Copyright © 1997 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1997. 35:
309-355 Copyright © 1997 by Annual Reviews. All rights reserved |
Excellent examples of SNe II-P are SN 1969L
(Ciatti et al
1971), SN 1986I
(Pennypacker et al
1989),
SN 1988A
(Turatto et al
1993a),
SN 1990E
(Schmidt et al
1993,
Benetti et al
1994), and
SN 1991G
(Blanton et al
1995).
At very early times, the spectrum is nearly featureless and quite blue,
indicating a high color temperature
(
10,000 K). Very
weak hydrogen Balmer lines and He I
5876
are often visible. Initially, the widths of the Balmer lines and the
blueshifts of their P Cygni absorption minima decrease noticeably in
some objects (e.g. SN 1987A;
Menzies 1991),
as the photosphere quickly recedes to the inner, more slowly moving layers
of the homologously expanding ejecta. The temperature rapidly decreases with
time, reaching ~ 5000 K within a few weeks, as expected from the
adiabatic expansion and associated
cooling of the ejecta. It remains roughly constant at this value during the
plateau, while the hydrogen recombination wave moves through the massive
(~ 10 M
)
hydrogen ejecta and releases the energy deposited by the shock. At this
stage, strong
Balmer lines and Ca II H&K with well-developed P Cygni profiles appear,
as do weaker lines of Fe II, Sc II, and other iron-group
elements. Subsequently,
as the light curve drops to the late-time tail, the spectrum gradually takes
on a nebular appearance; the continuum fades, but
H
becomes very strong,
and prominent emission lines of [O I], [Ca II], and Ca II also appear.
This behavior is well illustrated in Figure 12
with SN 1992H (see also
Clocchiatti et al
1996a,
who estimated the explosion date to be February 8, 1992). Although its
V-band plateau (
= 40-100 days, or perhaps 50-90 days, depending on one's definition) was
somewhat shorter than that of the most famous SNe II-P mentioned above and
declined slowly with time, SN 1992H can still be considered a SN II-P, and
its spectral development was quite typical. Weak He I
5876 was superposed on
a blue continuum on day 20.
H
absorption
was present, but the corresponding component of
H was weak or absent;
H emission, on the other
hand, was obvious. The
H
absorption line must have developed very rapidly, as it was strong by day
34, and the continuum was redder. The spectrum changed little between days
34 and 123, with the absorption lines gradually growing stronger; Na I D
became very prominent, and many lines of singly ionized metals were present.
The emergence of weak forbidden emission lines (day 105), most notably [Ca
II]
7291,
7324, roughly coincided with the end of the plateau phase; the Ca II near-IR
triplet and Na I D emission also became more prominent. By day 138, and
certainly by day 177, [O I]
5577 and [O I]
6300, 6364 were
unmistakable. At
1 year, when the
continuum was faint, the spectrum was dominated by
H, [Ca II]
7291, 7324, and [O I]
6300, 6364; weaker [Fe
II]
7155, Na D, Mg I]
4571, the Ca II
near-IR triplet, and blends of Fe II lines (especially near 5300 Å)
were also present.
 |
Figure 12. Montage of spectra of SN 1992H in NGC 5377 (cz = 1793 km s-1). Epochs (days) are given relative to the estimated date of explosion, February 8, 1992. |
SNe II-P are excellent distance indicators, using the "Expanding Photosphere Method" (a variant of the Baade-Wesselink method) described by Kirshner & Kwan (1974); see Schmidt et al (1994a, b), Eastman et al (1996), Filippenko (1997b), and references therein. This technique is independent of the various uncertain rungs in the cosmological distance ladder: It relies only on an accurate measurement of the effective temperature (from the measured colors, with appropriate modeling of deviations from a blackbody spectrum) and the velocity of the photosphere (from the wavelengths of weak absorption lines such as those of Sc II) during the plateau phase. An important check is that the object's derived distance should be independent of time.