Miércoles 15 de Febrero de 2006, Ip nº 140

Milky Way accused of million-star theft
Por Kimm Groshong

The Milky Way appears to have stolen about a million low-mass stars from a dense "globular cluster" in the constellation Ophiuchus, astronomers have discovered.

The finding suggests this cluster ventures closer to our galaxy's central bulge than previously thought, allowing the bulge's gravity to strip away many low-mass stars, while leaving the cluster's more massive stars behind.

A team led by Guido de Marchi, of the European Space Agency, measured the colour and luminosity of more than 16,000 stars in globular cluster Messier 12 using the Very Large Telescope in Chile. Now the team says the sphere of tightly packed stars is noticeably short on lower-mass stars – those about one-third as massive as the Sun.

"We found very few small stars," de Marchi says. Compared to current models, that is "much much less than we were expecting to find".

"It tells us that the orbit [for the cluster] we had before was simply wrong," de Marchi told New Scientist, and suggests the cluster's actual orbit brings it closer to the galaxy's centre. A closer pass would account for the high loss of low-mass stars.

Tidal tails
"One of the predictions for how globular clusters evolve over time is that they tend to become segregated, with the most massive stars ending up near the centre and the less massive stars in the outskirts," says Paul Martini, an astronomer at Ohio State University in Columbus, US. "Those [low-mass stars] are the ones this idea is predicting should be stripped away – and that's what these authors have found evidence of."

Astronomers have long hypothesised that the Milky Way strips stars from clusters, possibly adding them to the galaxy's spherical halo. Previous evidence includes observations of streams of stars, called "tidal tails", trailing behind their clusters. And scientists think thousands of globular clusters once orbited through the galaxy before most were pulled apart, leaving today's small population of about 200.

De Marchi says his team's new finding fits this theory and suggests the number of low-mass stars a cluster contains could give the best estimate of its multi-million-year orbit.

Astronomer Bill Harris of McMaster University in Ontario, Canada, says that while the galactic stripping theory can explain the lack of low-mass stars around Messier 12, it is important to note that current data cannot rule out an alternative scenario. It is thought less likely by astronomers, but the cluster might not have had any low-mass stars in the first place.

  08/02/2006. New Scientist Magazine.