The identity of the lens objects for most of the LMC microlensing events seen by the MACHO Project is unknown. The most popular explanations include a previously unknown population of old, cool white dwarfs in the Galactic halo or in a very thick disk, or a variation standard LMC models that would allow most events to be caused by faint LMC stars. This uncertainty exists because it is usually impossible to determine the lens distance from the observable features of a microlensing event. Distance estimates can be obtained by measuring the microlensing parallax effect with simultaneous observations of the events from Earth and from a small (~30cm) telescope located 1-2 AU from the Earth. Such a telescope has just been launched: the High Resolution Instrument on the flyby spacecraft of the Deep Impact (DI) Mission. This telescope has been placed in an ideal orbit for LMC microlensing parallax measurements, and the telescope will be at a distance of >1 AU from Earth when the DI prime mission ends this August. Our group plans to take advantage of this fortuitous circumstance and propose a "new science" extended mission for the DI flyby spacecraft to resolve the LMC microlensing puzzle with microlensing parallax observaions. This project is compatible with the DI Science Teams extended mission plans to visit a 2nd comet, and our extended mission proposal to NASA will be written in collaboration with the Deep Impact Science team. A crucial feature of these proposed microlensing parallax measurements is the determination of the absolute brightness of the source stars, which can only be resolved with HST images. The source star brightness must be measured over the entire sensitivity range of the Deep Impact High Resolution Instrument clear filter: 300-1000nm. We therefore request UBVriz HST images to resolve the blending of the microlensed LMC source stars observed by the Deep Impact 30cm telescope.

We are requesting 2 single orbit visits of 2 LMC microlensing events to be provided by the SuperMACHO, OGLE-III, and MOA-II microlensing surveys. The first visit for each target will be scheduled while the target star is magnified by microlensing, and the second orbit will be scheduled after the event is over. The scheduling of the first visit for each target will be more time critical, and so we have assumed that the first visit is scheduled at a time with only 55 minutes of target visibility, while the second visit can be scheduled at a time with a much greater target visibility period of 80 minutes or more. (Such extended visibility periods are common for our LMC targets, even in 2-gyro mode.) The exposure times for the first visit to each target may be shortened when the microlensing events occur if that will make prompt observatins easier to schedule. Since our main scientific goal is a comparison with observations from the DI/HRI through a clear filter, our exposure times will be selected to approximately match the throughput expected in the DI/HRI clear filter images. The first visit for each target will consist of ACS/HRC exposures totaling 360 seconds in each of the F435W, F555W, F625W, and F775W passbands with 4 CR split images per exposure and dithering to deal with hot pixels. It is important to have multi-color observations when the source star is still magnified by the microlens in order to test the possibility that the source may still be blended in the HST frames. (This was the case with microlensing event MOA-2002-BLG-33, which was imaged with HST while the microlensing event was still in progress.) An apparent color change of the source star between the two visits would be evidence of a blended source. We expect to observe 2 LMC microlensing events in the first year of the DI extended mission with at an average unmagnified magnitude of V = 21. These same exposures will be repeated in the second visit for each target, and exposures totaling 465 sec in each of the F330W and F850LP passbands will also be added in this longer visit. 12 false 0.0 false false false false false SUPPORTED STAR K III-I I= 18.6 J2000 00 00 0.0000 +00 00 0.00 J2000 POS-TARG DITHER DITHER 0.3 NIC-XSTRIP-DITH false false 8 0.0 POS-TARG NIC-SPIRAL-DITH 0.0 1 false POS-TARG DITHER 0.077 LINE false false 4 31.1 LINE 2 false POS-TARG DITHER 0.559017 LINE 0.559017 false false 143.1301 4 26.56505 WFPC2-BOX 3 false false -1 -1 false -1 -1 false -1 -1 Select either visit 02 or visit 03 based on guide star availability. false -1 -1 false -1 -1 Repeat of failed visit 04. false -1 -1