FEATURE ARTICLE
Protostars
"Stellar embryology" takes a step forward with the first detailed look at the youngest Sun-like stars
Thomas Greene
Youthful Energy
Our Galaxy appears to have a healthy lust for making stars, with many places that can be aptly called "stellar nurseries." Some of these nurseries may consist of merely a few young stars, whereas others may contain many hundreds. Among the more notable of these are the Orion Nebula and the star-forming clouds near the star Rho Ophiuchi. These regions have been studied for decades and have provided the observational basis for much of what we do know about the first stages of star formation.
The PMS stars were among the first young stellar objects to be observed, which is not surprising given that many of them are optically visible. In the 1950s, Merle Walker, then at the Mount Wilson and Palomar Observatories, noticed that some stars in the vicinity of dark clouds were intrinsically brighter (more luminous) than their main-sequence counterparts of the same temperature. Since an object's luminosity is proportional to its radius (R) and temperature (T), in the relation R2T4, it was apparent that these objects were larger than the main-sequence stars. This is precisely what theoreticians had predicted for a PMS star that was still in the process of contracting.
Protostars were not discovered until the pioneering infrared surveys of the 1960s and 1970s, which found several candidates embedded deep within the dark clouds. Space-based observations later showed that protostars are up to 10 times more luminous than PMS stars. But since protostars are not much bigger than PMS stars, it suggested that they had an extra source of energy that was contributing to the protostar's luminosity.
In the decades that followed, astronomers refined their observations of young stellar objects. Such studies included the object's spectral energy distribution—the amount of energy the object releases over a range of wavelengths (especially in the infrared part of the spectrum). As it happens, measurements of an object's spectral energy distribution are surprisingly informative.
In particular, the shape of the distribution reveals the evolutionary stage of the object (Figure 6). Since the various components of the object—the central star, the dusty envelope and the circumstellar disk—each have different temperatures, the amount of infrared radiation the object emits at a particular wavelength tells us which components are present. The spectral energy distribution of a protostar, for example, is dominated by an infrared emission near 100 micrometers, which represents the radiation from the large outer reaches of its chilly (30-kelvin) envelope. The presence of the envelope also explains why protostars are more luminous than PMS stars: Matter from the envelope is still falling onto the protostar. As the material slams into the protostar's surface, its gravitational energy is converted to thermal energy—the source of the infrared radiation.
In contrast, the spectral energy distribution of a PMS star with a circumstellar disk is dominated by the central star, with a peak emission near 2 micrometers. The disk itself shows up as a broad range of emissions, especially at longer wavelengths, representing the various temperatures of the disk (which decrease with distance from the central star). In the absence of the dusty envelope, the accretionary process has slowed to a trickle, if anything at all, in the PMS star.
As valuable as these measurements have been to our understanding of star formation, they only scratch the surface when it comes to revealing the physical properties of the young stars themselves. A much better probe is a star's true spectrum, which shows atomic and molecular absorption lines (Figure 8). Such spectra reveal the identity of the elements and molecules present in the star's atmosphere, as well as provide an accurate reading of its temperature, its radius and its rate of rotation.
Modern telescopes and infrared detectors have been able to show the absorption lines in the spectra of some young stars still embedded in dark clouds. These turn out to be PMS stars, which are very similar to their optical counterparts, except that they are still hidden in the dust. However, there are a number of young stars in the dark clouds that do not show strong spectral absorption features. The spectral energy distributions of these objects suggest that they are protostars, and they offer a special challenge for stellar spectroscopy.
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