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 échelle 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 per cent precision: M
A = 2.263 ± 0.012 M, M
B = 1.325 ± 0.007 M, R
A = 2.525 ± 0.011 R and R
B = 1.331 ± 0.006 R. We determine the abundance of 20 elements for the primary star, of which all except scandium are supersolar 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 (T
eff = 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.
ABSTRACT
If γ Dor-type pulsations are driven by the convective blocking mechanism, a convective envelope at a sufficient depth is essential. There are several hot γ Dor and hybrid star candidates in ...which there should not be an adequate convective envelope to excite the γ Dor-type oscillations. The existence of these hot objects needs an explanation. Therefore, we selected, observed, and studied 24 hot γ Dor and hybrid candidates to investigate their properties. The atmospheric parameters, chemical abundances, and vsin i values of the candidates were obtained using medium-resolution (R = 46 000) spectra taken with the Fibre-fed Échelle Spectrograph instrument mounted at the Nordic Optical Telescope. We also carried out frequency analyses of the Kepler long- and short-cadence data to determine the exact pulsation contents. We found only five bona fide hot γ Dor and three bona fide hot hybrid stars in our sample. The other 16 stars were found to be normal γ Dor, δ Sct, or hybrid variables. No chemical peculiarity was detected in the spectra of the bona fide hot γ Dor and hybrid stars. We investigated the interplay between rotation and pulsational modes. We also found that the hot γ Dor stars have higher Gaia luminosities and larger radii compared to main-sequence A–F stars.
Spots on Am stars Balona, L. A; Catanzaro, G; Abedigamba, O. P ...
Monthly notices of the Royal Astronomical Society,
04/2015, Letnik:
448, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We investigate the light variations of 15 Am stars using four years of high-precision photometry from the Kepler spacecraft and an additional 14 Am stars from the K2 Campaign 0 field. We find that ...most of the Am stars in the Kepler field have light curves characteristic of rotational modulation due to star-spots. Of the 29 Am stars observed, 12 are δ Scuti variables and one is a γ Doradus star. One star is an eclipsing binary and another was found to be a binary from time delay measurements. Two Am stars show evidence for flares which are unlikely to be due to a cool companion. The fact that 10 out of 29 Am stars are rotational variables and that some may even flare strongly suggests that Am stars possess significant magnetic fields. This is contrary to the current understanding that the enhanced metallicity in these stars is due to diffusion in the absence of a magnetic field. The fact that so many stars are δ Scuti variables is also at odds with the prediction of diffusion theory. We suggest that a viable alternative is that the metal enhancement could arise from accretion.
The EBLM project Triaud, A H M J; Hebb, L; Anderson, D R ...
Astronomy and astrophysics (Berlin),
1/2013, Letnik:
549
Journal Article, Web Resource
Recenzirano
Odprti dostop
This paper introduces a series of papers aiming to study the dozens of low-mass eclipsing binaries (EBLM), with F, G, K primaries, that have been discovered in the course of the WASP survey. Our ...objects are mostly single-line binaries whose eclipses have been detected by WASP and were initially followed up as potential planetary transit candidates. These have bright primaries, which facilitates spectroscopic observations during transit and allows the study of the spin-orbit distribution of F, G, K+M eclipsing binaries through the Rossiter-McLaughlin effect. Here we report on the spin-orbit angle of WASP-30b, a transiting brown dwarf, and improve its orbital parameters. We also present the mass, radius, spin-orbit angle and orbital parameters of a new eclipsing binary, J1219-39b (1SWAPJ121921.03-395125.6, TYC 7760484-1), which, with a mass of 95+ or -2 M sub(jup), is close to the limit between brown dwarfs and stars. We find that both objects have projected spin-orbit angles aligned with their primaries' rotation. Neither primaries are synchronous. J1219-39b has a modestly eccentric orbit and is in agreement with the theoretical mass-radius relationship, whereas WASP-30b lies above it.
We present the results of a study of a large sample of A and Am stars with spectral types from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and light curves from Wide Area ...Search for Planets (WASP). We find that, unlike normal A stars, ... Sct pulsations in Am stars are mostly confined to the effective temperature range 6900<Teff<7600 K. We find evidence that the incidence of pulsations in Am stars decreases with increasing metallicism (degree of chemical peculiarity). The maximum amplitude of the pulsations in Am stars does not appear to vary significantly with metallicism. The amplitude distributions of the principal pulsation frequencies for both A and Am stars appear very similar and agree with results obtained from Kepler photometry. We present evidence that suggests turbulent pressure is the main driving mechanism in pulsating Am stars, rather than the ...-mechanism, which is expected to be suppressed by gravitational settling in these stars. (ProQuest: ... denotes formulae/symbols omitted.)
We present a detailed spectroscopic study of 93 solar-type stars that are targets of the NASA/Kepler mission and provide detailed chemical composition of each target. We find that the overall ...metallicity is well represented by Fe lines. Relative abundances of light elements (CNO) and α elements are generally higher for low-metallicity stars. Our spectroscopic analysis benefits from the accurately measured surface gravity from the asteroseismic analysis of the Kepler light curves. The accuracy on the log g parameter is better than 0.03 dex and is held fixed in the analysis. We compare our T
eff determination with a recent colour calibration of V
T−K
S TYCHO V magnitude minus Two Micron All Sky Survey (2MASS) K
S magnitude and find very good agreement and a scatter of only 80 K, showing that for other nearby Kepler targets, this index can be used. The asteroseismic log g values agree very well with the classical determination using Fe i-Fe ii balance, although we find a small systematic offset of 0.08 dex (asteroseismic log g values are lower). The abundance patterns of metals, α elements and the light elements (CNO) show that a simple scaling by Fe/H is adequate to represent the metallicity of the stars, except for the stars with metallicity below −0.3, where α-enhancement becomes important. However, this is only important for a very small fraction of the Kepler sample. We therefore recommend that a simple scaling with Fe/H be employed in the asteroseismic analyses of large ensembles of solar-type stars.
Abstract
We report the discovery of WASP-166b, a super-Neptune planet with a mass of 0.1 MJup (1.9 MNep) and a bloated radius of 0.63 RJup. It transits a V = 9.36, F9V star in a 5.44-d orbit that is ...aligned with the stellar rotation axis (sky-projected obliquity angle λ = 3 ± 5 deg). Variations in the radial-velocity measurements are likely the result of magnetic activity over a 12-d stellar rotation period. WASP-166b appears to be a rare object within the ‘Neptune desert’.
WASP-19b has the shortest orbital period of any known exoplanet, orbiting at only 1.2 times the Roche tidal radius. By observing the Rossiter-McLaughlin effect we show that WASP-19b's orbit is ...aligned, with Delta *l = 46 ? 52. Using, in addition, a spectroscopic vsin I and the observed rotation period we conclude that the obliquity, Delta *v, is less than 20?. Further, the eccentricity of the orbit is less than 0.02. We argue that hot Jupiters with orbital periods as short as that of WASP-19b are two orders of magnitude less common than hot Jupiters at the 3-4 day 'pileup.' We discuss the evolution of WASP-19b's orbit and argue that most likely it was first moved to near twice the Roche limit by third-body interactions, and has since spiralled inward to its current location under tidal decay. This is compatible with a stellar tidal-dissipation quality factor, Q'*, of order 107.
We report on the discovery of WASP-12b, a new transiting extrasolar planet with R pl = 1.79+0.09 -0.09 RJ and M pl = 1.41+0.10 -0.10 M J. The planet and host star properties were derived from a Monte ...Carlo Markov Chain analysis of the transit photometry and radial velocity data. Furthermore, by comparing the stellar spectrum with theoretical spectra and stellar evolution models, we determined that the host star is a supersolar metallicity (M/H = 0.3+0.05 -0.15), late-F (T eff = 6300+200 -100 K) star which is evolving off the zero-age main sequence. The planet has an equilibrium temperature of T eq = 2516 K caused by its very short period orbit (P = 1.09 days) around the hot, twelfth magnitude host star. WASP-12b has the largest radius of any transiting planet yet detected. It is also the most heavily irradiated and the shortest period planet in the literature.
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