Two warm Neptunes transiting HIP 9618 revealed by TESS and Cheops

HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright (G = 9.0 mag) solar analogue. TESS photometry revealed the star to have two candidate planets with radii of 3.9 ± 0.044 R⊕ (HIP 9618 b) and 3.343 ± 0.039 R⊕ (HIP 9618 c). While the 20.77291 d period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-d gap in the time series, leaving many possibilities for the period. To solve this issue, CHEOPS performed targeted photometry of period aliases to attempt to recover the true period of planet c, and successfully determined the true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE, and CAFE revealed a mass of 10.0 ± 3.1M⊕ for HIP 9618 b, which, according to our interior structure models, corresponds to a 6.8 ± 1.4 per cent gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of <18M⊕. Follow-up and archival RV measurements also reveal a clear long-term trend which, when combined with imaging and astrometric information, reveal a low-mass companion (0.08+0.12-0.05 M⊙) orbiting at 26.0+19.0-11.0 au. This detection makes HIP 9618 one of only five bright (K < 8 mag) transiting multiplanet systems known to host a planet with P > 50 d, opening the door for the atmospheric characterization of warm (Teq < 750 K) sub-Neptunes.

Citation

Osborn , H P , Nowak , G , Hébrard , G , Masseron , T , Lillo-Box , J , Pallé , E , Bekkelien , A , Florén , H-G , Guterman , P , Simon , A E , Adibekyan , V , Bieryla , A , Borsato , L , Brandeker , A , Ciardi , D R , Collier Cameron , A , Collins , K A , Egger , J A , Gandolfi , D , Hooton , M J , Latham , D W , Lendl , M , Matthews , E C , Tuson , A , Ulmer-Moll , S , Vanderburg , A , Wilson , T G , Ziegler , C , Alibert , Y , Alonso , R , Anglada , G , Arnold , L , Asquier , J , Barrado y Navascues , D , Baumjohann , W , Beck , T , Belinski , A A , Benz , W , Biondi , F , Boisse , I , Bonfils , X , Broeg , C , Buchhave , L A , Bárczy , T , Barros , S C C , Cabrera , J , Cardona Guillen , C , Carleo , I , Castro-González , A , Charnoz , S , Christiansen , J , Cortes-Zuleta , P , Csizmadia , S , Dalal , S , Davies , M B , Deleuil , M , Delfosse , X , Delrez , L , Demory , B-O , Dunlavey , A B , Ehrenreich , D , Erikson , A , Fernandes , R B , Fortier , A , Forveille , T , Fossati , L , Fridlund , M , Gillon , M , Goeke , R F , Goliguzova , M V , Gonzales , E J , Günther , M N , Güdel , M , Heidari , N , Henze , C E , Howell , S , Hoyer , S , Frey , J I , Isaak , K G , Jenkins , J M , Kiefer , F , Kiss , L , Korth , J , Maxted , P F L , Laskar , J , Lecavelier des Etangs , A , Lovis , C , Lund , M B , Luque , R , Magrin , D , Almenara , J M , Martioli , E , Mecina , M , Medina , J V , Moldovan , D , Morales-Calderón , M , Morello , G , Moutou , C , Murgas , F , Jensen , E L N , Nascimbeni , V , Olofsson , G , Ottensamer , R , Pagano , I , Peter , G , Piotto , G , Pollacco , D , Queloz , D , Ragazzoni , R , Rando , N , Rauer , H , Ribas , I , Ricker , G , Demangeon , O D S , Smith , A M S , Santos , N , Scandariato , G , Seager , S , Sousa , S G , Steller , M , Szabó , G M , Ségransan , D , Thomas , N , Udry , S , Ulmer , B , Van Grootel , V , Vanderspek , R , Walton , N & Winn , J N 2023 , ' Two warm Neptunes transiting HIP 9618 revealed by TESS and Cheops ' , Monthly Notices of the Royal Astronomical Society , vol. 523 , no. 2 , pp. 3069-3089 . https://doi.org/10.1093/mnras/stad1319

Publication

Monthly Notices of the Royal Astronomical Society

Status

Peer reviewed

DOI

https://doi.org/10.1093/mnras/stad1319

ISSN

0035-8711

Type

Journal article

Rights

Copyright © 2023 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1093/mnras/stad1319.

Description

Funding: ACC acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant number ST/R003203/1.