Max Planck Institute for Astronomy - Annual Report 2005

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 for 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
for 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 for
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 for 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 for 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 for Surveys. The Advanced Camera for 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 formation
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 for the study
of the fainter ones. The high resolving power of ACS
is required so that crowding effects are avoided for
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 for the investigation of the
IMF and recent star formation in the LMC. Our program
hosts a collaborative project on recent star formation in
the MCs with colleagues from Leiden Observatory and
Eso. Similar observations with HST/ACS of NGC 346,
which is the brightest star forming 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
formation 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. Therefore, 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-forming
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 formation
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 for Surveys shows one of the most dynamic
and intricately detailed star-forming 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 formation processes. Credits: Nasa,
Esa and A. Nota (Esa/STScI).