A weak spectral signature of water vapour in the atmosphere of HD 179949 b at high spectral resolution in the L band

Rebecca K. Webb, Matteo Brogi, Siddharth Gandhi, Michael R. Line, Jayne L. Birkby, Katy L. Chubb, Ignas A.G. Snellen, Sergey N. Yurchenko

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


High-resolution spectroscopy (R ≽ 20 000) is currently the only known method to constrain the orbital solution and atmospheric properties of non-transiting hot Jupiters. It does so by resolving the spectral features of the planet into a forest of spectral lines and directly observing its Doppler shift while orbiting the host star. In this study, we analyse VLT/CRIRES (R = 100 000) L-band observations of the non-transiting giant planet HD 179949 b centred around 3.5 μm. We observe a weak (3.0σ, or S/N = 4.8) spectral signature of H2O in absorption contained within the radial velocity of the planet at superior-conjunction, with a mild dependence on the choice of line list used for the modelling. Combining this data with previous observations in the K band, we measure a detection significance of 8.4 σ for an atmosphere that is most consistent with a shallow lapse-rate, solar C/O ratio, and with CO and H2O being the only major sources of opacity in this wavelength range. As the two sets of data were taken 3 yr apart, this points to the absence of strong radial-velocity anomalies due, e.g. to variability in atmospheric circulation. We measure a projected orbital velocity for the planet of KP = (145.2 ± 2.0) km s−1 (1σ) and improve the error bars on this parameter by ∼70 per cent. However, we only marginally tighten constraints on orbital inclination (66.2+3.7-3.1 deg) and planet mass (0.963+0.0360.031 Jupiter masses), due to the dominant uncertainties of stellar mass and semimajor axis. Follow ups of radial-velocity planets are thus crucial to fully enable their accurate characterization via high-resolution spectroscopy.

Original languageEnglish (US)
Pages (from-to)108-119
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - May 1 2020


  • Planets
  • Planets and satellites: atmospheres
  • Planets and satellites: fundamental parameters
  • Satellites: individual: HD 179949b
  • Techniques: spectroscopic

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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