Centauri Dreams

Earlier in the week I talked about Astronomy Rewind, an ambitious citizen science project dedicated to recovering old astronomical imagery and digitizing it for comparison with new data. Now I’ve learned that another citizen science effort, Planet Finders, is working with simulated data from the Transiting Exoplanet Survey Satellite (TESS), planning to transition into real TESS data as soon as they become available. Have a look at this effort here if you are interested in becoming a beta tester. TESS will be a hugely significant exoplanet mission particularly in terms of nearby stars, so becoming a part of this project should be an exciting venture indeed.

On with today’s post, which I would have actually run yesterday if I had read the paper soon enough, as it offers insights into Wednesday’s entry on protoplanetary disks. As we’ve seen, these can become the discovery grounds for young planets. In the case of the 2-million year old CI Tau, that meant an already confirmed gas giant in a ‘hot Jupiter’ configuration, along with three other gas giants, two of them far from the central star. Hence the question: Where did the hot Jupiter CI Tau b form? Because if it migrated, it did so early in the history of this system.

Now we have a separate research effort attempting to show that planet migration within a protoplanetary disk can be identified through markers within the disk itself. The idea is to produce an observational marker that can guide future research and tell us whether, within a given system, a planet is moving inward through the disk or staying within its existing orbit.

Image: Astrophysicist Farzana Meru: Credit: University of Warwick.

Lead author Farzana Meru (University of Warwick, UK), working with colleagues at Cambridge University, describes an observational signature in young stellar system dust rings. As with the CI Tau work, the new study involves the Atacama Large Millimeter/submillimeter Array (ALMA), which is able to probe such disks in intricate detail, revealing structure in the form of clumps, gaps, spiral arms, crescents and rings. Even more to the point, by working at different millimeter frequencies, ALMA can identify clustering material in different particle sizes.

Image: Example of a protoplanetary disk. This is an ALMA image of the young star HL Tau and its disk. The image reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. The image was released Nov. 6, 2014. ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF).

The question, then, is what can variations in dust particle size tell us? The researchers perform simulations of movement within a ring of gas and dust in the presence of migrating planets of relatively low mass, from 12 to 60 Earth masses. The results show that the gas pressure profile of the disk varies significantly depending on whether the planet is migrating or stationary.

From the paper:

…the pressure perturbation exterior to the planet is weaker while that interior to the planet becomes more important for migrating planets. Dust can therefore be enhanced both interior or exterior to the planet and the result is governed by the relative values of the planet and dust velocities. For small sizes, the dust velocity in the outer disc is too small to keep up with the moving pressure maximum while in the inner disc it moves faster and can collect, forming a dust density enhancement interior to the planet.

For larger dust particles, the dust velocity in the disk exterior to the planet’s orbit is high enough to keep up with the pressure perturbation, producing what the authors describe as a ‘dust density enhancement’ in this region. We would thus expect smaller particles in the interior ring, larger particles in the exterior. By studying the inner and outer rings around the planetary orbit at different wavelengths, we should be able to show whether or not the planet is migrating.

We’re talking about small differences indeed — a migrating planet at 30 AU that is 30 times the mass of the Earth should, according to this modeling, be associated with an inner ring consisting of particles less than a millimeter in size, while those in the outer ring would measure slightly over a millimeter. But this is a workable observable, for as ALMA increases the wavelengths at which it observes, the inner dust ring fades while the exterior ring becomes brighter. This is because the emissivity of dust grains depends on the maximum grain size in the mixture.

Image: This is the paper’s Figure 7. Caption: “Dust density rendered simulation image of the disc with a 30M? migrating planet at Rp = 0.75 for dust with Stokes numbers of 0.02 (left) and 0.2 (right). The small dust forms a ring interior to the planet while the large dust forms a ring exterior to it.” The ‘Stokes number’ refers to the movement of particles in flow. What is significant here is the separation of small- and large-particle dust accumulations. Credit: Meru et al.

The movement of the dust is the key. In the inner ring, we are seeing smaller particles because these move inwards more slowly than the planet itself, accumulating in the inner ring, while large dust particles are found in the exterior ring because they move at higher velocity than the smaller particles and keep pace with the planet in its movement inward. Over time, the composition of the two rings is distinctive enough to become an ALMA observable.

We’re entering an era when protoplanetary disks are becoming the subject of intense scrutiny thanks to facilities like ALMA, along with the Spectro- Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on the Very Large Telescope (VLT) and the Gemini Planet Instrument (GPI) on the Gemini Telescope. And we’re learning that most such disks show sub-structure susceptible to such analysis. We need observational benchmarks like these because a hot Jupiter like CI Tau’s can be the result of migration, but it could also achieve its tight orbit because of gravitational interactions early in the development of the system.

Applying observational constraints to planetary migration could become a potent tool in untangling a young system’s evolution. The work is promising but still in its early stages. In principle, write the researchers, “…it may be possible to use the location of dust rings in order to detect planetary migration, although the feasibility of this measurement is yet to be established.”

The paper is Meru et al., “Is the ring inside or outside the planet?: The effect of planet migration on dust rings,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).

Given their ubiquity in the Milky Way, red dwarfs would seem to offer abundant opportunities for life to emerge. But we’re a long way from knowing how habitable the planets that orbit them might be. While mechanisms for moderating the climate on tidally locked worlds in tight habitable zones continue to be discussed, the issue of flares looms large. That makes a new survey of 12 young red dwarfs, and the project behind it, of unusual interest in terms of astrobiology.

What jumps out at the reader of Parke Loyd and team’s paper is the superflare their work caught that dwarfed anything ever seen from our own Sun, a much larger star. It was enough to set Loyd, a postdoctoral researcher at Arizona State University, back on his heels.

“When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, ‘Whoa.'” He adds: “With the Sun, we have a hundred years of good observations. And in that time, we’ve seen one, maybe two, flares that have an energy approaching that of the superflare. In a little less than a day’s worth of Hubble observations of these young stars, we caught the superflare. This means that we’re looking at superflares happening every day or even a few times a day.”

Image: Violent outbursts of seething gas from young red dwarfs may make conditions uninhabitable on fledgling planets. In this artist’s rendering, an active, young red dwarf (right) is stripping the atmosphere from an orbiting planet (left). ASU astronomers have found that flares from the youngest red dwarfs they surveyed — approximately 40 million years old — are 100 to 1000 times more energetic than when the stars are older. They also detected one of the most intense stellar flares ever observed in ultraviolet light — more energetic than the most powerful flare ever recorded from our Sun. Credit: NASA, ESA, and D. Player (STScI)

Loyd’s work is under the aegis of a program called HAZMAT, which stands for HAbitable Zones and M dwarf Activity across Time (ASU’s Evgenya Shkolnik is principal investigator for this project). The issue of time is significant, for HAZMAT will survey young, intermediate and old M-dwarfs using data from the Hubble Space Telescope, and this initial paper focuses on stars that are roughly 40 million years old, mere infants given that this category of star can burn for as long as a trillion years.

As to that superflare, it’s easy to see why it gave Loyd pause. His team detected 18 flares from its 12 target stars, but the superflare swamped them all, emitting 10^32.1 erg in the far ultraviolet. That exceeds the most energetic flare from an M-dwarf previously observed by Hubble by a factor of 30.

Have a look at the paper’s description of what the authors call the ‘Hazflare’ in comparison to other flare observations from the past:

This observation is of particular value because superflares are common on stars (e.g., Davenport 2016), yet spectrophotometry of such flares in the UV, the band most relevant to planetary atmospheric photochemistry, is rare. Superflares are estimated from Kepler data to occur on M0-M4 dwarfs at a frequency of a few per day (Yang et al. 2017). Photochemical models exploring the effects of flares on planetary atmospheres have thus far relied primarily on observations of the 1985 Great Flare on AD Leo (Hawley & Pettersen 1991; Segura et al. 2010; Tilley et al. 2017), a flare estimated to emit a bolometric energy of 10³⁴ erg. The Great Flare also showed a clear continuum in FUV emission, and overall the continuum was responsible for at least an order of magnitude more overall energy emitted by the flare than lines, consistent with the Hazflare. However, the Great Flare observations, made with the International Ultraviolet Explorer, saturated in the strongest emission lines, degrading their accuracy.

Image: Observations with the Hubble Space Telescope discovered a superflare (red line) that caused a red dwarf star’s brightness in the far ultraviolet to abruptly increase by a factor of nearly 200. Credit: P. Loyd/ASU.

So the HAZMAT observations prove valuable indeed. Superflares like the one described in this paper are far more common in young dwarfs, which can erupt up to 1,000 times more powerfully in their youth than after they have aged, and the Kepler data on their frequency (above) are noteworthy. The mechanism: Strong magnetic fields twisted by the churning atmosphere of the young star, causing them to break and reconnect, a process producing huge amounts of energy. Such violent activity is associated with M-dwarfs in the first 100 million years of their lifetime.

You would think we could just wait out flare activity and assume that older M-dwarfs were our best bet for habitable conditions, but a key question is what kind of damage will already have been done. Pummeling from flares like these could cause atmospheric damage, perhaps even stripping the atmosphere from what might have been promising worlds in the zone where liquid water could exist on the surface. Ultraviolet and X-ray radiation, even if it leaves the atmosphere more or less intact, would also be a huge factor in determining the emergence of surface life, possibly acting as an evolutionary spur or conceivably preventing it from appearing at all.

We also have to learn what processes can replenish the atmosphere of a planet if it undergoes a period of intense UV bombardment followed by a gradually calmer stellar environment. The sheer longevity of red dwarfs gives reason to hope that life could eventually emerge, but we won’t know until we can make the kind of atmospheric observations that will be coming our way through missions like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope and the European Space Agency’s ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) observatory. The latter, to be launched in 2028, will make a large-scale survey of the chemistry of exoplanet atmospheres using transit methods.

But back to HAZMAT, which will move on next to intermediate-age red dwarfs some 650 million years old, followed by analysis of the radiation environment around much older M-dwarfs. The evolution of that environment will help us refine the target list for the above missions as we focus on star systems more likely to have life. Given that most of the habitable zone planets in the galaxy will have had to withstand high flare activity, we need to make modeling the effects of flare erosion on atmospheres a high priority task.

The paper is Loyd et al., “HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet,” accepted for publication at the Astrophysical Journal (preprint).

We’ve never found a ‘hot Jupiter’ around a star as young as CI Tau. This well studied system, some 2 million years old, has drawn attention for its massive disk of dust and gas, one that extends hundreds of AU from the star. But radial velocity examination recently revealed CI Tau b, a hot Jupiter that in and of itself raises questions. Couple that to the likelihood of three other gas giant planets emerging in the disk with extreme differences in orbital radii and it’s clear that CI Tau challenges our ideas of how gas giants, especially hot Jupiters, emerge and evolve.

Can a hot Jupiter form in place, or is migration from a much more distant orbit the likely explanation? The latter seems likely, and in that case, what was the mechanism here around such a young star? Most hot Jupiter host stars have lost their protoplanetary disks, which means that astronomers have been working with theoretical formation models to produce the observed tight orbits. And because about 1 percent of main sequence solar type stars (CI Tau is a K4IV object) have hot Jupiters around them, the riddle of their formation demands resolution.

The new work on CI Tau comes via high resolution imaging at submillimeter wavelengths at the Atacama Large Millimeter/submillimeter Array (ALMA), where Cathie Clarke (Cambridge Institute of Astronomy) and colleagues have found three distinct gaps in the star’s protoplanetary disk at ? 13, 39 and 100 au. The team’s paper in Astrophysical Journal Letters reports on computer modeling showing that these gaps are likely caused by additional gas giant planets.

Image: From Figure 1 of the paper, showing the protoplanetary disk around CI Tau. Credit: Clarke et al.

Making the find even more intriguing is that while CI Tau b is in an orbit not dissimilar from Mercury’s, the farthest putative planet orbits at a distance three times that of Neptune. All are large objects — the two outer worlds are roughly the mass of Saturn, while the two inner planets weigh in at 1 Jupiter mass and 10 Jupiter masses for the hot Jupiter. We’re left with the question of how these other planets affected the hot Jupiter’s orbital position, and whether there is a mechanism at work here that could apply to hot Jupiters in other systems.

For that matter, how did the two Saturn-class planets emerge where they are?

“Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don’t necessarily fit the models,” said Clarke. “Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star. Most models will struggle to make planets of this mass at this distance.”

No doubt. Complicating the picture further is that we have only learned about these planet candidates because of their effects on the protoplanetary disk, so whether or not extreme orbital parameters like these are common in hot Jupiter systems remains an open question. After all, older systems like those we’ve found other hot Jupiters in have already lost their disks.

The Jovian-class worlds may turn out to be easier to explain. Despite CI Tau’s relative youth, the authors argue that its hot Jupiter would still have had time to make the migration into hot Jupiter range. From the paper:

The hot Jupiter… could have been formed by a variety of mechanisms; from the modeled masses in disc and planets and from the accretion on to the star the inferred timescale for its inward migration is ? 0.4 Myr (Dürmann & Kley 2015) so that there would have been plenty of time for it to have migrated from a range of outward lying locations. The roughly Jovian mass planet inferred at 14 au is also easy to account for in terms of existing planet formation models (i.e. core accretion models involving either planetesimal or pebble accretion.

But those two outer ‘Saturns’ will need further work. We have a number of planetary disks with well-defined substructure to look at (HL Tau, HD 163296 and HD 169142, among others), but none with a hot Jupiter. Is this orbital configuration one that can survive on billion-year timescales? The authors believe these planets may still end up at small radii, suggesting they could be eventually ejected from the system by gravitational interactions. It will take future imaging surveys to tell us whether systems like the emerging one at CI Tau can be long-lived.

The paper is Clarke et al., “High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” Astrophysical Journal Letters Vol. 866, No. 1 (4 October 2018). Abstract / preprint.

It was back in 2012 that Eric Mamajek (University of Rochester) and team discovered a possible ring system around the star J1407 in lightcurves originally taken in 2007, spawning subsequent work with Leiden Observatory’s Matthew Kenworthy. And what a ring system it would be if confirmed. The diameter, based on the lightcurve, would be nearly 120 million kilometers. This would be a ring system nearly 200 times larger than the rings of Saturn, one containing an Earth’s mass of dust particles, and in early studies, one housing over thirty separate rings.

Image: Artist’s conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b. The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. Credit: Ron Miller.

The possible J1407 ring system provides a nice segue from yesterday’s post on recovering astronomical images from a century’s worth of scientific journals, as Centauri Dreams reader Andrew Tribick was quick to note in the comments to that post. For Robin Mentel, working under Kenworthy at Leiden University, has made an analysis of hundreds of photographic plates containing J1407 from 1890 to 1990, finding no eclipses by the planet J1407b. Let’s dig into what that means with the help of a new paper.

Mentel began two years ago studying some 868 photographic plates containing the star, the oldest being data from the Harvard DASCH project (Digital Access to a Sky Century @ Harvard). These results were supplemented with more recent photometry to constrain the orbital period of J1407b. He also examined plates from collections held by Bamberg and Sonneberg observatories, comparing the brightness of J1407 with two equally bright stars nearby. An obscuration by eclipsing planet and rings would dim the star, but no such obscuration was found.

Thus we have useful information to add to what was a remarkable lightcurve, as discussed in a 2015 paper by Kenworthy and Mamajek. Have a look.

Image: From the 2015 paper. The caption reads: “Model ring fit to J1407 data. The image of the ring system around J1407b is shown as a series of nested red rings. The intensity of the colour corresponds to the transmission of the ring. The green line shows the path and diameter of the star J1407 behind the ring system. The grey rings denote where no photometric data constrain the model fit. The lower graph shows the model transmitted intensity I(t) as a function of HJD. The red points are the binned measured flux from J1407 normalised to unity outside the eclipse. Error bars in the photometry are shown as vertical red bars.” Credit: Matthew Kenworthy/Eric Mamajek.

In 2015 I quoted Kenworthy on what the lightcurve seemed to imply:

“The details that we see in the light curve are incredible. The eclipse lasted for several weeks, but you see rapid changes on time scales of tens of minutes as a result of fine structures in the rings. The star is much too far away to observe the rings directly, but we could make a detailed model based on the rapid brightness variations in the star light passing through the ring system. If we could replace Saturn’s rings with the rings around J1407b, they would be easily visible at night and be many times larger than the full moon.”

So what exactly are we dealing with here? The photographic plates allow Mentel and team to calculate how long the period between eclipses might be given gaps in the photographic record. Bear in mind that J1407 is not a circumpolar star, meaning we lack photometric data for more than 50% of the year, when the star was not visible. The paper explains the methodology in detail, including the rejection of numerous plates that lacked reliable photometry of J1407.

Factoring all this in, there remains the possibility of another eclipse between 2021 and 2024, but no guarantees. From the paper:

We cannot confidently rule out any period above 25 years. Since very few plates have been taken before 1900 – 107 years before the 2007 transit – this method effectively cannot rule out any periods above 100 years. We expect the next transit no earlier than Spring 2018 (corresponding to an orbital period of 11 years) and most plausibly between Spring 2021 (orbital period of 14 years) and Spring 2024 (orbital period of 17 years).

And while the new work allows the scientists to exclude a range of orbital periods, it also allows for speculation on the planet’s mass and the shape of its orbit. Moreover, it leaves open the question of what the original observations of 2007 actually recorded:

Elliptical orbits with periastron passage during May 2007 allow a range of possible masses from 5 to 20 M_Jup and infer an ellipticity of about 0.75. In this case, the rings would fill 70 to 100% of the Hill sphere of [the] companion, and exceed the shrinking hill sphere during periastron by at least a factor of two.

The Hill sphere referred to above is the region within which a planet dominates the attraction of satellites (named after astronomer George William Hill, 1838-1914). In other words, the moon of a planet should lie within the planet’s Hill sphere if the planet is to hold onto it. The paper goes on to make a significant statement about the possible ring detection:

Although the range of orbital periods has been reduced, the lack of the detection of another eclipse means that we still cannot confirm that the eclipse event in 2007 was due to an extended object on a bound orbit around J1407.

If they are there, the rings should show up in continued photometric monitoring of the star, and the authors suggest direct detection of the rings at submillimeter wavelengths by ALMA or at optical wavelengths through the polarization of reflected light from J1407. The broader picture is that we are beginning to put imagery from older observations to work in their newly digitized form. Bear in mind that the DASCH project alone takes in 500,000 photographic plates between 1885 and 1993, and you can see the potential for discovery as well as the immense challenge involved in assembling these materials from observatories and universities all over the world.

The paper is Mentel et al., “Constraining the period of the ringed secondary companion to the young star J1407 with photographic plates,” accepted at Astronomy & Astrophysics (abstract / preprint). The Kenworthy and Mamajek paper from 2015 is “Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?” Astrophysical Journal Vol. 800, No. 2 (abstract / preprint).