T and R Associations

The fate of an embedded cluster depends partially on how its gas is dispersed. In many cases, one or more high-mass stars drive off the interstellar matter relatively quickly. The result is the expanding group of stars known as an OB association. Other systems were born in dark cloud complexes that never contained massive stars. The extended distribution of low-mass stars in Taurus-Auriga (Figure 1.11) is not the product of rapid dispersal, but largely reflects the initial extent of the parent cloud. Not only are many of the visible stars too young to have spread far, but a significant number of even younger, embedded sources are commingled spatially with the others. Since the vast majority of the members in this system are T Tauri stars, the Taurus-Auriga stellar complex is designated generically as a T association. The term was introduced by V. Ambartsumian in 1949, four years after the identification of the T Tauri class by A. H. Joy.

4.2.1 T Tauri Star Birthsites

While the existence of an embedded cluster can be initially established by simply examining near-infrared images, finding a T association requires also the identification of T Tauri stars per se. These come in two varieties. The classical T Tauri stars are conspicuous spectroscopically for their strong optical emission lines in Ha, as well as the H and K-lines of Ca II at 3968 A and 3934 A, respectively. A practical and efficient search strategy, then, is to equip wide-field telescopes with objective prisms that can simultaneously record many stellar spectra over a few square degrees of the sky. Such surveys, however, capture only part of the population of interest. At least as numerous as the classical members are the weak-lined T Tauri stars. As their name implies, these stars lack strong emission lines, although the two types overlap substantially in age. The weak-lined population was actually discovered through its enhanced X-ray emission relative to main-sequence field stars. X-ray detection was made initially with the Einstein satellite, launched in 1979 and operational through 1981. The launch of the more sensitive ROSAT satellite in 1991 provided more weak-lined candidates.

Figure 4.7 shows the distribution of nearby T associations in the Galactic plane. The groups range from TW Hydrae, a small aggregate of T Tauri stars only 50 pc away, to the populous and highly active NGC 1333 in the Perseus cloud complex. Named for a bright reflection nebula, the latter region contains dozens of visible young stars and over a hundred embedded ones, including many that drive molecular outflows. Within the Serpens region, also depicted here, most visible stars are associated with the L572 cloud, which has a dense complement of infrared sources. Finally, the large molecular complex in Cygnus has both revealed T Tauri stars and embedded sources in the L984 and L988 clouds.

Our figure also includes major embedded clusters, such as p Ophiuchi and IC 348 (also part of the Perseus cloud). Indeed, all the regions shown in this map contain both obscured and visible objects. Within p Ophiuchi, for example, is a large group of revealed low-mass objects on the outskirts of the compact L1688 core. The numerous visible stars of IC 348 are more centrally concentrated. Surveys in Ha have discerned a scattered population of visible stars lying outside the four obscured clusters of Orion B. In summary, embedded clusters and T associations should be viewed as extremes along a continuum of morphological types.

The properties of T associations are well illustrated by the nearest prominent example, Taurus-Auriga, which has been scrutinized thoroughly in the infrared, optical, and X-ray regimes. Here, it has been possible to establish the membership of most stars kinematically. One first obtains a radial velocity, Vr, for each star by examining a convenient portion of its optical spectrum. We compare absorption lines with those of a standard star of the same spectral type, yielding the Doppler shift and hence Vr. In Taurus-Auriga, the results for both classical and weak-lined members agree well with the velocities of the local cloud gas, as obtained from molecular lines. Note that both cloud and stellar Vr-values change over the length of the complex, but the entity as a whole appears to be gravitationally bound.

Obtaining the orthogonal component of velocity, i. e., the proper motion, requires comparison of at least two wide-field images well separated in time. Such comparison shows that the proper motions of Taurus-Auriga stars cluster tightly about a single vector, as expected. The one-dimensional dispersion is from 2 to 3 km s_1. No velocity information is available for the more deeply embedded (Class I) members, which constitute about 10 percent of the population. The total number of kinematically confirmed members within the boundary of the complex shown in Figure 1.11 stands at about 100, with 60 being classical and the rest weak-lined T Tauri stars. This tally is probably complete down to a V-magnitude of+15.5.

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IC 348 ^NGC 1333

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