Transmission spectroscopy of transiting extrasolar planets

Coordinator: C. Moutou (LAM, France)

Co-Is: J.-C. Bouret, C. Foellmi


We propose to observe an transiting extra-solar planet to uncover its spectrum. During the transit of a planet in front of the parent star's disk, the planet atmosphere is seen in absorption. This allowed space observatories to detect NaI, HI, CII and OI in the atmosphere of HD 209458b (Charbonneau et al 2002, Vidal Madjar et al 2002 and 2003). This quest is more difficult from the ground, although several attemps were made at the VLT with UVES and ISAAC (Moutou et al 2000 and 2001). The recent discovery of HD 189733b (Bouchy et al 2005) opens new expectations since the planet is closer to its star and probably suffers more evaporation: the atmospheric spectral lines should be deeper. CIRIRES was also awaited for transit spectroscopy, as spectral resolution and infrared coverage is the best combination to differentially search for faint absorption lines in the stellar spectrum. We propose here to probe the planetary system during a 2.5 hour sequence, with half of it during the transit, and half of it out of transit, for comparison. With a resulting SNR of 6500, we could search for two narrow and deep absorption features of the planetary atmosphere, at 1243 and 1252 nm (Barman et al, private communication). This measurement could provide an estimate or upper limit of the abundance of KI in the atmosphere of hot Jupiters, a question which is still debated since the first transit spectroscopy results of Charbonneau et al (2002). This proposal would also provide useful information for the stability and ultimate performance of CRIRES over a sequence of 2.5 hours (or more!).

Allocated Time: 2.5 hours

Targets list

NameRA(2000)DEC(2000)Range (nm)Wavelength IDF_nu or F_lineDIT(s),NDIT
HD 18973320 00 43.71+22 42 39.11238-127045/0/nV = 7.5, G58x10s, 100

Project description/scientific objective:

Observations of KI lines at 1243 and 1252 nm in the atmosphere of a hot Jupiter extrasolar planet, during a transit. Comparison between the stellar spectrum obtained before transit and the star+planet specturm obtained during transit allows a differential detection of the faint atmospheric feature of the planet.

Observing strategy

The central wavelength is 1250nm. The detector gaps should not be placed at the regions where the narrow absorption lines are expected, at 1243 and 1252 nm. DITs of 10 sec give 32000 e/DIT following the online ETC. With NDIT=100 and 8 exposures, the total exposure time of 2.5 hours is achieved. The actual DIT should evidently be adapted to conditions and actual performance of CRIRES.

The sequence must be executed during the night 9-10 Aug 2006 where the transit takes place, with a center at 5:50 UT. The observability of the target is from 01:00UT to 6:00UT at airmass less than 2.0. At minimum, the sequence must cover the first half of the transit, i.e., from 4:35 to 5:50UT, and an equivalent time before the transit starts, i.e., from 03:20 to 4:35 UT. If time permits, the sequence could also be longer! This night is the last night of the CRIRES commissioning period. If it is impossible to schedule now, another transit epoch will be proposed for the science verification of October but the conditions will not be optimal then.

Point-like source, characterised by proper motion dRA=-2.49 dDEC=-250.81 mas/yr. Slit width 1.0''. Adaptive optics notes: reference star is the target itself.


Charbonneau et al 2002, ApJ 568, 377
Vidal Madjar et al, 2003, Nature 422, 143
Vidal Madjar et al, 2004, AjP 604, L69
Bouchy et al, 2005, AA 444, L15
Moutou et al, 2001, AA 371, 260
Moutou et al, 2003, AA 405, 341
Barman T. 2005, private communication