Max Planck Institute for Astronomy - Annual Report 2005

56 III. Scientific Work
The Field of the Large Magellanic Cloud with HST
Observations
We were recently able to present the first evidence
for flattening of the IMF toward the low-mass regime in
the field of the LMC (Gouliermis, Brandner & Henning
2006, ApJ, 641, 838). In this study, a large amount of archived
HST/WFPC2 photometric data of an area located
to the west of the Bar of the LMC, was used. This area
accounts for the general background stellar field of the
inner disk of the LMC and the data cover six overlapping
WFPC2 fields, reaching magnitudes as faint as V � 25
mag, and providing a large sample of more than 80 000
stars. This data enabled us to determine the PDMF of the
main-sequence stars in the field of LMC in detail down
to about 0.7 M 0 . This mass function is identical to the
Initial Mass Function (IMF) for stars with masses lower
than about 1 M 0 , since no evolutionary effects had time
to take place and alter the initial masses of these stars, or
remove them from the main sequence.
The highest mass found in the data is about 2 solar
masses, and the PDMF of main sequence stars up to this
limit in the LMC field is found not to have a uniform
slope throughout the observed mass range, meaning that
log [� (/log M/kpc 2 )]
7
6
5
4
–0.2 –0.1
Reconstructed IMF of all stars
IMF of red giants
PDMF of main-sequence stars
0 0.1
log [M/M � ]
0.2 0.3 0.4
Fig. III.1.7: The Mass Function of the stars in the general field
of the Large Magellanic Cloud to the west of its bar, observed
with HST/WFPC2. The Present-Day Mass Function (PDMF)
of the main-sequence stars was plotted with the green dotted
line. Several assumptions concerning the star formation history
of the LMC were taken into account for the reconstruction of
the Initial Mass Function of these stars. The IMF of the Red
Giant Branch stars (RGB) was constructed after taking the
evolutionary effects of these stars into account and was plotted
with the red dashed line. The small number of RGB stars in the
area and their narrow mass range introduce only small changes
to three specific mass intervals (red arrows), showing a trend
of the IMF to become slightly more shallow toward the limit of
about 1.6 M 0 . The IMF of all stars (both red giants and main
sequence stars) was plotted with the blue solid line. It was
reconstructed by the combination of the IMF of main sequence
and RGB stars. All mass functions shown were shifted to avoid
overlapping.
the PDMF slope of the LMC field does not follow a single
power law. This complies with what we found for the
general background population in the case of the LH 55
field and LH 52 Area. We established that a multi-power
law can represent the field PDMF, the slope of which
changes at about 1 M 0 to become shallower for stars
with smaller masses down to the lowest observed mass
of about 0.7 M 0 , and show clear indications of flattening
for even smaller masses. We statistically verified that the
IMF has a slope Γ of roughly – 2 for stars with masses
lower than 1 M 0 , with a Salpeter-like slope Γ � – 1.4
for stars in the mass range between 0.7 and 0.9 M 0 ,
while for more massive stars the main sequence PDMF
becomes much steeper with Γ � – 5. The main-sequence
luminosity function (LF) of the observed field closely
matches the previously found Galactic LF. Taking several
assumptions concerning evolutionary effects into
account, which should have changed the stellar content
of the observed field through time, we qualitatively reconstructed
its IMF for the whole observed mass range
(0.7 – 2.3 M 0 ). We found that the number of observed
evolved stars is not large enough to have significantly
affected the form of the IMF, which is almost identical to
the observed PDMF (Fig. III.1.7). Deeper observations
would certainly provide a better statistical stellar sample
with lower masses to verify this result.
The Pre-Main Sequence Population of Associations
in the Magellanic Clouds
Low-mass stars (with masses lower than 1 M 0 ) are
very important for addressing some of the most fundamental
problems in star formation, as they provide
a snapshot of the fossil star formation record of giant
molecular cloud complexes. Large scale surveys have
identified hundreds of low-mass members of nearby OB
associations in the Milky Way, and revealed that lowmass
stars exist wherever high-mass stars have recently
formed (Briceño et al. 2006, in Protostars & Planets
V). The low-mass stellar populations of Galactic OB
associations are considered to be a key to investigating
fundamental issues in the formation and early evolution
of stars. The shape of the low-mass IMF is still a debated
issue, and whether OB associations have low-mass
populations according to the field IMF of the Galaxy,
or if their IMF is truncated. Infrared studies have provided
strong/convincing evidence of primordial accretion
disks around low-mass stars, which dissipate on various
timescales of 1 to 20 or even 30 million years. These are
infant stars which are not yet on the main sequence and
therefore named pre-main- sequence (PMS) stars. Do
such stars exist in associations of the MCs? An answer
to this question is only possible with observations from
instruments equipped with high resolving power.
Our study using HST observations of the association
LH 52 revealed a low-mass stellar population which is