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The Discovery Of The Most Rapidly Rotating Magnetic B-Star HR5907/V1040 Scorpius

26/12/2011

Usually, strong magnetic fields are not expected to be observed in intermediate mass B-type stars (Grunhut et al. 2012). Such stars lack a significant convective outer envelope, and thus they do not generate the massive magnetic dynamos as observed on the Sun. Some B-type stars belong to areas known as OB associates,  large aggregates of hundreds to several thousand young O and B type stars. There exists many of such star aggregates, with one of the most famous being the Pleiades “seven-sisters”  association, but the subject of this article, the star V1040 Scorpius (HR5907*), can be located in the Upper right region of the Scorpius constellation.

N.B.* HR5907 (or Henry Draper Catalogue “HD142184”, V1040 Scorpius), is a bright early type emission line star, located in the nearby Upper Scorpius OB star association at a distance of 145 pc from Earth (Hernandez et al. 2005). Additionally, archival Fibre-fed, Extended Range, Échelle Spectrograph (FEROS) spectra suggested that the observed He [I] profile is morphologically more similar to the profiles of other magnetic He-strong stars (such as σ Orion E or HR7355) than to classical Be-type stars (Grunhur et al. 2012).

At the thermally hotter end of the B-star class are the main sequence, Helium-strong stars (e.g. Bohlender et al. 1987) that show significant variability and enhancement in their Helium (He) lines, as is found in the archetypical star σ Orion E (e.g. Landstreet & Borra 1978). In addition to this He variability, some He-strong stars show emission variability in Balmer lines, photometric brightness variations, variable UV resonance lines, and non-thermal radio emission, most of which vary with a single period, interpreted to be the rotational period of the star (e.g. Pedersen & Thomsen 1977; Shore & Brown 1990; Leone & Umana 1993). Many of these phenomena are thought to be due to the presence of a rigidly rotating, centrifugally supported magnetosphere – a region in the circumstellar environment where the stellar wind couples to the magnetic field and is forced to co-rotate with the star (Shore & Brown 1990; Townsend & Owocki 2005).

Although B-type stars are not expected to have strong magnetic fields, a theoretical consideration of electromagnetism would pertain to the fact that if a B-type star was rotating extremely fast (and such, charged particles were moving fast relative to an electric field) then a strong magnetic field could be observed. It was with this consideration of inducing magnetic fields which led Grunhut et al. (2012) to present their results on the detection of a largescale, organised magnetic field with a polar surface intensity of 10-16 kG in the B-type star HR5907 (Harvard spectral class B2.5V; Yerkes spectral classification Ib).

Combining newly measured Micro-variability and Oscillations of STars (MOST) survey photometry with archival Hipparcos measurements, Grunhur et al. (2012) obtained the photometric period of HD5907 to 0.508276 d, confirming its rapid movement. Their follow-up period search on the spectro-polarimetric data confi rms this period is also present in the equivalent width variations of Hα as well as the longitudinal magnetic field measurements. As such, Grunhut et al. (2012) began investigation into a possible B-star with a strong magnetic field.

These results, as interpreted by the team, reveal the shortest period, non-degenerate, magnetic massive star known to date, making HD5907 one of the only known rapidly-rotating magnetic massive stars that show strong emission variations due to a magnetosphere. The only other massive star with a comparable rotation period is HR7355, which is believed to have a magnetic geometry more similar to σ Ori E, which is very di fferent from this star. Grunthut et al. conclude that HR5907 is an ideal target for comparison with the predictions of rigidly rotating magnetosphere models as presented by Townsend & Owocki (2005) and again, by Townsend (2008). Hence, HR5907 provides a great testbed for studying the e ffects of the magnetic field orientation on angular momentum loss and magnetic spin-down.

In conjunction with a better understand of B-type stars, this work may give greater clues to the nature of pulsar formation and the properties of proto-neutron stars. Massive B-type stars such as HR5907, with masses in the range of   (Kiziltan 2011), are supernovae core-collapse candidates and as such will produce a compact stellar remnant of the neutron degenerate type (progenitor masses of more than approximately  (Shapiro & Teukolsky 1983) are expected to produce the more exotic black hole remnants).  As such, the discovery of an intense magnetic field within a neutron progenitor B-type star, and possible further discoveries of similar stars, could reveal greater information on the formation and evolution of the neutron star sub-class: rapidly-rotating, highly magnetic radio emitting pulsar stars.

Journal References:

  • Alecian, E. et al. (2011) First HARPSpol Discoveries Of Magnetic Fields In Massive StarsAstronomy & Astrophysics, 536: pp.L6.
  • Grunhut, J. H. et al. (2012) HR 5907: Discovery Of The Most Rapidly Rotating Magnetic Early B-type Star By The MiMeS Collaboration. Monthly Notices: Royal Astronomical Society, 419 (2): pp. 1610-1627.

Suggested Further Reading:

  • Bohlender, D.A. et al. (1987) Magnetic Field Measurements Of Helium-Strong StarsAstrophysical Journal, 323 (1): pp. 325-337.
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