* Astronomy

A team of Gemini Observatory astronomers led by Tom Geballe, and Spanish colleague Francisco Najarro (Instituto de Estructura de la Materia, Madrid), have proven that the star at the core of a spectacular cosmic bow shock near the Galactic Centre is indeed an evolved young massive star. It may also be the youngest, hottest and most massive star in the region of the Galactic Centre.



The nature of the Galactic Centre source IRS-8, one of the brightest compact mid-infrared sources in the central infrared cluster, was unknown until the advent of adaptive optics (AO) imaging in the infrared. Its dimensions on the sky are roughly 1 arcsecond across, and it appeared as a fuzzy blob to observers until its surprising structure was revealed at Gemini during early AO observations in 2000. In an previous article, Geballe et al. (2004) showed that the IRS-8 bow shock would be a straightforward consequence of the interaction of a dense, high velocity wind from a hot star that is traversing moderately dense interstellar gas. But at that time no detailed information was available about the star at the centre of IRS-8, now known as IRS-8*.

To test if IRS-8* is indeed a hot and massive star, the team used the adaptive optics system ALTAIR with the near infrared spectrograph NIRI on Gemini North to separate the spectrum of the central star of IRS-8, presumably powering the bow shock, from the bow shock itself. Indeed the stellar spectrum was found to be that of such a star, with emission and absorption lines characteristic of an O5-O6 giant or supergiant.
Use of computer models to analyse the spectrum indicates that IRS-8* has an effective temperature of 36,000 K, a luminosity more than 350,000 times that of the Sun, and a mass of 44.5 solar masses. If it is an isolated star its age would be only 3.5 million years old. Stars with these properties are observed to have wind mass loss rates of ~10^-5 solar masses per year, and thus IRS-8* could easily power the observed bow shock.
None of the above properties are unusual for a hot and massive star. However, within the central parsec of the Galaxy, all of the other hot stars that have been identified to date are believed to be approximately 6 million years old. The properties of IRS 8* conflict with the generally accepted picture of the Galactic centre in which all of the hot stars formed in a single burst. If IRS-8* is a close binary, however, exotic evolutionary scenarios involving mass exchange between the star pair could give it the appearance of a younger star than the other cluster members, even though it is actually the same age. In the near future the Gemini team intends to obtain more detailed spectra of IRS 8* which might reveal if the star is single or part of a massive binary system.


Title: The K-band Spectrum of The Hot Star in IRS~8: An Outsider in the Galactic Centre?
Authors: T. R. Geballe, F. Najarro, F. Rigaut, J.-R. Roy

Using adaptive optics at the Gemini North telescope we have obtained a K-band spectrum of the star near the centre of the luminous Galactic centre bowshock IRS8, as well as a spectrum of the bowshock itself. The stellar spectrum contains emission and absorption lines characteristic of an O5-O6 giant or supergiant. The wind from such a star is fully capable of producing the observed bowshock. However, both the early spectral type and the apparently young age of the star, if it is single, mark it as unique among hot stars within one parsec of the centre.

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