Centauri Dreams

Cataclysmic variable stars (CVs) are binary phenomena, usually consisting of a white dwarf that is accreting material out of a nearby companion star. As you would imagine, a wide range of CVs in various stages of accretion and subsequent outburst can be detected. When the accretion disk around the white dwarf becomes unstable, we get what is known as a dwarf nova (DN), and in systems with orbital periods less than two hours, there can be much more violent outbursts, feeding off orbital resonances in the orbit of the two stars.

Now we have a newly discovered cataclysmic variable (KSN:BS-C11a) in an interesting configuration, a white dwarf apparently feeding off a brown dwarf companion that is about 10 times less massive. The ‘super-outburst’ from the dwarf nova turned up in data from the decommissioned Kepler Space Telescope. Grad student Ryan Ridden-Harper (Australian National Observatory), lead author of the paper on this work, likes to refer to this cataclysmic variable as ‘a vampire star system.’ Whatever we call it, the level of activity here is noteworthy:

“The incredible data from Kepler reveals a 30-day period during which the dwarf nova rapidly became 1,600 times brighter before dimming quickly and gradually returning to its normal brightness. The spike in brightness was caused by material stripped from the brown dwarf that’s being coiled around the white dwarf in a disk. That disk reached up to 11,700 degrees Celsius at the peak of the super-outburst.”

Image: An artist’s impression of a star ‘feeding’ off a nearby brown dwarf. Credit: NASA and L. Hustak (STScI)

Still unexplained is the slow rise in brightness that preceded the outburst. Ridden-Harper has been working with colleagues at ANU as well as the Space Telescope Science Institute (STScI) and the University of Notre Dame. What catches my eye about the discovery of this bright transient is the data mining that turned it up. The team had been searching for new transients in the K2 and Kepler campaigns in a project known as K2: Background Survey. The idea here is that each science target in the data is accompanied by background pixels that have been observed at high cadence, meaning a short time period before re-observing the same target.

These background pixels, the authors note, can contain transient signals that have thus far gone undetected. The paper describes the K2: Background Survey as:

…a systematic search for transients in K2/Kepler background pixels. K2:BS independently analyses each pixel to detect abnormal behaviour. This is done by searching for pixels that rise above a brightness threshold set from the median brightness and standard deviation through a campaign. Telescope motion presents a challenge in candidate detection as science targets may drift into background pixel, triggering false events. False triggers are screened by vetting of events that last < 1 day, chosen for candidates with the 6 hourly telescope resets. Coincident pixels that pass the vetting procedure are grouped into an event mask. All candidate events are checked against the NASA/IPAC Extragalactic Database (NED) 1 and the SIMBAD database (Wenger et al. 2000) to identify potential hosts.

As to the parameters of the system, the brown dwarf orbits the white dwarf every 83 minutes at a distance of approximately 400,000 kilometers, close enough to allow the rapid growth of the accretion disk as material spirals from the brown dwarf inward toward the host star. The Kepler cadence of 30 minutes turns out to have been the key to making these observations, which are particularly useful because only about 100 such dwarf novae systems have been catalogued.

Image: This is Figure 3 from the paper. Caption: The K2/Kepler light curve of the transient KSN:BS-C11a shown with the 30-minute cadence. The time axis is shown in barycentric Julian days and the flux has been converted to Kepler magnitudes. Credit: Ridden-Harper et al.

Thus Kepler/K2 keeps on producing good science, in this case finding a transient whose rarity stems partially from the years or decades such a system can spend between outbursts. We shouldn’t be too surprised that Kepler data can produce such results — after all, the telescope was designed to study the transits of planets across the face of a star, making it ideal for spotting any objects that brighten or dim over time — but now we can see the potential for detecting other rare transient events both in archival data and data from ongoing missions like TESS.

The paper is Harper et al., “Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data,” Monthly Notices of the Royal Astronomical Society,” Vol. 490, Issue 4 pp. 5551-5559 (abstract).

Yesterday’s post on the Spitzer Space Telescope leads naturally to the targets it produced for its successor. For when Spitzer’s mission ends on January 30, we have the far more powerful James Webb Space Telescope, also operating at infrared wavelengths, in queue for a 2021 launch. In many ways, Spitzer has been the necessary precursor for JWST, for it was the need to operate a telescope at extremely low temperatures in order to maximize infrared sensitivity that drove Spitzer design. JWST must maintain its gold-coated beryllium mirror at similarly precise temperatures.

With over 8,700 scientific papers published based on Spitzer findings, a number that will continue to grow for many years, a path has been charted that JWST will follow in the form of observations early in its mission. Consider WASP-18b, a gas giant of ten times Jupiter mass in a tight orbit around its star. Data from both Spitzer and Hubble showed in 2017 that the planet is laden with carbon monoxide and all but devoid of water vapor. No other extrasolar planet can match this one in the way carbon monoxide dominates its upper atmosphere.

What’s going on in the atmosphere of this planet merits close study because it’s extreme even for ‘hot Jupiters’ in being so close to its star that it may not survive another million years. Expect a long look from JWST into the processes at work here. Nikku Madhusudhan (University of Cambridge) was a co-author on the 2017 paper describing the WASP-18b findings:

“The only consistent explanation for the data is an overabundance of carbon monoxide and very little water vapor in the atmosphere of WASP-18b, in addition to the presence of a stratosphere. This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres.”

The image below implies the method: Transmission spectroscopy. We can look at the light of the star passing through the atmosphere of the planet as it moves around the star in its orbit.

Image: A NASA-led team of scientists determined that WASP-18b, a “hot Jupiter” located 325 light-years from Earth, has a stratosphere that’s loaded with carbon monoxide, or CO, but has no signs of water. Credit: Goddard Space Flight Center.

At TRAPPIST-1, expect the fourth planet, TRAPPIST-1e, to receive early JWST scrutiny because of its density and surface gravity, both similar to Earth’s, in combination with incoming stellar flux sufficient to keep temperatures in the range needed for water on the surface. JWST should be able to tell us whether this planet does indeed have an atmosphere, and assuming it does, whether molecules like carbon dioxide or water vapor are present.

Here again Spitzer helped set the table, working with ground-based telescopes to confirm the first two candidates (found by the Transiting Planets and Planetesimals Small Telescope in Chile) and discover the other five. Our ideas of what these planets look like will change with the new data JWST, 1000 times more powerful than Spitzer, will bring in. The Hubble instrument has not been able to detect evidence for a hydrogen-dominated atmosphere on TRAPPIST-1d, e and f, making rocky composition likely. But it’s going to take JWST to further clarify the presence of atmospheres on the seven worlds and begin the study of their chemistry.

All of that should take what is a very fanciful image (below) and help us determine how far from reality it actually is. At present we’re simply injecting sparse data into the realm of art. As Nikole Lewis (Cornell University) says, “The diversity of atmospheres around terrestrial worlds is probably beyond our wildest imaginations. Getting any information about air on these planets is going to be very useful.”

Image: This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech.

Spitzer’s work on 55 Cancri e will also inform early JWST studies of the system. Spitzer was able to produce data leading to the first temperature map of a super-Earth, in this case an apparently rocky world about twice the size of our own. Lava flows may be the cause of the extreme temperature swings between one side of the planet and the other, as noted in 2016 by Brice Olivier Demory (University of Cambridge), who was lead author of the paper in Nature:

“Our view of this planet keeps evolving. The latest findings tell us the planet has hot nights and significantly hotter days. This indicates the planet inefficiently transports heat around the planet. We propose this could be explained by an atmosphere that would exist only on the day side of the planet, or by lava flows at the planet surface.”

Spitzer put in 80 hours of infrared telescope time in the 55 Cancri e work, watching this tidally locked world move about its star and allowing the construction of the temperature map. Mission scientists pushed Spitzer hard in accumulating their data, using novel calibration techniques to extract maximum results from a detector that had not been design to work at such high precision. Now JWST will help sharpen the map’s focus to explain its unusual temperature swings, which argue against a thick atmosphere and global temperature distribution.

Image: 55 Cancri e is tidally locked to its star, just as our moon is to Earth, which means that one side always sizzles under the heat of its star while the other side remains in the dark. If the planet were covered in lava, then the hot, sun-facing side of the planet would have liquid lava flows, while the colder, dark side would see solidified lava rock. The hardened lava would be unable to transport heat across the planet, explaining why Spitzer detected that the cold side of the planet is much colder than the hot side. Such a lava planet, if it exists, would have dust streaming off of it, as illustrated here. Radiation and winds from the nearby star would blow off the material. Scientists say that future observations with NASA’s upcoming James Webb Space Telescope should provide more details about the nature of this exotic world. Credit: NASA/JPL-Caltech.

Both Spitzer and Webb are sensitive to the infrared glow of gas and dust orbiting in circumstellar rings around stars, which means JWST will be able to extend our knowledge of planetary formation. The same sensitivity will make JWST the instrument of choice in the study of brown dwarfs, an area where Spitzer has already been able to examine clouds in brown dwarf atmospheres. What’s interesting here are the differences between the distribution and motion of clouds on brown dwarfs and the atmospheric boiling seen on true stars. JWST will investigate winds that seem reminiscent of the belts of Jupiter, Saturn, Uranus and Neptune.

I’ve focused on exoplanetary targets here, but of course the handoff from Spitzer to JWST will also involve using the large surveys of Spitzer and the Hubble instrument to furnish JWST with targets like GN-z11, now the most distant galaxy ever measured. Sean Carey is manager of the Spitzer Science Center at Caltech/IPAC in Pasadena:

“Spitzer surveyed thousands of galaxies, mapped the Milky Way and performed other groundbreaking feats by looking at large areas of the sky. Webb won’t have this capability, but it will revisit some of the most interesting targets in the Spitzer surveys to reveal them in amazing clarity.”

JWST’s higher sensitivity should make it possible to find galaxies that are even older. It will also home in on luminous infrared galaxies (LIRG), which Spitzer found to be producing far more energy per second than typical galaxies, most of it in the form of far-infrared light. Star formation and galactic mergers come into focus, as does the growth of supermassive black holes. All of this depends, of course, on getting a fabulously complex telescope plagued by cost overruns into its future home at the L2 Lagrangian point 1.5 million kilometers from the Earth.

All launches are scary, but this one more than most. We need Spitzer’s successor to fly.

The Spitzer Space Telescope, which is to end its mission on January 30, has a special place in my memory. I was making a trip to the Jet Propulsion Laboratory as part of the research for my Centauri Dreams book when I noticed on a monitor a countdown — still in days — for the launch of Spitzer, then known as the Space Infrared Telescope Facility (SIRTF).

The observatory was launched on the 25th of August, 2003. I remember hot Pasadena weather, a conversation with aerospace legend Adrian Hooke (he was a member of the Kennedy Space Center launch team for Apollo 9, 10, 11 and 12, among much else), a rousing talk with Humphrey “Hoppy” Price about interstellar possibilities. So many good conversations, some serious interviews, and a growing enthusiasm for interstellar flight.

But Spitzer had my attention because it was the next mission, one of the Great Observatory missions which included the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and Spitzer itself. The infrared observatory lasted longer than anyone expected and continued to function even after exhausting its helium coolant in 2009, at which point the ‘cold mission’ ended, but the ‘warm mission’ would last over a decade.

‘Warm’ and ‘cold’ are obviously relative terms when we talk about objects in space. With a full stock of coolant, the spacecraft could maintain temperatures as low as minus 267 degrees Celsius, so yes, that’s ‘cold.’ But Spitzer functioned a long way from Earth, so that even the ‘warm’ mission could operate at minus 244° C, and that enabled continued observations in two infrared wavelengths. We got more than 16 years of use out of Spitzer, another case among many of a mission’s science team working all the angles to keep their craft alive.

Image: The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility or SIRTF) is readied for launch at Cape Canaveral Air Force Station, in 2003. Credit: NASA.

The beauty of working at infrared wavelengths is that you can see things that are too cold to emit much visible light. For our purposes, that includes brown dwarfs and exoplanets, but the observatory has also excelled at studying distant galaxies, including (in conjunction with Hubble observations) the most distant galaxy yet observed, GN-z11, which is seen in the direction of Ursa Major as it was 13.4 billion years in the past. Again of direct relevance to Centauri Dreams readers is Spitzer’s work on interstellar dust, whose chemical composition, studied via spectroscopy, can help us learn more about what goes into planet formation.

This is the instrument that, in 2007, gave us the first identification of molecules in the atmosphere of an exoplanet — two of them, actually. HD 209458b and HD 189733b are ‘hot Jupiters’ and the use of transmission spectroscopy to study their atmospheres points us toward the future prospect of searching for biosignatures on small, rocky planets like Earth. In 2010, Spitzer was part of the discovery of a planet 13,000 light years away, found via gravitational microlensing. Back in 2005, it made direct observations of the ‘hot Jupiters’ HD 209458b and TrES-r1, the first telescope to directly observe light from a planet in another stellar system.

Image: Spitzer was the first telescope to directly observe light from a planet outside our solar system. Prior to that, exoplanets had been observed only indirectly. This accomplishment kicked off a new era in exoplanet science, and marked a major milestone on the journey toward detecting possible signs of life on rocky exoplanets. Two studies released in 2005 reported direct observations of the warm infrared glows from two previously detected “hot Jupiter” planets, designated HD 209458b and TrES-r1. Hot Jupiters are gas giants similar to Jupiter or Saturn, but are positioned extremely close to their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths. Credit: NASA/JPL-Caltech.

I won’t even try to go through all of Spitzer’s accomplishments, but I do need to mention the seven Earth-sized planets around TRAPPIST-1, which Spitzer data, over 500 hours worth, helped scientists identify via transit methods. And I have to include the ‘weather map’ of HD 189733b, a chart of temperature variations over the surface of a gas giant. Sean Carey, manager of the Spitzer Science Center at Caltech, has this to say about the instrument:

“When Spitzer was being designed, scientists had not yet found a single transiting exoplanet, and by the time Spitzer launched, we still knew about only a handful. The fact that Spitzer became such a powerful exoplanet tool, when that wasn’t something the original planners could have possibly prepared for, is really profound. And we generated some results that absolutely knocked our socks off.”

So true! From its Earth-trailing orbit, Spitzer was a long way from our planet’s heat and could offer greater sensitivity than other instruments, especially considering that it could detect some infrared wavelengths that could not be seen through the filter of our atmosphere. What a marvelous run for this observatory, whose decommissioning leads us directly into the launch of the infrared James Webb Space Telescope. We can only hope that the highly complex JWST makes it safely into space and that it lives to see as many mission extensions (5) as the Spitzer Space Telescope.

Image: This dazzling infrared image from NASA’s Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view. In this false-color picture, old and cool stars are blue, while dust features lit up by blazing hot, massive stars are shown in a reddish hue. Both bright and dark filamentary clouds can be seen, many of which harbor stellar nurseries. The plane of the Milky Way’s flat disk is apparent as the main, horizontal band of clouds. The brightest white spot in the middle is the very center of the galaxy, which also marks the site of a supermassive black hole. Credit: NASA/JPL-Caltech.

Spitzer was a pathfinder in many areas of astronomical research, but a kind of personal pathfinder for me, as I’ve always associated its observations with my work as a chronicler of this field. An observatory designed to study “the cold, the old and the dusty,” as NASA likes to say, taught us about objects as close as nearby asteroids and as far as the oldest galaxies. The frustrations of human spaceflight in the post-Apollo era have to be balanced against the brilliance of science missions like Spitzer, which have charted a path forward that should one day lead to direct observations of planets where life may even now be flourishing.