arXiv subjects

M. Oksala

Publications and source records attributed to M. Oksala.

8 recordsLinked to original sources

arXiv2026

New Observations of the Strongly Magnetic O-star NGC 1624-2 Reveal Its Magnetic South Pole

NCG 1624-2 has the strongest detected magnetic field of all known main-sequence O-type stars. It was originally found that its magnetospheric emission lines followed a $\sim$5 month periodicity, and the existing line-of-sight magnetic measurements were predominantly of strong positive (north) polarity. As such, the field's geometric interpretation has been a mildly tilted (with respect to the rotational axis) dipole, such that only the magnetic north pole is visible during a rotation cycle. However, S. Seadrow et al. (2026) recently reported that new magnetospheric observations no longer phased with the established ephemeris and that the period had to be decreased by a few days. S. Seadrow et al. (2026) also found that existing magnetic measurements did not rule out a period twice as long (306.56 d). This period suggests a different magnetic configuration with a larger dipolar tilt, making both magnetic poles visible over a single rotation. Because previous spectropolarimetric observations did not have sufficient phase coverage to distinguish between the geometries, both were equally viable. In this paper, we present new spectropolarimetric observations obtained specifically to resolve this ambiguity. Our new magnetic measurements have a strong negative (south) polarity, confirming that the rotational period of NGC 1624-2 is indeed nearly twice as long as previously thought. Our measurements show that both poles come within a similar angle to our line of sight and likely have roughly the same local magnetic field strength (with a dipolar strength of 15-20 kG or more depending on the inclination angle).

astro-ph.SR↗

arXiv2026

New Insights from Revisiting the Rotation Period of the Strongly Magnetic O Star, NGC 1624-2

NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star's currently accepted rotation period (157.99 d) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb-Scargle periodograms produced with new and archival, multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 $\pm$ 0.42 d and 306.56 $\pm$ 1.19 d are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the Oblique Rotator Model. The 306.56 d period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star's magnetic South pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.

astro-ph.SR↗

arXiv2022

Ultraviolet Spectropolarimetry With Polstar: Using Polstar to test Magnetospheric Mass-loss Quenching

Polstar is a proposed NASA MIDEX space telescope that will provide high-resolution, simultaneous full-Stokes spectropolarimetry in the far ultraviolet, together with low-resolution linear polarimetry in the near ultraviolet. This observatory offers unprecedented capabilities to obtain unique information on the magnetic and plasma properties of the magnetospheres of hot stars. We describe an observing program making use of the known population of magnetic hot stars to test the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

astro-ph.SR↗

arXiv2021

Ultraviolet Spectropolarimetry With Polstar: Hot Star Magnetospheres

Polstar is a proposed NASA MIDEX space telescope that will provide high-resolution, simultaneous full-Stokes spectropolarimetry in the far ultraviolet, together with low-resolution linear polarimetry in the near ultraviolet. In this white paper, we describe the unprecedented capabilities this observatory would offer in order to obtain unique information on the magnetic and plasma properties of the magnetospheres of hot stars. This would enable a test of the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

astro-ph.IM↗

arXiv2018

The magnetic field of evolved hot stars

About 10% of hot stars host a fossil magnetic field on the pre-main sequence and main sequence. However, the first magnetic evolved hot stars have been discovered only recently. An observing program has been set up to find more such objects. This will allow us to test how fossil fields evolve, and the impact of magnetism on stellar evolution. Already 7 evolved magnetic hot stars are now known and the rate of magnetic discoveries in the survey suggests that they host dynamo fields in addition to fossil fields. Finally, the weakness of the measured fields is compatible at first order with simple magnetic flux conservation, although the current statistics cannot exclude intrinsic decay or enhancement during stellar evolution.

astro-ph.SR↗

arXiv2015

Activity and Magnetic Field Structure of the Sun-Like Planet Hosting Star HD 1237

We analyse the magnetic activity characteristics of the planet hosting Sun-like star, HD 1237, using HARPS spectro-polarimetric time-series data. We find evidence of rotational modulation of the magnetic longitudinal field measurements consistent with our ZDI analysis, with a period of 7 days. We investigate the effect of customising the LSD mask to the line depths of the observed spectrum and find that it has a minimal effect on shape of the extracted Stokes V profile but does result in a small increase in the S/N ($\sim$ 7%). We find that using a Milne-Eddington solution to describe the local line profile provides a better fit to the LSD profiles in this slowly rotating star, which also impacts the recovered ZDI field distribution. We also introduce a fit-stopping criterion based on the information content (entropy) of the ZDI maps solution set. The recovered magnetic field maps show a strong (+90 G) ring-like azimuthal field distribution and a complex radial field dominating at mid latitudes ($\sim$45 degrees). Similar magnetic field maps are recovered from data acquired five months apart. Future work will investigate how this surface magnetic field distribution impacts the coronal magnetic field and extended environment around this planet-hosting star.

astro-ph.SR↗

arXiv2011

Discovery of the magnetic field of the B1/B2V star σLupi

In our search for new magnetic massive stars we use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurring in a velocity range symmetric around zero. Our aim is to obtain follow-up spectropolarimetry to search for a magnetic field in magnetic candidate stars. We quantify UV wind line variability, and analyse its time behaviour. The B1/B2V star sigma Lup emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The stellar wind line variations of sigma Lup are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a longitudinal magnetic field with varying strength and amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic and the optical line variations are consistent with the well-known photometric period of 3.02 days, which we identify with the rotation period of the star. Additional observations with ESPaDOnS at CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period. Further analysis revealed that $σ$ Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a spotted surface. We conclude that sigma Lup is a magnetic oblique rotator, and is a He-strong star. It is the 4th B star for which a magnetic field is discovered from studying only its wind variability. Like in the other magnetic B stars the wind emission originates in the magnetic equator, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.02 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field. A full paper will be submitted to A&A.

astro-ph.SR↗

arXiv2010

The MiMeS Project: Overview and Current Status

The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among many of the foremost international researchers of the physics of hot, massive stars, with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. At the time of writing, MiMeS Large Programs have acquired over 950 high-resolution polarised spectra of about 150 individual stars with spectral types from B5-O4, discovering new magnetic fields in a dozen hot, massive stars. The quality of this spectral and magnetic matériel is very high, and the Collaboration is keen to connect with colleagues capable of exploiting the data in new or unforeseen ways. In this paper we review the structure of the MiMeS observing programs and report the status of observations, data modeling and development of related theory.

astro-ph.SR↗