Kepler allows the measurement of starspot variability in a large sample of field red giants for the first time. With a new method that combines autocorrelation and wavelet decomposition, we measure ...361 rotation periods from the full set of 17 377 oscillating red giants in our sample. This represents 2.08% of the stars, consistent with the fraction of spectroscopically detected rapidly rotating giants in the field. The remaining stars do not show enough variability to allow us to measure a reliable surface rotation period. Because the stars with detected rotation periods have measured oscillations, we can infer their global properties, e.g. mass and radius, and quantitatively evaluate the predictions of standard stellar evolution models as a function of mass. Consistent with results for cluster giants when we consider only the 4881 intermediate-mass stars, M > 2.0 M⊙ from our full red giant sample, we do not find the enhanced rates of rapid rotation expected from angular momentum conservation. We therefore suggest that either enhanced angular momentum loss or radial differential rotation must be occurring in these stars. Finally, when we examine the 575 low-mass (M< 1.1 M⊙) red clump stars in our sample, which were expected to exhibit slow (non-detectable) rotation, 15% of them actually have detectable rotation. This suggests a high rate of interactions and stellar mergers on the red giant branch.
Context. In recent years the global seismic scaling relations for the frequency of maximum power, νmax ∝ g / √Teff $\nu_{\mathrm max} \propto g/\sqrt{T_{\mathrm eff}}$νmax∝g/Teff, and for the large ...frequency separation, Δν ∝ √ρ¯ ${{\mathrm \Delta}}\nu \propto \sqrt{\bar\rho}$Δν∝ρ¯ , have drawn attention in various fields of astrophysics. This is because these relations can be used to estimate parameters, such as the mass and radius of stars that show solar-like oscillations. With the exquisite photometry of Kepler, the uncertainties in the seismic observables are small enough to estimate masses and radii with a precision of only a few per cent. Even though this seems to work quite well for main-sequence stars, there is empirical evidence, mainly from studies of eclipsing binary systems, that the seismic scaling relations systematically overestimate the mass and radius of red giants by about 15% and 5%, respectively. Various model-based corrections of the Δν-scaling reduce the problem but do not solve it. Aims. Our goal is to define revised seismic scaling relations that account for the known systematic mass and radius discrepancies in a completely model-independent way. Methods. We use probabilistic methods to analyse the seismic data and to derive non-linear scaling relations based on a sample of six red giant branch (RGB) stars that are members of eclipsing binary systems and about 60 red giants on the RGB as well as in the core-helium burning red clump (RC) in the two open clusters NGC 6791 and NGC 6819. Results. We re-examine the global oscillation parameters of the giants in the binary systems in order to determine their seismic fundamental parameters and we find them to agree with the dynamic parameters from the literature if we adopt non-linear scalings. We note that a curvature and glitch corrected Δνcor should be preferred over a local or average value of Δν. We then compare the observed seismic parameters of the cluster giants to those scaled from independent measurements and find the same non-linear behaviour as for the eclipsing binaries. Our final proposed scaling relations are based on both samples and cover a broad range of evolutionary stages from RGB to RC stars: g / √Teff = (νmax / νmax,⊙)1.0075±0.0021 $g/\sqrt{T_{\mathrm eff}} = (\nu_{\mathrm max}/\nu_{\mathrm max,\odot})^{1.0075\pm0.0021}$g/Teff=(νmax/νmax,⊙)1.0075±0.0021 and √ρ¯ = (Δνcor / Δνcor,⊙)η − (0.0085 ± 0.0025) log2(Δνcor / Δνcor,⊙)−1 $\sqrt{\bar\rho} = ({{\mathrm \Delta}}\nu_{\mathrm cor}/{{\mathrm \Delta}}\nu_{\mathrm cor,\odot})\eta - (0.0085\pm0.0025) \log^2 ({{\mathrm \Delta}}\nu_{\mathrm cor}/{{\mathrm \Delta}}\nu_{\mathrm cor,\odot})^{-1}$ρ¯=(Δνcor/Δνcor,⊙)η−(0.0085±0.0025)log2(Δνcor/Δνcor,⊙)−1, where g, Teff, and ρ¯ $\bar\rho$ρ¯ are in solar units, νmax,⊙ = 3140 ± 5 μHz and Δνcor,⊙ = 135.08 ± 0.02 μHz, and η is equal to one in the case of RGB stars and 1.04 ± 0.01 for RC stars. Conclusions. A direct consequence of these new scaling relations is that the average mass of stars on the ascending giant branch reduces to 1.10 ± 0.03 M⊙ in NGC 6791 and 1.45 ± 0.06 M⊙ in NGC 6819, allowing us to revise the clusters’ distance modulus to 13.11 ± 0.03 and 11.91 ± 0.03 mag, respectively. We also find strong evidence that both clusters are significantly older than concluded from previous seismic investigations.
Context. The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. We are now able to probe the rotational behaviour in their deep interiors using the ...observations of mixed modes. Aims. We aim to measure the rotational splittings in red giants and to derive scaling relations for rotation related to seismic and fundamental stellar parameters. Methods. We have developed a dedicated method for automated measurements of the rotational splittings in a large number of red giants. Ensemble asteroseismology, namely the examination of a large number of red giants at different stages of their evolution, allows us to derive global information on stellar evolution. Results. We have measured rotational splittings in a sample of about 300 red giants. We have also shown that these splittings are dominated by the core rotation. Under the assumption that a linear analysis can provide the rotational splitting, we observe a small increase of the core rotation of stars ascending the red giant branch. Alternatively, an important slow down is observed for red-clump stars compared to the red giant branch. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass. Conclusions. Ensemble asteroseismology indicates what has been indirectly suspected for a while: our interpretation of the observed rotational splittings leads to the conclusion that the mean core rotation significantly slows down during the red giant phase. The slow-down occurs in the last stages of the red giant branch. This spinning down explains, for instance, the long rotation periods measured in white dwarfs.
Context. The detection of mixed modes that are split by rotation in Kepler red giants has made it possible to probe the internal rotation profiles of these stars, which brings new constraints on the ...transport of angular momentum in stars. Rotation rates in the central regions of intermediate-mass core helium burning stars (secondary clump stars) have recently been measured. Aims. Our aim is to exploit the rotational splittings of mixed modes to estimate the amount of radial differential rotation in the interior of secondary clump stars using Kepler data in order to place constraints on angular momentum transport in intermediate-mass stars. Methods. We select a subsample of Kepler secondary clump stars with mixed modes that are clearly rotationally split. By applying a thorough statistical analysis, we show that the splittings of gravity-dominated modes (trapped in central regions) and of p-dominated modes (trapped in the envelope) can be measured. We then use these splittings to estimate the amount of differential rotation by using inversion techniques and by applying a simplified approach based on asymptotic theory. Results. We obtain evidence for a weak radial differential rotation for six of the seven targets that were selected, with the central regions rotating from 1.8 ± 0.3 to 3.2 ± 1.0 times faster than the envelope. The last target is found to be consistent with a solid-body rotation. Conclusions. This demonstrates that an efficient redistribution of angular momentum occurs after the end of the main sequence in the interior of intermediate-mass stars, either during the short-lived subgiant phase or once He-burning has started in the core. In either case, this should bring constraints on the angular momentum transport mechanisms that are at work.
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•AFS-D successfully deposited layers wider than the diameter of the tool offering a new path for large scale AM components.•Aluminum oxide films were dispersed by tool features and ...provided increased interlayer mixing.•More homogenous grain structures occurring from repeated stirring in overlapping region.•As-deposited tensile strength was unaffected by the overlapping region compared to similar AFS-D AA6061 single row studies.•Undispersed oxides dominated the fracture surface in place of typical secondary phase continuative particles.
In this paper, the effect of overlapping parallel additive depositions on microstructure and mechanical properties in Additive Friction Stir Deposition (AFS-D) was examined. In particular, the AFS-D process was employed to make parallel depositions of AA6061 with a 6.35 mm overlap to effectively bond the two parallel layers. The AFS-D process draws on similar physics to friction stir welding in that frictional heat and plastic deformation is exploited to deposit metallic materials from the center of a hollow rotating tool as it traverses across the build table to produce consecutive metallurgically bonded layers. In this work, the microstructural aspects of the 6.35 mm overlapping raster interface were characterized using optical and scanning electron microscopy. Aluminum oxide particles were observed at the raster interface, which were located at layer boundaries. Additional grain refinement was also apparent as a direct result of multiple stirring passes within the overlapping deposition region. Mechanical characterization via microindentation and monotonic tensile testing observed a hardness gradient in the overlapping region, but the parallel deposition layers exhibited comparable tensile strength to a single row of deposited AA6061. The influence of existing oxides on the mechanical results was observed to have limited effect on the properties longitudinally across the raster. This study determined that parallel layers of AA6061 can be successfully deposited via the AFS-D technique. The resulting deposit exhibited a strong metallurgical bond across the parallel layers despite the presence of surface oxidation on the unprepared feedstock and substrate.
ABSTRACT
The internal structures and properties of oscillating red-giant stars can be accurately inferred through their global oscillation modes (asteroseismology). Based on 1460 d of Kepler ...observations we perform a thorough asteroseismic study to probe the stellar parameters and evolutionary stages of three red giants in eclipsing binary systems. We present the first detailed analysis of individual oscillation modes of the red-giant components of KIC 8410637, KIC 5640750, and KIC 9540226. We obtain estimates of their asteroseismic masses, radii, mean densities, and logarithmic surface gravities by using the asteroseismic scaling relations as well as grid-based modelling. As these red giants are in double-lined eclipsing binaries, it is possible to derive their independent dynamical masses and radii from the orbital solution and compare it with the seismically inferred values. For KIC 5640750 we compute the first spectroscopic orbit based on both components of this system. We use high-resolution spectroscopic data and light curves of the three systems to determine up-to-date values of the dynamical stellar parameters. With our comprehensive set of stellar parameters we explore consistencies between binary analysis and asteroseismic methods, and test the reliability of the well-known scaling relations. For the three red giants under study, we find agreement between dynamical and asteroseismic stellar parameters in cases where the asteroseismic methods account for metallicity, temperature, and mass dependence as well as surface effects. We are able to attain agreement from the scaling laws in all three systems if we use $\Delta \nu _{\rm ref,emp} = 130.8 \pm 0.9 \,\mu$Hz instead of the usual solar reference value.
ABSTRACT We study the dynamics of the stellar interior of the early red-giant star KIC 4448777 by asteroseismic inversion of 14 splittings of the dipole mixed modes obtained from Kepler observations. ...In order to overcome the complexity of the oscillation pattern typical of red-giant stars, we present a procedure to extract the rotational splittings from the power spectrum. We find not only that the core rotates from a minimum of 8 to a maximum of 17 times faster than the surface, confirming previous inversion results generated for other red giants (Deheuvels et al.), but we also estimate the variation of the angular velocity within the helium core with a spatial resolution of 0.001R and verify the hypothesis of a sharp discontinuity in the inner stellar rotation. The results show that the entire core rotates rigidly and provide evidence for an angular velocity gradient around the base of the hydrogen-burning shell; however, we do not succeed in characterizing the rotational slope, due to the intrinsic limits of the applied techniques. The angular velocity, from the edge of the core, appears to decrease with increasing distance from the center, reaching an average value in the convective envelope of 68 22 nHz. We conclude that a set of data that includes only dipolar modes is sufficient to infer quite accurately the rotation of a red giant not only in the dense core but also, with a lower level of confidence, in part of the radiative region and in the convective envelope.
In this study, the effect of post-deposition heat treatments on an Al–Mg–Si alloy processed with additive friction stir deposition (AFS-D), a solid-state additive manufacturing process, is examined. ...Results reveal that wrought-like microstructure and mechanical properties are achievable in AFS-D builds through the application of a post-deposition heat treatment.
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