Max Planck Institute for Astronomy - Annual Report 2005

Here, �(log m) is the IMF, which is constructed by
counting stellar masses in equal logarithmic intervals,
and Γ is its slope, which can be derived from the linear
regression between log �(log m) and log m. A reference
value for the IMF slope Γ, as found by Salpeter (1955,
ApJ, 121, 161) for the solar neighborhood and stars with
masses between 0.4 and 10 M 0 , is Γ � – 1.35.
The Massive Initial Mass Function
Ground-based investigations of a large sample of associations
in the MCs led to the conclusion that for massive
stars, the IMF in these systems is more or less the
same, and it has a slope which varies around a value of
Γ � – 1.5 � 0.1 (Massey et al. 1995, ApJ, 454, 151). This
value is not very different from the IMF slopes of typical
young compact LMC clusters for the same mass range
(e.g. Gouliermis et al. 2004, A&A, 416, 137). Thus, the
IMF of massive stars in young stellar systems appears
more or less to be universal, with a typical slope, which
does not differ significantly from the reference value
found by Salpeter. On the other hand, the IMF of massive
stars in the field away from any stellar system in both the
LMC and SMC appears to be steep with slope Γ � – 4,
the same value as for the Milky Way field. In general, the
IMF for massive stars shows variations from one region
of the galaxy to the other, which clearly suggest that environmental
conditions most likey significantly affect the
IMF in the high-mass regime. A characteristic example
is the case of the LMC association LH 95 (Gouliermis et
al. 2002, A&A, 381, 862), where a gradient of the IMF
slope was observed in the sense that the IMF of stars with
masses between 3 and 10 M 0 becomes steeper outwards
from the center of the system (Fig. III.1.3). This suggests
that there is a clear distinction between the population
Fig. III.1.3: The Mass Function of all main-sequence stars located
in the region of the association LH 95. The mass function
for all the stars within the association (distance less than 1.2 arc
minutes from its center) is shown in the left plot. The field population
is included. The corresponding mass function of stars
in the surrounding field is shown in the central plot. The lack
of stars more massive than 10 M � in the latter makes the mass
log [� (/log M/kpc 2 )]
5
4
3
2
� = –1.76 � 0.20
0.4
LH 95
III.1 Star Formation in the Magellanic Clouds 53
of the system, its surrounding field and the general field
of the LMC. In addition, there are stellar associations
which exhibit slopes of the massive IMF quite different
from each other with values varying between Γ � – 1
and – 2 (Parker et al. 1998, AJ, 116, 180).This variability
presumably originates in the differences in the star formation
process from one region to the other.
The IMF Toward the Low-Mass Regime
The picture of the stellar content of stellar associations
in the MCs from ground-based observations is limited
above 2 M 0 . Information on the low-mass stellar membership
and the corresponding IMF in these systems is still
incomplete. Considering that the MCs, being very close,
are the only extra-galactic targets where stars of sub-solar
masses can be observed with the advanced instruments
available today, this gap has only recently started to be
filled with our research based on observations with the
HubblE Space Telescope (HST). Our results open up a
new debate by addressing important questions such as:
(1) Are there any low-mass stars in stellar associations
and, if yes, then what is the low-mass slope of the IMF
in these systems? (2) What would be the lowest mass that
can be observed? Is there any specific low-mass cutoff in
the IMF of associations in the MCs? (3) What is the functional
form of the low-mass IMF in the MCs? Is there a
flattening of the IMF for sub-solar masses? and therefore
(4) Has the IMF got a constant slope or not through the
whole mass range detected? and finally (5) What is the
low-mass slope of the IMF in the general field and what
are the differences to those of young stellar systems? Is
there any dependence/relation of the IMF slope on/to the
environment, in which star formation does it take place?
Data from space observations of the MCs is available in
function steeper. This exemplifies the radial dependence of the
mass function slope, which becomes steeper outwards. The plot
on the right shows the IMF of the main sequence stars in the
association, after the contribution of stars, which belong to the
field has been subtracted. This IMF is comparable to a typical
Salpeter IMF. Adapted from Gouliermis et al. (2002).
LH 95 (field)
� = –2.94 � 0.15
� = –1.59 � 0.30
LH 95
(field subtracted)
0.8 1.2 1.6 0.4 0.8 1.2 1.6 0.4 0.8 1.2 1.6
log M