Matthew Kenworthy's Research
This page lists all my current research activities, follow the links below to jump to the latest results.
- Direct Imaging of Exoplanets
- The Young Suns Exoplanet Survey
- APP coronagraphs
- Phase Sorting Interferometry
- Exoring systems
- The J1407 exoring system
- The Beta Pictoris b ring project (bRing)
- The J0600 system
Direct Imaging of Exoplanets
Discovering and characterising alien worlds
The majority of planets outside our Solar System (exoplanets) have been detected indirectly - through the reflex motion on their parent star (radial velocity) or by the regular dimming as they pass between us and their star (transit).
My research tackles three approaches to directly imaging exoplanets around nearby stars - by finding the best target stars to carry out direct imaging searches, by minimising the glare of the parent star by using coronagraphs, and finally measuring and removing optical errors introduced by the telescope optics by using focal plane wavefront sensing.
The YSES survey
Graduate student Alex Bohn is carrying out a survey for directly imaged exoplanets using the VLT instrument SPHERE, called the Young Sun Exoplanet Survey (YSES). We have a sample of 72 young stars from the Sco-Cen association that were identified by Eric Mamajek and Mark Pecaut.
Starting in 2017, we have surveyed all the stars for a first epoch measurement and are now starting the second epoch observations, but we have discovered:
Two Directly Imaged, Wide-orbit Giant Planets around the Young, Solar Analog TYC 8998-760-1by Bohn, A. J., Kenworthy, M. A., Ginski, C., et al. (2020) in A&A
Discovery of a directly imaged disk in scattered light around the Sco-Cen member Wray 15-788 by Bohn, A. J., Kenworthy, M. A., Ginski, C., et al. (2019) in A&A.
The APP Coronagraph
The coronagraph was invented by Bernard Lyot in 1930 to enable observations of the Sun's outer atmosphere. The faintly glowing extended gas in the sun's corona was only visible during a Solar eclipse by the Moon, and Lyot wanted to carry out observations without waiting years at a time.
The optical layout of his Lyot coronagraph is shown below:
The telescope focuses light from both the star and the planet at the Focal Plane, where a mask then blocks out light from the star, but the planet (which is in a slightly different place on the sky) is not blocked by the mask. A lens takes the light from the FP and forms a Pupil Plane (PP). Here, the planet's pupil is an even disk of light, but for the star, the presence of the metal disk in the FP causes light to be redistributed into a series of bright rings. A mask with a diameter smaller than the full telescope pupil image diameter then blocks most of this scattered star light - this is called a Lyot stop. A second lens then reimages the pupil to form the final sky image on the detector.
The Lyot coronagraph is simple, but it has several disadvantages: (i) If the star image is not kept centred on the metal disk, starlight "leaks" around the edge of the disk and into the rest of the coronagraph, increasing unwanted light and decreasing the signal to noise at the location of the planet. (ii) The Lyot stop blocks the outer edge of the telescope pupil, and so the angular resolution of the telescope is reduced - the planet image gets larger in diameter, and (iii) the total throughput of the coronagraph decreases due to the Lyot stop's smaller diameter, and there is a trade-off between throughput and contrast.
Many coronagraphs are sensitive to pointing errors, and so require fast closed tip-tilt loops to ensure that the target star stays in the same position in the FP and so that the suppression works. By designing an optic that modifies only the phase in a pupil plane of a telescope, we remove this constraint. This Phase Apodization Coronagraphy (Codona and Angel 2004) is realised in the form of an
By having no FP mask, there is no spurious diffraction in the PP. Instead there is the APP Coronagraph, which modifies the point spread function (PSF) of all objects in the field of view of the coronagraph.
The APP is completely insensitive to vibrations in the telescope and the AO system tip-tilt correction, it is a very simple coronagraph, consisting of only one opti, and the phase pattern is a smoothly varying function of position, with no discontinuities or vortex singularities. Mathematically ideal coronagraphs with these types of discontinuity are challenging to manufacture and can result in degradation of the coronagraphic null and reduced imaging performance.
The suppression in the dark zone, the Inner Working Angle and the encircled energy transmission are related, and have to be traded off against each other. For most designs we keep 60 to 70% of the encircled energy within the Airy disk of the PSF.
The original APP coronagraph introduced phase variations in the wavefront using differing Optical Path Differences (OPDs) of Zinc Selenide. Since this is an inherently chromatic process, it restricted the bandwidth of the APP.
Panchratam phase can be introduced using the fast axis orientation of a birefringent liquid crystal, which then introduces a phase retardance depending on the orientation of the fast axis. Working with Michael Escuti at NCSU, we can then realise complex phase patterns that are not possible using the diamond turned Zinc Selenide technique.
The liquid crystal layers are still chromatic, but multiple layers of liquid crystal can be added on top of each other and the layers align themselves automatically, building up a more achromatic sandwich. Typically three layers will make a 100% bandwidth coronagraph.
We have tested this new Vector APP coronagraph in the laboratory at Leiden and we have demonstrated its performance over a 50% bandwidth with two layers in Otten et al. (2014), Optics Express.
By using a combination of quarter wave retarder and Wollaston prism, two simultaneous PSFs with complementary dark holes are produced, along with the broadband performance, making this a highly robust coronagraph for exoplanet detection and characterization.
The addition of a phase ramp in the APP phase pattern causes the two APP PSFs to split, with an additional third non-coronagraphic PSF appearing between the two. This PSF is called the leakage term, and is due to any deviations in performance from the liquid crystal layers acting as a half-wave plate. This optic therefore does not require any quarter wave plates or Wollaston prism, and is therefore very simple to install in any exoplanet camera.
We now build
Phase Sorting Interferometry
Several methods have been developed to estimate the point spread function (PSF) using the science camera images themselves, notably Angular Differential Imaging (ADI), Spectral Differential Imaging (SDI), Locally Optimized Combination of Images (LOCI), Principal Component Analysis (PCA), and combinations of all the above.
Johanan Codona (University of Arizona) and I developed a method for estimating the Science camera PSF directly using the Wavefront Sensor Camera telemetry on ground-based adaptive optics systems. It is called Phase Sorting Interferometry (PSI) and is described in Codona and Kenworthy (2013) ApJ, 767, 100. In this paper we demonstrate on-sky measurement of the PSF at the science camera, and also derive several measures of the atmospheric turbulence.
We are implementing PSI for the ERIS camera and it is considered as the baseline focal plane wavefront sensing method for the ELT instrument METIS.
The J1407 exoring system
J1407 is a nearby young star that underwent a complex series of eclipses that lasted for almost two months in 2007. Our best explanation is that an (as yet) unseen massive substellar companion with a giant ring system passed in front of the star, causing the eclipse. We haven't seen another eclipse towards J1407 (or its full name, 1SWASP J140747.93-394542.6), but data from the ALMA telescope array has detected what might be the dust making up the rings themselves.
We now give a brief history of what we've done since the discovery was announced in 2012.
Mark Pecaut, a then graduate student under the supervision of Eric Mamajek at the University of Rochester, discovered the unusual light curve towards the end of 2011. Mark was looking at the light curves of young stars as part of his thesis research. It is difficult to determine the ages of most stars, so several different pieces of evidence are needed to confirm a young star. Looking through the public light curve database of the SuperWASP collaboration, he found 1SWASP J140747.93-394542.6, a star almost the same mass as the Sun, with a light curve that showed a 3% variability that repeated every 3.2 days, almost certianly due to fainter star spots on the surface of the star rotating in and out of view as the star rotated.
J1407 also showed a very long lasting eclipse event from April through May 2007, where the star's changed brightness on a nightly basis, sometimes as much as 95% of the flux was blocked by something moving in front of the star. The discovery paper (Mamajek 2012) contained a simple model including four large rings about 0.6 au in diameter surrounding an unseen substellar companion, called J1407b, which orbited around the parent star.
Follow up observations were carried out to try and detect J1407b, which must be several tens of Jupiters in mass in order to keep such a large ring system held together and not ripped apart by the gravity of the star. These included looking for another eclipse in other photometric data (no other eclipses were found), looking for the reflex motion of the star as the gravity of the unseen companion tugged on it (the star is young and is very active, preventing the method from being sensitive enough), and directly detecting the thermal glow of the companion itself (no - even with the largest telescopes and an optical trick called sparse aperture masking we did not see anything).
All these non-detections were still useful, since they allowed us to rule out possible orbits and masses for J1407b, detailed in Kenworthy et al. 2015.
One interesting conclusion we had was that there were no circular orbits for J1407b since we needed to explain why the ring system seemed to be either very big or moving very fast in front of the star, implying that it was in an orbit close to the star. This presented one big problem: our ring system seemed to now be very large indeed, so large that the rings would spill away from J1407b and fall towards the star. Putting J1407b on an elliptical orbit seemed to make the problem even worse.
This inspired Steven Reider to simulate a ring system around a planet that itself was in an elliptical orbit around the star J1407. The new trick was one of a simple spin - if the rings were added so that they span around J1407b in the same direction that the planet orbited around the star (called prograde motion) then the rings fell apart within a few orbits when the planet passed close to the star.
However, if the rings were spinning in the opposite direction (a retrograde motion) then the rings held together for a much longer time, so much so that we could find possible masses for J1407b and orbital periods that would work!
Even if these orbits worked, then another problem presented itself, in that the rings were tilted with respect to the orbital plane of the planet. Most planets rotate, and the centripetal force generated by this rotation causes the planets to bulge out along their equators, distorting them from ideal spheres into oblate spheroids (M&M or Smarties if you like candy/sweeties). This deviation from an ideal sphere means that any ring system orbiting around the planet will tend to align with the plane of the equator, as is the case with Saturn. Since J1407b is younger and more massive than Jupiter, the bulge around the equator is proportionally much bigger, marchalling the rings into this plane.
The rings of J1407b are about 200 times larger than Saturn's rings, and for the rings a long way away from the planet, the tidal forces of the star J1407 will force them down into the orbital plane of J1407b, some 20 degrees away. The rings should no longer be flat, but have a large systematic warp to them, as calculated by Zanazzi et al. in 2017.
Photographs of the night sky have been taken for over a century, and one of the largest collections is at Harvard University. These photographic plates are being scanned in digitally and published onto the internet through the DASCH project. The process is slow and time consuming, especially as you work towards the Galactic plane, where there are far more stars crowded together and disentangling the signals is challenging, but they reached the location in the Galaxy where J1407 is located around 2017. This enabled the baseline for the J1407 lightcurve to be extended back to the 1900's, but still no eclipses were detected. The coverage of the photographic plates from DASCH were combined with others from Sonneborg and other more recent all sky surveys that looked towards J1407, but still a null result, written up in Mentel et al. 2018.
Meanwhile, the star is being observed every clear night by the dedicated observers of the AAVSO. They watch J1407 for when the next eclipse will start. The constraints they provide rule out successively more obital periods, leaving J1407b with fewer orbital periods to hide in.
If the planet J1407b could not be detected, then how about looking for the rings directly?
The rings are probably made of small rock or icy particles, ranging from one micron in size up to rocks and boulders. The surface area of the dust in the rings is vast, so much so that they radiate thermal energy at millimetere wavelengths that can be detected with radio telescopes on the Earth. The most sensitive sub-millimetre telescope is the Atacama Large Millimetre Array (ALMA) in the Atacama desert in Chile. We were able to get observing time on this array of telescopes in the Summer of 2018, and to our surprise we detected a sub-mm source with the brightness we predicted for J1407b's rings.
After our initial joy, we then became confused again - because the ALMA source wasn't exactly in the same position as the location of the star. At these very long wavelengths, the star J1407 is not visible in ALMA, so we had to calculated the expected location of the star and compare it to the ALMA source. If the source is the same physical object that caused the eclipse in 2007, we run into a big problem - the source cannot be in a gravitaionally bound orbit around J1407.... but there's an very unlikely alternative explanation - that the object we called J1407b is floating freely through the Galaxy, and just happened to pass in front of a very young star.
This is very very unlikely to happen, but there's a first time for everything.... and to confirm this tantalizing idea, we need another image from the ALMA array. And as of 2020, we have not been able to get the observing time to confirm or deny our latest idea. If this happens to be true, though, then we will never see another eclipse of J1407.
So that is the latest and greatest information about J1407 and the ring system around J1407b. We still think that the eclipse in 2007 was caused by something orbiting J1407, and that one day we will see another eclipse that will confirm the incomplete light curve from 2007. Once we see that eclipse, all the geometry of the ring system is solved almost immediately. Until then, we tide ourselves over and patiently wait for a triggering signal from an observer who says "This star has started fading again!"
The Beta Pictoris b ring project (bRing)
Looking at one star continuously for over one year
In 2017 and the start of 2018, the circumplanetary environment of the gas giant exoplanet Beta Pictoris b crossed in front of the bright young star Beta Pictoris, a naked eye 4th magnitude star in the Southern skies. We wanted to see if any circumplanetary material crossed in front of the star over this 300 day period, and so I started the bRing project.
The J0600 system
Last updated around April 2021.