HD 56414 b: A Warm Neptune Transiting an A-type Star

Abstract

We report the discovery in TESS data and validation of HD 56414 b (a.k.a. TOI-1228 b), a Neptune-size ($R_{\mathrm{p}}=3.71\pm 0.20R_{\oplus }$) planet with a 29-day orbital period transiting a young (Age = $420\pm 140$ Myr) A-type star in the TESS southern continuous viewing zone. HD 56414 is one of the hottest stars ($T_{\mathrm{eff}}=8500\pm 150\mathrm{K}$) to host a known sub-Jovian planet. HD 56414 b lies on the boundary of the hot Neptune desert in planet radius -- bolometric insolation flux space, suggesting that the planet may be experiencing mass loss. To explore this, we apply a photoevaporation model that incorporates the high near ultraviolet continuum emission of A-type stars. We find that the planet can retain most of its atmosphere over the typical 1-Gyr main sequence lifetime of an A-type star if its mass is $\ge 8M_{\oplus }$. Our model also predicts that close-in Neptune-size planets with masses $<14M_{\oplus }$ are susceptible to total atmospheric stripping over 1 Gyr, hinting that the hot Neptune desert, which has been previously observed around FGKM-type stars, likely extends to A-type stars.

arXiv comment12 pages, 4 figures, 1 table

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