MUSIC IS MY SOUL

Mass is constantly being recycled in the universe. One of the most common ways recycling is achieved is through stellar mass-loss. All stars exhibit some form of mass loss. Smaller stars can be quite stingy and hold onto their mass by burning away slowly. Other massive stars can lose prodigious amounts of mass each year. The rate at which a star loses mass depends on its initial mass, age, spectral type, luminosity class, rotation rate, evolutionary stage, even the proximity of a companion star.

The first generation of stars consisted mostly of hydrogen and helium. These stars then seeded the interstellar medium with heavier elements, shedding material via massive winds, planetary nebulae and supernova explosions.

This material then became the building blocks for the next generation of stars and planets. Understanding mass loss is essential for following the evolution of single stars, binaries, star clusters, and galaxies.

However, mass loss is among our weakest areas in understanding fundamental stellar processes. In large part, this is due to lack of detailed, direct observations of stellar photospheres and the mass-loss process. High-resolution optical interferometry with telescope arrays is beginning to provide these data, ushering in a promising a new era in mass-loss studies.

The angular resolution that a telescope can achieve is proportional to its diameter. The larger the telescope, the better its resolution. The purpose of astronomical interferometry is to mix signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection.

Recently, a team of French astronomers captured one of the sharpest color images ever made. The team members are Jean-Baptiste Le Bouquin and Antoine M�rand (ESO), Sylvestre Lacour and St�phanie Renard (LAOG, CNRS, Grenoble, France), and Eric Thi�baut (AIRI, Observatoire de Lyon, France).

The image was taken over several consecutive nights with ESO's Very Large Telescope Interferometer (VLTI), a virtual telescope about 100 meters across. The result is an amazing image that reveals a spherical molecular shell around an evolved Mira star, T Leporis, 500 light-years away!


Miras are giant pulsating variable stars. Evolved stars like this have used up most of their nuclear fuel, causing them to swell up to enormous radii and become unstable. This internal instability causes them to expand and contract more or less regularly. T Lep pulsates with a period of 380 days, and in the process loses the equivalent of the Earth�s mass every year to space. In the end, all that is left is a white dwarf, the ash of the core, surrounded by the gases lost to space from the star�s wind and tenuous grip on its outer layers. We see these stellar remnants as planetary nebula elsewhere in the galaxy. Our Sun will become a Mira in a few billion years, engulfing the inner planets in its final blaze of glory.

Mira stars are among the biggest factories of molecules and dust in the Universe. Since the molecules and dust are formed in the outer layers of a Mira's atmosphere, astronomers would like to be able to see these layers to better understand the processes of mass loss and dust formation. Until now, this has been all but impossible due to the distance to even the nearest stars. The angular size of even giant swollen stars like T Lep were too small to image from Earth.

�T Leporis looks so small from the Earth that only an interferometric facility, such as the VLTI at Paranal, can take an image of it. VLTI can resolve stars 15 times smaller than those resolved by the Hubble Space Telescope,� says Le Bouquin.

�We were able to construct an amazing image, and reveal the onion-like structure of the atmosphere of a giant star at a late stage of its life for the first time,� says team member, Antoine M�rand. �Numerical models and indirect data have allowed us to imagine the appearance of the star before, but it is quite astounding that we can now see it, and in color.

Obtaining images like these was one of the main motivations for building the Very Large Telescope Interferometer. We have now truly entered the era of stellar imaging.�

400 years after Galileo turned his tiny telescope towards the sky, we can now actually see stars atmospheres directly, using some of astronomy's most amazing new tools of discovery.

Okay, so we still don't find many stars in the CV 'period gap' with orbital periods of 2-3 hours, so in ten years since the last Wild Stars conference, and with all the new CVs discovered and measured in that time, this is a real phenomena.

We find a lot of stars piling up around three hours period and many of these are accreting at very high rates. The secondaries are losing prodigious amounts of mass to their cannibalistic white dwarf companions.

The leading theory explains this gap as the point where magnetic braking ceases and these binaries abruptly stop accreting matter from the secondary to the white dwarf. As these binaries continue to lose orbital energy through gravitational wave propagation they evolve through the period gap from 3 hours to two hours. At this point they've spiraled in close enough for the secondary to fill its Roche lobe and accretion starts up again.

From orbital period (Porb) 2 hours and less the only way the system loses orbital energy is through gravity waves. Typically these stars have low accretion rates and it takes a long time for them to build up enough material in the disk to go into outburst. So the secondaries are not losing mass very quickly, and we'd expect to see another spike in the population of CVs at shorter orbital periods.

One problem is the fact that these systems tend to be quite faint in quiescence, so they are harder to find than their bright actively outbursting friends at the longer Porbs. With the recent results of the Sloan Digital Sky Survey (SDSS) we've uncovered more ad more of these short period low accretion rate CVs, but there is still some debate about whether we have actually begun to pin down the actual spacial density of these objects. There aren't that many faint, short period CVs close to us, so they must indeed be rare objects.

CV theories also predict a minimum Porb of about 65 minutes. There may be something wrong with our models though, because observationally, the minimum seems to be closer to 80 minutes. Either we have a whole bunch of highly evolved, low accreting faint stars out there at 23rd magnitude (beyond the limit of SDSS) or there is something lacking in our understanding of the physics at this minimum threshold for CV period evolution.

To be honest, not much of this is new, or cutting edge astrophysics anymore. A lot of these same issues were being discussed ten years ago. We may have more observations and phenomenology, but we don't seem to have made any significant progress in our understanding of these Wild Stars.

That is a topic I want to interrogate Steve Howell, one of the local organizers, about tomorrow. I'll let you know what he thinks.

Much of variable stars research is related to stellar evolution. We have a pretty good handle on how single stars evolve over billions of years. They are born in clouds of dust and gas, contract due to gravity until they reach a critical limit at which nuclear processes begin converting hydrogen to helium and heavier elements. At this point a star is born.

The most important factor in the evolutionary track of a star is its initial mass. Giant stars burn up their fuel quickly and die spectacularly. Dwarf stars live for tens of billions of years, miserly using up their fuel while putting out a conservative amount of energy.

A majority of stars follow an evolutionary path that eventually causes them to swell to hundreds of times their original radius, throwing off layers of their outer atmosphere via stellar winds and from their losing their gravitational grip on the outer layers of their swollen stellar atmosphere. These stars eventually become planetary nebula with white dwarfs at their centers.

These degenerate white dwarf stars are fascinating objects. They do not produce new energy, like stars. They are the remaining ash of the core of evolved stars, slowly cooling through time from their original very high temperatures. They are small and extremely dense, planet sized objects with approximately the mass of our sun squeezed into their small frames.

Things get a lot more complicated when two or more stars in orbit around each other are involved. If the stars' orbits are wide enough, each star may be able to follow its normal evolutionary path for billions of years. However, cataclysmic variables are stellar pairs, typically containing a white dwarf and a swollen M dwarf, in orbit around each other so close that the orbital period can be measured in hours. Mass is exchanged from the secondary to the white dwarf via an accretion disk. This interaction has a profound effect on the evolution of the stars involved.

Some of these white dwarfs have extremely strong magnetic fields. The accretion process is interrupted in part in intermediate polars, or completely in polars (AM Her stars). No real accretion disk is formed. Instead the mass transferred in polars slams down onto a small area at the magnetic pole of the white dwarf, or goes into strange orbits following the magnetic field lines in an intermediate polar.


Fine. We can and do observe all these properties of these binary systems now, along with the outbursts and high and low states of activity CVs are well known for. But, the burning question of the day today was "How do they get this way?"

Where do these pairs come from? Are they born as normal stars in unremarkable circumstances that somehow evolve into wild pairs of objects orbiting each other so closely they are exchanging material? How long does this take? How do they lose their orbital energy, and what is that energy converted into? Even more perplexing is, where do the magnetic fields in CVs come from?

The generally accepted explanation is called the Common Envelope evolution scheme. As a pair of stars evolves, changes in the mass of one or both stars affects the orbital characteristics of the pair, and they lose energy and begin to spiral in towards each other. At a critical point in this process one star evolves to the point that it fills its Roche lobe and the pair becomes involved in a cloud of dust and gas shared by both called a 'common envelope'. We believe CVs evolve out of this phase into the semi-detached systems we see as dwarf novae and magnetic CVs.

Just how this happens is still not well understood, and how either star acquires a mega-Gauss magnetic field in the process is an even less understood process. The mystery was framed and discussed in a couple very interesting papers given in the afternoon session. James Liebert pointed out the fact that the Sloan Digital Sky Survey has found over 1200 close pairs containing a white dwarf and an M dwarf, precisely the kinds of pairs we believe are the progenitors of CVs, yet none of them have been found to contain a white dwarf with a strong magnetic field.

Nearly 25% of CVs are magnetic systems so where do they come from if not these pre-CV pairs? In fact, all highly magnetic white dwarfs appears as either single stars or components of CV binaries.

Christopher Tout proposed in the following paper that highly magnetic white dwarfs must be formed as a result of the common envelope phase of binary evolution. He went further to suggest that the single white dwarfs with the highest magnetic fields are the result of a pair of stars merging into one highly magnetic white dwarf from the common envelope phase. And the magnetic CVs we observe, polars and intermediate polars, are the result of systems that almost merge before eveolving into magnetic CVs.

There are also fundamental questions about the evolution of CV pairs. Do these stars continue to spiral in towards each other, reaching shorter and shorter periods? How does accretion and mass loss affect this evolution? How do we explain the well known 'period gap' where there are almost no actively accreting systems with orbital periods between 2 and 3 hours?


What is so special about this orbital period? What shuts off the accretion process at 3 hours yet lets it re-engage at less than 2 hours. What is the actual period minimum for CVs? Is it 65 minutes, as theory predicts, or is 80 minutes, as observations seem to imply?

There are lots of questions. I hope to get at least some of the answers this week. Stay tuned, and we'll find out together.

The images used in this article are from space artist Mark A. Garlick. Visit him on the web at www.space-art.co.uk and www.markgarlick.com

Steve Howell, head cowboy, and coiner of the famous acronym TOADs (tremendous outburst amplitude dwarf novae) welcomes everyone to the conference and explains where the bathrooms are.

His other main task for this conference seems to be getting everyone who wants one, a receipt for their expense reports. Poor Steve.

His poster on magnetic CVs has an awesome visualization. I'll try to get permission to reproduce it here. It is way cool....err, I mean hot.




Chritsian Knigge opens the paper session by reviewing what we know about the secondaries in CVs and their role in the evolution of these systems.

Check out the visualization of that bloated, star spotted, crazy looking secondary. Wild stars indeed!

More interesting is the fact that CV donor stars are larger and cooler than individual main sequence stars of equal mass. Observing these secondary properties may tell us a lot about the evolutionary track of these systems. Fascinating stuff presented very well. Two Simothumbs up for this one.

What's that? You say you don't understand magnetic braking? Don't worry, I'm in a room full of PhDs who will talk about it all day, but they don't understand it either!


Hands down winner of the animated visualizations for the conference thus far definitely goes to D.V. Bisikalo from the Russian Academy of Sciences, Moscow. His illustrations of accretion and the outflow of material into the envelope around the binary were fascinating to watch and quite detailed. The parameters and science used to achieve these results may be unrealistic, but the animations were glorious! Not only that, but watching the accretion overflow, I had an 'aha moment' for something I've been working on regarding Z Cam outbursts.




For those of you who remember my blog on 'Dusty Toads', here is one of the authors, Don Hoard, talking about surprising dusty environments around cataclysmic variables. They went hunting for information about the red secondary star of WZ Sge with the Spitzer Space Telescope and found so much dust they couldn't observe the secondary! A surprising result that may lead to, well, who knows?

I'll be interviewing Steve and Don about their dusty toads and where this new result may lead CV research.

On a personal note: it has been a lot of fun meeting the people and associating the names with the faces. I met several Japanese observers and important contributors to CVnet-Akira Arai, Hiroshima University, Izumi Hachisu, University of Tokyo, Akira Imada, Kagoshima University, Daisaku Nogami, Kwasan Observatory, Kyoto University. I also got to meet and talk with astronomers using AAVSO data for their papers here at this conference or elsewhere: Brad Schaefer, Louisiana State University, Christian Knigge, University of Southampton.

AAVSO was well represented with Arne Henden giving a poster presentation with hundreds of AAVSO light curves and Paula Szkody talking about pulsating white dwarfs in SDSS CVs.

Boris Gaensicke, who I met for the first time in Cambridge, UK last spring, seems to have his fingers in so many pies here it is quite remarkable. He is listed as a co-author or principle investigator on at least 40% (UNSCIENTIFIC SIMO-ESTIMATE) of the papers being presented.

And on a personal basis, I had the pleasure of meeting Kurtis Williams, of Professor Astronomy's Astronomy Blog .
He has been kind enough to support the AAVSO Writers Bureau with his blogs and is an all around nice guy who it is my pleasure to have met finally.

It's been a good time so far. More later.

It took us a while, but the second part of my discussion with Michael and Doug on Slacker Astronomy is now online.

You can download the podcast here.

In this episode, we talk about where research in cataclysmic variables is going, what astronomers are expecting to find and some of the surprises we've found along the way. I was fortunate to get three astronomers from the Wild Stars in the Old West II conference, Christopher Tout, Paula Szkody and Boris Gaensicke, to give up their coffee break time to let me interview them for this show.

My thanks to Michael and Doug for having me on the show, and to Paula, Christopher and Boris for giving me something interesting to contribute.

Boris Gaensicke / Christopher Tout /Paula Szkody

So check it out:

Podcast: Simo-Slacker Interviews Pt. II