Atmospheres & Climate

Into The Red: An M-band Study of the Chemistry and Rotation of β Pictoris b at High Spectral Resolution

By Keith Cowing
Status Report
May 16, 2024
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Into The Red: An M-band Study of the Chemistry and Rotation of β Pictoris b at High Spectral Resolution
Cross-correlation maps produced using models broadened to the measured rotational broadening of each molecule. These CCFs are predicted to show stronger detections as the models used more closely match the broadened planetary lines in the data, and the influence of noise sources on the scale of a single resolution element in the CCF are suppressed. Most notably, when cross-correlating with an SiO model broadened to the 22 km s−1 rotational broadening measured from the H2O CCF, the evidence for the tentative SiO signal increases to an S/N = 4.3, while other regions are suppressed, suggesting a potential planetary origin. The CO signal is boosted to S/N = 6.6 and the H2O signal is now above the S/N = 5 threshold for a detection, with an S/N = 5.7. Cross-correlation with a broadened model containing CO, H2O, and SiO produces the strongest detection of all models tested, with a S/N = 7.5. The CCFs produced using the H2O model and the model containing all three molecular opacities show an offset from the expected velocity, which is discussed further in Section 6.3. To reduce the impact of correlated signals the noise estimates for these CCFs are measured from a down-sampled CCF, with the planet signal excluded (see Section 4.7). For clarity, and following the convention for HCS, the CCFs are presented here at the 1.5 km s−1 CRIRES+ pixel resolution. — astro-ph.EP

High-resolution cross-correlation spectroscopy (HRCCS) combined with adaptive optics has been enormously successful in advancing our knowledge of exoplanet atmospheres, from chemistry to rotation and atmospheric dynamics.

This powerful technique now drives major science cases for ELT instrumentation including METIS/ELT, GMTNIRS/GMT and MICHI/TMT, targeting biosignatures on rocky planets at 3-5 μm, but remains untested beyond 3.5 μm where the sky thermal background begins to provide the dominant contribution to the noise. We present 3.51-5.21 μm M-band CRIRES+/VLT observations of the archetypal young directly imaged gas giant β Pictoris b, detecting CO absorption at S/N = 6.6 at 4.73 μm and H2O at S/N = 5.7, and thus extending the use of HRCCS into the thermal background noise dominated infrared.

Using this novel spectral range to search for more diverse chemistry we report marginal evidence of SiO at S/N = 4.3, potentially indicative that previously proposed magnesium-silicate clouds in the atmosphere are either patchy, transparent at M-band wavelengths, or possibly absent on the planetary hemisphere observed. The molecular detections are rotationally broadened by the spin of β Pic b, and we infer a planetary rotation velocity of vsin(i) = 22±2 km s−1 from the cross-correlation with the H2O model template, consistent with previous K-band studies.

We discuss the observational challenges posed by the thermal background and telluric contamination in the M-band, the custom analysis procedures required to mitigate these issues, and the opportunities to exploit this new infrared window for HRCCS using existing and next-generation instrumentation.

Luke T. Parker, Jayne L. Birkby, Rico Landman, Joost P. Wardenier, Mitchell E. Young, Sophia R. Vaughan, Lennart van Sluijs, Matteo Brogi, Vivien Parmentier, Michael R. Line

Comments: 17 pages, 20 figures. Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2405.08867 [astro-ph.EP] (or arXiv:2405.08867v1 [astro-ph.EP] for this version)
Submission history
From: Luke Parker
[v1] Tue, 14 May 2024 18:00:01 UTC (3,889 KB)


Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻