58 III. Scientific Work lower than 2 solar masses. This coincidence of the loci of low-mass PMS stars in the CMD of a galactic association with the observed sequence of PMS candidates seen in our CMD of LH 52 provides additional evidence that this sequence accounts <strong>for</strong> the PMS population of the association. In addition, it shows that the LMC PMS stars in stellar associations have a Galactic counterpart, most likely because they are of the same nature as PMS stars in Galactic associations. We constructed the distribution of the candidate PMS stars in LH 52 according to their masses as they are defined by limits provided by theoretical evolutionary tracks on the CMD (Fig. III.1.8). This distribution accounts <strong>for</strong> the IMF of these stars. Due to limitations in the detection of stars with very low masses (percentage of detection efficiency per magnitude range is given in Fig. III.1.8), the IMF can efficiently constructed <strong>for</strong> stars with masses down to about 0.8 M 0 . In addition, since we strictly selected the brightest limit of the PMS stars to be well below the turnoff, the numbers of counted stars in the higher mass ranges are also incomplete. The upper useful mass limit is thus defined at about 1.4 M 0 . The IMF <strong>for</strong> the mass range between 0.8 and 1.4 M 0 has a slope of Γ � – 1.26, which is very close to a typical Salpeter IMF. More complete data <strong>for</strong> PMS stars in MCs associations over a wider mass range will certainly provide a more complete picture of the IMF of the whole mass range of such objects. Future Prospects Our studies with HST/WFPC2 observations explored the scientific gain that would be achieved using very high spatial resolution 2D photometry on the basis of the newest instrument on-board HST, the Advanced Camera <strong>for</strong> Surveys. The Advanced Camera <strong>for</strong> Surveys (ACS), which was installed in HST in March 2002, increases the discovery efficiency of the HST by a factor of ten. It consists of three electronic cameras and a complement of filters that detect light from the ultraviolet to the near infrared (1200 – 10 000 Å). Deep observations with HST ACS of associations in the MCs will certainly enrich our knowledge on recent star <strong>for</strong>mation in their environment. Although stellar associations in the MCs are characterized by their loose structure, so their photometry does not suffer from significant confusion in identifying individual bright stars (crowding effects), high spatial resolution is required <strong>for</strong> the study of the fainter ones. The high resolving power of ACS is required so that crowding effects are avoided <strong>for</strong> stars down to about half a solar mass (corresponding to brightness V � 27 mag). In addition stellar associations are of the order of several tens parsec across, and thus, wide field imaging, which can also be provided by ACS, is required to observe a stellar association as a whole and its surrounding field. Taking these arguments and our results with WFPC2 into account, we proposed and were granted observations with HST/ACS of the LMC association LH 95 in HST Cycle 14. There are a number of facts that make this system a bona fide target <strong>for</strong> the investigation of the IMF and recent star <strong>for</strong>mation in the LMC. Our program hosts a collaborative project on recent star <strong>for</strong>mation in the MCs with colleagues from Leiden Observatory and Eso. Similar observations with HST/ACS of NGC 346, which is the brightest star <strong>for</strong>ming association in the SMC have recently revealed a rich population of low mass PMS stars in the area (Nota et al. 2006, ApJL, 640, L29). These stars are likely the product of a single star <strong>for</strong>mation 3 to 5 million years ago and they have masses between 0.6 and 3 solar masses. They appear mostly concentrated in the main part of the association, but they are also spread over a region across around 45 parsecs (Fig. III.1.9). The data of these observations, which are available in the HST Data Archive, include a large sample of high- and low-mass stars, covering an area about 5 arc minutes wide. The contribution of the field low-mass stars to the IMF of an association is so important that the field subtraction provides one of the major uncertainties in the determination of the IMF. There<strong>for</strong>e, observations of the general LMC field are as necessary as those of the system itself. Consequently, within our HST program (No. 10566) two fields, one centered on LH 95 and one on the general field, about 75 parsec west of the first, were observed within 10 orbits with ACS in wide-field mode in the V- and I-equivalent ACS filters (F555W and F814W). Parallel observations with NIcMos will help us to characterize any PMS population in the area. Several 10 000 low-mass main-sequence and premain-sequence stars are expected to be detected, giving a very clear picture of the sub-solar IMF in a star-<strong>for</strong>ming association of the LMC. We will take advantage of the wide-field high-resolution ability of ACS with the use of the archived data on NGC 346, as well as of our own data on LH 95, which were recently collected (Fig. III.1.10), to study the low-mass IMF and recent star <strong>for</strong>mation in associations of both the LMC and the SMC. It is almost certain that such deep observations will reveal the PMS population of these systems and, coordinated Fig. III.1.9: This HubblE Space Telescope image taken with the Advanced Camera <strong>for</strong> Surveys shows one of the most dynamic and intricately detailed star-<strong>for</strong>ming regions in the Small Magellanic Cloud. It is a composite of observations taken with the Wide-Field Channel of ACS in three wavelengths: V, I and H α . At the center of the region is the brilliant stellar association NGC 346. A structure of arched, ragged filaments with a distinct ridge, which is found to host infant PMS stars, surrounds the association. Such high-resolution observations of the Magellanic Clouds will considerably advance our knowledge on their recent star <strong>for</strong>mation processes. Credits: Nasa, Esa and A. Nota (Esa/STScI).
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