by Paul Gilster | Oct 24, 2016 | Exoplanetary Science |
Determining the age of a star is not easy, but one way of proceeding with at least some degree of confidence is to identify the star as a member of a stellar association. Here we’re talking about a loose cluster of stars of a common origin. Over time, the stars have begun to separate, but they still move together through space. It was the Armenian astronomer Viktor Ambartsumian, the founder of the Byurakan Observatory, who discovered the nature of these associations and demonstrated that they were composed of relatively young groups of stars.
Stellar associations, or young moving groups (YMGs), provide an outstanding place to study the evolution of protoplanetary disks around young stars, for all associated stars have a similar age. Indeed, their galactic motion can be traced back to their place of origin. Another benefit: Exoplanets in such infant systems are often still hot, well within the capabilities of our near-infrared direct imaging techniques. Many direct imaging and disk evolution surveys in recent years have focused on the members of young moving groups.
Without Ambartsumian’s discovery of these groups, we would be hard pressed to come up with the age of the interesting red dwarf tagged AWI0005x3s. We’ve just learned, through a team led by Steven Silverberg (University of Oklahoma), working with a group of citizen scientists using the Disk Detective site, that this star has a warm circumstellar disk, an interesting find in its own right because we haven’t found many disks around red dwarfs.
But AWI0005x3s is found in the Carina association, on the order of 200 light years away in the Carina nebula, and appears to be moving with it. That pegs the young red dwarf as highly unusual, as Silverberg explains:
“Most disks of this kind fade away in less than 30 million years. This particular red dwarf is a candidate member of the Carina stellar association, which would make it around 45 million years old [like the rest of the stars in that group]. It’s the oldest red dwarf system with a disk we’ve seen in one of these associations.”
Image: Artist’s concept of the newly discovered disk. Credit: Jonathan Holden.
Looking through the paper, I learned that the intriguing AWI0005x3s disk would be the oldest ever observed around an M-dwarf, assuming the star can be confirmed as a member of Carina (the authors argue that the star has a probability of over 90 percent of being part of the association). Astronomers have found a disk frequency of about 6 percent around M-dwarfs less than 40 million years old, dropping to 1.3 percent around older members of this stellar class.
So where does AWI0005x3s fit in? The paper contrasts what we see in M-dwarfs with other types of star:
…debris disks are detected around 32 ± 5% of young A stars with Spitzer/MIPS (Su et al. 2006), and around 1?6% of old (? 670 Myr) Sun-like (F5-K9) stars with Spitzer/MIPS (Urban et al. 2012). Survival models predict that M dwarf debris disks occur at a similar frequency as disks around Sun-like stars, and that the dearth of detections to date is either due to systems having blackbody-like dust close to their central star, or due to systems having a smaller amount of dust distributed over a larger orbital separation (Heng & Malik 2013).
But other possibilities are still in play, including accelerated disk dissipation through interactions with a young stellar wind. Its age places AWI0005x3s in a potentially useful place in relation to other M-dwarfs, as the paper makes clear:
Our new M dwarf debris disk would bridge the gap between YMG and field M dwarf disks. Given their common spectral type (both M5.5V), this system could be a young analog for the Proxima Centauri system (Anglada-Escudé et al. 2016), as well.
The authors believe that AWI0005x3s is a potential target for study via adaptive optics on large telescopes and should be within range for high-contrast imaging, which could allow us to resolve the structure of the disk and potentially identify exoplanet candidates.
As for Disk Detective, it’s well worth a look. In fact, some 30,000 people have gotten involved in viewing short videos from surveys like the Wide-field Infrared Survey Explorer mission (WISE) and Two-Micron All Sky Survey (2MASS) projects, with some two million classifications of celestial objects now achieved. Eight of the citizen scientists involved are listed as co-authors on the AWI0005x3s paper, which is now available online.
The paper is Silverberg et al., “A New M Dwarf Debris Disk Candidate in a Young Moving Group Discovered with Disk Detective,” Astrophysical Journal Letters Vol. 830, No. 2 (14 October 2016). Abstract / preprint.
by Paul Gilster | Oct 21, 2016 | Outer Solar System |
Witnessing Titan’s ever-changing seasons has been a major payoff of the Cassini mission, whose end is now close enough (September, 2017) to cause us to reflect on its accomplishments. We now see winter settling in firmly in the southern hemisphere, along with a strong vortex now developing over the south pole. When Cassini arrived in 2004, we saw much the same thing, only in the northern hemisphere. Athena Coustenis (Observatoire de Paris) is presenting results on Titan’s climate at the ongoing joint meeting of the American Astronomical Society’s Division for Planetary Sciences and 11th European Planetary Science Congress.
“Cassini’s long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time,” says Dr. Coustenis. “We arrived at the northern mid-winter and have now had the opportunity to monitor Titan’s atmospheric response through two full seasons. Since the equinox, where both hemispheres received equal heating from the Sun, we have seen rapid changes.”
The overall cycle of heat circulation on Titan is clearly defined. Warm gases rise at the summer pole as cold gases subside at its winter equivalent. The equinox occurred on Titan in 2009, and since then Cassini has observed a reversal of the system. A strong, revolving pattern of circulation, or vortex, has developed in the stratosphere over the south pole, one that is enriched in trace gases that are otherwise rarely found in Titan’s atmosphere. Cassini also revealed an atmospheric hot spot developing at high altitudes within months of the equinox, while its counterpart in the northern hemisphere had greatly diminished two years later.
Image: Slipping into shadow, the south polar vortex at Saturn’s moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan’s atmosphere. Images like this, from NASA’s Cassini spacecraft, lead scientists to conclude that the polar vortex clouds form at a much higher altitude — where sunlight can still reach — than the lower-altitude surrounding haze. This view looks towards the trailing hemisphere of Titan (5,150 kilometers across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013. Credit: NASA/JPL-Caltech/Space Science Institute.
Within the polar vortex over the south pole, trace gases are accumulating as sunlight diminishes. Here again the parallel is direct. We now see, according to this Europlanet news release, the appearance of complex hydrocarbons and nitriles like methylacetylene and benzene, which were before observed only at high northern latitudes. Coustenis again:
“We’ve had the chance to witness the onset of winter from the beginning and are approaching the peak time for these gas-production processes in the southern hemisphere. We are now looking for new molecules in the atmosphere above Titan’s south polar region that have been predicted by our computer models. Making these detections will help us understand the photochemistry going on.”
While the onset of winter led to a swift temperature drop of 40 degrees Celsius in the stratosphere over the southern pole, the warming effects in the northern hemisphere as the seasons change have been much more gradual, with a 6-degree rise since 2014. In these northerly regions, Cassini has found trace gases that persist into the summer. Although these should eventually disappear, Coustenis says an area of depleted molecular gas and aerosols has emerged across the entire northern hemisphere at an altitude of 400-500 kilometers.
High altitudes on Titan are, in other words, complicated, and while we’re developing a consistent picture thanks to Cassini’s twelve years of observations, these complex effects bear further study. Remember that although we’re entering Cassini’s last year, we have a two-part endgame to go through that involves a final close flyby of Titan to reshape the spacecraft’s orbit. In its new trajectory, Cassini will make 22 passes through the gap between the rings and the planet.
The so-called Grand Finale begins in April of 2017 and takes us to a first dive through the ring/planet gap on April 27. It should be quite a ride, with the closest observations ever made of Saturn, including mapping the planet’s magnetic and gravity fields at high precision, along with samples of particles in the main rings and gases from Saturn’s outer atmosphere. In addition, we should get spectacular views of the rings when, in November of this year, Cassini begins a series of 20 passes just beyond the outer edge of the main rings. Cassini has not gotten this close to the rings since its arrival at Saturn in 2004; we’ll see the ring structure at high resolution. The spacecraft’s final dive into Saturn is planned for September 15, 2017.
“While it will be sad to say goodbye, Cassini’s final act is like getting a whole new mission in its own right,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California. “The scientific value of the F ring and Grand Finale orbits is so compelling that you could imagine an entire mission to Saturn designed around what we’re about to do.”
by Paul Gilster | Oct 20, 2016 | Outer Solar System |
Considering how long we’ve been thinking about a massive planet in the outer Solar System — and I’m going all the way back to Percival Lowell’s Planet X here — the idea that we might find the hypothetical Planet Nine in just three years or so is a bit startling. But Caltech’s Mike Brown and colleague Konstantin Batygin, who predicted the existence of the planet last January based on its effects on Kuiper Belt objects, are continuing to search the putative planet’s likely orbital path, hoping for a hit within the next few years, a welcome discovery if it happens.
The duo are working with graduate student Elizabeth Bailey, lead author of a new study being discussed at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, which is occurring in conjunction with the European Planetary Science Congress. The new paper is all about angles and alignments, focusing on the fact that the relatively flat orbital plane of the planets is tilted about six degrees with respect to the Sun. That’s an oddity, and Planet Nine, hypothesized to be about ten times the mass of the Earth and in an orbit averaging 20 times Neptune’s distance from the Sun, just may be the cause.
The calculations on display in the new paper depict a planet some 30 degrees out of alignment with the orbital plane of the other planets. That can help to explain orbital observations of Kuiper Belt objects, but also the unusual system-wide tilt, which stands out because of the assumed formation of the planets through the collapse of a spinning cloud into a disk and, eventually, a collection of planets orbiting the Sun. We would expect the angular momentum of the planets to maintain a rough alignment with the Sun along the orbital plane.
Unless, of course, something is disrupting the system. Throw in the angular momentum of Planet Nine, based on its assumed mass and distance from the Sun, and profound effects on the system’s spin become evident, creating a long-term wobble that shows up in the system’s tilt. As Bailey puts it, “Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment.”
And this from the paper:
… a solar obliquity of order several degrees is an expected observable effect of Planet Nine. Moreover, for a range of masses and orbits of Planet Nine that are broadly consistent with those predicted by Batygin & Brown (2016); Brown & Batygin (2016), Planet Nine is capable of reproducing the observed solar obliquity of 6 degrees, from a nearly coplanar configuration. The existence of Planet Nine therefore provides a tangible explanation for the spin orbit misalignment of the solar system.
Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC).
The six-degree tilt we see between planetary disk and Sun thus fits into the team’s calculations regarding Planet Nine’s size and distance from the central star. And if this does indeed turn out to be the explanation, speculation will then center on how Planet Nine came to be so far out of line with the other planets. We know that gravitational interactions in young planetary systems can sometimes result in disruption, causing some planets to be thrown out of their systems, and others to be moved into distant orbits. Such gravitational byplay may well be the reason for Planet Nine’s unusual position. Now we just need to discover the planet.
I also want to mention that Renu Malhotra (University of Arizona) and team have continued their analysis of a possible Planet Nine, likewise presenting their results at the AAS/EPSC meeting in Pasadena. Through analysis of what they call ‘extreme Kuiper Belt Objects’ —on eccentric orbits with aphelia hundreds of AU out — the team finds a clustering of orbital parameters that may point to the existence of a planet of 10 Earth masses with an aphelion of more than 660 AU. Two orbital planes seem possible, one at 18 degrees offset from the mean plane, the other inclined at 48 degrees.
Dr. Malhotra confirmed in an email this morning that her own constraints on the current position of this possible planet line up with Mike Brown and team at Caltech. But her team continues to point out that we have no detection at this point, and much to learn about the orbits of the Kuiper Belt objects under study. From her paper:
…we note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, depending on the planet mass; if so, this would weaken the argument for a resonant planet orbit. In future work it would be useful to investigate scattering efficiency as a function of the planet mass, as well as dynamical lifetimes of non-resonant planet-crossing orbits in this region of the outer system. Nevertheless, the possibility that resonant orbital relations could be a useful aid to prediction and discovery of additional high mass planets in the distant solar system makes a stimulating case for renewed study of aspects of solar system dynamics, such as resonant dynamics in the high eccentricity regime, which have hitherto garnered insufficient attention.
The Bailey, Batygin & Brown paper is “Solar Obliquity Induced by Planet Nine,” accepted for publication in the Astrophysical Journal (preprint). The Malhotra paper is Malhotra, Volk & Wang, “Coralling a Distant Planet with Extreme Resonant Kuiper Belt Objects,” Astrophysical Journal Letters Vol. 824, No. 2 (20 June 2016). Abstract / preprint.
by Paul Gilster | Oct 19, 2016 | Outer Solar System |
We’re getting close on New Horizons data, all of which should be downlinked as of this weekend. Although that’s a welcome marker, it hardly means the end of news from the doughty spacecraft. For one thing, we have years of analysis ahead of us as we bring the abundant data from the spacecraft’s instrument packages into focus. For another, we’re still in business out there in the Kuiper Belt, heading for that interesting object 2014 MU69.
Who knows what will turn up at the latter, given our propensity to be surprised at every turn in interplanetary exploration, from Triton’s volcanic plains to fabulously fractured Miranda. And, of course, Pluto and Charon themselves, which turned out to be so interesting that Alan Stern, principal investigator for New Horizons, is already talking about future missions there.
But back to 2014 MU69, which has continued to be the subject of Hubble observations even as New Horizons homes in on the object. As this news release from the mission points out, MU69 is the smallest KBO ever to have its color measured, a reddish hue that confirms its identity as part of the ‘cold classical’ region of the Kuiper Belt. These are objects with low orbital eccentricity and inclination that are not in orbital resonance with Neptune. Reddish-brown tholins formed by solar radiation interacting with simple organic compounds are common here.
“The reddish color tells us the type of Kuiper Belt object 2014 MU69 is,” says Amanda Zangari, a New Horizons post-doctoral researcher from Southwest Research Institute. “The data confirms that on New Year’s Day 2019, New Horizons will be looking at one of the ancient building blocks of the planets.”
Image: 2014 MU69 travels diagonally across a dense field of stars and noise in the background. Credit: NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team.
New Horizons has now covered a third of its distance from Pluto to MU69, with the target approximately a billion kilometers away. The analysis of New Horizons data, meanwhile, is turning up interesting things on Charon, where we find landslides, a feature that has not yet been spotted on Pluto’s surface, although we’ve found them on worlds as diverse as Mars and Iapetus. The Charon landslides are the farthest ever observed from the Sun.
Image: Scientists from NASA’s New Horizons mission have spotted signs of long run-out landslides on Pluto’s largest moon, Charon. This image of Charon’s informally named Serenity Chasma was taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) on July 14, 2015, from a distance of 78,717 kilometers. Arrows mark indications of landslide activity. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Likewise, we learn that while Pluto’s atmosphere is hazy but largely cloud-free, a handful of possible clouds have turned up in New Horizons imagery. That would point to an atmosphere still more complex than expected. And the variations in surface brightness on Pluto itself are telling. Some of these regions, particularly in Pluto’s now famous heart-shaped region, are among the most reflective in the Solar System. This has implications for what may be occurring on another deep space object, says Bonnie Buratti (JPL), a co-investigator on the New Horizons science team:
“That brightness indicates surface activity. Because we see a pattern of high surface reflectivity equating to activity, we can infer that the dwarf planet Eris, which is known to be highly reflective, is also likely to be active.”
Image: Pluto’s present, hazy atmosphere is almost entirely free of clouds, though scientists from NASA’s New Horizons mission have identified some cloud candidates after examining images taken by the New Horizons Long Range Reconnaissance Imager and Multispectral Visible Imaging Camera, during the spacecraft’s July 2015 flight through the Pluto system. All are low-lying, isolated small features-no broad cloud decks or fields – and while none of the features can be confirmed with stereo imaging, scientists say they are suggestive of possible, rare condensation clouds. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
We’re a long way from through with New Horizons, which should make its flyby of 2014 MU69 on January 1, 2019 after the series of four course changes that adjusted its trajectory. We may have other outer system news to discuss as the joint meeting of the American Astronomical Society Division for Planetary Sciences and European Planetary Science Congress in Pasadena continues this week. But for now, I particularly like Alan Stern’s words:
“We’re excited about the exploration ahead for New Horizons, and also about what we are still discovering from Pluto flyby data. Now, with our spacecraft transmitting the last of its data from last summer’s flight through the Pluto system, we know that the next great exploration of Pluto will require another mission to be sent there.”
by Paul Gilster | Oct 18, 2016 | Uncategorized |
An antimatter sail, as described yesterday in the work of Gerald Jackson and Steve Howe, is an exciting idea particularly because it relies on only small amounts of antimatter, tapping its energies to create fission in a uranium-enriched sail. Thus the uranium is the fuel and the antimatter, as Jackson says, is the ‘spark plug.’ We reduce the needed amount of antimatter and define what the new Kickstarter campaign calls “…the first proposed antimatter-based propulsion system that is within the near-term ability of the human race to produce.”
The antimatter sail produces fission by allowing antimatter, stored probably as antihydrogen, to drift across to the sail, and as we saw yesterday, the potential for velocities up to 5 percent of lightspeed mean that such a sail could be deployed on interstellar missions. Proxima Centauri naturally emerges as a target, but Jackson and Howe’s work is not a result of recent interest in that star and its one known planet. The 2002 study in which they describe the antimatter sail was originally created for a probe to 250 AU (a Kuiper Belt and heliopause mission), drawing on work on a 10 kg instrument payload that was done at the Jet Propulsion Laboratory.
Image: Antiproton striking the depleted uranium coating on a carbon sail. Credit: Gerald Jackson/Hbar Technologies.
The just launched Kickstarter campaign is to re-think that earlier work in the context of an unmanned mission to a nearby solar system. Among the specific campaign goals is to create a detailed design for long-lived antimatter storage, a key issue in any such concept. After all, we have to store the antimatter in such a way that it does not annihilate with the normal matter surrounding it. We’ve known that this can be done for some time. In fact, it was back in 1984 that physicist Hans Dehmelt demonstrated how to hold a single positron in a cylinder using electric and magnetic fields. Dehmelt gave his device the name ‘Penning trap,’ a nod to Frans Penning, a Dutch physicist who saw that a magnetic field could steer electrons into tight orbits.
Dehmelt would go on to be awarded the Nobel Prize in Physics in 1989 for co-developing the Penning trap technique with Wolfgang Pauli (each shared one-half of the prize). By adjusting the voltage and strength of the magnetic field, the Penning trap would become a viable way to store tiny amounts of antimatter.
But as antimatter storage has become feasible, problems grow as we begin storing more and more of the stuff. Try to contain large amounts of positrons or antiprotons and their like charges repel. Thus large quantities of antimatter — and compared to current levels of production, we need large quantities indeed — experience repulsive forces between them that quickly become stronger than the magnetic container can handle. Soon the magnetic ‘bottle’ begins to leak and the antiparticles are destroyed.
In his book Antimatter (Oxford University Press, 2010), Frank Close notes that even a millionth of the amount of antimatter needed for a Mars trip would create tons of electric force on the walls of the tank. That’s a daunting thought, and this is for a nearby target, although Close isn’t thinking in terms of the antimatter sail concept, which minimizes the amount of antimatter needed. Even so, high-capacity storage of antimatter has to be addressed.
What Jackson and Howe have been looking at for their antimatter sail involves storing the antimatter in the form of antihydrogen (a positron orbiting an antiproton). Here we’re at the heart of the original work the duo did for NASA’s Institute for Advanced Concepts, from which the antimatter sail took root. A key goal of the new Kickstarter campaign is to produce a design report describing how to build an antihydrogen storage bottle that can be used aboard a spacecraft. Their extensive experience with the issues makes Jackson and Howe an ideal team to push spacecraft antimatter storage forward.
I’m looking back at the original NIAC report Steve Howe prepared for NASA, which envisions storing antihydrogen in the form of frozen pellets rather than in traps, the idea being to use integrated circuit chips of the kind we have become familiar with through today’s microprocessors. We would deploy the same kind of etching technology to create a series of tunnels on each chip, with wells at periodic intervals where the antihydrogen pellets would be held. Changing the voltage allows pellets to move down these tunnels from well to well.
Remember the concept here: The antimatter, as it emerges from the storage device (held some 12 meters behind the sail) is then accelerated so that it drifts out into contact with the sail. But Jackson and Howe’s thinking is clearly evolving on this matter. The plan discussed on the Kickstarter site is to create an antihydrogen storage bottle based on methods Robert Millikan and Harvey Fletcher used in the early 20th Century to measure the charge of the electron. As this involved observing charged oil droplets between two metal electrodes, its uses may supercede the kind of chip technology originally envisioned. And further research, Jackson says, may actually involve antilithium rather than antihydrogen as the optimum form of antimatter.
A key requirement, of course, is portability — this is a storage method that has to be applicable to spacecraft. At Hbar Technologies’ headquarters, Jackson and Howe have the first portable storage bottle for such uses, one able to store positrons or antiprotons at liquid helium temperatures in a hard vacuum. Jackson’s experience with storage also extends to the design and construction of particle accelerator storage rings. Forcing storage technology forward is the need to house amounts of antimatter too large to store as charged elementary particles.
Image: Original portable antimatter storage bottle built by Penn State University and JPL.
I’ve focused on storage, but it’s clear that a lot of things have to go right to get to an interstellar antimatter sail. Key parameters for the Kickstarter effort are to drill down further on antimatter storage issues while at the same time describing renewed antiproton production possibilities at Fermi National Accelerator Laboratory. The effort will recast the original study by way of firming up or adjusting earlier parameters in sail physics and engineering. The thrust-test apparatus developed through the NIAC grant will be upgraded and test facilities identified; here I would imagine experiments involving uranium-laden foils and antiproton interactions. Jackson and Howe also want to design an antihydrogen experiment to be funded in a later campaign.
To me, the possibility of a renewed and improved antiproton process at Fermilab is quite interesting, as we’ve seen what minute amounts our technologies are currently able to generate. In addition to an investigation like this, I would imagine Jackson and Howe will want to look at James Bickford’s ideas on natural antimatter production within the Solar System (see, for example, Antimatter Acquisition: Harvesting in Space, or search the archives here for more).
Bickford argues that space harvesting of antimatter is five orders of magnitude more cost effective than producing antimatter on Earth, an idea Jackson and Howe may want to contest if the highly restricted antimatter yields at Fermilab can be adjusted and improved. For more, see the Kickstarter page for this effort and keep an eye on Jackson and Howe’s Antimatter Drive site. The page there is not yet populated, but the intent is to archive all previous work on the antimatter sail, including a great deal of continuing studies on these storage concepts.
