X-rays from massive stars are ubiquitous yet not clearly understood. In an XMM-Newton observation devoted to observe the first site of star formation in the $rho$ Ophiuchi dark cloud, we detect smoothly variable X-ray emission from the B2IV+B2V syste
m of $rho$ Ophiuchi. Tentatively we assign the emission to the primary component. The light curve of the pn camera shows a first phase of low, almost steady rate, then a rise phase of duration of 10 ks, followed by a high rate phase. The variability is seen primarily in the band 1.0-8.0 keV while little variability is detected below 1 keV. The spectral analysis of the three phases reveals the presence of a hot component at 3.0 keV that adds up to two relatively cold components at 0.9 keV and 2.2 keV. We explain the smooth variability with the emergence of an extended active region on the surface of the primary star due to its fast rotation (v $sin~i sim315$ km/s). We estimate that the region has diameter in the range $0.5-0.6$ R$_*$. The hard X-ray emission and its variability hint a magnetic origin, as suggested for few other late-O$-$early-B type stars. We also discuss an alternative explanation based on the emergence from occultation of a young (5-10 Myr) low mass companion bright and hot in X-rays.
About 20% out of the $>1000$ known exoplanets are Jupiter analogs orbiting very close to their parent stars. It is still under debate to what detectable level such hot Jupiters possibly affect the activity of the host stars through tidal or magnetic
star-planet interaction. In this paper we report on an 87 ks Chandra observation of the hot Jupiter hosting star WASP-18. This system is composed of an F6 type star and a hot Jupiter of mass $10.4 M_{Jup}$ orbiting in less than 20 hr around the parent star. On the basis of an isochrone fitting, WASP-18 is thought to be 600 Myr old and within the range of uncertainty of 0.5-2 Gyr. The star is not detected in X-rays down to a luminosity limit of $4times10^{26}$ erg/s, more than two orders of magnitude lower than expected for a star of this age and mass. This value proves an unusual lack of activity for a star with estimated age around 600 Myr. We argue that the massive planet can play a crucial role in disrupting the stellar magnetic dynamo created within its thin convective layers. Another additional 212 X-ray sources are detected in the Chandra image. We list them and briefly discuss their nature.
The galactic Cepheid S Muscae has recently been added to the important list of Cepheids linked to open clusters, in this case the sparse young cluster ASCC 69. Low-mass members of a young cluster are expected to have rapid rotation and X-ray activity
, making X-ray emission an excellent way to discriminate them from old field stars. We have made an XMM-Newton observation centered on S Mus and identified (Table 1) a population of X-ray sources whose near-IR 2MASS counterparts lie at locations in the J, (J-K) color-magnitude diagram consistent with cluster membership at the distance of S Mus. Their median energy and X-ray luminosity are consistent with young cluster members as distinct from field stars. These strengthen the association of S Mus with the young cluster, making it a potential Leavitt Law (Period-Luminosity relation) calibrator.
