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

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.