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Дата изменения: Wed Apr 12 21:26:24 1972 Дата индексирования: Mon Jun 28 00:28:41 2010 Кодировка: Поисковые слова: molecular cloud |
Peremennye Zvezdy (Variable Stars) 30, No. 3, 2010 Received 4 May; accepted 5 May.
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SN 2008gy was discovered by P. Balanutsa on unfiltered CCD images
taken with a 335-mm telescope near Moscow on October 30.99 UT.
This telescope is a part of the large "MASTER Robotic Net"
(Lipunov et al. 2010). The new object was located at
,
(J2000.0),
which is
west and
north of the center of
PGC 1584648 (Lipunov 2008). Confirmation images were taken by
Kryachko and Korotkiy (2008) on November 2.94 UT with an 80-mm
refractor at Karachay-Cherkessia, Russia. Folatelli et al. (2008)
reported that they had obtained spectra (range 340-602 nm) of
SN 2008gy with the New Technology Telescope (+EFOSC2) at La Silla
on November 19.0-19.1 UT. The spectrum of SN 2008gy shows it to
be a type-Ia supernova, two to three weeks after maximum light, at
a redshift of 0.029.
SN 2008gy was imaged again with the 335-mm telescope near Moscow
on November 7.73 UT. The photometric monitoring of the SN in
filters was carried out at the 60-cm reflector of the
Crimean Observatory of Sternberg Astronomical Institute between
2008 November 9 and November 25. All image reductions and
photometry were made using IRAF1.
The image of SN 2008gy obtained at the 60-cm reflector in the
band is shown in Fig. 1. It is evident that the SN is located in a
group of three galaxies, which are labeled G1, G2, G3. We searched
the NED2 and
Hyperleda3 databases and found
out that G1 is PGC 1584648, G2 is PGC 1584876, and G3 is IC 1890.
Only for IC 1890 is the redshift known, it is 0.03412. The value
reported for SN 2008gy differs by 1500 km/s from this estimate,
which seems larger than possible peculiar velocities of galaxies
in a group or the velocity of the presupernova inside its host
galaxy. However, the redshift uncertainty for SN 2008gy was not
announced, and it can possibly account for most of this
difference. We calculated the angular distances from the SN to the
three galaxies on our CCD frames, respectively for G1, G2, G3:
,
,
. If we assume that the
redshift 0.029 corresponds to the distance for SN 2008gy and adopt
the distance modulus
, as reported in NED, the linear
projected distances of the SN from the centers of the galaxies are
12.2 kpc, 21.3 kpc, 35.7 kpc. According to these data, G1 is the
most likely host galaxy. But other galaxies are significantly
brighter and larger, and the relative projected radial distances,
defined as angular separation divided by the isophotal radius of
the galaxy, for the three galaxies are 2.27, 1.74, 2.06. Thus,
from this point of view, G2 is the most likely host galaxy, and G1
is the least probable host. Taking into account that the projected
distances are the lower limits to the spatial separations, we
conclude that there is nearly equal probability for SN 2008gy to
belong to any of the three galaxies, or to be located in the
intergalactic medium.
The local standard stars are also marked on Fig. 1. The magnitudes
of these stars are reported in Table 1, they were calibrated on
four photometric nights in November. It is clear that the galaxy
background has no effect on the photometry of SN 2008gy. The
magnitudes of the SN were derived by PSF fitting relative to a
sequence of local standard stars. We used the magnitudes to
calibrate the unfiltered CCD frames obtained at the 335-mm and
80-mm telescopes.
The results are presented in Table 2 and the light curves are shown in Fig. 2.
JD 2454000+ | ![]() |
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Tel. |
770.49 | 17.11 | 0.13 | 335-mm | ||||||
773.44 | 16.96 | 0.04 | 80-mm | ||||||
778.23 | 17.27 | 0.08 | 335-mm | ||||||
780.46 | 17.83 | 0.07 | 17.36 | 0.05 | 17.26 | 0.03 | 600-mm | ||
781.42 | 18.18 | 0.05 | 17.58 | 0.03 | 17.41 | 0.03 | 600-mm | ||
782.39 | 18.17 | 0.11 | 17.56 | 0.05 | 17.43 | 0.05 | 600-mm | ||
786.33 | 18.62 | 0.06 | 17.84 | 0.03 | 17.75 | 0.05 | 17.44 | 0.17 | 600-mm |
789.38 | 19.20 | 0.09 | 18.02 | 0.02 | 17.86 | 0.03 | 600-mm | ||
795.26 | 19.81 | 0.08 | 18.36 | 0.02 | 17.92 | 0.03 | 17.51 | 0.10 | 600-mm |
796.20 | 18.42 | 0.04 | 17.79 | 0.05 | 600-mm |
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Fig. 2.
The light curves of SN 2008gy in the
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Fig. 3.
The |
The data are best fitted by the light curves of moderately
fast-declining SN Ia 1994D (Richmond et al. 1995). From this fit,
we estimate that maximum light was reached around JD 2454775
(November 4) with ,
,
.
The color curves are shown in Fig. 3. The colors of SN 2008gy are
significantly redder than for SN 1994D, approximately by 0
44
in
and 0
22 in
. The galactic extinction for SN
2008gy is
,
,
according to NED. We cannot expect any significant extinction in
the parent galaxy because the SN is located outside the boundaries
of all possible host galaxies. So, we suppose that SN 2008gy is
intrinsically redder than SN 1994D at the phase of early decline
by about 0
28 in
and 0
11 in
. Assuming
and correcting only for the galactic extinction, the
absolute magnitudes of SN 2008gy are
. We
may suggest that the rate of early decline for this SN is not much
different from that for SN 1994D,
,
and then the luminosity of SN 2008gy at maximum is significantly
fainter than can be expected from the relations of
and
versus
as presented by Hicken et al. (2009).
However, the distance modulus corresponding to the redshift of IC
1890 is by 0
35 larger, and with absolute magnitudes brighter
by 0
35 mag SN 2008gy fits quite well to the relation between
maximum luminosity and rate of decline. Thus, we cannot make a
definite conclusion on the luminosity of SN 2008gy, but its
intrinsic red color is much more probable.
The possible lower luminosity and red color may be connected with the location of the SN far away from the host galaxy, where the characteristics of stellar population may be different from those closer to the galaxy center (see e.g. Sullivan et al. 2010 and references therein).
Acknowledgements. This work was supported by the Ministry of
Science of the Russian Federation (state contract No.
02.740.11.0249). The work of D.T. was partly supported by the
Leading Scientific Schools Foundation under grant NSh.433.2008.2
and by the RFBR grant 10-02-00249a. This research has made use of
the NASA/IPAC Extragalactic Database (NED) which is operated by
the Jet Propulsion Laboratory, California Institute of Technology,
under contract with NASA. We acknowledge the usage of the
HyperLeda database.
References:
Folatelli, G., Forster, F., Stritzinger, M., 2008, CBET, No. 1577
Hicken, M., Challis, P., Jha, S., et al., 2009, Astrophys. J., 700, 331
Lipunov, V., Kornilov, V., Gorbovskoy, E., et al., 2010, Advances in Astronomy, 2010, 1
Lipunov, V.M., 2008, CBET, No. 1565
Kryachko, T., Korotkiy, S., 2008, CBET, No. 1565
Richmond, M.W., Treffers, R.R., Filippenko, A.V., et al., 1995, Astron. J., 109, 2121
Sullivan, M., Conley, A., Howell, D.A., et al., 2010, preprint, arXiv:1003.5119