215 research outputs found
Probing the models: Abundances for high-mass stars in binaries
The complexity of composite spectra of close binary star system makes study
of the spectra of their component stars extremely difficult. For this reason
there exists very little information on the photospheric chemical composition
of stars in close binaries, despite its importance for informing our
understanding of the evolutionary processes of stars. In a long-term
observational project we aim to fill this gap with systematic abundance studies
for the variety of binary systems. The core of our analysis is the spectral
disentangling technique, which allows isolation of the individual component
star spectra from the time-series of observed spectra.
We present new results for high-mass stars in close binaries. So far, we have
measured detailed abundances for 22 stars in a dozen detached binary systems.
The parameter space for the stars in our sample comprises masses in the range
8--22 M_sun, surface gravities of 3.1--4.2 (c.g.s.) and projected rotational
velocities of 30--240 km/s. Whilst recent evolutionary models for rotating
single stars predict changes in photospheric abundances even during the main
sequence lifetime, no star in our sample shows signs of these predicted
changes. It is clear that other effects prevail in the chemical evolution of
components in binary stars even at the beginning of their evolution.Comment: 8 pages, 3 figures, to appear in the proceedings of the conference on
'Setting a New Standard in the Analysis of Binary Stars', K. Pavlovski, A.
Tkachenko and G. Torres, eds., EAS Publication Serie
Modelling of the B-type binaries CW Cep and U Oph: A critical view on dynamical masses, core boundary mixing, and core mass
Context: Intermediate-Mass stars are often overlooked as they are not
supernova progenitors but still host convective cores and complex atmospheres
which require computationally expensive treatment. Due to this, there is a
general lack of such stars modelled by state of the art stellar structure and
evolution codes. Aims: This paper aims to use high-quality spectroscopy to
update the dynamically obtained stellar parameters and produce a new
evolutionary assessment of the bright B0.5+B0.5 and B5V+B5V binary systems CW
Cep and U Oph. Methods: We use new spectroscopy obtained with the Hermes
spectrograph to revisit the photometric binary solution of the two systems. The
updated mass ratio and effective temperatures are incorporated to obtain new
dynamical masses for the primary and secondary. With these, we perform
isochrone-cloud based evolutionary modelling to investigate the core properties
of these stars. Results: We report the first abundances for CW Cep and U Oph as
well as report an updated dynamical solution for both systems. We find that we
cannot uniquely constrain the amount of core boundary mixing in any of the
stars we consider. Instead, we report their core masses and compare our results
to previous studies. Conclusions: We find that the per-cent level precision on
fundamental stellar quantities are accompanied with core mass estimates to
between ~ 5-15%. We find that differences in analysis techniques can lead to
substantially different evolutionary modeComment: 15 pages, 7 figures, two appendices with 4 figures each. Accepted for
publication in Astronomy & Astrophysic
Quantitative spectroscopy of close binary stars
The method of spectral disentangling has now created the opportunity for
studying the chemical composition in previously inaccessible components of
binary and multiple stars. This in turn makes it possible to trace their
chemical evolution, a vital aspect in understanding the evolution of stellar
systems. We review different ways to reconstruct individual spectra from
eclipsing and non-eclipsing systems, and then concentrate on some recent
applications to detached binaries with high-mass and intermediate-mass stars,
and Algol-type mass-transfer systems.Comment: To appear in the Proceedings of IAU Symposium 282 'From Interacting
Binaries to Exoplanets: Essential Modeling Tools
Absolute dimensions of detached eclipsing binaries. III. The metallic-lined system YZ Cassiopeiae
The bright binary system YZ Cassiopeiae is a remarkable laboratory for
studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2
dwarf on a circular orbit, which undergo total and annular eclipses. We present
an analysis of 15 published light curves and 42 new high-quality echelle
spectra, resulting in measurements of the masses, radii, effective temperatures
and photospheric chemical abundances of the two stars. The masses and radii are
measured to 0.5% precision: M_A = 2.263 +/- 0.012 Msun, M_B = 1.325 +/- 0.007
Msun, R_A = 2.525 +/- 0.011 Rsun and R_B = 1.331 +/- 0.006 Rsun. We determine
the abundance of 20 elements for the primary star, of which all except scandium
are super-solar by up to 1 dex. The temperature of this star (9520 +/- 120 K)
makes it one of the hottest Am stars. We also measure the abundances of 25
elements for its companion (Teff = 6880 +/- 240 K), finding all to be solar or
slightly above solar. The photospheric abundances of the secondary star should
be representative of the bulk composition of both stars. Theoretical stellar
evolutionary models are unable to match these properties: the masses, radii and
temperatures imply a half-solar chemical composition (Z = 0.009 +/- 0.003) and
an age of 490-550 Myr. YZ Cas therefore presents a challenge to stellar
evolutionary theory.Comment: Accepted for publication in MNRAS. 15 pages, 9 tables, 7 figure
Abundances from disentangled component spectra: the eclipsing binary V578 Mon
Chemical abundances of the early-B type components of the binary V578 Mon are
derived from disentangled component spectra. This is a pilot study showing
that, even with moderately high line-broadening, metal abundances can be
derived from disentangled spectra with a precision 0.1 dex, relative to
sharp-lined single stars of the same spectral type. This binary is well-suited
for such an assessment because of its youth as a member of the Rosette Nebula
cluster NGC 2244, strengthening the expectation of an unevolved ZAMS chemical
composition. The method is of interest to study rotational driven mixing in
main-sequence stars, with fundamental stellar parameters known with higher
accuracy in (eclipsing) binaries. The paper also includes an evaluation of the
bias that might be present in disentangled spectra.Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 2
figure
CoRoT 102918586: a Gamma Dor pulsator in a short period eccentric eclipsing binary
Pulsating stars in eclipsing binary systems are powerful tools to test
stellar models. Binarity enables to constrain the pulsating component physical
parameters, whose knowledge drastically improves the input physics for
asteroseismic studies. The study of stellar oscillations allows us, in its
turn, to improve our understanding of stellar interiors and evolution. The
space mission CoRoT discovered several promising objects suitable for these
studies, which have been photometrically observed with unprecedented accuracy,
but needed spectroscopic follow-up. A promising target was the relatively
bright eclipsing system CoRoT 102918586, which turned out to be a double-lined
spectroscopic binary and showed, as well, clear evidence of Gamma Dor type
pulsations. We obtained phase resolved high-resolution spectroscopy with the
Sandiford spectrograph at the McDonald 2.1m telescope and the FEROS
spectrograph at the ESO 2.2m telescope. Spectroscopy yielded both the radial
velocity curves and, after spectra disentangling, the component effective
temperatures, metallicity and line-of-sight projected rotational velocities.
The CoRoT light curve was analyzed with an iterative procedure, devised to
disentangle eclipses from pulsations. We obtained an accurate determination of
the system parameters, and by comparison with evolutionary models strict
constraints on the system age. Finally, the residuals obtained after
subtraction of the best fitting eclipsing binary model were analyzed to
determine the pulsator properties. We achieved a quite complete and consistent
description of the system. The primary star pulsates with typical {\gamma} Dor
frequencies and shows a splitting in period which is consistent with high order
g-mode pulsations in a star of the corresponding physical parameters. The value
of the splitting, in particular, is consistent with pulsations in l = 1 modes.Comment: 12 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Physical properties and CNO abundances for high-mass stars in four main-sequence detached eclipsing binaries: V478 Cyg, AH Cep, V453 Cyg, and V578 Mon
We present ten high-precision light curves of four transits in the XO-1 planetary system, obtained using u, g, r, redshifted Hα, I and z filters. We use these to measure the physical properties, orbital ephemeris, and a transmission spectrum of the planet covering the full optical wavelength range. We augment this with published HST/WFC3 observations to construct a transmission spectrum of the planet covering 0.37 to 1.65 μm. Our best-fitting model to this spectrum is for a H2/He-rich atmosphere containing water (3.05σ confidence), nitrogen-bearing molecules NH3 and HCN (1.5σ) and patchy cloud (1.3σ). We find that adding the optical to the near-infrared data does not lead to more precise constraints on the planetary atmosphere in this case. We conduct a detailed investigation into the effect of stellar limb darkening on our results, concluding that the choice of limb darkening law and coefficients is unimportant; such conclusions may not hold for other systems so should be reassessed for all high-quality datasets. The planet radius we measure in the g-band is anomalously low and should be investigated with future observations at a higher spectral resolution. From the measured times of transit we determine an improved orbital ephemeris, calculate a lower limit on the modified stellar tidal quality factor of Q′⋆>105.6, and rule out a previously postulated sinusoidal variation in the transit times
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