DATE | JD(-2450000) | B | V |
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B-V | t | Note |
March 5 | 878.24012 | 13.246 | 13.362 | 13.326 | -0.12 | -7 | |
March 6 | 879.19821 | 12.965 | 13.077 | 13.054 | -0.11 | -6 | * |
March 8 | 881.20014 | 12.676 | 12.729 | 12.712 | -0.05 | -4 | |
March 11 | 884.16831 | --- | 12.469 | 12.443 | --- | -1 | |
March 12 | 885.15253 | 12.462 | 12.435 | 12.371 | 0.03 | 0 | *,
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March 14 | 887.21179 | --- | 12.339 | 12.272 | --- | 2 | |
March 21 | 894.16584 | 12.490 | 12.313 | 12.187 | 0.18 | 9 | *,
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March 22 | 895.14534 | 12.601 | 12.328 | 12.199 | 0.27 | 10 | |
March 23 | 896.16741 | 12.609 | 12.361 | 12.217 | 0.25 | 11 | * |
March 24 | 897.14626 | 12.679 | 12.369 | 12.221 | 0.33 | 12 | |
March 25 | 898.13049 | 12.679 | 12.415 | 12.220 | 0.26 | 13 | |
March 26 | 899.15073 | 12.746 | 12.466 | 12.252 | 0.28 | 14 | * |
March 27 | 900.24368 | 12.883 | 12.471 | 12.300 | 0.41 | 15 | |
March 28 | 901.10917 | 12.865 | 12.490 | 12.294 | 0.37 | 16 | |
March 29 | 902.17131 | --- | --- | 12.396 | --- | 17 | |
March 30 | 903.17039 | 12.962 | 12.559 | --- | 0.40 | 18 | |
April 7 | 911.12118 | 13.382 | 12.863 | 12.561 | 0.52 | 26 | |
April 8 | 912.13229 | 13.475 | 12.933 | 12.606 | 0.54 | 27 | |
April 12 | 916.1162 | 13.649 | --- | 12.688 | --- | 31 | |
April 14 | 918.09834 | 13.768 | 13.148 | 12.774 | 0.62 | 33 | |
April 18 | 922.13403 | 14.029 | 13.329 | 12.895 | 0.70 | 37 | |
April 21 | 925.06375 | 14.263 | 13.327 | 12.961 | 0.94 | 40 | |
April 24 | 928.09568 | --- | 13.538 | 13.035 | --- | 43 | * |
May 2 | 936.05209 | 14.675 | 13.666 | 13.231 | 1.01 | 51 | |
May 7 | 941.04237 | 15.000 | 13.839 | 13.363 | 1.16 | 56 | |
May 20 | 954.06493 | 15.606 | --- | 13.833 | --- | 69 | |
May 25 | 959.04054 | --- | 14.762 | 14.212 | --- | 74 | * |
May 28 | 962.02126 | --- | 14.741 | --- | --- | 77 | |
May 29 | 963.06325 | 16.235 | 14.884 | 14.330 | 1.35 | 78 | |
June 5 | 970.04291 | --- | 14.957 | 14.390 | --- | 85 | |
June 18 | 983.05329 | --- | --- | 14.604 | --- | 98 | |
July 2 | 997.03228 | --- | 15.391 | 14.790 | --- | 112 | * |
SN 1998S was rising when we began to do photometry on 1998 March 5. B rose quickly (-0.19
mag day-1) from March 5 to March 8, then it rose more slowly, reaching a maximum around 1998 March
12. Although the maximum light may occur between March 12 and 21 because there is a 9-day
observational gap between them, we still take March 12 as the maximum for convenience. During the first
70 days after maximum, the light curves show a steep, almost linear, decline. A linear fit of the data gives
= 5.4
0.2,
= 3.6
0.3,
= 2.7
0.3 mag (100 d)-1,
which are typical of linear SNII (Patat et al. 1994). The fitting of the observed B magnitudes with the light curve
of SN 1979C clearly shows that this SN belongs to the same subclass of SN II. It will be useful to note that the
luminosity of SN 1998S at 230 days after the maximum is consistent with that of SN 1979C expected from the
interpolation of the B and V light curves.
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Figure 1: Light curves of SN 1998S in NGC 3877. The lines represent the schematic B and V light curves of SN 1979C in NGC 4321 (Barbon et al. 1982), which have been arbitrarily shifted along both axes in order to obtain the best fit to the observations of SN 1998S. The solid symbols are obtained at BAO, and the open symbols are taken from IAU circulars |
Adopting for the parent galaxy NGC 3877 a distance modulus
=
31.15 and reddening
(Tully 1988), we derive an absolute magnitude at maximum
,
not considering the extinction
in the parent galaxy. The maximum magnitude of SN 1998S is more than 2 mag brighter than the regular
SNII and indicates this SN is a member of the rare class of Bright SNII (Patat et al. 1994), which include
both type II-L SNe 1979C, 1980K, and 1990K and type IIn SNe 1983K, 1987F, and 1988Z.
The (B-V) is only 0.03 at maximum light (Fig. 2). Patat et al. (1994) found that the behaviour of (B-V) in
SNe II-P and II-L is relatively homogeneous in the early 50 days. In general, the maximum (B-V) of II-L
is about +1.0, but the Type II-L SNe 1980K and 1959D reach a maximum (B-V) value of +1.4 and 1.7
respectively. The maximum (B-V) of another SN II-L 1990K also reached 1.45 (Cappellaro et al. 1995).
The B-V color exhibits a rapid evolution from blue to red and then linearly increased to a high value of 1.35 on
May 29 at t= 78 days after maximum light, comparable with Type II-L SNe 1959D, 1980K, and 1990K.
In Fig. 2, we compare the B-V color curve of SN 1998S with that of the well-studied type II-L SN 1980K in NGC 6946
(Barbon et al. 1982). Because SN 1980K was discovered after maximum light, we have shifted
the colours 7 days along x-axis in order to obtain the best fit to the observations of SN 1998S. We can see
from the figure that the colour curve of SN 1998S is well fitted to that of SN 1980K. Although the photometric
observations of SN 1998S only covered an epoch of about four months, the B-V color evolution at early epoch
is also typical of linear SN II due to its
= 1.8 (Patat et al. 1994) and similarity to SN 1980K.
Date | t | JD (-2450000) | Exposure (s) | Range (Å) | dispersion (Å/mm) |
March 6 | -6 | 879.245 | 1200 | 3677-8644 | 200 |
March 12 | 0 | 885.238 | 600 | 3586-8408 | 200 |
March 21 | 9 | 894.225 | 600 | 3764-8730 | 200 |
March 23 | 1 | 896.198 | 600 | 3730-8696 | 200 |
March 26 | 14 | 899.186 | 600 | 3660-9264 | 400 |
April 14 | 33 | 918.174 | 600 | 3525-8493 | 200 |
April 24 | 43 | 928.172 | 600 | 3548-8514 | 200 |
May 25 | 74 | 959.126 | 900 | 3633-8505 | 200 |
July 2 | 112 | 997.124 | 1500 | 3599-9015 | 400 |
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Figure 3: Spectral evolution of the supernova in NGC 3877. Wavelength is in the rest frame of the parent galaxy |
The first spectrum of SN 1998S (Fig. 3), obtained about a week before maximum light, shows it to be quite
unusual. The most prominent lines are readily identified with He II 4686, the H I Balmer
series, and the
N III
4634, 4640, 4641 and probable C III 4640 blend. The overall character is one of quite high
excitation, strikingly similar to that observed in Wolf-Rayet stars (Garnavich et al. 1998a;
Leonard et al. 1999), as noted by Niemela et al. (1985) in SN 1983K. The strengths of He II
4686 Å and N III/C III are comparable to the flux of H
.
These features are identical with those seen in SN 1983K (Niemela et al. 1985), which was
discovered two weeks before maximum light, but with a much weaker H
emission intensity. A broad
emission feature peaking at 5800 Å may be C IV, also found in Wolf-Rayet stars. Another emission at
5696 Å is likely to be C III line. There are also weaker He II lines seen at 5411 and 4541 Å, as well as the
presence of a hint of weak emission of He I at 4471, 5876 and 6678 Å. Na I absorption with an
equivalent
width of 0.7 Å is measured. Although the H-Balmer lines are relatively narrow, they are still wider than
those of typical IIn SNe 1988Z and 1994W are.
The second spectrum, taken at the maximum light, i.e., March 12, displays a nearly featureless
blue continuum, superimposed by weak Balmer lines, H
to H
.
The narrow component of H
initially
quite luminous, is now much weaker. The W-R star features near 4670 Å (He II and N III/C III) and
5800 Å (C IV) in the spectrum of March 6 are hardly discernable now. The weak He I
6678 and 5876
can be detected. The most striking feature is the H
and H
emission which had developed a notable red
asymmetry, and the blue wings become steeper with little evidence of the P Cygni absorption. The
equivalent widths of H
and H
are -8.3 Å and -2.4 Å, respectively, compared to -32 Å and -13 Å on
March 6. The full width at the base of H
is 74 Å, implying an expansion velocity of 3380
.
The next three spectra were taken between March 21 and 26. Because of the similarity of the
three spectra, we have averaged them to improve signal-to-noise ratio. The extraordinary feature of
the spectra is the narrow emission P Cygni profile superposed on a much broader absorption or P
Cygni structure in all five Balmer lines, H
to H
.
Such lines dominate the spectrum. The H
line
shows a narrow P Cygni emission, narrow absorption shifted by -1100
from the emission,
and a
broad, shallow absorption extending to -10000
with a minimum at -4700
.
One month later, the continuum becomes significantly fainter and redder. The character of the
spectrum changes radically. The forbidden lines of [O I]
5577 first appear as the density of the line
emitting region decreases. The narrow component of H
nearly disappeared by April 14. Like
typical II-L SNe, no obvious P Cygni profile was present on the H
line of SN 1998S. The overall feature of
the spectra is similar to those of SNe 1979C and 1984E obtained a month after maximum light. The
blueshifts of the absorption components of H
and Fe II
5018 are comparable with those of SN 1984E,
but smaller than those of SN 1979C. We also notice that the Ca II near-IR triple is apparent although the
spectra do not cover the full line.
The last two spectra, respectively taken on May 25 and July 2, show the further development of
the April spectra. Broad emission line of the Ca II near-IR triplet (
8498, 8542,
and 8662) is very
strong, and Ca II H, K lines are discernible. The H
and the Ca II near-IR triplet are the dominant feature.
The forbidden lines of [Ca II]
7291 and 7324 and [O I]
6300 and 6364 first appear
(but can trace back to the April spectra) and progressively grows in strength.
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