Dwarf Novae

We have a guest post from Martin Still. Martin is Deputy Science Team Lead and Guest Observer Office Director for Kepler. He’s writing today to tell you about an interesting class of objects  you might encounter when classifying Kepler light curves

Dwarf novae are binary stars consisting of a white dwarf and main sequence companion. The binary orbit of period a few hours is small enough that the outer atmosphere of the main sequence star is being stripped through gravitational influence by the white dwarf, and gas is falling in a steady stream towards the surface of the white dwarf. The light from the binary stars is dominated not by the stellar components but the accreting material configured around the white dwarf within an accretion disk. The brightness of the accretion disk is coupled strongly to the temperature and density of the disk. Brightness changes over the timescales of a few days and several magnitudes indicate changes in the density and temperature of the disk. Coherent oscillations in the light curves of dwarf novae on timescales of a few hours indicate the orbital period of the binary and tidally-driven distortions in the accretion disk. To identify dwarf novae, look for targets that brighten by an order of magnitude or more over a few days and decay on a similar timescale. Coherent modulation on periods of a few hours are also expected, but not essential for characterization as a dwarf nova. 16 dwarf novae are currently known in the Kepler field, perhaps several hundred more are suspected to reside there. Some of them will be faint, blended background sources behind brighter Kepler targets. Dwarf novae are scientifically important because they are the cleanest objects in the galaxy for studying accretion disks – those structures that surround e.g. active galactic nuclei, cataclysmic variables and symbiotic stars. Planets around stars form from similar disks around proto-stars. Without a detailed understanding of accretion disks, the evolution and structure of the universe on many scales cannot be understood.
If you spot light curves like these, please post them in Talk  here.
Here’s an example of a dwarf novae light curve, you can see the huge brightness humps are when the outbursts are occuring
or you might spot in one of the light curves you’re reviewing on the main Planet Hunters site something like this:
The one above is KIC 4378554 a dwarf novae found and discussed in a recent paper by Barclay et al (2012). 

7 responses to “Dwarf Novae”

  1. VJOM says :

    Poseo una colección de todas las Estrellas eruptivas observadas, espero que les se útil… http://talk.planethunters.org/collections/CPHS0005an

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