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

We present radial velocity measurements of the very bright (\(V\sim5.7\)) 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, \(N_\textrm{p}\), 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 \(P_\textrm{b} = 3.4982^{+0.0015}_{-0.0027}\) d and corresponding minimum mass of \(m_\textrm{b}\,\textrm{sin}\,i = 12.2^{+1.8}_{-1.9}\) M\(_\oplus\). Without further high time resolution observations over a longer timescale, we cannot definitively rule out the purely Keplerian model with 2 candidates planets with \(P_\textrm{b} = 2.4570^{+0.0026}_{-0.0462}\) d, minimum mass \(m_\textrm{b}\,\textrm{sin}\,i = 8.0^{+1.1}_{-1.5}\) M\(_\oplus\) 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\(_\oplus\). 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.