scholarly journals Internal Rotation, Mixing and Lithium Abundances

1998 ◽  
Vol 185 ◽  
pp. 25-36
Author(s):  
Brian Chaboyer
Keyword(s):  
Internal Rotation ◽  
Transport Process ◽  
Main Sequence ◽  
Convection Zone ◽  
Nuclear Reactions ◽  
Open Clusters ◽  
Current Status ◽  
The Sun ◽  
Rotation Rates ◽  
Destruction Zone

Lithium is an excellent tracer of mixing in stars as it is destroyed (by nuclear reactions) at a temperature around ~ 2.5 × 106 K. The lithium destruction zone is typically located in the radiative region of a star. If the radiative regions are stable, the observed surface value of lithium should remain constant with time. However, comparison of the meteoritic and photospheric Li abundances in the Sun indicate that the surface abundance of Li in the Sun has been depleted by more than two orders of magnitude. This is not predicted by solar models and is a long standing problem. Observations of Li in open clusters indicate that Li depletion is occurring on the main sequence. Furthermore, there is now compelling observational evidence that a spread of lithium abundances is present in nearly identical stars. This suggests that some transport process is occurring in stellar radiative regions. Helioseismic inversions support this conclusion, for they suggest that standard solar models need to be modified below the base of the convection zone. There are a number of possible theoretical explanations for this transport process. The relation between Li abundances, rotation rates and the presence of a tidally locked companion along with the observed internal rotation in the Sun indicate that the mixing is most likely induced by rotation. The current status of non-standard (particularly rotational) stellar models which attempt to account for the lithium observations are reviewed.

scholarly journals Asteroseismology of evolved stars to constrain the internal transport of angular momentum

2019 ◽  
Vol 621 ◽  
pp. A66 ◽  
Author(s):  
P. Eggenberger ◽  
S. Deheuvels ◽  
A. Miglio ◽  
S. Ekström ◽  
C. Georgy ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Transport Process ◽  
Main Sequence ◽  
Red Giants ◽  
Rotation Rates ◽  
Giant Stars ◽  
Evolved Stars ◽  
Surface Rotation ◽  
Red Giant ◽  
Internal Transport

Context. The observations of solar-like oscillations in evolved stars have brought important constraints on their internal rotation rates. To correctly reproduce these data, an efficient transport mechanism is needed in addition to the transport of angular momentum by meridional circulation and shear instability. The efficiency of this undetermined process is found to increase both with the mass and the evolutionary stage during the red giant phase. Aims. We study the efficiency of the transport of angular momentum during the subgiant phase. Methods. The efficiency of the unknown transport mechanism is determined during the subgiant phase by comparing rotating models computed with an additional corresponding viscosity to the asteroseismic measurements of both core and surface-rotation rates for six subgiants observed by the Kepler spacecraft. We then investigate the change in the efficiency of this transport of angular momentum with stellar mass and evolution during the subgiant phase. Results. The precise asteroseismic measurements of both core and surface-rotation rates available for the six Kepler targets enable a precise determination of the efficiency of the transport of angular momentum needed for each of these subgiants. These results are found to be insensitive to all the uncertainties related to the modelling of rotational effects before the post-main sequence (poMS) phase. An interesting exception in this context is the case of young subgiants (typical values of log(g) close to 4), because their rotational properties are sensitive to the degree of radial differential rotation on the main sequence (MS). These young subgiants constitute therefore perfect targets to constrain the transport of angular momentum on the MS from asteroseismic observations of evolved stars. As for red giants, we find that the efficiency of the additional transport process increases with the mass of the star during the subgiant phase. However, the efficiency of this undetermined mechanism decreases with evolution during the subgiant phase, contrary to what is found for red giants. Consequently, a transport process with an efficiency that increases with the degree of radial differential rotation cannot account for the core-rotation rates of subgiants, while it correctly reproduces the rotation rates of red giant stars. This suggests that the physical nature of the additional mechanism needed for the internal transport of angular momentum may be different in subgiant and red giant stars.

scholarly journals Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars

2020 ◽  
Vol 642 ◽  
pp. A180
Author(s):  
Ingrid Pelisoli ◽  
Joris Vos ◽  
Stephan Geier ◽  
Veronika Schaffenroth ◽  
Andrzej S. Baran
Keyword(s):  
Proper Motion ◽  
Binary Systems ◽  
Main Sequence ◽  
Open Clusters ◽  
Binary Interaction ◽  
Spectral Energy ◽  
Wide Binary ◽  
Single Star ◽  
Rotation Rates ◽  
Hot Subdwarfs

Context. Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. Aims. We investigate the possibility that hot subdwarfs could form without interaction by studying wide binary systems. If single formation scenarios were possible, there should be hot subdwarfs in wide binaries that have undergone no interaction. Methods. Angular momentum accretion during interaction is predicted to cause the hot subdwarf companion to spin up to the critical velocity. The effect of this should still be observable given the timescales of the hot subdwarf phase. To study the rotation rates of companions, we have analysed light curves from the Transiting Exoplanet Survey Satellite for all known hot subdwarfs showing composite spectral energy distributions indicating the presence of a main sequence wide binary companion. If formation without interaction were possible, that would also imply the existence of hot subdwarfs in very wide binaries that are not predicted to interact. To identify such systems, we have searched for common proper motion companions with projected orbital distances of up to 0.1 pc to all known spectroscopically confirmed hot subdwarfs using Gaia DR2 astrometry. Results. We find that the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. We also report a shortage of hot subdwarfs with candidate common proper motion companions. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. Conclusions. The observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. Additionally, there is a shortage of hot subdwarfs in common proper motion pairs, considering the frequency of such systems among progenitors. These results suggest that binary interaction is always required for the formation of hot subdwarfs.

scholarly journals The Present Data Situation for Stars in Open Clusters. II

1980 ◽  
Vol 85 ◽  
pp. 129-133
Author(s):  
J.-C. Mermilliod
Keyword(s):  
Present State ◽  
Spectral Type ◽  
Main Sequence ◽  
Open Clusters ◽  
Photometric Data ◽  
Main Sequence Star ◽  
The Sun ◽  
Previous Version

The present state of astrometric, spectroscopic and photometric data for stars in 63 open clusters nearer to the Sun than 750 pc is summarized. Table I reports the limits of the available data in terms of the apparent V magnitude, in the same way as the previous version (Mermilliod 1977). Information on the apparent V magnitude of the brightest main sequence star (Vbr) and on the number of stars brighter than V=10 (n∗) has been included. In addition, the bluest spectral type on the main sequence has been used as an age estimator (TS).

scholarly journals Extended main sequence turnoffs in open clusters as seen by Gaia – II. The enigma of NGC 2509

2019 ◽  
Vol 491 (2) ◽  
pp. 2129-2136 ◽  
Author(s):  
M de Juan Ovelar ◽  
S Gossage ◽  
S Kamann ◽  
N Bastian ◽  
C Usher ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Narrow Range ◽  
Open Cluster ◽  
Open Clusters ◽  
Stellar Rotation ◽  
Rotation Rates ◽  
Magnitude Diagram ◽  
Stellar Density

ABSTRACT We investigate the morphology of the colour–magnitude diagram (CMD) of the open cluster NGC 2509 in comparison with other Galactic open clusters of similar age using Gaia photometry. At ${\sim}900\,\rm {Myr}$ Galactic open clusters in our sample all show an extended main sequence turnoff (eMSTO) with the exception of NGC 2509, which presents an exceptionally narrow CMD. Our analysis of the Gaia data rules out differential extinction, stellar density, and binaries as a cause for the singular MSTO morphology in this cluster. We interpret this feature as a consequence of the stellar rotation distribution within the cluster and present the analysis with mesa Isochrones and Stellar Tracks (MIST) stellar evolution models that include the effect of stellar rotation on which we based our conclusion. In particular, these models point to an unusually narrow range of stellar rotation rates (Ω/Ωcrit, ZAMS = [0.4, 0.6]) within the cluster as the cause of this singular feature in the CMD of NGC 2509. Interestingly, models that do not include rotation are not as good at reproducing the morphology of the observed CMD in this cluster.

scholarly journals The Pennsylvania-Toruń search for planets around evolved stars with HET

2010 ◽  
Vol 6 (S276) ◽  
pp. 445-447
Author(s):  
Andrzej Niedzielski ◽  
Alex Wolszczan ◽  
Grzegorz Nowak ◽  
Paweł Zieliński ◽  
Monika Adamów ◽  
...  
Keyword(s):  
Main Sequence ◽  
Convincing Evidence ◽  
Current Status ◽  
Brown Dwarf ◽  
Solar Mass ◽  
Rotation Rates ◽  
Evolved Stars ◽  
Intermediate Mass Stars ◽  
Red Dwarfs ◽  
Formation And Evolution

AbstractSearches for planets around giants represent an essential complement to ’traditional’ surveys, because they furnish information about properties of planetary systems around stars that are the descendants of the A-F main sequence (MS) stars with masses as high as ~5 M⊙. As the stars evolve off the MS, their effective temperatures and rotation rates decrease to the point that their radial velocity variations can be measured with a few ms−1 precision. This offers an excellent opportunity to improve our understanding of the population of planets around stars that are significantly more massive than the Sun, without which it would be difficult to produce abroad, integrated picture of planet formation and evolution. Since 2001, about 30 such objects have been identified, including our five published HET detections (Niedzielski et al. 2007; Niedzielski et al. 2009a; Niedzielski et al. 2009b). Our work has produced the tightest orbit of a planet orbiting a K-giant identified so far (0.6 AU), and the first convincing evidence for a multiplanet system around such as star (Niedzielski et al. 2009a). Our most recent discoveries (Niedzielski et al. 2009b) have identified new multiplanet systems, including a very intriguing one of two brown dwarf-mass bodies orbiting a 2.8M⊙, K2 giant. This particular detection challenges the standard interpretation of the so-called brown dwarf desert known to exist in the case of solar-mass stars. Along with discoveries supplied by other groups, our work has substantially added to the emerging evidence that stellar mass positively correlates with masses of substellar companions, all the way from red dwarfs to intermediate-mass stars. We present current status and forthcoming results from the Pennsylvania-Toruń Search for Planets performed with the Hobby-Eberly Telescope (HET) since 2004.

scholarly journals 2-D and 3-D models of convective turbulence and oscillations in intermediate-mass main-sequence stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 540-543
Author(s):  
Joyce A. Guzik ◽  
T. H. Morgan ◽  
N. J. Nelson ◽  
C. Lovekin ◽  
K. Kosak ◽  
...  
Keyword(s):  
Main Sequence ◽  
Convection Zone ◽  
Low Frequency ◽  
Main Sequence Stars ◽  
Low Frequencies ◽  
Rotation Rates ◽  
Multidimensional Modeling ◽  
Core Convection ◽  
Radiative Zone

AbstractWe present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid γ Dor/δ Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations of 1-2 M⊙ stars gained by multidimensional modeling.

scholarly journals Microturbulent velocities and abundances for A and F dwarfs in open clusters

2006 ◽  
Vol 2 (S239) ◽  
pp. 160-162
Author(s):  
M. Gebran ◽  
R. Monier
Keyword(s):  
Main Sequence ◽  
Chemical Elements ◽  
Open Clusters ◽  
Current Status ◽  
High Signal ◽  
Signal To Noise ◽  
Synthetic Spectra ◽  
Noise Spectroscopy ◽  
Microturbulent Velocity ◽  
Alpha Persei

AbstractThe current status of microturbulent velocity and abundance determinations for A and F dwarfs in open clusters is reviewed. A programme to observe several tens of A and F dwarfs in open clusters of various ages was initiated several years ago. We have performed high resolution high signal-to-noise spectroscopy of stars well distributed in mass along the Main Sequence. Microturbulent velocities and abundances of several chemical elements have been derived iteratively by fitting grids of synthetic spectra calculated in LTE to the observed spectra. Curve of growths were used in a few instances as well. The results obtained are reviewed for Coma Berenices, the Pleiades, Alpha Persei and the Ursa Major moving group.The microturbulent velocities exhibit a broad maximum in the range A5V to about F0V as indicated in Smalley (2004).

scholarly journals The evolution of Earth’s magnetosphere during the solar main sequence

2019 ◽  
Vol 489 (4) ◽  
pp. 5784-5801 ◽  
Author(s):  
S Carolan ◽  
A A Vidotto ◽  
C Loesch ◽  
P Coogan
Keyword(s):  
Solar Wind ◽  
Main Sequence ◽  
Solar Rotation ◽  
Strong Shock ◽  
Bow Shock ◽  
Base Temperature ◽  
The Sun ◽  
Earth’S Magnetosphere ◽  
Rotation Rates ◽  
Earth's Magnetosphere

ABSTRACT As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how Earth’s magnetosphere has responded to the change in the solar wind. Earth’s magnetosphere is simulated using 3D magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. The solar wind, on the other hand, is modelled in 1.5D for a range of rotation rates Ω from 50 to 0.8 times the present-day solar rotation (Ω⊙). Our solar wind model uses empirical values for magnetic field strengths, base temperature, and density, which are derived from observations of solar-like stars. We find that for rotation rates ≃10 Ω⊙, Earth’s magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. As the Sun spins-down, the magnetopause standoff distance varies with Ω−0.27 for higher rotation rates (early ages, ≥1.4 Ω⊙) and with Ω−2.04 for lower rotation rates (older ages, <1.4 Ω⊙). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between the magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (≤10 Ω⊙). It is possible that a young fast rotating Sun would have had rotation rates as high as 30–50 Ω⊙. In these speculative scenarios, at 30 Ω⊙, a weak shock would have been formed, but for 50 Ω⊙, we find that no bow shock could be present around Earth’s magnetosphere. This implies that with the Sun continuing to spin-down, a strong shock would have developed around our planet and remained for most of the duration of the solar main sequence.

scholarly journals Binary Stars in Praesepe

1992 ◽  
Vol 135 ◽  
pp. 231-233
Author(s):  
Pavel Kroupa ◽  
Christopher A. Tout
Keyword(s):  
Binary Stars ◽  
Binary Systems ◽  
Main Sequence ◽  
Equal Mass ◽  
Open Clusters ◽  
Independent Component ◽  
The Sun ◽  
Single Star ◽  
Low Mass ◽  
Simple Models

AbstractIn open clusters the scatter about the single-star main sequence is usually negligible. However, unresolved binary systems are brighter and redder than single stars, and thus some ‘stars’ appear shifted away from the main sequence. We make very simple models of the Praesepe cluster, and find evidence that low-mass systems prefer independent component masses, whereas systems of higher mass than the sun appear to favour some correlation towards equal-mass components.

scholarly journals The Relevance of Nuclear Reactions for Standard Solar Models Construction

2021 ◽  
Vol 7 ◽  
Author(s):  
Francesco L. Villante ◽  
Aldo Serenelli
Keyword(s):  
Nuclear Reaction ◽  
Cross Sections ◽  
Nuclear Reactions ◽  
Direct Detection ◽  
Reaction Rates ◽  
Precise Determination ◽  
Reaction Cross ◽  
Solar Interior ◽  
Current Status ◽  
The Sun

The fundamental processes by which nuclear energy is generated in the Sun have been known for many years. However, continuous progress in areas such as neutrino experiments, stellar spectroscopy and helioseismic data and techniques requires ever more accurate and precise determination of nuclear reaction cross sections, a fundamental physical input for solar models. In this work, we review the current status of (standard) solar models and present a complete discussion on the relevance of nuclear reactions for detailed predictions of solar properties. In addition, we also provide an analytical model that helps understanding the relation between nuclear cross sections, neutrino fluxes and the possibility they offer for determining physical characteristics of the solar interior. The latter is of particular relevance in the context of the conundrum posed by the solar composition, the solar abundance problem, and in the light of the first ever direct detection of solar CN neutrinos recently obtained by the Borexino collaboration. Finally, we present a short list of wishes about the precision with which nuclear reaction rates should be determined to allow for further progress in our understanding of the Sun.

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