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Figure 4.7 Distribution within the Galactic plane of nearby T associations and embedded clusters.

Note the longitude convention: the Galactic center is at l = 0°, while Cygnus lies near l = 90°.

Note also the symbol representing the Sun's location.

Figure 4.7 Distribution within the Galactic plane of nearby T associations and embedded clusters.

Note the longitude convention: the Galactic center is at l = 0°, while Cygnus lies near l = 90°.

Note also the symbol representing the Sun's location.

Although associations do not exhibit the high degree of central concentration seen in embedded clusters, all large molecular clouds, including dark cloud complexes, are intrinsically clumpy. Hence, some degree of clustering is to be expected in the stars born from such structures. Figure 4.8 confirms this hypothesis in the case of Taurus-Auriga. The heavy contours are lines of constant stellar surface density in the plane of the sky, while the lighter curve is the CO boundary from Figure 1.11. Each of the 6 groups, containing 5 to 20 stars each, has a projected radius under 1.0 pc and an internal, radial velocity dispersion of 0.5 to 1.0 km s_1. Thus, most of the total measured dispersion across the complex actually stems from the relative motion between these subunits.

4.2.2 HR Diagrams: Main-Sequence Turnon

A particularly effective means of visualizing the composition and evolutionary status of a T association is to place its optically visible members in the theoretical HR diagram. Figure 4.9 displays the diagrams for four associations. In most cases, the quantity L* has been extrapolated from the luminosity in the J-band. To this monochromatic value was added a bolometric correction. Strictly speaking, the latter is appropriate only for a main-sequence star. Detailed comparisons find, however, that the resulting L*-values are accurate to within an error of about 20 percent.

Right Ascension a

Figure 4.8 Clumping of stars in Taurus-Auriga. The heavy contours represent stellar surface density, while the solid grey contour is the border of CO emission from Figure 1.11.

Right Ascension a

Figure 4.8 Clumping of stars in Taurus-Auriga. The heavy contours represent stellar surface density, while the solid grey contour is the border of CO emission from Figure 1.11.

Beginning with Taurus-Auriga (Figure 4.9a), we see that many stars are crowded near the birthline, also shown in each panel. These members have only recently dispersed their obscuring envelopes of dust and gas. An even younger population is represented by the embedded, infrared sources, which, in the absence of a measurable effective temperature, cannot be placed in a conventional HR diagram. The youngest optical members are mostly classical T Tauri stars, symbolized by the filled circles, but contain an admixture of weak-lined members (open circles). Below the birthline, the number of low-mass stars drops off before the main sequence is reached, at isochrones corresponding to several million years. Thus, the complex as a whole began forming stars at that epoch and continues to produce them today.

It is also apparent that the proportion of non-emission stars increases markedly as a function of age. This trend is consistent with the fact that the strong emission lines present in the classical T Tauri population are absent in ZAMS stars of the same mass. In addition, most of the weak-lined stars, including the ones shown here, lack the infrared excess of their classical counterparts. 1 Thus, both the hot gas (T ~ 104 K) creating optical emission lines and the cooler

1 An infrared excess always signifies the presence of circumstellar dust. Hence those weak-lined stars with main-sequence spectral energy distributions in the infrared were formerly designated "naked" T Tauri stars. The open circles in Figure 4.9 also include some older, "post-T Tauri" stars, to be defined shortly.

Figure 4.9 HR diagrams for four stellar associations. In panels (a)-(c), closed circles represent classical T Tauri stars, while open circles are weak-lined and post-T Tauri stars. For NGC 2264, we show both classical T Tauri and Herbig Ae/Be stars, as well as main-sequence objects. The upper and lower solid curves in each panel are the birthline and ZAMS, respectively.

dust (T ~ 102 - 103 K) emitting in the near- and mid-infrared regimes gradually disappear in a low-mass pre-main-sequence object.

The highest stellar density in a nearby T association occurs in the Lupus constellation in the southern sky. Because of its location, about 10° south of p Ophiuchi, this large and active association is less well-studied than Taurus-Auriga. Star formation is mainly confined to four subgroups embedded in an extended dark cloud complex (see Figure 4.10). The total mass in molecular gas is 3 x 104 Mq , close to that for Taurus-Auriga, with a substantial fraction located in the isolated B228 cloud. The greatest concentrations of CO emission correspond to filamentary dust lanes apparent in optical photographs.

Originally, the young stars catalogued in Lupus were classical T Tauri's discovered in objective prism surveys. Later X-ray studies found other, weak-lined objects. In addition, there is a Herbig Ae star of 71 Lq. Fully half of the association members are in the Lupus 3 subgroup; these stars are only indicated schematically in Figure 4.10. The Ae star (HR 5999) is part of a binary pair at the center of this highly compact stellar birthplace. The HR diagram

Galactic Longitude I

Figure 4.10 Stars and molecular gas in the Lupus association. The contours trace 12C16O intensity. Large star symbols represent tight clusters.

Galactic Longitude I

Figure 4.10 Stars and molecular gas in the Lupus association. The contours trace 12C16O intensity. Large star symbols represent tight clusters.

for all the subgroups together (Figure 4.9b) again shows a population near the birthline, indicating current star formation activity. The picture will be more complete once infrared and weak-lined sources are studied more systematically. Finally, we note that a polarization map of background starlight reveals a well-ordered magnetic field oriented roughly perpendicular to the most prominent filaments.

In the case of Taurus-Auriga, the distance of 140 pc can be determined reliably by comparing the absolute and dereddened apparent magnitudes of main-sequence stars that have reflection nebulae, and are therefore physically associated with the dark clouds. For the more sparsely sampled Lupus region, a more uncertain figure of 150 pc follows from its proximity to the Scorpius-Centaurus OB association. Yet a third T association at a similar distance is that of

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