Tom Megeath
March 19, 2003
This is the final plan for the total IRAC survey of the Orion A and B clouds. The goals of this program are to:
The total survey is plotted over a dust map of the Orion Molecular clouds, shown below, overlayed onto maps of the dust column density from Schlegel, Finkbeiner & Davis 1998 (courtesy of the skyview virtual observatory). The left panel shows the map, the right panel shows the maps with the IRAC FOVs overlayed with SPOT. The northern field is the NGC2068/NGC2071 region of Orion B, the middle field is the NGC2024/NGC2023 region of Orion B, and the bottom map covers the Orion Nebula Cluster and L1641.
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The Orion region presents a special problem since we would like to make long-narrow (i.e. high aspect ratio) maps with a very specific orientation in the sky. IRAC does allow celestical coordinate grid maps, where the user defines a fixed rectangular grid of points in the sky and a position angle for the grid. If the spacecraft rotation is not optimally aligned, then holes can appear between the images, even with substantial overlaps in the grid. This is demonstrated in the image below, where I have shown the section of an IRAC celestical grid map in which the grid orientation and detector orientation are not well matched. The image shows the number of detector pixels which have sampled a particular regions of sky, with 0 pixels showing as a dark grey, 1 pixel showing as grey, and 2 pixels showing as a light gray.
Based on a suggestion by Bill Reach, that a triangular grid pattern may work well for celestial orienation maps, I have interleaved two grids. The spacing in each grid in the "Y" direction is roughly twice that in the "X" direction. The grids are interleaved, so that the centers of adjacent arrays form a triangular pattern in the sky, as shown below
When the Orion region is visible, the orientation of the array will rotate from -17 degrees to 4 degree (the angle is the position angle east of celestial north of the axis defined by the centers of the two IRAC field of views). Using the interleaved positions, holes do not appear as they do in a normal rectangular grid. As the array rotates to an angle of -17 degrees, we get the following pattern:
To map the "streamer" of the L1641 cloud, it is necessary to rotate the map grid a position angle of -40 degrees (again, rotation is east of celestial north). For such a grid, the following examples show the the sampling of the sky for rotation angles of -17, -10, and 3 degrees. Notice that a healthy amount of overlap is maintained.
Using these patterns, I have made several AORs which cover most of the L1641 cloud, and a significant fraction of the L1630 cloud. These are shown below for both FOVs. The rotation angle of the array is assumed to be 17 degrees. Below are the maps for the L1641 cloud. The known clusters are marked with asterisks. There is a small gap in L1641 which may require a small AOR to fill in a region of a few FOVs. The coordinates are RA and Dec in degrees, J2000.
The following maps were generated by SPOT. The assumed dates were Sept 14, 2003 for Epoch 1 and Feb 14, 2004 for Epoch 2.
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| Above: Ch1/Ch3: First Epoch | Ch2/Ch4: First Epoch |  |
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| Above: Ch1/Ch3: Second Epoch | Ch2/Ch4: Second Epoch |
The following overlays were generated by SPOT. For epoch 1, the assumed dates were Sept 14, 2003 for the Orion Nebula map and Oct 25, 2003 for the map of the L1641 "Streamer" (diagonal part of the map). For epoch 2 the dates were Feb 14, 2004 for the Orion Nebula map, and March 1 and March 25 2004 for the streamer (each part of the map is done twice per epoch).
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| Above: 13CO map with known stellar groups and clusters are marked by asterisks. | Bell Labs 13CO map with pseudocolor color scale |
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| Above: Ch1/Ch3: First Epoch | Ch2/Ch4: First Epoch |  |
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| Above: Ch1/Ch3: Second Epoch | Ch2/Ch4: Second Epoch |
 | Left: Bell Labs 13CO Map of the L1630 Cloud. There are two main regions of bright 13CO emission, the northern region contains NGC 2071, NGC 2068 and LBS 23, the southern region NGC 2024, NGC 2023 and the Horsehead Nebula. The maps are designed to cover all these known star forming regions and the surrounding areas. The overlays were generated with SPOT assuming Epoch 1 was imaged on Sept 14, 2003 and Epoch 2 on Feb 16, 2003. The map to the left only shows a subsection of the entire map, which is displayed below with the overlays. |  |
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| Above: Ch1/Ch3: First Epoch | Ch2/Ch4: First Epoch |  |
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| Above: Ch1/Ch3: Second Epoch | Ch2/Ch4: Second Epoch |