PDS 70b Shows Stellar-like Carbon-to-oxygen Ratio

Creators: Hsu, Chih-Chun¹; Wang 王, Jason J. 劲飞¹; Blake, Geoffrey A.²; Xuan, Jerry W.²; Zhang, Yapeng²; Ruffio, Jean-Baptiste³; Horstman, Katelyn²; Cronin, Julianne¹; Sappey, Ben³; Xin, Yinzi²; Finnerty, Luke⁴; Echeverri, Daniel²; Mawet, Dimitri^{2, 5}; Jovanovic, Nemanja²; Do Ó, Clarissa R.³; Baker, Ashley²; Bartos, Randall⁵; Calvin, Benjamin^{2, 4}; Cetre, Sylvain⁶; Delorme, Jacques-Robert⁶; Doppmann, Gregory W.⁶; Fitzgerald, Michael P.⁴; Liberman, Joshua⁷; López, Ronald A.⁴; Morris, Evan⁸; Pezzato-Rovner, Jacklyn²; Schofield, Tobias²; Skemer, Andrew⁸; Wallace, J. Kent⁵; Wang 王, Ji 吉⁹

1. Northwestern University
2. California Institute of Technology
3. University of California, San Diego
4. University of California, Los Angeles
5. Jet Propulsion Lab
6. W.M. Keck Observatory
7. University of Arizona
8. University of California, Santa Cruz
9. The Ohio State University

Abstract

The ~5 Myr PDS 70 is the only known system with protoplanets residing in the cavity of the circumstellar disk from which they formed, ideal for studying exoplanet formation and evolution within its natal environment. Here, we report the first spin constraint and C/O measurement of PDS 70b from Keck/KPIC high-resolution spectroscopy. We detected CO (3.8σ) and H₂O (3.5σ) molecules in the PDS 70b atmosphere via cross correlation, with a combined CO and H₂O template detection significance of 4.2σ. Our forward-model fits, using BT-Settl model grids, provide an upper limit for the spin rate of PDS 70b (<29 km s⁻¹). The atmospheric retrievals constrain the PDS 70b C/O ratio to 0.28_(−0.12)^(+0.20) (<0.63 under 95% confidence level) and a metallicity [C/H] of −0.2_(−0.5)^(+0.8) dex, consistent with that of its host star. The following scenarios can explain our measured C/O of PDS 70b in contrast with that of the gas-rich outer disk (for which C/O ≳ 1). First, the bulk composition of PDS 70b might be dominated by dust+ice aggregates rather than disk gas. Another possible explanation is that the disk became carbon enriched after PDS 70b was formed, as predicted in models of disk chemical evolution and as observed in both very low-mass stars and older disk systems with JWST/MIRI. Because PDS 70b continues to accrete and its chemical evolution is not yet complete, more sophisticated modeling of the planet and the disk, and higher-quality observations of PDS 70b (and possibly PDS 70c), are necessary to validate these scenarios.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The authors thank Yayaati Chachan, Stefano Facchini, and Bruce Macintosh for extremely useful discussions on the physical interpretation of planet and disk chemistry, which have significantly improved this manuscript. The authors thank the anonymous referee for useful comments, which improved the original manuscript. The authors thank the Keck observing assistant Arina Rostopchina for her help in obtaining the Keck/KPIC spectra. W. M. Keck Observatory access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). This research was supported in part through the computational resources and staff contributions provided for the Quest high-performance computing facility at Northwestern University which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. This work used computing resources provided by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Funding for KPIC has been provided by the California Institute of Technology, the Jet Propulsion Laboratory, the Heising-Simons Foundation (grant #2015-129, #2017-318, #2019-1312, and #2023-4598), the Simons Foundation (through the Caltech Center for Comparative Planetary Evolution), and the NSF under grant AST-1611623. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the significant cultural role and reverence that the summit of Maunakea has with the indigenous Hawaiian community, and that the W. M. Keck Observatory stands on Crown and Government Lands that the State of Hawai'i is obligated to protect and preserve for future generations of indigenous Hawaiians.

Facilities

Keck:II - (KPIC), Keck:II - (NIRSPEC), Keck:II - KECK II Telescope (NIRC2).

Software References

Astropy (Astropy Collaboration et al. 2013, 2018), DYNESTY (J. S. Speagle 2020), corner (D. Foreman-Mackey 2016), emcee (D. Foreman-Mackey et al. 2013), Matplotlib (J. D. Hunter 2007), Numpy (C. R. Harris et al. 2020), petitRADTRANS (P. Molliére et al. 2019), Scipy (P. Virtanen et al. 2020), seaborn (M. L. Waskom 2021), SMART (C.-C. Hsu et al. 2021a, 2021b).

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