A new binary asteroid and interesting lightcurve

[higginsdj]

Looks like my latest target, 2121 Sevastopol, is a binary asteroid. Analysis has revealed something most of us haven't seen before in a lightcurve - 3 eclipsing events.

We believe that we are seeing the moon transit, moon shadow transit and eclipse of the moon. We are fortunate to have the target geometry in just the right position (high phase angle and near edge on view of the orbital plane).

We should have enough data in the next lunation to publish a CBET. You can view the lightcurves on my lightcurve results page:
http://www.david-higgins.com/Astrono...ightcurves.htm
http://www.asu.cas.cz/~asteroid/02121_porb.png is the official version of the orbit curve.

Cheers

[Outbackmanyep]

Hi David,
Well done, congrats!
Are you using Occultwatcher as well?

[higginsdj]

Yes, I have occult watcher installed though I have not been observing any occultations. I find that if conditions are suited to occultations then they are suited to my MP photometric work as well! Maybe one day I will be able to afford a second scope to fit all my occultation observing gear to

[higginsdj]

Note if people keep an eye on the second link I provided above they will see the events quickly evolving as the phase angle reduces. The Moon and shadow transit events are merging. Quite exciting

Oh and just to give some perspective to this, 2121 is an inner main belt asteroid who's diameter is under 12km....

[Outbackmanyep]

Hi David,
So you determine your lightcurves from monitoring the brightness of the asteroid, not so much as in occultations of stars, how long does this take and how do you predict the events you're looking for, or is this more or less a by-product of your monitoring routine?

[higginsdj]

Yes, it is all based on lightcurve analysis. In it's simplest form, we first derive the primary period. If you look at my analysis (just from my own data) we see the raw light curve:
http://www.david-higgins.com/Astrono...ges/2121lc.GIF

From experience I can tell right off that this is a binary (or my observing setup has a major hiccup). You can see the primary period sticks out but there are a lot of data points that don't fit. Now since I am 'assuming' a binary, that means this lightcurve is comprised of 2 or more overlapping period curves.

Step 1 - remove the primary period from the data and retest

After removing the primary period and retesting we end up with:
http://www.david-higgins.com/Astrono.../2121_pOrb.GIF

As you can see, there are 3 events in this curve (we usually only see 2 events but as I said, we got lucky with this one). This is the same type of curve as we see for synchronous binary asteroids and separated binary stars (or event extra solar planet transits).

Step 2: Now that we have P_Orb, we then go back to the original data and subtract P_Orb so that we can refine the Primary period and we end up with:
http://www.david-higgins.com/Astrono...es/2121_p1.GIF

Then we repeat the whole process for as many iterations as we can to refine the curves. Obviously the more data we get, the better.

As a final step, I subtract the Primary Period P1 AND the Orbit Period P_Orb and see if I can derive P2, the rotation period of the moon itself. Its very noisy but I get:
http://www.david-higgins.com/Astrono...s/2121_p2a.GIF

(Now before ANYONE says - Wow, that sounds complicated - it isn't. Remember, I just took images, measured them using differential photometry then used functionality in a piece of off the shelf software to get the results!)

From the P_Orb curve, we measure the smaller of the eclipse events and derive the Diameter ration of the 2 objects. Since we have not completed observations of the smaller event, we can only constrain it to be not less than 0.38 (the moon is 0.38 the diameter of the Parent). With this information we can also determine most of other characteristics of the system.

After a period of time and obtaining lightcurves for the system over a few apparitions we will be able to derive the shape of both parent and moon, H and G values and pretty good values for the exact size and mass of both objects. All without having to undertake direct imaging and without the need to park a spacecraft nearby

Now all this can by done by amateurs with sophisticated amateur software. If you look at my curves you will note that data was compiled over 6 nights spanning a month (note that I observe more than 1 object each night). The software is MPO_Canopus written by my mentor Brian Warner as a result of our ("our" meaning the amateur members of the ProAm BINAST collaboration) with Dr's Alan Harris, Petr Pravec, Joseph Durech and Mikko Kaaselainen.

Cheers

[higginsdj]

I would also like to stress that ANYONE can do this. It has been documented that 15% of all NEO's and small inner main belt asteroids are binaries. It may well hold that 15% or ALL small asteroids are binaries (small as in H>12 and Inner Main belt where a < 2.5au)

This means that you do not have to be part of a Professional Collaboration. You can, if you wish, collaborate amongst yourselves and unlike 'normal' asteroid lightcurves, the discovery/uncovering of Asteroid moons still warrants CBETs. Who knows, one day we might even be allowed to name them

If you want to try - I'm happy to help/mentor/advise as time permits. PM me if you want.

Cheers

[AstroJunk (Jonathan)]

That looks great David,

I'm certainly planning to spend some time looking at the variable nature of asteroids myself (once my house is finished and I can spend more time at the observatory!). My interst in this area was sparked when I captured a graze occultation of 32 Pomona which blinked out twice and which suggested something more than the simple model that had been produced from its light curve.

I think it would be good idea to do some co-ordinated research on asteroids that are known variables, both photometrically and occulting.

Out of interest, do you know what is the smallest that an asteroid can be to capture a moon? This will be directly linked to the rotational period of course! I guess something like a rubble pile like the tiny (700m)Itakawa or bizarly shaped Hertha at 100km would give similarly complex light curves (and I stress GUESS). Ultimately, untill they turn the radar onto them, we can only speculate!

[higginsdj]

Hey Astrojunk.

Therein lies a common misconception. Most asteroids moons are not captured - they are made (even now) and that is the primary reason we are observing these small ones. There are far to many binary systems and most are too small to have been captured/capture events. "What size is the smallest for capture to be possible" - I'll have to dig through a few papers to find that one out - will take some time.

The small asteroids are simply gravitationally held rubble piles. As an asteroid is spun up by YORP they approach what we call the spin barrier (around 2 hours) at which point a process called rotation fission occurs. The asteroid is spinning so fast that landslides occur toward its equator and matter is thrown off. In most cases the forces involved cause the asteroid to lose rotational speed and a moon enters orbit around the parent where tidal forces then start to come into play affecting the rotation of the parent the moon and the orbit. In most cases the moon becomes synchronised to the orbit (like our own moon)

But there are always exceptions to the 'rule'. I've been told to keep an eye out on Nature when it comes out at the end of the week for an interesting read.

[AstroJunk (Jonathan)]

Which also suggests that these rubble piles must actually be fairly cohesive as the very slow acceleration due to YORP would surely cause oblate or disk shaped objects if they were truly loose agglommerations.

The very little known about these emotive objects makes their study by capable amateurs even more valuable!

[higginsdj]

Oblate - yes, disk - no. In order to achieve any 'extreme' shapes, the spin rate would need to exceed the spin barrier. Throwing off mass, in general, will cause the objects rotation to slow (yes there may be exceptions). Also note that YORP can spin up - but it can also have the opposite effect.

Have a look at this:
http://www.socastrosci.org/videos/harris.wmv

Its a presentation made by Dr Alan Harris at the 2010 SAS symposium. For those interested in Asteroid science within the reach of amateurs, I recommend looking at this presentation as well:
http://www.socastrosci.org/videos/Buchheim.wmv

Cheers

[AstroJunk (Jonathan)]

Very interesting site. I shall watch those later

[Outbackmanyep]

Hi David and Jonathan,
I am intrigued!
I have never done photometry on anything, occultations is as far as i have got, but i did see one of Gordon Garradd's asteroid video's showing rotation of an asteroid and you can see just by watching it the rotation and "lumpiness" of the asteroid in the reflected light, over long periods of time this would be negligible to the eye but watching the fast playback you can see it by eye!
So not only can we see the light brighten and dim you can also "see" the shape of the asteroid just by doing the lightcurve.

It is very interesting indeed and i am sure that if Jon takes it up then i will seriously consider this as a sideline to what i already do!

I hope you will be at NACAA in 2012 to do a talk on the subject and get more involved, awesome stuff!
Cheers!

[higginsdj]

Very few asteroids will have lightcurve variations (amplitude) that are visible with the naked eye. Most amplitudes are < 0.5 mag and most of our binary targets are < 0.2 mag. Also, shape modelling requires a few curves - onecurve doesn't tell you a great deal about shape.

To get started, find a short period target that has already been done and see how you fare with your results. Data on previously covered targets is always useful for spin axis determination and shape modelling so covering 'known' targets is never a waste of time.

[higginsdj]

Well I have drafted the CBET and sent it on to Dan Green so it should be out in the next day or 2. If you take a look at the orbit lightcurve now you will see that we have coverage of the 0.25 phase event and it too shows the interaction of the shadow on the occulting event. So due to the high phase angle we have been able to detect all 4 eclipse/occultation events.

[PeterM]

Congratulations David,
It has indeed been announced in CBET 2427 today.
Another Australian Amateur astronomer contributing to the science of Astronomy. Top shelf stuff.
PeterM.

[CraigS]

Good onya David (& team)!

Congratulations !!
Great stuff !

Cheers