Peaks, Valleys and Plateaus

Ideas more or less related to teaching and learning physics at Luther College.

I started my career as an x-ray astronomer, building new instrumentation. Arriving at a liberal arts college I re-invented myself as an optical observer, intending to keep both projects going. But there was no way to give both of them the attention they needed. So now I am an optical astronomer working on the observing projects I have written about repeatedly in this space. This kind of transition from one research area to another is exciting because everything is new and there is plenty to learn. Sometimes, however, it can be tough to know if what one is seeing is something well-known or not. An interesting feature we observe might have been written about extensively 100 years ago and is now in the category of “common knowledge” so that references to the effect are hard to come by. All we can do is keep digging and learning all we can.

One of the things we deduced along the way is that most of the 60 or so semi-regular variable stars we discovered are multiperiodic. We also found that those displaying a single strong period very often showed regular “plateaus” or “stalls” in their brightening but not their dimming. Several example light curves are shown below, with the plateau region examined more closely for one of them.

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Now, we had no idea just how common this phenomenon was and we are still working on deducing that. We don’t know why they only appear in the monoperiodic stars. It could be that the light curves of the multiperiodic stars are too chaotic to see the plateaus but that doesn’t seem to be the case. We do know that the plateau stars all have periods between 365 and 410 days, about twice as long as a typical period for our variable stars. Of course, the monoperiodic stars are the longest period stars in our data set so it’s hard to tell what is driving what there. More importantly, we have yet to figure out what these plateaus are telling us about what is happening physically in these stars and if there is any crucial evidence here to help us understand the evolution of these systems from one type of system (e.g., multiperiodic) to another (e.g., monoperiodic).

This summer one of the few monoperiodic stars not previously showing a plateau has demonstrated clear evidence of a strong plateau. See the purple triangles in the graphs below. It is likely that we had data too sparse to be certain of plateaus in earlier years. The region of pulsation phase we are examining shifts slowly year-to-year (since the period is a little longer than a year) and we are now in a position to examine the plateau in this star’s light curve. This is the kind of thing that keeps us doing what we are doing night after night, year after year.

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