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Дата изменения: Tue Feb 23 12:44:58 1999 Дата индексирования: Sat Sep 11 22:14:26 2010 Кодировка: Поисковые слова: star |
In Fig. 2 the spectrum of the host galaxy HS1035+4758 is shown, in
which typical HII-galaxy narrow emission lines are seen: H
4861, [OIII]
4959,5007, H
6562, [NII]
6548,6584 and [SII]
6716,6731 Å. The equivalent widths of the strongest emission
lines, H
,
[OIII] 4959,5007 and H
,
are 20, 20, 60 and 90 Å,
respectively. The galaxy redshift as measured using these strong lines is
z=0.0054
0.0001.
The absolute blue magnitude
(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
(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
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
(in mag/100d) 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
0.10 on
6.04.1998 and B(SN)=17.20
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
.
This
leads to
B=
and
= 1.17
mag/100d. This value is so much less than 3.5 that even allowing
for a possible uncertainty in
B of up to
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
MB=-15.4. Then M
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
absorption.
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
5876 Å.
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It is interesting to estimate the extent of narrow H
line emission
in the host galxy, indicating the extent of ionized gas (Fig.4).
The region which emits in H
can be traced
on our long-slit spectrum over
24 pixels = 96
(
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
0.5 kpc from the center of the galaxy. The intensity distribution of the narrow
[OIII] line
5007 Å is similar to that of narrow H
,
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 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
-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
1.5 kpc.
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