As we consider progressively more massive young stars, any cloud material in proximity not only reflects starlight, but starts to generate its own optical radiation. This emission stems from ionization created by the ultraviolet component of the stellar spectrum. The O and early-B stars capable of such ionization are themselves often found in loose collections of a few dozen members. Although their boundaries are often difficult to locate with any precision, these OB associations extend over regions that can be as small as ordinary open clusters, or as large as several hundred parsecs in diameter.
Historically, the tendency for O and B-type stars to cluster was recognized as soon as precise spectral classification became available, at the beginning of the 20th century. Spectroscopic and proper motion studies gave a physical basis to the observed grouping by establishing common spatial velocities for bright stars in Orion, Perseus, and the Scorpius-Centaurus region. It gradually became clear that their large internal velocities, typically about 4 km s-1, doom these systems to expansion and eventual dispersal. Observational verification of the expansion came in 1952, when A. Blaauw measured proper motions in what is now called Per OB2 (signifying the second OB association in the Perseus region). With this discovery came understanding of the great size range of these systems. The largest are the oldest, with maximum inferred ages of about 3 x 107 yr.
None of the massive stars in OB associations have optically visible pre-main-sequence contraction phases. In any given system, therefore, most of the luminous members lie on the main sequence, while a smaller fraction are supergiants caught in the act of leaving it. This fact allows determination of the distance to the association, through the technique known as spectroscopic parallax. The first step is to obtain, through analysis of absorption lines, the spectral types of as many member stars as possible. Photometry in the V-band then allows placement of the association in a diagram plotting mV against spectral type. At this point, we again utilize the fact that the absolute magnitude MV is a known function of spectral type along the main sequence (Table 1.1). The difference between MV and mV for each star then yields the distance via equation (2.12). Of course, this equation cannot be used without knowledge of the interstellar extinction AV. Since the extinction is itself proportional to distance, one must obtain a self-consistent solution through trial and error.
For individual stars located outside associations, a variant of the method is still feasible. First, one uses spectroscopic analysis to verify that the object is on the main sequence. Photometry in two wavebands then establishes the apparent color. This may be compared with the intrinsic color index to yield the reddening and extinction, and hence the distance. Note that a basic assumption underlying spectroscopic parallax is that the standard relations hold between reddening and extinction, and between extinction and distance. The method is therefore un-suited for investigating clumpy clouds or those with anomalously large grains, although such local effects presumably fade in significance with greater distance. Indeed, many visible OB associations are too far away for accurate spectroscopy. In these cases, photometry in three wavebands suffices to place the member stars in a color-color diagram analogous to Figure 4.2. If the stars lie off the appropriate main-sequence curve, one may draw the reddening vector to read off the extinction and thus the distance. When applied to the (U — B, B — V) diagram, this procedure is traditionally known as the Q method.2
Such techniques have facilitated the location of hundreds of O and B stars throughout the Galactic disk. Early efforts yielded the first convincing delineation of the local spiral arms, a discovery soon corroborated by researchers employing the 21 cm line of HI. OB associations trace the spiral structure as reliably as the interstellar gas because their constituent stars are too young to have moved far from their birthsites. Out of 200 O stars within 3 kpc of the Sun, some 75 percent are within associations. The figure falls to 50 percent for B0 - B2 stars, whose population is not as completely sampled. Modern study of OB associations has been greatly facilitated by the remarkable sensitivity of charge-coupled detectors (CCDs). In addition, near-infrared arrays and X-ray detectors have enabled us to probe the lower-mass component of these systems.
2 Consider, for any observed star, the quantity Eu—b/Eb—V. The relation analogous to equation (4.1) together with the interstellar extinction curve (Figure 2.7) imply that this ratio has the numerical value 0.71, independently of the actual degree of reddening to the star or its spectral type. From the definition of the color excess (equation (2.14)), it follows that
(U - B) - 0.71 (B - V) = (U - B)0 - 0.71 (B - V)◦ = Q.
The quantity Q is thus also reddening-independent, but varies with spectral type. Note that (U - B)0 and (B - V)◦ are functionally related along the main sequence. Knowledge of Q from the apparent magnitudes therefore also yields the star's intrinsic colors and hence the reddening.
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