Regaining the FORS: making optical ground-based transmission spectroscopy of exoplanets with VLT+FORS2 possible again

Transmission spectroscopy facilitates the detection of molecules and/or clouds in the atmospheres of exoplanets. Such studies rely heavily on space-based or large ground-based observatories, as one needs to perform time-resolved, high signal-to-noise spectroscopy. The FORS2 instrument at ESO's Very Large Telescope is the obvious choice for performing such studies, and was indeed pioneering the field in 2010. After that, however, it was shown to suffer from systematic errors caused by the Longitudinal Atmospheric Dispersion Corrector (LADC). This was successfully addressed, leading to a renewed interest for this instrument as shown by the number of proposals submitted to perform transmission spectroscopy of exoplanets. We present here the context, the problem and how we solved it, as well as the recent results obtained. We finish by providing tips for an optimum strategy to do transmission spectroscopy with FORS2, in the hope that FORS2 may become the instrument of choice for ground-based transmission spectroscopy of exoplanets.

[1]  David Charbonneau,et al.  THE GJ1214 SUPER-EARTH SYSTEM: STELLAR VARIABILITY, NEW TRANSITS, AND A SEARCH FOR ADDITIONAL PLANETS , 2010, 1012.0518.

[2]  Travis Barman,et al.  Warm ice giant GJ 3470b. I. A flat transmission spectrum indicates a hazy, low-methane, and/or metal-rich atmosphere , 2013, 1308.6580.

[3]  Andreas Seifahrt,et al.  A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C , 2015 .

[4]  Bernard Muschielok,et al.  Successful Commissioning of FORS1 - the First Optical Instrument on the VLT , 1998 .

[5]  Drake Deming,et al.  A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion , 2016, Nature.

[6]  Sabine Moehler,et al.  FORS2 Rotating Flat Field Systematics Fixed -- Recent Exchange of FORS LADC Prisms Improves the Long-known Flat-fielding Problem , 2016 .

[7]  G. Rupprecht,et al.  Correction of Field Rotator-Induced Flat-Field Systematics—A Case Study Using Archived VLT-FORS Data , 2010, 1001.1099.

[8]  A. Burrows.  Highlights in the study of exoplanet atmospheres , 2014, Nature.

[9]  N. Gibson,et al.  Making FORS2 fit for exoplanet observations (again) , 2015 .

[10]  Jacob L. Bean,et al.  Characterizing Transiting Exoplanet Atmospheres with Gemini/GMOS: First Results , 2015 .

[11]  S. Aigrain,et al.  A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720 nm , 2012, 1210.7798.

[12]  M. Lendl,et al.  FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b , 2015, 1512.06698.

[13]  Andreas Seifahrt,et al.  TRANSMISSION SPECTROSCOPY OF THE HOT JUPITER WASP-12b FROM 0.7 TO 5 μm , 2013, 1305.1670.

[14]  Klaus G. Strassmeier,et al.  Transmission spectroscopy of HAT-P-32b with the LBT: confirmation of clouds/hazes in the planetary atmosphere , 2016, 1603.09136.

[15]  Drake Deming,et al.  Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b , 2013, Nature.

[16]  Santiago de Chile,et al.  Potassium detection in the clear atmosphere of a hot-Jupiter. FORS2 transmission spectroscopy of WASP-17b , 2016, 1609.03906.

[17]  N. Gibson,et al.  Regaining the FORS: optical ground-based transmission spectroscopy of the exoplanet WASP-19b with VLT+FORS2 , 2015, 1503.04155.

[18]  Gerardo Avila,et al.  Atmospheric dispersion correction for the FORS Focal Reducers at the ESO VLT , 1997, Other Conferences.

[19]  Tristan Guillot,et al.  BULK COMPOSITION OF GJ 1214b AND OTHER SUB-NEPTUNE EXOPLANETS , 2013, 1305.2629.

[20]  S. Aigrain,et al.  The optical transmission spectrum of the hot Jupiter HAT-P-32b: clouds explain the absence of broad spectral features? , 2013, 1309.6998.

[21]  Jacob L. Bean,et al.  A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b , 2010, Nature.

[22]  Sabine Moehler,et al.  Twenty years of FORS science operations on the VLT , 2010 .

[23]  Portugal,et al.  FURTHER CONSTRAINTS ON THE OPTICAL TRANSMISSION SPECTRUM OF HAT-P-1b , 2015, 1508.06696.

[24]  Suzanne Aigrain,et al.  The GTC exoplanet transit spectroscopy survey II: An overly-large Rayleigh-like feature for exoplanet TrES-3b , 2015, 1510.04988.

[25]  Andrew Szentgyorgyi,et al.  A GROUND-BASED OPTICAL TRANSMISSION SPECTRUM OF WASP-6b , 2013, 1310.6048.

[26]  S. Dreizler,et al.  The GTC exoplanet transit spectroscopy survey. IV. Confirmation of the flat transmission spectrum of HAT-P-32b , 2016, 1604.06041.

[27]  Sara Seager,et al.  THE OPTICAL AND NEAR-INFRARED TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ 1214b: FURTHER EVIDENCE FOR A METAL-RICH ATMOSPHERE , 2011, 1109.0582.

[28]  A. Cabrera-Lavers,et al.  The GTC exoplanet transit spectroscopy survey - I. OSIRIS transmission spectroscopy of the short period planet WASP-43b , 2014, 1401.3692.

[29]  I. Ribas,et al.  Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b , 2015, 1506.05685.

[30]  Andrew Szentgyorgyi,et al.  GROUND-BASED TRANSIT SPECTROSCOPY OF THE HOT-JUPITER WASP-19b IN THE NEAR-INFRARED , 2013, 1303.1094.