55
5.1.2 Determining Age and Extinction for Star Forming
Regions
The observed FUV
,
B and log N
Lyc
L
FUV
for each region are compared to the
appropriate cluster model allowing a simultaneous estimation of both the cluster
age and internal extinction. Before making the comparison, the observed values of
FUV
,
B and log N
Lyc
L
FUV
are corrected with an array of EB
,
V's assuming
a LMC or SMC extinction curve. The internal extinction curve is chosen under
the assumption that the shape of the curve correlates with metallicity Verter &
Rickard 1998. In the FUV, A
FUV
EB
,
V=9.04 for the LMC extinction curve
and 17.72 for the SMC extinction curve Hill
et
al.
1997. The array of corrected
observations for each region is compared to the model plot; the point at which
a corrected value agrees with the model curve estimates the extinction since an
EB
,
V is tied to each corrected data point and the age since the model curve
is age dependent. To illustrate this method, the LMC model for t= 0
,
50
Myr is shown in Fig. 5.8 with several regions from HoII. The region number
represents the uncorrected data point; the dashed line that connects it to the
model represents the array of corrected values. Therefore, the length of the dashed
line is a measure of the internal extinction and the point at which it crosses the
model is the approximate age of the cluster. Since the corrected data points
often fall between timesteps, the age estimates are approximations; absolute age
determinations are not allowed using this method. However, an age range can
con dently be assigned to each region for comparison purposes to identify the
pattern of star formation in a galaxy. The age groups result from both the shape
of the model and how the observations are naturally grouped with one another.
The age groups are shown in Fig. 5.8 by the alternating dotted and solid lines of
the model: group 1 rst dotted line = 0
,
3.5 Myr, group 2 rst solid line
= 3.5
,
4.5 Myr, group 3 next dotted line = 4.5
,
6.3 Myr; group 4 last
