THE VANNISHING L2 PUPPIS


A variable star once ranging from as bright as 2.6 down to 6.2 magnitudes now ranges from 6.9-8.0 mag.


L2 Puppis is a semi-regular variable star in the southern constellation of Puppis; a rich constellation spanning a great deal of the sky between the bright stars Canopus and Sirius. L2 Pup is marked on most star maps (at least, so far), and is found at position 7h 13.5m -44o 39’. But it is disappearing rapidly behind a cloud of dust and silicates.


Below is a light curve spanning ten thousand days (until today, 9/5/07), made up of thousands of observations by many observers, and lodged with the American Association of Variable Star Observers. Note the vertical lines of dots; at this scale each line is a pulsation period of around 140 days from dim to bright and back again.



This curve shows the star disappearing before our eyes; getting fainter over the last five thousand days. This type of star – a red giant - has unstable envelopes, which eventually will be shaken from the core of the star altogether, leaving a white dwarf star and an expanding cloud of dust and gas. In this case there is not a lot of dispersion, and L2 Pup is being obscured by dust at an ever increasing rate.


My Norton’s Star Atlas quotes the range 2.6-6.2 magnitudes, and even over the history of the star presented above, it has never been that bright. (Note that the lower the number in the magnitude scale, the brighter the object is.) What’s more, the visual amplitude is diminishing, too. The listed range of 3.5 magnitudes from bright to faint in the literature is apparently now just over 1 magnitude.


The curve below is the last four hundred days of this star, and here the 140 day pulsation period and smaller amplitude range are clearly seen. The star has never been seen fainter, nor showed smaller amplitude; now the range is seen to be roughly 6.9-8 magnitudes.



How far will it continue? Who knows. I’m sure it will continue to be optimistically charted on bright star atlases for a long time to come, in the hope of it brightening. If you wish to observe this star, contact the Variable Star Section of the Sydney City SkyWatchers for charts and instruction. So far, only a small telescope or medium binoculars are needed.


Alan Plummer.


Further reading,

‘The light curve of the semiregular variable L2 Puppis: I. A recent dimming event from dust’ Bedding et al., 2002MNRAS.337...79B (available for free through http://lanl.arxiv.org/find/astro-ph)


Also see www.aavso.org