One of the benefits of having Alpha Centauri as our closest stellar neighbor is that this system comprises three different kinds of star. We have the familiar Centauri A, a G-class star much like our Sun, along with the smaller Centauri B, a K-class star with about 90 percent of the Sun’s mass. Proxima Centauri gives us an M-dwarf, along with the (so far) only known planet in the system, Proxima b. Questions of habitability here are numerous. Along with possible tidal locking, another major issue is radiation, since M-dwarfs are known for their flare activity.
As we learn more about the entire Alpha Centauri system, though, we’re learning that the two primary stars are much more clement. They may have issues of their own — in particular, although stable orbits can be found around both Centauri A and B, we still don’t know whether planets are likely to have formed there — but scientists studying data from the Chandra X-ray Observatory have found that levels of X-ray radiation are far lower here than around Proxima Centauri.
This is good news, because high radiation levels could prove fatal for surface life, with the additional effect of possible damage to planetary atmospheres. Chandra has been involved in a multi-year campaign targeting Centauri A and B stretching back to 2005, with observations every six months. No other X-ray observatory is capable of resolving the two primary stars during their current close orbital approach. What we wind up with is a look at radiation activity over time, covering a period analogous to our own Sun’s 11-year sunspot cycle.
Image: A new study involving long-term monitoring of Alpha Centauri by NASA’s Chandra X-ray Observatory indicates that any planets orbiting the two brightest stars are likely not being pummeled by large amounts of X-ray radiation from their host stars. This is important for the viability of life in the nearest star system outside the Solar System. Chandra data from May 2nd, 2017 are seen in the pull-out, which is shown in context of a visible-light image taken from the ground of the Alpha Centauri system and its surroundings. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zden?k Bardon/ESO.
Tom Ayres (University of Colorado Boulder) presented these results at the just concluded meeting of the American Astronomical Society in Denver. Any planets in the habitable zone of Centauri A would actually receive a lower dose of X-rays, on average, than planets around the Sun, while the X-ray dosage for a planetary companion of Centauri B is about 5 times higher than the Sun. This contrasts sharply with Proxima Centauri’s planet, which would receive an average dosage 500 times larger than the Earth, rising to 50,000 times higher during a major flare. If we find planets around either A or B, it may be that Breakthrough Starshot will want to prioritize these at the expense of the more endangered Proxima b.
In the animation below, we can see the proper motion of Centauri A and B.
Image: This movie shows Chandra observations of Alpha Centauri A and B taken about every 6 months between 2005 and 2018. Alpha Cen A is the star to the upper left. The motion of the pair from left to right is their “proper motion”, showing the movement of the pair in our galaxy with respect to the solar system. The change in relative positions of the pair shows the motion in their 80 year long orbit and the wobbles show the small apparent motion (called parallax) caused by the year long orbit of the Earth around the Sun. The Chandra images are shown in black and white. To place these semi-annual images in context, the two colored circles show the expected motion of Alpha Cen A (yellow) and Alpha Cen B (orange) when taking account of proper motion, orbital motion and parallax. The size of the circles is proportional to the X-ray brightness of the source. Credit: Thomas Ayres.
Ayres has also written up some of the results in Research Notes of the American Astronomical Society, where I learned that the central AB pair has actually been under X-ray study for almost four decades, dating back to the late 1970s and the HEAO-2 satellite (also known as the Einstein Observatory), which was the first fully imaging X-ray telescope ever put into space. Subsequent observations were conducted by ROSAT (Röntgen-Satellit), XMM-Newton and now Chandra. Here, Ayres explains why X-ray studies may help us learn about habitability in this system as well as giving us information closer to home:
The modest coronae (106 K) of ? Cen AB are on par with our own Sun’s. X-ray studies of these objects can help us understand how the “Dynamo” in the stellar interior produces the episodic surface magnetic eruptions at the core of solar activity and “Space Weather.” The hard radiation and particle bombardment from flares and coronal mass ejections can affect Planet Earth, so the interest is not solely academic. Exoplanets of other sunlike stars can be exposed to analogous extreme high-energy transients from their hosts, with perhaps serious repercussions for habitability.
Image: Figure 1 from the Ayres note. Caption: X-ray light curves of a Cen AB and the Sun 1995–2018. Credit: T. R. Ayres.
I was fortunate enough to be in the audience when Ayres spoke to Breakthrough Discuss in 2016 in a presentation called “The Ups and Downs of Alpha Centauri.” Here’s Breakthrough’s video of that talk, which I highly recommend.
The research note is Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (22 January 2018). Full text.
Some 150 globular clusters are associated with the Milky Way, great collections of stars inhabiting the galactic halo. Their stars have long been assumed to be ancient, making the question of life there intriguing: If life caught hold in these tightly packed clusters early in the universe’s evolution, could ancient civilizations have formed that might persist even today? I know of only one planet that has yet been found in a globular cluster, but we’re obviously early in the game, and planets have been discovered in open clusters, which are much less densely packed.
Just how little we know about globular clusters, though, is made apparent by the work of Elizabeth Stanway (University of Warwick), whose new paper argues that such clusters could be billions of years younger than we have thought. Working with JJ Eldridge (University of Auckland), Stanway invokes a model called Binary Population and Spectral Synthesis (BPASS). In play here is the evolution of binary stars within globular clusters, a line of research that has been put to work in prior studies of young stellar populations in the Milky Way and elsewhere.
Image: The globular cluster Terzan 1. Clusters like these have been thought to contain some of the oldest known stars. New research puts their age into question. Credit: Judy Schmidt/ESA/NASA.
A significant problem in observing globular clusters is described in the paper. Having discussed using the spatially resolved light of individual stars to determine stellar properties, the authors note:
However, in the majority of old stellar populations, such detailed investigation of resolved stellar properties is impossible due to a lack of depth or angular resolution, and the integrated light of the unresolved stars must instead be used to constrain the properties of the population as a whole. In this scenario observable characteristics of the source spectral energy distribution (SED), including photometric colours and spectroscopic emission lines or indices, are compared to those determined for models of known age and composition.
And here’s the crux, which is why the age of such significant galactic features as these remains in doubt:
Either a best fitting template or a relation calibrated on such templates is then used to characterise the population. As a result, such analyses are strongly dependent on the properties of the template stellar population models.
Out of this understanding has sprung a new generation of models for the study of galactic evolution. But as the authors point out, binary interactions can have marked effects on a star’s development, and current research indicates that multiple star systems are ubiquitous in globular clusters, making single star models problematic when conclusions are being drawn from the integrated light of the entire cluster. And it turns out that the model populations that best fit observations are substantially younger than those derived from older spectral models.
Thus the utility of BPASS, which the authors have upgraded from a version described in an earlier paper. The BPASS upgrade is targeted at refining age estimates for older populations of stars. The researchers examine the elements in the spectra of binary stars, looking at systems where the larger star expands into a giant while the smaller star strips away its atmosphere. In this model, both stars are assumed to have formed at the same time as the globular cluster.
From the paper:
…incorporating binary stellar evolution pathways, together with the most up-to-date stellar evolution and atmosphere models for single stars, into stellar population synthesis models can make a significant difference in the interpretation of their integrated light properties. The lifetimes of stars in different mass ranges can be modified, by mass transfer and mixing, as can their temperatures and gravities. Mass transfer onto a secondary can produce more massive, and therefore brighter, stars at late ages, even if their stellar atmospheres are typical of cool red giants. A population can also incorporate stellar types simply not found in the absence of binary evolution.
?Image: Binary star evolution within a globular cluster. Credit & copyright: Mark A. Garlick/University of Warwick.
Given the dependence of the age estimates of older star populations on evolutionary models, it’s helpful to see that the BPASS work can reproduce observed values in globular clusters and galaxies. And as the model is continuing to be refined, it produces this result:
Model fits to photometry and spectroscopic indices yield a consistently younger fit, often at slightly higher metallicity, than fits to older calibrations, when new stellar atmosphere models and binary stellar evolution pathways are included.
And this:
At its most basic level, this means that we are able to reproduce the photometry of mature, quiescent galaxies and clusters at younger ages than previous model sets (i.e. ? 5 ? 8 Gyr, rather than 10-14 Gyr).
Stellar interactions, then, may tell the tale, perhaps adjusting our estimates of cluster age by billions of years. Still ancient, these vast ‘cities of stars’ pose huge questions — how stable are planetary orbits, for example, given the interactions between such tightly packed stars? But the vision of places like Lagash, the planet in Isaac Asimov’s “Nightfall” (called Kalgash in the later novel), its skies always ablaze with stars, keeps the science fiction fan in me speculating. What would it be like to be on a planet deep within something as splendid as the image below?
Image: Thousands and thousands of brilliant stars make up this globular cluster, Messier 53, captured with crystal clarity in this image from the NASA/ESA Hubble Space Telescope. Bound tightly by gravity, the cluster is roughly spherical and becomes denser towards its heart. These enormous sparkling spheres are by no means rare, and over 150 exist in the Milky Way alone, including Messier 53. It lies on the outer edges of the galaxy, where many other globular clusters are found, almost equally distant from both the centre of our galaxy and the Sun. Although they are relatively common, the famous astronomer William Herschel, not at all known for his poetic nature, once described a globular cluster as “one of the most beautiful objects I remember to have seen in the heavens.” This picture was put together from visible and infrared exposures taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys.
Our understanding of globular clusters is, of course, a work in progress. Says Stanway:.
“It’s important to note that there is still a lot of work to do – in particular looking at those very nearby systems where we can resolve individual stars rather than just considering the integrated light of a cluster – but this is an interesting and intriguing result. If true, it changes our picture of the early stages of galaxy evolution and where the stars that have ended up in today’s massive galaxies, such as the Milky Way, may have formed. We aim to follow up this research in future, exploring both improvements in modelling and the observable predictions which arise from them.”
The paper is Stanway & Eldridge, “Reevaluating Old Stellar Populations,” in press at Monthly Notices of the Royal Astronomical Society (preprint).
Things always get interesting when the American Astronomical Society meets, which it is now doing in Denver, in sessions that will run until June 7. There should be no shortage of topics emerging from the meeting, but the first that caught my eye was a different approach to the putative world some are calling Planet Nine. Teasing out the existence of a planet at the outer edges of the Solar System has involved looking at gravitational interactions among objects that we do know about, and extrapolating the presence of a far more massive body.
But the methodology may be flawed, if new work from Ann-Marie Madigan and colleagues at the University of Colorado Boulder is correct. At a press briefing at the AAS meeting, the team presented its view that objects like Sedna, an outlier that takes more than 11,000 years to complete an orbit around the Sun, should be considered in relation to other so-called ‘detached bodies.’ Almost 13 billion kilometers out, Sedna is one of a collection of such objects that appear in some ways to be in another category from the more conventional inner worlds.
Image: An artist’s rendering of Sedna, which looks reddish in color in telescope images. Credit: NASA/JPL-Caltech.
Sedna and its ilk come nowhere near the larger planets of our system, and their orbits may tell a tale. As this CU-Boulder news release explains, it was an undergraduate student named Jacob Fleisig who began to model a significant pattern known as ‘inclination instability’ that Madigan had previously described in the literature. Fleisig’s computer modeling illustrates how inclination instability can ease Sedna’s orbit from oval to circular over time.
In the model, accumulating gravitational forces drive growth in the orbital inclinations of objects in eccentric orbits. From the new work:
…secular (orbit-averaged) gravitational torques between orbits in the disk drive exponential growth of their inclinations. As the orbits’ inclinations grow, they tilt in the same way with respect to the disk plane. This leads to clustering in their angles of pericenter and the initially thin disk expands into a cone shape. Concurrently, the orbital eccentricities decrease and perihelion distances increase.
If such a mechanism is at work in our own system, we would expect it to occur between minor planets originally scattered to large orbital eccentricities via interactions with the giant planets. Such objects then become gravitationally detached from those planets. The authors believe this mechanism can explain the orbits of high perihelia objects like Sedna. Current studies posit anywhere from 1 to 10 Earth masses of cometary material existing at hundreds of AU from the Sun. The authors see such objects being originally scattered by the giant planets and their orbits decoupled by perturbations from cluster gas and nearby stars.
Perhaps this is all we need to explain the orbits of detached objects. There would be no need for a ‘Planet Nine’ at the edge of the Solar System. Instead, the detached objects achieve their present orbits through a series of small-scale interactions.
“There are so many of these bodies out there. What does their collective gravity do?” asks Madigan. “We can solve a lot of these problems by just taking into account that question.”
A great deal of work is ahead as the authors apply findings from their current computer simulations — “focused on the linear phase of the inclination instability in an idealized set-up” — to the outer Solar System. The paper notes their intent to model the gravitational influences of the giant planets and to focus on individual minor planets — especially those whose orbits become retrograde — instead of averaging their results for many hypothetical objects.
The paper is Madigan et al., “On the Dynamics of the Inclination Instability,” submitted to The Astrophysical Journal (preprint).
Breakthrough Starshot held an ‘industry day’ on Wednesday May 23rd devoted to its lightsail project to take nanocraft to another star, framing the release of a Request for Proposals during its early concepts and analysis phase. The RFP focuses on the sail itself, investigating sail materials and stability under thrust. Step A proposals are due June 22, step B proposals on July 10, with finalists to be notified and contracts awarded this summer. The intent of the RFP is laid out in documents and slides from the meeting that Breakthrough has now placed online.
From the RFP itself:
The scope of this RFP addresses the Technology Development phase – to explore LightSail concepts, materials, fabrication and measurement methods, with accompanying analysis and simulation that creates advances toward a viable path to a scalable and ultimately deployable LightSail.
We’ve been talking about Breakthrough Starshot in these pages for a long time, as a search through the archives will reveal. The intention is to send gram-scale probes commonly referred to as ‘starchips’ attached to sails on the scale of meters to a nearby star to investigate its planets, with the highly interesting Proxima Centauri b an obvious target given that it is also the closest star to the Sun. Propelling the sail will be a gigawatt-scale ground-based laser. 20 years of flight time sets up the flyby, with data transmission returning images of the star system.
$100 million in research and development is to be spent over the next 5 years to determine the feasibility of both laser and sail. Overall, the five-year technology development period will be managed by three groups: Starshot’s sail committee, a photon engine committee and a systems engineering committee. After that, the goal from years 6 to 11 is to build a low-power prototype for space testing. After that comes a full scale laser system (called the ‘photon engine’) over the next 20 years, with launch of an interstellar mission occurring in approximately 30 years.
It’s an ambitious schedule, to be sure, but the early conceptual steps are now being taken, with initial sail work investigating candidate materials and sail stability. Keeping a sail stable under the high accelerations induced by the laser array is obviously critical and has already involved discussions and papers on different sail configurations, while the equally critical issue of sail materials is likewise considered in the RFP, which is Phase 1 of the sail’s development.
Phase 2 of the technology development program for the lightsail will validate lightsail materials and stable designs by proof of concept laboratory demonstrations, while Phase 3 takes us into laboratory testing of scalable prototypes. All of this points toward the eventual goal of a prototype mission that would launch nanocraft to a target here in the Solar System.
With this RFP focusing on Phase 1, Breakthrough Starshot seeks proposals for quantitative models that can produce testable predictions of sail performance, mathematical models defining the necessary conditions for sail stability, experimental methods for lightsail material fabrication and precision measurements to ‘validate optical, thermal and mechanical stability of materials.’ Here is the RFP statement of key issues involved in identifying candidate materials and designs:
Design of a reflector consistent with the mission requirement of achieving 0.2c for ~1g payload and LightSail area of >1 m2
Design of passive, adaptive or active features that enable or enhance stability, damage resistance, thermal robustness, and durability under deformation
Assessment of candidate materials (including thin-films, micro/nanopatterned structures, 2D materials) for thermal/mechanical stability
Development of measurement techniques and protocols for LightSail material properties (absorption, reflectivity, temperature, stress state, etc.)
Identification of materials for which scale-up and manufacture at the >1 m2 scale is feasible
Materials that facilitate integration of the LightSail with the Starchip
Development of the next generation Starchip scale spacecraft with a path towards incorporation into the Nanocraft
The second objective of the RFP is to identify and assess optimally-shaped designs for a stable sail that can withstand the temperatures and accelerations involved in pushing nanoncraft to 20 percent of the speed of light in a matter of minutes. The RFP notes the key issues here as:
Validation via, e.g., multi-physics simulation (optical, mechanical, thermal, etc.) of LightSail durability and dynamic stability and sensitivity to Photon Engine laser propulsion beam geometry and ground demonstrations
Evaluation of spacecraft stability in the context of a LightSail integrated with a Starchip payload
Development of optimization-based tools for evaluating LightSail designs matched to corresponding laser beam profiles.
Defining a roadmap for test and verification, including:
o Measurement techniques for thin membranes at a small (<1 cm2) scale
o Developing diagnostics and instrumentation needed for LightSail stability
measurements
You can download the RFP from the Breakthrough Starshot site for bidding information; note that multiple awards are anticipated. A layout of the proposal process and requirements is provided there, with submission information. The procurement is a two-step process with initial (short) white paper proposal evaluated by the Starshot lightsail committee and experts in beamed propulsion, and a second round in which finalists are invited to make final proposals. Contract negotiations will then be performed by the Starshot lightsail committee.
I’m looking forward to the buildup as New Horizons gets ever closer to Kuiper Belt Object MU69 and whatever surprises will attend the flyby. But the ongoing operations of the Dawn spacecraft orbiting Ceres equally command the attention. The image below is one of the first images Dawn has returned in more than a year, a stark view of surface features taken on May 16 of this year. The altitude here is 440 kilometers — for scale, the large crater near the horizon is about 35 kilometers wide. The foreground crater is about 120 kilometers from that crater, within a jumbled landscape suggestive of ancient terrain underlying the more recent impact.
Image: On the way to its lowest-ever and final orbit, NASA’s Dawn spacecraft is observing Ceres and returning new compositional data (infrared spectra) and images of the dwarf planet’s surface, such as this dramatic image of Ceres’ limb. Dawn has returned many limb images of Ceres in the course of its mission. These images offer complementary perspective to the images generally obtained by imaging the surface directly beneath the spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
As with Pluto/Charon, so with Ceres — we’ve named many surface features, and can thus say that the image, a view of terrain at 23 degrees north latitude, 350 degrees east longitude, is in the neighborhood of mountainous terrain that includes Kwanzaa Tholus. A tholus is a type of small mountain — we now have six named tholi and montes, somewhat bigger mountains, in this region of the dwarf world.
Backing out the view in the image below, you can see Kwanzaa Tholus marked at image center, though its gradual rise makes it difficult to pick out. Here we’re looking at a 2017 mosaic taken by Dawn in its high-altitude mapping orbit at about 1470 kilometers above the surface.
Image: Scientists say Kwanzaa Tholus may have once been as prominent as Ahuna Mons, the tallest and most noticeable mountain on Ceres. Ahuna Mons is likely a cryovolcano, a volcano formed by the gradual accumulation of thick, slowly flowing icy materials. Because ice is not strong enough to preserve an elevated structure for extended periods, cryovolcanoes on Ceres are expected to gradually collapse over tens of millions of years. This means Kwanzaa Tholus and other tholi in that area could be degraded mountains, which also formed from cryovolcanic activity. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
But back to the present, for Dawn is drawing closer to Ceres as it moves into its final orbit, which it will reach in early June. This orbit will take the spacecraft as close as 50 kilometers above the surface, so we can expect the spectacular imagery to continue. It’s worth noting how complex this orbital adjustment is, involving examination of more than 45,000 possible trajectories before engineers chose the final plan. Along with the images, Dawn will be collecting gamma ray and neutron spectra (using the GRaND gamma ray and neutron detector instrument) by way of gaining insights into the chemical makeup of Ceres’ upper crust.
“The team is eagerly awaiting the detailed composition and high-resolution imaging from the new, up-close examination,” said Dawn’s Principal Investigator Carol Raymond of NASA’s Jet Propulsion Laboratory, Pasadena, California. “These new high-resolution data allow us to test theories formulated from the previous data sets and discover new features of this fascinating dwarf planet.”
We’re now going into what is known as XM07, or extended mission orbit 7, a designation that, as mission director and chief engineer Marc Rayman wryly observes, “illustrates the team’s flair for the dramatic.” Since XM05 the spacecraft’s orbit has been elliptical, for Dawn was not designed to operate at low altitude, and we’re dealing with a spacecraft whose reaction wheels have failed, making controlling its orientation an issue. Dawn must fire its small hydrazine-fueled thrusters to control its orientation in space. In his JPL blog, Rayman describes the orbit:
Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well.
Image: Mission director and chief engineer Marc Rayman, whose mission updates are indispensable for anyone following Dawn. Both Dawn and New Horizons have done a superb job in keeping mission activities visible to the public. Credit: JPL.
The XM07 ellipse will take Dawn from less than 50 kilometers back out to 4,000 kilometers, with each revolution lasting 27 hours and 13 minutes. Rayman notes that XM07 will place extreme demands on Dawn’s photography because of the high speed of the spacecraft this near to the surface and the pointing problems involved in capturing a specific target. For more on this and a full description of XM07 and its consequences, I’ll send you to Rayman’s blog, which in any case should be on your radar as an informative and witty look deep inside the mission.
Image: This view of Juling Crater was constructed from pictures Dawn took from XM06 at an altitude of 385 kilometers. The science team has presented other views of this 20-kilometer crater, including last month, when Rayman and company described the discovery that the amount of ice on the shadowed northern wall changed over six months in 2016. Ceres is not a static world. When Dawn dives down lower in June, it will obtain sharper images than this (at other locations). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
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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