Hot Jupiters are subject to strong irradiation from the host stars and, as a consequence, they do evaporate. They can also interact with the parent stars by means of tides and magnetic fields. Both phenomena have strong implications for the evolution
of these systems. Here we present time resolved spectroscopy of HD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST. The star has been observed during five consecutive HST orbits, starting at a secondary transit of the planet ($phi$ ~0.50-0.63). Two main episodes of variability of ion lines of Si, C, N and O are detected, with an increase of line fluxes. Si IV lines show the highest degree of variability. The FUV variability is a signature of enhanced activity in phase with the planet motion, occurring after the planet egress, as already observed three times in X-rays. With the support of MHD simulations, we propose the following interpretation: a stream of gas evaporating from the planet is actively and almost steadily accreting onto the stellar surface, impacting at $70-90deg$ ahead of the sub-planetary point.
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.
Abridged. Here we report on the X-ray activity of the primary star, HD189733 A, using a new XMM-Newton observation and a comparison with the previous X-ray observations. The spectrum in the quiescent intervals is described by two temperatures at 0.2
keV and 0.7 keV, while during the flares a third component at 0.9 keV is detected. We obtain estimates of the electron density in the range $n_e = 1.6 - 13 times 10^{10}$ cm$^{-3}$ and thus the corona of HD189733 A appears denser than the solar one. {For the third time, we observe a large flare that occurred just after the eclipse of the planet. Together with the flares observed in 2009 and 2011, the events are restricted to a small planetary phase range of $phi = 0.55-0.65$. Although we do not find conclusive evidence of a significant excess of flares after the secondary transits, we suggest that the planet might trigger such flares when it passes close to locally high magnetic field of the underlying star at particular combinations of stellar rotational phases and orbital planetary phases. For the most recent flares, a wavelet analysis of the light curve suggests a loop of length of four stellar radii at the location of the bright flare, and a local magnetic field of order of 40-100 G, in agreement with the global field measured in other studies. The loop size suggests an interaction of magnetic nature between planet and star, separated by only $sim8 R_*$. We also detect the stellar companion (HD 189733 B, $sim12$ from the primary star) in this XMM observation. Its very low X-ray luminosity ($L_X = 3.4times 10^{26}$ erg s$^{-1}$) confirms the old age of this star and of the binary system. The high activity of the primary star is best explained by a transfer of angular momentum from the planet to the star.
We present an XMM-Newton survey of the part of Orion A cloud south of the Orion Nebula. This survey includes the Lynds 1641 (L1641) dark cloud, a region of the Orion A cloud with very few massive stars and hence a relatively low ambient UV flux, and
the region around the O9 III star Iota Orionis. In addition to proprietary data, we used archival XMM data of the Orion Nebula Cluster (ONC) to extend our analysis to a major fraction of the Orion A cloud. We have detected 1060 X-ray sources in L1641 and Iota Ori region. About 94% of the sources have 2MASS & Spitzer counterparts, 204 and 23 being Class II and Class I or protostars objects, respectively. In addition, we have identified 489 X-ray sources as counterparts to Class III candidates, given they are bright in X-rays and appear as normal photospheres at mid-IR wavelengths. The remaining 205 X-ray sources are likely distant AGNs or other galactic sources not related to Orion A. We find that Class III candidates appear more concentrated in two main clusters in L1641. The first cluster of Class III stars is found toward the northern part of L1641, concentrated around Iota Ori. The stars in this cluster are more evolved than those in the Orion Nebula. We estimate a distance of 300-320 pc for this cluster and thus it is closer than the Orion A cloud. Another cluster rich in Class III stars is located in L1641 South and appears to be a slightly older cluster embedded in the Orion A cloud. Furthermore, other evolved Class III stars are found north of the ONC toward NGC 1977.
We report on the follow-up XMM-Newton observation of the planet-hosting star HD 189733 we obtained in April 2011. We observe a flare just after the secondary transit of the hot Jupiter. This event shares the same phase and many of the characteristics
of the flare we observed in 2009. We suggest that a systematic interaction between planet and stellar magnetic fields when the planet passes close to active regions on the star can lead to periodic variability phased with planetary motion. By mean of high resolution X-ray spectroscopy with RGS we determine that the corona of this star is unusually dense.
