FEATURE ARTICLE
Protostars
"Stellar embryology" takes a step forward with the first detailed look at the youngest Sun-like stars
Thomas Greene
Spinning in the Cradle
Among the more interesting physical properties that absorption-line spectra allow us to measure is the star's rate of rotation. All stars spin on their axes, a quality that is believed to be inherited from the rotation of the dense core as it collapses in the prenatal dark cloud. Our Sun has a rotation period of about 26 days, which corresponds to a velocity of about 2 kilometers per second at its equator. This actually corresponds to a tiny fraction (one percent) of the angular momentum in our solar system, even though the Sun contains the bulk of the system's mass. Instead, most of the solar system's angular momentum lies in the planets, especially Jupiter and Saturn. One of the mysteries of solar-system formation is how the bulk of the mass gets concentrated in the center, while almost all of the angular momentum is transferred to the periphery. One road to answering this question is to observe how the rate of rotation changes during the early stages of a star's formation.
The key to measuring a protostar's rotation is the carbon monoxide molecule. At a wavelength of about 2.3 micrometers, the absorption line of carbon monoxide has detailed structure that is very sensitive to the rotation of the protostar, but is not altered by the star's other physical properties (Figure 10). By comparing the observed shapes of this feature to synthetic spectra with different rotation velocities we can determine the object's rate of rotation. Actually, what we do measure is the object's projected rotation velocity. Since we don't know the exact alignment of the object's axis to our line of sight, we are not getting a full measure of the object's rate of rotation. Even so, this information is extremely valuable because the orientation of these objects should be more or less random with respect to the Earth.
The results are intriguing. Nearly all of the protostars that we observed are rotating quickly, with projected velocities greater than 20 kilometers per second. On the other hand, most PMS stars with circumstellar disks are rotating slowly, with projected velocities less than 20 kilometers per second. What accounts for the differences in rotation velocities?
Several theorists have shown that the rotation velocity of a young star can be controlled by its circumstellar disk through the star's magnetic field. If this process occurs in both types of young stellar object, it means that the protostars must be coupled to regions of their disks that are spinning faster than those of PMS stars (Figure 11). Since Kepler's third law dictates that the circumstellar material that orbits a star most quickly must be closest to the star, our results suggest that protostars are coupled to nearby parts of their disks. This may arise because protostars are still accreting large amounts of matter directly from their disks. Or it may be that the accreting matter carries a substantial angular momentum, which "spins up" the protostars.
In contrast, PMS stars are no longer accreting much matter and instead may have experienced some braking. "Disk braking" is thought to be produced by ionized gas in the circumstellar disk which tugs on the star's magnetic field, slowing the spinning star. The phenomenon can be likened to pulling on a sticky wad of bubble gum that is attached to a spinning baseball. The ball may not stop spinning, but it will slow down. Whatever the mechanism, our results suggest that the transition from protostar to PMS star involves a drop in rotational velocity and a coincident drop in angular momentum.
The absorption-line spectra also reveal some other interesting phenomena involving the accretion process. When we look at an object's spectrum we must distinguish whether it arises from the atmosphere of the central star or from its circumstellar disk. Since the accretion process will make the disk hot and relatively dense, it should have its own emission spectrum and absorption lines. Fortunately, the physical differences between the disk and the star help us to discriminate between their spectra. The disk is less dense than the star, so its gas is under less pressure, which means that certain elements will have different levels of ionization (which are apparent in the spectrum). Moreover, disks rotate at different speeds at each radial distance, whereas stars rotate essentially as a solid body. These physical differences translate into substantial differences in the relative strengths of the absorption lines and the velocity profiles of the carbon monoxide features.
The spectral features of nearly all protostars that my colleagues and I have studied clearly indicate that they arise from the stars' atmospheres and not their circumstellar disks. However, we have found a handful of young stars that intermittently show disk-like features. They turn out to be FU Orionis variables, which also show large, episodic changes in their luminosity. One possible explanation for this puzzling observation is that the FU Orionis protostars accrete material through their disks in brief spurts. Most of the time they would be in low-accretion states, during which they would show star-like spectral features. Occasionally, however, they would undergo periods of high accretion, when their luminosity increases and their spectra become dominated by disk-like features. This idea, largely championed by Lee Hartmann of the Smithsonian Astrophysical Observatory, is currently the best explanation out there.
Might all protostars go through an FU Orionis phase? So far the infrared spectra of protostars and hundreds of PMS stars show little or no evidence of FU Orionis-like features. If the FU Orionis phase of evolution were common to all young stars, one would expect at least a few of these stars to show some evidence of accretionary spurts. Perhaps the FU Orionis stars are merely freaks.
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