325 research outputs found
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 ()
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 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 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- 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
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