An inventive approach suggests that the solar neighborhood is a mix of nursery and nursing home.Just how old are the stars near the Sun? When did our nearest few thousand neighbors start their lives as stars? These are some very basic questions that are difficult to answer because of one simple fact: it’s really, really hard to estimate the age of any given star in the Milky Way. But knowing the rate at which stars form over time, also known as the “star formation history,” is vitally important for many astronomical investigations, from understanding the formation of our galaxy to estimating the number of exoplanets that are nearby.
Recently, Nicholas Rowell (Dundee University, UK) has developed a new tool for estimating the star formation history of the solar neighborhood — and he’s come up with an intriguing answer.
Appearing in an upcoming Monthly Notices of the Royal Astronomical Society, Rowell's work takes advantage of a class of stars known as white dwarfs. These stars are the final remnants of stars like our own Sun. Once a star has finished fusing the hydrogen and helium in its core, it sloughs off its outer layers, leaving the glowing core behind. These cores often have masses about 60% that of the Sun but vary greatly in temperature, ranging from about half to 5 times that of the Sun.
These temperatures are extremely useful for astronomers. White dwarfs are rather simple objects, and they cool as they age in a methodical fashion. If you can estimate a white dwarf’s temperature and mass from observations, then you can estimate how long it has been a white dwarf (and therefore how much time has passed since its parent star died).
Rowell took advantage of this property to infer the underlying rate at which the nearest white-dwarf-birthing stars formed. The method works in this way. First, identify the coolest white dwarfs seen near the Sun. Those stars were the first to become white dwarfs because they have had the longest time to cool, so they set the limit on how far back in time you can look. (In this study, Rowell was limited to about 10 billion years ago.) Next, Rowell examined the number of white dwarfs in the second-coolest bin. These stars haven't had quite enough time to cool off, so they set constraints on the star formation history at a more recent time, say 8 billion years ago. He repeated this process over and over, until he reached the hottest white dwarfs, which have formed within the last billion years.
Rowell tested his method using a variety of mock stellar populations. He generated a synthetic solar neighborhood, where he could tweak the rate of star formation with time, as well as the intrinsic stellar properties of the stars that formed white dwarfs, such as their compositions and masses. He then "observed" these fake stars, and used the method described above to infer the rate of star formation. He found that the model performed quite well at recovering the star formation history, although it depended on the exact type of cooling track models that were used.
Finally, Rowell used his method on two samples of nearby white dwarfs, coming from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey. Each sample contained several thousand white dwarfs. Using his new technique, Rowell inferred that star formation within 650 light-years of the Sun has been ongoing for the last 10 billion years or so, with two major peaks. The first happened 7 to 9 billion years ago, and the other was just 2 to 3 billion years ago.
Previous studies using normal, hydrogen-fusing stars have found the more recent star-formation bump, but they’ve disagreed on stars’ earlier history. If the white-dwarf method holds up to scrutiny, it could be a new way to place strong constraints on a property of our galaxy that has been largely unknown. With larger samples of white dwarfs on the way (from surveys like LSST), this technique could be a powerful tool for studying our galaxy’s history.
Reference: N. Rowell. "The Star Formation History of the Solar Neighbourhood from the White Dwarf Luminosity Function." Accepted to the Monthly Notices of the Royal Astronomical Society.