Early-type Magnetic Stars

Magnetic fields in stars are common. The fields are understood as being formed by the so-called α-Ω dynamo process in the convective inner stellar regions. The problem with early-type hot stars is that they don't have convective envelopes, and so they were theoretically not expected to have magnetic fields. Yet, some of them do. Since that discovery, quite a few mechanisms to explain those fields have been proposed: core convection dynamos, dynamos working on other internal instabilities than convection, fossil fields, i.e. a remnant from the star formation, and a few others.

There are actally too many field generating mechanism proposed to be really satisifed. So the observer's task is now to provide constraints which of them may work, which may not, and which require what sort of tuning to work.

The field generation is not the only research topic on those stars, however. In hot stars, magnetic fields have a considerable influence of the circumstellar environment. The close regions are usually well populated by the stellar wind, which is typically to large fractions, if not completely, ionized. The wind and the field will, therefore, interact. These processes are physically interesting by themselves, but also should give at least some hints towards the field generation mechanism acting in those stars.

σ Ori E: The prototype

FEROS data of σ Ori E

Periodic Hα variations.

Periodic SiIII4553 variations.

This star is the early magnetic B-star, the first discovered in 1970. It's field is so strong that it not only causes the chemical elements on the surface to fractionate into abundance spots, but completely controls the circumstellar environment. The field is not aligned with the rotational axis, but oblique to it. This causes an interesting effect: Although the star, as a B2V star, would be early enough to have a certain wind, it in fact has two magnetically bound clouds, namely where the magnetic and the rotational equator cross. As the star rotates, the way we see these clouds in the spectrum changes periodically, as seen in the upper panel of the figure left.

The stellar rotation also causes variations of the photospheric profiles, as the abundance spots rotate across the visible stellar hemisphere. This is seen in the lower part of the figure, as featires corssing from the left (blueward) to the right (redward) side of the photospgeric profile, broadened by the Doppler effect of the stellar rotation.

While earlier works concentrated on the abundance spots, and direct measurements of the field strength and geometry, the circumstellar geometry and its variations have gained some focus.

Due to its strength, the magnetic field can be considered to dominate the circumstellar environment, i.e. the wind, which around a B star is completely ionized, follows the field lines until it settles in a local minimum of the potential: The intersection line of the magnetic and rotational equatorial planes. This is the basis of the Rigidly Rotating Magnetosphere model by R. Townsend, which was able to explain the observed variation of σ Ori E in unprecedented detail.

However, at some point there will be so much material that the field will not be strong enough anymore to control it. Magnetic fieldlines have to be closed, and so a smooth outflow is not possible. Rather the material breaks out of its magnetic confinement, causing a flare. These flares have only recently, in 2004, been discovered.
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As is usual with prototypes, however, they are not quite like the serial model: σ Ori E has a very high rotational velocity. Not as high as Be stars, but astonishingly high for a magnetic star, that has its field to brake it down: Several hundred kilometers per second. Before our descovery of HR 7355 it actually used to be the record holder.

θ¹ Ori C: The hottest one in the sky

The earliest star known to have a magnetic field is the O-star θ¹ Ori C, one of trapezium stars in the central cluster of the Orion nebula of spectral type O6. Its spectrum was found by the Hot-star group of the Landessternwarte to show a very peculiar periodal variation of 15.4 days, both far to long and far to regular for the typical O-type star. The star was well known to have a variable wind, but for quite some time this variability was believed to be of irregular nature. Before the advent of service observing, it was actally quite hard to obtain spectroscopic data suited to search for such periods, since a typical observing run would be much shorter, and indivudual runs be much farther apart than a few weeks.

Luckily, we could secure the now retired ESO 50cm telescope for two to three months in a row during several years in the mid-90s, to use with our own echelle spectrograph, and so found the "irregular" variation of the wind emission to be strictly periodic. With the help of archival IUE data we could trace back the periodicity more than a decade. There is no clock inside an O-star that could produce such a stable period, expect one: Rotation. However, why should the rotation influence the wind? To do so, one more ingredient had to be added to the model, a magnetic field.

Later observations by colleagues not only measured the actual magnetic field, but turned up other interesting aspects related to the field, such as X-ray activity. With FEROS data similar to the one shown above for σ Ori E we could show the presence of abundance variations on the stellar surface, further putting θ¹ Ori C in one row with other magnetic and chemically peculiar stars.

HR 7355: The new record holder

HeI4388 of HR7355 in the two spectra.

Many years σ Ori E was not only the first, but also the most rapidly rotating He-strong star. It has a projected rotation speed of v sin i = 140 km/s and a period of 1.19 days. That's rapid, but still well away from the so-called critical speed, at which stars become rotationally unstable. The non-existence of more rapidly rotating magnetic stars was thought to be an immediate consequence of the magnetic field: The field would help the star to get rid of angular momentum by forcing the wind into co-rotation, and so to brake down the star.

HR 7355 is now a star which was observed by two groups for different purposes. First by the FEROS-consortium, as it was a B star with rapid rotation and we then searched for the signature of pulsation in such stars. I noticed the presence of quite broad emission then, but otherwise found the star not very interesting. Only later, when a visiting scientist woke my interest in that star again because of the somewhat peculiar emission, I found a second spectrum, taken a few years later with the same instrument. Comparing the two spectra, there was the obvious signature of a He-rich star, which is actually one of the names observers give to early type magnetic stars.

The sensation now is that HR7355 would not be just "yet another one", but with v sin i = 320 km/s and a preliminary period of 0.52 days, estimated from HIPPARCOS data, the most rapidly rotating early-type magnetic star, namely almost at the critical limit.

However, many scientists consider the comparably slow rotation of magnetic stars to be a paradigm, something that is a basic property that has to be accounted for, by all attempts to explain early-type magnetic stars. HR 7355 seems to bring down this paradigm. As "strong claims require strong evidence", two spectra are admittedly somewhat weak. Hence, we are going to continue observing this object, hoping to give the theoreticians something more solid they can "dent their teeth in", if I may quote one of those theoreticians.