Revisiting the HD 21749 planetary system with stellar activity modelling

Gan, Tianjun and Wang, Sharon Xuesong and Teske, Johanna K. and Mao, Shude and Howard, Ward S. and Law, Nicholas M. and Batalha, Natasha E. and Vanderburg, Andrew and Dragomir, Diana and Huang, Chelsea X. ORCID: and Feng, Fabo and Butler, R. Paul and Crane, Jeffrey D. and Shectman, Stephen A. and Beletsky, Yuri and Shporer, Avi and Montet, Benjamin T. and Burt, Jennifer A. and Feinstein, Adina D. and Flowers, Erin and Nandakumar, Sangeetha and Barbieri, Mauro and Corbett, Hank and Ratzloff, Jeffrey K. and Galliher, Nathan and Chavez, Ramses Gonzalez and Vasquez, Alan and Glazier, Amy and Haislip, Joshua (2020) Revisiting the HD 21749 planetary system with stellar activity modelling. Monthly Notices of the Royal Astronomical Society, 501 (4). pp. 6042-6061. ISSN 0035-8711

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HD 21749 is a bright (V = 8.1 mag) K dwarf at 16 pc known to host an inner terrestrial planet HD 21749c as well as an outer sub-Neptune HD 21749b, both delivered by Transiting Exoplanet Survey Satellite (TESS). Follow-up spectroscopic observations measured the mass of HD 21749b to be 22.7 ± 2.2 M with a density of 7.0^{+1.6}_{-1.3} g cm-3, making it one of the densest sub-Neptunes. However, the mass measurement was suspected to be influenced by stellar rotation. Here, we present new high-cadence PFS RV data to disentangle the stellar activity signal from the planetary signal. We find that HD 21749 has a similar rotational time-scale as the planet's orbital period, and the amplitude of the planetary orbital RV signal is estimated to be similar to that of the stellar activity signal. We perform Gaussian process regression on the photometry and RVs from HARPS and PFS to model the stellar activity signal. Our new models reveal that HD 21749b has a radius of 2.86 ± 0.20 R, an orbital period of 35.6133 ± 0.0005 d with a mass of Mb = 20.0 ± 2.7 M and a density of 4.8^{+2.0}_{-1.4} g cm-3 on an eccentric orbit with e = 0.16 ± 0.06, which is consistent with the most recent values published for this system. HD 21749c has an orbital period of 7.7902 ± 0.0006 d, a radius of 1.13 ± 0.10 R, and a 3σ mass upper limit of 3.5 M. Our Monte Carlo simulations confirm that without properly taking stellar activity signals into account, the mass measurement of HD 21749b is likely to arrive at a significantly underestimated error bar.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Published version deposited in accordance with the copyright policy of the publisher.
Faculty/School / Institute/Centre: No Faculty
Faculty/School / Institute/Centre: No Faculty
Date Deposited: 04 Apr 2022 04:04
Last Modified: 30 May 2022 06:26
Uncontrolled Keywords: stars: individual: HD 21749; planetary systems; Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Solar and Stellar Astrophysics
Fields of Research (2020): 51 PHYSICAL SCIENCES > 5101 Astronomical sciences > 510109 Stellar astronomy and planetary systems
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