Clusters of optically revealed stars are the scant remains of much more populous systems created within the dense interiors of molecular cloud complexes. Although no complete census exists, observations are consistent with the hypothesis that most stars form in such environments. We shall use the term "embedded cluster" generically, to signify any group of physically related stars so obscured by ambient molecular gas that most can be detected only at infrared and longer wavelengths. In applying this terminology, we recognize that the issue of whether any particular group will remain gravitationally bound following dispersal of its gas is rarely, if ever, known with confidence.
The Formation of Stars. Steven W. Stahler and Francesco Palla Copyright © 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-40559-3
The discovery of embedded stellar aggregates resulted from a key technological advance in infrared astronomy. Prior to the early 1980s, observational surveys at these longer wavelengths could not attain the fine detail available with optical instruments. The situation dramatically changed with the advent of near-infrared array detectors. These solid state devices provide in a relatively short exposure time detailed views of embedded systems, even those of large angular size. A filter in one of the standard wavelengths precedes the detector, so that a monochromatic image results. Combining several such images also allows one to produce a composite, false-color rendition. We will later show examples of both types.
The utility of near-infrared radiation for penetrating large columns of molecular cloud gas is evident from the interstellar extinction curve (Figure 2.7). It can be seen that a photon in the K band, centered at 2.22 pm, has an extinction 0.1 times that of a V-band photon at 0.555 pm. Consider now a representative T Tauri star of spectral type K7, with MV = +6.5 and Mk = +2.2. If such a star were at a distance of 200 pc, equation (2.12) tells us that it would have an apparent V-magnitude above a reasonable detection threshold, e. g., mV = +25, only if the associated cloud extinction AV were under 12 mag. On the other hand, the same star could be inside a cloud with AV = 100 and still be detectable at K, where the limiting magnitude at large, groundbased telescopes is currently about +20. Of course, observations in the mid- and far-infrared regimes would be even more effective in this regard. Such radiation, however, is so strongly absorbed by the Earth's atmosphere that its detection, at the longest wavelengths, requires spaceborne instruments. The 1983 launch of IRAS first allowed reconstruction of the nearly complete spectral distribution of emitted energy from numerous embedded stars.
Clusters inside molecular clouds have thus far mostly been discovered through surveys in a single near-infrared waveband. A cluster is usually first identified as a region with a significant overdensity of sources compared with nearby fields. However, this initial reconnaissance work is never sufficient to establish the true membership. Many, if not most, of the stars in the group will be intrinsically fainter than the ones first seen. Besides accounting for completeness, it is also necessary to separate out background objects that are reddened by the same cloud. One may estimate the total cluster population statistically by using off-cloud observations to subtract the expected number of background and foreground stars in the region. Individual cluster members can be selected in principle by their proper motion (movement in the sky relative to the background), but one needs at least two observations widely separated in time. Other identification techniques include spectroscopy and multicolor photometry.
Let us consider further the photometric method, which often combines K-band observations with those at J (1.25 pm) and H (1.65 pm). The principal tool in such studies is a near-infrared color-color diagram. As illustrated in Figure 4.1, one plots the J — H color on the vertical axis and H - K horizontally. The magnitude conventions imply that the numerical values of both J — H and H — K increase for redder, cooler stars. In any individual case, the observed colors depend both on the photospheric properties and on the extinction provided by the cloud. Background stars, however, exhibit a well-defined relationship between the two colors.
To obtain this relationship, we assume that all sources outside the cluster are either main-sequence stars or the rarer but more luminous red giants. If stellar surfaces radiated as perfect blackbodies, equation (2.29) indicates that the ratio of emergent fluxes at any two wavelengths would be a unique function of the temperature Teff. The dotted line in Figure 4.1 displays the blackbody values of J—H and H—K for the indicated range of Teff. Stellar photospheres depart
Figure 4.1 Near-infrared color-color diagram. The J — H and H — K color indices are displayed as the vertical and horizontal axes, respectively. The solid curve shows the relation between these indices for main-sequence stars (lower branch) and giants (upper branch). The dotted curve shows the colors of blackbody spectra at the indicated temperatures. Straight dashed lines indicate the relative color changes due to interstellar reddening.
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