arXiv subjects

M. E. Shultz

Publications and source records attributed to M. E. Shultz.

40 records · Page 3Linked to original sources

arXiv2019

Detection of coherent emission from the Bp star HD 142990 at uGMRT frequencies

HD 142990 is a Bp-type star with a nearly dipolar surface magnetic field of kG strength. Recently Lenc et al. (2018) reported the tentative discovery of Electron Cyclotron Maser Emission (ECME) from this star at 200 MHz. This type of emission has been observed from only three other hot magnetic stars. In this paper, we present our observations of HD 142990 with the upgraded Giant Metrewave Radio telescope (uGMRT) at 550-804 MHz and with the legacy GMRT at 1420 MHz near the rotational phases of the nulls of the longitudinal magnetic field. We found strong enhancements in flux density in both circular polarisations suggesting an ECME bandwith of at least 1.2 GHz (200-1420 MHz). In one of the observation sessions, we observed enhancements with opposite circular polarisations from the same magnetic pole. This has not been reported in any other hot magnetic star known to exhibit ECME. In order to explain this unusual finding, we suggest a scenario that involves a transition of the dominant mode of ECME between the magneto-ionic modes.

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arXiv2019

The Magnetic Early B-type Stars II: stellar atmospheric parameters in the era of Gaia

Atmospheric parameters determined via spectral modelling are unavailable for many of the known magnetic early B-type stars. We utilized high-resolution spectra together with NLTE models to measure effective temperatures $T_{\rm eff}$ and surface gravities $\log{g}$ of stars for which these measurements are not yet available. We find good agreement between our $T_{\rm eff}$ measurements and previous results obtained both photometrically and spectroscopically. For $\log{g}$, our results are compatible with previous spectroscopic measurements; however, surface gravities of stars previously determined photometrically have been substantially revised. We furthermore find that $\log{g}$ measurements obtained with HARPSpol are typically about 0.1 dex lower than those from comparable instruments. Luminosities were determined using Gaia Data Release 2 parallaxes. We find Gaia parallaxes to be unreliable for bright stars ($V<6$ mag) and for binaries; in these cases we reverted to Hipparcos parallaxes. In general we find luminosities systematically lower than those previously reported. Comparison of $\log{g}$ and $\log{L}$ to available rotational and magnetic measurements shows no correlation between either parameter with magnetic data, but a clear slow-down in rotation with both decreasing $\log{g}$ and increasing $\log{L}$, a result compatible with the expectation that magnetic braking should lead to rapid magnetic spindown that accelerates with increasing mass-loss.

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arXiv2018

The Magnetic Early B-type Stars I: Magnetometry and Rotation

The rotational and magnetic properties of many magnetic hot stars are poorly characterized, therefore the MiMeS and BinaMIcS collaborations have collected extensive high-dispersion spectropolarimetric datasets of these targets. We present longitudinal magnetic field measurements $\langle B_{\rm z}\rangle$ for 52 early B-type stars (B5 to B0), with which we attempt to determine their rotational periods $P_{\rm rot}$. Supplemented with high-resolution spectroscopy, low-resolution DAO circular spectropolarimetry, and archival Hipparcos photometry, we determined $P_{\rm rot}$ for 10 stars, leaving only 5 stars for which $P_{\rm rot}$ could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of $P_{\rm rot}$ is very similar to that observed for the cooler Ap/Bp stars. We also measured $v\sin{i}$ and $v_{\rm mac}$ for all stars. Comparison to non-magnetic stars shows that $v\sin{i}$ is much lower for magnetic stars, an expected consequence of magnetic braking. We also find evidence that $v_{\rm mac}$ is lower for magnetic stars. LSD profiles extracted using single-element masks revealed widespread, systematic discrepancies in $\langle B_{\rm z}\rangle$ between different elements: this effect is apparent only for chemically peculiar stars, suggesting it is a consequence of chemical spots. Sinusoidal fits to H line $\langle B_{\rm z}\rangle$ measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in 6 stars for which this has not previously been reported; however, in all 6 cases the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.

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arXiv2015

The MiMeS Survey of Magnetism in Massive Stars: Introduction and overview

The MiMeS project is a large-scale, high resolution, sensitive spectropolarimetric investigation of the magnetic properties of O and early B type stars. Initiated in 2008 and completed in 2013, the project was supported by 3 Large Program allocations, as well as various programs initiated by independent PIs and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS at the Canada-France-Hawaii Telescope, Narval at the Télescope Bernard Lyot, and HARPSpol at the European Southern Observatory La Silla 3.6m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.

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