Wild Stars- Day Four

Now that it is done, I am feeling a mix of relief and sadness. Relief- because four days of intensive research results presented session after session is like cramming a semester's worth of astronomy into your brain in four days. Sadness- because I could take maybe one more day, just to finish meeting and talking with some of the people I really wanted to talk to one on one. There just wasn't enough time. I think this was the most worthwhile, well-organized and relevant conference I have ever attended.

I'm not knocking any of the organizers of variable star meetings I've been to in the past, but this was all about CVs, all the rock stars of CV research were here, and I had back stage passes. It was frakkin' cool. The last day of the conference was all about things I am interested in- TOADs, Recurrent Novae and Classical Novae.

The talks of the day were also book-ended by two impressive astronomers from Japan's VSNET, Daisaku Nogami and Izumi Hachisu.

First thing in the morning session Daisaku Nogami presented impressive results of the superoutburst evolution of three very interesting CVs- WZ Sge, GW Lib and V455 And. These are three of the most significant outbursts of dwarf novae in the last couple years and Nogami had it all nailed down. The following talks on GW Lib and V455 And were almost redundant after the impressive amount of optical photometric and spectroscopic information Daisaku presented first thing in the morning.

The last talk of the morning session was about the source of negative superhumps, by Michele Montgomery. I had the pleasure of meeting her and talking for an hour or so Tuesday night, at the NOAO reception. She is an excellent presenter and had a great talk, chock full of teasers about her upcoming publication.

You just have to be intrigued by a woman who can teach you all about the minimum accretion disk tilt needed to generate negative superhumps in light curves, the parameters that affect negative superhump signal strength, and the location in the disk that contributes to this phenomena. That is sexy stuff!

Not to dismiss the other presenters who talked about classical novae and recurrent novae after lunch, but Brad Schaefer stole the show with his talk. Brad does not need a microphone to be heard in a room of 200 people. He is enthusiastic and knowledgeable about his topic, recurrent novae, and he knows how to work a room.

Some of the most exciting news from Brad were previously undiscovered eruptions of RN in past years. He and his graduate assistant relentlessly scoured all the archival data in the world to find plates and observations of recurrent novae outbursts. They found three eruptions of U Sco (1917, 1945, 1969) RS Oph in 1907, V2487 Oph in 1900 and CI Aql in 1941.

From these intensive searches, and modeling the results, Brad is able to make some bold predictions about the timing of future eruptions. In the next decade he predicts no less than five recurrent novae eruptions- V2487 Oph, V394 CrA, V745 Sco V3890 Sgr and the one he is most excited about U Sco. U Sco is predicted to erupt in 2009.3, which is RIGHT NOW!
After lunch we were shown amazing results of x-ray observations of recurrent novae and novae. There is a lot of interest in novae eruptions by professionals, more than I realized. What's more, they are making x-ray observations of these objects months and years after the initial outburst in an attempt to observe just when the accretion disk reforms and accretion begins again and what happens at this phase of the outburst.

From this I realized, amateurs need to follow many of these objects for much longer than we typically do as best we can. The AAVSO light curves sort of peter out after six months or so, and rarely is there much follow-up when a star reappears from solar conjunction. It's like we've all forgotten about them and moved on to the next big thing. Unfortunately, this is about the time they become really interesting to professionals, and from what I was seeing there may be some very interesting observational phenomena we are totally missing, like sudden rebrightenings of the systems or flickering. Amateur CV sleuths take note!One of the final talks of the conference was from Izumi Hachisu. He presented the methodology and results of his search for a relationship between the t3 time of decay (the 3 magnitude decay time from optical maximum) and the turn-on and turn-off times of supersoft x-ray emission. He presented detailed light curve analysis of classical novae detected in x-rays, and proposed best fit models that reproduce the optical and supersoft x-ray observations. If all that sounds impressive, you're right, it was. The fact that most of the data was collected with relatively small telescopes makes it that much more impressive.

That's about all I can dig out of my CV overloaded memory and notes at this point. I'm going to let this all digest for a day or two and write a summary review. I know I am looking forward to the next CV conference, which is planned for 2010 in Kyoto, Japan!

T Scorpii, Our Long Lost Friend

On May 21, 1860 a nova burst forth in the constellation Scorpius. This nova was notable in the fact that it seemed lie either in or in our direct line of sight with the globular cluster NGC 6093, also known as Messier 80 (M80). At peak brightness it shone at magnitude 7.0, brighter than the rest of the entire cluster. It faded more rapidly than a 'typical' nova and disappeared. The nova was given the variable star designation T Scorpii and hasn't been seen or heard from since.

In recent years, studies of globular clusters have yielded some surprising results, among them a class of stars called 'blue straggler stars'. Blue straggler stars (BSSs) are stars observed to be hotter and bluer than other stars with the same luminosity in their environment. As such, they appear to be much younger than the rest of the stars around them.  This presents a problem for stellar evolution theory, since all the stars in a cluster ae believed to have formed at roughly the same time, so they should all be the same age and have evolved differently mainly (if not only) because of their initial mass.

The three main theories suggested that could produce such stars are: (1) collisions between stars in clusters or (2) mass transfer between, or the merger of components of short-period binaries, or (3) the progenitors of BSSs are formed in primordial triple star systems. Processes such as tidal friction might create very close inner binaries. Angular momentum loss in a magnetized wind or stellar evolution could then lead to the merger of these binaries, or to mass transfer between them, ultimately producing BSSs.

Messier 80
Credit: NOAO/AURA/NSF

Star clusters, both open and globular, are great astrophysical laboratories for astronomers, because all the stars are assumed to be at roughly the same distance and approximately the same age, so differences in the population of stars are assumed to be real, not the result of how far away they are or how old. These blue stragglers are just one hot topic that has caused astronomers to look at clusters like M80 much closer with space telescopes like Chandra and Hubble space telescopes.

Recently, astronomers studying stellar populations in M80 in the ultraviolet, using the HST, were able to identify most of the known x-ray sources in the core of the cluster. While the main point of their investigation was to study the populations of BSS's, one of these sources happened to be a dwarf nova in outburst at the time of the observations. More interesting than that, it appears to be consistent with the location of the 1860 nova, T Scorpii. Given its position, X-ray and UV brightness and variability, this source is almost certainly the true counterpart to T Sco. After 150 years we have finally found an old friend.

Welcome back, T Scorpii.

Unprecedented Eruption Catches Astronomers By Surprise

An alert was raised March 11 when Japanese amateur astronomers announced what might have been the discovery of a new 8th magnitude nova in the constellation of Cygnus. It was soon realized that this eruption was not what it appeared to be. It was actually the unexpected nova-like eruption of a known variable star, V407 Cygni. Typically varying between 12th and 14th magnitude, V407 Cyg is a rather mundane variable star. So what caused this well-behaved star to suddenly go ballistic?

Artist rendering of a symbiotic recurrent nova. Image credit: David A. Hardy & PPARC

V407 Cyg is a symbiotic variable. These are close, interacting binary pairs usually containing a red giant and a hotter, smaller white dwarf. They orbit a common center of gravity inside a shared nebulosity. A typical symbiotic variable consists of an M type giant transferring matter to a hot white dwarf via its stellar wind. This wind is ionized by the white dwarf, giving rise to the symbiotic nebula.

Symbiotic variables are complex systems with many sources of variability. They can vary periodically due to the binary motion, the red giant can vary due to pulsation, the stars may be obscured by circumstellar dust, or the light emitted my change due to the formation of giant star spots.

The white dwarf component may glow more or less constantly as it accretes material from the red giant and heats it up at a steady rate, or the material may form an accretion disk around the white dwarf, like in dwarf novae. Mass accreted onto the white dwarf can result in flickering and quasi-periodic oscillations. If there is a sudden increase in the rate of accretion, or the material in the accretion disk reaches a point of instability and crashes down onto the surface of the white dwarf the symbiotic system may undergo a nova-like eruption.

About 20% of symbiotics consist of a Mira-type variable as the giant of the pair. These binaries reside in much dustier envelopes. V407 Cyg is one of these dusty, Mira-type symbiotics. Its typical variation of a few magnitudes is due mainly to the pulsation of the Mira component of the system. Astronomers had never before witnessed a nova-like outburst of this interacting binary. You can imagine their surprise when Japanese amateurs, searching for novae along the galactic plane, suddenly detected this mild mannered, dusty Mira, symbiotic variable glowing nearly 100 times brighter than ever before.

That was just the beginning of the story. The first new spectra taken of the system, on March 13th, was different from any ever recorded for this star or any other symbiotic Mira variable in outburst. The normal absorption spectra of the Mira star was completely overwhelmed by the blue continuum of the outbursting white dwarf. The characteristics of the emission spectra revealed two distinct types of activity. One was the relatively slow ionized wind of the Mira star. The other looked like the fast expanding ejecta of a nova outburst. In fact, the spectrum looked remarkably similar to the symbiotic recurrent novae, RS Ophiuchi.

Typical outbursts of known symbiotic binaries, and symbiotic Miras in particular, usually exhibit a very slow rise to maximum, taking months, and no real significant mass ejection. This appears to be a much more quickly evolving and violent event, more like the eruptions of the recurrent novae RS Oph and T CrB. V407 Cyg may join this rare class of symbiotic recurrent novae.

As if that weren�t enough, another twist was added to the story on March 19th, when the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope may have detected the star in gamma-rays, something never observed in a symbiotic system before. The gamma-rays could be caused by shock driven acceleration of the ejected material, and its capture by strong magnetic fields within the system.

Unlike many novae and recurrent novae outbursts, this eruption may last for weeks or months and the variation in light output could be quite complex and interesting. Because the giant secondary is losing mass, the system is likely to have a large amount of circumstellar material. The ejected shell from the nova explosion on the white dwarf will interact with this material as the shell propagates outward, and will likely produce a wide variety of variable phenomena.

V407 Cyg has our attention now, and professional and amateur astronomers will be keeping a close eye on it from now on.