Thursday, April 22, 2010

A Cataclysmic Variable in the Field of the Kepler Mission

The field of stars with our new-found cataclysmic variable star

In September of 2008, I was sitting at the controls of the McDonald Observatory 2.1-meter Struve Telescope.  I was there to help some of my colleagues who work on a team associated with NASA's Kepler Mission.  This team, the Kepler Astroseismic Science Consortium, isn't on the lookout for planets.  They are studying the stars themselves, looking for variations in the light from stars caused by sound waves in the star.  The study of these sound waves, known as asteroseismology, allows astronomers to probe the interior structure of a star, just like geologists use seismic waves from earthquakes to study the interior of the Earth.

My specific goal at the telescope was to look for pulsating white dwarfs.  While around a hundred pulsating white dwarfs are known in the sky, none are known in the patch of sky where the Kepler mission stares.  I had a long list of candidates, some that I chose, and some that our European collaborators selected.  One of the stars selected by Viennese astronomer Gerald Handler and his collaborators,  the star in the center of the picture above, had been observed with the Telescopio Nazionale Galileo in the Canary Islands, and looked "interesting".

So, my job was to follow up on the interesting star. I pointed the telescope and started taking pictures, measuring how bright the star was every 10 seconds.  The pulsating white dwarfs we look for vary their brightness in a very regular fashion, getting brighter and fainter by a few percent every few minutes.   At first, this star looked like it was doing exactly that, and then, in a space of just a couple of minutes, the star got a whopping 20% brighter.  I could actually see on the computer screen that the star had brightened relative to the other stars (this is very rare; we need computers to tell us about the few percent variations).

I knew then that this was not a pulsating white dwarf, and I suspected it was a cataclysmic variable, or CV, where a white dwarf is pulling material off of another star.  I suspected this because CVs often show this behavior of getting suddenly brighter and fainter.  The rest of that night, and some of the following night, I saw this variability of the star.  I notified our European contacts, who looked with a telescope in Bologna and verified the variability.  At this point I also started to blog about it, but was then asked not to say much.  If we had found something new, we wanted to make sure we got the data we needed to publish a paper and not be scooped by someone else. 

Our next step was to make sure that we had actually discovered something new, so we scoured journal articles, online databases, and various catalogs, and while we found the star listed in some basic catalogs, nobody had ever studied it or determined that the star was a CV.  What we needed, though, was a spectrum of the object.  Spectroscopy, the analysis of the different colors of light coming from an object,  allows us to identify uniquely each object, as well as garner a lot of additional information.

None of us had time on a telescope with a spectrograph before the star went behind the sun for the winter.  But two of my friends and colleagues were using the MMT and its spectrograph later that month, so I asked them if they might look at it.  The first of them didn't have much time but grabbed a quick spectrum, which he misinterpreted as a "normal" white dwarf star, so he went on with his program.  That's fine, these things happen.  But we got lucky with my other colleague, Patrick Dufour (who has made some very interesting discoveries on his own).  When he got on the telescope, the wind was coming out of the east so fast that he could not safely point the telescope any direction but west, and all of his targets were in the east.  Our star was in the west.  So, Patrick very kindly pointed the telescope west and took about 90 minutes of spectral data.  (He could just as easily closed the dome and gone to bed, and we would have been none the wiser). 

Patrick's high-quality data proved that this star is a cataclysmic variable, and even allowed us to start to classify what kind of CV it is.   CVs come in many flavors, so one of the most basic bits of analysis for a new CV is to fit it into the existing classification scheme, if possible.  So, we managed to narrow down its class somewhat, though there is still more work that needs to be done.  This star is definitely a nova-like variable of the UX Uma class, and it may belong to the SW Sextantis subclass.  (To compare to a topic that may be more familiar, this is like saying we found a creature that we know is a mammal, we know is a primate, and we know is a great ape, but we're not yet sure if it is a gorilla or a chimpanzee.) To make a better classification, we need better data.  But since this star is outside my area of expertise, we published the information that we have so other people can go and dig deeper.

To me, the exciting thing about this discovery is that the CV is in the field of view of the Kepler mission (I think four or five others are known in the Kepler field).  Kepler is just a phenomenal instrument, and its ability to stare at an object for months on end and to obtain very high precision measurements mean that people who study CVs can try new avenues of analysis that have never been done before.  This is illustrated by the recent discovery by Kepler of Doppler boosting, a brightening of light from an object due to its motion toward you (like a Doppler shift).  This boosting effect is very tiny for the majority of objects in the Universe, but Kepler can measure it.  In fact, Kepler has already looked at this system for about a month, and those data should be available for download very soon.

So, in short, we found an interesting cataclysmic variable in the same region of the sky that the Kepler mission is looking.  We don't have more than a basic description, and some of the important parameters and classification of the system remain to be done.  However, since I only dabble in the field of cataclysmic variables, I probably won't be doing much more with this star.  Some of my collaborators may well be working on it, and because we've published it other researchers can feel free to follow up on it, too.  All of them can do a better and more efficient job studying this star than I can as a newbie.  So, friends, have at it!
Kurtis A. Williams, Domitilla de Martino, Roberto Silvotti, Ivan Bruni, Patrick Dufour, Thomas S. Riecken, Martin Kronberg, Anjum Mukadam, & G. Handler (2010). Discovery of a Nova-Like Cataclysmic Variable in the Kepler Mission Field The Astronomical Journal, in press, (arXiv: 1004.3743v1)


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  2. Michael Suede1:07 PM

    I think your theories of stars are wrong.

    In order to understand CVs, one must first understand the Sun.

    Given the current theories about our Sun, any postulates about other stars, particularly peculiar stars such as CVs are bound to be wrong.

    Allow me to highlight some facts:

    1. There is no proper explanation that agrees with the known laws of physics for the inverted temperature gradient of the Sun.

    The corona is hotter than the photosphere, the photosphere is hotter than Sun spots, and as far as we can demonstrate, this inverted temperature gradient continues all the way down to the center of the Sun.

    2. There is no evidence that the interior of the Sun is a hydrogen to Helium fusion reaction at all. The neutrino counts observed are far to low to agree with nuclear fusion postulates. The observed fusion in the corona has nothing to do with hydrogen to helium reactions.

    3. The standing theory of stars is an obtuse mix of "dynamo" theory, magnetic reconnection, and hypothetical fusion reactions, how can this be an acceptable model? How can one draw conclusions about other stars if our own Sun is not fully understood?

    4. The photosphere is granulated and exhibits classic anode tufting just like we see in typical plasma anode discharges here on earth. There is no plausible reason as to why a hydrogen to helium fusion reaction should cause such tufting on a relatively cool surface. Nor why a "dynamo" should produce the magnetic fields responsible for them.

    5. The Sun is x-ray variable, which again is totally unexplained by the notion that a hydrogen to helium reaction is the cause of solar discharge. What causes this variability?

    6. The differential rotation observed by latitude and depth is totally unexplained by the standard model of the Sun. If this rotation is not fully explained, how can one possibly speculate about distant stars reacting to "sucked in material from a near by star"? "Convective currents" do not produce even differential rotation. Such currents themselves are entirely hypothetical.

    7. Why should a hydrogen to helium reaction produce a "dynamo" magnetic field? What science backs this postulate other than wild speculation? How are the charged particles formed that supposedly make up this dynamo? Why should such action cause Sun spots, the Sun spot penumbra, and other observed features? Is there any laboratory proven science at all to demonstrate how this is possible?

    Please sir, the Sun needs to be understood first before theories of CVs are postulated.

    I'll give you generous head start over your colleagues in this regard. The physicist that won the Nobel prize for formulating MHD theory also did some work on modeling stars. His work on stars has been expounded upon by other physicists and presents an alternative model that can explain the afore mentioned observations in a way that agrees with laboratory experimentation and the known laws of physics.

    Please give it some serious consideration.