A Lucky Imaging search for stellar sources near 74 transit hosts

Many transiting planet host stars lack high resolution imaging and thus close stellar sources can be missed. Those unknown stars potentially bias the derivation of the planetary and stellar parameters from the transit light curve, no matter if they are bound or not. In addition, bound stellar companions interact gravitationally with the exoplanet host star, the disk and the planets and can thus influence the formation and evolution of the planetary system strongly. We extended our high-resolution Lucky Imaging survey for close stellar sources by 74 transiting planet host stars. 39 of these stars lack previous high-resolution imaging, 23 are follow up observations of companions or companion candidates, and the remaining stars have been observed by others with AO imaging though in different bands. We determine the separation of all new and known companion candidates and estimate the flux ratio in the observed bands. All observations were carried out with the Lucky Imaging camera AstraLux Norte at the Calar Alto 2.2 m telescope in i' and z' passbands. We find new stellar sources within 1 arcsec to HAT-P-27, HAT-P-28, HAT-P-35, WASP-76, and WASP-103, and between 1 and 4 arcsec to HAT-P-29 and WASP-56.Comment: 6 pages, 1 figure, accepted for publication in Astronomy & Astrophysic

VLT spectra of the companion candidate Cha Ha 5/cc 1

We obtained optical and infrared spectra of Cha Ha 5/cc 1, a faint possibly sub-stellar companion candidate next to the M6-type brown dwarf candidate Cha Ha 5 in Cha I, using FORS1 and ISAAC at the VLT. The VRIJHK colors of Cha Ha 5/cc 1 are consistent with either an L-type companion or a K-type background giant. Our spectra show that the companion candidate actually is a background star.Comment: IAU 211 Symp. "Brown dwarfs" poster proceedings (in press

Feedback regulated star formation: II. dual constraints on the SFE and the age spread of stars in massive clusters

We show that the termination of the star formation process by winds from massive stars in protocluster forming clumps imposes dual constraints on the star formation efficiencies (SFEs) and stellar age spreads ($\Delta \tau_{*}$) in stellar clusters. We have considered two main classes of clump models. One class of models in one in which the core formation efficiency (CFE) per unit time and as a consequence the star formation rate (SFR) is constant in time and another class of models in which the CFE per unit time, and as a consequence the SFR, increases with time. Models with an increasing mode of star formation yield shorter age spreads (a few 0.1 Myrs) and typically higher SFEs than models in which star formation is uniform in time. We find that the former models reproduce remarkably well the SFE$-\Delta \tau_{*}$ values of starburst clusters such as NGC 3603 YC and Westerlund 1, while the latter describe better the star formation process in lower density environments such as in the Orion Nebula Cluster. We also show that the SFE and $\Delta \tau_{*}$ of massive clusters are expected to be higher in low metallicity environments. This could be tested with future large extragalactic surveys of stellar clusters. We advocate that placing a stellar cluster on the SFE-$\Delta \tau_{*}$ diagram is a powerful method to distinguish between different stellar clusters formation scenarios such as between generic gravitational instability of a gas cloud/clump or as the result of cloud-cloud collisions. It is also a very useful tool for testing star formation theories and numerical models versus the observations.Comment: Accepted to MNRA

A Lucky Imaging search for stellar companions to transiting planet host stars

The presence of stellar companions around planet hosting stars influences the architecture of their planetary systems. To find and characterise these companions and determine their orbits is thus an important consideration to understand planet formation and evolution. For transiting systems even unbound field stars are of interest if they are within the photometric aperture of the light curve measurement. Then they contribute a constant flux offset to the transit light curve and bias the derivation of the stellar and planetary parameters if their existence is unknown. Close stellar sources are, however, easily overlooked by common planet surveys due to their limited spatial resolution. We therefore performed high angular resolution imaging of 49 transiting exoplanet hosts to identify unresolved binaries, characterize their spectral type, and determine their separation. The observations were carried out with the Calar Alto 2.2m telescope using the Lucky Imaging camera AstraLux Norte. All targets were imaged in i' and z' passbands. We found new companion candidates to WASP-14 and WASP-58, and we re-observed the stellar companion candidates to CoRoT-2, CoRoT-3, CoRoT-11, HAT-P-7, HAT-P-8, HAT-P-41, KIC 10905746, TrES-2, TrES-4, and WASP-2. We deduce from the stellar density around all sources that two companion candidates out of the targets with the first position measurement (CoRoT-11, HAT-P-41, KIC 10905746, WASP-14 and WASP-58) are probably unbound. In addition, we re-analyse the influence of the sources close to WASP-14 and WASP-58 on the planetary parameters given in the literature and find no significant changes

Star formation environments and the distribution of binary separations

We have carried out K-band speckle observations of a sample of 114 X-ray selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB association. We find that for binary T Tauri stars closely associated to the early type stars in Upper Scorpius, the youngest subgroup of the OB association, the peak in the distribution of binary separations is at 90 A.U. For binary T Tauri stars located in the direction of an older subgroup, but not closely associated to early type stars, the peak in the distribution is at 215 A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not result from the same distibution at a significance level of 98%. Apparently, the same physical conditions which facilitate the formation of massive stars also facilitate the formation of closer binaries among low-mass stars, whereas physical conditions unfavorable for the formation of massive stars lead to the formation of wider binaries among low-mass stars. The outcome of the binary formation process might be related to the internal turbulence and the angular momentum of molecular cloud cores, magnetic field, the initial temperature within a cloud, or - most likely - a combination of all of these. We conclude that the distribution of binary separations is not a universal quantity, and that the broad distribution of binary separations observed among main-sequence stars can be explained by a superposition of more peaked binary distributions resulting from various star forming environments. The overall binary frequency among pre-main-sequence stars in individual star forming regions is not necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for publication in ApJ Letter