Systematic Bias in Helioseismic Measurements of Meridional Circulation Arising from Nonlocal Averaging Kernels

Meridional circulation in the solar convection zone plays a profound role in regulating the interior dynamics of the Sun and its magnetism. While it is well accepted that meridional flows move from the equator towards the poles at the Sun's surface, helioseismic observations have yet to provide a definitive answer for the depth at which those flows return to the equator, or the number of circulation cells in depth. In this work, we investigate whether the discrepancies regarding the nature of the return flow are intrinsic to how helioseismic observations are made. We examine the seismic signature of possible meridional flow profiles by convolving time-distance averaging kernels with the mean flows obtained from 3-D hydrodynamic simulations of the solar convection zone. At mid and high latitudes, we find that weak flow structures in the deeper regions of the convection zone can be strongly obscured by signal from the much stronger surface flows. This contamination is the result of extended side lobes in the averaging kernels and generates a spurious equatorward signal of 2--3 m s$^{-1}$ at those latitudes, and at $\approx 70~\mathrm{Mm}$ depth. At low latitudes, however, the flows in the simulations tend to be stronger and multiple cells across the shell depth can produce a sufficiently strong seismic signal to survive the convolution process. The signal associated with the deep equatorward return flow in the Sun is expected to be weak and in the same sense as the contamination from the surface. Hence, the return flow needs to exceed $\sim 2$--$3~ \mathrm{m~s^{-1}}$ in magnitude for reported detections to be considered significant.Comment: Submitted to AAS Journal

The white dwarf population of NGC 6397

Context. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.; Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions.; Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.; Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8(-0.75)(+0.50) Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.; Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.Peer Reviewe

The white dwarf population of NGC 6397

Context. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.; Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color-magnitude diagram and the corresponding magnitude and color distributions.; Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.; Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8(-0.75)(+0.50) Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.; Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters.Peer Reviewe

White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

Context. White dwarfs are nowadays routinely used as reliable cosmochronometers, allowing several stellar populations to be dated.; Aims. We present new white dwarf evolutionary sequences for low-metallicity progenitors. This is motivated by the recent finding that residual H burning in low-mass white dwarfs resulting from Z = 0.0001 progenitors is the main energy source over a significant part of their evolution.; Methods. White dwarf sequences have been derived from full evolutionary calculations that take the entire history of progenitor stars into account, including the thermally pulsing and the post-asymptotic giant branch (AGB) phases.; Results. We show that for progenitor metallicities in the range 0.00003 less than or similar to Z less than or similar to 0.001, and in the absence of carbon enrichment from the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases and in the planetary nebulae stage.; Conclusions. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.Peer Reviewe

Assessing the Observability of Deep Meridional Flow Cells in the Solar Interior

Abstract: Meridional circulation regulates the Sun’s interior dynamics and magnetism. While it is well accepted that meridional flows are poleward at the Sun’s surface, helioseismic observations have yet to provide a definitive answer for the depth at which those flows return to the equator, or the number of circulation cells in depth. Here, we explore the observability of multiple circulation cells stacked in radius. Specifically, we examine the seismic signature of several meridional flow profiles by convolving time–distance averaging kernels with mean flows obtained from a suite of 3D hydrodynamic simulations. At mid and high latitudes, we find that weak flow structures in the deep convection zone can be obscured by signals from the much stronger surface flows. This contamination of 1–2 m s−1 is caused by extended side lobes in the averaging kernels, which produce a spurious equatorward signal with flow speeds that are 1 order of magnitude stronger than the original flow speeds in the simulations. At low latitudes, the flows in the deep layers of the simulations are stronger (>2 m s−1) and multiple cells across the convection zone can produce a sufficiently strong signal to survive the convolution process. Now that meridional flows can be measured over two decades of data, the uncertainties arising from convective noise have fallen to a level where they are comparable in magnitude to the systematic biases caused by nonlocal features in the averaging kernels. Hence, these systematic errors are beginning to influence current helioseismic deductions and need broader consideration

Updated evolutionary sequences for hydrogen-deficient white dwarfs

We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmospheres. We take into account the evolutionary history of the progenitor stars, all the relevant energy sources involved in the cooling, element diffusion in the very outer layers, and outer boundary conditions provided by new and detailed non-gray white dwarf model atmospheres for pure helium composition. These model atmospheres are based on the most up-to-date physical inputs. Our calculations extend down to very low effective temperatures, of ~2500 K, provide a homogeneous set of evolutionary cooling tracks that are appropriate for mass and age determinations of old hydrogen-deficient white dwarfs, and represent a clear improvement over previous efforts, which were computed using gray atmospheres.Peer Reviewe

The effect of 22Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity prgenitors

© 2016. The American Astronomical Society. All rights reserved. Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the Brunt-Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 derived from full evolutionary computations of their progenitor stars, starting at the zero-Age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities (), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.Peer Reviewe