Centauri Dreams

I’m always interested in ways readers can dig directly into data from our telescopes, and this morning I can point to two. I’ll begin with the Lick Carnegie Exoplanet Survey, which has just released 60,949 precision Doppler velocities for 1,624 stars. The data draw on observations using HIRES (the High Resolution Echelle Spectrometer) on the Keck 1 telescope on Mauna Kea (Hawaii). As exoplanet hunter Greg Laughlin (UC-Santa Cruz) explains on his systemic site, the data contain hundreds of possibly planetary signals.

If you’d like to dig into this material, which includes hints of a super-Earth around the fourth closest star to the Sun (Lalande 21185), I’ll remind you of Stefano Meschiari’s Systemic Console, developed with Laughlin as a way of exploring exoplanetary data. The latest version completely reworks the older Console and provides the tools needed to study the Lick Carnegie material. Versions of this open source software are available here, and a visit to the Earthbound Planet Search website will yield links to the Lick Carnegie data release.

As to the stars under examination, they are primarily the nearest and brightest F, G, K and M-class stars visible from Mauna Kea. All are within 100 parsecs (326 light years), though some include a relatively short range of data, while others offer a deeper dataset. Accompanying the data release is an upcoming paper, “The LCES HIRES/Keck Precision Radial Velocity Exoplanet Survey,” slated for publication at The Astronomical Journal (preprint available).

Discussing the data release on systemic, Laughlin noted an interesting fact as he placed the Lick Carnegie work in context:

…as the breakthroughs rolled in, the Keck I Telescope was gradually accumulating Doppler measurements of hundreds of nearby Sun-like stars with HD designations and magnitudes measured in the sevens and eights. This data is as important for what it shows (scores of planets) as for what it doesn’t show (a profusion of planets with Jupiter-like masses and orbits). There are several reasons why our Solar System is unusual, and Jupiter is one of them.

Image: From Greg’s systemic discussion, drawing on Rowan 2016.

Just how unique is our Solar System? It’s an intriguing question to keep in mind, and one that provides additional motivation for digging into the data in classrooms and in the home. Laughlin has spoken before about ‘democratizing’ the search for planets, which is all about making not just the data but the software tools available for trying out possible planetary orbits that would explain what we’re seeing. Expect much more as this dataset is examined.

Backyard Worlds: Planet 9

NASA’s Wide-field Infrared Survey Explorer (WISE) mission captured a lot of attention in these pages as it scanned the entire sky between 2010 and 2011. Although its primary mission ended in 2011, WISE was reactivated in 2013 with the charter of finding near-Earth objects that could be potentially hazardous. The latter mission was named the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).

A lot of us were keeping an eye on M-class dwarfs and, especially, brown dwarfs, wondering whether a dim object might not exist much closer to the Earth than the Alpha Centauri stars. If WISE had found something like this, it would have given us a much closer target for an interstellar mission, but no such target has yet emerged. Even so, we’re not quite through with the WISE data. There may be brown dwarfs waiting to be found therein, not to mention distant objects in our own Solar System including, if it exists, the as yet undetected Planet Nine.

Image: This diagram shows the orbits of several Kuiper Belt objects that were used to infer the existence of Planet 9. If it exists, Planet 9 may reveal itself in WISE infrared images. Credit: ASU.

Making such finds now comes down to looking for moving objects in WISE images, an effort in which human eyes have a role to play, because computers can trip over image artifacts in crowded parts of the sky. Thus we get Backyard Worlds: Planet 9, a new website that puts computer users — citizen scientists — to work doing what Clyde Tombaugh did in the search for Pluto, studying imagery to pick out the moving object while disregarding the artifacts.

The site works through ‘flipbooks’ — brief animations that show the changes in small parts of the sky over time. Objects spotted by sharp-eyed observers will be prioritized for future investigation. The image below illustrates the process with a real WISE discovery.

Image: Previously known brown dwarf WISE 0855-0714 is seen here in this Backyard World flipbook as a moving orange dot at upper left. Citizen scientists will be asked to inspect images just like this to search for new objects in the solar neighborhood. Credit: NASA.

Working in the infrared, WISE produced data on nearly 750 million individual sources in the sky, working at infrared wavelengths that could reveal objects emitting at low temperatures. In other words, there’s plenty of work here for those patient enough to study these images. “There are just over four light-years between Neptune and Proxima Centauri, the nearest star, and much of this vast territory is unexplored,” said the lead researcher for Backyard Worlds: Planet 9, Marc Kuchner, an astrophysicist at NASA’s Goddard Space Flight Center.

Bringing the human ability to spot movement against a fixed background, and bringing in large numbers of volunteers to work the project, allows all of us to get involved with these ongoing investigations. Can we find what the computers have missed amidst the virtual noise?

It’s no surprise that planets can affect the stars they orbit. We’ve used that fact for several decades now, relying on radial velocity studies that showed the movement of a star toward us and then away again as it was tugged on by the planet under investigation. But now we’re hearing about another kind of planetary effect, one whose future uses may be intriguing. We’re seeing a star’s brightness change in evident synchrony with a planetary orbit.

The star is some 370 light years away from the Earth. The planet in question is HAT-P-2b, a ‘hot Jupiter’ in a highly elliptical orbit that makes its closest approach to the star every 5.6 days. The planet, discovered by the automated HATNet project (Hungarian Automated Telescope Network), is about 8 times Jupiter’s mass. The temperature changes its orbit should induce in its atmosphere led indirectly to the brightness discovery, for researchers led by Julien de Wit (MIT) wanted to learn about the circulation of energy in the planet’s atmosphere, causing them to turn to the Spitzer infrared telescope for data.

The dataset de Wit and colleagues consulted contained some 350 hours of observation between July of 2011 and November of 2015. The planet’s passage close to and then behind the star as seen from Earth was the key window of interest, as changes to the star’s brightness can reveal how much energy it is delivering to the planet. The method has been used with success before on planets like HD 149026b, likewise studied with Spitzer data.

What the researchers found was a bit of a surprise. When the planet passed behind the star, oscillations in the star’s light with a period of 87 minutes became visible. The signals were tiny — de Wit compares them to the sound of a mosquito next to a jet engine — but they demanded attention because their period was an exact multiple of the planet’s orbital frequency.

The paper recounts the team’s exhaustive analysis of possible effects in the Spitzer instrument itself, and explains why the only conclusion they could reach was that this was a stellar, not an instrumental, effect. Bear in mind that while HAT-P-2b is a massive planet, it is dwarfed by its host star, which is more than 100 times more massive. A pulsation effect at this scale seems unusual, showing how much we have to learn about planet/star interactions.

“This is really exciting because, if our interpretations are correct, it tells us that planets can have a significant impact on physical phenomena operating in their host-stars,” says co-author Victoria Antoci, a postdoc at Aarhus University in Denmark. “In other words, the star ‘knows’ about its planet and reacts to its presence.”

Image: For the first time, astronomers have observed a star pulsing in response to its orbiting planet. The star, HAT-P-2, pictured, tracks its star in a highly eccentric orbit, flying extremely close to and around the star, then hurtling far out before eventually circling back around. Credit: NASA (edited by MIT News).

The find is interesting indeed, but fraught with issues. For one thing, pulsations on a star are far from unusual. Could what de Wit and company observed simply be a natural phenomenon unrelated to the planet? The answer is almost certainly no, because the oscillations are only observed during the planetary occultations, and the correspondence between the pulsations and the planet’s orbital frequency implies a strong connection between the two.

Pulsations caused by tidal effects have been seen before in binary star systems (these are known as ‘heartbeat’ stars), and the researchers turn to tidal effects as an explanation. From the paper:

The photometric observations also show no sign of orbit-to-orbit variability nor of orbital evolution but reveals high-frequency low-amplitude stellar pulsations that correspond to harmonics of the planet’s orbital frequency, supporting their tidal origin.

But the explanation only goes so far:

Current stellar models are however unable to reproduce these pulsations. HAT-P-2’s RV [radial velocity] measurements exhibit a high level of jitter and support the orbital evolution of HAT-P- 2 b inconsistent with the ultra-precise eclipse times. The inability of current stellar models to reproduce the observed pulsations and the exotic behavior of HAT-P-2’s RV indicate that additional observations and theoretical developments are required to understand the processes at play in this system.

We have, in other words, to gather more data, presumably through future missions like TESS, PLATO and CHEOPS, to understand these intriguing interactions. The star’s pulsations are the tiniest variations of light from any source that Spitzer has ever measured. But on a broader front, are we seeing the development of a new kind of exoplanet discovery tool, one that would work regardless of the orbital inclination of an undetected planet? The paper speculates on the possibility, noting that such detections could flag stars that we would then want to submit to examination with direct imaging and radial velocity techniques.

The paper is De Wit, “Planet-Induced Stellar Pulsations in HAT-P-2’s Eccentric System,” Astrophysical Journal Letters 14 February 2017 (preprint).

We’re beginning to find evidence of objects like those in the Kuiper Belt beyond our own solar system. In this case, the work involves a white dwarf whose atmosphere has been recently polluted by an infalling object, giving us valuable data on the object’s composition. The work involves the white dwarf WD 1425+540, whose atmosphere has been found to contain carbon, nitrogen, oxygen and hydrogen. The findings are unusual because white dwarfs are the dense remnants of normal stars, with gravitational fields strong enough to pull elements like these out of their atmospheres and into their interiors, where they are immune from detection by our instruments. And that implies a relatively recent origin for these elements.

Lead author Siyi Xu (European Southern Observatory) and team worked with spectroscopic observations from HIRES (the High Resolution Echelle Spectrometer) on the Keck Telescope and included data from the Hubble instrument. The researchers believe the white dwarf’s atmosphere has been enriched by the breakup and eventual spiral into the star of a minor planet, whose composition mimics what we find in Kuiper Belt objects. WD 1425+540, some 200 light years away in the constellation Boötes, thus absorbed a body that is calculated to have been composed of 30 percent water and other ices and 70 percent rocky material.

The event, which would have involved the gravitational disruption of the object’s orbit, causing its infall, disintegration and the eventual absorption of its elements by the star, may have occurred as recently as 100,000 years ago. We could be seeing a process that has implications for the existence of life on planets in the inner system like our own. The Earth may well have been dry when it first formed, with life’s building blocks delivered as the result of collisions with other objects. Siyi Xu sees this as a process that can occur anywhere:

“Now we’re seeing in a planetary system outside our solar system that there are minor planets where water, nitrogen and carbon are present in abundance, as in our solar system’s Kuiper belt. If Earth obtained its water, nitrogen and carbon from the impact of such objects, then rocky planets in other planetary systems could also obtain their water, nitrogen and carbon this way. We would like to know whether in other planetary systems Kuiper belts exist with large quantities of water that could be added to otherwise dry planets. Our research suggests this is likely.”

Image: Rendering of a white dwarf star (bright white spot), with rocky debris from former asteroids or a minor planet that has been broken apart by gravity (red rings). Credit: University of Warwick.

Temperatures in the protoplanetary disk of our own system are thought to have determined the distribution of water, with dry rocky worlds inside the snow line, and water ice available beyond it, much of it still to be found in asteroids, comets and Kuiper Belt objects. Nitrogen ice can also be found in comets from the outer regions of the Kuiper Belt and in the Oort Cloud. The assumption has been that similar conditions prevailed around other stars, but our knowledge of analogs to Kuiper Belt objects in other systems has remained scant.

This makes white dwarfs like WD 1425+540 a useful tool, for the detection of heavy elements in their atmosphere has to point to an external source, giving us a way to measure the broad composition of objects in the system. And indeed, the paper notes that there are at least a dozen ‘polluted’ white dwarfs whose accreted material has been measured in detail. Excess oxygen that can be attributed to water-rich objects has been detected in only three.

From the paper:

The accreting material observed in WD 1425+540 provides direct evidence for the presence of KBO analogs around stars other than the Sun. In addition, WD 1425+540 has a K dwarf companion at 40.0 arcsec (2240 au) away (Wegner 1981). The presence of a distant stellar companion can impact the stability of extended planetary systems and, thus, enhance the chances of perturbing objects – that initially orbit far from a white dwarf – into its tidal radius via the Kozai-Lidov mechanism (Zuckerman 2014; Bonsor & Veras 2015; Naoz 2016).

Interestingly, the authors have worked parameters for the disrupted object in the system. When it was on the main sequence, WD 1425+540 would have been about twice as massive as the Sun and 10 times more luminous. Its nitrogen-bearing KBO analogs would have been found at about 120 AU, or three times further from the star than the Sun’s Kuiper Belt objects. Add in the star’s red giant phase and its Kuiper Belt moves out by a factor of 3. Thus the object that was accreted by WD 1425+540 likely orbited beyond 300 AU before experiencing the gravitational disruption that drove it inward toward the star and its eventual destruction.

Most material accreted onto white dwarfs in previous studies of polluted atmospheres has come from dry minor planets. This paper argues that the spectroscopic work on WD 1425+540 gives us strong evidence for volatile-rich planetesimals around other stars:

With this new dataset, we can conclude that extrasolar terrestrial planets could have volatile element and water abundances provided by KBO analogs that are comparable to those on Earth.

The paper is Xu et al., “The Chemical Composition of an Extrasolar Kuiper-Belt-Object,” Astrophysical Journal Letters 836, L7 (2017). Abstract / preprint.

When it comes to the nearest stars, our focus of late has been on Proxima Centauri and its intriguing planet. But of course the work on Centauri A and B continues at a good clip. The prospects in this system are enticing — a G-class star like our own, a K-class dwarf likewise capable of hosting planets, and the red dwarf Proxima a scant 15000 AU away. Project Blue examines how we might image planets here as our radial velocity studies proceed.

But we have much to learn, and not just about possible planets. A new paper by Pierre Kervella (Observatoire de Paris), working with Lionel Bigot and Fréderic Thévenin (both at the Observatoire de la Cote d’Azur), reminds us of the importance of firming up our stellar data.

We need to learn as much as possible about Centauri A and B not just because we’d like to find planets there but also because the work has implications for space missions, including the ESA’s Gaia, which will tighten our distance measurements to many stars. The Alpha Centauri stars are important benchmarks for Gaia, putting the emphasis on an accurate calibration of the basic stellar parameters in this system.

Image: The Alpha Centauri stars in comparison to our Sun and other Solar System objects. Credit: Pierre Kervella.

Kervella and team have used new observations of Centauri A and B with the Very Large Telescope Interferometer equipped with the PIONIER (Precision Integrated Optics Near-infrared Imaging ExpeRiment) beam combiner to operate in the near infrared. Their paper reports on improved measurements of both stars’ angular diameters in relation to the phenomenon known as limb darkening. The latter results point to the need to improve our models as we study the photospheres of stars including our own Sun.

Limb darkening refers to the gradual decrease in brightness that we see as we look away from the center of a star toward its outer edge, or limb. Have a look at the image below to see the effect, which is easily visible in photographs of the Sun. When we look at the center of the Sun’s disk, we see the greatest light emission because we are viewing the deepest, and hottest layers, while at the limb, we are seeing only cooler layers that produce less light.

The phenomenon is important because we can use it to study how a star’s atmosphere is structured, but it turns up as a factor in everything from eclipsing binary stars to gravitational microlensing. Moreover, limb darkening will affect the shape of the transit curve produced by a planet moving in front of its star. The planet blocks a smaller part of the star’s light when it is near the limb, and a greater fraction as it moves toward the center of the star. The center of a transit, in other words, is always going to be deeper than the edges, something that would not happen if a star had a uniform brightness (there the transit ‘curve’ would appear flat).

Image: A filtered image of the Sun in visible light, showing the limb-darkening effect as a dimmer luminosity towards the edge or limb of the solar disk. The image was taken during the 2012 transit of Venus (seen here as the dark spot at the upper right). Credit: Brocken Inaglory / Wikimedia Commons.

What the researchers find is that there is a discrepancy between the measured limb darkening parameters and the models previously discussed in the literature — these overestimate limb darkening for both Centauri A and B. While the difference is small and does not affect the team’s measurements of the stars’ angular diameter, the limb darkening models need adjustment, a problem that is not limited to Alpha Centauri. From the paper:

This implies that the underlying atmosphere models deviate from the real intensity profiles of α Cen, and we note that similar discrepancies are observed on the Sun. The observed discrepancies indicate that the predictive accuracy of the current generation of model atmospheres may be significantly lower than expected. This is likely to be more critically the case for stars with parameters that are very different from those of the Sun (e.g., cooler stars with molecular envelopes) and for wavelength regions more complex to model than the near-infrared (e.g., the ultraviolet and visible).

Our interest in cooler stars like Proxima Centauri, and the useful transit depths that could be observed with exoplanets around nearby red dwarfs mean we need as accurate a limb darkening model as possible in order to measure their transits precisely. These observations for Centauri A and B also offer us benchmarks we can use to firm up our atmospheric models for more distant stars.

Image: Proxima Centauri in relation to familiar objects in our own system. Credit: Pierre Kervella.

Meanwhile, the radii of Centauri A and B are shown to line up with earlier work. For Centauri A, we get a radius of 1.2234 ± 0.0013 ± 0.0051 R_☉ (where R_☉ is the radius of the Sun). Centauri B yields 0.8632 ± 0.0009 ± 0.0036R_☉. As the paper notes, “Together with the parallax and masses recently reported by Kervella et al. (2016), as well as spectroscopic studies, the determined radii complete the calibration of the fundamental parameters of both components of α Centauri.”

The paper is Kervella, Bigot and Thévenin “The radii and limb darkenings of α Centauri A and B: Interferometric measurements with VLTI/PIONIER,” Astronomy & Astrophysics 597, A137 (2017). Abstract / preprint.