We present observations of ∼7 square degrees of the North American and Pelican Nebulae region at 24, 70, and 160 µm with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). We incorporate the MIPS observations with earlier Spitzer Infrared Array Camera (IRAC) observations, as well as archival near-infrared (IR) and optical data. We use the MIPS data to identify 1286 young stellar object (YSO) candidates. IRAC data alone can identify 806 more YSO candidates, for a total of 2076 YSO candidates. Prior to the Spitzer observations, there were only ∼200 YSOs known in this region. Three subregions within the complex are highlighted as clusters: the Gulf of Mexico, the Pelican, and the Pelican's Hat. The Gulf of Mexico cluster is subject to the highest extinction (A V at least ∼30) and has the widest range of infrared colors of the three clusters, including the largest excesses and by far the most point-source detections at 70 µm. Just 3% of the cluster members were previously identified; we have redefined this cluster as about 10-100 times larger (in projected area) than was previously realized.

We have considered the problem of elastic wave velocities in a matrix containing aligned ellipsoidal fluid-filled cracks. This problem is relevant to a variety of geophysical applications, including crustal and mantle seismology and the behavior of stressed and dilatant rock. When the cracks are ellipsoids of revolution, the composite is transversely isotropic and is describable with five elastic constants. For aligned oblate spheroids the major reduction in velocity occurs along the axis of symmetry. The opening of new cracks, the widening of old cracks, or the reduction of pore pressure accompanying crustal dilatancy can be expected to cause a large decrease in compressional velocity and considerable compressional wave anisotropy.Laboratory experiments indicate that crack porosity significantly depresses the seismic velocities even in lowporosity igneous rocks [Birch, 1960]. Anderson and Spetzle•' [1970], using the Eshelby-Walsh theory, showed that flat cracks were much more effective in reducing the elastic constants than spherical pores were. Thus they were able to show that the properties of the low-velocity zone could be explained with a small amount of partial melting. Likewise, the low velocities in crystalline rocks at low pressures can be under- Most of the work to date on the properties of composites or solids containing cracks has assumed isotropy, i.e., random orientation of grains and cracks. Geophysical aggregates can be expected to be anisotropic even if the matrix is isotropic, due to preferred orientation of cracks. The crack fabric of rock can be caused by tectonic stress fields or temperature gradients. Layering, recrystallization, cooling, and tectonic deformation can all be expected to result in cracks with a preferred orientation, both on microscopic (intergranular cracks) and on macroscopic (preferred orientation of dikes, joints) scales. Nur [1971] has discussed the problem of stressinduced crack orientation and gives a general treatment for velocities in a matrix containing oriented dry cracks.In the case of crustal deformation the orientation of cracks is controlled by the orientation of the principal stresses. In particular, the occurrence of dilatancy should be associated with cracks aligned in a statistical sense. We shall investigate here the extreme case in which the cracks are parallel.The overall elastic symmetry of a material containing parallel penny-shaped cracks is axial or transversely isotropic. Only five elastic constants are then independent. The extreme case of a laminated medium has been examined by Postma Copyright ¸ 1974 by the American Geophysical Union. •11ipsoidal inhomogeneities. This theory has been used by Walsh [1969] to calculate the elastic constants of a solid containing penny-shaped fluid-filled cracks with random orientation. Such a material is isotropic in the large and is described by two elastic constants.We consider an isotropic matrix containing ellipsoidal fluidfilled zones where the axial ratios are a/b = a/c = a < 1. The Eshelby theory neglects i...

We present a 9 deg 2 map of the North American and Pelican Nebulae regions obtained in all four Infrared Array Camera (IRAC) channels with the Spitzer Space Telescope. The resulting photometry is merged with that at JHK s from Two Micron All Sky Survey and a more spatially limited BVI survey from previous ground-based work. We use a mixture of color-color diagrams to select a minimally contaminated set of more than 1600 objects that we claim are young stellar objects (YSOs) associated with the star-forming region. Because our selection technique uses infrared excess as a requirement, our sample is strongly biased against inclusion of Class III YSOs. The distribution of IRAC spectral slopes for our YSOs indicates that most of these objects are Class II, with a peak toward steeper spectral slopes but a substantial contribution from a tail of Flat spectrum and Class I type objects. By studying the small fraction of the sample that is optically visible, we infer a typical age of a few Myr for the low-mass population. The young stars are clustered, with about a third of them located in eight clusters that are located within or near the LDN 935 dark cloud. Half of the YSOs are located in regions with surface densities higher than 1000 YSOs/deg 2. The Class I objects are more clustered than the Class II stars.

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We present small-aperture (1B5) photometry and new high-resolution images at 10 m (N band) for 87 Seyfert galaxies from the Extended 12 m Sample drawn from the IRAS database. With this data we hope to test the predictions of the unified model for active galactic nuclei and to search for bright, extended circumnuclear 10 m emission. We detected 62 Seyfert galaxies, 18 of which have no previously published small-aperture photometry. All the detected sources, both Seyfert 1's and Seyfert 2's, show a central point source. The 31 Seyfert 1's and 31 Seyfert 2's that we detected have similar luminosity distributions. Except for previously known bright extended 10 m structure around Arp 220, NGC 1068, and NGC 7469, we see definitive evidence for bright extended emission around only one new object: Mrk 1239. Four other Seyfert 1's and six other Seyfert 2's show evidence of faint, low-level extended emission. One Seyfert 1 and two Seyfert 2's show evidence of significantly increased flux over previously published small-aperture values. We also compared the N-band data with the JÀK s color that we derived from the Two Micron All Sky Survey (2MASS). There is a distinct trend of redder central bulge JÀK s colors corresponding to brighter absolute N-band magnitudes. In color-magnitude space there is a definite grouping of Seyfert 1's and Seyfert 2's, with two sets of outliers.

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