Original Paper: http://arxiv.org/abs/1409.7259v1
Steve Preston — http://www.asteroidoccultation.com
Derek Breit — http://www.poyntsource.com/New/Global.htm
IOTA — http://www.lunar-occultations.com/iota/iotandx.htm
Occult — http://www.lunar-occultations.com/iota/occult4.htm
Description: In this apogee podcast, Cosmic discusses asteroidal occultations and a newly discovered series of rings around Centaur asteroid (10199) Chariklo
Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2014, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at firstname.lastname@example.org.
Hello! This is Cosmic, and welcome to the Apogee podcast! The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal. Sometimes topics within these articles send me off on temporary tangents, but they’re always relevant to the overall topic. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.
If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.
The music you hear in the background are my own compositions, which can be downloaded from my soundcloud channel ‘cosmiclettuce’. I hope you enjoy listening to space music as much as I do.
The apogee for this podcast takes place today, 18 October 2014, at 06:06 UTC. The lunar distance at that time is 404897 km, which is 948 km closer than last apogee on 20 September, and 561 km further away than the next apogee on 15 November.
Tomorrow, 19 October 2014, is an important day for astronomy. Comet Siding Spring makes its close encounter with Mars — a paper thin 139300km. Closest approach is expected to be at 18:28 UTC on that day, which makes direct observations from Earth impossible since Mars at that time is only visible from the daytime side of the planet. It also looks like Siding Spring is going to stay pretty faint which means it won’t be visible unless you have access to a fairly large telescope. A real bummer. But I’m still very excited to see what happens, and even more excited about the data that will be collected from various spacecraft at Mars observing the event.
Ok, on with the paper!
This apogee, I’m taking a look at a paper entitled “A ring system detected around the Centaur (10199) Chariklo”. There are 64 authors on this paper representing 32 insititutions which is an indication of how much effort went into this work. This paper was published in the journal Nature on 3 April 2014.
On 3 June 2013, the 250km diameter asteroid (10199) Chariklo — the largest of the known Centaur asteroids — occulted the 12.4 R magnitude star UCAC4 248-108672 which was visible over a large region of South America (Brazil, Argentina, Uruguay, and Chile).
What is an occultation? Well, an occultation is an astronomical event when one object passes in front of another, the former blocking (or partially blocking) the view of the latter. So, for instance, the upcoming Solar eclipse on 23 October is an occultation event since the moon will block our view of the sun. This is one of many kinds of occultations. To list a few:
1. The moon occults a planet about once a year.
2. The moon regularly occults stars (many binary star systems have been discovered this way).
3. Planets and other moons will occult stars.
[Many years ago I watched Saturn and it’s rings occult a background star and was delighted when I saw the star flicker as its light passed through the rings.]
4. Other solar system bodies like asteroids and comets will also occult stars.
[In fact, the Kepler observatory in orbit around the Sun has discovered nearly 1000 extra-solar planets by very carefully measuring the light from distant stars to watch for the dimming effects of a planet transiting (partially occulting) the star.]
Most occultations are dimming events. The result is a light curve (imagine a graph of brightness versus time) that shows the brighter background object dimming as the fainter (often invisible) foreground object occults it.
According to the International Astronomical Union Minor Planet Center, there are 611630 Main Belt asteroids. Those are asteroids that orbit the sun between Mars and Jupiter. There are thousands of other asteroids grouped in families including the 5684 Apollos (inner solar system), the 6224 Trojans (associated with Jupiter), and the 520 Centaurs (in unstable orbits somewhere between Jupiter and Neptune).
These hundreds of thousands of known and unknown asteroids, of course, are always on the move. From our point of view on the Earth they can, from time to time, pass directly between us and a background star. This is what’s called an asteroidal occultation of a star.
The actual number of stars being occulted by asteroids at any given time depends on the very uneven distribution of stars and asteroids in the sky, and how the asteroids move relative to the background stars (their orbits). It turns out to be only a few per day visible from somewhere on the surface of the Earth, and a few per year from a particular location on the Earth.
A great resource for predicting asteroidal occultations for any location on Earth is a piece of free software called Occult v4.1.1. I’ll put a link to that software in the podcast notes. Two excellent online resources are Steve Preston’s asteroid occultations page and Derek Breit’s Global Asteroid Events page — I’ll include links to these in the podcast notes as well. Another excellent online resource is the International Occultation Timing Association (IOTA), which is also the organization that collects and distributes occultation observations.
If you want to get into observing and timing asteroidal occultations all you need is a decent-sized telescope (25cm or larger), a recording device, an accurate time, and a little luck. When I’ve done occultation observations, the first thing I do is locate the star that is to be occulted. This can be challenging and requires knowledge of how to read a star chart and how to translate that into what you’re seeing through the telescope. I start about 90 minutes before the predicted time of the event. Once I find the star and become familiar with the field of view, I can set up the rest of my equipment. I use a digital audio recorder located next to my shortwave radio tuned to one of the WWV channels. Since I know when the predicted time of the occultation is suppose to take place, I start observing about five minutes before and I’ll observe for about five minutes after. I do this because predictions based on inaccurate orbits can be different than the actual time of the occultation, and there’s always a chance of some kind of a surprise. When I see the star vanish, I give out a yell. When I see the star return, I give out another yell. For those of us who are used to observing static astronomical objects through a telescope, it certainly provides a rush of adrenalin to actually see something happen out there. Anyhow, these yells are recorded along with the exact time. I can then use some simple audio software to determine exactly what time I yelled and I can use that to measure the time and duration of the occultation. The only correction that I need to make to the timing data is my reaction time, which I’ve estimated to be 1/3 second plus or minus 1/8th of a second. I report my observations and data to IOTA. This timing data can be used to improve the orbital elements of the asteroid, as well as determining the size and shape of the asteroid. There are better, more precise methods of making occultation observations, and you can read about those on the websites listed above. Sophisticated occultation observers, for example, use video cameras with very accurate clocks to record the event. This allows for very precise timing measurements to be made and can more easily detect any unusual “secondary events” that might take place. But the fact is that with some simple devices, a keen eye, and a little know-how, anyone can contrubute valueable data to this knowledge base.
One of the first questions I asked myself when I saw this paper was, “How did they know that Chariklo was going to be so interesting?” It takes a lot of effort to coordinate an observation with several observatories. I expect it took them months to put it all together — all for an event that was going to last no more than a few tens of seconds! Well they had good reason to be very interested in observing this occultation. Chariklo was discovered in 1997 at Spacewatch on Kitt Peak. Since then, this asteroid not only has varied in brightness, but also changes in it’s spectrum have been observed. Given these mysteries, any astronomer would jump at the chance to observe this asteroid occult a bright star. The light from the background star can act as an optical probe to look at the environment around the asteroid. Centaurs are known to sometimes form a comet-like coma. Perhaps this is what was going on with Chariklo.
So at 06:25 UTC on 3 June 2013, sixteen instruments were poised to collect data. Extended Data Table 1 in the paper gives details of each instrument. Seven instruments detected an occultation, with a couple showing a surprising result. At least two instruments detected two short and faint “secondary events” a few seconds before and after the main event. You can see that in Figure 1 of the paper.
As an aside, a part of the music you’re hearing in the background is some more of my “data music”. I’ve taken the data from Figure 1 — the single “primary event” and the two “secondary events” — and turned it into an arpeggio that repeats through the entire song. Can you hear it?
Analysis of the data and fitting to various computer models led the authors to conclude that what they were seeing was a complex ring of material encircling the asteroid. Complex because it appears to be a ring with a gap in it. The distance from the asteroid to the rings were measured to be about 391km and 405km, respectively, with a gap of about 8.7km between them. The widths of the rings were measured to be 6.6km and 3.5km, respectively. This is the first minor-body ring system ever detected! The only other objects in our solar system known to have rings are the four planets Jupiter, Saturn, Uranus, and Neptune. How cool is that???
It turns out that a ring system around Chariklo explains not only the variations in brightness that’s been observed over the years, but also the variations in spectral features. As the asteroid and the Earth orbit the sun, our view of it changes. We see this same effect when we look at the rings of Saturn: sometimes we see the rings more “face-on” and at other times the rings are “edge-on”. Astronomers call this the “opening angle”. Chariklo and it’s system of rings are unresolved — meaning that we’re not able to directly see the rings or any surface features — but we can still detect their light. Sometimes we see more of the rings’ light than at other times because the opening angle is larger. This, the authors argue, explains the variability in brightness. The changing spectrum can be explained if the material that makes up the rings is different than the material on Chariklo itself. The more or less of the rings we can see (a larger or smaller opening angle) the more or less the light from the rings show up in the spectrum. Also from the spectral data, the authors speculate that the rings are a debris disk partially composed of water ice.
The fact that there’s an apparent gap in the rings allows the authors to further speculate that there are as yet undetected kilometer-sized shepherd moons in some kind of orbit around Chariklo that give the rings their complex structure.
The Supplementary Information at the end of the paper gives a great example of the authors trying to fit various computer models to actual data. It’s apparent that a lot of effort was put into creating and working with their models. It’s very nice to see this kind of difficult and detailed work being done.
Unfortunately, it’s very rare for a particular asteroid to occult a bright star that has to be seen at a particular location on Earth at a particular time. So obtaining more data like this is very unlikely. However, it’s not impossible. Large telescopes are not required to observe occultations. You don’t need to see the occulting object (like an asteroid), just the star that it’s occulting. The only things required is a telescope large enough to see the occulted star, and a recording device like an audio recorder or a camera attached to the telescope. Seeing an occultation with the unaided eye is extremely rare. However, observing an occultation of a 12th-16th magnitude star within 100km of a particular location happens several times a year. I encourage you all with the right equipment and know-how to make observing and measuring astroidal occultations of stars part of your regular observing routine. Very little is known about the Centaur asteroids, and just like the authors of this paper, you never know what you might discover.
Until next apogee — I bid you Peace.
End of podcast:
365 Days of Astronomy
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