CFHT, Current Image of the Week


April 24th, 2000

A Strange Collection of Stars at the Galactic Center

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A Strange Collection of Stars at the Galactic Center

Credit: Image courtesy of J.P. Maillard & T. Paumard

Institut d'Astrophysique de Paris, France

On February 28th an image of the center of our Galaxy was the featured picture of the week. The presence of a Black Hole was suggested at the center of the Galaxy to explain the rapid stellar motions that are measured in the field next to Sgr A*. This week we feature the Galactic Center again, but for a different reason: the number of peculiar stars it hosts!

Recently, a team of French Astronomers lead by Jean-Pierre Maillard used the Fourier Transform Spectrograph (FTS) on the Canada-France-Hawaii Telescope to image the Galactic Center and search for objects known as Helium Stars. These stars are rare because very short-lived. Oddly, a large number of them was found near the Galactic center.

We believe that one of the "side effects" of having a black hole at the center of a galaxy is to produce a "starburst", an episode of very intense star formation. Howewer, the presence of the rare Helium stars in such large number is difficult to understand with this model.

The images obtained by the group of astronomers allowed to understand better the nature of these objects. Because FTS is an imaging spectrograph, it is possible to efficiently select only those stars that are, for example, Helium-rich among the large number of objects present in the field-of-view. Indeed, 16 sources are detected by FTS in the Helium line close to the Galactic center. This is more than anywhere else in the Galaxy! The 16 sources are identified in this week's image, and some of them are also identified here for comparison and orientation with the image of the week of February 28th. Note that this image was obtained with Adaptive Optics. It has better angular resolution but lacks the spectrosocpic capacity of FTS. Both techniques complement each other nicely.

Further clues to the nature of these Helium stars were revealed by the images presented here. Interestingly, two classes of Helium stars seem to be present and distributed differently. Bright Helium stars are grouped in a compact cluster in the immediate neighbourhood of the Sgr A* (the Galactic center, indicated by the red + sign at the center of the image). They are found mainly in the IRS-16 cluster. On the contrary, the fainter Helium stars are located slightly further from the center, within a radius of ~0.4parsec (~1.3 light-years). The existence of these two distinct groups probably traces a difference of evolution, with the more evolved and brighter stars likely in the Wolf-Rayet phase of their evolution and located in the central cluster.

Technical description:

The image presented this week was obtained in July 1997 with the imaging Fourier Transform Spectrograph (FTS) attached to the Canada-France-Hawaii Telescope. The instrument, called BEAR, operates in the near infrared and allows integral field spectroscopy over a field of view of 24 arcseconds.

In the image presented here, the instrument was tuned to measure a neutral Helium line at a wavelength of 2.058 microns. A cube of 300 images of 10 second exposure time was needed to produce this picture. The stellar continuum adjacent to the line was carefully subtracted to better allow identification of the Helium stars. Extended patches of diffuse Helium emission remain (visible in green). They trace interstellar gas.

next week: The Mystery of Zeta Cancri Revealed

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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.