Global magnetic fields of early-type stars are commonly characterised by the mean longitudinal magnetic field 〈 B z 〉 and the mean field modulus 〈 B 〉, derived from the circular polarisation and ...intensity spectra, respectively. Observational studies often report a root mean square (rms) of 〈 B z 〉 and an average value of 〈 B 〉. In this work, I used numerical simulations to establish statistical relationships between these cumulative magnetic observables and the polar strength, B d , of a dipolar magnetic field. I show that in the limit of many measurements randomly distributed in rotational phase, 〈 B z 〉 rms = 0.179 −0.043 +0.031 B d and 〈 B 〉 avg = 0.691 −0.023 +0.020 B d . The same values can be recovered with only three measurements, provided that the observations are distributed uniformly in the rotational phase. These conversion factors are suitable for ensemble analyses of large stellar samples, where each target is covered by a small number of magnetic measurements.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context. Doppler imaging (DI) is a powerful spectroscopic inversion technique that enables conversion of a line profile time series into a two-dimensional map of the stellar surface inhomogeneities. ...DI has been repeatedly applied to reconstruct chemical spot topologies of magnetic Ap/Bp stars with the goal of understanding variability of these objects and gaining an insight into the physical processes responsible for spot formation. Aims. In this paper we investigate the accuracy of chemical abundance DI and assess the impact of several different systematic errors on the reconstructed spot maps. Methods. We have simulated spectroscopic observational data for two different Fe spot distributions with a surface abundance contrast of 1.5 dex in the presence of a moderately strong dipolar magnetic field. We then reconstructed chemical maps using different sets of spectral lines and making different assumptions about line formation in the inversion calculations. Results. Our numerical experiments demonstrate that a modern DI code successfully recovers the input chemical spot distributions comprised of multiple circular spots at different latitudes or an element overabundance belt at the magnetic equator. For the optimal reconstruction based on half a dozen spectral intervals, the average reconstruction errors do not exceed ~0.10 dex. The errors increase to about 0.15 dex when abundance distributions are recovered from a few and/or blended spectral lines. Ignoring a 2.5 kG dipolar magnetic field in chemical abundance DI leads to an average relative error of 0.2 dex and maximum errors of 0.3 dex. Similar errors are encountered if a DI inversion is carried out neglecting a non-uniform continuum brightness distribution and variation of the local atmospheric structure. None of the considered systematic effects lead to major spurious features in the recovered abundance maps. Conclusions. This series of numerical DI simulations proves that inversions based on one or two spectral lines with simplifying assumptions of the non-magnetic radiative transfer and a single model atmosphere are generally reliable provided that the stellar magnetic field is not much stronger than 2-3 kG and the recovered spot map has a contrast of at least ~0.3 dex. In the light of these findings, we assess magnetic field strengths of several dozen Ap/Bp stars previously studied with DI methods, concluding that the vast majority of the published chemical spot maps are unaffected by the systematic errors addressed in this paper.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Hidden magnetic fields of young suns Kochukhov, O.; Hackman, T.; Lehtinen, J. J. ...
Astronomy and astrophysics (Berlin),
03/2020, Volume:
635
Journal Article
Peer reviewed
Open access
Global magnetic fields of active solar-like stars are, nowadays, routinely detected with spectropolarimetric measurements and are mapped with Zeeman Doppler imaging (ZDI). However, due to the ...cancellation of opposite field polarities, polarimetry only captures a tiny fraction of the magnetic flux and cannot assess the overall stellar surface magnetic field if it is dominated by a small-scale component. The analysis of Zeeman broadening in high-resolution intensity spectra can reveal these hidden complex magnetic fields. Historically, there were very few attempts to obtain such measurements for G dwarf stars due to the difficulty of disentangling the Zeeman effect from other broadening mechanisms affecting spectral lines. Here, we developed a new magnetic field diagnostic method based on relative Zeeman intensification of optical atomic lines with different magnetic sensitivity. By using this technique, we obtained 78 field strength measurements for 15 Sun-like stars, including some of the best-studied young solar twins. We find that the average magnetic field strength
B
f
drops from 1.3−2.0 kG in stars younger than about 120 Myr to 0.2−0.8 kG in older stars. The mean field strength shows a clear correlation with the Rossby number and with the coronal and chromospheric emission indicators. Our results suggest that magnetic regions have roughly the same local field strength
B
≈ 3.2 kG in all stars, with the filling factor
f
of these regions systematically increasing with stellar activity. In comparing our results with the spectropolarimetric analyses of global magnetic fields in the same stars, we find that ZDI recovers about 1% of the total magnetic field energy in the most active stars. This figure drops to just 0.01% for the least active targets.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context. The brightest magnetic chemically peculiar stars θ Aur and ε UMa were targeted by numerous studies of their photometric and spectroscopic variability. Detailed maps of chemical abundance ...spots were repeatedly derived for both stars. However, owing to the weakness of their surface magnetic fields, very little information on the magnetic field geometries of these stars is available. Aims. In this study we aim to determine detailed magnetic field topologies of θ Aur and ε UMa based on modern, high-resolution spectropolarimetric observations. Methods. Both targets were observed in all four Stokes parameters using the Narval and ESPaDOnS spectropolarimeters. A multi-line technique of least-squares deconvolution was employed to detect polarisation signatures in spectral lines. These signatures were modelled with a Zeeman-Doppler imaging code. Results. We succeed in detecting variable circular and linear polarisation signatures for θ Aur. Only circular polarisation was detected for ε UMa. We obtain new sets of high-precision longitudinal magnetic field measurements using mean circular polarisation metal line profiles as well as hydrogen line cores, which are consistent with historical data. Magnetic inversions revealed distorted dipolar geometries in both stars. The Fe and Cr abundance distributions, reconstructed simultaneously with magnetic mapping, do not show a clear correlation with the local magnetic field properties, with the exception of a relative element underabundance in the horizontal field regions along the magnetic equators. Conclusions. Our study provides the first ever detailed surface magnetic field maps for broad-line, weak-field chemically peculiar stars, showing that their field topologies are qualitatively similar to those found in stronger field stars. The Fe and Cr chemical abundance maps reconstructed for θ Aur and ε UMa are at odds with the predictions of current theoretical atomic diffusion calculations.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the Magnetism in Massive Stars (MiMeS) Survey. Mean least-squares deconvolved Stokes I ...and V line profiles were extracted for each observation, from which we measured the radial velocity, rotational and non-rotational broadening velocities, and longitudinal magnetic field B sub( l). The investigation of the Stokes I profiles led to the discovery of two new multiline spectroscopic systems (HD 46106, HD 204827) and confirmed the presence of a suspected companion in HD 37041. We present a modified strategy of the least-squares deconvolution technique aimed at optimizing the detection of magnetic signatures while minimizing the detection of spurious signatures in Stokes V. Using this analysis, we confirm the detection of a magnetic field in six targets previously reported as magnetic by the MiMeS collaboration (HD 108, HD 47129A2, HD 57682, HD 148937, CPD-28 2561, and NGC 1624-2), as well as report the presence of signal in Stokes V in three new magnetic candidates (HD 36486, HD 162978, and HD 199579). Overall, we find a magnetic incidence rate of 7 plus or minus 3 per cent, for 108 individual O stars (including all O-type components part of multiline systems), with a median uncertainty of the B sub( l) measurements of about 50 G. An inspection of the data reveals no obvious biases affecting the incidence rate or the preference for detecting magnetic signatures in the magnetic stars. Similar to A- and B-type stars, we find no link between the stars' physical properties (e.g. T sub( eff), mass, and age) and the presence of a magnetic field. However, the Of?p stars represent a distinct class of magnetic O-type stars.
Context.
M dwarfs are key targets for high-resolution spectroscopy and model atmosphere analyses because of the high incidence of these stars in the solar neighbourhood and their importance as ...exoplanetary hosts. Several methodological challenges make such analyses difficult, leading to significant discrepancies in the published results.
Aims.
The aim of our work is to compare M dwarf parameters derived by recent high-resolution near-infrared studies with each other and with fundamental stellar parameters. We also assess to what extent deviations from local thermodynamic equilibrium (LTE) for iron and potassium influence the outcome of these studies.
Methods.
We carry out line formation calculations based on a modern model atmosphere grid appropriate for M dwarfs along with a synthetic spectrum synthesis code that treats formation of atomic and molecular lines in cool-star atmospheres including departures from LTE. We use near-infrared spectra collected with the CRIRES instrument at the ESO VLT as reference observational data.
Results.
We find that the effective temperatures obtained with spectroscopic techniques in different studies mostly agree to better than 100 K and are mostly consistent with the fundamental temperatures derived from interferometric radii and bolometric fluxes. At the same time, much worse agreement in the surface gravities and metallicities is evident. Significant discrepancies in the latter parameters appear when results of the studies based on the optical and near-infrared observations are intercompared. We demonstrate that non-LTE effects are negligible for Fe
I
in M-dwarf atmospheres but are important for K
I
, which has a number of strong lines in the near-infrared spectra of these stars. These effects, leading to potassium abundance and metallicity corrections on the order of 0.2 dex, may be responsible for some of the discrepancies in the published analyses. Differences in the temperature–pressure structures of the atmospheric models may be another factor contributing to the deviations between the spectroscopic studies, in particular at low metallicities and high effective temperatures.
Conclusions.
High-resolution spectroscopic studies of M dwarfs are yet to reach the level of consistency and reproducibility typical of similar investigations of FGK stars. Attention should be given to details of the line formation physics as well as input atomic and molecular data. Collecting high-quality spectra with a wide wavelength coverage of M dwarfs with known fundamental parameters is an essential step in benchmarking spectroscopic parameter determination of low-mass stars.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Information about stellar magnetic field topologies is obtained primarily from high-resolution circular polarisation (Stokes V) observations. Because of their generally complex morphologies, the ...stellar Stokes V profiles are usually interpreted with elaborate inversion techniques such as Zeeman Doppler imaging (ZDI). Here we further develop a new method for interpreting circular polarisation signatures in spectral lines using cumulative Stokes V profiles (anti-derivative of Stokes V). This method is complimentary to ZDI and can be applied to validate the inversion results or when the available observational data are insufficient for an inversion. Based on the rigorous treatment of polarised line formation in the weak-field regime, we show that, for rapidly rotating stars, the cumulative Stokes V profiles contain information about the spatially resolved longitudinal magnetic field density. Rotational modulation of these profiles can be employed for a simple, qualitative characterisation of the stellar magnetic field topologies. We apply this diagnostic method to the archival observations of the weak-line T Tauri star V410 Tau and Bp He-strong star HD 37776. We show that the magnetic field in V410 Tau is dominated by an azimuthal component, in agreement with the ZDI map that we recover from the same data set. For HD 37776 the cumulative Stokes V profile variation indicates the presence of multiple regions of positive and negative field polarity. This behaviour agrees with the ZDI results, but contradicts the popular hypothesis that the magnetic field of this star is dominated by an axisymmetric quadrupolar component.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.The photospheres of about 5–10% of the upper main sequence stars exhibit remarkable chemical anomalies. Many of these chemically peculiar (CP) stars have a global magnetic field, the origin ...of which is still a matter of debate. Aims.We present a comprehensive statistical investigation of the evolution of magnetic CP stars, aimed at providing constraints to the theories that deal with the origin of the magnetic field in these stars. Methods.We have collected from the literature data for 150 magnetic CP stars with accurate Hipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1 observations of circularly polarized spectra for 100 stars. From these spectra we have measured the mean longitudinal magnetic field, and discovered 48 new magnetic CP stars (five of which belonging to the rare class of rapidly oscillating Ap stars). We have determined effective temperature and luminosity, then mass and position in the H-R diagram for a final sample of 194 magnetic CP stars. Results.We found that magnetic stars with $M > 3 ~M_\odot$ are homogeneously distributed along the main sequence. Instead, there are statistical indications that lower mass stars (especially those with $M \le 2~M_\odot$) tend to concentrate in the centre of the main sequence band. We show that this inhomogeneous age distribution cannot be attributed to the effects of random errors and small number statistics. Our data suggest also that the surface magnetic flux of CP stars increases with stellar age and mass, and correlates with the rotation period. For stars with $M > 3~M_\odot$, rotation periods decrease with age in a way consistent with the conservation of the angular momentum, while for less massive magnetic CP stars an angular momentum loss cannot be ruled out. Conclusions.The mechanism that originates and sustains the magnetic field in the upper main sequence stars may be different in CP stars of different mass.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Magnetic fields are ubiquitous in active cool stars, but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular ...polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1-3.5 times higher. The overall complexity is also increased as the field energy is shifted toward higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the large-scale field component.
ABSTRACT
The primary component of the multiple star φ Dra is one of the brightest magnetic chemically peculiar stars in the northern sky. Here, we report results of a comprehensive study of the ...rotational photometric variability, binarity, magnetic field geometry, and surface chemical spot structure for this star. We derived a precise photometric rotational period of 1.71650213(21) d based on 1 yr of TESS nearly continuous space observations and discovered modulation of the stellar light curve with the phase of the 127.9-d binary orbit due to the light time travel effect. We revised parameters of the binary orbit and detected spectroscopic contribution of the secondary. A tomographic mapping technique was applied to the average intensity and circular polarization profiles derived from Narval high-resolution spectropolarimetric observations. This analysis yielded a detailed map of the global magnetic field topology together with the surface distributions of Si, Cr, and Fe abundances. Magnetic mapping demonstrates that the surface field structure of φ Dra is dominated by a distorted dipolar component with a peak field strength of 1.4 kG and a large asymmetry between the poles. Chemical maps show an enhancement of Cr, Fe and, to a lesser extent, Si in a series of spots encircling intersections of the magnetic and rotational equators. These chemical spot geometries do not directly correlate with either the local field strength or the field inclination.
PDF
