Observing Challenge: Variable Star R Aquarii

[mark.m]

There's a pretty cool variable star that needs visual observations: R Aqr (Aquarius). This star is actually a binary pair, separated by about the distance between Saturn and our sun. The two stars started life with somewhat different masses. The more massive one (now called star "B"), reached the end of its life before the less massive one (now called star "A"), and has turned into a white dwarf no longer undergoing fusion. (It also lost some of its mass at end of life, and is now less massive than "A".) The pair was stable like that for a while, but now star A has also finished burning the hydrogen in its core. At this point the core shrank, triggering a series of "helium flashes" that have dramatically increased star A's total energy production. All that energy has caused A to swell into a red giant, with the little white dwarf B continuing to orbit swollen A. The evolution into a red giant has given star "A" a very strong solar wind, with a significant amount of its mass being blown into space. But dwarf B is close enough with a strong enough gravitational field to start capturing mass tossed off from the red giant A. (The artist' rendering below even shows some parts of the red giant close enough to the white dwarf that the little star is actually sucking material right off the red giant.)

The mass transfer from A to B isn't a quiet, gentle exchange. Conservation of momentum prevents the transferred material from just gravitationally "falling" onto B – the transferred material is being whipped about, generating both an accretion disk (sort of like Saturn's rings) around B and a strong jet of material that is being expelled from the pair. The jet is now interacting with the old material that B shed a long time ago when it reached end-of-life, creating a complex system. This is the closest known example of a symbiotic star system, where there is strong interaction between members of a binary pair. (See also the Wikipedia article on R Aqr.)


Artist' Rendering of R Aqr, showing large red giant, tiny white dwarf, blue accretion disk, and narrow ejected jets
Credit: Dana Berry (STScI) via ESA/Hubble

Earth-based observation of the system is unable to separate the two stars. We do see a complex nebulosity surrounding the system. (Any astrophotographers want to try to image it??) Dr. Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) is studying this system with the Hubble and Chandra space telescopes, and has requested ground-based observations through the American Association of Variable Star Observers (AAVSO) to supplement her observing time in space. The light curve of the pair tends to be dominated by the long-period variability of the red giant.
Just about any small telescope will suffice for observing this star visually. At its faintest it gets to about magnitude 12.5. (Which is close to the limiting magnitude of our library scopes.) Right now, it is around magnitude 10 or so. R Aqr is low to the south, so you do need pretty good access to the southern horizon.

To observe this star, you need to fetch a star chart from the AAVSO, and then get (or make) a finder chart. (I used maps 3 and 4 in my trusty old Norton's Star Atlas to start my star-hopping. I've attached one of the fine charts from Taki, which also works.) Find a star-hopping path to get to the right field, and then shift to the AAVSO chart. (I went from Formalhaut north about 7 degrees, then east-northeast to a group of mag 5 stars, then north to R Aqr and stars labeled "53" and "50" on the AAVSO chart.) You estimate the brightness of R Aqr by finding a brighter comparison star and a fainter comparison star and then interpolating. (When I looked a couple days ago, I found R Aqr dimmer than the "103" comparison star – the "103" means mag 10.3 – and brighter than the "95" comparison star. I estimated that R Aqr was half way between the two, so I declared my estimate to be mag 9.9.)

If you give it a try, post your experience (joyous or frustrating) here!
- Mark M

[Stargrrl]

Thank you Mark for posting this cool star system and the fascinating story behind it! A good challenge for anyone who'd like to start doing some variable star observing.

Rebekah

[Paul D]

Very interesting talk on variables. I was also thinking that a method I use when trying to determine the magnitude of comets might be used as well. Using my existing star field I simple defocus my scope and compare the defocused blobs of stars with the defocused comet core and then match up the two blobs in brightness to each other and then identify the star that mostly matches the defocused brightness. I find by not having a sharp image but a blob my eyes seem to look at the whole picture per say and not colors or diffraction spikes on focused stars.

[mark.m]

Paul:
Yes! To quote from the AAVSO visual observing manual:

"Another technique that is strongly recommended for making magnitude estimates of red stars is
called the “out-of-focus method.” That is, the eyepiece must be drawn out of focus so far that
the stars become visible as colorless disks. In this way a systematic error due to the Purkinje effect is
avoided. If the color of the variable is visible even when the stars are out-of-focus, you may need
to use a smaller telescope or an aperture mask."

Very interesting talk on variables. I was also thinking that a method I use when trying to determine the magnitude of comets might be used as well. Using my existing star field I simple defocus my scope and compare the defocused blobs of stars with the defocused comet core and then match up the two blobs in brightness to each other and then identify the star that mostly matches the defocused brightness. I find by not having a sharp image but a blob my eyes seem to look at the whole picture per say and not colors or diffraction spikes on focused stars.