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ACS sweeps field

Probing the star-filled, ancient hub of our Milky Way, NASA's Hubble Space Telescope has found a rare class of oddball stars called blue stragglers, the first time such objects have been detected within our galaxy's bulge. The size and nature of the blue straggler population detected in the bulge will allow astronomers to better understand if the bulge is exclusively old stars, or a mixture of both young and old stars. In addition, the discovery provides a new test case for formation models of the blue stragglers themselves. Blue stragglers, so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed, were first found inside ancient globular star clusters half a century ago. They have been detected in many globular and open clusters, as well as among the stars in the solar neighborhood. But they have never been seen inside the core of our galaxy until Hubble was trained on the region. Hubble astronomers found blue straggler stars in an extensive set of Hubble exposures of the Milky Way's crowded hub. Blue stragglers are much hotter, and hence bluer, than they should be for the aging neighborhood in which they live.

Now that blue stragglers have at last been found within the bulge, the size and characteristics of this population will allow astronomers to better understand the still-controversial processes of star formation within the bulge.The results, to be published in The Astrophysical Journal, are being reported by lead author Will Clarkson of Indiana University and the University of California, Los Angeles, at the American Astronomical Society meeting in Boston, Mass.These results support the idea that the Milky Way's central bulge stopped making stars billions of years ago. It is now home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there exploded as supernovae billions of years ago. If our galaxy were the size of a dinner plate, the central bulge would be roughly the size of a grapefruit placed in the middle of the plate.

This discovery is a spin-off from a seven-day-long survey conducted in 2006 called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Hubble peered at and obtained variability information for 180,000 stars in the crowded central bulge of our galaxy, 26,000 light-years away. The survey was intended to find hot Jupiter-class planets that orbit very close to their stars. But the SWEEPS team also uncovered 42 oddball blue stars among the bulge population with brightness and temperatures typical for stars much younger than ordinary bulge stars. Blue stragglers have long been suspected to be living in the bulge. Until now, it has never been proven because younger stars in the disk of our galaxy lie along the line-of-sight to the core, confusing and contaminating the view. But Hubble's view is so sharp that astronomers could distinguish the motion of the core population from foreground stars in the Milky Way. Bulge stars orbit the galactic nucleus at a different speed than foreground stars. Plotting their motion required returning to the SWEEPS target region with Hubble two years after the first-epoch observations were made.

Hence, the blue stragglers were identified as moving along with the other stars in the bulge. It's like looking into a deep, clear pond where the fish at the bottom of the pond are swimming at a faster rate than the fish closer to the surface. "The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm's length, and within this region, Hubble sees about a quarter million stars towards the bulge," Clarkson says. "Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field". From the 42 candidate blue stragglers, the investigators estimate 18 to 37 of them are likely to be genuine blue stragglers, with the remainder consisting of a mixture of foreground objects and at most a small population of genuinely young bulge stars. It's not clear how blue stragglers form, or if there is more than one mechanism at work. A common idea is that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, the less massive star accretes material from the companion. This stirs up hydrogen fuel and causes the accreting star to undergo nuclear fusion at a faster rate. It burns hotter and bluer.

The seven-day observation allowed the fraction of blue straggler candidates presently in close binaries to be estimated by virtue of their changing light-curve. This is caused by the change of shape induced in one star due to the tidal gravitational pull of its companion. "The SWEEPS program was designed to detect transiting planets through small light variations. Therefore the program could easily detect the variability of binary pairs, which was crucial in confirming these are indeed blue stragglers", says Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md., the principal investigator of the SWEEPS program. The observations clearly indicate that if there is a young star population in the bulge, it is very small, and it was not detected in the SWEEPS program. "Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood", Clarkson says. "While the consensus is that the bulge largely stopped forming stars long ago, many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge".

Credit: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI)

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