1,484 research outputs found
Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta a system
to derive astrophysical parameters such as the age, reddening and distance
modulus for open clusters. The Delta a system samples the flux depression at
520 nm which is highly efficient to detect chemically peculiar (CP) objects of
the upper main sequence. The evolutionary status of CP stars is still a matter
of debate and very important to test, for example, the dynamo and diffusion
theories. In fact, the dynamo or fossil origin of the magnetic fields present
in this kind of stars it still not clear. Using the stellar evolutionary models
by Claret (1995), a grid of isochrones with different initial chemical
compositions for the Delta a system was generated. The published data of 23
open clusters were used to fit these isochrones with astrophysical parameters
(age, reddening and distance modulus) from the literature. As an additional
test, isochrones with the same parameters for Johnson UBV data of these open
clusters were also considered. The fits show a good agreement between the
observations and the theoretical grid. We find that the accuracy of fitting
isochrones to Delta a data without the knowledge of the cluster parameters is
between 5 and 15%.Comment: 5 pages, 4 figures, accepted by A&
Chemically peculiar stars in the Large Magellanic Cloud
The detection of magnetic chemically peculiar (CP2) stars in open clusters of
extragalactic systems can give observational answers to many unsolved
questions. The mean percentage of CP2 stars in the Milky Way is of the order of
5% for the spectral range from early B- to F-type, luminosity class V objects.
The origin of the CP2 phenomenon seems to be closely connected to the overall
metallicity and global magnetic field environment. The theoretical models are
still only tested by observations in the Milky Way. It is therefore essential
to provide high quality observations in rather different global environments.
The young clusters NGC 2136/7 were observed in the Delta a photometric system.
This intermediate band photometric system samples the depth of the 520nm flux
depression by comparing the flux at the center with the adjacent regions with
bandwidths of 11nm to 23nm. The Delta a photometric system is most suitable for
detecting CP2 stars with high efficiency, but is also capable of detecting a
small percentage of non-magnetic CP objects. We present high precision
photometric Delta a observations of 417 objects in NGC 2136/7 and its
surrounding field, of which five turned out to be bona fide magnetic CP stars.
In addition, we discovered two Be/Ae stars. From our investigations of NGC
1711, NGC 1866, NGC 2136/7, their surroundings, and one independent field of
the LMC population, we derive an occurrence of classical chemically peculiar
stars of 2.2(6)% in the LMC, which is only half the value found in the Milky
Way. The mass and age distribution of the photometrically detected CP stars is
not different from that of similar objects in galactic open clusters.Comment: 5 pages, 4 figures, accepted by A&
High-precision photometry by telescope defocussing. III. The transiting planetary system WASP-2
We present high-precision photometry of three transits of the extrasolar
planetary system WASP-2, obtained by defocussing the telescope, and achieving
point-to-point scatters of between 0.42 and 0.73 mmag. These data are modelled
using the JKTEBOP code, and taking into account the light from the
recently-discovered faint star close to the system. The physical properties of
the WASP-2 system are derived using tabulated predictions from five different
sets of stellar evolutionary models, allowing both statistical and systematic
errorbars to be specified. We find the mass and radius of the planet to be M_b
= 0.847 +/- 0.038 +/- 0.024 Mjup and R_b = 1.044 +/- 0.029 +/- 0.015 Rjup. It
has a low equilibrium temperature of 1280 +/- 21 K, in agreement with a recent
finding that it does not have an atmospheric temperature inversion. The first
of our transit datasets has a scatter of only 0.42 mmag with respect to the
best-fitting light curve model, which to our knowledge is a record for
ground-based observations of a transiting extrasolar planet.Comment: Accepted for publication in MNRAS. 9 pages, 3 figures, 10 table
On the Selection of Photometric Planetary Transits
We present a new method for differentiating between planetary transits and
eclipsing binaries based on the presence of the ellipsoidal light variations.
These variations can be used to detect stellar secondaries with masses ~0.2
M_sun orbiting sun-like stars at a photometric accuracy level which has already
been achieved in transit surveys. By removing candidates exhibiting this effect
it is possible to greatly reduce the number of objects requiring spectroscopic
follow up with large telescopes. Unlike the usual candidate selection method,
which are primarily based on the estimated radius of the orbiting object, this
technique is not biased against bona-fide planets and brown dwarfs with large
radii, because the amplitude of the effect depends on the transiting object's
mass and orbital distance. In many binary systems, where a candidate planetary
transit is actually due to the partial eclipse of two normal stars, the
presence of flux variations due to the gravity darkening effect will show the
true nature of these systems. We show that many of the recent OGLE-III
photometric transit candidates exhibit the presence of significant variations
in their light curves and are likely to be due to stellar secondaries. We find
that the light curves of white dwarf transits will generally not mimic those of
small planets because of significant gravitationally induced flux variations.
We discuss the relative merits of methods used to detect transit candidates
which are due to stellar blends rather than planets. We outline how photometric
observations taken in two bands can be used to detect the presence of stellar
blends.Comment: ApJ, 11 pages, 2 figures, 1 table, replaced with accepted versio
A method to simulate inhomogeneously irradiated objects with a superposition of 1D models
In close binary systems the atmosphere of one or both components can be
significantly influenced by irradiation from the companion. Often the
irradiated atmosphere is simulated with a single-temperature approximation for
the entire half-sphere. We present a scheme to take the varying irradiation
angle into account by combining several separate 1D models. This is independent
of the actual code which provides the separate stellar spectra. We calculate
the projected area of zones with given irradiation angle and use this
geometrical factor to scale separate 1D models. As an example we calculate two
different irradiation scenarios with the PHOENIX code. The scheme to calculate
the projected area is applicable independent of the physical mechanism that
forms these zones. In the case of irradiation by a primary with T=125000 K, the
secondary forms ions at different ionisation states for different irradiation
angles. No single irradiation angle exists which provides an accurate
description of the spectrum. We show a similar simulation for weaker
irradiation, where the profile of the H line depends on the irradiation
angle.Comment: published in A&
Absolute dimensions of the unevolved B-type eclipsing binary GG Orionis
We present photometric observations in B and V as well as spectroscopic
observations of the detached, eccentric 6.6-day double-lined eclipsing binary
GG Ori, a member of the Orion OB1 association. Absolute dimensions of the
components, which are virtually identical, are determined to high accuracy
(better than 1% in the masses and better than 2% in the radii) for the purpose
of testing various aspects of theoretical modeling. We obtain M(A) = 2.342 +/-
0.016 solar masses and R(A) = 1.852 +/- 0.025 solar radii for the primary, and
M(B) = 2.338 +/- 0.017 solar masses and R(B) = 1.830 +/- 0.025 solar radii for
the secondary. The effective temperature of both stars is 9950 +/- 200 K,
corresponding to a spectral type of B9.5. GG Ori is very close to the ZAMS, and
comparison with current stellar evolution models gives ages of 65-82 Myr or 7.7
Myr depending on whether the system is considered to be burning hydrogen on the
main sequence or still in the final stages of pre-main sequence contraction. We
have detected apsidal motion in the binary at a rate of dw/dt = 0.00061 +/-
0.00025 degrees per cycle, corresponding to an apsidal period of U = 10700 +/-
4500 yr. A substantial fraction of this (approximately 70%) is due to the
contribution from General Relativity.Comment: To appear in The Astronomical Journal, December 200
CCD photometric search for peculiar stars in open clusters. VII. Berkeley 11, Berkeley 94, Haffner 15, Lynga 1, NGC 6031, NGC 6405, NGC 6834 and Ruprecht 130
The detection of magnetic chemically peculiar (CP2) stars in open clusters of
the Milky Way can be used to study the influence of different galactic
environments on the (non-)presence of peculiarities, which has to be taken into
account in stellar evolution models. Furthermore it is still unknown if the CP2
phenomenon evolves, i.e. does the strength of the peculiarity feature at 5200A,
increase or decrease with age. We have observed eight young to intermediate age
open clusters in the Delta a photometric system. This intermediate band
photometric system samples the depth of the 5200A, flux depression by comparing
the flux at the center with the adjacent regions having bandwidths of 110A, to
230A. The Delta a photometric system is most suitable to detect CP2 stars with
high efficiency, but is also capable of detecting a small percentage of
non-magnetic CP objects. Also, the groups of (metal-weak) lambda Bootis, as
well as classical Be/shell stars, can be successfully investigated. This
photometric system allows one to determine the age, reddening and distance
modulus by fitting isochrones. Among the presented sample of eight galactic
clusters, we have detected twenty three CP2, eight Be/Ae and eight metal-weak
stars. Another six objects show a peculiar behaviour which is most probably due
to a non-membership,variability or duplicity. Fitting isochrones to Delta a
photometry yields estimates of the age, reddening and distance that are in
excellent agreement with published values
Absolute dimensions of solar-type eclipsing binaries. EF Aquarii: a G0 test for stellar evolution models
Recent studies have shown that stellar chromospheric activity, and its effect
on convective energy transport in the envelope, is most likely the cause of
significant radius and temperature discrepancies between theoretical evolution
models and observations. We aim to determine absolute dimensions and abundances
for the solar-type detached eclipsing binary EF Aqr, and to perform a detailed
comparison with results from recent stellar evolutionary models. uvby-beta
standard photometry was obtained with the Stromgren Automatic Telescope. The
broadening function formalism was applied on spectra observed with HERMES at
the Mercator telescope in La Palma, to obtain radial velocity curves. Masses
and radii with a precision of 0.6% and 1.0% respectively have been established
for both components of EF Aqr. The active 0.956 M_sol secondary shows star
spots and strong Ca II H and K emission lines. The 1.224 M_sol primary shows
signs of activity as well, but at a lower level. An [Fe/H] abundance of
0.00+-0.10 is derived with similar abundances for Si, Ca, Sc, Ti, V, Cr, Co,
and Ni. Solar calibrated evolutionary models such as Yonsei-Yale,
Victoria-Regina and BaSTI isochrones and evolutionary tracks are unable to
reproduce EF Aqr, especially for the secondary, which is 9% larger and 400 K
cooler than predicted. Models adopting significantly lower mixing length
parameters l/H_p remove these discrepancies, as seen in other solar type
binaries. For the observed metallicity, Granada models with a mixing length of
l/H_p=1.30 (primary) and 1.05 (secondary) reproduce both components at a common
age of 1.5+-0.6 Gyr. Observations of EF Aqr suggests that magnetic activity,
and its effect on envelope convection, is likely to be the cause of
discrepancies in both radius and temperature, which can be removed by adjusting
the mixing length parameter of the models downwards.Comment: 11 pages, 8 figures, accepted for publication by A&
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