Starship Asterisk*

I recalled that Betelgeuse (Alpha Ori) was quite variable so I wanted to check this with archival NSL data. The first bad news is that Betelgeuse saturates the 65K ceiling of the CCD chip, as shown by CI C1-B counts above 60K at many times. We have learned not to trust saturated data, even for C25. The first good news, though, is that when the moon is up and the NSL exposure is only 20 seconds (as opposed to 180 when the moon in down), then Betelgeuse does not saturate. Far from it! I trolled the archive and found a clear moonlit night on 2004 January 7.

The second bad news is that over two hours I was unable to find variability in the 20 sec data. The data showed a slow increase but so did the comparison star, Beta Ori. The ratio also showed an increase but the angular distance between their altitudes was changing and sometimes high. So the change in brightness might be easily attributed to a change in altitude.

The second good news is that data looked pretty well behaved, even though the moon was up and looking pretty ominous in NSL CI frames. Here is a quick plot:

This plot looks better than a previous plot because I realized that making the plot really big before copying it to the MS Paint program saves more pixels (duh!). The BMP file might be large but when Photoshop makes it a GIF the resulting file ends up really small -- 10K or less.

For Beteleguese, I think a better search with a better comparison technique over a longer time period can confirm that NSL data can see Betelgeuse variability.

- RJN

Bob, Betelgeuse is a difficult object for the CONCAMs because it is a LONG PERIOD semi-regular variable. There is no "period" proper; the variability, as in many supergiants, is of hundreds of days. See for example http://www.solstation.com/x-objects/betelgeuse.htm or http://www.aavso.org/vstar/vsots/1200.shtml, and in particular http://www.aavso.org/images/LTbetelgeuse.gif for the historical light curve from 1911! This will demonstrate that (a) in the last 50 years or so the amplitude is smaller, ~0.5 mag, and (b) there is no proper period.

Cheers,
Noah Brosch

Thanks, Noah! Interesting. While chasing your links I chanced across this PASP paper:
http://adsbit.harvard.edu/cgi-bin/nph-i ... ..96..366G
that is also interesting.

I think Betelgeuse would make a good NSL project for a student. NSL data gives the most continuous and most uniform stream of observations for Betelgeuse ever. At the least NSL data can quanitfy what variability Betelgeuse is showing this year. At the best the new data can be fit to new supergiant models and perhaps a better physical understanding might emerge. Since the stellar surface is thought to be complex, perhaps occasional bright surface features can be tracked photometrically as they cross the surface of the star.

One way to follow the longer term variability of Betelgeuse is to troll the archives for 20 second observations done at the same sidereal time of day (OK, night). Then the stars will be in the same positions but the relative positions of the Sun and Moon will have moved. Betelgeuse should even appear over the same pixels, removing some sources of systematic error.

- RJN

Bob:

"I think Betelgeuse would make a good NSL project for a student. NSL data gives the most continuous and most uniform stream of observations for Betelgeuse ever. At the least NSL data can quanitfy what variability Betelgeuse is showing this year."

I agree and, just because Betelgeuse is very bright, this means that it is NOT properly observed.

"One way to follow the longer term variability of Betelgeuse is to troll the archives for 20 second observations done at the same sidereal time of day (OK, night). Then the stars will be in the same positions but the relative positions of the Sun and Moon will have moved. Betelgeuse should even appear over the same pixels, removing some sources of systematic error. "

The problem might be that there are no stars of similar brightness in the vicinity to serve as comparison stars. You either must be able to do whole-sky photometry, then the ensemble of stars will serve as standards, or at least, for what you propose above, do photometry for stars in the same altitude strip around the image center (same atmospheric extinction if no clouds).

Noah