* Astronomy

Blobrana · L

RE: tau Scorpii

Blobrana · L

Title: Tau Sco: discovery of the clones
Authors: V. Petit, D. L. Massa, W. L. F. Marcolino, G. A. Wade, R. Ignace, the MiMeS Collaboration

The B0.2 V magnetic star tau Sco stands out from the larger population of massive magnetic OB stars due to its high X-ray activity and remarkable wind, apparently related to its peculiar magnetic field - a field which is far more complex than the mostly-dipolar fields usually observed in magnetic OB stars. tau Sco is therefore a puzzling outlier in the larger picture of stellar magnetism - a star that still defies interpretation in terms of a physically coherent model. Recently, two early B-type stars were discovered as tau Sco analogues, identified by the striking similarity of their UV spectra to that of tau Sco, which was - until now - unique among OB stars. We present the recent detection of their magnetic fields by the MiMeS collaboration, reinforcing the connection between the presence of a magnetic field and wind anomalies (Petit et al. 2010). We will also present ongoing observational efforts undertaken to establish the precise magnetic topology, in order to provide additional constrains for existing models attempting to reproduce the unique wind structure of tau Sco-like stars.

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Blobrana · L

Title: The surprising magnetic topology of tauSco: fossil remnant or dynamo output?
Authors: JF Donati, ID Howarth, MM Jardine, P Petit, C Catala, JD Landstreet, JC Bouret, E Alecian, JR Barnes, T Forveille, F Paletou, N Manset

Researchers report the discovery of a medium-strength (~0.5kG) magnetic field on the young, massive star tauSco (B0.2V), which becomes the third-hottest magnetic star known. Circularly polarised Zeeman signatures are clearly detected in observations collected mostly with the ESPaDOnS spectropolarimeter, recently installed on the 3.6-m Canada-France-Hawaii Telescope; temporal variability is also clearly established in the polarimetry, and can be unambiguously attributed to rotational modulation with a period close to 41d. Archival UV spectra confirm that this modulation repeats over timescales of decades.
By reconstructing the large-scale structure of its magnetic topology, they find that the magnetic structure is unusually complex for a hot star. The surface topology is dominated by a potential field, although a moderate toroidal component is probably present. They fail to detect intrinsic temporal variability of the magnetic structure over the 1.5-yr period of their spectropolarimetric observations (in agreement with the stable temporal variations of the UV spectra), and infer that any differential surface rotation must be very small.
The topology of the extended magnetic field that the researchers derive from the photospheric magnetic maps is also more complex than a global dipole, and features in particular a significantly warped torus of closed magnetic loops encircling the star (tilted at about 90deg to the rotation axis), with additional, smaller, networks of closed field lines. This topology appears to be consistent with the exceptional Xray properties of tauSco and also provides a natural explanation of the variability observed in wind-formed UV lines.
They conclude that its magnetic field is most probably a fossil remnant from the star-formation stage.

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Blobrana · L

An international team of astronomers has discovered that the naked-eye star, tau Scorpii, unexpectedly hosts a complex network of magnetic field lines over its surface.

Our Sun has its explosive flares and spots and high speed wind, but it is a placid star compared to some. Stars that are much more massive live fast and die young, with blue-white, intensely hot surfaces that emit energy at a rate millions of times greater than that of the Sun. These stars are so bright that their light alone propels outflowing stellar winds - up to a billion times stronger than the solar wind - at speeds of up to 30,000 km/s, or one per cent of the speed of light.

Tau Scorpii has been known for some time to emit X-rays at an unusually high rate and to rotate more slowly than most otherwise similar stars. The newly discovered magnetic field, presumably a relic from the star’s formation stage, goes some way to explaining both characteristics, although the mechanism by which the magnetic field slowed down tau Scorpii's rotation so strongly remains mysterious.
These results will be published in the Monthly Notices of the Royal Astronomical Society.
The processes by which hot, massive stars expel their surface layers through their strong outflowing winds have a major impact on a star's long-term fate. The cast-off material can also interact with other nearby stars, contribute matter and energy to the surrounding interstellar medium, and even induce bursts of new star formation. Hot massive stars are thus key actors in the life of a galaxy.

One such hot star is tau Scorpii, whose intrinsic brightness is so great that it is easily visible with the naked eye, despite its distance of over 400 light-years. Weighing as much as 15 Suns, tau Scorpii is 5 to 6 times bigger and hotter than our own star. Such massive stars are relatively few in number compared to stars like the Sun, and tau Scorpii is actually one of our closest massive neighbours.
Massive stars are thought to emit X-rays because of supersonic shocks occurring within their outflowing winds. However, tau Scorpii is an unusually strong X-ray source compared to stars which are otherwise similar.
The reason for this enhanced activity was a puzzle until the present discovery, which revealed that the star hosts a complex network of magnetic field lines over its surface. According to the discovery team, this field is most probably a relic from the star's formation stage.

IMAGE
The most interesting aspect, though, is how the field interacts with the wind, forcing it to flow along magnetic field lines, like beads along wires. Wind streams along 'open' magnetic field lines (shown in blue) freely escape the star, something that wind streams in magnetic 'arcades' (shown in white) cannot achieve. The result is that, within each magnetic arcade, wind flows from both footprints collide with each other at the loop summits, producing tremendously energetic shocks and turning the wind material into blobs of million-degree, X-ray emitting plasma tied to the magnetic loops.

This model provides a natural explanation of why tau Scorpii is such an intense X-ray emitter. However, it is not yet clear how the magnetic field succeeded in slowing down the rotation rate of the star to less than one-tenth that of otherwise similar, non-magnetic, massive stars.
Sun-like stars can be slowed down through their magnetic wind, just as ice-skaters are spun down when outstretching their arms. Tau Scorpii does not, however, lose material fast enough to have its rotation modified within its very short lifetime of a few million years.

The researchers discovered and examined the magnetic field of the star by looking at the tiny, very specific polarisation signals that magnetic fields induce in the light of magnetic stars. To do this, they used ESPaDOnS, by far the most powerful instrument in the world for carrying out this kind of research. This new instrument, currently attached to the Canada-France-Hawaii Telescope on Hawaii, was specially designed at the Observatoire Midi-Pyrénées in France for observing and studying magnetic fields in stars other than the Sun.

Source RAS

Tau Scorpii is a star in the constellation Scorpius. Tau Scorpii also has the traditional name Alniyat or Al Niyat, which it shares with σ Scorpii.
The star is a blue-white B-type dwarf with an apparent magnitude of +2.82. It is approximately 430 light years from Earth.

Position(2000): RA = 16h 35m 53.0s Dec = −28° 12' 58"