stars, probably do not exist in real stars. This finding agrees with an outcome of the MHD simulations of fossil field evolution in stably stratified stellar interiors.
Abstract
In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of 19 previously known magnetic O, B, and A stars ...observed in 2-min cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS.
Context. The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known ...field Ap stars. Aims. We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands. Methods. We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T_{\rm {eff}}, L/L_\odot, and M/M_\odot. With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars. Results. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_\odot the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3\times10 super(7) yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10 super(8) yr. Conclusions. Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise rms fields, will help to clarify the results obtained so far. Further field observations are in progress.
ABSTRACT
We present a spectropolarimetric analysis of the hot star V352 Peg. We have acquired 18 spectropolarimetric observations of the star with ESPaDOnS at the CFHT between 2018 and 2019 and ...completed our data set with one archival ESPaDOnS measurement obtained in 2011. Our analysis of the spectra shows that the star is on the main sequence and chemically peculiar, i.e. it is a Bp star, with overabundances of iron peak elements (Ti, Cr, and Fe) and underabundance of He and O. Through a Least-Square Deconvolution of each spectrum, we extracted the mean Zeeman signature and mean line profile of thousands of spectral lines and detected a magnetic field in V352 Peg. By modelling the Stokes I and V profiles and using the Oblique Rotator Model, we determined the geometrical configuration of V352 Peg. We also performed Zeeman–Doppler Imaging (ZDI) to provide a more detailed characterization of the magnetic field of V352 Peg and its surface chemical distributions. We find a magnetic field that is mainly dipolar, dominantly poloidal, and largely non-axisymmetric with a dipole field strength of ∼9 kG and a magnetic axis almost perpendicular to the rotation axis. The strong variability of Stokes I profiles also suggests the presence of chemical spots at the stellar surface.
ABSTRACT
W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large data set of high-resolution spectropolarimetry. The star is a previously ...unreported spectroscopic binary, consisting of two B2 stars with a mass ratio of 1.8, masses of 12 and 6.2 M⊙, in an eccentric 110-d orbit. The magnetic field belongs to the secondary, W 601 B. The H α emission is consistent with an origin in W 601 B’s centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disc. However, the low value of log g = 3.8 determined via spectroscopic analysis and the star’s membership in the young NGC 6611 cluster are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the H α line and the longitudinal magnetic field 〈Bz〉 is 1.13 d. Modelling of Stokes V and 〈Bz〉 indicates a surface dipolar magnetic field Bd between 6 and 11 kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to H α-bright magnetic B-type stars such as σ Ori E. By contrast, the primary is an apparently non-magnetic (Bd < 300 G) pre-main-sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.
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