Older stars can birth planets

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Older stars can birth planets

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 23 April, 2009

Recalculating the ages of young clusters of stars seems to solve apparent discrepancies between star ages and planetary disk formation times, according to Dr Tim Naylor of Exeter University, who presented his work at the European Week of Astronomy and Space Science.

The stellar populations of star clusters have been found to be higher than we had realised. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA)-ESA/Hubble collaboration.

Main sequence stars are those like our Sun that are in the 'hydrogen-burning' phase of their lives (stars spend 90 percent of their lifetimes on the main sequence). Very massive stars last only a fraction of the ten billion years or so of a star like our Sun, whilst lower mass stars last much longer. They also join the main sequence at different times depending on their mass.

The relationship between brightness and temperature of main sequence stars is related to their size, and can be plotted on a colour-magnitude graph. Many of these diagrams for star clusters show a line of main sequence stars running broadly from top left (hottest, brightest and most massive giant stars) to bottom right (dim, low-mass cool dwarfs).

The problem with calculating the ages of stars from when they join the main sequence to when they leave it as they grow old, is that the figure you get depends on what initial conditions you use in your models of star formation.

In his talk, Naylor explained that to get the absolute ages of stars in a cluster it was helpful to use a statistical distribution area and see how well the stars fit into it. In particular he looked at the massive, upper main sequence stars, as these have short-lived main sequence lifetimes. From this Naylor found that "Every single star came out older by 1.5 to two times."

At first it seemed that something had gone wrong. But then Naylor did the same thing with other young clusters and found the same discrepancies. Crucially, the figures also matched previous ages for upper main sequence stars (though Naylor's results are more precise). Speaking to Astronomy Now, Naylor says, "It takes about nine million years to form a planet, and the protoplanetary disc disappears in four or five million years. The new ages therefore give enough time for planet formation.