by Paul Gilster | Feb 11, 2022 | Exoplanetary Science |
The apparent discovery of a new planet around Proxima Centauri moves what would have been today’s post (on laser-thermal interstellar propulsion concepts) to early next week. Although not yet confirmed, the data analysis on what will be called Proxima Centauri d seems strong, in the hands of João Faria (Instituto de Astrofísica e Ciências do Espaço, Portugal) and colleagues. The work has just been published in Astronomy & Astrophysics.
It’s good to hear that Faria describes Proxima Centauri as being “within reach of further study and future exploration.” That last bit, of course, is a nod to the fact that this is the nearest star to the Sun, and while 4.2 light years is its own kind of immensity, any future interstellar probe will naturally focus either here or on Centauri A and B.
Years are short on Proxima d – the putative planet circles Proxima every five days. That’s a tenth of Mercury’s distance from the Sun, closer to the star than to the inner edge of the habitable zone. Despite some press reports, this is not a habitable zone world.
Where this work is significant isn’t simply in providing us with a third world at Proxima, but in the method of detection. When Guillem Anglada-Escudé and team found Proxima Centauri b back in 2016, they were working with data taken with the HARPS spectrograph mounted on the European Southern Observatory’s 3.6-meter telescope at La Silla. Confirming Proxima b demanded the newer ESPRESSO spectrograph, fed by the four Unit Telescopes (UTs) of the Very Large Telescope at Cerro Paranal. That work was accomplished in 2020 by researchers from the University of Geneva.
For the work on Proxima Centauri d, Faria and team used more than 100 observations of Proxima Centauri’s spectrum over two years, using ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) to turn up the radial velocity signature of a planet in a five day orbit. The weak signal went, with further observations, from just a hint of a new world to a viable planet candidate as the team made sure they weren’t picking up an obscuring asteroseismological signal from the star itself.
The fact that Proxima Centauri d is so small – probably smaller than Earth, and no less than 26% of its mass – emphasizes the magnitude of the achievement. This is the lightest exoplanet ever measured using radial velocity techniques, surpassing the recent find at L 98-50. The planet’s RV signature demands that ESPRESSO pick up a stellar motion of no more than 40 centimeters per second. As the paper notes:
Even in the presence of stellar activity signals causing RV variations of the order of m s?1, it is now possible to detect and measure precise masses for very low-mass planets that induce RV signals of only a few tens of cm s?1.
Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile, notes the significance of the find:
“This achievement is extremely important. It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it.”
Image: A team of astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile have found evidence of another planet orbiting Proxima Centauri, the closest star to our Solar System. This candidate planet is the third detected in the system and the lightest yet discovered orbiting this star. At just a quarter of Earth’s mass, the planet is also one of the lightest exoplanets ever found. Credit: ESO.
In an email this morning, Guillem Anglada-Escudé told me that Proxima Centauri d was not necessarily a surprising discovery given how many exoplanets we are now finding, but it was nonetheless ‘a very beautiful one.’ He finds the work solid:
“It is just because ESPRESSO is a new machine and Proxima has been used to benchmark it that there might be some caveats, but I find the signal strong and very convincing. Also, the only reason to be cautious here is because it is Proxima and it has scientific and cultural relevance. In summary, I would have claimed the same as the authors did. A high cadence, regularly sampled campaign should be able to confirm it with a little more effort. Unsure the ESPRESSO folks will want to invest more time on that or not.”
Anglada-Escudé went on to make another important point: The paper shows that the scientists were able to precisely measure the signal against the stellar activity background, thus separating the planetary find from the noise. The surface of the star may be marked by dark spots and convective activity. The process of ‘detrending’ cleans up the signal to eliminate spurious artifacts so that the planetary signature can be measured. Spurious Doppler shifts affect the line width and the symmetry of a signal. Detrending, said the scientist, only works well when such changes can be measured more accurately than the Doppler shifts themselves.
Thus the power of ESPRESSO. The implications for future studies are heartening:
“…this anticipates very exciting discoveries, as that should enable the detrending of RVs, especially on more sun-like stars. Whether or not ESPRESSO will be the key to solve the ‘stellar activity’ noise floor remains to be seen, but to me, it now seems to have the tools and sensitivity to achieve that.”
The paper is Faria, et al., “A Short-Period Sub-Earth Orbiting Proxima Centauri,” Astronomy & Astrophysics 658 (4 January 2022), A115 (full text).
by Paul Gilster | Feb 9, 2022 | Culture and Society |
Anyone who ever had the pleasure of talking to Freeman Dyson knows that he was a gracious man deeply committed to helping others. My own all too few exchanges with him were on the phone or via email, but he always gave of his time no matter how busy his schedule. In the article below, Colin Warn offers an example, one I asked him for permission to publish so as to preserve these Dysonian nuggets for a wider audience. Colin is an Associate Propulsion Component Engineer at Maxar, with a Bachelor of Science in mechanical engineering from Washington State University. His research interests dip into in everything from electric spacecraft propulsion to small satellite development, machine learning and machine vision applications for microrobotics. Thus far in his young career, he has published two papers on the topics of nuclear gas core rockets and interstellar braking mechanisms in the Journal of the British Interplanetary Society. He tells me that when he’s not working on interstellar research, he can be found teaching music production classes or practicing martial arts.
by Colin Warn
Three years ago, I decided to make a switch from being a part time dance music ghost producer to study something that would help advance humanity’s knowledge of the stars. Eventually, I decided that something would be mechanical engineering, a switch which was in no small part due to space podcasts that introduced me to cool technologies such as Nuclear Pulsed Propulsion (NPP): Rockets propelled by small mini-nuclear explosions. The man behind this technology? Freeman Dyson.
Dyson worked on Project Orion for four years, deeply involved in studies that produced the world’s first and only prototype spacecraft powered by NPP. Due to the 1963 Partial Test Ban Treaty, which he supported, humanity’s best bet for interstellar travel was filed away. Yet, something about the audacity of this project resonated with me decades later when I uncovered it, especially when I found out that Dyson was in charge of this project despite not being a PhD.
So, as a bright-eyed and optimistic freshman entering his first year of college, figuring that out of anyone in the world he would have the best insights on what technology would lead humanity to the stars, I decided to send him this email:
Hello Professor Dyson,
My name is Colin Warn, and I’m a freshman pursuing a degree in mechanical engineering/physics.
Had a few questions for you regarding how I should structure my career path. My ambitions are to work on interstellar propulsion technologies, and I figured you might know a thing or two about the skill set required.
If you have the time, here’s what I’d love to hear your opinion on:
1. What research/internships would you suggest I focus on as an undergraduate to learn the skills that will be needed for working on advanced propulsion technologies? Especially in my freshman and sophomore years?
2. For my initial undergrad years, would you suggest that I focus more on taking physics or engineering courses initially?
Thank you so much in advance for your time. Been reading the book your son wrote about Orion. Let’s just say the reactions I’ve been getting from my friends when I tell them what I’m reading about is already quite fun to observe.
Regards,
-Colin
I sent it to his Princeton email, as I’ve sent many emails in the past to fairly high-caliber people, without a hope of getting anything in return.
Two days later, I woke up to find this email in my inbox.
Dear Colin Warn,
I will try to answer your two questions and then go on to more general remarks.
1. So far as I know, the only techniques for interstellar propulsion that are likely to be cost-effective are laser-propelled sails and microwave-propelled sails. Yuri Milner has put some real money into his Starshot project using a high-powered laser beam. Bob Forward many years ago proposed the Starwisp spacecraft using a microwave beam. Either way, the power of the beam has to be tens of Megawatts for a miniature instrument payload of the order of a gram, or tens of Terawatts for a human payload of the order of a ton. My conclusion, the manned mission is not feasible for the next century, the instrument mission might be feasible.
For the instrument mission, the propulsion system is the easy part, and the miniaturization of the payload is the difficult part. Therefore, you should aim to join a group working on miniaturization of instruments, optical sensors, transmitters and receivers, navigation and information handling systems. These are all the technologies that were developed to make cell-phones and surveillance drones. An interstellar mission is basically a glorified surveillance drone. You should go where the action is in the development of micro-drones. I do not know where that is. Probably a commercial business attached to a technical university.
2. For undergraduate courses, I would prefer engineering to physics. Some general background in physics is necessary, but specialized physics courses are not. More important is computer science, applied mathematics, electrical engineering and optics, chemistry of optical and electronic materials, microchip engineering. I would add some courses in molecular biology and neurology, with the possibility in mind that these sciences may be the basis for big future advances in miniaturization. We still have a lot to learn by studying how Nature does miniaturization in living cells and brains.
This email contained more detailed insights to my questions than I could have ever hoped for. Then to top it all off, he still had one more piece of advice for me to a question I hadn’t even asked.
General remarks. In my own career I never made long-range plans. I would advise you not to stick to plans. Always be prepared to grab at unexpected opportunities as they arise.Be prepared to switch fields whenever you have the chance to work with somebody who is doing exciting stuff. My daughter Esther, who is a successful venture capitalist running her own business, puts at the bottom of every E-mail her motto, “Always make new mistakes”. That is a good rule if you want to have an interesting life.
With all good wishes for you and your career, yours sincerely,
Freeman Dyson.
Upon reading this email in 2018, I promised myself that one day I’d put myself in a position to thank him in person. Sadly I’ll never get the opportunity. I discovered watching an old YouTube video featuring him that he died in February of 2020.
So this article is my way of saying thank you to him. For creating literal star-shot projects to inspire a new generation. For being someone who always questioned the status quo. But most of all, for still being down to earth enough to email some amazingly insightful answers to a freshman’s cold-email. I hope one day I’m in a position where I can pass on the favor.
by Paul Gilster | Feb 8, 2022 | Exoplanetary Science |
It would explain a lot if two recent discoveries involving ‘mini-Neptunes’ turned out to be representative of what happens to their entire class. For Michael Zhang (Caltech) and colleagues, in two just published papers, have found that mini-Neptunes can lose gas to their parent star, possibly indicating their transformation into a ‘super-Earth.’ If such changes are common, then we have a path to get from a dense but Neptune-like world to a super-Earth, a planet roughly 1.6 times the size of the Earth and part of a category of worlds we do not see represented in our Solar System.
As we drill down toward finding smaller worlds, we’ve been finding a lot of mini-Neptunes as well as super-Earths, with the former two to four times the size of the Earth. Thus we have a bimodal gap in exoplanet observation. Where are the worlds between 1.6 and 2-4 times the size of Earth? The new work examines two mini-Neptunes around the TESS object TOI 560, located about a hundred light-years from Earth, and a pair of mini-Neptunes orbiting HD 63433, about 70 light years away. At TOI 560, the planets have periods of 6.4 days and 18.9 days; at HD 63433, the periods are 7.1 and 20.5 days.
At both stars we find a planet whose atmosphere is being stripped away, creating a large cocoon of gas. At TOI 560, it is the innermost mini-Neptune that is losing atmosphere; at HD 63433, the process is occurring on the outer world. Zhang, who is lead author of the two papers on this work, speculates that at the latter, the inner world may already have had its atmosphere stripped away; while the signature of hydrogen is found at the outer mini-Neptune, it is not detected at HD 63433 b, the inner world. The paper notes:
The predicted mass-loss timescale for planet c is longer than the age of the system, but the corresponding mass-loss timescale for planet b is significantly shorter. This implies that c could have retained a primordial H/He atmosphere, while b probably did not.
These planets are Neptune-like in having a rocky core surrounded by a thick envelope of what is thought to be hydrogen and helium. Using Hubble and Keck data for TOI 560 and HD 63433 respectively, the scientists found that at least in these systems, hot Neptunes can transform into super-Earths.
A small enough mini-Neptune close enough to its star undergoes atmospheric loss under the bombardment of stellar X-rays and ultraviolet radiation. The remnant world would be smaller in radius, while any planet in the ‘radius gap’ between 1.6 and 2-4 Earth radii would be in transition, in the process of losing much of its atmosphere over a period of hundreds of millions of years.
“Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres,” adds Zhang. “But nobody had ever caught one in the process of doing so until now.”
Image: This is an artist’s Illustration of the mini-Neptune TOI 560.01, located 103 light-years away in the Hydra constellation. The planet, which orbits closely to its star, is losing its puffy atmosphere and may ultimately transform into a super-Earth. Credit: Artwork: Adam Makarenko (Keck Observatory).
We have yet to determine whether the process is common, because other scenarios are possible. It is conceivable that some of the mini-Neptunes we observe are actually water worlds that are not enshrouded in hydrogen at all. As the paper on TOI 560 notes:
An alternate explanation for the radius gap is that it has nothing to do with mass loss, but is instead because cores have a broad mass distribution, with the smaller cores having never accreted gas in the first place (Lee & Connors 2021). It is also possible that some mini-Neptunes have no hydrogen-rich envelopes at all, but instead formed with substantial water-rich envelopes (e.g., Mousis et al. 2020). This could dramatically change the mass-loss rates, especially that of helium, which would have been already lost to space alongside the primordial hydrogen.
But TOI 560.01 and HD 63433 c are in the spotlight because they offer the first evidence for the theory that mini-Neptunes do become super-Earths. That evidence is strengthened by the the speed of gasses in their atmospheres. Helium at TOI 560.01 is moving as fast as 20 km/sec, while hydrogen at HD 63433 c reaches 50 km/sec.
These data are the result of transmission spectroscopy, in which light from the star is observed passing through a planetary atmosphere, thus carrying information about its composition and characteristics. The degree of motion here precludes retention by the planet, a fact that is bolstered by the size of the gas cocoons around both worlds. At TOI 560.01, the gas is detected in a radius 3.5 times that of the planet, while at HD 63433 c hydrogen is found at a distance at least twelve times the radius of the planet.
The work on TOI 560.01 involved two transits, both of which showed strong helium absorption and some evidence of variability in the atmospheric outflow. Bear in mind that this is the first mini-Neptune with a helium detection, and given that this system contains two worlds where a potential transformation into a super-Earth is possible, we have a new way to explore what Zhang calls ‘exoplanet demographics.’ From the paper:
TOI 560 is a two-planet system, and TOI 560.02 is also a transiting mini-Neptune. This makes the system an excellent test for mass-loss models. The two planets share the same contemporary X-ray/EUV environment, as well as the same irradiation history. In addition, planets of similar size located in adjacent orbits might be expected to have largely similar formation and/or migration histories, and therefore it is reasonable to expect that their primordial atmospheric compositions would be quite similar. This is supported by observational studies of the masses and radii of multi-planet systems in the Kepler sample, which suggest that planets in the same system tend to have similar masses and radii (the “peas in a pod” theory; Weiss et al. 2018).
An intriguing aspect of the situation at TOI 560 is that the innermost world shows a gas outflow that seems to be moving toward the central star. It will take future observations of other mini-Neptunes to find out just how anomalous this may be.
The papers are Zhang et al., “Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune.” Astronomical Journal Vol. 163 No. 2 (17 January 2022) 68 (full text); and Zhang et al., “Escaping Helium from TOI 560.01, a Young Mini-Neptune,” Astronomical Journal Vol. 163 No. 2 (17 January 2022) 67 (full text).
by Paul Gilster | Feb 4, 2022 | Exoplanetary Science |
Given our recent discussion of exomoon candidate Kepler-1708 b-i, a possible moon 2.6 times the mass of Earth orbiting a gas giant, I want to be sure to work in Miki Nakajima’s work on how moons form. Nakajima (University of Rochester) is first author of the paper describing this work. It’s a significant contribution because it points to a way to refine the target list for exomoon searches, one that may help us better understand where to look as we begin to flesh out a catalog of these objects..
And flesh it out we will, as the precedent of the rapidly growing exoplanet count makes clear. What I want to do today is consider how we’ve thus far proceeded. You’ll recall that when David Kipping and team performed their deep analysis of the data leading to Kepler-1708 b-i, they chose gas giants on orbits with a period of 400 days or more, so-called ‘cool worlds’ more like Jupiter than the ‘hot Jupiters’ found so frequently in early exoplanet studies. The method produced a strong candidate indeed.
But Nakajima’s work suggests that when it comes to fractionally large moons, super-Earths should be high on the target list. We need as large a moon as possible, one having the maximum gravitational effect on its planet so that it produces the most observable signature in terms of transit timing variations of the host world around its star. Working with colleagues at Tokyo Institute of Technology and the University of Arizona, Nakajima has concluded that rocky planets larger than 6 Earth masses are unlikely to produce large moons, as are icy planets larger than a single Earth mass.
Image: This is Figure 6 from the paper, titled “Schematic view of the mass range in which a fractionally large exomoon can form by an impact.” Caption: The horizontal axis represents the mantle composition and the vertical axis represents the planetary mass normalized by the Earth mass M?. Rocky planets smaller than 6?M? and icy planets smaller than 1?M? are capable of forming fractionally large moons as indicated by the orange shading. Our prediction is consistent with planet–moon systems in the solar system. Credit: Nakajima et al.
These conclusions grow out of computer simulations, modeling the kind of impact scientists generally believe produced our own fractionally large moon. In this view, the so-called Theia impactor, an object perhaps the size of Mars, struck the Earth early in its development, creating a partially vaporized disk around the planet that eventually produced the Moon. Nakajima’s team ran simulated rocky planets as well as icy worlds through collision scenarios, varying their masses and examining the resulting disks.
As to the parameters of the simulation, note this:
In this work, we use the fixed impact angle (??=?48.6°) and the impact velocity (vimp?=?vesc), where vesc is the mutual escape velocity. The impact angle and velocity are similar to those for the canonical moon-forming impact models. The reason why we explore different mass ranges for the rocky and icy planets is that the required mass for complete vaporization is different between them…
The key to the results is the nature of the debris disk. A partially vaporized disk, like the one that presumably produced our own Moon, cools in these simulations and allows accreting ‘moonlets’ to emerge. These will eventually aggregate into a moon. But when the collisional disk is fully vaporized, these constituent parts experience strong drag from the vapor, causing them to disperse and fall onto the planet before forming a moon.
The six Earth mass cutoff marks the point in these simulations at which these planetary collisions begin to produce fully vaporized disks, rendering them incapable of forming the kind of fractionally large moons that will be easiest to detect. This finding could shake things up, because thus far we have been examining a search space largely defined by gas giants. From the paper:
Our model predicts that the moon-forming disk needs to be initially liquid or solid rich, supporting the canonical moon-forming impact model. Moreover, this work will help narrow down planetary candidates that may host exomoons; we predict that planets whose radii are smaller than ~1.6R? would be good candidates to host fractionally large exomoons (?6 M? for rocky planets and ?1 M? for icy planets). These relatively small exoplanets are understudied (only four out of 57 exoplanets surveyed by the HEK [Hunt for Exomoons with Kepler] project are under this radius limit), which can potentially explain the lack of exomoon detection to date.
Image: This is Figure 1 from the paper, titled ‘Snapshots of giant impacts.’ Caption: The top two rows represent an impact between two rocky planets. The red-orange colors represent the entropy of the mantle material (forsterite). The iron core is shown in gray. The bottom two rows represent an impact between two icy planets. The blue-sky-blue colors represent the entropy of water ice, and the orange color represents forsterite. The scale represents 107?m. Credit: Nakajima et al.
Refining our target list is crucial for maximizing the return on telescope time using our various space-based assets, including the soon to be functioning James Webb Space Telescope. The paper continues:
Super-Earths are likely better candidates than mini-Neptunes to host exomoons due to their generally lower masses and potentially lower H/He gas contribution to the disk. This narrower parameter space may help constrain exomoon search in data from various telescopes, including Kepler, the Hubble space telescope, CHaraterising ExOPlanet Satellite (CHEOPS), and the James Webb Space Telescope (JWST).
As you would imagine, I went to David Kipping (Columbia University) for his thoughts on the Nakajima et al. result, he being the most visible exponent of exomoon research. His response:
“My main comment is that I’m pleased to see a testable theory regarding exomoons around terrestrial planets finally emerge. If scaled-up versions of the Earth-Moon system are out there up to 6 Earth masses, then we have a great shot at detecting those using HST/JWST.”
That science fictional trope of an Earth-sized moon orbiting a habitable zone gas giant may soon be joined by this new vision, large exomoons around super-Earths — even ‘double planets’ – which offer even more plot fodder for writers looking for exotic interstellar venues. Robert Forward’s Rocheworld inevitably comes to mind.
The paper is Nakajima et al., “Large planets may not form fractionally large moons,” Nature Communications 13, 568 (2022). Full text.
by Paul Gilster | Feb 3, 2022 | Exoplanetary Science |
Half a century ago, we were wondering if other stars had planets, and although we assumed so, there was always the possibility that planets were rare. Now we know that they’re all over the place. In fact, recent research out of Katholieke Universiteit Leuven in Belgium suggests that under certain circumstances, planets can form around stars that are going through their death throes, beginning the transition from red giant to white dwarf. The new work homes in on certain binary stars, and therein hangs a tale.
After a red giant star has gone through the stage of helium burning at its core, it is referred to as an asymptotic giant branch star (AGB), on a path that takes it through a period of expansion and cooling prior to its becoming a white dwarf. These expanding stars lose mass as the result of stellar wind, up to 50 to 70 percent of the total mass of the star. The result: An extended envelope of material collecting around the object that will become a planetary nebula, a glowing shell of ionized gas.
In binary systems, that stellar material coming off the star can evolve in interesting ways. While our standard view of planet formation involves circumstellar disks and planets emerging not all that long after the birth of their star, the KU Leuven work, led by Jacques Kluska, notes that in binary stars, a second star can gravitationally shape the gas and dust being ejected by a late stage red giant. From an observational perspective, this disk looks much like the disks that form around young stars.
To analyze the matter, the KU Leuven team assembled a catalog of all known post-AGB binaries that showed disks, compiling spectral energy distributions of 85 systems and examining the infrared characteristics of the various disk formations. Out of this emerged a catalog of different disk types. This is where things get intriguing. Between 8 and 12 percent of the cataloged systems are surrounded by what the paper calls ‘transition disks,’ meaning disks that show little or no low-infrared excess.
At the same time, these disks demonstrate a link with the depletion of refractory elements (metals highly resistant to heat) on the surface of the red giant star.
Jacques Kluska explains the significance of this result:
“In ten per cent of the evolved binary stars with discs we studied, we see a large cavity in the disc. This is an indication that something is floating around there that has collected all matter in the area of the cavity… In the evolved binary stars with a large cavity in the disc, we saw that heavy elements such as iron were very scarce on the surface of the dying star. This observation leads one to suspect that dust particles rich in these elements were trapped by a planet.”
Image: Discs surrounding so-called evolved binary stars not uncommonly show signs that could point to planet formation. Credit: N. Stecki.
In this scenario, it is possible that not just single planets but an entire system’s worth could eventually grow. We’ll have to see whether planets can be confirmed in some of these systems, and to do that the team intends to home in on the ten pairs of binary stars whose disks demonstrate a large cavity. Such confirmation would denote yet another method of planet formation, to add to what we’re learning about planets in white dwarf systems. The tendency of stars to grow planets, it seems, can manifest itself even in stellar death, a useful fact given that perhaps a third of all stars in the Milky Way are found in multiple star systems. And there is precedent:
…if the planetary explanation is correct for transition disks, it would mean that there is a pressure maximum outside the planet’s orbit, trapping the dust and creating a favorable environment for second-generation planet formation… The possibility of second-generation planet formation is further supported by the detection of two giant planets around the white-dwarf binary system NN Ser. These planets are candidates for having been formed in such second-generation disks… If the planetary scenario is confirmed, these disks would become a promising site for studying second-generation planet formation and, hence, planet formation scenarios in an unprecedented parameter space.
About 1700 light years away, NN Serpentis is a system containing an eclipsing white dwarf and an M-class dwarf. The existence of what are evidently two gas giants here has been inferred from transit timing variations that can be attributed to one planet of about 6 times Jupiter’s mass, the other of about 1.6 times Jupiter’s mass, both in circumbinary orbits around this extremely tight binary. This is an interesting system for models of planet formation that go well beyond the infancy of the host stars.
The paper is Kluska et al., “A population of transition disks around evolved stars: fingerprints of planets. Catalog of disks surrounding Galactic post-AGB binaries,” Astronomy & Astrophysics Vol. 658, A36 (01 February 2021). Full text. On NN Serpentis, see Völschow et al. ”Second generation planet formation in NN?Serpentis?” Astronomy & Astrophysics Vol. 562 (February 2014) A19 (abstract). For more on late binary systems, see Winckel, “Binary post-AGB stars as tracers of stellar evolution,” in Beccari, ed., The Impact of Binary Stars on Stellar Evolution, Cambridge University Press, 2019 (preprint).
