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

Abstract

We present radial velocity measurements of the very bright ($V\sim 5.7$) 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 $\sim 10$ ms${}^{-1}$ 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, $N_{\text{p}}$, 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 $P_{\text{b}}=3.4982_{-0.0027}^{+0.0015}$ d and corresponding minimum mass of $m_{\text{b}}\text{sin}i=12.2_{-1.9}^{+1.8}$ M${}_{\oplus }$. Without further high time resolution observations over a longer timescale, we cannot definitively rule out the purely Keplerian model with 2 candidates planets with $P_{\text{b}}=2.4570_{-0.0462}^{+0.0026}$ d, minimum mass $m_{\text{b}}\text{sin}i=8.0_{-1.5}^{+1.1}$ M${}_{\oplus }$ and $P_{\text{c}}=5.4196_{-0.0030}^{+0.6766}$ d and corresponding minimum mass of $m_{\text{b}}\text{sin}i=12.2_{-1.6}^{+1.4}$ M${}_{\oplus }$. 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.

19 pages, 11 figures. Accepted for publication in MNRAS

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