Characterizing Pre-Main Sequence Populations in Stellar Associations of the Large Magellanic Cloud
The Large Magellanic Cloud (LMC) offers an extremely rich sample of resolved low-mass stars (below 1 Solar Mass) in the act of formation that has not been explored sufficiently yet. These pre-main sequence (PMS) stars provide a unique snapshot of the star formation process, as it is being recorded for the last ~20 Myr, and they give important information on the low-mass Initial Mass Function (IMF) of their host stellar systems. Studies of young, rich LMC clusters like 30 Doradus are crowding limited, even at the angular resolution facilitated by HST in the optical. To learn more about low-mass PMS stars in the LMC, one has to study less crowded regions like young stellar assocations. We propose to employ WFPC2 to obtain deep photometry (V ~ 25.5 mag) of four selected LMC stellar associations in order to perform an original optical analysis of their red PMS and blue bright MS stellar populations. With these observations we aim at a comprehensive study, which will add substantial information on the most recent star formation and the IMF in the LMC. The data reduction and analysis will be performed with a 2D photometry software package especially developped by us for WFPC2 imaging of extended stellar associations with variable background. Our targets have been selected optimizing a combination of criteria, namely spatial resolution, crowding, low extinction, nebular contamination, and background confusion in comparison to other regions in the Local Group. Parallel NICMOS imaging will provide additional information on near-infrared properties of the stellar population in the regions surrounding these systems.
WFPC2 imaging of four star-forming regions of the LMC will be performed. Two WFPC2 pointings for each region will be needed for the coverage of each stellar system and its surroundings. The first pointing will be centered on the system, while the second will be such, that the local background field of the galaxy will be well covered. The system- and field-pointings of every target will overlap each other, so that it will be possible to construct a mosaic of the whole area of each star-forming region. The observations will be performed in a suitable LINE dithering pattern for each pointing in order to improve the spatial resolution, since we wish to eliminate crowding effects. Observations in four broad-band filters, F300W, F450W, F555W and F814W, and the narrow-band filter F656N will be taken for the four pointings on the systems. For the four pointings on the corresponding fields, observations only in F555W and F814W are required for the determination of the stellar populations of the local LMC field.
Total exposure times of about 500 sec in both F300W and F450W, 950 sec in F555W, 800 sec in F814W and about 1,150 sec in F656N are necessary. The exposures will be splitted in three intervals for the removal of cosmic rays without any significant cost in the S/N of the final combined image. Short exposures in both F300W and F450W filters will be also necessary to obtain the photometry of the brightest early-type stars in the regions, and to accomodate the available ground-based photometry. Parallel observations with NICMOS (NIC2) in F110W , F160W and F205W bands will be also performed. The consideration of acquisition and overhead times for our exposures results to one set of four orbits for each system-pointing and two orbits for each field-pointing. Each set of these orbits will make up one visit for every pointing. Consequently, two visits will be considered for every target, one for the system and one for the field. For the construction of a reasonable mosaic for every target, each visit on a field should have the same orientation with the one of the corresponding system. In total for our four targets eight visits containing 24 orbits are required for the success of this program.
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:internal-dur-at-end 0
:dark-offset NIL
:dark-utilization 0
:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 3281
:pcs-gap 0
)
(Tic-Orbit
:proposal-id 11547
:visit-id "08"
:orbit-number 2
:pcs-time 355
:si-vis-utilization 2880
:al-overhead-at-end 180
:internal-dur-at-end 98
:dark-offset NIL
:dark-utilization 0
:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 3281
:pcs-gap 0
)
true
2
0