Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.sao.ru/precise/Laboratory/Publications/1998/HS1035_SAO/SN_for_SAO/node3.html
Дата изменения: Tue Feb 23 12:44:58 1999
Дата индексирования: Sat Sep 11 22:14:26 2010
Кодировка:
Поисковые слова: п п п п п п п п п п п п п

Results and Discussion

In Fig. 2 the spectrum of the host galaxy HS1035+4758 is shown, in which typical HII-galaxy narrow emission lines are seen: H$\beta$ $\lambda$4861, [OIII] $\lambda\lambda$4959,5007, H$\alpha$$\lambda$6562, [NII] $\lambda\lambda$6548,6584 and [SII] $\lambda\lambda$6716,6731 Å. The equivalent widths of the strongest emission lines, H$\beta$, [OIII] 4959,5007 and H$\alpha$, are 20, 20, 60 and 90 Å, respectively. The galaxy redshift as measured using these strong lines is z=0.0054$\pm$0.0001.

The absolute blue magnitude ${\rm M_{B}=-15\fm5}$ (for linear model of Virgo flow correction and adopted Hubble constant 75 km s^-1Mpc^-1, which corresponds to a distance of 29.6 Mpc) and the total size of about $20\arcsec$ (2 kpc) are typical of BCGs, galaxies with strong HII-type emission indicating superclusters of massive stars in regions of current intense star formation.

The spectral evolution of SN 1998bv is illustrated in Fig. 3. The spectrum clearly indicates SN II type with strong broad hydrogen emission lines with PCyg profiles (see e.g. review by Filippenko, 1997). Having 3 spectra with a total time lag of 18 days and estimates of the blue magnitude on those dates, obtained through convolution of calibrated spectra with the B passband, we can attempt to make finer classification of this SN as one of the well known subclasses (Linear or Plateau).

The change of the spectrum between April 6 and 19 with clear reddening shortward of H$\beta$ during these 2 weeks indicates seemingly that the 1st spectrum was observed in the period between the 20th and 30th days after the peak of the light curve. Spectra for April 6 and 24, 1998 were obtained under photometric conditions. Thus we can use the estimates of B-magnitudes obtained to get an idea on the decline rate $\beta_{100}^{B}$ (in mag/100^d) which discriminates by its value of 3.5 between Plateau and Linear shapes of the light curve (Patat et al., 1994). The calculations give B(SN)=16.99$\pm$0.10 on 6.04.1998 and B(SN)=17.20$\pm$0.07 on 24.04.1998, where r.m.s. errors are intrinsic ones estimated from the noise counts. We estimate that the external accuracy is about $0\fm15-0\fm2$. This leads to $\Delta$B=$0\fm21$ and $\beta_{100}^{B}$ = 1.17 mag/100^d. This value is so much less than 3.5 that even allowing for a possible uncertainty in $\Delta$B of up to $0\fm2-0\fm3$ it would not change the classification of this SN II as Plateau subclass. The absolute blue magnitude for this SN on the discovery date is M_B=-15.4. Then M $_{max}^{B} \approx -$16.4, which agrees well with parameters of Regular SNeII-P.

Now we briefly describe some characteristic spectral features and their parameters related to ejecta motion. Hydrogen Balmer lines with pronounced PCyg profiles are the most prominent features which are used for determination of the expansion velocity.

The characteristic expansion velocities of the ejected shells are measured from the positions of the deepest features of the PCyg absorption. They are presented in Table 2 for the 3 dates of observations. The upper limit of the expansion velocity was derived from the wavelength of the blue wing edge of the H$\alpha$ absorption.

 

Table 2: The expansion velocities (km s^-1)

	April 6	April 19	April 24
H$\alpha$	...	6350	6125
H$\beta$	6600	5500	4940
H$\gamma$	5800	5050	4115
H$\delta$	4740	...	...
CaII K	6215	...	...
CaII H	6419	5597	...

Besides the PCyg absorptions in the Balmer series, several other broad absorptions are observed, including some FeII multiplets and CaII H and K lines (also typical of the Plateau phase, which is transitional between photosphere and nebular phases). A remarkable feature of this SN is the absence of detectable HeI line at $\lambda$ 5876 Å.

  

Figure 3: Spectra of SN 1998bv obtained with the 6 m telescope. Spectra for April 6 and April 19, 1998 are shifted on the flux axis. The absorption line identifications are indicated, adopting an expansion velocity of 5000 km s^-1. The spectra are corrected to the galaxy rest frame.

It is interesting to estimate the extent of narrow H$\alpha$ line emission in the host galxy, indicating the extent of ionized gas (Fig.4). The region which emits in H$\alpha$ can be traced on our long-slit spectrum over $\approx$24 pixels = 96 ($\approx$1 kpc) with the peak of brightness near the center of this interval. The SN is located at the edge of this giant HII-region, at a distance of $\approx$ 0.5 kpc from the center of the galaxy. The intensity distribution of the narrow [OIII] line $\lambda$5007 Å is similar to that of narrow H$\alpha$, hence the excitation of HII region is more or less similar to that near the current SF region and near the position of the SN.

At first glance there is apparent analogy of the periphery position of the SN 1998bv in the galaxy HS 1035+4758 with the periphery positions of the majority of other SNe II observed in spiral galaxies. However, the deeper insight into the situation shows that such an analogy will lead rather to a more central position of the SN in HS 1035+4758. The periphery positions of SNe II in spirals are related to the regions of high current SF rate (and of massive stars, progenitors of SNe II, in particular) in spiral arms. The displacement of the SN 1998bv position by $\approx$0.5 kpc from the central supercluster of massive stars which formed in the course of the current SF burst is therefore quite suprising and needs more careful elaboration of possible models of SF propagation. Observations of the area around the SN position after SN fading will allow one to elucidate the status of the underlying stellar population and probably better understand such apparent spatial discontinuity in production of massive stars.

We would like to note that the situation with SN 1998bv is quite similar to that with SN 1995ah -- the first SN in the blue compact galaxy. The only difference is the peak luminosity (M $_{B} \approx$ -18.5) and the classification of its light curve as Linear. As described by Popescu et al. (1997), SN II is also displaced by about 0.7 kpc, approximately to the north of the center of BCG. But it is still situated inside the boundaries of a very large high excitation HII-region with a total size of $\approx$1.5 kpc.

  

Figure: Perspective view of two-dimensional spectrum near H$\alpha$ region (the range 6400-6700 Å). The broad H$\alpha$ of SN and narrow H$\alpha$ through the body of host galaxy are shown.