139,752 research outputs found
Nitrogen depletion in field red giants: Mixing during the He flash?
No full textWe combine simultaneous constraints on stellar evolutionary status from asteroseismology, and on nitrogen abundances derived from large spectroscopic surveys, to follow nitrogen surface abundances all along the evolution of a low-mass star, comparing model expectations with data. After testing and calibrating the observed yields from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we first show that nitrogen surface abundances follow the expected trend after the first dredge-up occurred, i.e. that the more massive is the star, the more nitrogen is enhanced. Moreover, the behaviour of nitrogen data along the evolution confirms the existence of non-canonical extramixing on the red giant branch (RGB) for all low-mass stars in the field. But more surprisingly, the data indicate that nitrogen has been depleted between the RGB tip and the red clump. This may suggest that some nitrogen has been burnt near or at the He flash episode
Investigating Gaia EDR3 parallax systematics using asteroseismology of cool giant stars observed by Kepler, K2, and TESS. II. Deciphering Gaia parallax systematics using red clump stars
No full textWe analyse Gaia EDR3 parallax systematics as a function of magnitude and sky location using a recently published catalogue of 12 500 asteroseismic red-giant star distances. We selected ~3500 red clump (RC) stars of similar chemical composition as the optimal subsample for this purpose because (1) their similar luminosity allows for straightforward interpretation of trends with apparent magnitude; (2) RC stars are the most distant stars in our sample at a given apparent magnitude, so uncertainties related to asteroseismic radii and distances are the smallest; (3) and they provide the largest sample of intrinsically similar stars. We performed a detailed assessment of systematic uncertainties relevant for parallax offset estimation based on the asteroseismic distances. Specifically, we investigated (1) the impact of measuring the basic asteroseismic quantities νmax and 〈Δν〉 using different pipelines, (2) uncertainties related to extinction, (3) the impact of adopting spectroscopic information from different surveys, and (4) blending issues related to photometry. Following this assessment, we adopted for our baseline analysis the asteroseismic parameters measured in Elsworth et al. (2020, Res. Notes Am. Astron. Soc., 4, 177) and spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (DR17), and we further restricted the sample to low-extinction (AV ≤ 0.5 mag) RC stars with quality astrometric solutions from Gaia EDR3, as indicated by RUWE < 1.4. We then investigated both the parallax offset relative to the published Gaia EDR3 parallaxes and the residual parallax offset after correcting Gaia EDR3 parallaxes following Lindegren et al. (2021, A&A, 649, A4). We found residual parallax offsets very close to zero (-1.6 ± 0.5 (stat.)±10 (syst.) μas) for stars fainter than G > 11 mag in the initial Kepler field, suggesting that the Lindegren parallax offset corrections are adequate in this magnitude range. For 17 K2 campaigns in the same magnitude range, the residual parallax offset is +16.5 ± 1.7 (stat.)±10 (syst.) μas. At brighter magnitudes (G ≤ 11 mag), we found inconsistent residual parallax offsets between the Kepler field, 17 K2 campaigns, and the TESS southern continuous viewing zone, with differences of up to 60 μas. This contradicts the studies that suggest a monotonic trend between magnitude and residual parallax offsets and instead suggests a significant dependence on sky location at bright magnitudes due to a lack of bright physical pairs being available to determine the parallax offset corrections. Inspection of the 17 K2 campaigns allowed for investigation of parallax offsets as a function of ecliptic longitude and revealed a possible signal. Finally, we estimated the absolute magnitude of the red clump and obtained MKsRC = -1.650 ± 0.025 mag in the 2MASS Ks band and MGRC = (0.432 ± 0.004) - (0.821 ± 0.033) · (Teff [K]-4800 K)/1000 K [mag] in the Gaia G-band
Frictional strength and strain weakening in simulated fault gouge: Competition between geometrical weakening and chemical strengthening
No full textDespite the importance of hydromechanical effects in fault processes, not much is known about the interplay of chemical and mechanical processes, in part because the conditions are difficult to simulate in the laboratory. We report results from an experimental study of simulated fault gouge composed of rock salt sheared under conditions where pressure solution is known to operate. At sliding velocities above 10 m/s and high shear strains (>5), friction measurements show that layers of rock salt weaken significantly and ultimately slide unstably (i.e., stick-slip). Microstructural observations show the presence of a zone of comminuted grains along shear zone boundaries, forming boundary-parallel Y shears at high sliding velocities. Samples deformed at low sliding velocities do not show boundary-parallel shear but rather exhibit low porosity passive regions isolated by dilational zones in the Riedel shear orientation. We posit that the significant strain weakening observed at high sliding velocities is caused by severe grain size reduction as shear localization develops, i.e., by frictional wear, ultimately leading to the development of a throughgoing boundary parallel Y shear. Unstable slip is probably related to rupture on this Y shear surface with intermittent healing of the asperities by pressure solution. Furthermore, the data show that the weakening and subsequent unstable slip can be delayed (i.e., occur at higher strains) by lower sliding velocities, larger initial grain sizes, lower normal stresses, and the presence of fluids. This suggests a competition between mechanical wear and chemical processes. Our data highlight the importance of hydrothermal processes in tectonic faulting. Copyright 2010 by the American Geophysical Union
Peakbagging in the open cluster NGC 6819: Opening a treasure chest or Pandora's box?
No full textHere we report on an extensive peakbagging effort on the evolved red giant stars of the open cluster NGC 6819. This consists of around 50 stars spanning all the way up the red giant branch (RGB) and down to and including the red clump (RC). These stars represent a unique sample because of their common distance, metallicity and age. By employing sophisticated pre-processing of the time series and Markov Chain Monte Carlo (MCMC) techniques, we have extracted individual frequencies, heights, and line widths for hundreds of individual oscillation modes in the sample of stars. We show that average asteroseismic parameters derived from these can be used to distinguish the stellar evolutionary state between RGB and RC stars without having to measure the often difficult dipole modes. Furthermore, we show how the fitting of some of these dipole modes can improve the detectability of acoustic glitches arising from the helium II ionization zone and how this can potentially be used to constrain the helium content in the cluster. We also discuss some of the difficulties facing similar studies in the future, where it seems that detailed studies of star clusters are facing some difficult times ahead
Why should we correct reported pulsation frequencies for stellar line-of-sight Doppler velocity shifts?
No full textIn the age of Kepler and CoRoT, extended observations have provided estimates of stellar pulsation frequencies that have achieved newlevels of precision, regularly exceeding fractional levels of a few parts in 104. These high levels of precision now in principle exceed the point where one can ignore the Doppler shift of pulsation frequencies caused by the motion of a star relative to the observer. We present a correction for these Doppler shifts and use previously published pulsation frequencies to demonstrate the significance of the effect. We suggest that reported pulsation frequencies should be routinely corrected for stellar line-of-sight velocity Doppler shifts, or if a line-of-sight velocity estimate is not available, the frame of reference in which the frequencies are reported should be clearly stated
An Exploration of One Educational Career: The Differing Experience of Compulsory, Post-16 and Vocational Study
No full textEmily began her educational career as most children do in primary school. The majority of the work that she completed was “hands on really. We had to do lots of, erm, topic books” and this, she found, engaging and interesting, perhaps because the goal oriented perception of the work she was completing complimented the theoretical and academic aspect of it. Alongside this, Emily explains that she had a solid friendship base at primary school and this was enhanced by her positive relationships with her teachers who “were easy to talk to teachers [who] […] made class fun”. This approachability in teaching staff clearly enhanced Emily’s perception of school and it could, therefore, be argued that it is the practices of the teachers that enabled Emily to have a positive and engaging working relationship with them. This act of mutual recognition between participants is one that Wenger (1998) cites as being a key element in relationships between individuals as a way of governing their social interactions through understanding each others’ roles and intentions. It is this ability to successfully participate within the community of the primary school classroom with teachers and peers that will enable a child to learn comfortably (Dean, 2000). In line with the Vygotskian theory of passage through the Zone of Proximal Development, Emily has illustrated here that it is through her relationships with her “more competent peers”, the teachers, that she became engaged in learning at primary school (Moll, 1992)
Solar core rotation: Low-degree solar p-mode rotational splitting results from BiSON
No full textIn a recent paper (Elsworth et al.), we presented rotational splitting measurements of low-degree, low-frequency solar p modes, which, for the first time, show visually well-separated components at frequencies as low as similar to 1.5 mHz. At the present time, these data appear to rule out a rapidly rotating solar core. Here, we expand our analysis of these data to a full discussion of the statistical properties of the measured splittings. We have considered two fully independent Fourier spectra, generated from Doppler velocity residuals collected by the six-station Birmingham Solar-Oscillations Network (BiSON) between 1992 January 1 and 1994 August 23 (Fourier spectra 16A and 16C from Elsworth et al.). We present formal uncertainties from maximum-likelihood fits to the mode multiplets, analyse and compare the statistical properties of the sets, and search for possible trends in the splitting measures with n
Prospects for detecting asteroseismic binaries in Kepler data
No full textAsteroseismology may in principle be used to detect unresolved stellar binary systems comprised of solar-type stars and/or red giants. This novel method relies on the detection of the presence of two solar-like oscillation spectra in the frequency spectrum of a single light curve. Here, we make predictions of the numbers of systems that may be detectable in data already collected by the NASA Kepler Mission. Our predictions, which are based upon TRILEGAL and BiSEPS simulations of the Kepler field of view, indicate that as many as 200 or more "asteroseismic binaries" may be detectable in this manner. Most of these binaries should be comprised of two He-core-burning red giants. Owing largely to the limited numbers of targets with the requisite short-cadence Kepler data, we expect only a small number of detected binaries containing solar-type stars. The predicted yield of detections is sensitive to the assumed initial mass ratio distribution (IMRD) of the binary components and therefore represents a sensitive calibration of the much debated IMRD near mass ratio unity. © 2014. The American Astronomical Society. All rights reserved
Super-Nyquist asteroseismology of solar-like oscillators with Kepler and K2 - expanding the asteroseismic cohort at the base of the red giant branch
No full textWe consider the prospects for detecting solar-like oscillations in the 'super-Nyquist' regime of long-cadence (LC) Kepler photometry, i.e. above the associated Nyquist frequency of ≃ 283 μHz. Targets of interest are cool, evolved subgiants and stars lying at the base of the red giant branch. These stars would ordinarily be studied using the short-cadence (SC) data, since the associated SC Nyquist frequency lies well above the frequencies of the detectable oscillations. However, the number of available SC target slots is quite limited. This imposes a severe restriction on the size of the ensemble available for SC asteroseismic study.We find that archival Kepler LC data from the nominal mission may be utilized for asteroseismic studies of targets whose dominant oscillation frequencies lie as high as ≃500 μHz, i.e. about 1.75-times the LC Nyquist frequency. The frequency detection threshold for the shorter duration science campaigns of the re-purposed Kepler mission, K2, is lower. The maximum threshold will probably lie somewhere between ≃400 and 450 μHz. The potential to exploit the archival Kepler and K2 LC data in this manner opens the door to increasing significantly the number of subgiant and low-luminosity red giant targets amenable to asteroseismic analysis, overcoming target limitations imposed by the small number of SC slots.We estimate that around 400 such targets are now available for study in the Kepler LC archive. That number could potentially be a lot higher for K2, since there will be a new target list for each of its campaigns
NGC6819: Testing the asteroseismic mass scale, mass loss and evidence for products of non-standard evolution
No full textWe present an extensive peakbagging effort on Kepler data of ~50 red giant stars in the open star cluster NGC6819. By employing sophisticated pre-processing of the time series and Markov chain Monte Carlo techniques we extracted individual frequencies, heights and line widths for hundreds of oscillation modes.We show that the 'average' asteroseismic parameter δv02, derived from these, can be used to distinguish the stellar evolutionary state between the red giant branch (RGB) stars and red clump (RC) stars. Masses and radii are estimated using asteroseismic scaling relations, both empirically corrected to obtain self-consistency and agreement with independent measures of distance, and using updated theoretical corrections. Remarkable agreement is found, allowing the evolutionary state of the giants to be determined exclusively from the empirical correction to the scaling relations. We find a mean mass of the RGB stars and RC stars in NGC6819 to be 1.61 ± 0.02 and 1.64 ± 0.02M⊙, respectively. The difference ΔM=-0.03 ± 0.01M⊙ is almost insensitive to systematics, suggesting very little RGB mass loss, if any. Stars that are outliers relative to the ensemble reveal overmassive members that likely evolved via mass transfer in a blue straggler phase. We suggest that KIC 4937011, a low-mass Li-rich giant, is a cluster member in the RC phase that experienced very high mass loss during its evolution. Such over- and undermassive stars need to be considered when studying field giants, since the true age of such stars cannot be known and there is currently no way to distinguish them from normal stars
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