We present the discovery of KELT-21b , a hot Jupiter transiting the V = 10.5 A8V star HD 332124 .
The planet has an orbital period of P = 3.6127647 \pm 0.0000033 days and a radius of 1.586 _ { -0.040 } ^ { +0.039 } R _ { J } .
We set an upper limit on the planetary mass of M _ { P } < 3.91 M _ { J } at 3 \sigma confidence .
We confirmed the planetary nature of the transiting companion using this mass limit and Doppler tomographic observations to verify that the companion transits HD 332124 .
These data also demonstrate that the planetary orbit is well-aligned with the stellar spin , with a sky-projected spin-orbit misalignment of \lambda = -5.6 _ { -1.9 } ^ { +1.7 \circ } .
The star has T _ { eff } = 7598 _ { -84 } ^ { +81 } K , M _ { * } = 1.458 _ { -0.028 } ^ { +0.029 } M _ { \Sun } , R _ { * } = 1.638 \pm 0.034 R _ { \Sun } , and v \sin { I _ { * } } = 146 km s ^ { -1 } , the highest projected rotation velocity of any star known to host a transiting hot Jupiter .
The star also appears to be somewhat metal-poor and \alpha -enhanced , with \left [ { Fe } / { H } \right ] = -0.405 _ { -0.033 } ^ { +0.032 } and [ \alpha /Fe ] = 0.145 \pm 0.053 ; these abundances are unusual , but not extraordinary , for a young star with thin-disk kinematics like KELT-21 .
High-resolution imaging observations revealed the presence of a pair of stellar companions to KELT-21 , located at a separation of 1 \farcs 2 and with a combined contrast of \Delta K _ { S } = 6.39 \pm 0.06 with respect to the primary .
Although these companions are most likely physically associated with KELT-21 , we can not confirm this with our current data .
If associated , the candidate companions KELT-21 B and C would each have masses of \sim 0.12 M _ { \Sun } , a projected mutual separation of \sim 20 AU , and a projected separation of \sim 500 AU from KELT-21 .
KELT-21b may be one of only a handful of known transiting planets in hierarchical triple stellar systems .