DMPP-4: candidate sub-Neptune mass planets orbiting a naked-eye star

Barnes, J. R.; Standing, M. R.; Haswell, C. A.; Staab, D.; Doherty, J. P. J.; Waller-Bridge, M.; Fossati, L.; Soto, M.; Anglada-Escudé, G.; Llama, J.; McCune, C. and Lewis, F. W. (2023). DMPP-4: candidate sub-Neptune mass planets orbiting a naked-eye star. Monthly Notices of the Royal Astronomical Society, 524(4) pp. 5196–5212.



We present radial velocity measurements of the very bright (V ∼ 5.7) nearby F star, DMPP-4 (HD 184960). The anomalously low Ca ii H&K emission suggests mass loss from planets orbiting a low activity host star. Periodic radial velocity variability with ∼10 m s−1 amplitude is found to persist over a > 4 year timescale. Although the non-simultaneous photometric variability in four TESS sectors supports the view of an inactive star, we identify periodic photometric signals and also find spectroscopic evidence for stellar activity. We used a posterior sampling algorithm that includes the number of Keplerian signals, Np, as a free parameter to test and compare (1) purely Keplerian models (2) a Keplerian model with linear activity correlation and (3) Keplerian models with Gaussian processes. A preferred model, with one Keplerian and quasi-periodic Gaussian process indicates a planet with a period of $P_\textrm {b} = 3.4982^{+0.0015}_{-0.0027}$ d and corresponding minimum mass of $m_\textrm {b}\, \textrm {sin}\, i = 12.2^{+1.8}_{-1.9}$ M⊕. Without further high time resolution observations over a longer timescale, we cannot definitively rule out the purely Keplerian model with 2 candidates planets with $P_\textrm {b} = 2.4570^{+0.0026}_{-0.0462}$ d, minimum mass $m_\textrm {b}\, \textrm {sin}\, i = 8.0^{+1.1}_{-1.5}$ M⊕ and $P_\textrm {c} = 5.4196^{+0.6766}_{-0.0030}$ d and corresponding minimum mass of $m_\textrm {b}\, \textrm {sin}\, i = 12.2^{+1.4}_{-1.6}$ M⊕. The candidate planets lie in the region below the lower-envelope of the Neptune Desert. Continued mass loss may originate from the highly irradiated planets or from an as yet undetected body in the system.

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