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Date: Fri, 16 Apr 93 08:33:01 PDT
From: alex%avf.hepnet@Lbl.Gov (ALEX FILIPPENKO, UC BERKELEY, 510-642-1813)
Subject: SN 1993J updates
4-16-93
SN 1993J'ers:
Here are three messages about various topics:
(1) From Kurt Weiler (kweiler@shimmer.nrl.navy.mil)
Subj: SN1993J Radio Information Update
N. Panagia, Space Telescope Science Institute and University of
Catania; S. D. Van Dyk and K. W. Weiler, Naval Research Laboratory
report that the available radio observations of SN 1993J are already
sufficient for initial modelling of the progenitor star and projection
of the likely evolution of the radio emission. Applying the Chevalier
mini-shell model which has been shown appropriate for other radio
supernovae (Weiler, et al. 1986, ApJ 301, 790) with the usual
assumptions for the stellar wind from the red supergiant progenitor
star and assigning a normal spectral index of -0.7 and an index for the
emission decay rate of -0.7, the model yields a presupernova mass loss
rate of about 2 x 10 ^-6 Msun/yr. The stellar evolution models of
Maeder & Meynet (1988, A&AS, 76, 411) then imply a ZAMS mass
for the progenitor of SN1993J of <15 Msun. Comparison with other
known radio supernovae, however, indicate that SN1993J is quite
similar in radio properties to SN1980K and SN1981K, and may
represent the lower end of the mass range of stars which can explode
as Type II supernovae. This relative consistency of properties may
indicate a relatively firm lower limit to the radio properties of Type
II supernovae, and therefore a possible minimum distance indicator.
Our modelling also results in predictions of the radio flux density to
be expected from SN1993J at various frequencies to assist planning
observations at other telescopes. Taking 26 March 1993 as Day = 1,
the expected peak flux density at different radio frequencies is ~20
mJy on Day 20 (14 April) at 23 GHz, ~20 mJy on Day 25 (19 April)
at 15 GHz, ~25 mJy on Day 40 (4 May) at 8.4 GHz, ~30 mJy on Day
55 (19 May) at 5 GHz, and ~35 mJy on Day 130 (2 August) at 1.4
GHz. After peak, the flux density at each frequency should decline
approximately as t^-0.7.
****
(2) From Janet Mattei (aavso@cfa0.harvard.edu)
CONSISTENT COMPARISON STAR MAGNITUDE MEASUREMENTS DESPERATELY NEEDED
for SN 1993J in NGC 3031
Photoelectric (V) and preferably also B, R, I measurements of the comparison
stars shown on the Thompson-Bryan and AAVSO preliminary charts are
desperately needed in order to obtain homogeneous brightness estimates of
Supernova 1993J in NGC 3031. This need was also pointed out by Dr. Gerard
de Vaucouleurs earlier.
Presently there are the following finder charts of the NGC 3031 field:
1. AAVSO PRELIMINARY CHART, indicated with designation 0947+69. Made
in 1979 by T. Fetterman, an active AAVSO observer and supernova searcher.
In the absence of a chart of this galaxy for visual observers, it was
developed to aid observers identify the field stars and to provide approximate
brightness estimates for these stars. The bright magnitudes (brighter than
about 9.0 to 9.5) are from the SAO Catalog, in which magnitudes come from
various sources and so are often not homogeneous. The faint magnitudes are
visual eye estimates by T. Fetterman. These visual eye estimates may have a
zero point problem and/or inconsistancies. Due to uncertainties in the
magnitudes of the comparison stars, this chart was indicated by the AAVSO as
preliminary and thus subject to revision. The circulation of this chart was
very limited.
2. THOMPSON-BRYAN CHART. Published by Cambridge University Press,
well-drafted field of NGC 3031, printed with white stars on black background.
The comparison star magnitudes come from the following sources:
a. Photoelectric sequences
i. One set [underlined] is from A. R. Sandage (Astron. Journ. 89, 621,
1984), and includes mostly stars northeast of NGC 3031. Observers have
reported that these magnitudes are consistent within themselves.
ii. One set [overlined] is from H. G. Corwin, and includes stars in
the field of the galaxy and close to the supernova. Observers have reported
brightness differences for some of the stars in this set.
b. Visual magnitudes
From AAVSO preliminary chart 0947+69 [double-underlined]. Not all
the stars on the preliminary chart are entered on the Thompson-Bryan chart.
c. Source(s) unknown
Also on this Thompson-Bryan chart are magnitudes for which the source
is not indicated. These stars do not have any markings (e.g., underlining or
overlining).
Thompson and Bryan circulated a limited number of an early edition of their
finder chart, with stars indicated as black dots on a white background. The
chart of SN 1993J distributed with AAVSO Alert Notice 169 is this chart.
Regrettably, we noticed after distribution that this early version is
different from the published one, in that some comparison star magnitudes in
the early version are different from the ones in the published chart. Why
these magnitudes were changed and what their source is are not known to me.
3. GUIDE STAR CATALOG CHART. Magnitudes were independently obtained by the
Space Telescope Science Institute. For some stars shown in this field, the
GSC magnitudes are quite different from photoelectric magnitudes mentioned
above.
Thus, in the finder charts mentioned above, there are at least five sources
of comparison star magnitudes. These magnitudes are not always consistent
within and between sources.
In order to revise the brightness estimates reported by visual observers and
those doing differential photometry, and to follow the optical behavior of
this supernova, photoelectric measurements of ALL comparison stars with
magnitudes on the Thompson-Bryan chart need to be made as soon as possible.
Also, it is essential that all observers report what comparison stars they
have used and are using in making magnitude estimates.
Janet A. Mattei
AAVSO Director
****
(3) From Roberta Humphreys (roberta@aps1.spa.umn.edu)
Notes on the Progenitor: Reddening, Variability, and Luminosity
Roberta Humphreys, Greg Aldering, and Kris Davidson
University of Minnesota
1. Nature of progenitor:
Determining the nature of the progenitor obviously depends on
whether we can resolve a single image,and if not, fit the observed
energy distribution with a single composite spectrum.
The energy distribution from the UBVR photometry we reported in
IAU Circ # 5739 can be fit by an unreddened late F -type
supergiant, an early F-type supergiant with Av 1.2 mag or an early A
type supergiant with Av about 2 mag. Taken at face value our colors
are consistent with a late A or early F-type supergiant with
Av 1.2 -1.5 mag.
The BVR colors reported by Perelmuter ( Mar 1990) are consistent with an
unreddened late G-type supergiant or a reddened mid to late F -type
supergiant with Av 1 -1.5 mag.
If we look only at the V-R colors and assume no reddening then a late
K-type supergiant is possible. But with any reddening even the V-R
color becomes too blue for a red supergiant.
Most people are reporting V and V-R photometry for the possible
progenitor but multicolor photometry ( especially B and U) is
needed to constrain the nature of the progenitor and to determine
the reddening.
Also the progenitor image is being described as stellar. With our
digitized scans we have been careful to isolate what appears to be
a stellar image from possible background contamination and nearby
stars, and on each plate we measured an image that was basically
stellar with the same x,y center to 0.15 arcsec with respect to
a reference grid. Evenso the image on the blue plate was more
elliptical. Thus, at the distance of M81 it is probable that
this image is not single.
We have now scanned additional plates of M81 including a short
exposure J (blue) and F (red) plates from March 21 1985.
The plates were scanned in multi-level isodensity thresholds.
On the J plate the image separates into at least two components.
One appears to still be blended and the other agrees in position
to within 0.13 arcsec with the centroid of the short R image
( which did not resolve into separate images). This positional
agreement corresponds to 2 parsecs at the distance of M81. At
the highest isodensity levels, the B and R images have diameters
20 microns which is 6 parsecs in M81. Thus these images may
still be blended. The corresponding magnitudes for the images on
these short exposure plates are B=21.8 and R=19.9 mag.
If V-R =0.8mag ( see #3 below), then the corresponding V is
20.7 mag and B-V = 1.1 mag.
[ The scale of these 4-meter plates is 18.6 arcsec/mm. The
encoders on the APS read to 0.366 micron with a
repeatability of 1 micron.]
Observers with CCD images may be able to deconvolve their
images.
Not surprisingly, it is possible to fit the observed energy
distribution of the progenitor by a wide range of composite spectra
from a mid K star ( the latest) plus an early B star (the earliest)
to a late G star plus an early A star. These fits assume no reddening.
Increased reddening restricts the range of allowed composite spectra
by decreasing any contribution from a red supergiant.
However the possible variability may put important constraints on
any proposed composite.
2. Reddening:
As Richmond reported in IAU Circ 5739 there is high reddening across
the face of M81, as indicated from the UV images- Av 1.5 mag ( Hill et
al 1992); the radio and Halpha measurements by Kaufman et al (1987)
show an average reddening of 1.1 mag +/- 0.4 with no gradient from
the nucleus. In a study of the red supergiants Humphreys et al (1986)
found Av from 1 -1.5 mag. Thus there is significant reddening
throughout M81.
SN1993J may be in a hole, but given its location in a spiral arm
near a prominent dust lane this seems unlikely. Reddening should
be considered when discussing the luminosity of the supernova and
the possible progenitor.
3. Variability:
The photometry reported for the possible progenitor shows considerable
evidence for variability with the R mag posssibly ranging from 20.7 to
19.0.
Summary of photometry for possible progenitor:
Date V V-R (R) B-V U-B Source
Mar 21-23 1982 20.8 0.9 19.9 0.6 0.3 4-meter plates
Jan 21 1985 21.5 0.8 20.7 CCD Pal 1.5-meter
(transformed from r,g-r)
Mar 21 1985 (20.7) 19.9 1.1 4-meter plates
Mar 1990 20.0 0.7 19.3 1.1 CCD KPNO
Jan 24 1992 19.7: 0.8 19.0: CCD Hiltner 2.5 meter
Oct 2 1992 20.0 CCD KPNO 0.9 meter
Mar 16-23 1993 20.0 Haute-Provence CCD
There are several very important points about the variability:
1. The V-R color has remained essentially constant at 0.8 +/- 0.1 mag.
This is very remarkable. All luminous irregular variables show
changes in color coinciding with the expansion and contraction
of their atmospheres. This is also true for M supergiants.
No change in color with this much variation in magnitude means
a real change in the total luminosity of the star.
This could be explained by the ejection of shells.
2. An R mag of about 20 appears three or more times in this list.
If the star is varying between max. and min., one would not expect
random measurements to give the same magnitude. This may mean
that an R of 20 and a corresponding V of 20.8 represent some
kind of quiescent state for the star, and are indicative of
the star's true luminosity.
3. In a 1984 paper Sandage ( AJ,89,621) lists the magnitudes for
several irregular blue variables in M81 for the period 1910
to about 1955. One of these I2 is quite close to SN1993J. He
lists variations for this star of a few 0.1's. Thus if the
progenitor was varying at that time by one or more magnitudes,
I think he would have noticed it.
4. Finally, the variablity may not be real and only due to differences
in reduction. Because the object is obviously in a crowded region,
the magnitudes may depend on where the background is set.
4. Luminosity:
Using the unreddened distance modulus for M81 of 27.5 - 27.7 and no
reddening for the progenitor then Mv is -7.6 at visual max.
But with 1 -1.5 mag Av, then Mv becomes -8.6 to -9.1 and the star
is in the regime of the cool hypergiants near the upper luminosity
boundary in the HRD.
The above luminosities are all for the brightest observed
mv of near 20 mag. A V mag near 20.8mag may be more representative
of the progenitor's true luminosity. If V=20.8 then Mv is -6.8 or
about -7.8 with 1 mag of reddening.
For the SN, if max light was mv 10.3 the Mv was -17.3 or
withh reddening Mv -18.3 to -18.8.
5. Needed:
There are numerous photographic plates of M81 as well as CCD images
of this famous galaxy. It is critical to measure as many as possible
to document this stars variability. According to a paper by Sandage
(1984) plates exist as far back as 1910!
Zickgraf et al (1990 PASP,102,925) have published a faint UBVR
CCD sequence for M81 and Sandage (1984 AJ 89,621) has a BV photoelectric
sequence that can be used for calibration.
It is especially important to measure magnitudes in B and U.
Efforts should be made to deconvolve the image.
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