Photometric monitoring of the young star Par 1724 in Orion

We report new photometric observations of the 200000 year old naked weak-line run-away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90cm Dutch telescope on La Silla, the 80cm Wendelstein telescope, and a 25cm telescope of the University Observatory Jena in Grossschwabhausen near Jena. The photometric data in V and R are consistent with a 5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long-term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars.Comment: AN in press (eps or ps files on http://www.astro.uni-jena.de/Observations/gsh/gsh_papers.htm

Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies

Spectroscopic and spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007, 2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. Brightness and surface magnetic field topologies were reconstructed for the star using the technique of least-squares deconvolution to increase the signal-to-noise of the data. The reconstructed brightness maps show that HD 141943 had a weak polar spot and a significant amount of low latitude features, with little change in the latitude distribution of the spots over the 4 years of observations. The surface magnetic field was reconstructed at three of the epochs from a high order (l <= 30) spherical harmonic expansion of the spectropolarimetric observations. The reconstructed magnetic topologies show that in 2007 and 2010 the surface magnetic field was reasonably balanced between poloidal and toroidal components. However we find tentative evidence of a change in the poloidal/toroidal ratio in 2009 with the poloidal component becoming more dominant. At all epochs the radial magnetic field is predominantly non-axisymmetric while the azimuthal field is predominantly axisymmetric with a ring of positive azimuthal field around the pole similar to that seen on other active stars.Comment: 18 pages, 17 figures, accepted by MNRA

Close Binaries with Infrared Excess: Destroyers of Worlds?

We present the results of a Spitzer photometric investigation into the IR excesses of close binary systems. In a sample of 10 objects, excesses in IRAC and MIPS24 bands implying the presence of warm dust are found for 3. For 2 objects we do not find excesses reported in earlier IRAS studies. We discuss the results in the context of the scenario suggested by Rhee and co-workers, in which warm dust is continuously created by destructive collisions between planetary bodies. A simple numerical model for the steady-state distribution of dust in one IR excess system shows a central clearing of radius 0.22 AU caused by dynamical perturbations from the binary star. This is consistent with the size of the central clearing derived from the Spitzer spectral energy distribution. We conclude that close binaries could be efficient "destroyers of worlds", lead to destabilize the orbits of their planetary progeny by magnetically-driven angular momentum loss and secular shrinkage of the binary separation.Comment: Accepted by The Astrophysical Journa

Doppler imaging an X-ray flare on the ultrafast rotator BO Mic - A contemporaneous multiwavelength study using XMM-Newton and VLT

We present an analysis of contemporaneous photospheric, chromospheric and coronal structures on the highly active K-dwarf star BO Mic (Speedy Mic). We localize a moderate flare in the stellar atmosphere and study its energetics, size and thermal behaviour. The analysis is based on strictly simultaneous X-ray, UV- and optical observations carried out by XMM-Newton and the VLT. We use Doppler imaging and related methods for the localization of features. Based on X-ray spectroscopy we study the the coronal plasma in and outside the flare. The flare emits in X-rays and UV, but is not detected in white light; it is located at intermediate latitude between an extended spot group and the weakly spotted pole. We estimate its height below 0.4 stellar radii making it clearly distinct in longitude and height from the prominences found about two stellar radii above the surface. While BO Mic's photospheric brightness is modulated due to extended starspots, neither the chromospheric nor the X-ray emission show a pronounced rotational modulation.Comment: Accepted by A&

On the photometric behaviour of the Pleiades K dwarf HII 1883

Photometric data from a variety of sources have been analysed to check if effects of differential rotation or evolutionary spin-down can be detected in HII 1883. Although the same light- curve form may be maintained for several years, shape changes and phase shifts do not allow sufficiently long time-bases for the required accuracy of period determination. One form of light-curve may have a tendency to recur with phase coherence. Using this observation allows determination of three potentially more accurate periods relative to the one previously established, but the choice of any revised value is subject to imprecise knowledge of the integer number of cycles over the time-base. The phase-locked tendency, if confirmed, requires some mechanism for spots to recur at a given longitude. A suggestion of the star being a magnetic oblique rotator is mooted but other observational and theoretical support would be required to promote this notion further. Evidence of small short-term changes occurring over just a few periods is explored. Comparison of the contemporaneous light- curve with the H &#945; line emission RV variations reveals a phase difference of 90&#176;, so making a connection that the disturbances producing the photometric signals have the same location as those generating at least some part of the H &#945; emission. Finally a multi-spot model is presented to mimic the observed light-curves

RACE-OC Project: Rotation and variability in young stellar associations within 100 pc

Our goal is to determine the rotational and magnetic-related activity properties of stars at different stages of evolution. We have focussed our attention on 6 young loose stellar associations within 100 pc and ages in the range 8-70 Myr: TW Hydrae (~8 Myr), beta Pictoris (~10 Myr), Tucana/Horologium, Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional data on alpha Persei and the Pleiades from the literature is also considered. Rotational periods of stars showing rotational modulation due to photospheric magnetic activity (i.e. starspots) have been determined applying the Lomb-Scargle periodogram technique to photometric time-series obtained by the All Sky Automated Survey (ASAS). The magnetic activity level has been derived from the amplitude of the V lightcurves. We detected the rotational modulation and measured the rotation periods of 93 stars for the first time, and confirmed the periods of 41 stars already known from the literature. For further 10 stars we revised the period determinations by other authors. The sample was augmented with periods of 21 additional stars retrieved from the literature. In this way, for the first time we were able to determine largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, as well as increase by 150\% the number of known periodic members of AB Dor.The analysis of the rotation periods in young stellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264 data from the literature, has allowed us to find that in the 0.6 - 1.2 solar masses range the most significant variations of the rotation period distribution are the spin-up between 9 and 30 Myr and the spin-down between 70 and 110 Myr. Variations between 30 and 70 Myr are rather doubtful, despite the median period indicates a significant spin-up.Comment: Accepted by Astronomy and Astrophysic