Max Planck Institute for Astronomy - Annual Report 2005

log [��(log M/kpc 2 )]
7
6
5
4
3
2
–0.2 0 0.2
log M
0.4
LH 52 Area
LH 55 Field
0.6 0.8
Fig. III.1.5: Mass Function of the main sequence stars of the
LH 52 area and the LH 55 Field from observations with HST/
WFPC2. The mass function is constructed by using the massluminosity
relation provided by the theoretical stellar evolution
models. The stars were counted in logarithmic (base 10) mass
intervals. The numbers were corrected for incompleteness and
normalized to a surface of one square kiloparsec. The errors reflect
the statistical uncertainties. The arrows indicate the level
of 50 % success in the detection of stars in the LH 52 area (blue
arrow) and the LH 55 Field (red arrow). The mass function was
found to change shape for stars with masses lower than 2 M � .
The large majority of the observed low-mass stars
with masses between 1 and 2 M 0 in both fields seems
to be a feature of the general field population of the
LMC. The mass function of these stars closely resembles
their IMF, since low-mass stars evolve very slowly
and no evolutionary effects are expected to have taken
place, changing their original mass and forcing them
to leave the main sequence within the life-span of the
LMC general field. This time-scale is revealed from the
corresponding Color-Magnitude Diagrams (CMDs) of
the LH 52 Area and of LH 55 field. In this investigation,
it could be shown for the first time that the Present-Day
Mass Function (PDMF) of the main-sequence stars with
low and intermediate masses in the LMC is not a single
power-law, but that it is steeper for masses lower than
about 2 M 0 with slopes between – 4 and – 6, while for
stars with masses greater than about 2 M 0 the mass
function slope is more shallow, between – 1 and – 2 (Fig.
III.1.5). This implies that the contribution of the lowmass
stars in the main sequence to the mass function of a
system is higher than expected, when assuming that the
mass function follows the same slope for masses smaller
than about 2 M 0 . The mass function of the stars found
in the general background field of the LMC is used for
correcting the contribution of the background field to the
stars found in the observed area of the association LH
52. Applying this correction, the field-subtracted mass
function of the main-sequence stars found with WFPC2
in LH 52 is constructed. This PDMF covers only the
stars which are members of the association and therefore
log [��(log M/kpc –2 )]
6
5
4
3
2
III.1 Star Formation in the Magellanic Clouds 55
� = –1.12 � 0.24
(Gouliermis et al. 2005)
� = –1.12
(Hill et al. 2005)
0 0.2 0.4 0.6
log M
0.8 1 1.2 1.4
Fig. III.1.6: The Initial Mass Function of the association LH 52
down to about 1 M � from observations with HST/WFPC2.
Results from studies on both high- (Hill et al. 1995) and lowmass
(Gouliermis et al. 2005) stellar populations combined
for the first time for the construction of the IMF through the
whole observed mass range of a young stellar system in the
Magellanic Clouds for stars with masses up to 18 solar masses.
The IMF of the association is found to be constant through the
whole mass range and it has a slope comparable, but a bit more
shallow than a typical Salpeter mass function (plotted with the
dashed line for reference).
is the IMF of the system, since it is considered to be very
young. Its reconstruction was possible for masses down
to about 1 M 0 , due to observational limitations in the
LH 55 Field.
The slope of this IMF is found to be comparable,
maybe somewhat shallower than a typical Salpeter IMF
(Γ � – 1.12 � 0.24) in the mass range between 1 and 9
M 0 . Previous, modest, ground-based CCD imaging by
Hill et al. (1995, ApJ, 446, 622) led to the construction
of the IMF of the same association for stars of intermediate
and high masses (between 3 and 18 M 0 ). The
combination of our results on the low-mass regime of
the IMF based on HST observations with the ones on its
high-mass end from ground-based imaging enabled us to
construct the IMF of a young LMC association throughout
its full observed mass range of 1 to 18 M 0 (Fig.
III.1.6) for the first time. Its slope is found to be constant
for the whole mass range, providing clear indication
of a »top-heavy« IMF, which is well represented by a
single-power law with slope Γ � – 1.1. Conclusively,
the Present-Day Mass Function slope of the field of
the Large Magellanic Clouds is found to be determined
by the low-mass population, while the Initial Mass
Function of the association LH 52 by its more massive
members. This clearly suggests the local star formation
conditions in LMC associations may favor the formation
of higher-mass stars. No evidence for flattening of the
IMF toward the low-mass regime was detected in our
data, neither was a lower mass cutoff in the IMF down
to the observed limit of about 1 M 0 .