One of the beauties of the Spitzer Space Telescope is that it can pinpoint the swirling dust disks around distant stars. Such dust, heated by the star, puts out an infrared signature that Spitzer can analyze to a degree hitherto unattainable. Now a team of astronomers has observed some 500 young T Tauri stars in the star-forming regions of the Orion nebula. They’ve been looking at how young stars spin, and the effects that dusty disks have on slowing their rotation.
T Tauri stars are ideal for this kind of work. They’re young objects (less than 10 million years old) that are still in the process of gravitational contraction. Such stars often show large accretion disks, but a variant called weak-lined T Tauri stars have little or no disk. Figuring out the various phases of T Tauri formation and how they relate to planets is thus a substantial challenge.
The answers Spitzer has provided are intriguing even if they leave many questions unanswered. Slow-spinning stars are five times more likely to have dust disks than fast-spinning ones. Which implies that the disks, the early construction zones for planets, have a role in slowing the star. But other factors also appear to be involved, including stellar winds. “We can now say that disks play some kind of role in slowing down stars in at least one region, but there could be a host of other factors operating in tandem. And stars might behave differently in different environments,” said Luisa Rebull (Spitzer Science Center, Pasadena).
Image: How does a disk put the brakes on its star? It is thought to yank on the star’s magnetic fields (green lines). When a star’s magnetic fields pass through a disk, they are thought to get bogged down like a spoon in molasses. This locks a star’s rotation to the slower-turning disk, so the star, while continuing to shrink, does not spin faster. Credit: NASA/JPL-Caltech/R. Hurt (SSC).
What we’re aiming for is an understanding of how a star’s rotation rate factors into the formation of planetary systems around it. And things aren’t nearly as clearcut as they might seem — nobody is arguing that fast-spinning stars can’t develop planets. Indeed, Rebull says that a slow spinner may simply take more time than other stars to clear out its disk and begin planet formation.
The exoplanets we’ve learned about so far all circle slowly turning stars; our own Sun rotates once every 28 days, a relatively sedate pace. Finding planets around stars that rotate more quickly is thus key to understanding how rotation and planet formation are intertwined in young stars. That leaves plenty of work for the next generation of space and ground-based telescopes.
The paper is Rebull, Stauffer, Megeath et al., “A Correlation between Pre-Main-Sequence Stellar Rotation Rates and IRAC Excesses in Orion,” Astrophysical Journal 646 (20 July 2006), pp. 297 ff., available online.
“The exoplanets we’ve learned about so far all circle slowly turning stars; our own Sun rotates once every 28 days, a relatively sedate pace. Finding planets around stars that rotate more quickly is thus key to understanding how rotation and planet formation are intertwined in young stars. That leaves plenty of work for the next generation of space and ground-based telescopes.”
Isn’t that simply observational bias? They would have broadened spectral lines and would be excluded from most radial velocity surveys.
Of course COROT and Kepler will use the transit method so they should find these planets if they exist.
Struve pointed out years ago that some stars rotate slow, while heavier stars are fast rotators. For years people felt there had to be a connection between slow stars and planet formation, a holdover from Laplace’s original theory, and the magnetic braking concept has been around since the early 50s. But I’d really like to see figures backing it up theoretically and observationally. Measuring the magnetic fields of T-Tauri stars has been rather tricky, so it’d be good to see more data. I wonder if the Square Kilometre array will allow more detail to be teased out of the data.
Re the observational bias point above, the answer is yes — we’re limited by existing radial velocity methods and need new tools to study what happens around faster rotating stars. I suspect our notions of planet formation are in for more than a few surprises as we learn more about the rotation issue.
Title: A Possible Stellar Metallic Enhancement in Post-T Tauri Stars by a Planetesimal Bombardment
Authors: O.C. Winter, R. de la Reza, R.C. Domingos, L.A.G. Boldrin, C. Chavero
(Submitted on 23 Apr 2007)
Abstract: The photospheres of stars hosting planets have larger metallicity than stars lacking planets. In the present work we study the possibility of an earlier metal enrichment of the photospheres by means of impacting planetesimals during the first 20-30Myr. Here we explore this contamination process by simulating the interactions of an inward migrating planet with a disc of planetesimal interior to its orbit. The results show the percentage of planetesimals that fall on the star. We identified the dependence of the planet’s eccentricity ($e_p$) and time scale of migration ($\tau$) on the rate of infalling planetesimals. For very fast migrations ($\tau=10^2$yr and $\tau=10^3$yr) there is no capture in mean motion resonances, independently of the value of $e_p$. Then, due to the planet’s migration the planetesimals suffer close approaches with the planet and more than 80% of them are ejected from the system. For slow migrations ($\tau=10^5$yr and $\tau=10^6$yr) the percentage of collisions with the planet decrease with the increase of the planet’s eccentricity. For $e_p=0$ and $e_p=0.1$ most of the planetesimals were captured in the 2:1 resonance and more than 65% of them collided with the star. Whereas migration of a Jupiter mass planet to very short pericentric distances requires unrealistic high disc masses, these requirements are much smaller for smaller migrating planets. Our simulations for a slowly migrating 0.1 $M_{\rm Jupiter}$ planet, even demanding a possible primitive disc three times more massive than a primitive solar nebula, produces maximum [Fe/H] enrichments of the order of 0.18 dex. These calculations open possibilities to explain hot Jupiters exoplanets metallicities.
Comments:
Accepted for publication by Monthly Notices of the Royal Astronomical Society
Subjects:
Astrophysics (astro-ph)
Cite as:
arXiv:0704.2997v1 [astro-ph]
Submission history
From: Othon Cabo Winter [view email]
[v1] Mon, 23 Apr 2007 13:10:36 GMT (174kb)
http://arxiv.org/abs/0704.2997
How is it that our sun, containing 98% of the matter in the solar system has only 2% of the angular momentum? Even the proposed magnetic arms that would slow the protosun would also slow the protoplanets/dust cloud. There is no evidence of the sweeping action of protoplanets. There is the Poyinting-Roberson effect of our sun sweeping out dust debris. The fact that there still IS inter-planetary dust is an indication of a young solar system. I noticed several speculative phrases such as “It is thought,” “they are thought,” “Things aren’t nearly as clearcut as they might seem.” Even the “picture” was only and artist’s rendition. Science should deal in facts not warm fuzzy stories spun to comfort groping philosophers. whitely@catt.com
Am I missing something here? If the star’s magnetic arms were slowing the rotation of the star, wouldn’t the kinetic energy of the star be transfered to the swirling dust disk. After all, the dust disk is not fixed to an anchor. If this theory were true the disk would also have to contain enough mass to make a significant counterweight to the star. Since in our solar system this would not be the case, most of the mass being contained in the sun, is it reasonable to believe it would be the case in the average proto solar system? The transfer of energy to the disk would greatly increase it rotational speed, possibly sending it away from the star at escape velocity, and probably eliminating any chance of planetesimal formation.
Hi Ernst & Gerald
Kinetic energy is lost to the disk as a star deccelerates its angular velocity and sheds momentum – that’s what drives the polar outflows seen in young stars, particularly the T-Tauri phase of formation. Planetesimals don’t begin their formation until after the angular momentum is shed.
Disks are typically assumed to be about 1-10% of a star’s mass, sufficient to soak up the excess angular momentum because the radius of the disk is so much bigger than the radius of the star.
Gerald, the PR effect, and related relativistic effects powered by sunlight, does indicate relatively youthful origins for the dust – spectroscopic examination allows us to identify asteroidal families that it originates from, and orbital back-tracking of those families allows the original break-up event to be dated. Current dust is from break-ups from a few million years ago at most.
As for the angular momentum problem the magnetic braking scenario has been confirmed both observationally and theoretically, so there is currently no problem as such. What is puzzling cosmogonists is just how the originally fine dust managed to clump into rocks big enough to become planetesimals, but new simulations are showing how that can happen too.
There is no evidence for the Solar System, as a whole, having a youthful origin.
I realize that this is an old article and that the intended target of this statement will probably never see this, but I feel I must respond to Gerald’s last statement.
“Science should deal in facts not warm fuzzy stories spun to comfort groping philosophers.”
The point of science is to answer questions to the best of the current abilities of the scientists and to allow for question and revision. Once things are declared as fact then those things are no longer science because inquiry has been eliminated. Thus, as long as this topic can be questioned it is still science, which means that science does in fact deal with warm fuzzy stories and wild stretches of the imagination. Thank you for contributing to science by questioning the aforementioned theories; that’s how we get stuff done.
Ernst’s points are valid. There must be a mechanism to “slow” the proto sun/planets’ rotation (Newton’s 1st law). I am puzzled by Adam’s idea of “shedding of angular momentum.” Again, there must be a mechanism that produces the “shedding.” (Magnetic fields – in the vacuum of space – neither “slow” nor “speed up.”) The fact that spectroscopic analysis of the dust and the asteroids is identical does not “prove” accretion of the dust into asteroids any more than it “proves” that the asteroids disintegrated into the dust. Actually, to be rather unorthodox, the identical spectroscopic fingerprints could “prove” that they were both “made” at the same time from the same material. There, I’ve come right out and said it! As Jacob observed, we should continue searching for answers – wherever the answers may lead. I feel that some “answers” are considered “out of bounds” simply because they are not mainstream. whitely@catt.com