Spatially resolved IUE spectra (1150 to 2000 A) taken at the position of SN 1987A in March 1987 show that the 12th mag B3 I star Sk -69 deg 202 disappeared. Only the fainter companion stars (Star 2 and Star 3) are present near the site of the supernova. It is concluded that Sk -69 deg 202 exploded to produce SN 1987A. The known characteristics of Sk -69 deg 202 are consistent with the interpretation that the progenitor was a relatively compact star, having a high-velocity low-density stellar wind prior to the outburst. Recent IUE spectra of SN 1987A (May 1988) show no evidence that Sk -69 deg 202 still exists inside the expanding ejecta

Sonneborn, G.

English

NASA Technical Reports Server

N89-10697 107
CONTRIBUTED PAPER
THE PROGENITOR OF SN 1987A
G. Sonueboi n
Astronomy Programs, Conltmle.r Scic'nees Corporation,
and IUE Observatory, (;oddard Space l:light Center
ABSTI{ACT
Spatially resolved IUE spectra (1150- 2000/_ ) taken at,
the position of SN 1987A m March 1987 show that the 12th
mag B3 1 star Sk -6!)° 202 has disappeared. Only the two
fainter companion stars (Star 2 and Star 3) ate now present
near tim site of the supernova Sk -69 ° 202 is tile star which
exploded to produce SN 1987A. The known characteristics
of Sk .69 ° 202 are consistent with the interpretation that
the progenitor was a relatively compact star, having a high-
velocity low-density stellar wind pimr to the outburst. Re-
cent IUE spectra of SN 1987A (May I!)88) show no evidence
that Sk -69 ° 202 still exists inside the expanding ejecta
Keywords: Supernovae, ultraviolet spectra
I. 1NTIIODUCTION
It came as a svrprise, to those following the early ob-
servations of SN 1987A that the definitive identification of
the progenitor wot, ld be made in the ultraviolet. However,
the discovery that the supernova's far ultraviolet flux faded
by three orders of magnitude in the first four days of the
outburst (/{eL 1), revealing a constant, stellar background
flux, presented us with an unexpected opportunity to ex-
amine the supernova's immediate surroundings for tile sur-
vivors. By 27 February 1!187 the far UV flux had dropped
to a level where a constant, stellar background was detected
shortwa,'d of 1500 7_ .
Analysis of preoutbu,st plates revealed an excellent po-
sitional coincidence 1)etwcen tile supernova and the 12th
mag B3 1 stai Sk -69 ° 202 (llef. 2) and the presence of a
second star 3 arc sec 1.o the NW (l{ef. 3). Similarities be-
tween the IUE spectrum of the UV source at the position of
the supernova and that of a luminous early-type star seemed
to imply (Ref. ,1) that Sk -69 ° 202 might have survived the
explosion.
It was soon discovered t]lat IUE Sl)ect, ra of SN 1987A
were broadened pelpcndiculdr to the dispersion shortward
of A 1500, indicai.mg lhe presence of ii,ol(', than one source in
tim 10" x 20" at)crl ilrc during tile supcl nov*_ exposure <, (Ref.
5). The detection of two early-tyl)e UV stellar spectra at
the sin." of the fading supernova and tim identification of two
blue stars in the Sk -69 ° 202 systenl seemed to imply that
tile spectra came from the two stars: Sk -69 ° 202 (Star 1)
and Star 2.
Subsequently, high-precision measurements and image
syntheses of the Sk -69 ° 202 tield (lIef. 6) demonstrated
the reality of ,t llm'd star and l)rovided accurate relative
positions for the three members of the Sk -69 ° 202 system.
Star 3 was about two magnitudes brighter than l)rewously
thought (my = 15.5), about 1.6 arc sec SE of Stal 1, and
also blue in color. This review summarizes the work (Refs.
7 and 8) which demonstrates that the objects detected near
the supernova in IUE spectra are Stars 2 and 3 and that
Sk -69 ° 202 is no longer present.
1I. OBSERVATIONS AND DATA ANALYSIS
The identity arid chal'acteristms of the stars within sev-
eral arc seconds of the supernova, and detected in IUE spec-
tra, are determined by analysis of SWP low-dispersion spec-
tra taken after 1 March 1987. During this period tile far
UV flux of the supernova was much weaker than the other
sources in the al)erture and the projected separation of Stars
2 and 3 was sufficiently large to allow deconvolution of their
spectra. The analysis described below makes use of IUE
spatially-resolved low-dispersion spectra, tire so-called Ex-
tended Line-By-Line (ELBL) spectra (see Refs. 9 and 10).
The xelative positions of Stars 1, 2, and 3 from Ref. 6
were adopted for comparison with the IUE data. The ori-
entation of the th,'ee stars and the large aperture are shown
to scale in Figure 1. The aperture orientation on the sky
is determined by the satellit&s solar array orientation. Be-
cause the Large Magellanic Cloud is located near the south
ecliptic pole, IUE's array orientation changes about one de-
gree per day. It was completely fortuitous that the stars
within a few arc seconds of SN 1987A were in near-optimal
alignment with the spectrograph's spatial direction at the
time when the supernowl's UV flux faded.
A Decade of UV Astronomy with IUE, Proc Celebratory Sympostum, GSFC, Greenbelt, USA, 12-15 April 1988, ESA SP-281, llol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001326 2020-03-20T05:58:02+00:00Z
108 G SONNEBORN
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A RIGHT ASCENSION ARCSECONDS)
Figure 1 Changing IUE aperture orientation with respect to tile
SN t987A field 'File astrometrte potations (Ref 6) of Stars 2 and 3
are shown relatwe to Star 1 The outhne of the aperture Is shown for
three dates 27 February, 13 and 22 March 1987 Tile stra@lt hne is
tile direction perpcndlcular to the dlspermon for the 13 March date
(sohd hne) (From Ref. 8)
The 1250 - 1600 _ ELBL data from 13 March (SWP
30512) are compared in Figure 2 with simdar data for a
single point source. The spectral data have been averaged
in 50 _ intervals; no binning or smoothing has been applied
in the spatial direction. The presence of two sources in the
aperture during the supernova exposure is apparent. The
FWHM of the IUE point-spread function (PSF) in SWP
spectra varies between 4.6 arc sec at 1350 _ to 6.0 arc see
at 1900 /_ (Ref. 11). A good photographic representation
of the spatially-resolved spectra of Stars 2 and 3 is shown
in Ref. 7.
Numerical proceduies were developed to analyze over-
lapping spectra in IUE low-dispersion images (Ref. 12).
Multiple PSF's are fit to the spatial profile of the ELBL
data with a multi-variable least squares fitting technique,
adopting the skewed-gaussian form of the IUE PSF (Ref.
11). Figure 3 compares the variation in the mean separa-
tion between two PFSs with that expected for Stars 1 and 2
and for Stars 2 and 3. A more detailed discussion of the fit-
ting procedure is given in Ref. 8. The variation depicted in
Figure 3 is the result of changing IUE aperture orientation
due to solar array constraints.
3
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O
o
t/]
o 2
o2
0
50
MAR 1 APR 1 MAY 1 dUN 1 dUL 1 AUG
i i i i i i
, I I _ V',,_I I I
75 100 125 150 175 200 225
OA¥Or YEAR(1987)
Figure 3 The measured (diamonds) and predicted (sohd dots
and line) separation between Stars 2 and 3 as a function of time The
expected separation between Stars [ and 2 is also shown (large dashed
hne). The small dashed line represents a 4 ° change in tile Star 2-3
positron angle to 133.°7 (From Ref 8)
i i i "
b
O
+
d
-10 -5 o 5
ARCSECONDS
, 1575
1525
/ 1475
/ 1425
1375
1325
/ 1275
I
10 -10 -5 0 5 10
ARCSECONDS
Figure 2 Low-dispersion SWP spatially-resolved spectra a) The
ELBL data for a single point source is shown at 50 _ intervals from
1275 to 1575/_ . b) The ELBL data for one of the supernova manges
(SWP 30512, I3 March) shows the presence of two sources close to the
position of SN 1987A . (From Ref. 8)
II1. RESULTS
The measured separations between the two spectra are
in very good agreement with the astrometry of Walborn
et al. (Ref. 6) only if the observed stars are Stars 2 and 3
The results are not consistent with the expected separation
between Stars 1 and 2. The error in the component sep-
arations, which includes the estimated uncertainty in the
IUE spatial scale and the rms error in the least-squares
fits to each 25 ,&. section of the spatial profile, are small,
generally less than 5% of the gaussian FWHM. We do not
know the relative contributions to the small systematic dif-
ferences between measured and predicted separations from
measurement error in the astrometric positions and in the
IUE analysis and spatial scale. However, these differences
can be accounted for by a 4 ° rotation of the Star 2 - Star
3 position angle, as shown in Figure 3.
Two point sources produce an excellent fit to the ELBL
data ()_ < 1500) taken after about t March, with flux resid-
uals (data - computed fit) of several percent of the height
PROGENITOROFSN198-/A 109
of the gaussian PSF (Fig. 4a). llowever, there was a pe-
riod of a day or two at the end of February when Stars 2
arid 3 and SN 1987A had comparable UV flux levels below
1500 _ . A two component fit to these data yields large [lux
residuals of 25-30% (Fig. 41)) When three point sources
are fit to the same data an excellent result is obtained, wit, h
flux residuals again close to several percent (Fig. 4c). The
third point source is located between Stats 2 and '3, 2.9:k0.:]
arc sec from Star 2, in good agreement with the position of
Sk -69 ° 202 / SN 1987A wtth respect to Stars 2 and .3.
The gausslan fitting procedure also determines the rel-
ative contribution of Stars 2 ,uld 3 l.o the total flux for the
entire SWP wavelength range. The deconvolved spectra of
stars 2 and 3 are shown in Figure 5. They have the ap-
pearance of early-l] spectral lypes. The strength of the nu-
merous interstellar lines in both spectra is consistent with
IUE low-dispersion spectra of other I,MC early-type stars.
Both Stars 2 and 3 must I)e located in the LMC, and are
not foreground objects.
Dereddened fluxes for Stars 2 and 3 have been deter-
mined by comparing thetr deconvolved spectra with the Ku-
rncz (Ref. 13 and unl)ublished ) model atmosphere grid.
The reddening was estimated fi'om the colors of Sk -69 ° 202
(l{ef. 14) and recent work on mtrinsic cok)rs of LMC su-
pergtants (Ref. 15). A two component extinction has been
adopted: E(/3-V)GAL =0.08 flora the Galaxy (Ref. 16)
and E(I3-V)LMC = 0.10 from the LMC (30 Doradus curve
from Ref. 17), for a total 12(13 - V) of 0.18. The one-thlrd
solar abundance Kurucz models winch give the best fit to
the contimlunl slopes are: Tef f = 20000K and 25000K (:i:
2000K) and log g = 3.0 and ,1.5 (:t: 0.5) for Stats 2 and 3,
respectively. This suggests that Star 2 might be slightly
evolved (luminosity class IV or Ill). flowever, the decon-
volved sl)ectra are noisy and not of high enough quahty to
reliably assign spectral types and luminosity classes.
In summary, only two of the originM three stars comlms -
ing the Sk -69 ° 202 system are detected in spatially-resolved
UV spectra taken at the potation of SN 1987A. The tempo-
ral variation of the spectral separation is in good agreement
with theft expected for the potations of St_trs 2 ,rod 3 mea-
sured on preoutburst plates. Furthermore, in late February
1987 the location of the supernova spectrum shortward of
1500 _ , relative to Stars 2 and 3, was in excellent agree-
ment with the expected position of Star 1. Sk -69 ° 202 is
absent from the field and was therefore the progenitor of
SN 1987A. The known characteristms of Sk -69 ° 202 are
consistent with the interpretation that the progenitor was
a relatively compact star (F[ef. 18), having a high-velocity
low-density stellar wind prior to tit(," outburst (Ref. 19). Re-
cent IUE spectra of SN 1987A (May 1988) show no evidence
that Sk -69 ° 202 still exists inside the expanding ejecta.
z
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hA __ A m
I I I
I L I
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I I I
I I I
c
I I I
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ARCSECONDS
1
lO
I"lgure 4 Least-square fits of IUE PSFs to .,,patrol profiles of
tile 1350 - 1:175 /l regmn of ELBI, data a) Two components are
fit to SWP 30512 The computed fit (sohd hne), observatmns (open
circles), lndlwdual PSFs (dashed lines), and residuals (sohd squares)
are shown The left- and right-hand components correspond to Stars
2 and 3, respecUvely b) A two-component tlt to SWP 30408 (27 Feb
1987), where SN 1987A and Stars 2 and 3 had similar flux levels The
component separatmn m 4 50 arc see c) A three-component fit to the
same data a.s 111Fig 4b is in excellent agreement with tile observations
The center colnl)onent corresponds 1o SN 1987A (From Ref 8)
o<£
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E
o
7
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r
v
x
=,
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6
5
4
3
2
1
0
12OO
I 1 I I I I I
I I I I I I I
1300 1400 1500 1600 1700 1800 1900 2000
o
WAVELENGTH ( A )
Figure b The deconvolved spectra of Stars 2 and 3 Strong
interstellar absorptmn hnes between 1250 and 1500 /_ are evident,
even m low dBperMon, unplymg both stars are m the LMC The xs
mark portions of the spectrum effected by camera reseaux
110 G SONNEBORN
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The Progenitor of SN 1987A

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