The mean magnetic field modulus of Ap stars

Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.; Manfroid, Jean

doi:10.1051/aas:1997103

Cited by 242 publications

(270 citation statements)

References 43 publications

(96 reference statements)

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“…Our results are in excellent agreement with the literature, illustrating the well-known peak around log P = 0.4 days (compare, for example, Fig. 7 of RM09) and the magnitude range of the variations (Mathys & Manfroid 1985). It is worthwhile pointing out, though, that the observed sharp decrease in numbers below a V magnitude range <0.03 mag is at least partially due to observational bias, since the ASAS-3 measurement uncertainties approach this value for the fainter objects, thus preventing the detection of very low-amplitude variables among the fainter stars.…”

Section: Statistical Analysessupporting

confidence: 92%

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

Bernhard

Hümmerich

Otero

et al. 2015

A&A

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Context. The (magnetic) chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of magnetic fields on the stellar surface because they produce abundance inhomogeneities (spots), which results in photometric variability that is explained in terms of the oblique rotator model. CP stars exhibiting this phenomenon are normally classified as α 2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. An increased sample size will contribute to the understanding of the CP stars' evolution in time. Aims. We aim at discovering new ACV variables in the public data of the third phase of the All Sky Automated Survey (ASAS-3). Furthermore, by analysis of the available photometric data, we intend to derive rotational periods of the stars. Methods. The ASAS-3 data were cross-correlated with the Catalogue of Ap, HgMn, and Am stars in order to analyse the light curves of bona fide CP and related stars. The light curves were downloaded and cleaned of outliers and data points with a flag indicating bad quality. Promising candidates showing a larger scatter than observed for constant stars in the corresponding magnitude range were searched for periodic signals using a standard Fourier technique. Objects exhibiting periodic signals well above the noise level were considered and visually inspected, whereas borderline cases were rejected. Results. In total, we found 323 variables, from which 246 are reported here for the first time, and 77 were probably wrongly classified before. The observed variability pattern of most stars is in accordance with an ACV classification. For some cases, it is difficult to distinguish between the light curves of double-waved ACVs and the variability induced by orbital motion (ellipsoidal variables/eclipsing variables), especially for objects exhibiting very small amplitudes and/or significant scatter in their light curves. Thus, some eclipsing or rotating ellipsoidal variables might be present. However, we are confident that the given periods are the correct ones. There seems to be a possible weak correlation between the rotational period and colour, in the sense that cooler magnetic CP stars rotate more slowly. However, this correlation seems to disappear when correcting for the interstellar reddening. Conclusions. The next steps have to include a compilation of all available rotational periods from the literature and a detailed investigation of the astrophysical parameters of these stars. This includes a determination of the individual masses, luminosities, ages, and inclination angles. However, this information cannot be straightforwardly determined from photometric data alone.

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Section: Statistical Analysessupporting

confidence: 92%

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

Bernhard

Hümmerich

Otero

et al. 2015

A&A

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“…Although very strong magnetic fields have been measured for a handful of lower mass ApBp stars (e.g. 34 kG Babcock's star; see Mathys et al 1997), NGC 1624-2 has by far the strongest magnetic field ever measured on an O-type star: all the others have B pole < 2.5 kG. According to the properties derived by W12a, summarised here in Table 1, NGC 1624-2's magnetosphere is estimated to extend to RA ≈ 11R hence trapping 95% of the outflowing wind, much more than other magnetic O stars which have RA of just a few stellar radii.…”

Section: Stellar Properties Of Ngc 1624-2mentioning

confidence: 99%

X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2

Petit

Cohen

Wade

et al. 2015

Mon. Not. R. Astron. Soc.

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We observed NGC 1624-2, the O-type star with the largest known magnetic field (B p ∼ 20 kG), in X-rays with the ACIS-S camera onboard the Chandra X-ray Observatory. Our two observations were obtained at the minimum and maximum of the periodic Hα emission cycle, corresponding to the rotational phases where the magnetic field is the closest to equator-on and pole-on, respectively. With these observations, we aim to characterise the star's magnetosphere via the X-ray emission produced by magnetically confined wind shocks. Our main findings are: (i) The observed spectrum of NGC 1624-2 is hard, similar to the magnetic O-type star θ 1 Ori C, with only a few photons detected below 0.8 keV. The emergent X-ray flux is 30% lower at the Hα minimum phase. (ii) Our modelling indicated that this seemingly hard spectrum is in fact a consequence of relatively soft intrinsic emission, similar to other magnetic Of?p stars, combined with a large amount of local absorption (∼1-3×10 22 cm −2 ). This combination is necessary to reproduce both the prominent Mg and Si spectral features, and the lack of flux at low energies. NGC 1624-2 is intrinsically luminous in X-rays (log L em X ∼33.4) but 70-95% of the X-ray emission produced by magnetically confined wind shocks is absorbed before it escapes the magnetosphere (log L ISMcor X ∼32.5). (iii) The high X-ray luminosity, its variation with stellar rotation, and its large attenuation are all consistent with a large dynamical magnetosphere with magnetically confined wind shocks.

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“…The former method, exploited by Mathys et al (1997), gives the average of the magnetic field modulus over the visible stellar disk. Zeeman splitting may be detected only under quite special circumstances, i.e., v e sin i at most a few km s −1 , and field strength at least 2 kG, that are not met in most of the known magnetic Ap stars.…”

Section: Magnetic Fieldmentioning

confidence: 99%

Searching for links between magnetic fields and stellar evolution

Landstreet¹,

Bagnulo²,

Andretta³

et al. 2007

A&A

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177

249

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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 eff , L/L , and M/M . 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 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 × 10 7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10 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.

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The mean magnetic field modulus of Ap stars

Cited by 242 publications

References 43 publications

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2

Searching for links between magnetic fields and stellar evolution

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