by Paul Gilster | Aug 20, 2021 | Deep Sky Astronomy & Telescopes |
Anomalies in our models are productive. Often they can be explained by errors in analysis or sometimes systematic issues with equipment. In any case, they force us to examine assumptions and suggest hypotheses to explain them, as in the case of the unusual acceleration of stars that has turned up in two areas. Greg Matloff has written about one of them in these pages, the so-called Parenago’s Discontinuity that flags an unusual fact about stellar motion: Cool stars, including the Sun, revolve around galactic center faster than hotter ones.
This shift in star velocities occurs around (B-V) = 0.62, which corresponds to late F- or early G-class stars and extends down to M-dwarfs. In other words, stars with (B-V) greater than 0.61 revolve faster. The (B-V) statement refers to a color index that is used to quantify the colors of stars using two filters. One, the blue (B) filter, lets only a narrow range of wavelengths centered on blue colors through, while the (V) visual filter only passes wavelengths close to the green-yellow band. Hot stars have (B-V) indices closer to 0 (or negative); cool M-dwarf stars come out closer to 2.0. Proxima Centauri’s (B-V) number, for example, is 1.90. The (B–V) index for the Sun is about 0.65.
What the Soviet space scientist Pavel Parenago (1906-1960) had observed is that cooler stars, at least in the Sun’s neighborhood, revolve about 20 kilometers per second faster around the galactic center than hotter, more massive stars. Work in the 1990s using Hipparcos data on over 5,000 nearby main sequence stars clearly showed the discontinuity. An early question was whether or not this was an effect local to the Sun, or one that would prove common throughout the galaxy.
Image: Soviet space scientist Pavel Parenago. Credit: Visuotin? Lietuvi?.
The answer came in the form of a paper by Veniamin Vityazev (St. Petersburg State University) and colleagues in 2018. The scientists hugely expanded the stars in their sample by using the Gaia Data Release 1 dataset,. The Gaia release included some 1,260,000 stars on the main sequence and a further 500,000 red giants. As we saw yesterday, the Gaia mission, which launched in 2013, is continuing to gather data at the Earth-Sun L2 Lagrange point, using astrometric methods to determine the positions and motions of a billion stars in the galaxy.
You may remember that the Vityazev paper (citation below) and its consequences was the subject of a 2019 dialogue in these pages between Greg Matloff and Alex Tolley (see Probing Parenago: A Dialogue on Stellar Discontinuity). And for good reason: The Gaia dataset allowed the Russian scientist and colleagues to demonstrate that Parenago’s Discontinuity was not a local phenomenon, but was indeed galactic in nature. All of this naturally leads to questions about the mechanisms at work here — Matloff’s interest in panpsychism as one explanation is well known and discussed in his book Starlight, Starbright (Curtis, 2019).
But in a new paper with Matloff, Roman Ya. Kezerashvili (New York City College of Technology) and Kelvin Long (Interstellar Research Centre, UK) point to another noteworthy datapoint. The extensive Gaia dataset showed a feature that turned up in the Hipparcos data as well. There was what the authors call a ‘strange bump in the curve of galactic revolution velocity versus (B-V) color index between (B-V) ? 0.55 and (B-V) ? 0.9…’ This would take in stars of the spectral classes G1 through K2, and the authors add that the bump is apparent only in the direction of stellar galactic revolution, like Parenago’s Discontinuity itself. Is this to be construed as a subset of the discontinuity or a separate effect? We don’t yet know.
What we do have is apparent acceleration in the direction that stars rotate around the galaxy, an unusual effect that adds up: In a billion years, the positional shift between a star without this acceleration vs. with the acceleration is about 1,500 light years.
This second apparent acceleration has yet to be confirmed by other researchers, as the authors acknowledge. But as we await that result, the new paper investigates the physical processes that could account for it. If we consider a star expelling material in a specific direction, we can model the system by analogy to a rocket and its expelled fuel, and thus examine velocity change with the Tsiolkovsky rocket equation. From the paper:
There are a number of physical mechanisms responsible for expelling mass during the stars’ life. These include but may not be limited to: unidirectional or focused electromagnetic (EM) flux, solar wind, accelerated solar wind, coronal mass ejections (CMEs), unidirectional neutrino flux, and solar wind thermonuclear fusion. Each mechanism can be treated as an exhaust velocity Vex with a mass flow rate so that the acceleration of the star is analogous to a rocket thrust F = VexMs.
What stands out here is how big the force F = 1016 N — the force needed to accelerate stars like the Sun to this level — is from our terrestrial perspective. It is the equivalent of the force exerted by two billion 106 kg rockets, each accelerating at 3g. Backing out to cosmic scale, though, the force appears less daunting, being 5 orders of magnitude less than the mutual gravitational force between the Earth and the Sun.
The mechanisms of mass loss are many and varied, as noted above, and the paper goes to work on eight of them:
- Unidirectional or focused stellar electromagnetic flux
- Galactic cannibalism
- Stellar mass loss by thermonuclear fusion
- Non-isotropic stellar wind
- Accelerated unidirectional stellar wind
- Coronal mass ejections
- Unidirectional neutrino flux
- Solar wind thermonuclear fusion
Note how many of these demand an existing advanced technology to implement them. In fact, of the eight, only two have solutions that are what Matloff calls ‘mechanistic;’ i.e. operational within nature without technological intervention. These are galactic cannibalism and stellar sass loss by thermonuclear fusion. Neither show promise as a cause for stellar acceleration of this order.
Galactic cannibalism refers to interactions between dwarf galaxies and large galaxies like the Milky Way, with the satellite galaxies being absorbed. This adds to the mass of the primary galaxy and may increase star orbiting velocities, but the authors discount the process because stellar nurseries are also accelerated by galaxy mergers, just as pre-existing stars. Stellar mass loss by thermonuclear fusion due not only to fusion within the core but via the stellar wind — which is in any case omnidirectional — likewise fails to produce the observed acceleration effect across classes of stars.
The other six options all involve the application of advanced technologies to create stellar motion in a specific direction, and some of these don’t work either. Unidirectional coronal mass ejections, for example, even if they could be shaped as thrust, fail to provide the needed energies. What emerge as viable candidates are the combined effects of a unidirectional neutrino jet and an accelerated unidirectional stellar wind “in conjunction with the unidirectional photon jet ejected from the star and non-isotropic stellar wind.” Thus on the accelerated unidirectional stellar wind:
Consider the case of a dual-purpose Dyson/Stapledon megastructure constructed around a star. Confirmation of the existence of such a megastructure would constitute evidence for the existence of a Kardashev Level 2 or higher civilization. A fraction of the star’s radiant output is used to accelerate a fraction of the star’s stellar wind as a unidirectional jet.
Obviously these options take us into science fiction territory, such as the recent Greg Benford/Larry Niven collaboration beginning with Bowl of Heaven (Tor, 2012), where a star in directed motion carries with it a vast environmental shell. So as an example of just one way to do all this, let me quote from an article Benford wrote for Centauri Dreams in 2014 (see Building the Bowl of Heaven):
Our Bowl is a shell more than a hundred million miles across, held to a star by gravity and some electrodynamic forces. The star produces a long jet of hot gas, which is magnetically confined so well it spears through a hole at the crown of the cup-shaped shell. This jet propels the entire system forward – literally, a star turned into the engine of a “ship” that is the shell, the Bowl. On the shell’s inner face, a sprawling civilization dwells…
The jet passes through a Knothole at the “bottom” of the Bowl, out into space, as exhaust. Magnetic fields, entrained on the star surface, wrap around the outgoing jet plasma and confine it, so it does not flare out and paint the interior face of the Bowl — where a whole living ecology thrives, immensely larger than Earth’s area.
Image: Moving a star as depicted in Bowl of Heaven. Credit: Don Davis.
But the idea of moving stars via technology has a long pedigree. As just one example, let me quote Fritz Zwicky, that fiercely independent and prescient thinker, from an article he published in June, 1961 in Engineering and Science:
In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields…
So could an advanced culture pull this off? The authors are quick to point out that the eight mechanisms they evaluate do not exhaust the range of possibilities, but thus far the physical mechanisms under investigation fail the test unless assisted by a technology. The first order of business, of course, will be to verify the apparent stellar acceleration and then to ponder stellar kinematics in their combined light.
But while we can envision advanced cultures developing the tools to move individual stars, as breathtaking as the concept seems, it appears unlikely in the extreme that entire stellar classes are being moved about simultaneously by megastructures or other technologies.
I think the existence of an explanation within the realm of nature has to be assumed. The question, of course, is just where it is to be found. This spirited paper, cheerfully speculative, contains an open call to the community to investigate anomalous stellar accelerations and produce other hypotheses explaining this curious behavior.
The paper is Kezerashvili, Matloff & Long, “Anomalous Stellar Acceleration: Causes and Consequences,” JBIS Vol. 74 (2021), pp. 269-275 (full text). The Vityazev paper is V. V. Vityazev et al., “New Features of Parenago’s Discontinuity from Gaia DR1 Data,” Astronomy Letters 44, 629-644 (2018). Abstract.
by Paul Gilster | Aug 18, 2021 | Deep Sky Astronomy & Telescopes |
Infrared imagery drawn from Spitzer Space Telescope data, coupled with the massive Gaia Early third Data Release (EDR3), have just given us a new insight into our galaxy’s spiral structure. The Milky Way’s Sagittarius Arm is now shown to have a ‘spur’ of star-forming gas and young stars emerging at a steep angle and stretching some 3,000 light years. The authors of the paper on this work refer to it as “unprecedented in the context of the generally adopted model of the Milky Way spiral structure.”
The spur was a tricky catch, because from our position within the galactic disk we can only see the full spiral structure in galaxies other than our own. But the authors point out that in these galaxies, spiral arms often show smaller-scale structures including ‘spurs,’ which are luminous groupings of stars, and ‘feathers,’ which are dust features. We also find branching in the main arms. Now we’ve identified a spur structure in the Milky Way.
Image: Artist’s concept of the Milky Way. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity. The new structure (not shown here) has been found to extend from the Sagittarius Arm. Credit: NASA/JPL-Caltech.
Early work on galactic structure going back to the 1950s homed in on nearby star-forming regions in work that helped to define the Sagittarius Arm in the first accepted galactic map. Today we have a great deal more data to work with than earlier researchers. Michael Kuhn (Caltech) and collaborators homed in on the Sagittarius Arm, looking for data on infant stars — the paper refers to ‘young stellar objects’ (YSO) — still nestled in the molecular clouds of their birth nebulae; these are thought to align with the shape of the arms in which they are found. Indeed, in other spiral galaxies star formation follows the features of the spiral pattern.
The researchers drew on a Spitzer-derived catalog called SPICY (Spitzer/IRAC Candidate YSO) to study a nearby portion of the Sagittarius Arm, using the information to map out star-forming regions and compare their actual distribution with models of the arm. They also used a catalog derived from Spitzer data called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) containing more than 100,000 newborn stars. But it took Gaia to map the result in 3-D.
The European Space Agency’s mission tracks the motions, luminosity, temperature and composition of stars down to magnitude 20. Its most recent data release, EDR3, contains astrometry information on the distances and velocities of these objects. Using EDR3, the authors inferred the parallax and proper motions for their star-forming regions and estimated their radial velocities. The team rightly points to the “renaissance in investigations of Galactic spiral structure within a few kiloparsecs of the Sun” that Gaia’s positional and radial velocity measurements have produced. The Gaia achievement is not exaggerated: Thanks to this mission, we can now examine the position and kinematics of over a billion stars.
Out of all this comes evidence for a long, thin structure of young stars moving roughly with the Sagittarius Arm and in the same direction. Kuhn points out that as the structure of a spiral galaxy becomes more open, its pitch angle increases. If a circle has a pitch angle of 0 degrees, the most recent models of the Milky Way show that the Sagittarius Arm has a pitch angle of 12 degrees. The new structure breaks radically here, with an angle of nearly 60 degrees.
Previous studies have suggested a linear structure in this region, but the high pitch angle of this spur has not previously been noted. According to the authors’ models, the structure would appear as a bright stellar feature from another galaxy, paralleling the same kind of high pitch-angle structures found in many. Gravitational instabilities may account for their formation, with mass concentrations within the arms shearing as a consequence of the galaxy’s rotation. In any case, the authors see the structure as “an excellent laboratory for examining star formation on scales large enough to be compared to extragalactic observations, but with the ability to resolve the mass function, spatial distribution, and kinematics of the individual sources.”
Robert Benjamin (University of Wisconsin-Whitewater) is principal investigator for the GLIMPSE survey:
“Ultimately, this is a reminder that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we want to understand that bigger picture. This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”
Image: These four nebulae (star-forming clouds of gas and dust) are known for their breathtaking beauty: the Eagle Nebula (which contains the Pillars of Creation), the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula. In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure. In the new study, astronomers have shown that these nebulae are part of a substructure within the arm that is angled differently from the rest of the arm. Credit: NASA/JPL-Caltech.
The paper is Kuhn et al., “A high pitch angle structure in the Sagittarius Arm,” Astronomy & Astrophysics Vol. 651, L10 (July 2021). Full text.
by Paul Gilster | Aug 17, 2021 | Outer Solar System |
Rotorcraft have certainly been in the news lately, with Ingenuity, the Mars helicopter, commanding our attention. The Dragonfly mission to Titan involves a far more complex rotorcraft capable of visiting numerous destinations on the surface. In fact, Dragonfly makes use of eight rotors and depends upon an atmosphere more helpful than what Ingenuity has to work with on Mars. Titan’s atmosphere is four times denser than what we have on Earth, allowing Dragonfly to move its entire science payload from one location to another as it examines surface landing zones while operating on a world whose gravity is but one-seventh that of Earth.
I want to call your attention to the publication of the science team that just appeared in the Planetary Science Journal, because it lays out the rationale for the various decisions made thus far about operations on and above Titan’s surface. It’s a straightforward, interesting read, and makes clear how much work we have to do here. Yes, we had Cassini for a breathtaking tour that lasted 13 years, with repeated flybys and investigations on Titan using radar, but while we know a lot about structures like lakes and mountains on the surface, we know all too little about their composition.
In fact, as Alex Hayes points out, we didn’t know at the time Cassini launched whether the Huygens probe would find a global ocean at Titan or a solid surface of ice and organics. Because of the uncertainty, the Huygens science experiments were primarily atmospheric, meant to function during the descent phase. Hayes (Cornell University) is a co-investigator for Dragonfly. He adds:
“The science questions we have for Titan are very broad because we don’t know much about what is actually going on at the surface yet. For every question we answered during the Cassini mission’s exploration of Titan from Saturn orbit, we gained 10 new ones.”
Image: What we do know. This is Figure 1 from the paper. Caption: Dragonfly will image from the surface to provide context for sampling and measurements, as well as in flight to identify sites of interest at a variety of locations. (Left) Huygens image of Titan’s surface; cobbles are 10-15 cm across and may be water ice (Tomasko et al. 2005; Keller et al. 2008; Karkoschka & Schröder 2016a). (Right) Huygens aerial view of terrain akin to the diverse equatorial landscapes that Dragonfly will traverse and image at higher resolution. Credit: NASA/ESA/Barnes et al.
Dragonfly’s 2.7 year mission, starting upon arrival at Titan in 2034 during winter in the northern hemisphere, will commence at a landing site that was chosen for its safety factors (broad, relatively flat terrain) as well as its proximity to nearby interesting scientific targets. The goal is to set down at the equatorial dune fields called Shangri-La, which NASA notes are similar to the dunes found in Namibia on Earth. A series of short flights will explore this area before longer flights of up to eight kilometers begin, the beauty of the design being that Dragonfly will be able to sample interesting surface areas along the route to its destination, the Selk impact crater.
As the mission now stands, the lander should log on the order of 175 kilometers across Titan enroute to Selk. The latter is an interesting place because there is evidence here of past liquid water as well as organics, complex molecules containing carbon, along with hydrogen, oxygen and nitrogen. Methane rain and a snow of organics keep Titan’s weather systems complex amidst a landscape containing the building blocks of life.
But let’s get back to that landing site. The paper refers to Shangri-La as an “organic sand sea,” with the touchdown site located 134 kilometers south of Selk Crater, and approximately 175 kilometers north-northwest of the Huygens Landing Site. The image below is Figure 7 from the paper, giving the landing site in context.
Image: Dragonfly landing site. Credit: Barnes et al.
As the paper notes, a ‘sand sea’ is only partially sand. Dunes can be separated by flat sand-free areas called ‘interdunes,’ a feature likewise common to Namibia, where the Namib sand sea is covered only 40 percent by sand. The interdunes that make up the balance are primarily gravel. Cassini was able to resolve Titan’s interdunes to reveal their predominantly icy character, one that matches the spectral properties at the Huygens landing site. The authors find the correlation interesting because it implies the Shangri-La interdunes will include water-ice gravels, “potentially a fine-grained layer damp with condensed methane,” and thus offering a chance for Dragonfly to sample both Titan’s organic sands and materials with a water ice component.
Image: This illustration shows NASA’s Dragonfly rotorcraft-lander approaching a site on Saturn’s exotic moon, Titan. Taking advantage of Titan’s dense atmosphere and low gravity, Dragonfly will explore dozens of locations across the icy world, sampling and measuring the compositions of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry. Credit: NASA/JHU-APL
The path to Selk Crater should take in a variety of terrain with different compositions, which will include the edge of the crater’s ejecta deposits. From Cassini data, the authors believe this material is similar in composition to the Huygens landing site, representing an area likely to feature water ice. Selk itself is 80 kilometers in diameter. Cassini data along with the Dragonfly team’s modeling show the spectral signature of organic sand in the interior and water-ice around the edges of the crater floor.
As you can see, the astrobiological examinations Dragonfly will engage in are both water-based and hydrocarbon-based, meaning a potential biosignature is possible from impact melt deposits or interactions with the interior ocean — this would be life as we know it — or from a form of life we have yet to discover that draws on liquid hydrocarbons within Titan’s lakes, seas and aquifers. The mission is designed around the ability to seek out both, as the paper explains:
We designed the science of Dragonfly around the themes of prebiotic chemistry, habitability, and the search for biosignatures, with an explicit consideration of both water and hydrocarbon solvents. To address prebiotic chemistry, we will determine the inventory of prebiotically relevant organic and inorganic molecules and reactions on Titan. In the realm of habitability, we will determine the role of Titan’s tropical atmosphere and shallow subsurface reservoirs in the global methane cycle, determine the rates of processes modifying Titan’s surface and rates of material transport, and constrain what physical processes mix surface organics with subsurface ocean and/or melted liquid-water reservoirs. Our search for biosignatures will entail a broad-based search for signatures indicative of past or extant biological processes.
The paper is Barnes et al., “Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander,” Planetary Science Journal Vol. 2, No. 4 (full text).
by Paul Gilster | Aug 13, 2021 | Asteroid and Comet Deflection |
In addition to its sample return mission at asteroid Bennu, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has tightened our projections about the object’s future trajectory. Although the impact possibility on Earth through the year 2300 is on the order of 1 in 1750 (0.057%), it’s an object we want to keep an eye on, because in 2135 Bennu will make a close approach to Earth that could nudge its trajectory in ways that are difficult to anticipate.
OSIRIS-REx spent more than two years working near the 500-meter wide asteroid, studying its mass and composition while tracking its spin and orbital trajectory. In terms of the latter, even factors as tiny as the force the spacecraft exerted during its sample collection event in October of 2020, a mere touch-and-go, had to be considered (the study confirms that the effect was negligible). Far more significant is the Yarkovsky effect, which occurs as solar heating eases on the nightside during the asteroid’s rotation and is radiated away as infrared energy, generating thrust.
It’s the tiniest of effects at Bennu, says Steve Chesley (JPL), a co-investigator on the study that has just appeared in Icarus, but of course it builds over time and has consequences for the asteroid’s path:
“The Yarkovsky effect will act on all asteroids of all sizes, and while it has been measured for a small fraction of the asteroid population from afar, OSIRIS-REx gave us the first opportunity to measure it in detail as Bennu traveled around the Sun. The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennu’s future impact chances over the decades and centuries to come.”
Imagine how tricky it is to measure the cumulative effects of the Yarkovsky effect on a rotating object of uneven shape. Three close encounters in 1999, 2005 and 2011 were extensively tracked from the ground, but OSIRIS-REx data from the object itself have now allowed researchers to model an asteroid’s trajectory to the highest level of precision ever. Bennu’s future path is well known up to 2135. The question during the close encounter in that year will be whether it will pass through a dangerous ‘gravitational keyhole,’ an area where, as it responds to the effects of Earth’s gravity, the asteroid could become more likely to present an impact threat in the future. There is one ‘keyhole’ solution that would result in an Earth impact in 2182, with an impact probability of 1 in 2,700 (or about 0.037%).
In addition to the Yarkovsky factor, scientists have to consider the gravitational influence of the Sun and other objects in the Solar System including more than 300 asteroids that could have some effect. The solar wind streaming at variable rates outward from the Sun has to be considered as a source of pressure, and remember that OSIRIS-REx also discovered that Bennu was shedding rock particles, likely the result of thermal fracturing due to heating and cooling during rotation. These events along with the drag caused by interplanetary dust all affect the future trajectory.
Image: This mosaic of Bennu was created using observations made by NASA’s OSIRIS-REx spacecraft that was in close proximity to the asteroid for over two years. Credit: NASA/Goddard/University of Arizona.
The authors note that a close approach to Earth that will occur in 2037 will be the next opportunity to collect radar data and therefore gauge the accuracy of their work while improving the trajectory projections even more. In their conclusion they add this (and note that we are dealing with not one but numerous gravitational keyholes)::
…improved orbital knowledge allowed us to refine the impact hazard assessment, which we extended through 2300. The dense structure of keyholes on the B-plane of the 2135 encounter with Earth (Chesley et al., 2014) made it unlikely to avoid all possible pathways to impact. Still, the uncertainties for the 2135 encounter decreased by a factor of about 20, and so many of the most significant impacts found by Chesley et al. (2014) are now ruled out.
The fact that we have a sample return on the way adds another plus for OSIRIS-REx. Dante Lauretta (University of Arizona) is principal investigator for the mission:
“The spacecraft is now returning home, carrying a precious sample from this fascinating ancient object that will help us better understand not only the history of the solar system but also the role of sunlight in altering Bennu’s orbit since we will measure the asteroid’s thermal properties at unprecedented scales in laboratories on Earth.”
The opportunity given us by OSIRIS-REx to test models and calculate future trajectory probabilities shows what can be done with objects with even a remote chance of striking the planet. The trajectory changes coming in 2135’s close approach will be watched carefully by way of further understanding how to tighten such calculations. By then, in the highly unlikely event of a dangerous deflection toward impact, we can hope to have methods in place to force a further trajectory change of our own.
The paper is Farnocchia et al., “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” published online by Icarus 10 August 2011 (full text).
by Paul Gilster | Aug 12, 2021 | Exoplanetary Science |
Looking at flare activity in young M-dwarf stars, as we did in the last post, brings out a notable difference between these fast-spinning stars and stars like the Sun. Across stellar classifications from M- to F-, G- and K-class stars, there is commonality in the fusion of hydrogen into helium in the stellar cores. But the Sun has a zone at which energy carried toward the surface as radiative photons is absorbed or scattered by dense matter.
At this point, convection begins as colder matter moves downward and hot matter rises. This radiative zone giving way to convection is distinctive — stars in the M-class range, a third of the mass of the Sun and lower, do not possess a radiative core, but undergo convection throughout their interior.
Image: Interior structure of the Sun. Credit: kelvinsong / Wikimedia Commons CC BY-SA 3.0.
If we’re going to account for magnetic phenomena like starspots, flares and coronal mass ejections, we can come up with a model that fits stars with a radiative core, but fully convective stars might be expected to have a different kind of magnetic dynamo. What stands out in the data, however, is that the relationship between the star’s rotation and its magnetic activity appears the same for stars on both sides of what I might call the ‘convective divide.’ In both cases, the magnetic dynamo seems to be efficient despite the fact that M-dwarfs are fully convective.
Digging further into the subject is a new paper out of Rice University, where modeling of these phenomena examines the linkage between the rotation of stars and the behavior of their surface magnetic flux. The flux in turn governs the luminosity of the star at X-ray wavelengths, giving us a way to probe magnetic activity and its potential effects on planets in these systems. The paper explaining the new model has just run in The Astrophysical Journal. Lead author Alison Farrish comments on the implications over time as rotation periods change:
“All stars spin down over their lifetimes as they shed angular momentum, and they get less active as a result. We think the sun in the past was more active and that might have affected the early atmospheric chemistry of Earth. So thinking about how the higher energy emissions from stars change over long timescales is pretty important to exoplanet studies.”
Image: Rice University scientists have shown that “cool” stars like the sun share dynamic surface behaviors that influence their energetic and magnetic environments. Stellar magnetic activity is key to whether a given star can host planets that support life. Credit: NASA.
Farrish and team used the Rossby number of our own star to model the behavior of other stars. This value measures stellar activity through the combination of rotational speed and subsurface liquid flows that influence how the magnetic flux is distributed on the stellar surface. Presumably the magnetic field in stars with a radiative zone is generated at the interface between the interior radiative region and the outer convective zone. The Rossby number relates the rotation of the star — determined through observation — to the internal convective activity of the star.
The results affirm that the mechanisms producing local ‘space weather’ are common across different stellar classes, meaning we can with some confidence examine planetary systems around M, F, G and K stars using the same model. The process generating a star’s magnetic field may thus turn out to be similar despite the presence, or lack, of a radiative core. Adds co-author Christopher Johns-Krull:
“A lot of ideas about how stars generate a magnetic field rely on there being a boundary between the radiative and the convection zones, so you would expect stars that don’t have that boundary to behave differently. This paper shows that in many ways, they behave just like the sun, once you adjust for their own peculiarities.”
Thus an M-dwarf with a Rossby number typical for its class shows magnetic behaviors close enough to the Sun for us to make predictions about their effect on its planets. The stellar magnetic field data are, in turn, affected by the activity cycle of individual stars, which the model does not include because this would demand lengthy observational study for each star. But from the perspective of magnetically active stars, the new model from Farrish and team can be applied to interactions within their systems.
We’re a long way from knowing whether M-dwarf systems like that at Proxima Centauri or the intriguing TRAPPIST-1 and L 98-59 could support living planets, but our models for their magnetic interactions can draw on what we see in our own Sun, despite its differences in age and stellar class. Refining that model for these systems will help us determine the most likely M-dwarf candidates for habitability.
The paper is Farrish et al., “Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars,” Astrophysical Journal Vol. 916, No. 2 (3 August 2021). Abstract.
