I would like to thank all of you who have stepped up and reported observations
of V1280 Sco in the past 2 years. I'd further like to tell you that your data
have helped us understand the system, but that would be an exaggeration. If
anything, you have contributed to the mystery (which is a very important thing).
V1280 Sco, formerly Nova Sco 2007, was a bright classical nova that reached
nearly 4th magnitude in the V band. (Novae are thermonuclear runaways -
essentially hydrogen bombs - on the surfaces of white dwarf stars accreting
material in a binary star system.)
The possible precursor of V1280 Sco in the POSS images has V~17.
Early spectra of the nova showed it to be a slow, optically-thick
(Fe II class) nova. The nova formed dust; the V-band brightness dropped to less
than V=16.2 about 4 months after maximum and then recovered over the next
3 months to about V=10 (see Figure 1).
Mystery #1 is that the nova settled down to V~10 for over 10 years, about
7 magnitudes brighter than its pre-explosion level. All novae fade, and many
fade slowly, but none that I know of have gotten stuck this much brighter than
the pre-explosion level.
This may be tied into another mystery: the optical spectra do not resemble
other fading novae. Most novae, after the ejecta have become transparent,
show strong "forbidden" emission lines, from the low density expanding ejecta
excited by the still hot white dwarf. Other novae resemble accretion disks,
with emission He II and other hot lines (like the nova-like CVs). But V1280
Sco resembles neither. Its "quiescent" spectrum has strong and narrow lines
of hydrogen and neutral helium, but also many narrow lines of singly-ionized
iron. This is unprecedented in a nova so far past peak.
At the end of the 2017 observing season, about 10 years post-peak and 9 years
after returning to "quiescence", the photometry started to show some
variability, dipping by over 1 magnitude at all 7 bands (BVRIJHKs - see
Figure 2). This dip lasted less than about 20 days, and recovery seemed well
underway as nova moved behind the Sun (though the last point suggests further
dipping). Two short dips with full recovery occurred near the beginning and
end of the 2018 observing season (because of instrumental problems with Andicam,
the AAVSO data are critical to letting us define the third dip). Nearly all
the 2019 observing season has been in a dip, about 1 mag below "quiescence",
but highly variable and with one near-recovery (Figure 3).
The fascinating thing about these dips are that they are grey: the depth of
the dip in the B band is essentially the same as the dip in the Ks band
(2.2 microns). This cannot be caused by dust absorption; it must be due to
absorption by objects much larger than dust or some completely different
mechanism.
Simultaneous spectra provide some clues. When the star is bright, the
spectrum is dominated by the cool lines of single ionized iron (Fe II). There
are no emission lines from gas hotter than 10-20,000K.
But as the star dips, hot lines of singly ionized helium (He II),
doubly and triply ionized carbon (C III, C IV),
and forbidden lines of double ionized oxygen ([O III] and 6-times ionized
iron [Fe VII] appear (see Figure 4).
The equivalent widths of the Fe II lines do not change. My working hypothesis
is that the opaque cool shell is starting to crack, giving a view into a hot
low density cavity. Such behavior has never been reported in any nova, and
bears watching.
We (think we) understand the basic nova mechanism and how the nova
ejecta interacts with the surrounding interstellar medium. Why continue to
study V1280 Sco years after its peak? Because we can learn a lot more from
studying a few pathological cases than we can from studying hundreds of
cases that behave as expected, reinforcing our limited view of nature.
Someday V1280 Sco will return to true quiescence, at about V=17.
When it goes there, and how it gets there, will provide unique insights into
the nova shell. After that happens we'll be able to see the underlying binary star and figure out just what exploded. Please keep this star on your observing list.
Thanks to all observers who have contributed, and please keep those data flowing
Fred Walter
Stony Brook University
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Observing Details
Observations in the B, V, Rc, and Ic bands are most useful, because they can
be compared directly with extant data. Regular monitoring is useful. The data
in the figures is generally at cadences of one to a few days. It might be
useful to sit on the target in one filter for a night, to see how fast it can vary.
Please report your observations promptly - If the star returns to its bright level (V<10.4), or fades by a magnitude or more below that (V>11.5), I will trigger specroscopic observations.
Figure captions.
Figure 1: The full optical/near-IR light curve of V1280 Sco.
Open circles are from the AAVSO; filled circles are from SMARTS/Andicam;
the plus signs are other data from the literature. The photometric bands
are color-coded.
The brightness had largely stabilized by Julian day 2,455,500 (only the last
4 digits are shown for clarity), though lingering effects of
the dust (a slow brightening at B and V and fading in K) are seen through
about JD 7500. There are no large color variations after about JD 5500.
The stable magnitudes give way to more chaotic behavior about JD 7900 (see
Figure 2).
The regular gaps in the data are due to Solar conjunction; there is an
unfortunate gap in the near-IR photometry around the time of the dust dip.
Figure 2: The optical/near-IR light curve of V1280 Sco since JD 7800.
Open circles are from the AAVSO; filled circles are from SMARTS/Andicam.
The photometric bands are color-coded. The mean V magnitude prior to the start
of the dips was about 10.3. At least 4 "dips" are clearly seen; the light
curve is nearly identical in each of the 7 bands, indicating that normal dust
is not responsible (if so the dips would be deeper at shorter wavelengths).
Figure 3: The light curve of the current observing season.
Symbols are as in Figure 2. The star is varying by about 1 magnitude in all
bands, on timescales of days to weeks. Note that since JD 8610 Andicam has
been limited to I-band data in the optical, so continued AAVSO monitoring is important.
Figure 4 Brightness-related spectral variations in V1280 Sco.
These spectra are from the SMARTS Chiron echelle spectrograph, at a
resolution of 27,800. Spectra are color-coded by Julian date (less 2,450,000).
The continua are set at the V magnitude within half a day of when the spectra
were obtained.
I only show a small portion of the full (4080-8900 Angstrom) spectrum.
When bright (top spectrum), the emission is dominated by centrally-reversed
singly-ionized iron (Fe II) lines. These lines are omni-present. The narrow
emission line at 4713 Angstroms is neutral helium (He I). When the star is
bright, there is an outflow - a stellar wind - evidenced by the dip below the
continuum level on the blue (short wavelength) side of the line.
When the star fades, strong emission of doubly ionized carbon (C III;
4645-4650 Angstroms) and ionized helium (He II) at 4686 Angstroms grow.
At longer wavelengths other hot lines appear, including [O III], C IV, and [Fe VII].