The Case of the Disappearing Dust

Astronomy has always taught us that planets form from vast clouds of dust and gas orbiting young stars. It’s a gradual process of accretion that takes hundreds of thousands, perhaps even millions, of years… or does it?

During a 1983 sky survey with the Infrared Astronomical Satellite (IRAS) astronomers identified a young Sun-like star with a large cloud of dust surrounding it. The star, named TYC 8241 2652 1, is 450 light years away and what they had found around it was thought to be the beginnings of a solar system – the protoplanetary disc from which planets form.

Fast forward to 2008. Astronomers observed at the same star with a different infrared telescope, the Gemini South Observatory in Chile. What was observed looked a lot like what was previously seen in ’83.

Then, in 2009, they looked again. Curiously, the brightness of the dust cloud was only a third of what it was the year before. And in WISE observations made the very next year, it had disappeared entirely.

“It’s like the classic magician’s trick: now you see it, now you don’t. Only in this case we’re talking about enough dust to fill an inner solar system, and it really is gone.”

– Carl Melis, lead author and postdoctoral fellow at UC San Diego


“It’s as if you took a conventional picture of the planet Saturn today and then came back two years later and found that its rings had disappeared,” said study co-author and circumstellar disk expert Ben Zuckerman of UCLA.

It’s always been thought that planets take some time to form, in the order of hundreds of thousands of years. Although that may seem like forever to humans, it’s quick in cosmic time scales. But if what they’ve seen here with TYC 8241 is in fact planetary formation, well… it may happen a lot faster than anyone thought.

On the other hand, the star could have somehow blown all the dust out of the system. More research will be needed to see if that was the case.

The really interesting thing here is that astronomers have traditionally looked for these kinds of dust clouds around stars to spot planetary formation in action. But if planets form quicker than we thought, and the dust clouds are only fleeting features, then there may be a lot more solar systems out there that we can’t directly observe.

“People often calculate the percentage of stars that have a large amount of dust to get a reasonable estimate of the percentage of stars with planetary systems, but if the dust avalanche model is correct, we cannot do that anymore,” said study co-author Inseok Song, assistant professor of physics and astronomy at the University of Georgia. “Many stars without any detectable dust may have mature planetary systems that are simply undetectable.”

Read more in the news release from the University of Georgia.

Top image: Gemini Observatory/AURA artwork by Lynette Cook.

M33’s “Object-X”

Often times, objects that are unremarkable in one portion of the spectra, can often be vivid in others. In M33, the Triangulum Galaxy, a star that’s barely visible in the optical, stands out as the second brightest source (and single brightest single star) in the mid-infrared. This unusual star has been the target of a recent study, led by Rubab Khan at the Ohio State University and may help astronomers to understand an unusual supernova from 2008.

The supernova 2008S occurred February first in NGC 6946, the Fireworks Galaxy. Since it happened in a galaxy that is relatively nearby, astronomers seized the opportunity to explore the progenitor star in archival images. Yet images from the Large Binocular Telescope and other optical observatories could not find a star that could be identified as a parent. Instead, the detection of the star responsible came from Spitzer, an infrared observatory. Observations from this instrument indicated that the star responsible may have been unexpectedly low mass for such a powerful explosion leading other astronomers to question whether or not SN 2008S was a true supernova, or merely an impostor in the form of an eruption of a Luminous Blue Variable (LBV), which tend to be more massive stars and would be in stark contradiction to the Spitzer findings.

Yet, regardless of the nature of the nature of SN 2008S, teams all seemed to agree that the progenitor had only been detected in the infrared because it was veiled by a thick curtain of dust. So to help better understand this class of dusty stars, astronomers have been working to uncover more of them, against which they can test their hypotheses.

To find these objects, astronomers have been searching the infrared portion of the spectrum for objects that are exceptionally bright yet lack optical counterparts. The brightest of the stellar sources in M33 features faint star in the red portion of the optical spectrum from the Local Group Galaxies Survey published in 2007, but no star at all in archival records with similar limiting magnitudes from 1949 and 1991. The authors of the new study have dubbed this odd source, Object-X.

The team rules out the possibility that the object could be a young stellar object (YSO), blocked by a thick dust disc along the line of sight, noting that models of even the thickest dust discs still predict more light to be scattered back along the line of sight. Instead, the team concludes that Object-X must be a self-obscured star that has undergone relatively recent mass loss which has cooled to form either graphite or silicate dust. Depending on which type of dust is predominant, the team was able to fit the data to two wildly different temperatures for the star: either 5000 K for graphite, or 20,000 K for the silicate. In all cases, the predicted mass for the central star was always greater than 30 solar masses.

In general, there are two mechanisms by which a star can eject material to form such a curtain. The first is through stellar winds, which increase as the star enters the red giant phase, swelling up and lowering the force of gravity near the surface. The second is “impulsive mass ejections” in which stars shudder and throw mass off that way. A classical example of this is Eta Carinae. The team predicts from the features they found, that Object-X is most likely a cool hypergiant. The fact that the star was completely obscured until very recently hints that the mass loss is not constant (as stellar wind), but patchy, coming from frequent eruptions. As the shell of dust expands, the star should reemerge in the optical, becoming visible again in the next few decades.

Could Mars Dust Be “Levitated” Away?

What could potentially be the biggest problem during a human mission to Mars? One NASA study says, surprisingly, that dust could be the number one risk for both humans and equipment. Human explorers could inhale the extremely fine but rough dust particles causing severe respiratory problems, and high winds on Mars could disperse the dust to coat solar panels, penetrate through seals and interfere with machinery. But scientists at the University of Vermont may have come up with a new way to combat dust: acoustic levitation. But will it work on Mars?

The researchers conducted a feasibility study to develop an acoustic dust removing system for use in space stations or habitations on the Moon or Mars. They found a high-pitched (13.8 kHz, 128 dB) standing wave of sound emitted from a 3 cm aperture tweeter and focused on a reflector 9 cm away was strong enough to dislodge and move extremely fine (<2 µm diameter) dust particles on the reflector surface. The sound waves overcome the van der Waals adhesive force that binds dust particles to the surface, and creates enough pressure to levitate the dust, which is then blown away. The team tested the system on a solar panel coated with mock lunar and Martian dust. The output of the clean panel was 4 volts, but when coated with dust it produced only 0.4 volts. After four minutes of acoustic levitation treatment the output returned to 98.4% of the maximum. Mars dust, although fine, is rougher that Earth dust, and likely is more similar to the dust that covers the Moon. The thin atmosphere on Mars means dust particles are not as rounded as they would be on Earth and can remain quite sharp and abrasive. [/caption] Mars dust, as we have found with the Mars rovers, has a high electrostatic charge, which means the fine dust clings to everything. The dust has severely decreased the efficiency of solar panels on the rovers, and over time has likely caused other problems with the mechanical operation on the rovers as well. We've had several articles here on Universe Today discussing the problems of dust on the solar panels of the Mars Exploration Rovers, and inevitably we get comments from readers suggesting "wiper blades" or other types of cleaning solutions for the solar panels. Amazingly, Mars itself has cleaned the rovers' solar panels several times with gusts of wind from the almost ubiquitous Martian dust devils. Acoustic levitation could be a solution, as it would be cheap and easily built. But there is a problem, and it is a big one: it will only work when it is sealed inside a space station or other habitation. It will not work where there is no atmosphere (such as the moon) or where the atmosphere is low pressure and thin (such as Mars) because sound is a pressure wave that travels through the air. So, we might be stuck with having to resort to wiper blades, or devising a way to mimic the dust devils and gusts of wind that have repeatedly benefited the Mars rovers. Unless we can figure out a way to get dust to levitate without sound. Nirvana anyone? Source: PhysOrg

Mysterious Alien Dust Hints at Violent Planet Formation

Image credit: Lynette Cook for Gemini Observatory/AURA

An artist’s rendition of colliding planets, the most likely explanation for the warm dust observed around HD 131488. Image credit: Lynette Cook for Gemini Observatory/AURA

Five-hundred light years away, worlds are colliding, and they’re made of nothing we’ve ever seen.

Last week at the 215th American Astronomical Society meeting, UCLA astronomers announced that they had found warm dust – evidence for the violent collision of rocky planets – around a star called HD 131488. The strange thing is, the composition of the dust has little in common with the composition of rocky bodies in any other known system.

“Typically, dust debris around other stars, or our own Sun, is of the olivine, pyroxene, or silica variety, minerals commonly found on Earth,” said Dr. Carl Melis, who led the research as a graduate student at UCLA. “The material orbiting HD 131488 is not one of these dust types. We have yet to identify what species it is – it really appears to be a completely alien type of dust.”

The warm dust in the HD 131488 system is concentrated in an area close to the star, where temperatures are similar to those on Earth. The researchers concluded that the most likely source for dust in that part of the system would be the collision of two rocky planetary bodies. Only five other stars like HD 131488 with dust in their terrestrial planet zone are known. “Interestingly, all five of these stars have ages in the range of 10-30 million years,” Melis said. “This finding indicates that the epoch of final catastrophic mass accretion for terrestrial planets, the likes of which could have resulted in the formation of the Earth-Moon system in our own Solar System, occurs in this narrow age range for stars somewhat more massive than the Sun.”

The team also discovered a unique second dusty region in the outer reaches of the HD 131488 system, comparable to the location of Pluto and other Kuiper Belt objects in our own solar system.
Image Credit: Lynette Cook for Gemini Observatory/AURA

Top: Illustration depicting the location of the warm and cold dust rings in the HD131488 system. Bottom: Comparable regions in our own solar system, with the orbits of the outer planets for scale. Image Credit: Lynette Cook for Gemini Observatory/AURA

“The hot dust almost certainly came from a recent catastrophic collision between two large rocky bodies in HD 131488’s inner planetary system,” Melis said. “The cooler dust, however, is unlikely to have been produced in a catastrophic collision and is probably left over from planet formation that took place farther away from HD 131488.”

“…for some reason stars that have large amounts of orbiting warm dust do not also show evidence for the presence of cold dust. HD 131488 dramatically breaks this pattern,” said Dr. Benjamin Zuckerman, a co-author on the paper and a professor of physics and astronomy at UCLA.

With its unusual dust composition and unique combination of warm and cold dust regions, the HD 131488 system is now under intense scrutiny. Melis and colleagues plan to continue trying to determine the composition of the dust, and will search for other stars with the dusty evidence for planet formation.

Source: Gemini Observatory