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\IBVShead{4229}{15 August 1995}

\title{Multicolor Photoelectric Photometry of SN~1993J}

\begintext

We present $UBV(RI)_c$ photometry of supernova 1993J which erupted in the 
spiral galaxy M81 (NGC 3031) during late March 1993.  The supernova was 
discovered on 28 March 1993 by Francisco Garcia during the star's initial 
rapid rise (Garcia {\em et al.}, 1993).  This was the brightest supernova in 
the northern hemisphere in about 40 years and because of its brightness 
($m_{v(max)} \simeq +10.3$) and high declination of +69$\de$, SN~1993J has 
been extensively observed with ground based instruments and orbiting 
satellites over a wide range of wavelengths.  Thus, SN~1993J has joined 
SN~1987A as one of the most extensively observed supernovae.

\vskip 2mm
\begin{center}
Table 1
\end{center}
\normalsize
\begin{center}
\begin{tabular}{cccccc}
\hline
HJD (2449000+) & $U$   & $B$   & $V$   & $R_c$   & $I_c$  \\
\hline
\\
 79.6292 & +11.02  & +11.69 & +11.42 & +11.26 & +10.87 \\
 80.6343 & +11.42  & +11.95 & +11.63 & +11.30 & +10.99 \\
 81.6344 & +11.67  & +12.18 & +11.82 & +11.43 & +11.14 \\
 82.6353 & +11.79  & +12.23 & +11.84 & +11.44 & +11.17 \\
 84.6339 & +11.77  & +12.17 & +11.69 & +11.32 & +11.09 \\
 86.6368 & +11.71  & +11.94 & +11.50 & +11.11 & +10.96 \\
 88.6377 & +11.54  & +11.72 & +11.28 & +10.92 & +10.80 \\
 93.6400 & +11.33  & +11.40 & +10.89 & +10.57 & +10.53 \\
 94.6448 & +11.36  & +11.40 & +10.86 & +10.53 & +10.50 \\
 96.6366 & +11.48  & +11.47 & +10.86 & +10.52 & +10.46 \\
 97.6371 & +11.60  & +11.53 & +10.89 & +10.54 & +10.48 \\
101.6337 & +12.41  & +12.08 & +11.19 & +10.72 & +10.56 \\
102.6342 & +12.64  & +12.24 & +11.28 & +10.79 & +10.64 \\
103.6348 & +12.76  & +12.37 & +11.37 & +10.85 & +10.67 \\
105.6358 & +13.08  & +12.61 & +11.54 & +10.97 & +10.77 \\
106.6354 & +13.33  & +12.73 & +11.61 & +11.02 & +10.80 \\
107.6329 & +13.45  & +12.86 & +11.68 & +11.07 & +10.86 \\
109.6330 & +13.47  & +13.00 & +11.82 & +11.16 & +10.93 \\
110.6335 & +13.81  & +13.19 & +11.91 & +11.25 & +10.95 \\
111.6350 & +13.61  & +13.10 & +11.91 & +11.23 & +10.97 \\
112.6346 & +13.91: & +13.15 & +11.91 & +11.27 & +10.95 \\
113.6352 & +13.60: & +13.19 & +12.00 & +11.32 & +11.02 \\
114.6358 & +13.86: & +13.14 & +12.00 & +11.29 & +11.10 \\
115.6363 & +14.04: & +13.27 & +12.09 & +11.44 & +11.17 \\
116.6368 & +13.96: & +13.31 & +12.12 & +11.51 & +11.26 \\
117.6375 & +13.82: & +13.34 & +12.15 & +11.49 & +11.27 \\
118.6388 & +13.95: & +13.34 & +12.17 & +11.49 & +11.15 \\
120.6400 & +14.21: & +13.43 & +12.21 & +11.51 & +11.12 \\
121.6406 & +14.28: & +13.37 & +12.23 & +11.53 & +11.17 \\
\hline
\end{tabular}
\end{center}
\large
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\centerline{2}
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\epsfysize=10cm
\centerline{\epsffile{4229-f1.ps}}
\centerline{Figure 1.  A plot of our data from Table~1.}

\epsfysize=10cm
\centerline{\epsffile{4229-f2.ps}}
\begin{center}
Figure 2.  A plot of all the data collected from the literature, including 
our own observations, to create a single light curve of SN~1993J in the 
$UBV(RI)_c$ bandpasses.
\end{center}
\vfill \eject

\centerline{3}
\epsfysize=8.5cm
\centerline{\epsffile{4229-f3.ps}}
Figure 3.  A plot comparing SN~1993J and SN~1987A by plotting the absolute 
visual magnitude vs. time using the same time scale for both objects.  A 
distance modulus of 27.8 mag and an intermediate value for interstellar 
absorption, $A_v \simeq$ 0.5 mag (see Richmond {\em et al.}, 1994) were used 
for calculating the absolute magnitude of SN~1993J.  For SN~1987A, a distance 
modulus of 18.5 mag and an $A_v$ of 0.25 mag (see Menzies {\em et al.}, 
1987) were used.  SN~1993J's light curve developed much more rapidly than 
SN~1987A's light curve. Also, SN~1993J was about 2 magnitudes brighter in 
absolute magnitude than SN~1987A at their respective peaks.
\vskip 3mm
We carried out $UBV(RI)_c$ photoelectric photometry from early April through 
May 1993 using the Four College Consortium 0.8m Automatic Photoelectric 
Telescope (FCC 0.8m APT) located at Mt. Hopkins, AZ.  Differential photometry 
was conducted through a 40 arcsec diaphragm using the following comparison 
stars:  HD\,85458 (F5, V = +8.7, B$-$V = +0.5), HD\,86677 (F5, V = +7.876, 
B$-$V = +0.510); and HD\,85828 (K0, V = +8.0, B$-$V = +1.0) served as a check 
star.  Because of the faintness of the supernova, blind offsets were made to 
the position of the supernova.  The supernova was observed using the usual 
pattern of {\em sky - comparison - check - variable - comparison - sky}\ 
with an integration time of 20 seconds.  The data were reduced using 
software developed at Villanova University by G.P. McCook.  The details of 
the reduction procedure have been described elsewhere by Guinan {\em et al.}\ 
(1987).  The data were then transformed to the $UBV(RI)_c$ system and 
corrections were made to account for the effect of fainter stars included in 
the aperture with the supernova as it faded.  In order to fully correct for 
stars included in the diaphragm with the supernova, we observed the same 
field one year later, after the supernova had faded below the telescope's 
limiting magnitude, to measure the background light directly.  We computed 
nightly means from the individual measures of SN~1993J and these values are 
tabulated in Table~1 and a plot of our observations vs. Heliocentric Julian 
Day number is given in Figure~1.  The estimated errors of the observations 
in the $BV(RI)_c$ are on the order of 0.01--0.02 magnitudes; the scatter 
in the $U$ observations was similar until the star rapidly faded in this 
bandpass and the errors became large.  We included these $U$-band measures 
in Table~1 with colons after the magnitude values.
\vfill \eject

\centerline{4}
\vskip 1mm

We also collected all the available visual and photometric measurements of 
SN~1993J from the {\em IAU Circulars}, those published in the literature, 
and other sources through the end of 1994.  Extensive compilations of 
optical photometry of SN~1993J are given by Lewis {\em et al.} (1994) and 
Richmond {\em et al.} (1994).  After removing some obviously inaccurate 
measures, these data are included in Figure~2 along with our own 
observations.  A comparison of the absolute visual magnitudes of SN~1993J is 
made to the LMC supernova SN~1987A in Figure~3 where both stars are plotted 
on the same magnitude and time scales.


The rapid expansion of the progenitor star's interior and the interaction of 
this shock front with its envelope and photosphere caused the initial rapid 
rise of SN~1993J to a peak magnitude of V = +10.3.  The first decline in 
brightness occurred in early April 1993, dropping to V = +11.9, as the thin 
envelope of the star lost heat from the shock wave.  During mid-April there 
was a second increase in brightness to V = +10.5; this time the increase was 
powered by the energy released by the decays of radioactive cobalt into 
nickel and iron.  As the energy from these short-lived decay products 
decreased, the star slowly dimmed and cooled.  By December 1993, the 
supernova's magnitude had decreased to approximately V = +17.

The progenitor of SN~1993J has been identified as a late G or early K 
supergiant (Aldering, Humphreys, Richmond, 1994) with $V_{mag} \simeq 
+20.75$.  The progenitor appears to be a member of a binary or multiple star 
system having fainter nearby blue components.  The spectroscopic and 
photometric behavior of SN~1993J indicate that the progenitor star had its 
hydrogen envelope partially stripped away by strong winds, or more likely, 
by a close binary companion.  Possible analogs of SN~1993J could be 
$\zeta$ Aurigae systems which are composed of K-supergiant components and 
early B-main sequence companions.
\vskip 1cm

\beginauthors
\authorsrightcol
Edward F. GUINAN
James J. MARSHALL
Department of Astronomy
and Astrophysics
Villanova University
Villanova, PA  19085,
USA 
\endauthors

\references

Aldering, G., Humphreys, R.M., Richmond, M.,  1994, {\it AJ}, 107, 662

Garcia, F., {\em et al.}, 1993, {\it I.A.U. Circular}, No.\,5731

Guinan, E.F., Najafi, S., Zamani-Noor, F., Boyd, P.T., and Carroll, S.M.,  
1987,  {\it I.B.V.S.}, No.\,3070

Lewis, J.R. {\em et al.},  1994,  {\it MNRAS}, {\bf 266}, L27

Menzies, J.W., {\em et al.},  1987, {\it MNRAS}, {\bf 227}, 39p

Richmond, M.W., {\em et al.}, 1994, {\it AJ}, {\bf 107}, 1022


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