April 2010 Archives
...well, not quite, we still have a few reports and video entries to file over the next couple of days, so do check back here next week. We've had a great time reporting from the conference and meeting lots of really inspirational people, and we hope you've enjoyed following our reports. Now, with volcanic ash suspending all flights out of Scotland, I know how I'll be getting home...!
(Taken in Glasgow City Centre)
How do we know what kinds of stars explode as supernovae in distant galaxies? A new in-depth study has attempted to tackle this thorny problem by searching for the suspected progenitor stars of type Ib/c supernovae. This supernovae are the collapse of masses stars that seem to possess no hydrogen. There are stars like this, called Wolf-Rayet stars, which are some of the most massive stars in the Universe (above 20 solar masses), and have strong stellar winds that blow away their outer envelope of hydrogen. This exposes the inner layers of carbon and nitrogen.
During a session of lectures at NAM today, entitled 'Explosions in the Distant Universe', Joanne Bibby of the University of Sheffield presented results of a survey of 11 galaxies all within a distance of 33 million light years that are being searched in detail for Wolf-Rayet stars. For one galaxy in particular, NGC 7793, which is 13 million light years away 52 Wolf-Rayet stars were found, 27 being Wolf-Rayet stars with exposed nitrogen that are believed to be the progenitors of Type Ib supernovae, and 25 with exposed carbon layers that make Type Ic supernovae. In total Bibby estimates that 80 percent of all nitrogen Wolf-Rayet stars, and 90 percent of all carbon Wolf-Rayet stars, have now been found in NGC 7793. The upshot of this is that the next time there is a Type Ib/c supernova in NGC 7743, the chances are that we will know which star it is that has exploded, allowing us to put more constraints on our understanding of which stars explode, and why. Image: ESO.
Astronomy Now: How were MBCs first discovered and what sets them aside from asteroids?
Henry Hsieh: The first MBC, 133P/Elst-Pizarro, was discovered as a comet in 1996 and attracted some attention at the time for its strange asteroid-like orbit, but eventually faded from interest mainly because I think people just didn't know what to make of it, and as a single fluke object, it was easy enough to ignore. In 2002, David Jewitt and I reobserved the return of activity in the comet, strongly suggesting that it was the result of a sublimation-driven process, rather than an impact debris cloud (since it would be exceedingly strange if one object experienced an impact in the same part of its orbit in the span of six years when we don't see impacts at anywhere near the same frequency for other asteroids).
Since dynamically it was shown to be very difficult for an "ordinary" comet (i.e. from the outer Solar System) to evolve onto a main-belt orbit, we reasoned that 133P was probably native to the main belt. If this were true (that 133P was an "ordinary" asteroid), other asteroids might show similar behaviour, though it might be fairly rare and difficult to detect (i.e. requiring large telescopes). At the University of Hawaii, time on large telescopes was one thing that we had an abundance of, so we set out to survey a large number of carefully selected main-belt asteroids for comet-like activity. In October 2005, another comet, P/2005 U1 (Read), was discovered orbiting in the main belt, and then coincidentally in November 2005, our survey turned up a third, 176P/LINEAR (so-named because the original asteroid was discovered by the asteroid survey project LINEAR, even though we discovered that it was actually a comet). With now three known comets in the main belt, we had demonstrated that 133P was no fluke, and furthermore calculated that there could be around 100 more such objects in the asteroid belt. We published the paper announcing our findings in 2006, naming the new group of objects "main-belt comets". Since then, just one more MBC has come to light, bringing the current total to four known MBCs. These are 133P/Elst-Pizarro, P/2005 U1 (Read) (P/Read, for short), 176P/LINEAR, and P/2008 R1 (Garradd) (P/Garradd, for short). When P/2010 A2 (LINEAR) was discovered recently, it was thought to be a 5th MBC, but initial indications are that it is not in fact a cometary body (i.e. emitting dust due to ice sublimation) but is in fact the result of an asteroid collision, which is interesting in its own right, but just not for tracing ice in the asteroid belt.
Two of the known MBCs imaged using the UH 2.2m telescope.
More images on Hsieh's website - http://star.pst.qub.ac.uk/~hhh/mbcs.shtml
As far as we can tell so far, not much separates the MBCs from other asteroids, at least the ones in the same region of the main belt. Main-belt asteroids show a fair amount of variation depending on their distance from the Sun, and so the MBCs are certainly different from objects in the inner belt, but among asteroids beyond 3AU from the Sun (where the MBCs are found), all measurements so far show that they are mostly unremarkable, except for the fact that they emit dust like comets from time to time. One main difficulty in finding new MBCs is that activity is transient, lasting over only ~1/4 of the orbit for an "activated" asteroid (i.e., that has received an impact recently) and probably only lasting 100-1000 years after an activation has occurred. After that, the activity is thought to die away, and then a new impact is required to renew the activity. As such, one of our main research goals is to find a "special" characteristic of MBCs that set them apart from other asteroids (e.g. a particular spectral feature, or maybe membership in a young, recently-fragmented asteroid family as mentioned in my talk) that would allow us to rapidly pinpoint new MBC candidates in the absence of actual cometary activity. Since "dormant" or "inactive" MBCs are really just asteroids with ice that doesn't happen to be currently sublimating, identifying these really just means we will be able to identify icy asteroids and chart their distribution in the asteroid belt, which is actually our primary goal in all this.
AN: Is it likely that they formed in situ in the asteroid belt, or migrated in as e.g. trans-Neptunian Objects (TNOs) from the outer Solar System?
HH: The strong similarity of the MBCs to other main-belt asteroids in addition to their general orbital stability (except for P/Garradd) strongly suggests that they formed in situ in the asteroid belt. It is extremely unlikely that they recently migrated as TNOs from the outer Solar System. Given the current configuration of the major planets, there is simply no clear and consistent way for an object to make such a orbital transition. When only one MBC was known (133P), people could argue that some unique circumstances (very close approaches to terrestrial planets for example, which are chaotic and are therefore essentially impossible to predict the outcome of) had deposited it into the asteroid belt, but once three MBCs were known (with the likelihood of many more), it became far more difficult to argue for this scenario. Something implausible (in principle) can always happen once, but when it must have happened more than once, you're on much shakier ground. More likely, they are native to the main belt.
The one possible exception is a scenario suggested by Levison et al.'s so-called "Nice model" which suggests that a large number of Kuiper Belt Objects may have been deposited into the main belt during a period of planetary migration (primarily Jupiter and Saturn) linked to the so-called Late Heavy Bombardment (about 3.8 billion years ago). This is a purely theoretical result so far though, as no clear links have been observationally demonstrated between KBOs and main-belt asteroids. Nonetheless, it emphasises the need to keep an open mind as to the origin of the MBCs, but for now, we believe them to be native main-belt objects.
AN: Does their presence in the Asteroid Belt have any implications for theories of Solar System formation?
HH: Their presence in the asteroid belt does not actually present a significant challenge to current theories of Solar System formation. Asteroids in the outer belt have been long known to show evidence of past water/ice in the form of hydrated minerals (i.e. minerals that formed in the presence of water). These minerals were thought to have formed long ago, however, meaning that the water only needed to be present long ago. Over 4.5 billion years (the age of the Solar System), any water was thought to be long baked away by the Sun, so it's probably more accurate to say that the MBCs are telling us something interesting about Solar System evolution, rather than formation, specifically about the survivability of ice in close proximity to the Sun on small (kilometre-scale) bodies.
That said, MBCs provide an opportunity for constraining theories of Solar System formation. We can see if their pristine ice is consistent with the chemical and isotopic composition of hydrated minerals that we've been using thus far to infer the nature of the early Solar System. Once we find enough MBCs to provide a clear picture of the current distribution of ice in the asteroid belt, we can compare this with predictions of various formation models and weed out the inconsistent ones, and so on.
AN: You mentioned that the observed sublimation of ices from the MBCs was likely triggered by impact. Would these have been recent impact events and on what sorts of scale?
HH: We would expect each "pocket" of subsurface ice that is exposed by a single impact on an MBC to eventually all sublimate away, so yes, current activity must be the result of a recent impact. We tentatively estimate that metre-sized (as opposed to 10m-sized or cm-size) impacting bodies (which excavate areas of a few hundred square metres) are responsible for MBC activity, and we expect activity to die away after roughly 100-1000 years. Such collisions probably occur roughly every 10,000 years, so if an object is sufficiently icy that each impact actually triggers activity (an excavating impact won't create an MBC if all it excavates is more rock), each object could be active over roughly 1%-10% of its life.
AN: How likely is it that many other asteroids are concealing ices beneath their surfaces and therefore might also be MBCs?
HH: We believe that many other asteroids might be concealing ices beneath their surfaces, at least in the outer main belt (outside of about 3 AU from the Sun). How much ice is actually contained in these bodies (0.1%? 1%? 10%? 50%?) is unknown though, as is the distance of that ice from the surface of each icy body. Over time, ice in the upper surface layers should sublimate away, even if only slowly, and as ice recedes farther and farther down, it will become more difficult for an impact to excavate it to create an active MBC. I didn't mention this in my talk, but there is also the possibility that large asteroids (10km or larger) may never show observable cometary activity due to their size. We estimated the speed of the dust being emitted from the current MBCs and found it to be generally fairly slow (1-2 metres per second), which as it turns out, is too slow to actually escape the gravity of asteroids much larger than the known MBCs (about 5 km across and smaller), just as rockets must be launched at a certain speed to escape Earth's gravity. This slow dust ejection speed is probably due to small amounts of ice that are sublimating. Much less power is behind MBC dust ejection, relative to other comets, so therefore you get slower ejection speeds. If these speeds are typical of all icy main-belt asteroids, it is entirely possible that large icy asteroids could have their ice excavated and sublimate away all the time, but never produce observable comet-like activity because dust launched by the gas released from ice sublimation simply falls back down to the surface of these larger asteroids. Again, this is an effect we have to keep in mind when using cometary activity as a proxy for tracing water ice in the asteroid belt.
AN: What sort of quantities of ice might be hosted by the MBCs, and are there any astrobiological implications of MBCs in the inner Solar System?
HH: An excellent question, but unfortunately one for which I have no answers for at the moment! Best guess, I'd say MBCs could be a few percent to as much as 10% ice, but this is a *pure* guess. Much more work is needed before we can get anywhere close to a real answer for this. This is actually one of over-arching goals of this research...figuring out how much present-day ice is present in a single MBC, how many MBCs are in the main belt, and therefore how much present-day ice is in the entire asteroid belt. Astrobiology however is mainly concerned with the primordial water content of the main belt (since most of Earth's water was probably delivered soon after its formation, probably during the aforementioned Late Heavy Bombardment, which doesn't require ice to survive for *that* long in the main belt -- hundreds of millions of years, instead of billions of years, which is a fairly significant difference), so we will need to use thermal modelling to extract the original ice distribution of the asteroid belt from the present-day distribution (once we figure *that* out), since ice is likely to have decreased in abundance over the last 4.5 billion years, though given the existence of the MBCs, clearly not as quickly as we previously thought. As mentioned above, the fact that some water is currently stable as ice in main belt asteroids is not terribly relevant to terrestrial formation or astrobiological scenarios, but it *does* represent an intriguing present-day opportunity to probe the cousins of the ancient objects that *did* participate in the delivery of water to Earth all those billions of years ago.
Herschel Space Observatory scientists today met to discuss the progress of their mission, which was launched less than a year ago. AN's deputy editor Emily Baldwin speaks to one of the Principal Investigators, Matt Griffin about what the mission has achieved so far.