Publications

Vanderburg & Johnson — “A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission”

Posted by admin on September 15, 2014
Posted in: Publications, Uncategorized.
Comparison between raw K2 and corrected photometry. Figure 5 for Vanderburg & Johnson (2014).

Comparison between raw K2 and corrected photometry. Figure 5 for Vanderburg & Johnson (2014).

Read a neat paper from Andrew Vanderburg of John Johnson’s Exoplanets group at Harvard about working with data from the upcoming K2 mission.

Having suffered failures of two of its reaction wheels, required to accurately point the telescope, the Kepler mission has ended its nominal science investigation. However, clever engineering will allow the spacecraft to keep operating and doing exciting astronomy as the K2 mission.

Among other goals, the K2 mission will continue to look for transiting exoplanets, which involves looking for the shadows of planets as the occult their host stars. However, small attitude tweaks needed to keep the K2 spacecraft accurately pointed result in fairly large artificial variations in the measured brightnesses of target stars. Removing the effects of these tweaks from K2’s data can be quite challenging, but Vanderburg & Johnson’s recent paper describes one technique for doing that.

By carefully tracking the centers of target stars as they drift across the K2 CCD camera, their technique allows them to remove quite large and complex artificial variability and to recover the actual brightness variations of target stars.

From the paper, the figure at left shows how well they do: the blue dots at the top show the raw measurements, with the artificial variations from attitude tweaks apparent as discontinuous jumps. By carefully modeling the exact position of the target star and removing the effects of its motion across the CCD, the technique produces a much more accurate measurement of the actual brightness variations of the target star, orange dots at bottom.

Applying these techniques to many target stars monitored during one of the K2 engineering tests, Vanderburg & Johnson showed they can produce data nearly as precise as the original Kepler mission — almost as good as sending an astronaut repair team to fix the Kepler spacecraft, but all it took was some sophisticated numerical modeling and a laptop.

Valsecchi+ (2014) — “From Hot Jupiters to Super-Earths Via Roche Lobe Overflow”

Posted by admin on August 21, 2014
Posted in: Publications.
Artist's conception of tidal disruption of a gas giant planet.

Artist’s conception of tidal disruption of a gas giant planet.

Neat paper today from Francesca Valsecchi and colleagues at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics. They looked at the final fates of gas giant planets that wander too close to their host stars, sometime called “hot Jupiters”.

This unexpected but apparently fairly common class of planet consists of massive planets made mostly of hydrogen and helium, like Jupiter, but, unlike Jupiter, these planets orbit tens or even hundreds of times closer to their host stars than the Earth orbits the Sun. Consequently, many of these hot Jupiters are fated to spiral closer and closer to their stars, eventually getting so close that their host stars’ gravity rips them apart, drawing their atmospheres into thin accretion disk around the star. This process is called “Roche lobe overflow” (RLO).

Once the progenitor hot Jupiter has lost its atmosphere, what’s left behind is probably the rocky/icy core at its center, and following up studies by other groups, Valsecchi and colleagues point out that RLO of hot Jupiters may help explain the puzzling presence of small rocky planets also found orbiting very close to their host stars. These little bodies may indeed be the skeletal remnants of unspeakable astrophysical violence.

Mandell et al. (2013) — Exoplanet Transit Spectroscopy of WASP-12 b, WASP-17 b, and WASP-19 b

Posted by admin on December 4, 2013
Posted in: Publications.
Fig. 3 from Mandel+ (2013) showing the combined-light time series for WASP-12 during transit.

Fig. 3 from Mandel+ (2013) showing the combined-light time series for WASP-12 during transit.

Very cool result that, for some reason, only just appeared in the press. Using the Hubble Space Telescope, Avi Mandell and co-authors detected spectral signatures of water in the atmospheres of several very hot, transiting exoplanets.

The figure at left shows the transit signal for WASP-12 b, a very hot gas giant planet that is so close to its host star that the star may be ripping the planet apart.

To get a sense for how impressive these detections are, consider the following: given the temperature and radius for the host star WASP-17, which is about 1,000 lightyears away, we receive about 1 pico-Watt per square meter from the star here on Earth [ (1.38 R_sun/1000 lightyears)^2 (sigma) (6509 K)^4 ~ 1 pW/m^2].

That’s about the same amount of energy we’d receive from a 1000-Watt lightbulb suspended in space 10,000 km from the Earth [1000 W / 4 pi (10000 km)^2 ~ 1 pW/m^2].

The planet WASP-17 b has a radius roughly a tenth that of its host star, giving a transit depth [(radius of the planet)^2/(radius of the star)^2] of about 1% (as seen in the figure). For comparison, the radius of a standard lightbulb is about 3 cm and that of a fruitfly is about 2 mm.

So being able to measure a spectrum for WASP-17 b in-transit is a bit like watching a fruitfly pass in front of a lit lightbulb at a distance of 10,000 km from Earth and being able to tell what color the fly’s wings are. Very cool stuff.

Mandell’s paper is here: http://adsabs.harvard.edu/abs/2013arXiv1310.2949M.