Discovery of water at high spectral resolution in the atmosphere of 51 Peg b (1701.07257v1)

Abstract: We report the detection of water absorption features in the dayside spectrum of the first-known hot Jupiter, 51 Peg b, confirming the star-planet system to be a double-lined spectroscopic binary. We used high-resolution (R~100,000), 3.2 micron spectra taken with CRIRES/VLT to trace the radial-velocity shift of the water features in the planet's dayside atmosphere during 4 hours of its 4.23-day orbit after superior conjunction. We detect the signature of molecular absorption by water at a significance of 5.6 sigma at a systemic velocity of Vsys=-33+/-2 km/s, coincident with the host star, with a corresponding orbital velocity Kp = 133^+4.3_-3.5 km/s. This translates directly to a planet mass of Mp=0.476^+0.032_-0.031MJ, placing it at the transition boundary between Jovian and Neptunian worlds. We determine upper and lower limits on the orbital inclination of the system of 70<i (deg)<82.2. We also provide an updated orbital solution for 51 Peg b, using an extensive set of 639 stellar radial velocities measured between 1994 and 2013, finding no significant evidence of an eccentric orbit. We find no evidence of significant absorption or emission from other major carbon-bearing molecules of the planet, including methane and carbon dioxide. The atmosphere is non-inverted in the temperature-pressure region probed by these observations. The deepest absorption lines reach an observed relative contrast of 0.9x10^-3 with respect to the host star continuum flux, at an angular separation of 3 milliarcseconds. This work is consistent with a previous tentative report of K-band molecular absorption for 51 Peg b by Brogi et al. (2013).

• The paper reports the detection of water absorption features using CRIRES/VLT, achieving a 5.6σ confidence level to refine 51 Peg b's orbital properties.
• The analysis employed high-resolution infrared spectroscopy at 3.2μm over 3.7 hours, revealing a non-inverted temperature-pressure profile in the planet's atmosphere.
• The non-detection of methane and CO2 emphasizes the capability of precise spectral techniques in constraining exoplanetary atmospheric compositions.

Detection of Water in the Atmosphere of 51 Peg b at High Spectral Resolution

The paper presents a detailed analysis in which the authors report the detection of molecular water features in the atmosphere of the well-known exoplanet 51 Pegasi b, using high-resolution spectroscopy. Using the CRIRES instrument at the Very Large Telescope (VLT), the team successfully detected these features with a significant cross-correlation signal at a 5.6σ confidence level, confirming 51 Pegasi b as a double-lined spectroscopic binary. The planet was observed over a period of 3.7 hours at a wavelength around 3.2μm, using high-resolution ($R \approx 100,000$) infrared spectroscopy.

The spectroscopic analysis focused on the radial velocity changes in the observable water features on the planet’s day side during its orbit, particularly when it was near superior conjunction. The results showed molecular absorption by water, measuring a systemic velocity close to that of the host star and an orbital velocity $K_P = 133^{+4.3}_{-3.5}$ km/s. This analysis translates to a planet mass of $M_p = 0.476^{+0.032}_{-0.031}M_J$, situating it near the boundary between Jovian and Neptunian planets.

Key Findings and Analysis

1. Orbital Characterization: By analyzing the data, the authors refined the orbital parameters of 51 Peg b, offering important insights into its inclination, now estimated to range between $70^{\circ}$ and $82.2^{\circ}$. The inclination and mass findings are consistent with earlier tentative research, reinforcing the characterization of 51 Peg b.
2. Non-Inverted Atmospheric Profile: The spectral analysis did not reveal any inversion layer in the atmosphere for the pressures probed, indicating a non-inverted temperature-pressure profile. This adds to the data on exoplanet atmospheres where temperature inversions seem to be absent based on the infrared analysis at high resolutions.
3. Non-Detection of Other Molecules: The paper found no significant evidence for methane (CH$_4$) and carbon dioxide (CO$_2$) in the observable spectrum, potentially pointing to chemical compositions or atmospheric dynamics limiting the presence of these molecules or grounding them at very low abundances.
4. Epistemic Confirmation: This work lends independent empirical support to prior tentative findings by Brogi et al. (2013) regarding potential water absorption at different wavelengths (2.3μm), enhancing confidence in methodologies of atmospheric paper via high spectral resolution.

Implications and Future Directions

The detection of atmospheric water is a compelling aspect not only because it adds to the understanding of 51 Peg b itself but also because it illustrates the potential of high-resolution spectroscopic techniques in studying non-transiting planets. Continued improved measurement of such exoplanetary atmospheres could refine our grasp of their temperature structures, atmospheric compositions, and dynamic processes.

Furthermore, the findings encourage future work on improving the models of exoplanet atmospheres, especially for resolving the role of varied atmospheric conditions during different orbital phases, to confirm radial shifts. Moreover, upcoming instruments and missions equipped with sophisticated high-resolution spectrographs will amplify these capabilities further, potentially clarifying not only molecular compositions but also velocity offsets indicative of atmospheric dynamics and rotation.

Such investigations will have profound implications for the development and evolution of exoplanetary modeling and the precision characterization of their atmospheres. The data emphasize a need for stereo-temporal observations at multi-wavelengths, bridging observations with advanced theoretical modeling to elucidate the physical state and chemical properties of distant worlds.