601 research outputs found
Binarity of Transit Host Stars - Implications on Planetary Parameters
Straight-forward derivation of planetary parameters can only be achieved in
transiting planetary systems. However, planetary attributes such as radius and
mass strongly depend on stellar host parameters. Discovering a transit host
star to be multiple leads to a necessary revision of the derived stellar and
planetary parameters. Based on our observations of 14 transiting exoplanet
hosts, we derive parameters of the individual components of three transit host
stars (WASP-2, TrES-2, and TrES-4) which we detected to be binaries. Two of
these have not been known to be multiple before. Parameters of the
corresponding exoplanets are revised. High-resolution "Lucky Imaging" with
AstraLux at the 2.2m Calar Alto telescope provided near diffraction limited
images in i' and z' passbands. These results have been combined with existing
planetary data in order to recalibrate planetary attributes. Despite the
faintness (delta mag ~ 4) of the discovered stellar companions to TrES-2,
TrES-4, and WASP-2, light-curve deduced parameters change by up to more than
1sigma. We discuss a possible relation between binary separation and planetary
properties, which - if confirmed - could hint at the influence of binarity on
the planet formation process.Comment: 9 pages, 3 Figures. Accepted by A&
Holographic Imaging of Crowded Fields: High Angular Resolution Imaging with Excellent Quality at Very Low Cost
We present a method for speckle holography that is optimised for crowded
fields. Its two key features are an iterativ improvement of the instantaneous
Point Spread Functions (PSFs) extracted from each speckle frame and the
(optional) simultaneous use of multiple reference stars. In this way, high
signal-to-noise and accuracy can be achieved on the PSF for each short
exposure, which results in sensitive, high-Strehl re- constructed images. We
have tested our method with different instruments, on a range of targets, and
from the N- to the I-band. In terms of PSF cosmetics, stability and Strehl
ratio, holographic imaging can be equal, and even superior, to the capabilities
of currently available Adaptive Optics (AO) systems, particularly at short
near-infrared to optical wavelengths. It outperforms lucky imaging because it
makes use of the entire PSF and reduces the need for frame selection, thus
leading to higher Strehl and improved sensitivity. Image reconstruction a
posteriori, the possibility to use multiple reference stars and the fact that
these reference stars can be rather faint means that holographic imaging offers
a simple way to image large, dense stellar fields near the diffraction limit of
large telescopes, similar to, but much less technologically demanding than, the
capabilities of a multi-conjugate adaptive optics system. The method can be
used with a large range of already existing imaging instruments and can also be
combined with AO imaging when the corrected PSF is unstable.Comment: Accepted for publication in MNRAS on 15 Nov 201
Lucky Imaging survey for southern M dwarf binaries
While M dwarfs are the most abundant stars in the Milky Way, there is still
large uncertainty about their basic physical properties (mass, luminosity,
radius, etc.) as well as their formation environment. Precise knowledge of
multiplicity characteristics and how they change in this transitional mass
region, between Sun-like stars on the one side and very low mass stars and
brown dwarfs on the other, provide constraints on low mass star and brown dwarf
formation. In the largest M dwarf binary survey to date, we search for
companions to active, and thus preferentially young, M dwarfs in the solar
neighbourhood. We study their binary/multiple properties, such as the
multiplicity frequency and distributions of mass ratio and separation, and
identify short period visual binaries, for which orbital parameters and hence
dynamical mass estimates can be derived in the near future. The observations
are carried out in the SDSS i' and z' band using the Lucky Imaging camera
AstraLux Sur at the ESO 3.5 m New Technology Telescope. In the first part of
the survey, we observed 124 M dwarfs of integrated spectral types M0-M6 and
identified 34 new and 17 previously known companions to 44 stars. We derived
relative astrometry and component photometry for these systems. More than half
of the binaries have separations smaller than 1 arcsec and would have been
missed in a simply seeing-limited survey. Correcting our sample for selection
effects yields a multiplicity fraction of 32+/-6% for 108 M dwarfs within 52 pc
and with angular separations of 0.1-6.0 arcsec, corresponding to projected
separation 3-180 AU at median distance 30 pc. Compared to early-type M dwarfs
(M>0.3M_Sun), later type (and hence lower mass) M dwarf binaries appear to have
closer separations, and more similar masses.Comment: 18 pages, 9 figures. Minor corrections and changes. Revised to match
accepted A&A versio
GRAVITY: getting to the event horizon of Sgr A*
We present the second-generation VLTI instrument GRAVITY, which currently is
in the preliminary design phase. GRAVITY is specifically designed to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the
massive black hole at center of the Milky Way. We have identified the key
design features needed to achieve this goal and present the resulting
instrument concept. It includes an integrated optics, 4-telescope, dual feed
beam combiner operated in a cryogenic vessel; near infrared wavefront sensing
adaptive optics; fringe tracking on secondary sources within the field of view
of the VLTI and a novel metrology concept. Simulations show that the planned
design matches the scientific needs; in particular that 10 microarcsecond
astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given
the availability of suitable phase reference sources.Comment: 13 pages, 11 figures, to appear in the conference proceedings of SPIE
Astronomical Instrumentation, 23-28 June 2008, Marseille, Franc
The Rapid Outbursting Star GM Cep: An EX-or in Tr 37?
We present optical, IR and millimeter observations of the solar-type star
13-277, also known as GM Cep, in the 4 Myr-old cluster Tr 37. GM Cep
experiences rapid magnitude variations of more than 2 mag at optical
wavelengths. We explore the causes of the variability, which seem to be
dominated by strong increases in the accretion, being similar to EX-or
episodes. The star shows high, variable accretion rates (up to ~10
Msun/yr), signs of powerful winds, and it is a very fast rotator (Vsini~43
km/s). Its strong mid-IR excesses reveal a very flared disk and/or a remnant
envelope, most likely out of hydrostatic equilibrium. The 1.3 millimeter fluxes
suggest a relatively massive disk (Mdisk~0.1 Msun). Nevertheless, the
millimeter mass is not enough to sustain increased accretion episodes over
large timescales, unless the mass is underestimated due to significant grain
growth. We finally explore the possibility of GM Cep having a binary companion,
which could trigger disk instabilities producing the enhanced accretion
episodes.Comment: 43 pages, including 10 figures, ApJ in pres
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
Direct imaging of the young spectroscopic binary HD 160934
We report on the direct detection of a close companion to HD 160934, a young
active star, SB1 spectroscopic binary, and suggested member of the AB Doradus
moving group. High angular resolution at the Calar Alto 2.2m telescope was
achieved by means of the Lucky Imaging technique, allowing direct imaging close
to the diffraction limit in the SDSS z' band. Our results are combined with
pre-discovery HST archive data, own UBVRI broadband photometry, published JHK
magnitudes, and available radial velocity measurements to constrain the
physical properties of the HD 160934 close binary. We suggest that the direct
detection may be identical to the spectroscopically discovered companion.Comment: 5 pages, 1 figure, submitted to A&
Masses, radii, and orbits of small Kepler planets : The transition from gaseous to rocky planets
We report on the masses, sizes, and orbits of the planets orbiting 22 Kepler stars. There are 49 planet candidates around these stars, including 42 detected through transits and 7 revealed by precise Doppler measurements of the host stars. Based on an analysis of the Kepler brightness measurements, along with high-resolution imaging and spectroscopy, Doppler spectroscopy, and (for 11 stars) asteroseismology, we establish low false-positive probabilities (FPPs) for all of the transiting planets (41 of 42 have an FPP under 1%), and we constrain their sizes and masses. Most of the transiting planets are smaller than three times the size of Earth. For 16 planets, the Doppler signal was securely detected, providing a direct measurement of the planet's mass. For the other 26 planets we provide either marginal mass measurements or upper limits to their masses and densities; in many cases we can rule out a rocky composition. We identify six planets with densities above 5 g cm-3, suggesting a mostly rocky interior for them. Indeed, the only planets that are compatible with a purely rocky composition are smaller than 2 R ⊕. Larger planets evidently contain a larger fraction of low-density material (H, He, and H2O).Peer reviewedFinal Accepted Versio
Euclid: On the reduced shear approximation and magnification bias for Stage IV cosmic shear experiments
Stage IV weak lensing experiments will offer more than an order of magnitude leap in precision. We must therefore ensure that our analyses remain accurate in this new era. Accordingly, previously ignored systematic effects must be addressed. In this work, we evaluate the impact of the reduced shear approximation and magnification bias, on the information obtained from the angular power spectrum. To first-order, the statistics of reduced shear, a combination of shear and convergence, are taken to be equal to those of shear. However, this approximation can induce a bias in the cosmological parameters that can no longer be neglected. A separate bias arises from the statistics of shear being altered by the preferential selection of galaxies and the dilution of their surface densities, in high-magnification regions. The corrections for these systematic effects take similar forms, allowing them to be treated together. We calculate the impact of neglecting these effects on the cosmological parameters that would be determined from Euclid, using cosmic shear tomography. We also demonstrate how the reduced shear correction can be calculated using a lognormal field forward modelling approach. These effects cause significant biases in Omega_m, n_s, sigma_8, Omega_DE, w_0, and w_a of -0.51 sigma, -0.36 sigma, 0.37 sigma, 1.36 sigma, -0.66 sigma, and 1.21 sigma, respectively. We then show that these lensing biases interact with another systematic: the intrinsic alignment of galaxies. Accordingly, we develop the formalism for an intrinsic alignment-enhanced lensing bias correction. Applying this to Euclid, we find that the additional terms introduced by this correction are sub-dominant
Euclid: The reduced shear approximation and magnification bias for Stage IV cosmic shear experiments
Context: Stage IV weak lensing experiments will offer more than an order of magnitude leap in precision. We must therefore ensure that our analyses remain accurate in this new era. Accordingly, previously ignored systematic effects must be addressed.
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Aims: In this work, we evaluate the impact of the reduced shear approximation and magnification bias on information obtained from the angular power spectrum. To first-order, the statistics of reduced shear, a combination of shear and convergence, are taken to be equal to those of shear. However, this approximation can induce a bias in the cosmological parameters that can no longer be neglected. A separate bias arises from the statistics of shear being altered by the preferential selection of galaxies and the dilution of their surface densities in high-magnification regions.
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Methods: The corrections for these systematic effects take similar forms, allowing them to be treated together. We calculated the impact of neglecting these effects on the cosmological parameters that would be determined from Euclid, using cosmic shear tomography. To do so, we employed the Fisher matrix formalism, and included the impact of the super-sample covariance. We also demonstrate how the reduced shear correction can be calculated using a lognormal field forward modelling approach.
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Results: These effects cause significant biases in Ωm, σ8, ns, ΩDE, w0, and wa of −0.53σ, 0.43σ, −0.34σ, 1.36σ, −0.68σ, and 1.21σ, respectively. We then show that these lensing biases interact with another systematic effect: the intrinsic alignment of galaxies. Accordingly, we have developed the formalism for an intrinsic alignment-enhanced lensing bias correction. Applying this to Euclid, we find that the additional terms introduced by this correction are sub-dominant
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