Map of Taurus-Auriga in the 2.6 mm line of 12C16O. Prominent dark clouds are
Molecular Cloud (see Fig. 1.1), but only because it is closer. At a distance of 140 pc, the entire region measures about 30 pc in linear extent. As before, we have labeled some of the main dark clouds, which show up as local peaks in the CO emission; Barnard's optical image is centered on L1521 and L1495. The dense core TMC-1 has been closely studied for its wealth of interstellar molecules. Shown also in Figure 1.9 is the famous star T Tauri, to which we shall return shortly. Note finally that the elongated structure to the north, including L1459 and L1434, and the high-intensity area to the west containing Barnard 5 (B5) and NGC 1333, are not physically associated with the Taurus-Auriga system, but are more distant, massive clouds seen in projection.
In passing from Orion to Taurus-Auriga, we have not only diminished the physical scale of the star forming region but also the mode of stellar production. The gas in Taurus-Auriga is not part of a giant complex; the total mass in molecular hydrogen is of order 104 Mq . Nor is the region forming high-mass stars, with their attendent reflection nebulae and HII regions. Finally, the existing low-mass stars are nowhere clustered as densely as in the Trapezium or even the less populous groups of Orion B.
Let us now focus on the dark clouds of the region, using two different tracers. The upper panel of Figure 1.10 is a map of the central portion of Taurus-Auriga, covering TMC-1, TMC-2 and the dark cloud L1495 in Figure 1.9. The image shows emission from 13C16O. This isotope
is excited at the same volume density as the more common 12C16O. However, it effectively highlights regions of greater column density, which tend to trap the radiation from the more abundant species. Notice that the dark clouds have an elongated appearance, not unlike those in Orion (Figure 1.3).
As before, the infrared emission from warm dust grains is another useful tracer of molecular gas. The lower panel of Figure 1.10 is an IRAS map of the same region as in the upper panel. What is shown here is the ratio of the fluxes at 100 ^m and 60 ^m, a measure which emphasizes the contribution from the colder (near 30 K) grains. There is evidently an excellent match between the distributions of this dust component and the gaseous structures traced in 13C16O.
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