{
  "id": "2304.00854",
  "version": "v1",
  "published": "2023-04-03T10:05:32.000Z",
  "updated": "2023-04-03T10:05:32.000Z",
  "title": "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"
  ],
  "comment": "6 pages, 7 figures, A&A accepted",
  "categories": [
    "astro-ph.SR",
    "astro-ph.EP",
    "astro-ph.HE"
  ],
  "abstract": "X-rays emitted by the coronae of solar-type stars are a feature present in up to late-A types during the main sequence phase. F stars, either with or without hot Jupiters, are usually X-ray emitters. The very low level of X-ray emission of the F5 star WASP-18 despite its relatively young age and spectral type is thus quite peculiar. [Abridged] We observed KELT-24 with \\xmm\\ for a total of 43 ks in order to test if the X-ray activity of this star is depressed by the interaction with its massive hot Jupiter, as is the case of WASP-18. KELT-24 is detected in combined EPIC images with a high significance level. Its average coronal spectrum is well described by a cool component at 0.36 keV and a hotter component at 0.98 keV. We detected a flare with a duration of about 2 ks, during which the coronal temperature reached 3.5 keV. The unabsorbed quiescent flux in 0.3-8.0 keV is $\\sim1.33\\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$, corresponding to a luminosity of $1.5\\times10^{29}$ erg s$^{-1}$ at the distance of the star. The luminosity is well within the range of the typical X-ray luminosity of F stars in Hyades, which are coeval. We conclude that the activity of KELT-24 appears normal, as expected, and is not affected by any star--planet interaction. From the analysis of TESS light curves, we infer a distribution of optical flares for KELT-24 and WASP-18. Small optical flickering similar to flares is recognized in WASP-18 but at lower levels of energy and amplitude than in KELT-24. We discuss the causes of the low activity of WASP-18. 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. This latter hypothesis would make WASP-18 among the few candidates showing such a quench of stellar activity.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2023-04-03T10:05:32.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "hot jupiter",
      "x-ray activity",
      "solar maunder minimum",
      "average coronal spectrum",
      "high significance level"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 6,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}