scholarly journals Hydrodynamical Simulations of Turbulent Convection in a Rotating Red Giant Star

2008 ◽  
Vol 4 (S252) ◽  
pp. 175-182 ◽  
Author(s):  
A. Palacios ◽  
A. S. Brun
Keyword(s):  
Angular Momentum ◽  
Rotation Rate ◽  
Turbulent Convection ◽  
Convective Envelope ◽  
Giant Star ◽  
Giant Stars ◽  
Low Mass ◽  
Red Giant ◽  
Hydrodynamical Simulations

AbstractWe present 3-D hydrodynamical simulations of the extended turbulent convective envelope of a low-mass red giant star. These simulations, computed with the ASH code, aim at understanding the redistribution of angular momentum and heat in extended turbulent convection zones of these giant stars. We focus our study on the effects of turbulence and of the rotation rate on the convective patterns and on the distribution of angular momentum within the inner 50% of the convective envelope of such stars.

scholarly journals Active red giants: Close binaries versus single rapid rotators

2020 ◽  
Vol 639 ◽  
pp. A63
Author(s):  
Patrick Gaulme ◽  
Jason Jackiewicz ◽  
Federico Spada ◽  
Drew Chojnowski ◽  
Benoît Mosser ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Close Binaries ◽  
Red Giants ◽  
Intermediate Mass ◽  
Giant Stars ◽  
Rotational Modulation ◽  
Low Mass ◽  
Red Giant Stars ◽  
Red Giant ◽  
Giant Branch Stars

Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, which enables detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, which relies on the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We selected a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (∼8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 nonoscillators are in this group of binaries. On the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of the stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones are less active. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measured are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets that is not available in the Sun.

scholarly journals Red giant stars: from mixed modes to angular momentum

2019 ◽  
Vol 82 ◽  
pp. 189-211
Author(s):  
K. Belkacem
Keyword(s):  
Angular Momentum ◽  
Current Knowledge ◽  
Red Giants ◽  
Low Mass Stars ◽  
Giant Stars ◽  
Physical Mechanisms ◽  
Star Evolution ◽  
Low Mass ◽  
Mixed Modes ◽  
Red Giant

Solar-like oscillations are ubiquitous to low-mass stars from the main-sequence to the red-giant branch as demonstrated by the space-borne missions CoRoT and Kepler. Understanding the physical mechanisms governing their amplitudes as well as their behavior along with the star evolution is a prerequisite for interpreting the wealth of seismic data and for inferring stellar internal structure. In this paper, I discuss our current knowledge of mode amplitudes with particular emphasis on non-radial modes in red giants (hereafter mixed modes). Then, I will show how these modes permit to unveil the rotation of the inner-most layers of low-mass stars and how they put stringent constraints on the redistribution of angular momentum.

scholarly journals Rotating Models of Low Mass Giants: Rotational Evolution and Surface Abundance Anomalies

2004 ◽  
Vol 215 ◽  
pp. 438-439 ◽  
Author(s):  
Julio Chanamé ◽  
Marc Pinsonneault ◽  
Don Terndrup
Keyword(s):  
Angular Momentum ◽  
Initial Conditions ◽  
Local Conservation ◽  
Case Scenario ◽  
Convective Envelope ◽  
First Results ◽  
Rotational Evolution ◽  
Solar Metallicity ◽  
Low Mass ◽  
Red Giant

We present some first results of stellar models of low mass, solar metallicity giants that include rotation and the mixing associated with it. We base our choice of initial conditions and input physics on observational constraints obtained for main sequence (MS) and horizontal branch (HB) stars, and obtain what can be regarded as a “best case scenario” for rotational mixing. These observations suggest local conservation of angular momentum in the cores and constant specific angular momentum in convective regions on the red giant branch (RGB). We show that under these circumstances and with moderate rotation rates at the turnoff it is possible to obtain enough mixing in order to reproduce the anomalous abundance ratios of the CNO species seen in giants. It is also shown that solid body rotation in the convective envelope fails to account for the observations. It is found that rotational mixing is inefficient on the lower RGB, so there is no need to invoke any inhibiting role of μ–gradients.

scholarly journals Core rotation braking on the red giant branch for various mass ranges

2018 ◽  
Vol 616 ◽  
pp. A24 ◽  
Author(s):  
C Gehan ◽  
B. Mosser ◽  
E. Michel ◽  
R. Samadi ◽  
T. Kallinger
Keyword(s):  
Angular Momentum ◽  
Red Giants ◽  
Regular Pattern ◽  
Convective Envelope ◽  
Large Sample ◽  
The Mean ◽  
Mixed Modes ◽  
Red Giant ◽  
Core Rotation ◽  
Dipole Gravity

Context. Asteroseismology allows us to probe stellar interiors. In the case of red giant stars, conditions in the stellar interior are such as to allow for the existence of mixed modes, consisting in a coupling between gravity waves in the radiative interior and pressure waves in the convective envelope. Mixed modes can thus be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for red giant core rotation. Thus large-scale measurements of red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. Aims. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. Methods. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with the same azimuthal order are expected to be almost equally spaced in stretched period, with a spacing equal to the pure dipole gravity mode period spacing. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. Results. We automatically identify the rotational multiplet components of 1183 stars on the red giant branch with a success rate of 69% with respect to our initial sample. As no information on the internal rotation can be deduced for stars seen pole-on, we obtain mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 M⊙, allowing us to test the dependence of the core slow-down rate on the stellar mass. Conclusions. Disentangling rotational splittings from mixed modes is now possible in an automated way for stars on the red giant branch, even for the most complicated cases, where the rotational splittings exceed half the mixed-mode spacing. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow-down, our results suggest rotation is constant along the red giant branch, with values independent of the mass.

scholarly journals Weighing the stellar constituents of the galactic halo with APOGEE red giant stars

2020 ◽  
Vol 492 (3) ◽  
pp. 3631-3646 ◽  
Author(s):  
J Ted Mackereth ◽  
Jo Bovy
Keyword(s):  
Milky Way ◽  
Galactic Halo ◽  
Power Laws ◽  
Stellar Mass ◽  
Giant Star ◽  
Giant Stars ◽  
Stellar Halo ◽  
Tentative Evidence ◽  
History Of ◽  
Red Giant

ABSTRACT The stellar mass in the halo of the Milky Way is notoriously difficult to determine, owing to the paucity of its stars in the solar neighbourhood. With tentative evidence from Gaia that the nearby stellar halo is dominated by a massive accretion event – referred to as Gaia-Enceladus or Sausage – these constraints are now increasingly urgent. We measure the mass in kinematically selected mono-abundance populations (MAPs) of the stellar halo between −3 < [Fe/H] < −1 and 0.0 < [Mg/Fe] < 0.4 using red giant star counts from APOGEE DR14. We find that MAPs are well fit by single power laws on triaxial ellipsoidal surfaces, and we show that that the power-law slope α changes such that high [Mg/Fe] populations have α ∼ 4, whereas low [Mg/Fe] MAPs are more extended with shallow slopes, α ∼ 2. We estimate the total stellar mass to be $M_{*,\mathrm{tot}} = 1.3^{+0.3}_{-0.2}\times 10^{9}\ \mathrm{M_{\odot}}$, of which we estimate ${\sim}0.9^{+0.2}_{-0.1} \times 10^{9}\ \mathrm{M_{\odot}}$ to be accreted. We estimate that the mass of accreted stars with e > 0.7 is M*,accreted, e > 0.7 = 3 ± 1 (stat.) ± 1 (syst.) × 108 M⊙, or ${\sim}30{-}50{{\ \rm per\ cent}}$ of the accreted halo mass. If the majority of these stars are the progeny of a massive accreted dwarf, this places an upper limit on its stellar mass, and implies a halo mass for the progenitor of ∼1010.2 ± 0.2 M⊙. This constraint not only shows that the Gaia-Enceladus/Sausage progenitor may not be as massive as originally suggested, but that the majority of the Milky Way stellar halo was accreted. These measurements are an important step towards fully reconstructing the assembly history of the Milky Way.

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 Asteroseismology of red giants to constrain angular momentum transport

2014 ◽  
Vol 9 (S307) ◽  
pp. 165-170
Author(s):  
P. Eggenberger
Keyword(s):  
Angular Momentum ◽  
Rotational Frequency ◽  
Momentum Transport ◽  
Red Giants ◽  
Angular Momentum Transport ◽  
Rotation Rates ◽  
Giant Stars ◽  
Red Giant ◽  
Core Rotation

AbstractAsteroseismic data obtained by theKeplerspacecraft have led to the recent detection and characterization of rotational frequency splittings of mixed modes in red-giant stars. This has opened the way to the determination of the core rotation rates for these stars, which is of prime importance to progress in our understanding of internal angular momentum transport. In this contribution, we discuss which constraints can be brought by these asteroseismic measurements on the modelling of angular momentum transport in stellar radiative zones.

scholarly journals Fluorescence in the Outer Atmospheres of Red Giant Stars

1989 ◽  
Vol 106 ◽  
pp. 372-372
Author(s):  
K.G. Carpenter
Keyword(s):  
Radiative Decay ◽  
Energy Levels ◽  
Uv Spectrum ◽  
Line Radiation ◽  
Giant Star ◽  
Lyman Alpha ◽  
Giant Stars ◽  
Chromospheric Emission ◽  
Outer Atmosphere ◽  
Red Giant

The outer atmosphere of a cool red giant star is an ideal locale for the operation of line fluorescence processes. Low plasma densities imply low rates of collisional de-excitation and thus allow radiative decay of levels populated by selective radiative pumping. There are many strong sources of line radiation (i.e. possible pumps) and numerous possible upward transitions from highly populated low-lying levels of abundant elements such as Fe II, thus providing many chance coincidences between potential pumps and lines to be pumped. These conditions ensure that many of the chromospheric emission features observed in the UV spectrum of such a star are affected by fluorescence. Many of the observed emission features originate from energy levels populated solely by radiative fluorescent excitation, including strong lines of S I, O I, CO, Ni II, Si I, Fe I and Fe II, as well as weaker lines from Cr II and Co II. Important pumps active in these atmospheres include hydrogen Lyman alpha, and individual lines of 0 I, C I, Si II, Fe II, and Mg II. In the case of Fe II, there are many additional features arising from upper levels whose populations, although primarily maintained by collisions, are also significantly affected by radiative fluorescent excitation. In fact, there may be virtually no level in Fe II not affected to one degree or another by direct decays or cascades down from levels populated by fluorescence, driven either by Lyman alpha or, in some cases, by lines of Fe II itself (“self-fluorescence“).

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