Science + Technology

Astronomy’s Case of the Missing Disks


Astronomersannounced Jan. 10 that they have a lead in the case of the missing disks. The report was presented by UCLA graduatestudent and Ph.D. candidate Peter Plavchan; hisadviser, Michael Jura; and Sarah Lipscy,now at Ball Aerospace, to the American Astronomical Society meeting in SanDiego. This lead may account for the missing evidence of red dwarfs formingplanetary systems.

The evidence

Red dwarfs (or MDwarfs) are stars like our Sun in many respects but smaller, less massive andfainter. Approximately 70 percent of all the stars in our galaxy are reddwarfs.

"We would liketo understand whether these stars form planets, as the other stars in ourgalaxy do," said Plavchan, who leads this researchinvestigation.

Approximatelyhalf of all newborn stars are known to possess the materials to make planets.When stars are born, the leftover materials form what astronomers refer to as aprimordial disk surrounding the star. From this primordial disk, composed ofgas and small grains of solid material astronomers call "dust," planets canstart to grow. As these "planetesimals" grow byaccreting nearby material in the primordial disk, they also collide with oneanother. These collisions are frequent and violent, producing more dust forminga new disk of debris after the star is about 5–10 million years old. In our ownsolar system, we see evidence everywhere of these violent collisions that tookplace more than 4 billion years ago — such as the craters on the moon.

The debris diskof "dust" left over from these ancient collisions in our own solar system haslong since dissipated. Astronomers, however, have discovered many young starsin the local part of our galaxy where these debris disks still can be seen.These stars are caught in the act of forming planets and are of great interestto astronomers who want to understand how this process works. Curiously though,only two of these stars with debris disks were found to be red dwarfs: AU Microscopium (AU Mic) and GJ 182,located 32.4 light-years and approximately 85 light‑years from Earth,respectively.

Despite reddwarfs holding a solid majority among the different kinds of stars in ourgalaxy, only two have been found with evidence of debris disks. If half of allred dwarfs started with the material to form planets, what happened to the restof them? Where did the material and dust surrounding these stars go? Factorssuch as the ages, smaller sizes and faintness of red dwarfs do not fullyaccount for these missing disks.

The investigation

In December 2002and April 2003, Plavchan, Juraand Lipscy observed a sample of nine nearby reddwarfs with the Long Wavelength Spectrometer, an infrared camera on the10-meter telescope at the Keck Observatory on Mauna Kea, Hawaii. These ninestars all are located within 100 light-years of Earth and were thoughtpotentially to possess debris disks. None, however, showed any evidence for thepresence of warm dust produced by the collisions of forming planets.

Backed by theprevious research investigations that also came up empty-handed, theresearchers considered what makes red dwarfs different from other bigger,brighter stars that have been found with debris disks.

"We have toconsider how the dust in these young red dwarfs gets removed and where itgoes," said Jura, Plavchan'sthesis adviser.

In other young,more massive stars — A-, F- and G-types — the dust primarily is removed by Poynting-Robertson drag, radiativeblowout and collisions.

"These first twoprocesses are simply ineffective for red dwarfs, so something else must begoing on to explain the disappearance of the debris disks," Plavchansaid.

Under Poynting‑Robertson drag, a consequence of specialrelativity, the dust slowly spirals in towards the star until it heats up andsublimates.

The new lead in the case

Plavchan, Jura and Lipscy have discovered that there is another processsimilar to Poynting-Robertson drag that potentiallycan solve the case of the missing red dwarf debris disks: stellar wind drag.

Stars like ourSun and red dwarfs possess a stellar wind — protons and other particles thatare driven by the magnetic fields in the outer layers of a star to speeds inexcess of a few hundred miles per second and expelled out into space. In ourown solar system, the solar wind is responsible for shaping comets' tails andproducing the Aurorae Borealis on Earth.

This stellarwind also can produce a drag on dust grains surrounding a star. Astronomershave long known about this drag force, but it is less important than Poynting-Robertson drag for our own Sun. Red dwarfs,however, experience stronger magnetic storms and consequently have strongerstellar winds. Furthermore, X-ray data show that the red dwarf winds are evenstronger when the stars are very young and planets are forming.

"Stellar winddrag can 'erase' the evidence of forming planets around red dwarfs by removingthe dust that is produced in the collisions that are taking place. Withoutstellar wind drag, the debris disk would still be there and we would be able tosee it with current technology," Plavchan said.

This researchpotentially solves the case of the missing disks, but more work is needed. Astronomers know little about the strength ofstellar winds around young stars and red dwarfs. While further observations of red dwarfs bythe Spitzer Infrared Telescope Facility have supported this research, this casewill not be closed until we can directly measure the strength of stellar windsaround young red dwarfs.

This researchhas been submitted to The Astrophysical Journal for publication and issupported by funding from NASA.



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