Effects of rotation on the evolution and asteroseismic properties of red giants

The influence of rotation on the properties of red giants is studied in the context of the asteroseismic modelling of these stars. While red giants exhibit low surface rotational velocities, we find that the rotational history of the star has a large impact on its properties during the red giant phase. In particular, for stars massive enough to ignite He burning in non-degenerate conditions, rotational mixing induces a significant increase of the stellar luminosity and shifts the location of the core helium burning phase to a higher luminosity in the HR diagram. This of course results in a change of the seismic properties of red giants at the same evolutionary state. As a consequence the inclusion of rotation significantly changes the fundamental parameters of a red giant star as determined by performing an asteroseismic calibration. In particular rotation decreases the derived stellar mass and increases the age. Depending on the rotation law assumed in the convective envelope and on the initial velocity of the star, non-negligible values of rotational splitting can be reached, which may complicate the observation and identification of non-radial oscillation modes for red giants exhibiting moderate surface rotational velocities. By comparing the effects of rotation and overshooting, we find that the main-sequence widening and the increase of the H-burning lifetime induced by rotation (Vini=150 km/s) are well reproduced by non-rotating models with an overshooting parameter of 0.1, while the increase of luminosity during the post-main sequence evolution is better reproduced by non-rotating models with overshooting parameters twice as large. This is due to the fact that rotation not only increases the size of the convective core but also changes the chemical composition of the radiative zone.Comment: 9 pages, 13 figures, accepted for publication in A&

Why should we correct reported pulsation frequencies for stellar line-of-sight Doppler velocity shifts?

In the age of Kepler and Corot, extended observations have provided estimates of stellar pulsation frequencies that have achieved new levels of precision, regularly exceeding fractional levels of a few parts in $10^{4}$. 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.Comment: 5 pages, 1 figure, accepted for publication in MNRAS Letter

New Information in Naturalistic Data Is Also Signalled by Pitch Movement: An Analysis from Monolingual English/Spanish and Bilingual Spanish Speakers

New Information in Naturalistic Data Is Also Signalled by Pitch Movement:  An Analysis from Monolingual English/Spanish and Bilingual Spanish SpeakersIn communication, speakers and listeners need ways to highlight certain information and relegate other information to the background. They also need to keep track of what information they (think they) have already communicated to the listener, and of the listeners' (supposed) knowledge of topics and referents. This knowledge and its layout in the utterance is commonly referred to as information structure, i.e., the degree to which propositions and referents are given or new. All languages have 'chosen' different ways to encode such information structure, for instance by modifying the pitch or intensity of the vocal signal or the order of words in a sentence. In this study, we assess whether the use of pitch to signal new information holds in typologically different languages such as English and Spanish by analyzing three population group monolingual California English speakers, bilingual speakers of English and Spanish from California (Chicano Spanish), and monolingual Mexican Spanish speakers from Mexico City. Our study goes beyond previous work in several respects. First, most current work is based on sentences just read or elicited in response to highly standardized and often somewhat artificial stimuli whose generalizability to more naturalistic settings may be questionable. We opted instead to use semidirected interviews whose more naturalistic setting provides data with a higher degree of authenticity. Second, in order to deal with the resulting higher degree of noise in the data as well as the inherent multifactoriality of the data, we are using state-of-the-art statistical methods to explore our data, namely generalized linear mixed-effects modeling, to accommodate speaker- and lexically-specific variability. Despite the noisy data, we find that contour tones including H+L or L+H sequences signal new information, and that items encoding new information also exhibit proportionally longer stressed vowels, than those encoding given information. We also find cross-dialectal variation between monolingual Mexican Spanish speakers on the one hand and monolingual English speakers and Chicanos on the other: Mexican Spanish speakers modify pitch contours less than monolingual English speakers, whereas the English patterns affect even the Spanish pronunciation of early bilinguals. Our findings, therefore, corroborate Gussenhoven's theory (2002) that some aspects of intonation are shared cross-linguistically (longer vowel length & higher pitch for new info), whereas others are encoded language-specifically and vary even across dialects (pitch excursion & the packaging of information structure)

Super-Nyquist asteroseismology of solar-like oscillators with Kepler and K2 - expanding the asteroseismic cohort at the base of the red-giant branch

We 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 approximately 283 {\mu}Hz. 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 approximately 500 {\mu}Hz, 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 approximately 400 and 450 {\mu}Hz. 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.Comment: Accepted for publication in MNRAS; 11 pages, 7 figures; reference list update

NGC 6819: testing the asteroseismic mass scale, mass loss, and evidence for products of non-standard evolution

We present an extensive peakbagging effort on Kepler data of $\sim$50 red giant stars in the open star cluster NGC 6819. By employing sophisticated pre-processing of the time series and Markov Chain Monte Carlo techniques we extracted individual frequencies, heights and linewidths for hundreds of oscillation modes. We show that the "average" asteroseismic parameter $\delta\nu_{02}$, 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 as well as 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 NGC 6819 to be $1.61\pm0.02\,\textrm{M}_\odot$ and $1.64{\pm}0.02\,\textrm{M}_\odot$, respectively. The difference $\Delta M=-0.03\pm0.01\,\textrm{M}_\odot$ 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.Comment: 21 pages, 11 figure

Driving and damping mechanisms in hybrid pressure-gravity modes pulsators

We study the energetic aspects of hybrid pressure-gravity modes pulsations. The case of hybrid beta Cephei-SPB pulsators is considered with special attention. In addition to the already known sensitivity of the driving mechanism to the heavy elements mixture (mainly the iron abundance), we show that the characteristics of the propagation and evanescent regions play also a major role, determining the extension of the stable gap in the frequency domain between the unstable low order pressure and high order gravity modes. Finally, we consider the case of hybrid delta Sct-gamma Dor pulsators.Comment: 7 pages, 9 figures, in the proceedings of the Helas II Conference: "Helioseismology, Asteroseismology and MHD Connections", Goettingen, August 200

Nitrogen depletion in field red giants: mixing during the He flash?

We 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.This work was partly supported by the European Union FP7 programme through ERC grant number 320360. NL acknowledges financial support from the Marie Curie Intra-European fellowship (FP7-PEOPLE-2012-IEF) and the CNES postdoctoral fellowship 2016. AM and YE acknowledge the support of the UK Science and Technology Facilities Council (STFC). Funding for the Stellar Astrophysics Centre (SAC) is provided by The Danish National Research Foundation (Grant agreement no. DNRF106)

Development of energy system model of the Caspian region

The main objective of the project is to develop and to use the model of energy system of Central Asia and Caspian region (CAC): Azerbaijan (AZJ), Kazakhstan (KZK), Turkmenistan (TKM) and Uzbekistan (UZB) - TIMES-CAC-4R and to assess quantitatively the direct economic benefits of cooperation in export of hydrocarbons among CAC countries

Impact of magnetic activity on inferred stellar properties of main-sequence Sun-like stars

The oscillation frequencies observed in Sun-like stars are susceptible to being shifted by magnetic activity effects. The measured shifts depend on a complex relationship involving the mode type, the field strength, and spatial distribution of activity, as well as the inclination angle of the star. Evidence of these shifts is also present in frequency separation ratios that are often used when inferring global properties of stars in order to avoid surface effects. However, one assumption when using frequency ratios for this purpose is that there are no near-surface perturbations that are non-spherically symmetric. In this work, we studied the impact on inferred stellar properties when using frequency ratios that are influenced by non-homogeneous activity distributions. We generate several sets of artificial oscillation frequencies with various amounts of shift and determine stellar properties using two separate pipelines. We find that for asteroseismic observations of Sun-like targets we can expect magnetic activity to affect mode frequencies that will bias the results from stellar modelling analysis. Although for most stellar properties this offset should be small, typically less than 0.5 per cent in mass, estimates of age and central hydrogen content can have an error of up to 5 per cent and 3 per cent, respectively. We expect a larger frequency shift and therefore larger bias for more active stars. We also warn that for stars with very high or low inclination angles, the response of modes to activity is more easily observable in the separation ratios and hence will incur a larger bias