NGC 3314 Variable Object
By Bill Keel (University of Alabama, Tuscaloosa)
and Lisa Frattare (Hubble Heritage Project, STScI)
NGC 3314 with variable object
marked.
Tracking down a stellar
explosion
We first noticed a new object in NGC 3314
while discussing the details of color rendition
for the Hubble Heritage release, incorporating
data from both the 1999 and 2000 observations
in all four filters. There was a prominent
green starlike object in one of the spiral
arms. "Prominent" is relatve here; the visual
magnitude was 21.6. Green stars are not found
in nature, because the spectral band we see
as green is narrow compared to the amount
of light stars put out in various wavelengths,
so this was a signal of something quite unusual.
This was quickly traced to an object appearing
only in the March 2000 observations, so that
combining data from some filters in which
it was there and some in which it wasn't produced
the giveaway color. The fact that we had blue-light
images from both sets of data let us show
that it had indeed newly appeared, and was
not some object with a bizarre spectrum that
vanished in some filters. In asking what kind
of new object this was, we could quickly eliminate
several possibilities: |
| B
filter images of nucleus of NGC 3314 taken
in April 1999 (without object present) and
March 2000 (showing the new object).
|
Solar system object, such as an asteroid, comet,
or Kuiper-Edgeworth belt object? No, because
the orbit of HST (essentially spanning the diameter
of the Earth) produces a parallactic ellipse when
observing any nearby object unless the telescope
is specifically tracking expected motion (as in
this
example of an asteroid trail). Even an object
50 astronomical units from the Sun, well beyond
the orbit of Pluto, would show such a motion by
an obvious 0.3 arcseconds (3 pixels on the WFPC2
CCD) during each orbit's observations, plus a still
larger shift of several arcseconds due to the Earth's
orbital motion over the 2-hour duration of the whole
observation sequence. Objects other than comets
would also be unlikely (though not impossible) because
this part of the sky lies 33 degrees south of the
ecliptic plane, and very few asteroids have high
enough orbital inclinations to be seen so far from
the plane.
A previously undetected star with such high
proper motion that only during this year did it
wander in front of NGC 3314? Such a star would
be very interesting, almost certainly being among
our nearest stellar neighbors, since the current
record holder - Barnard's Star in Ophiuchus - moves
across our sky at about 10 arcseconds per year,
which is about the apparent separation between the
new object and the foreground nucleus of NGC 3314a.
We checked the rest of the field of view and do
not see any star which was somewhere else in 1999
and then vanished, so any rapidly moving star would
have to be going across the sky at more than 40
arcseconds per year. Existing sky surveys don't
go deep enough for us to absolutely rule out such
an object, but it would be extremely unlikely because
a star so close would be much brighter than the
visual magnitude of 21.6 for the object, and we
see it as red, but not nearly red enough to be a
cool red dwarf (or cooler brown dwarf) that could
appear close and simultaneously so dim.
Gravitational microlensing? The deflection
of starlight by mass, one of the most stunning predictions
of Einstein's general theory of relativity, has
been observed for galaxies,
quasars,
and individual stars in the Milky Way and Large
Magellanic Cloud. A direct superposition of two
luminous galaxies is a fertile place to seek lensing,
so we estimated the expected number and brightness
of lensing events. A back-of-the-envelope calculation
(well, by now this a scratch-file calculation, as
we move to the electronic paperless office) suggests
that star-star microlensing goes on all the time
in such a galaxy superposition - there is, on average,a
two-magnitude event always going on. But two magnitudes
won't cut it. This object would have visual magnitude
-11.4 if it's in NGC 3314a, and brighter for 3314b
(especially since there might well be extra dust
in the way). For even a red giant in NGC 3314b,
that means we need an amplification of 10 magnitudes
(which is to say, 10,000 times in intensity), and
a foreground star in NGC 3314a wouldn't lens all
of a background red giant very effectively - they're
too big. Such an event should happen only about
once every 10,000 years - and if we were that lucky,
we would have already won big in Las Vegas. What's
worse, the random motions of stars in the two galaxies
would make such a strong amplification last only
about 20 minutes, less than the two hours we know
about, again unless we were ridiculously fortunate.
Also, the colors of the object don't fit a normal
star even with foreground dust. Still, if we could
reliably detect variable objects that are much fainter,
this effect could in principle offer a way to measure
the population of very faint stars and stellar remnants
(neutron stars, black holes, white dwarfs) much
as the optical lensing experiments are now doing
for our own galaxy's halo.
An ordinary nova? Nova outbursts result
from the buildup of material on the surface of a
dense white dwarf star, after being gradually funnelled
from a close companion star. The absolute magnitude
of the new object is too bright for a typical nova,
about 3 magnitudes brighter than well-known nova
outbursts. Still, there have been a few cases of
outbursts between the ranges ordinarily associated
with novae and supernovae, one of which stayed brght
for several years. So the possibility exists that
we saw such an anomalous nova-like object close
to its peak brightness.
A supernova? That would have been most
galaxy observers' first guess. The object is actually
a good hundred times dimmer than a supernova should
be near maximum light in either component of NGC
3314, but there's no reasons we would have happened
to look just then, so it would most likely be on
the way up or down. Supernova brighten very rapidly,
so it would be more likely for us to miss the initial
outburst and see its slow fading. The colors we
see between the various filters make sense for a
fading supernova with strong hydrogen emission,
as seen in type II supernovae from massive stars
such as occur in spiral arms. Furthermore, the red
color might indicate that the supernova went off
in the background galaxy NGC 3314b, and thus its
light underwent additional dimming and reddening
passing through the disk of NGC 3314a. The main
problem with simply assuming it's a supernova is
that nobody saw a supernova go through peak brightness
in this system within the last few months. Some
supernovae (such as SN 1987A in the Large Magellanic
Cloud) do fall below the typical behavior in brightness,
but not by this much.
To get this observation on the record, and seek
confirming or prediscovery observations, we reported
it to the Central Bureau for Astronomical Telegrams,
who included it in
International Astronomical Union Circular 7388.
To date we've gotten no reports of other observations,
or of helpful non-detections in 1999 or 2000. So
if you happened to take a deep image of NGC 3314
during this period... |
