Joe Patterson

Post date: 27-Mar-2009 16:44:25

Joe Patterson is a professor of astronomy at Columbia University. One of his

major research goals is to understand the structure and evolution of cataclysmic variables, particularly WZ Sge, AM CVn and DQ Her (intermediate polar) type stars. Up until the early 90's Joe's observing was done primarily at the National Observatories and with various X-ray and other satellites.

In 1991, Joe formed the Center for Backyard Astrophysics (CBA), a network of

primarily amateur astronomers spanning the globe and collaborating on

observation of variable stars, using CCDs mounted on backyard telescopes.

Since then, they have conducted observing campaigns and long-term

photometric studies of several systems and published papers with the results

(a selected list follows this interview).

We're going to ask Joe about the CBA's collaboration between professional

and amateur astronomers.

CVnet: After more than a decade of observing superhumps in SU UMa systems, don't we know just about everything there is to know about UGSU type CVs? What is left to understand, and how can observations with small telescopes help in unraveling the remaining mysteries?

Joe: The dwarf-nova outburst itself has become generally well understood -- in the sense that there's a theory which successfully reproduces the observed phenomena. But it's noteworthy that all of that theory was crafted to fit previously known data -- it has never actually predicted something not known in advance. So that higher standard of scientific worthiness has never been met. In addition, the theory breaks down when applied to dwarf novae in quiescence -- it predicts about a thousand times less accretion than is actually observed! Plenty of thought needed there.

The UGSUs are of high interest because of the very regular periodic signals they show -- superhumps and their relatives. These are caused by the (gravitational) tugging of the secondary star on the outer accretion disk. Their periods, if measured with sufficient precision, reveal the mass of the secondary pretty accurately. That is quite hard to do in any other way, since the secondary is usually invisible in the spectrum.

Professional astronomers typically observe the UGSUs for a a few days at one site - their data is very limited in time and very afflicted by aliasing (since they are "off the air" for ~20 out of every 24 hours). They go very faint (of course) but their data on periodic signals is often of low accuracy and/or aliased. Single large telescopes proudly performing their hijinks for a few days at a time... that just doesn't produce enough accuracy (or reliability, because of the aliasing problem) for time-series studies. The best time-series data sets are nearly always those of amateur astronomers.

CVnet: What makes nova-like variables (and old novae) such rewarding targets for CBA?

Joe: Oh, we like these because they don't show much night-to-night variability. This makes the time-series studies extremely sensitive. They also are the only CVs you can confidently model with the "steady disk" assumption.

CVnet: FY Per and FS Aur seem to be frustrating your attempts to understand them.

What makes them so interesting, and what has made them so difficult to

study?

Joe: Well, at present "annoying" would be a better word. Both show stable photometric periods greatly discrepant from their spectroscopic periods -- yet not fitting any interpretation previously recognized in other CVs (superhumps, rotation, precession). They're not difficult to study - we just haven't figured 'em out! When a good idea arrives, that will be a reason to upgrade "annoying" to "interesting".

CVnet: What are the CBA's current observing targets, and why have they been selected for study?

Joe: We change targets every month or so, shown on the homepage (and detailed more in the "News" category). The reasons are varied. Some of the targets are selected for very long-term study (studying a periodic signal as it evolves over years), others are more "object of the month" (new dwarf novae).

CVnet: How can CCD observers contribute to CBA? How do you sign up or get involved?

Joe: If you have an 8" or larger scope, a CCD, appropriate software for time-series photometry (repititive images, >100 for a night), and a love of doing science, then we want you! Best to write to cba-info@cba.phys.columbia.edu; this goes to myself and Jonathan Kemp. You should then select a current target, get a few long nights of differential photometry, and send the data (Var-Comp versus JD) to cba-data@cba.phys.columbia.edu (same destinations). I'll write back with comments. The CBA site also has emails of members; you might want to write them for advice.

CVnet: How can visual observers contribute? Are there any particular stars you are waiting for an outburst or unusual activity that you would like to be notified of?

Joe: The recognized UGWZ stars are of huge interest - every outburst is of high importance. Since they erupt so rarely, the astronomical community needs a lot of help in finding the outbursts. I couldn't stress that enough! In addition, there are a few dozen faint CVs thought to be UGWZ stars, but which have never actually erupted. Any eruption of theirs would be an even bigger deal.

This is awfully tough work - staring at a patch of sky and seeing nothing for years or decades on end. Yet VSOers do it. Amazing.

CVnet: What has been the biggest advantage to collaborating with mostly amateurs on observing projects like CBA?

Joe: Well, after collaborating with professional astronomers for 15 years, I couldn't help but notice a few things: they get rained on a lot, they're very nervous about money, there aren't many of them, and their life stories and skills are pretty redundant with mine. Amateur astronomers seemed like a better and more interesting investment...