A large part of the fascination of astronomy is the discovery of objects that don’t fit our standard definitions. KIC 8462852 — ‘Tabby’s Star’ — is deeply mysterious and high on my watchlist. But yesterday we also looked at CX330, a so-called FUor of the kind that brightens enormously over years of observation. Today we have another strange one, a system called AR Scorpii, where a white dwarf star in a binary system is releasing a blast of radiation onto a nearby red dwarf. The entire system brightens and fades every 1.97 minutes, a phenomenon that has only recently been properly understood.
“AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in June 2015,” says Tom Marsh (University of Warwick), lead author of the paper on this work. “We realised we were seeing something extraordinary the more we progressed with our observations.”
Those observations proceeded with data from the European Southern Observatory’s Very Large Telescope (Chile) and the Isaac Newton Group of telescopes at La Palma (Canary Islands), along with other ground-based resources and the Hubble and Swift instruments in space. European amateur astronomers also played a key role in studying the star’s behavior.
AR Scorpii is about 380 light years from Earth. Its white dwarf component is roughly Earth-sized though containing 200,000 times the mass, and the associated red dwarf is about one-third the mass of the Sun. This is a tight system, with the two objects orbiting one another every 3.6 hours. What researchers have found is that the white dwarf is accelerating electrons almost to the speed of light in a beam that sweeps across the face of the M-dwarf. The brightening is dramatic, but the emissions range all the way from X-rays to radio wavelengths.
Image: This artist’s impression shows the strange object AR Scorpii. In this unique double star a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star (left) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. Credit: M. Garlick/University of Warwick, ESA/Hubble.
AR Scorpii was classified in the 1970s as a Delta Scuti variable, a kind of star (also known as a dwarf cepheid) that shows luminosity variations due to pulsations on the star’s surface. Such variables are useful as standard candles that help astronomers calculate stellar distances. But Marsh and team discovered that AR Scorpii’s pulsations were not the result of a single variable star but a binary system in which intense brightness variations were occurring. The pulses are strong enough that the star’s optical flux can increase by a factor of four within 30 seconds.
The nature of the AR Scorpii pulses is what intrigues the researchers. From the paper:
Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions, and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies.
The team’s calculations show that the 1.97 minute brightness pulsations reflect the spin of a magnetic white dwarf, one that is slowing down on a timescale of 107 years.
Although the pulsations are driven by the white dwarf’s spin, they originate in large part from the cool star. AR Sco’s broad-band spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.
Synchrotron radiation involves the acceleration of charged particles in a magnetic field, but as the quote above shows, what the researchers don’t yet know is the source of the electrons. The kind of pulsations observed here have been seen before in neutron stars but AR Scorpii is the first white dwarf system to show similar behavior. The paper notes that white dwarfs and neutron stars are the only two types of object that can support a misaligned magnetic dipole and spin fast enough to match the observed pulsations. The paper goes to some length to demonstrate that the AR Scorpii pulsations are consistent only with a white dwarf and not a neutron star.
The paper is Marsh et al., “A radio pulsing white dwarf binary star,” published online in Nature 27 July 2016 (abstract).
Given that we have fewer than a dozen examples, highly variable stars like the recently discovered CX330 have much to teach us. These stars have been nicknamed FUors, after FU Orionis, a pre-main sequence star that has shown huge variations in magnitude over the past century. Eruptions like these may be common, as Alan Boss argued last year (see A Disruptive Pathway for Planet Formation), but as we learn more about them, we have to account for dramatic changes, as when the star V1057 Cyg increased in brightness by 5.5 magnitudes over the course of a few years. What does this do to an associated circumstellar disk?
As we ponder these questions, we also have to account for CX330, which comes into the news this week because of its odd isolation. Star-forming clouds packed with young stars are rich in gas and dust, and it is in these that we find all other examples in the FU Orionis category. But CX330 is a thousand light years away from the closest region of star formation, and because it is thought to be no more than a million years old, it’s hard to explain how it could have migrated from a star-forming region without losing much of the disk it now seems to be consuming.
From a new paper on CX330:
In the YSO [Young Stellar Object] interpretation, the most interesting characteristic of CX330 is its position on the sky. It is 2? above the Galactic Plane, and well outside of any known star forming region. All known large magnitude outbursting YSOs such as FUors are located in star forming regions, likely in part because of selection biases but the actual distribution is also concentrated in these regions… Additionally, CX330 must have formed in situ, since its measured proper motion with the VVV survey has an upper limit of 3.3 mas [milliarcseconds] yr?1. The phase of stellar evolution in which disk instability events occur is generally thought to be within 106 years after the formation of the disk…so that it can have drifted less than a degree since the time of formation.
Image: This artist’s concept shows an unusual celestial object called CX330 that was first detected as a source of X-ray light in 2009 by NASA’s Chandra X-Ray Observatory while it was surveying the bulge in the central region of the Milky Way. A 2016 study in Monthly Notices of the Royal Astronomical Society found that CX330 is the most isolated young star that has been discovered. Researchers compared NASA’s Wide-field Infrared Survey Explorer (WISE) data from 2010 with NASA’s Spitzer Space Telescope data from 2007 to come to this conclusion. Credit: NASA/JPL-Caltech.
What we can say is that CX330, isolated as it is, is in the midst of an outbursting period that allowed its detection at X-ray wavelengths as well as visible light, with surveys like the above mentioned VVV (VISTA Variables in The Via Lactea) helping to measure its output. Infrared studies with data from the Wide-Field Infrared Survey Explorer (WISE) made it clear to Chris Britt (Texas Tech) that the object was associated with a large amount of warm dust that would have been heated in the outburst. From 2007 to 2010, according to WISE and Spitzer data, the star increased in brightness by several hundred times. CX330 appears more massive than known FU Orionis objects, with strong interactions with the gas and dust around it as material falls onto the star.
“CX330 is both more intense and more isolated than any of these young outbursting objects that we’ve ever seen,” said Joel Green, study co-author and researcher at the Space Telescope Science Institute in Baltimore. “This could be the tip of the iceberg — these objects may be everywhere.”
Which leads me back to Alan Boss’ 2015 paper. In it, Boss (Carnegie Institution for Science) modeled interactions between protoplanetary disks and their stars, studying in particular periods of gravitational instability in the disk that can scatter larger objects (1 to 10 meters) away from the star. What we wind up with is a star that consumes about half the gaseous disk mass but allows these larger objects to grow into planetesimals. Thus we have a mechanism for planet formation despite infalling gas and violent upheavals in the star’s accretion disk.
If outbursts like these are relatively short in astronomical terms, we wouldn’t have observed many of them, and Joel Green’s comment could be right on target. In fact, we may learn that most or all stars go through periods of violent development in their youth. This would follow a hierarchical model of star development in which a surrounding gas cloud reaches a critical density and begins to collapse onto the young star. The paper argues that this may explain stars like CX330:
Hierarchical SF [star formation] posits that stars form in a fractal distribution on a smoothly varying range of scales dominated by turbulence rather than magnetic support… There is evidence of hierarchical SF as a result of supernovae shocks…and in at least one association of massive stars in a larger SF region… If star formation proceeds in a fractal distribution of scales, some few stars should be observed to form even in total isolation from the cloud complexes which have been the focus of large star formation studies.
In any case, the effect of outbursts from young stars could be a vital element in planet formation. Finding more stars in the FU Orionis class would be useful, particularly since no FU Orionis star has ever been observed closing down from the high luminosity phase. We’re dealing with a short period in a star’s evolution but one that holds clues to the growth of planetary systems.
The paper is Britt et al., “Discovery of a Long-Lived, High Amplitude Dusty Infrared Transient,” Monthly Notices of the Royal Astronomical Society, Vol. 460, Issue 3 (18 May, 2016). Abstract available. The Alan Boss paper is “Orbital Survival of Meter-size and Larger Bodies During Gravitationally Unstable Phases of Protoplanetary Disk Evolution,” Astrophysical Journal Vol. 807 (July 1, 2015), No. 1, 10 (abstract).
Speculating about what an advanced extraterrestrial civilization might do has kept us occupied for the last two days, with gas giants like Jupiter the primary topic of conversation. We don’t know if it’s possible to ignite a gas giant to provide new sources of energy. But with Juno getting ready to measure Jupiter’s aurorae, we’re looking at naturally produced energy today, and now we have interesting work on the planet’s Great Red Spot that comes out of Earth-based observations. The enormous storm turns out to be a key factor in heating Jupiter’s atmosphere.
And what a storm it is. We knew about the Great Red Spot as early as the 17th Century because its span — three Earth diameters — qualifies this highly visible maelstrom as the largest hurricane we know of. Winds can take six days to complete one circuit of the Great Red Spot, which has varied in size and color ever since it was discovered. It is now observed to span 22,000 km by 12,000 km in longitude and latitude, respectively.
The Great Red Spot gives us a source of energy to heat Jupiter’s upper atmosphere but thus far we have lacked evidence of its effect upon temperatures. Now an analysis based upon new infrared data is changing our view of temperatures high above Jupiter’s visible disk.
Image: Acquiring Jovian spectra. Bright regions at the poles result from auroral emissions; the contrast at low- and midlatitudes has been enhanced for visibility. Great Red Spot (GRS) emissions at mid latitudes are indicated by the red arrow. Additional info: The vertical dark line in the middle of the image indicates the position of the spectrometer slit, which was aligned along the rotational axis of Jupiter. Image shown is taken from the slit (slit-jaw imaging) using the “L-filter” (3.13 – 3.53 ?m). Credit: J. O’Donoghue, NASA Infrared Telescope Facility (IRTF).
The results come from James O’Donoghue (Boston University) and colleagues, who report their findings on the matter today in Nature, using 2012 data from the SpeX spectrometer on NASA’s Infrared Telescope Facility in Hawaii. The issue caught the astronomers’ attention because at mid- to low latitudes, temperatures in Jupiter’s upper atmosphere are hundreds of degrees warmer than heating from the Sun can explain. We’re looking at non-Solar energy whose sources could be studied by creating heat maps of the entire planet.
This is what the O’Donoghue set out to do, realizing that what his team refers to as an ‘energy crisis’ occurs not just on Jupiter but on other giant planets as well. One explanation for Jupiter has been auroral heating mechanisms that pump energy into the upper atmosphere. But the low to mid-latitudes lack this kind of heat source and yet remain 600 K warmer than can be explained by solar heating. The paper makes the case for a different kind of source:
A more likely energy source is acoustic waves that heat from below (also via viscous dissipation); this form of heating requires vertical propagation of disturbances in the low-altitude atmosphere. Acoustic waves are produced above thunderstorms, and the subsequent waves have been modelled to heat the Jovian upper atmosphere by 10K per day and on Earth have been observed to heat the thermosphere over the Andes mountains. On Jupiter, acoustic-wave heating has been modelled to potentially impart hundreds of degrees of heating to the upper atmosphere. However, to the best of our knowledge, no such coupling between the lower and upper atmosphere has been directly observed for the outer planets, so vertical coupling has not been seriously considered as a solution to the giant-planet energy crisis.
The team found that high altitude temperatures on Jupiter are greater than expected when the Great Red Spot is directly below. In fact, the atmosphere above this region is hundreds of degrees hotter than anywhere else on the planet. The temperature spike above the Great Red Spot points to coupling between lower and upper atmosphere. The authors believe the heating is caused by atmospheric turbulence that rises because of the shear between the storm and surrounding atmosphere, with propagating waves depositing their energies high above.
Image: Turbulent atmospheric flows above the storm produce both gravity waves and acoustic waves. Gravity waves are much like how a guitar string moves when plucked, while acoustic waves are compressions of the air (sound waves). Heating in the upper atmosphere 800 kilometers above the storm is thought to be caused by a combination of these two wave types ‘crashing’ like ocean waves on a beach. Credit: Art by Karen Teramura, UH IfA, James O’Donoghue
Co-author Tom Stallard (University of Leicester) puts the work into the context of ongoing missions like Juno:
“This fantastic result, showing how the upper atmosphere is heated from below, was produced directly from Leicester’s 2012 observing campaign, which was designed to try and answer why Jupiter’s upper atmosphere is so hot. Juno will be measuring the aurora and its sources, and we expected the auroral energy to flow from the pole to the equator. Instead, we find the equator appears to be heated from plumes of energy coming from Jupiter’s vast equatorial storms.”
The paper is O’Donoghue et al., “Heating of Jupiter’s upper atmosphere above the Great Red Spot,” Nature, published online 27 July 2016 (abstract).
When Nikolai Kardashev looked into the question of where to find advanced extraterrestrial civilizations, he argued that the obvious starting point would be in the vicinity of extreme astrophysics. Active galactic nuclei (AGN) come to mind, or even the centers of comparatively quiet galaxies like our own. Clément Vidal picked up the same point in his The Beginning and the End (Springer, 2014), arguing persuasively that we should consider how black holes could be used, perhaps by manipulating the merger of such objects. And yes, this is astroengineering utterly beyond our skills, but possibly not those of an advanced ETI.
Using black holes for energy is extreme, but Roger Penrose has imagined a super-civilization extracting black hole rotational energy by the injection of matter, and there are a number of other propositions on how such advanced engineering might work. Extracting energy from a black hole’s accretion disk might be the most efficient method, but lower-grade operations could exist around neutron stars. To that idea we might add, as Milan ?irkovi? does in the paper we looked at yesterday, the exploitation of X-ray binaries or quasars like SS433.
The new ?irkovi? paper homes in on gas giants and brown dwarfs, with the possibility of making either into a star. It’s an idea with a popular pedigree, the ignition of Jupiter having gone viral with the film 2010, but the real action is beyond the Solar System entirely. Consider that the number of substellar objects in interstellar space has been estimated to be as high as 105 times greater than the number of main sequence stars (see Island-Hopping to the Stars for more on this estimate and the kind of substellar objects it references).
Image: Artist’s impression of a free-floating gas giant. Credit: NASA/JPL-Caltech.
Could a technologically manipulated gas giant or brown dwarf be a SETI observable? Here we can look at several possibilities. The luminosity of a ‘stellified’ object should be greater than its mass would lead us to expect for natural objects. And, taking the long view, its luminosity should evolve differently from natural stars. Moreover, we might find anomalies in the spectra of such objects, especially early and late in their astronomically-brief lifetimes. And bear in mind that stellified objects would be bright power sources, unlike artificial orbital habitats or other large structures which would only reflect light or become apparent through their thermal emissions.
In other words, if such objects exist, they would be useful targets for Dysonian SETI investigation. ?irkovi? notes that determining the mass of a field star is a tricky proposition, and if we’re considering possible artifacts, we can’t just try to position the object on the Hertzsprung-Russell diagram as if it were a normal main sequence star. An accurate mass determination would require a multiple star system, and even here the measurement is fraught with uncertainties. But where we can determine it, mass is worth pursuing.
From the ?irkovi? paper, referring to Martyn Fogg’s 1989 paper on stellified gas giants:
The outliers from the low-mass stellar luminosity-mass relation deserve our best observational scrutiny, especially if the anomaly is extreme. In Fogg’s model, for instance, luminosity of [a] stellified Jupiter will fairly soon after the beginning of the process be ten orders of magnitude or so above the expected luminosity of such a low-mass free-floating object. If such artefacts are numerous in the Galaxy, their considerably easier detectability could deform and leave an imprint on the substellar mass function.
The evolution of luminosity in such objects would be challenging to gauge because of the timeframes involved — Fogg estimated a 50 million year span of exponentially increasing luminosity in the case of a stellified gas giant like Jupiter. As ?irkovi? notes, we could easily have such artifacts in our stellar catalogs now, for depending on their stage of stellification, they would simply mimic a particular type of star. We seem to be best off in hunting for stellified objects at the beginning and end of their lifetimes, looking for gamma and X-ray flares, for example, during the early stages of a planet’s transformation into a star.
Even so, we still are dealing with relatively transient phenomena compared to main sequence star lifetimes, and advanced technologies about which we can only speculate. Remember that the scenario Martyn Fogg originally came up with assumes using a small black hole, merging it with Jupiter in a carefully controlled orbit that eventually brings it toward the planet’s center. 100 million years of habitability are provided to the Jovian satellite system, but several hundred million years later, runaway accretion would have to be prevented. ?irkovi? comments:
For the initial phase, the period of unstable bursting and flaring can be shortened by sufficiently gently placing the mini-black hole into the substellar objects; for the final phase, duration and properties of instabilities depend on the manner of removing the excess mass from the black hole, as well as on the existing installation/swarm surrounding it. If such anomalies are observed in a planetary system containing at least one planet in the circumstellar habitable zone, this could be further incentive to give it high priority as a SETI target.
My take on all this is that as we are at the beginning of Dysonian SETI, we’re early in the process of developing the necessary parameters. ?irkovi? speaks to this point near the end of this paper, calling for improved quantitative models for the kind of astroengineering projects we can imagine and their possible SETI signatures. The advantage of stellified gas giants is that they are larger and simpler than many of the conjectured astroengineering projects that have been proposed, but we would want to have sound models for a wide range of possibilities.
And there’s the nub of the problem: We’d like to be able to observe an anomaly in our astronomical data and relate it swiftly to a potential technology, using what we believe to be its observables. But Dysonian SETI is built around the concept of abandoning anthropocentrism and simply observing. How does a Kardashev sub-Type I culture like ours envision what a Type II might do? Our conjectures invariably grow out of our preconceptions, and the models we build can only be crude templates. Rather than abandoning the process, we have to keep all this in mind, continually adjusting our assumptions while being alert for data that fit no previous niche.
The ?irkovi? paper is “Stellified Planets and Brown Dwarfs as Novel Dysonian SETI Signals,” in press at JBIS. I also referenced a Nikolai Kardashev paper above; it’s “On the inevitability and possible forms of supercivilizations”, in The Search for Extraterrestrial Life: Recent Developments, ed. M.D. Papagiannis, IAU, Dordrecht, pp.497-504, 1985.
The SETI concepts now called ‘Dysonian’ are to my mind some of the most exhilarating ideas in the field. Dysonian SETI gets its name from the ‘Dyson spheres’ and ‘Dyson swarms’ analyzed by Freeman Dyson in a 1960 paper. This is a technology that an advanced civilization might use to harvest the energy of its star. You can see how this plays off Nikolai Kardashev’s classification of civilizations; Kardashev suggested that energy use is a way to describe civilizations at the broadest level. A Type II society is one that can use all the energy of its star.
In the film 2010, director Peter Hyams’ 1984 adaptation of Arthur C. Clarke’s novel 2010: Odyssey Two (Del Rey, 1982), we see an instance of this kind of technology at work, though it has nothing to do with a Dyson sphere. In the film, a dark patch appearing on Jupiter signals the onset of what Martyn Fogg has called ‘stellification,’ the conversion of a gas giant into a small star. Rapidly replicating von Neumann machines — the famous monoliths — increase Jupiter’s density enroute to triggering nuclear fusion.
A new star is born, with consequences entertainingly explored in the novel’s epilogue. Without monoliths to work with, Fogg described another way of triggering a gas giant’s fusion reaction in a 1989 paper. A small black hole could be put into orbit around the planet, its orbit gradually sinking toward the planetary center. Accretion will eventually cause the new star to shine like a red dwarf, its brightness steadily increasing over a 50 million year period. Parts of the Jovian satellite system could be rendered continuously habitable over a period of about 100 million years, even as the star-builders exploit its energies via orbiting power stations.
Image: 2010’s cinematic depiction of runaway replication in progress on Jupiter. Credit: Peter Hyams/Metro-Goldwyn-Mayer.
True, the process would one day have to be arrested, for runaway accretion will eventually, according to Fogg’s calculations, present a danger to these worlds, though presumably the civilization that can create the new star in the first place can also figure out how to tame it. These timeframes are extravagant, of course, and the engineering is far beyond our own, but as Milan ?irkovi? points out in a new paper, we should consider such stellified objects as potential SETI signatures. Dysonian SETI thus expands to a broad search for anomalous uses of energy.
Having never observed an extraterrestrial civilization, can we plausibly look for one? Here’s how ?irkovi?, the author of The Astrobiological Landscape (Cambridge University Press, 2012) and numerous papers, frames the question:
Copernicanism implies that we should reason as if humanity is a typical member of the set of all intelligent species evolved in naturalistic manner in all epochs. Therefore, what we expect in humanity’s future is also likely to occur at some point in the evolutionary trajectory of at least a significant subset of other intelligent species, both those present in the Galaxy nowadays, and those from past or future. If humans could perform an engineering feat X at some point in our future for clearly utilitarian reasons, we should expect at least some other intelligent species in the Galaxy to have already performed the same (or similar enough) X, provided they are sufficiently older from us. In accordance with such “mirroring” of human future and possible evolutionary trajectories of advanced extraterrestrial civilizations in the Galaxy, we may wish to investigate how the procedure of stellification might look from afar and consider it a new form of detection signature in the sense of SETI studies.
Notice that whatever the target, Dysonian SETI makes no assumptions about communications or contact with other civilizations. When we work at radio or optical wavelengths, we are looking for ephemeral signals, most likely some kind of a beacon that announces the existence of the culture that built it. The new Dysonian strategy puts detection times into a much deeper timeframe. We make no social or cultural assumptions and, in fact, can make no conjectures about the beings behind any artifact we find in our searches. One exciting consequence is that a SETI detection may already be present in our abundant stores of astronomical data.
The study of the anomalous star KIC 8462852 likewise touches on Dysonian SETI. While there have been brief attempts to study this object for evidence of power beaming (see SETI: No Signal Detected from KIC 8462852), the star has also been the subject of intense investigation historically, with researchers like Bradley Schaeffer and Michael Hippke reaching different conclusions about whether or not old photographic plates show a steady dimming. Here we’re using astrophysics with no cultural assumptions to delve into a phenomenon that is probably natural, but one so mysterious that we still can’t rule out advanced engineering.
But back to stellification and the question of energy. Let’s ask this: If there were a civilization capable of engineering at a solar system-wide scale, what would it do? The creation of a small star within a solar system is one way to proceed, and in Clarke’s novel it paves the way for the creation of new life on Europa. But the material for stellification is hardly confined to a single system. Usefully, we have large numbers of brown dwarfs and unbound, ‘rogue’ planets between the stars. As ?irkovi? notes, we have resources here not just for fuel but for habitation and industry with significant amounts of metals in relatively shallow gravitational wells.
The key question is, what sort of signature would this kind of stellification produce? More on this tomorrow, as we look a little deeper into Dysonian methods and speculate not only on the uses of thermonuclear fusion but the utilization of other kinds of energy. For if we’re trying to find evidence of astroengineering, extreme astrophysical sources may be the places to look.
The paper is ?irkovi?, “Stellified Planets and Brown Dwarfs as Novel Dysonian SETI Signals,” in press at JBIS. Martyn Fogg’s paper is “Stellifying Jupiter: A first step to terraforming the Galilean satellites,” JBIS 42 (1989), 587-592..
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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