A magnetic cycle of tau Bootis? The coronal and chromospheric view

Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter. During the last years spectropolarimetric observations led to the hypothesis that this star may host a global magnetic field that switches its polarity once per year, indicating a very short activity cycle of only one year duration. In our ongoing observational campaign, we have collected several X-ray observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to characterize tau Boo's corona and chromosphere over the course of the supposed one-year cycle. Contrary to the spectropolarimetric reconstructions, our observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29 (2012

Discovery of the secondary eclipse of HAT-P-11 b

We report the detection of the secondary eclipse of HAT-P-11 b, a Neptune-sized planet orbiting an active K4 dwarf. Using all available short-cadence data of the Kepler mission, we derive refined planetary ephemeris increasing their precision by more than an order of magnitude. Our simultaneous primary and secondary transit modeling results in improved transit and orbital parameters. In particular, the precise timing of the secondary eclipse allows to pin down the orbital eccentricity to $0.26459_{-0.00048}^{+0.00069}$. The secondary eclipse depth of $6.09_{-1.11}^{+1.12}$ ppm corresponds to a $5.5\sigma$ detection and results in a geometric albedo of $0.39\pm0.07$ for HAT-P-11 b, close to Neptune's value, which may indicate further resemblances between these two bodies. Due to the substantial orbital eccentricity, the planetary equilibrium temperature is expected to change significantly with orbital position and ought to vary between $630^\circ$ K and $950^\circ$ K, depending on the details of heat redistribution in the atmosphere of HAT-P-11 b.Comment: Accepted by A&A, 27/10/201

Structure and variability in the corona of the ultrafast rotator LO Peg

Low-mass ultrafast rotators show the typical signatures of magnetic activity and are known to produce flares, probably as a result of magnetic reconnection. As a consequence, the coronae of these stars exhibit very large X-ray luminosities and high plasma temperatures, as well as a pronounced inverse FIP effect. To probe the relationship between the coronal properties with a spectral type of ultra-fast rotators with rotation period P < 1d, we analyse the K3 rapid-rotator LO Peg observed with XMM-Newton and compare it with other low-mass rapid rotators of spectral types G9-M1. We investigate the temporal evolution of coronal properties like the temperatures, emission measures, abundances, densities and the morphology of the involved coronal structures. We find two distinguishable levels of activity in the XMM-Newton observation of LO~Peg, which shows significant X-ray variability both in phase and amplitude, implying the presence of an evolving active region on the surface. The X-ray flux varies by 28%, possibly due to rotational modulation. During our observation, a large X-ray flare with a peak X-ray luminosity of 2E30 erg/s and an energy of 7.3E33 erg was observed. At the flare onset we obtain clear signatures for the occurrence of the Neupert effect. The flare plasma also shows an enhancement of iron by a factor of 2 during the rise and peak phase of the flare. Our modeling analysis suggests that the scale size of the flaring X-ray plasma is smaller than 0.5 R_star. Further, the flare loop length appears to be smaller than the pressure scale height of the flaring plasma. Our studies show that the X-ray properties of the LO~Peg are very similar to those of other low-mass ultrafast rotators, i.e., the X-ray luminosity is very close to saturation, its coronal abundances follow a trend of increasing abundance with increasing first ionisation potential, the so-called inverse FIP effect.Comment: 11 pages, 15 figures and 4 tables. Accepted for publication by Astronomy and Astrophysic

Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths

We present new X-ray observations obtained with Chandra ACIS-S of the HD 189733 system, consisting of a K-type star orbited by a transiting Hot Jupiter and an M-type stellar companion. We report a detection of the planetary transit in soft X-rays with a significantly larger transit depth than observed in the optical. The X-ray data favor a transit depth of 6-8%, versus a broadband optical transit depth of 2.41%. While we are able to exclude several possible stellar origins for this deep transit, additional observations will be necessary to fully exclude the possibility that coronal inhomogeneities influence the result. From the available data, we interpret the deep X-ray transit to be caused by a thin outer planetary atmosphere which is transparent at optical wavelengths, but dense enough to be opaque to X-rays. The X-ray radius appears to be larger than the radius observed at far-UV wavelengths, most likely due to high temperatures in the outer atmosphere at which hydrogen is mostly ionized. We furthermore detect the stellar companion HD 189733B in X-rays for the first time with an X-ray luminosity of log LX = 26.67 erg/s. We show that the magnetic activity level of the companion is at odds with the activity level observed for the planet-hosting primary. The discrepancy may be caused by tidal interaction between the Hot Jupiter and its host star.Comment: 15 pages, accepted for publication in The Astrophysical Journa

Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R$^{+}_{\rm{HK}}$ in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R$^{+}_{\rm{HK}}$ values are derived from the respective S-indices and related to the rotation periods in various $B-V$-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R$^{+}_{\rm{HK}}$ approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R$^{+}_{\rm{HK}}$, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.Comment: 13 pages, 7 figures, accepted for publication in A&

The initial temporal evolution of a feedback dynamo for Mercury

Various possibilities are currently under discussion to explain the observed weakness of the intrinsic magnetic field of planet Mercury. One of the possible dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its weak magnetic field Mercury exhibits a small magnetosphere whose subsolar magnetopause distance is only about 1.7 Hermean radii. We consider the magnetic field due to magnetopause currents in the dynamo region. Since the external field of magnetospheric origin is antiparallel to the dipole component of the dynamo field, a negative feedback results. For an alpha-omega-dynamo two stationary solutions of such a feedback dynamo emerge, one with a weak and the other with a strong magnetic field. The question, however, is how these solutions can be realized. To address this problem, we discuss various scenarios for a simple dynamo model and the conditions under which a steady weak magnetic field can be reached. We find that the feedback mechanism quenches the overall field to a low value of about 100 to 150 nT if the dynamo is not driven too strongly

Jet directions in Seyfert galaxies: B and I imaging data

We present the results of broad-band B and I imaging observations for a sample of 88 Seyfert galaxies (29 Seyfert 1's and 59 Seyfert 2's), selected from a mostly isotropic property, the flux at 60$\mu$m. We also present the B and I imaging results for an additional sample of 20 Seyfert galaxies (7 Seyfert 1's and 13 Seyfert 2's), selected from the literature and known to have extended radio emission. The I band images are fitted with ellipses to determine the position angle and ellipticity of the host galaxy major axis. This information will be used in a future paper, combined with information from radio observations, to study the orientation of radio jets relative to the plane of their host galaxies (Kinney et al. 2000). Here we present surface brightness profiles and magnitudes in the B and I bands, as well as mean ellipticities and major axis position angles.Comment: To appear in The Astrophysical Journal Supplement Series, June 2000. 48 pages, 7 tables, 19 gif and 11 postscript figures. Better quality figures can be obtained with the autho