Wild Stars Day Two

You better show up wide awake for these conferences. Tuesday morning we jumped in feet first into the crazy world of AM CVn type stars. This is a rare class of stars that has been garnering more attention lately due to their extremely short orbital periods, (we're talking 10-60 minutes here), and the fact they are sources for low frequency gravitational waves.

AM CVn spectra are totally devoid of Hydrogen lines. They show a rich Helium spectrum along with processed heavy element lines. This makes them exciting to astronomers because supernovae spectra don't show hydrogen, so these stars may be supernovae progenitors. If you want to get grant support or telescope time these days, it helps if you're proposal has something to do with the sexy topics of exoplanets, supernovae or dark matter.

There are several proposed channels for the evolution of these stars. By the third paper of the morning we had heard about all the possible ways these stars can be born and how they may die as helium Ia supernovae. AM CVn's are thought to form via 2-3 different "channels".

1-A detached white dwarf (DWD) system, formed through a series of Common Envelope evolutions, shrinks as a result of angular momentum losses due to Gravitational wave Radiation (GWR). Eventually, the less massive star fills its Roche radius and mass transfer commences. The system then evolves to higher periods due to redistribution of angular momentum.

...or, 2- a low mass helium donor transfers mass to a white dwarf accretor. The system passes through a minimum in period of ~ 10 minutes. The period increases after this minimum and mass transfer keeps falling. During this process the helium donor goes from being a non-degenerate to a degenerate star.

...or , 3- they may evolve from cataclysmic variables with evolved donors. After significant mass loss, the exposed Helium core of the donor in a CV evolves similar to #2 Helium star track.

What's truly amazing, and mind bending if you haven't had enough coffee, is the fact that we just can't find helium core white dwarfs right now. Since these may be members of binary progenitors for both supernovae and classical novae, astronomers are forced to model how these stars contribute to colossal cosmic explosions using math, physics and imagination. Then they have to figure out a way to explain it to other astronomers and survive the question and answer session after they present their talk.

Kudos to graduate student Ken Shen for his paper on Unstable Helium Shell Burning on Accreting White Dwarfs. This young man knows his stuff and can give a presentation. I predict good things for his future.

After lunch we started hearing talks closer to my areas of interest. I already mentioned the awesome 3D Gas Dynamic Modeling movies shown in the talk given by D.V. Bisikalo. Then Don Hoard talked about Dusty Toads, a topic we have seen here before a few times.

SW Sex stars is another class of stars I wanted to learn more about. It seems that all eclipsing nova-like stars in the 3-4 hour period range are SW Sex stars. But eclipses are simply a line of sight effect, so they can't be considered a pre-requisite for inclusion in the SW Sex club. So Linda Schmidtobreick and her colleagues looked at a large sample of non-eclipsing stars in the 3-4 hour period range to see if they had the rest of the required characteristics for inclusion in the SW Sex category. What they found was that most of these stars are indeed SW Sex stars. Remember, the CV period gap is from 2-3 hours, so these stars may represent an important group of stars, with periods just above the gap, in a high mass transfer state which may cause the binaries to lose contact and stop accreting as they evolve through the period gap.

Boris Gaensicke has become a rock star in the CV community. It is almost unfair to have him start the final afternoon session and then expect three more people to deliver talks on essentially the same topic- 'recent results of CV population studies and the space density of classes of CVs'. I'm just glad it wasn't me, because that is exactly what happened.

Boris hit it out of the park with his presentation. He presented results from the new CVs discovered by SDSS. The main points of his talk are that we've now determined where the missing 80 minute period spike stars predicted by CV theory are. Paula Szkody and SDSS have found them down to around 19th magnitude. They do exist, and they are significantly different from the rest of the CV population.

The spectra of the majority of these stars reveal slowly accreting, white dwarf dominated, WZ Sge-like stars. But, we have still not found the "period bouncers"; those stars with periods less than 80 minutes that are near the predicted 65 minute limit where these CVs will begin to evolve back to longer orbital periods. Boris says they will be found if we just dig another couple magnitudes deeper, and I believe him.