Ground-based spectroscopy for WASP-19 b’s atmosphere


An outline of the paper: Ground-based transit spectroscopy of the hot Jupiter WASP-19 b in the near-infrared by Jacob Bean et al.

Starting with the initial detection of atomic sodium in the atmosphere of HD209458 b in 2002, the first decade of exoplanet atmospheric characterisation has been dominated by contributions from space telescopes using the techniques of transmission and emission spectroscopy. Similar measurements from the ground are much harder due to the Earth’s atmosphere, with effects like turbulence and time-varying moisture content introducing systematics that swamp the tiny signals from exoplanet atmospheres.

Multi-object spectroscopy is a method that addresses many of these traditional difficulties, and is starting to make an increasingly important contribution to the field (e.g. these previous observations for GJ1214 b and recent studies by Gibson et al 2012 and Stevenson et al 2013). The basic idea is to take simultaneous sequences of spectra of the target star and one or more nearby comparison stars of similar brightness during the transit. By dividing the sequence of the target star by the summed sequences of the comparison stars, one can in theory calibrate away the systematics that affect all of the stars. Crucially, this includes the time-varying atmospheric effects that are essentially constant across the telescope’s field of view.

In this paper, Jacob Bean and collaborators present new results from their ground-based multi-object spectroscopy program. They used the MMIRS instrument on the southern Magellan telescope in Chile to observe two transits and two eclipses of the hot Jupiter WASP-19 b in the J, H and K infrared bands.


The black points in the plot above are the measured data for WASP-19 in one of the wavelength channels, and the grey shaded regions show when the secondary eclipse occurs. Clearly, the systematic features caused by time-varying atmospheric and instrumental conditions make it impossible to detect a shallow eclipse with any confidence in this raw data. The red lines, however, show the calibration functions that are obtained by summing the flux time series of a few nearby comparison stars and then fitting a second order polynomial in time to account for smooth residual trends. Here’s the corrected lightcurve that Bean et al got when they divided the black points by the red calibration curve:


Similar lightcurves were obtained in 8 other wavelength channels between 1.25-2.35 microns, allowing the authors to build up an emission spectrum for the planet by measuring the eclipse depth as a function of wavelength. Unfortunately, in terms of constraints on the physics of the atmosphere of WASP-19, it turns out these new data don’t reveal terribly much new. But they’re reassuringly consistent with previous observations. An acceptable fit to the ensemble of current data is obtained if  the planet’s dayside is assumed to radiate as a blackbody with a temperature of 2250K. The data is not yet precise enough to infer any absorption features caused by molecules in the atmosphere.

Meanwhile, the error bars that Bean et al. achieve in the transmission spectrum are equivalent to 1-2 atmospheric scale heights, which is roughly the amplitude of the absorption features they were hoping to detect evidence for. So pretty much all the authors can say at this point is that they rule out absorption features with amplitudes larger than this.

Although these new data for WASP-19 b aren’t as constraining as we might hope, the fact that ground-based observations are gaining momentum in the field of exoplanet atmospheric characterisation is encouraging. The beauty is that observations from the ground are much cheaper than observations from space, so this could prove to be a rewarding approach, even using humble 2-4m aperture telescopes.



About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.