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Date: Wed, 31 Mar 93 13:02:12 PST
From: alex%avf.hepnet@Lbl.Gov (ALEX FILIPPENKO, UC BERKELEY, 510-642-1813)
Subject: sorry, you were not on my original distribution list of observers!
3-31-93
Dear Nova and Supernova observers,
Several items concerning SN 1993J in M81:
(1) We have some information about the stars near SN 1993J, which will
be used for calibration purposes. I append a note from Michael Richmond
(with apologies to those of you who have already seen it). We agree that
for photometry, we should use as many comparison stars as possible for the
time being. Any stars that are found to be variable can be thrown out
later on.
(2) Regarding the local spectroscopic standard that should be observed
whenever you observe the supernova: Spectroscopy by Jim Liebert and
Jay Holberg at the Steward 90" shows that Star B is a late F star with
pretty strong Balmer lines. Star C may be a bit later, perhaps a G.
Until further notice, then, let's adopt Star B as our spectroscopic
comparison star. As already noted, Star B is better than Star A anyway,
in terms of having smaller background starlight contamination.
I will try to find a nearby star with an even earlier spectral type,
but I don't have any observing time with a spectrograph until April 12.
Late F (Star B) should be pretty good, though --- we'll be able to
see continuum deviations from what it SHOULD look like, except near
the strong Balmer lines.
The basic idea is as follows: observe and reduce this star in the
same way that you do SN 1993J (using normal KPNO or Oke/Gunn standards).
Then e-mail me the reduced spectra of SN 1993J AND Star B. I'll then
carefully check all the spectra when systematically looking at the
complete data set. Any deviations of Star B's spectrum from the
adopted one (which defines what it SHOULD look like) will then be
used to correct the spectrum of SN 1993J. Be sure to send me BOTH
the narrow-slit and wide-slit spectra, if you are able to take data
through two slit widths. (The extraction window should be large for
the wide-slit spectrum, of course.)
I emphasize once again that you should try to go as far down to
the near-UV as possible, to connect up with the IUE spectra.
By the way, I would prefer that any spectra that you wish to
contribute to the collaboration be e-mailed to me in 2-column format
(e.g., MONGO PL format), with the first column being wavelength
and the second being f-nu or f-lambda.
(3) Brian Schmidt and Pilar Ruiz-lapuente (CfA) have sent me the following
messages for distribution. I'm sorry if some of you have already received
them.
(4) If you would like to be taken OFF my distribution list, please
let me know by e-mail. Similarly, if you want to be put on it, send
me a message.
Best of luck with your observations! Remember --- keep at it even if
skies are cloudy!
Alex Filippenko
***
From Brian Schmidt:
A PostScript Finding chart of SN 1993J is available through
anonymous ftp at cfa0.harvard.edu (128.103.40.1). It is in
the directory pub/brian/ and you can get the compressed
version 93J.finder.ps.Z (use UNIX uncompress), or the full
PostScript File "93J.finder.ps". Print it out on a PostScript
printer with 1 Mbyte of memory or more.
This SN~II - because it is in a galaxy in which a Cepheid distance
will be shortly forthcoming from HST observations (Mould et al) -
is especially interesting for application of the Expanding Photosphere Method
to determine its distance (Schmidt Eastman and Kirshner 1992 ApJ).
To do the best job on this SN, spectroscopic and photometric observations
through the bright-time are strongly encouraged, as are all other observations.
This SN has probably nearly reached its B-maximum, which at
m_B = 10.5 - 11 is typical for a SN II (Distance Modulus for M81
is 27.5 Freedman 1990 - average Sn~II have M_B = -16 to -18). We can
expect it to brighten in the near-IR for several more weeks, and it
will probably remain more-or-less steady in VR and I for the next hundred
days.
***
From Pilar Ruiz-lapuente:
An account with images of the SN in M81 taken at Mount Hopkins Observatory
has been set up for those interested in getting an accurate position to the
object. The way to access to this account is making an anonymous ftp to
cfa0.harvard.edu. The images have the names *.SN_M81 and are located in
the subdirectory /incoming/pilar.
***
From Michael Richmond:
Astrometry and Photometry of SN 1993J
------------------------------------------------------------
I've gone through the Leuschner images from last night
(Mar30 UT) and derived a position based on the values for
nearby stars given in the Guide Star Catalog. Here is the
position I derived, together with the one from the IAU
Circular 5731:
RA (2000.0) Dec (2000.0) RA (1950.0) Dec (1950.0)
================================================================
me 09:55:25.0 +69:01:13.3 09:51:19.25 +69:15:26.4
IAU 09:55:25.02 +69:01:12.6 09:51:19.27 +69:15:25.7
You can take your pick. They're pretty close.
The offsets of the SN may be of more use to observers.
The best object to use is star C (from my previous chart: if
you don't already have it, you can get it via anonymous FTP
from "spiff.princeton.edu"; cd to "richmond" and get the file
"chart"), which is very close to the SN; however, some observers
may want to use the nucleus of the galaxy. Since the nucleus is
very extended, however, it may not be as easy to measure an offset
precisely.
SN offset from RA (arcsec) Dec (arcsec)
==============================================================
star C 7.4 E 24.3 N
nucleus 44.7 W +/- 0.05 161.1 S +/- 0.3
Note that the offset from the nucleus in declination may be off by
up to 0.3 arcsec or so, due to the large distance and a slight
uncertainty in our plate scale. The offsets from star C
should be good to less than 0.05 arcsec, since they are
so small.
Now, let me GUESS at the colors of the stars in the field.
Our images showed four possible comparison stars, called
A,B,C and F in the finding chart I mailed out earlier.
As Alex has mentioned, star A is immersed in a brighter
portion of M81 than either B or C; and star F is likewise
in a bright region. Nonetheless, I measured the RELATIVE
magnitudes of the stars and the SN in our images (2 each
of VRI), NOT making any color corrections, and came up with
the following uncalibrated relative colors:
Uncalibrated magnitudes and colors relative to star A
star V (V-R) (R-I) GSC "V"
==========================================================
B 0.51 -0.09 +0.06 -0.9
C 3.10 +0.01 +0.04 +2.3
F 3.27 +0.16 +0.04
SN -0.69 -0.31 -0.22
From which I conclude that
a) the SN is blue (no surprise)
b) stars A, B, C all have pretty similar colors,
with star F being slightly redder.
Let me point out, however, that the Guide Star Catalog's
magnitude values for stars A, B, and C are VERY different
from the ones I expected from our images. It's possible
that one of stars A or B is a variable, with quite a large
amplitude. We should be careful to make photometric measurements
relative to as many stars as possible in every frame!
Alex has already suggested that we use stars "B" and "C" as our
photometric comparisons - I suggest that we use all three
of A, B and C, at least until enough time passes that we
can get better photometry and assure ourselves that some
stars are non-varying.
Finally, a request: the Hubble Atlas picture of M81 shows
several very faint point sources near the location of the SN,
in the midst of the spiral arm. It is POSSIBLE that one of
them might be a supergiant progenitor. If anyone knows of
high-resolution images of this section of the galaxy that might
already exist in the literature, please let us know. This
COULD be the second SN progenitor to be identified!
(actually third, if you include SN 1961V --- Alex).
ADDENDUM BY ALEX: IAUC 5736 notes the presence of a possible
progenitor (or tight cluster containing the progenitor) on CFHT plates
obtained in 1980 and on KPNO images obtained in 1990. The object appears
stellar to 1". At V = 20.0, this object would have an absolute V
magnitude of -7.5 (adopting a distance modulus of 27.5 from Freedman
1990) --- not totally outrageous for a supergiant. Any other CCD images
or plates should also be examined, as suggested by Michael Richmond above.
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