To B(e) or not to B(e)

via Astronomy Now.

B(e) stars are stars that peak in the B part of the spectrum in rather archaic astronomical units (OBAFGKM are the various spectral peak ranges, O is the hottest, M the coolest), they are roughly four times as hot as the Sun. They also include lines in their spectrum known as “forbidden lines” (which is what the (e) means), because there is so low a probability that an emission will be seen from them in the lab, it was believed they were among the lines that couldn’t emit.

Credit: ESO/F Millour et al

One such B(e) star is HD 87643 (from the Henry Draper catalogue of stars). This has recently been the subject of some intense observations. In visible light, high resolution images show the star enveloped in dust from which arc like reflections can be seen. That’s all fine and well, but the arcs are separated and on the same side, suggesting that they are from separate shells in the gas. The shells are believed to be hydrogen in the outer layers of the star ejected by some mechanism or other. Looking at the size of the shells and their velocities away from the star, a team of observers using the European Southern Observatory’s 2.2 metre telescope at La Silla Observatory calculated that there were two periods of outbursts, with one occurring every 15 years or so and one every 50 years or less.

Credit: ESO/F Millour et al

Further investigations were undertaken with ESO’s Very Large Telescope interferometer, which adds the signals from multiple radio telescopes to provide resolution normally reserved for larger telescopes, though not the same sensitivity. The team, led by Florentin Millour of the Max Planck Institute in Germany, found that they were able to spot not one but two stars where HD 87643 should be, separated by a distance equivalent to 51 times the separation of the Sun and the Earth (51AU). The wavelength used by the telescope sees the nebula as transparent, allowing a view of the stars within. The team believe that the outbursts of material from this B(e) star are linked to the orbit of the companion star, which may draw material out as it approaches the giant star. But does this mean all B(e) stars have a companion? That’s for further observations to determine.