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"Stellar embryology" takes a step forward with the first detailed look at the youngest Sun-like stars

Thomas Greene

Playing Peek-a-Boo with Protostars

It's been said that protostars are the "Holy Grail" of infrared astronomy, and this statement pretty much sums up the difficulty of the pursuit. The dust that surrounds a protostar dims its visible light by a factor of one billion, and even its near-infrared emissions (wavelengths around 2 micrometers) are dimmed by a factor of 10. Much of this dust is very close to the star (about 5 stellar diameters away), and is heated to a temperature of nearly 1,500 kelvins by the star's radiation and the energy liberated by matter falling in from the envelope and disk. The glow of the warm dust usually emits several times as much near-infrared radiation as the central star itself. The protostar is effectively washed out—or veiled—by its surroundings. What is an infrared astronomer to do?

The solution lies in bigger telescopes and extremely sensitive infrared detectors. Fortunately, such equipment now exists. Three of the more powerful infrared telescopes are located atop Mauna Kea on the big island of Hawaii: the 3-meter NASA Infrared Telescope Facility (IRTF), the 3.8-meter United Kingdom Infrared Telescope and the giant 10-meter Keck II telescope. These telescopes collect infrared radiation with their large primary mirrors, which focus the light into spectrographs. These spectrographs use large diffraction gratings to disperse the light (into its component wavelengths), which is then focused again onto sophisticated infrared detector arrays (Figure 7). The optics and the detectors must be cooled to as low as 30 kelvins (with liquid nitrogen and mechanical coolers) to prevent the infrared radiation of the instruments from overpowering the faint signals from the young stars.

Charles Lada, of the Smithsonian Astrophysical Observatory, and I have recently used the IRTF and Keck telescopes to make the first detections of infrared absorption lines in Sun-like protostars. The task required a large amount of observing time (about a week per year) to build up a sufficient signal. So far we have recorded about a dozen high-resolution spectra of strongly veiled protostars in the dark clouds near Rho Ophiuchi.

Interestingly, the atomic spectra of these objects are similar to main-sequence stars that are very cool—except that their absorption features are much weaker. (Main-sequence stars such as red dwarfs can have very cool surfaces, even though their core temperatures are hot enough for thermonuclear fusion.) We interpret this to mean that the protostars have surface temperatures that are similar to the cool stars, about 3,500 to 4,000 kelvins. This is about the same as the PMS stars in the same clouds, which have temperatures as high as 4,500 kelvins. We need more observations to see whether there are truly any significant differences in the temperatures or sizes of these young stars.

We have also quantified the veiling of these protostars by measuring the depth of their absorption lines relative to main-sequence stars and PMS stars of equivalent temperatures. At wavelengths of about 2 micrometers, some of the veiling is as much as three times brighter than the star itself! We think this "excess" emission is caused by the active accretion of material either onto the protostar or within its circumstellar disk. In some instances the veiling is even greater than a threefold brightness of the central star. We interpret this higher level of veiling as the accretion of matter from the circumstellar envelope.

By subtracting the excess emissions we can also determine how much infrared radiation comes from the central star itself. This allows us to estimate the protostar's intrinsic brightness, which reveals a luminosity comparable to the PMS stars in the same cloud. This suggests that protostars and PMS stars are about the same size. These measures of the protostar's luminosity may appear to be in slight conflict with their spectral energy distributions, which indicate that protostars are several times brighter than PMS stars. However, the spectral energy distributions of the protostars also include the accretion of matter onto their central stars, a process that releases gravitational energy.

Figure 9. Hertzsprung-Russell diagram . . .Click to Enlarge Image

The temperatures and luminosities of the protostars can also be converted to estimates of their ages and their masses. Theorists generally model the evolution of PMS stars, but are now beginning to consider the effects of accretion so that their models can be applied to protostars. This was recently done by several independent groups of astrophysicists, including three Italian theorists, Francesco Palla, Francesca D'Antona and Italo Mazzitelli, as well as Steven Stahler of the University of California, Berkeley, and Isabelle Baraffe and Lionel Siess of France. Their sophisticated calculations allow us to place the protostars on a temperature-luminosity—or Hertzsprung-Russell (H-R)—diagram for the first time ever (Figure 9). This is no small accomplishment, since the H-R diagram is the key to providing the age and mass of a star.

All of the protostars in the plotted sample come from the same dark cloud near Rho Ophiuchi. They appear to be very young, effectively defining the observational birthline where stars first appear on the H-R diagram, and all appear to be lightweights of about 0.5 solar mass. Similarly, PMS stars in the same cloud appear to span a range of masses with a peak near 0.5 solar mass. The masses of protostars and PMS stars are generally expected to be similar because the protostars should have accreted nearly all of their final mass at this stage of their evolution. However, many more protostars will have to be studied to determine whether there is a significant difference between their masses and those of PMS stars.

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