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NGC 3314 Variable Object

By Bill Keel (University of Alabama, Tuscaloosa)
and Lisa Frattare (Hubble Heritage Project, STScI)

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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:



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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...