# The X-ray activity of F stars with hot Jupiters: KELT-24 versus WASP-18

**Authors:** I. Pillitteri, S. Colombo, G. Micela, S. J. Wolk  
**arXiv:** [2304.00854](https://arxiv.org/abs/2304.00854)  
**Date:** 2023-04-03  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2304.00854v1/

## Key points

- X-ray activity is typically present in solar-type stars up to late-A types during the main sequence phase
- F stars, with or without hot Jupiters, are usually X-ray emitters
- The F5 star WASP-18 exhibits a very low level of X-ray emission despite its relatively young age and spectral type
- KELT-24 was observed to test if its X-ray activity is depressed by the interaction with its massive hot Jupiter, similar to WASP-18
- KELT-24 was detected in combined EPIC images with high significance and exhibited an average coronal spectrum described by a cool component at 0.36 keV and a hotter component at 0.98 keV
- A flare lasting about 2 ks was also detected during which the coronal temperature reached 3.5 keV
- The unabsorbed quiescent flux for KELT-24 falls within the range of typical X-ray luminosity for F stars in Hyades that are coeval with KELT‑24
- From TESS light curve analysis it was inferred that there is a distribution of optical flares for both KELT‑24 and WASP‑18; however small optical flickering similar to flares were recognized in WASP‑18 but at lower levels of energy and amplitude than in KELT‑24.
- The causes behind the low activity level exhibited by WASP‑18 were discussed: either WASP‑18b could hamper the formation of a corona bright in X-rays in its host star through some form of tidal interaction or the star has entered a minimum of activity similar to the solar Maunder minimum; making WASP‑18 one among few candidates showing such quench of stellar activity.

## Summary

The X-ray activity of solar-type stars is typically present up to late-A types during the main sequence phase. F stars, with or without hot Jupiters, are usually X-ray emitters. However, the F5 star WASP-18 exhibits a very low level of X-ray emission despite its relatively young age and spectral type, which is quite peculiar. In contrast, KELT-24 was observed with \xmm\ for a total of 43 ks to test if its X-ray activity is depressed by the interaction with its massive hot Jupiter, similar to WASP-18. KELT-24 was detected in combined EPIC images with high significance and exhibited an average coronal spectrum described by a cool component at 0.36 keV and a hotter component at 0.98 keV. A flare lasting about 2 ks was also detected during which the coronal temperature reached 3.5 keV. The unabsorbed quiescent flux in the range of 0.3–8.0 keV for KELT-24 was found to be $\sim1.33\times10^{−13}$ erg s$^{−1}$ cm$^{−2}$, corresponding to a luminosity of $1.5\times10^{29}$ erg s$^{−1}$ at the distance of the star, which falls within the range of typical X-ray luminosity for F stars in Hyades that are coeval with KELT‑24. From TESS light curve analysis it was inferred that there is a distribution of optical flares for both KELT‑24 and WASP‑18; however small optical flickering similar to flares were recognized in WASP‑18 but at lower levels of energy and amplitude than in KELT‑24. The causes behind the low activity level exhibited by WASP‑18 were discussed: either WASP‑18b could hamper the formation of a corona bright in X‑rays in its host star through some form of tidal interaction or the star has entered a minimum of activity similar to the solar Maunder minimum; making WASP‑18 one among few candidates showing such quench of stellar activity.