Low in the south in the summer sky shines the constellation Scorpius and the bright, red supergiant star Antares. Many of the brightest stars in Scorpius, and hundreds of its fainter stars, are among the youngest stars found near the earth, and a new analysis of them may result in a rethinking of both their ages and the ages of other groups of stars.
New research by astrophysicists from the University of Rochester focused on stars in the north part of the constellation, known as Upper Scorpius, which is a part of the Scorpius-Centaurus OB association, one of our best studied groups of young stars and a benchmark sample for investigating the early lives of stars and the evolution of their planet-spawning disks.
The Upper Scorpius stellar group lies roughly 470 light years from Earth.
While those stars have been thought to be just five million years old, the team concludes that those stars are actually more than twice as old, at 11 million years of age. The findings are surprising given Upper Scorpius's status as one of the best-studied samples of young stars in the sky.
The findings by graduate student Mark Pecaut and Assistant Professor Eric Mamajek of Rochester, and Assistant Professor Eric Bubar of Marymount University, were accepted for publication in the Astrophysical Journal.
The scientists came to their conclusions after analyzing hundreds of optical spectra measured with the SMARTS 1.5-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, as well as reanalyzing previously published data on the stars.
"We combined our new estimates for the temperatures of the stars based on our spectra, with data on the brightnesses and distances to estimate accurate luminosities," said Pecaut. "Then we used state-of-the-art stellar evolution models to determine the ages."
The Scorpius OB association is a loose group of stars. This group contains many hot, extremely luminous OB-type stars. It is the site of recent star formation.
The stars in such groups are mostly not gravitationally bound but are expanding away from some common center, which presumably marks their birthplace.
A recent study indicates that the Scorpius association has had 20 supernova explosions over the past 11 million years.
The new analysis shows that stars over a wide range of masses in Upper Scorpius - from slightly more massive than our Sun, up to the mass of the bright star Antares (17 times the mass of our Sun) are giving ages consistent with a mean age of 11 million years.
"For one thing, the distances to the stars are now much more accurately known," said Pecaut. "Also, the newer computer models take into account the rotation of the stars and its effect on the mixing on the star's hydrogen - its nuclear fuel source."
"The first criticism that we heard of the work was that our age estimates for the stars more massive than the Sun in Upper Scorpius disagreed drastically with previously published ages for the smaller stars in the group," said Mamajek.
"However, we think the stellar parameters and models are on much firmer footing for the higher mass stars than for the lowest mass stars.
"The computer models of stars have trouble predicting the correct masses of low-mass stars when they are dynamically measureable, as well as the rate at which the low-mass stars consume their lithium through nuclear reactions.
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