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Subsections

2 Observations and results

2.1 Photometry

The photometric data of SN 1998S were obtained on a total of 32 nights between 1998 March 5 and 1998 July 2 with a 0.6 m telescope at Xinglong Station of the BAO. The CCD camera attached at the prime focus is a Texas Instruments TI-215, which has 1024$\times$1024 pixels and a field of view of $16\hbox{$.\mkern-4mu^\prime$ }8\times 16\hbox{$.\mkern-4mu^\prime$ }8$. The broadband filters used for the photometry of SN 1998S are B and V and Cousins' $R_{{\rm c}}$. Since there is no guider on the 0.6 m telescope, the exposure time could not exceed 300 s. We primarily used exposures of 120 s when the SN was around maximum and 300 s when it dimmed. The data were reduced using the IRAF package. The photometry for 1998S in B, V and R bands is listed in Table 1, and plotted in Fig. 1, together with the photometry reported in the IAU circulars (Nakamura et al. 1998; Garnavich et al. 1998; Garnavich et al. 1998b). The solid lines are schematic light curves of SN 1979C in the B and V systems, a typical SN II-L in NGC 4321 (Barbon et al. 1982).


 

 
Table 1: Photometric observations of SN 1998S

DATE
JD(-2450000) B V $R_{{\rm c}}$ 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 *, $B_{{\rm max}}$
March 14 887.21179 --- 12.339 12.272 --- 2  
March 21 894.16584 12.490 12.313 12.187 0.18 9 *, $V_{{\rm max}}$, $R_{{\rm max}}$
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 $\beta_{100}^{B}$ = 5.4 $\pm$ 0.2, $\beta_{100}^{V}$ = 3.6 $\pm$ 0.3, $\beta_{100}^{R}$ = 2.7 $\pm$ 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.


  \begin{figure}
\includegraphics[]{9539f1.eps}\end{figure} 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


  \begin{figure}
\includegraphics[]{9539f2.eps}\end{figure} Figure 2: Comparison of the B-V color curve of SN 1998S with that of the well-studied type II-L SN 1980K in NGC 6946. The solid circles are obtained at BAO, and the open circle is taken from Garnavich et al. (1998b). The stars are the B-V colour of SN 1980K (Barbon et al. 1982), which have been arbitrarily shifted along x-axis in order to obtain the best fit to the observations of SN 1998S

Adopting for the parent galaxy NGC 3877 a distance modulus $\mu$ = 31.15 and reddening $\ A\ _{B}^{b} = ~0.01$ (Tully 1988), we derive an absolute magnitude at maximum $M_{B}^{0}\leq -18.7$, 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 $\beta_{100}^{B-V}$= 1.8 (Patat et al. 1994) and similarity to SN 1980K.

2.2 Spectroscopy

The optical and near-infrared low-dispersion spectra of SN 1998S were obtained with the OMR spectrograph at the BAO 2.16 m telescope. The CCD is a Tektronix 1024 $\times$1024. We used two low- dispersion gratings, the dispersions of which are about 200 and 400 Å mm-1, respectively. The spectral coverage of the two gratings is about 3800-8200 and 3800-9200 Å. Their resolutions are about 4.9 and 9.8 Å pixel-1, respectively. All the data were bias-subtracted, flat-fielded, and extracted with the IRAF packages. Wavelength calibrations were performed using the spectra of FeAr or HeNeAr lamps. The supernova flux was calibrated in reference to observations of spectroscopic standards (Oke & Gunn 1983).


 

 
Table 2: Journal of spectroscopic observations of SN 1998S

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

         



  \begin{figure}
\includegraphics[]{9539f3.eps}\end{figure} 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 $\lambda$4686, the H I Balmer series, and the N III $\lambda \lambda$ 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$\beta$. 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$\alpha$ 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$\alpha$ to H$\epsilon$. The narrow component of H$\alpha$ 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 $\lambda \lambda$ 6678 and 5876 can be detected. The most striking feature is the H$\alpha$ and H$\beta$ 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$\alpha$ and H$\beta$ 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$\alpha$ is 74 Å, implying an expansion velocity of 3380 ${\rm km~s}^{-1}$.

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$\alpha$ to H$\epsilon$. Such lines dominate the spectrum. The H$\alpha$ line shows a narrow P Cygni emission, narrow absorption shifted by -1100 ${\rm km~s}^{-1}$ from the emission, and a broad, shallow absorption extending to -10000 ${\rm km~s}^{-1}$ with a minimum at -4700 ${\rm km~s}^{-1}$.

One month later, the continuum becomes significantly fainter and redder. The character of the spectrum changes radically. The forbidden lines of [O I] $\lambda$ 5577 first appear as the density of the line emitting region decreases. The narrow component of H$\alpha$ nearly disappeared by April 14. Like typical II-L SNe, no obvious P Cygni profile was present on the H$\alpha$ 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$\beta$ and Fe II $\lambda$ 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 ( $\lambda \lambda$ 8498, 8542, and 8662) is very strong, and Ca II H, K lines are discernible. The H$\alpha$ and the Ca II near-IR triplet are the dominant feature. The forbidden lines of [Ca II] $\lambda \lambda$ 7291 and 7324 and [O I] $\lambda \lambda$ 6300 and 6364 first appear (but can trace back to the April spectra) and progressively grows in strength.


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