The orbital motion, absolute mass, and high-altitude winds of exoplanet HD209458b (1006.4364v1)

Abstract: For extrasolar planets discovered using the radial velocity method, the spectral characterization of the host star leads to a mass-estimate of the star and subsequently of the orbiting planet. In contrast, if also the orbital velocity of the planet would be known, the masses of both star and planet could be determined directly using Newton's law of gravity, just as in the case of stellar double-line eclipsing binaries. Here we report on the detection of the orbital velocity of extrasolar planet HD209458b. High dispersion ground-based spectroscopy during a transit of this planet reveals absorption lines from carbon monoxide produced in the planet atmosphere, which shift significantly in wavelength due to the change in the radial component of the planet orbital velocity. These observations result in a mass determination of the star and planet of 1.00+-0.22 Msun and 0.64+-0.09 Mjup respectively. A ~2 km/sec blueshift of the carbon monoxide signal with respect to the systemic velocity of the host star suggests the presence of a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet within the 0.01-0.1 mbar atmospheric pressure range probed by these observations. The strength of the carbon monoxide signal suggests a CO mixing ratio of 1-3x10-3 in this planet's upper atmosphere.

• The paper determines HD209458b's orbital velocity (140±10 km/s) and absolute mass (0.64±0.09 Mjup) using high-dispersion infrared spectroscopy and Newton’s laws.
• The study reveals strong atmospheric dynamics through a 2 km/s blueshift in CO absorption, indicating high-altitude winds with a mixing ratio of 1–3×10⁻³.
• The paper notes unsuccessful detections of H2O and CH4, suggesting these molecules are either low in abundance or obscured by telluric contamination, warranting further research.

Analyzing the Orbital Dynamics and Atmospheric Composition of Exoplanet HD209458b

This paper presents a significant advancement in the characterization of exoplanet HD209458b, particularly focusing on its orbital motion, absolute mass, and atmospheric conditions. The novel detection of the orbital velocity of this extrasolar planet via high-resolution spectroscopy provides a more accurate determination of its mass and the mass of its host star using Newton's laws, analogous to the methodology employed for stellar double-line eclipsing binaries.

Methodology and Observational Details

The authors employed high-dispersion infrared spectroscopy using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) at the Very Large Telescope at Cerro Paranal. This enabled the detailed observation of carbon monoxide (CO) absorption lines in the atmosphere of HD209458b. The resulting wavelength shifts, due to the planet's radial velocity change during transit, were analyzed to determine the planet's orbital velocity with a significant blueshift indicating atmospheric wind patterns.

The observational setup collected 51 spectra with a spectral resolution of 100,000 over a wavelength range of 2291 to 2349 nm. The key novelty in the methodology lies in the detection of the CO signal via cross-correlation with a model spectrum of expected absorption lines, paving the way for directly deriving the planet's atmospheric properties.

Key Findings

1. Mass and Orbital Velocity: The planetary mass of HD209458b is determined as 0.64±0.09 Mjup, and the mass of the host star as 1.00±0.22 Msun. The derived orbital velocity of the planet during the transit is 140±10 km/s, which directly contributes to these mass determinations.
2. Atmospheric Conditions: The presence of CO with a volume mixing ratio in the upper atmosphere of 1-3x10^-3 is confirmed. The findings indicate a robust atmospheric dynamic system with winds moving from the day side to the night side, marked by a statistically significant 2 km/s blueshift of the CO signal.
3. Absence of Other Molecules: Attempts to detect other molecules using H2O and CH4 templates were unsuccessful, which implies a potentially low abundance of these molecules or stronger masking by telluric contamination.

Implications and Future Directions

The implications of these findings are twofold. Practically, the paper demonstrates the effectiveness of high-dispersion spectroscopy in determining exoplanetary atmospheric conditions, offering a more reliable approach to characterize exoplanets outside our solar system. Theoretically, it provides insights into the atmospheric dynamics of hot Jupiters, suggesting significant wind patterns driven by stellar irradiance.

In future research, extending this methodology to other exoplanets could further refine our understanding of planetary atmospheres and their evolution. Additionally, addressing limitations due to telluric contamination and enhancing spectral resolution could uncover signatures of other atmospheric constituents, contributing to a more comprehensive understanding of exoplanetary atmospheres.

The substantial precision achieved in determining planetary and stellar masses presents a promising avenue for more complex models of exoplanet formation and atmospheric dynamics, potentially informing broader astrophysical inquiries into the nature of planetary systems.