Wobble?

Hi asciiadam, First, congratulations on your brand new telescope. I remember my first real telescope and the many nights I spent looking at the moon and planets with it (especially the moon!), before I moved on to more distant targets. You ask an important question. The wobble you refer to is the motion of the bright star around the common center of mass; much like a teeter-totter, the stars will show little orbits based on how massive each is. For example, in our own solar system, Jupiter is sufficiently massive that the center of mass is not at the center of the Sun, but instead just inside the outer edge of the Sun, so that the Sun executes a little wobble with a diameter that about equals the diameter of the Sun, with a period equal to the orbital period of Jupiter. For epsilon Aurigae, you have two massive objects about 30 AU (astronomical unit; the radius of the earth's orbit) apart. Since they have about the same mass, each follows an orbit with diameter 30 AU, over the course of 27 years. To determine whether we could measure this depends on a number of factors. First, the best astrometric measures from the ground are about one milliarcsecond (mas). This is pretty small; it basically means that we could barely detect a 1 AU wobble at 1000 parsecs (pc; 3.26 light years/parsec); a more reliable detection would require the object to be quite a bit closer. So the 27-year wobble of epsilon Aurigae with its 30AU motion should be pretty simple to detect, as long as the star system is not much more distant than 1000pc. The Hipparcos satellite measured the distance to epsilon Aurigae using parallax (using the earth's orbit to give a measurable motion of nearby objects with comparison to a more distant background of stars, much like our separated eyes measure distance). It determined a parallax of 1.60 mas, or about 625pc. However, the error in that measurement is 1.16mas, so the true distance, with 68% certainty, could be as close as 362pc, or as far as 2270pc. Even at 2270pc, you should be able to see the wobble due to orbital motion, if you wanted to follow the system for the next 27 years. If we assume the 625pc distance and the 30AU orbit, then the astrometric motion (or wobble) would be about 48mas; a piece of cake for modern telescopes. No one has attempted this, but it ought to be done! Arne

I'll answer my own post... There is a paper by Wulff Heintz and Bruce Cantor (http://adsabs.harvard.edu/abs/1994PASP..106..363H) which discusses the available astrometry for epsilon Aurigae, using the long astrometric photographic plate series from the Sproul refractor; 363 nights spread over 44 years. They yield a semiamplitude of 22.4mas (+/- 1.8mas, or a full amplitude of about 45mas), with an ascending node at 264degrees (an interesting value) and an inclination of 87 degrees (very weakly determined). These values fall very close to the predicted ones in my previous posting, and so mesh well with the existing distance estimate, orbital size, etc. As I said, 48mas should be easy to measure, and it looks like early astrometrists did just that. Peter van de Kamp also wrote a paper on the astrometric orbit; he was a real pioneer in astrometry, and you will see his name come up prominently whenever you study photographic techniques. So the short answer to your question: yes, there should be a wobble, and yes, the wobble has been seen. Fun stuff! Arne

Thanks a lot! That was a ton of information for me but I will read it a few times. A follow up question if I could, if we have detected the wobble shouldn't that disprove the dust theory? As dust would not have the gravitational mass to make a star wobble, correct?

Hi asciiadam, A small amount of dust, like that caused by a tractor in a farm field, doesn't have much mass. However, the obscuring object in the epsilon Aurigae system is, well, astronomical in size. It probably consists of some massive star that is surrounded by this immense cloud of dust. The cloud is likely to have more mass than the Sun, but most of the mass of the obscuring object is that embedded star, hidden from our view by the dust. The dust+star combination has about 14 times the mass of the Sun, about the same as the visible star in the eps Aur system, and that is why it has enough mass to make the star wobble. This is all theory - all that we really know are the observations. We know the likely distance; we know the velocity of the components due to spectroscopy; we know the size of the orbits based on astrometric measurements. What the obscuring object consists of is still a mystery. It could be a single star surrounded by dust. It could be a pair of smaller stars, whose mass totals 14 solar masses, again surrounded by a dust cloud but with a central hole cleared by the orbital motion of the pair. When something is "dark", you have to infer its properties by how it influences its surroundings. Arne

Again, Thanks so much. I just wish I enjoyed learning this much while I was in School.