PRESS RELEASE, September 2011
A BABY STAR HAS BEEN LEFT ON THE SUN'S DOORSTEP
Dr. Todd J. Henry
What if someone left a baby on your doorstep? What if the baby were a star? That seems to be just what has happened in the Sun's neighborhood.
In the October issue of The Astronomical Journal, a team of astronomers from the Research Consortium on Nearby Stars (RECONS, www.recons.org), headquartered at Georgia State University, report the discovery of the closest known star to our Solar System that is still in the process of forming. Working with collaborators at the Australian National University, the University of California, San Diego, and the Cerro Tololo Inter-american Observatory, the team has found that the variable star AP Columbae is only 8.4 parsecs (27 light years) from the Sun --- very close, in astronomical terms --- and only 40 million years old. "What makes the star so unusual is that it is still gravitationally contracting and not generating all of its light by fusion, as the Sun does. Stars like AP Col are so young that they are still collapsing from a cloud of gas and dust, and are powered by, essentially, their outer layers falling in." says lead author Adric Riedel. Compared to our middle-aged Sun, the small red dwarf star AP Col is a violent place, with enormous stellar flares, giant sunspots, and tremendous X-ray radiation levels.
The star originally came to the attention of the RECONS group as a potentially nearby star with a predicted distance as close as 4.6 parsecs (15 light years). The RECONS team have been searching for stars within 10 pc (33 light years) since 1999 and count to their credit 31 such systems. "We started observing AP Col in 2004 because our first estimate of its distance indicated it might be quite close, ranking in the nearest few dozen star systems," says Dr. Todd Henry, RECONS Director. AP Col was a tantalizing prospect, but it took a few years for RECONS to measure a distance that revealed AP Col to be nearly twice as far away as anticipated, ultimately ranking it as the 167th nearest system. Stars might appear to be nearer than thought because they are intrinsically brighter than estimated. This might happen if the star were an evolved giant star, different in composition than most stars of similar temperatures, multiple rather than single, or very young.
To understand what kind of star AP Col is, the team attacked the problem from several angles. "A spectrum of the star, much like a fingerprint, revealed that it wasn't a giant star or radically different in composition from other red dwarf stars," says Riedel, eliminating those two possibilities. "It was tougher to determine if it was a close multiple star, but we eventually ruled out that option by examining our best images for a close companion, and by using various techniques to check for the pull of an unseen companion. In the end, we didn't find any evidence of a companion." That left the option that the star was very young.
Hints that AP Col might be a young star caught the attention of Simon Murphy of the Australian National University and Carl Melis at UC San Diego. They observed AP Col with high-resolution spectroscopy, and were able to measure its chemical composition, surface gravity, and the minute redshifts associated with the star's line-of-sight "radial" velocity, the same basic measurement commonly used to detect planets. Determining the youth of a small star is difficult because many indicators of youth aren't conclusive. The team, however, ticked off every available box and the pieces came together to form a coherent picture. AP Col has been found to be a prodigious emitter of x-rays and shows emission lines of hydrogen, both indicators of stellar youth. The star is brighter than expected because it is larger than expected, thereby exhibiting low surface gravity, a trait of young stars that are still collapsing. The team also detected the chemical element lithium in AP Col's spectrum. Lithium, a product of the Big Bang, is quickly destroyed in stars and not replenished, so the lithium measurement suggested AP Col was very young indeed.
The last, most intriguing, piece of the puzzle is linked to work Murphy and colleagues in Australia have refined over the years. Murphy compared AP Col's lithium content and surface gravity to nearby clusters of known ages and found that AP Col was somewhere between 12 and 50 million years old. When Riedel calculated AP Col's position and motion in our Galaxy, it matched a group of 40-million year old stars called the Argus association. The twist is that stars in the group are typically more than 100 parsecs (326 light years) away. The authors suggest that Argus is an enormous stellar nursery primed by star formation in a related cluster of stars called IC2391. Linking AP Col to Argus is critical because they should have similar characteristics. Thus, the baby star on the Sun's doorstep can provide a close-up example of what young stars look like at a distance close enough for detailed study. Says Russel White, also at Georgia State University but not part of the study, "The discovery of a young star here in our backyard will provide us with unique opportunities to study it in unprecedented detail, and may allow us to discover planets in the process of or soon after formation. AP Col is now destined to become one of the most intensively studied young stars in the sky." Efforts to see if AP Col has any planetary companions are already under way. Says Riedel, "The idea of a star that's only as old as, say, crocodiles, is cool. That it's sitting right next to us is VERY cool."
This research effort has been supported by the National Science Foundation and via observations made possible by the SMARTS Consortium.