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typical system consists of a dozen or so A- and B-type stars spread out over perhaps 10 pc. Intermingled with these objects is a larger population of low-mass stars, many of which are T Tauri's. The majority of intermediate-mass stars in the association are on the main sequence, but some have the emission lines indicative of pre-main-sequence contraction. From Figure 1.18, these latter members have nearly identical luminosities as ZAMS stars of the same mass but lower effective temperatures. One well-known R association is Mon R1, located near the NGC 2264 cluster. Other examples include Ori R1 and Ori R2. The first is within the L1630 region of the Orion B molecular complex (Figure 1.3), while the second is a more widely dispersed collection of reflection nebulae centered on the Trapezium in Orion A. Table 4.1 lists all the R associations within 1 kpc. Here we give both the distance and the number of identified B stars.

Photometric techniques are employed to study the properties of the dust within R associations. Recall that stars already on the main sequence have well-known absolute magnitudes and intrinsic colors as a function of their spectral type. Given the latter, the apparent B - V color of a main-sequence star illuminating a reflection nebula immediately yields its color excess EB-V. Suppose now that the distance to the association is already established. Then the apparent V-magnitude of the same member star gives its associated AV through equation (2.12). One common result of such investigations is that AV varies substantially from one star to another within the R association, indicating a clumpy distribution of dust. More intriguing is the fact that the ratio AV/EB-V is often higher than the fiducial interstellar value given by equation (2.16). Such "greyer" extinction is a sign that the typical grain is abnormally large, the result presumably of continuing mantle growth within the denser portions of the enveloping clouds.

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