The Milky Way appears to have stolen about a million low-mass stars from a dense 鈥済lobular cluster鈥� in the constellation Ophiuchus, astronomers have discovered.
The finding suggests this cluster ventures closer to our galaxy鈥檚 central bulge than previously thought, allowing the bulge鈥檚 gravity to strip away many low-mass stars, while leaving the cluster鈥檚 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 鈥� those about one-third as massive as the Sun.
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鈥淲e found very few small stars,鈥� de Marchi says. Compared to current models, that is 鈥渕uch much less than we were expecting to find鈥�.
鈥淚t tells us that the orbit [for the cluster] we had before was simply wrong,鈥� de Marchi told New 杏吧原创, and suggests the cluster鈥檚 actual orbit brings it closer to the galaxy鈥檚 centre. A closer pass would account for the high loss of low-mass stars.
Tidal tails
鈥淥ne 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,鈥� says Paul Martini, an astronomer at Ohio State University in Columbus, US. 鈥淭hose [low-mass stars] are the ones this idea is predicting should be stripped away 鈥� and that鈥檚 what these authors have found evidence of.鈥�
Astronomers have long hypothesised that the Milky Way strips stars from clusters, possibly adding them to the galaxy鈥檚 spherical halo. Previous evidence includes observations of streams of stars, called 鈥渢idal tails鈥�, trailing behind their clusters. And scientists think thousands of globular clusters once orbited through the galaxy before most were pulled apart, leaving today鈥檚 small population of about 200.
De Marchi says his team鈥檚 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.
