Section 4.1 For the relative birthrates of stars in different cluster environments, see
Miller, G. E. & Scalo, J. M. 1978, PASP, 90, 506. Two reviews of embedded clusters, both stressing near-infrared observations, are
Zinnecker, H., McCaughrean, M. J., & Wilking, B. A. 1993, in Protostars and Planets III, ed. E. H. Levy and J. I. Lunine (Tucson: U. of Arizona Press), p. 429
The 2MASS (Two Micron All Sky Survey) project has provided a view of the Milky Way in three near-infrared bands:
Skrutskie, M. F. et al. 1997, in The Impact of Large Scale Near-Infrared Surveys, ed. F. Garzon et al. (Dordrecht: Reidel), p. 25. The 2MASS data were released in 2003. For the original classification of embedded stars by their infrared spectral index, see
Lada, C. J. 1987, in Star Forming Regions, ed. M. Peimbert and J. Jugaku (Dordrecht: Reidel), p. 1.
Section 4.2 The objects now called classical T Tauri stars were first recognized as a distinct group by
Joy, A. H. 1945, ApJ, 102, 168, while their weak-lined counterparts were classified by Walter, F. W. 1986, ApJ, 306, 573.
Note that Walter actually identified the "naked" group, i.e., those weak-lined T Tauri stars lacking near-infrared excess. For the morphology of nearby T associations, see the discussion of Chamaeleon by
Schwarz, R. D. 1991, in Low-Mass Star Formation in Southern Molecular Clouds, ed. B. Reipurth (ESO Publication), p. 93, and that of Taurus-Auriga in
Palla, F. & Stabler, S. W. 2002, ApJ, 581, 1194. The identification of R associations is due to Van den Bergh, S. 1966, AJ, 71, 990.
Section 4.3 Two contributions of historical interest on the dynamical expansion of OB associations are
Ambartsumian, V. A. 1955, Observatory, 75, 72. A modern review of these associations is
Garmany, C. D. 1994, PASP, 106, 25. The relationship of these groups to molecular clouds is analyzed in Williams, J. P. & McKee, C. F. 1997, ApJ, 476, 166.
Section 4.4 The reader wishing to see how spectroscopic parallax is used in practice to derive the distances to young clusters may consult
Perez, M. R., Thé, P. S., & Westerlund, B. E. 1987, PASP, 99, 1050.
For properties of the Galactic system of open clusters, see
Janes, K. A., Tilley, C., & Lyngâ, G. 1988, AJ, 95, 771.
A theoretical work that lucidly discusses the main issues in their evolution is
Section 4.5 The concept of the initial mass function and its first determination are due to
Salpeter, E. E. 1955, ApJ, 121, 161. A comprehensive review that delves into the many subtleties in this continuing endeavor is
Scalo, J. M. 1986, Fund. Cosm. Phys., 11, 1. For a more recent discussion, see
Plate 1 left: Optical photograph of the NGC 2024 cluster in Orion B. The image covers an angular size of 4' x 10', or 0.4 x 1 pc. right: Near-infrared image of the same cluster. The vertical scale matches that of the optical image. This is a composite picture combining three separate mosaics in the J, H, and K wavebands. The color coding is blue, green, and red, respectively.
Plate 2 Near-infrared image of the S106 bipolar nebula. This is a composite of the J, H, and K wavebands. The color coding in this and all other composite, near-infrared images is the same as in Plate 1.
Plate 3 Negative optical photograph of three HII regions in the Gem OB1 association. The regions span a total distance of 9 pc.
Plate 4 Expanded near-infrared (J, H, and X) image of Gem OB1. The bright nebula in the center lies between the left and center HII regions of Plate 3, and is invisible optically.
Plate 5 Composite, near-infrared image (J, H, and K) of the region surrounding the Herbig Be star BD+4004124. This object is the brightest central spot. The most prominent companion is an emission-line star, V1686 Cyg, that is also optically visible, but most of the other, nearby objects can only be seen in the near-infrared.
Plate 6 Near-infrared image (J, H, and K) of NGC 7538. The entire image covers 12' x 12', or 9.5 x 9.5 pc at the 2.7 kpc distance of the region. The red patches are embedded clusters that appear to be younger than the prominent HII region.
Plate 7 Composite J, H, and K-band near-infrared image of the dense cluster NGC 3603. The field of view is 3/5 x 3/5, or 6 x 6 pc at a distance of 6 kpc.
Plate 8 Composite image in B, V, and ^-bands of 30 Doradus in the Large Magellanic Cloud. The gas filaments surrounding the central cluster span about 50 pc.
Physical Processes in Molecular Clouds
The Formation of Stars
Steven W. Stahler and Francesco Palla © 2004 WILEY-VCH Verlag GmbH & Co.
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