Images of NGC 1365 in the F160BW, F336W, F547M, F555W, and F814W
bands are available from two different programs, one of which is the HST
Extragalactic Distance Scale Key Project. Unfortunately, the Key Project
images for this galaxy did not include the site of the SN Ic 1983V. The
exposure times for the two sets of UV images were too short to be
useful, and we did not consider them further. The F547M, F555W, and
F814W images have reasonable S/N ratio after coaddition, such that we
could produce limited color-magnitude diagrams for the SN environment.
The absolute position from Lindblad & Grosbol (1983) was used to isolate
the SN site, and we show the SN environment in Figure 8. Van Dyk et al.
(1996) had found that the SN occurred within a bright H II region of
about 5" radius. From the HST image we see the stars and small clusters
that are presumably ionizing the region.
We performed PSF-fitting photometry on the F555W and F814W images
(the F547M band does not provide any additional color information and is
less sensitive in the same wavelength range than the F555W band). In
Figure 9 we show the resulting CMD. The reddening to SN 1983V may be
between E(B-V) ~ 0.18 and 0.4 (Clocchiatti et al. 1997). We assume both
values of the reddening, applying them to the theoretical isochrones. We
also apply to the isochrones the distance modulus to NGC 1365 determined
from Cepheids using HST, m - M = 31.31 (Silbermann et al. 1999).
The positions of the stars on the CMD in Figure 9 appear to agree
better with the lower reddening than the higher. One can see that the
three objects in or very near the error circle around the SN position
appear too bright to be individual stars and are probably compact blue
star clusters. The resolved stars near the SN site have ages about 4-10
Myr. We can age-date the compact clusters by using the color evolution
models for starbursts from Leitherer & Heckman (1995). In Figure 10 we
show the colors of these probable star clusters, compared with the color
evolution models for an instantaneous starburst from Leitherer &
Heckman, transformed to WFPC2 synthetic colors, and reddened by
E(B-V) = 0.18. The line conventions in Figure 10 are the same as
Figure 1 of Leitherer & Heckman.
These clusters are evidently bluer than the model predictions.
Ideally, one would make this comparison having more color information.
However, we find that the cluster colors are consistent with very young
starbursts, of ages <~5 Myr. If the progenitor of SN 1983V was
associated with these clusters, then it may have had a comparably young
age, implying a very high initial mass of about 40 M_sun_. Stars of this
mass generally evolve to the Wolf-Rayet stage toward the end of their
lives. Thus, analysis of the environment of SN 1983V is consistent with
the Wolf-Rayet progenitor model for SNe Ib/c, but the data are of
limited sensitivity and color information.