How old is too old? Pro football players tend to peak in their late 20s, and few continue their careers beyond the age of 35. For young stars, the peak age for planet formation is around 1 to 3 million years. By 10 million years old, their resources are exhausted and they retire to a life on the stellar "main sequence."

Like pro football players, young stars also peak early. Most planets form when stars are 1 to 3 million years old. By 10 million years, the dusty disk is gone and planet formation ends. Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

Using telescopes on the ground and in space, a team of astronomers led by Lee W. Hartmann and Aurora Sicilia-Aguilar (Harvard-Smithsonian Center for Astrophysics) is studying Sun-like stars in their waning formative years, within clusters older than previously explored. They seek to refine our understanding of planet formation by studying dusty protoplanetary disks around such stars. Their results, presented this week at the 204th meeting of the American Astronomical Society in Denver, Colorado, better define the time span during which planets might form.

"While the planets that may be forming cannot be detected directly," said Sicilia-Aguilar, "we can see changes in the circumstellar dusty accretion disks caused as the planets sweep up and accumulate mass."

"The data also has shown dramatic differences between stars of 3 and 10 million years of age: the younger stars frequently have dusty disks capable of forming planets, while such disks are essentially absent in the older population," she continued.

The team used data from the Smithsonian Institution's Whipple Observatory telescopes, the WIYN telescope at Kitt Peak National Observatory, and from the Spitzer Space Telescope (the latter made available as part of the Guaranteed Time Program of Infrared Array Camera PI Giovanni Fazio), to make these findings.

"We are trying to understand the evolution of protoplanetary disks around stars not too different from the Sun," said team leader Lee W. Hartmann. "Many stars about 1 million years old have disks, but by 10 million years, almost none have disks. We are trying to find stars at an in-between age and 'catch them in the act' of forming planets."

Circumstellar dust disks enshroud young stars, and astronomers understand this to be a common feature of stellar evolution and of possible planetary system formation. The initial protoplanetary disks contain the gas and dust that provide the raw materials for the formation of later planetary systems.

"After stars form planets in their disks and clear out most of the material-either by accretion onto the star, accretion onto planets, or ejection-small amounts of dust can remain in so-called 'debris disks.' Most or all of this debris dust is thought to be continuously generated by the collision of small bodies, much like the zodiacal light in our solar system," said Hartmann.

The team is presenting the first identification of low mass stars in the young clusters Trumpler 37 and NGC 7160. (These clusters are loose associations of stars that have formed together in the comparatively recent past.) "The cluster members confirm the age estimates of 1 to 5 million years for Tr37 and 10 million years for NGC 7160," said Sicilia-Aguilar.

"We do find active accretion in some of the stars in Tr37. The average accretion rate is equivalent to swallowing up 10 Jupiter masses in a million years," said Sicilia-Aguilar. "This is consistent with models of viscous disk evolution."

"In comparison, we have detected no signs of active accretion so far in the older cluster NGC 7160, suggesting that disk accretion ends within 10 million years. This probably coincides with the major phase of giant planet formation."

Trumpler 37 is of more immediate interest, said Hartmann, because we hope to find stars with Jupiter-size planets that are still accumulating material from the disks, so the disks are not completely cleared out yet. However, there may be a few objects in the 10 million-year-old cluster NGC 7160 that are also still forming their giant planets. Not all disks evolve at the same rate.

"Thus we expect eventually to find out more about the frequency of debris disks, and the rate at which the dust in such disks is removed, by studying the 10-million-year-old cluster NGC 7160 and comparing it to Trumpler 37," said Hartmann.

In addition to Sicilia-Aguilar and Hartmann, team members include Cesar Briceno (Centro de Investigaciones de Astronomia), James Muzerolle (University of Arizona), and Nuria Calvet (Smithsonian Astrophysical Observatory). This work was supported by NASA grant NAG5-9670.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.