Kavli Affiliate: Sara Seager
| First 5 Authors: Steven Giacalone, Courtney D. Dressing, Antonio García Muñoz, Matthew J. Hooton, Keivan G. Stassun
| Summary:
We report the discovery in TESS data and validation of HD 56414 b (a.k.a.
TOI-1228 b), a Neptune-size ($R_{rm p} = 3.71 pm 0.20, R_oplus$) 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 ($T_{rm eff} = 8500 pm 150 , {rm K}$) 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 $ge 8 ,
M_oplus$. Our model also predicts that close-in Neptune-size planets with
masses $< 14 , M_oplus$ 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.
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