Issue 
A&A
Volume 548, December 2012



Article Number  A10  
Number of page(s)  14  
Section  Stellar structure and evolution  
DOI  https://doi.org/10.1051/00046361/201220106  
Published online  13 November 2012 
Online material
Appendix A: How rotational splittings are fitted
A.1. Large separation and gravity mode spacing
The first step for identifying the red giant oscillation spectrum is, as for all stars showing solarlike oscillations, the correct identification of the radial mode pattern, in order to locate precisely the location of the theoretical pure dipole pressure modes. The fit of the radial modes depends mainly on the accurate determination of the large separation. According to the universal red giant oscillation pattern (Mosser et al. 2011b), the surface offset and the curvature of the ridge are functions of the large separation. In practice, small residuals due to glitches (Miglio et al. 2010) can induce a frequency offset of about, typically, Δν/50. Thus, a second free parameter, simply a frequency offset, or equivalently an offset of ε less than 0.02 (Eq. (1)), is useful for providing the best fit of the radial ridge. The location of the dipole ridge with respect to the radial ridge is given by the small separation d_{01} (Eq. (1)), which is a function of the large separation (Mosser et al. 2011b).
The fit of the mixedmode pattern is based on two free parameters: the period spacing ΔΠ_{1} and the coupling constant q, as defined by Eq. (9) of Mosser et al. (2012c), which closely follows the formalism of mixed modes given by Unno et al. (1989). In order to determine ΔΠ_{1} on the RGB, it is worthwhile to consider that this period is a function of the large separation. For the lowmass stars of the RGB with a degenerate helium core, a convenient proxy is given by the polynomial development (A.1)with Δν in μHz and ΔΠ_{1} in s, according to Fig. 3 of Mosser et al. (2012c). When the rotational splitting is larger than half the mixed mode spacing at ν_{max}, this step cannot be done independent of the next one.
Fig. A.1
Fit of rotational splittings, for the RGB star KIC 6144777, with an échelle diagram as a function of the reduced frequency ν/Δν − (n_{p} + ε). The radial orders are indicated on the yaxis. Radial modes (highlighted in red) are centered on 0, quadrupole modes (highlighted in green), near − 0.12 (with a radial order n_{p} − 1), and ℓ = 3 modes, sometimes observed, (highlighted in hell blue) near 0.20. Rotational splittings are identified with the frequency of the m = 0 component given by the asymptotic relation of mixed modes, in μHz. The fit is based on peaks showing a height larger than eight times the mean background value (grey dashed lines). In order to enhance the appearance of the multiplets, highest peaks have been truncated; to enhance the shortlived radial and quadrupole modes, a smoothed spectrum is also shown, superimposed on the corresponding peaks. 

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Fig. A.2
Same as Fig. A.1, for the RGB star 5858947. In such a spectrum where the total splitting 2δν_{rot} is equal to half the mixedmode spacing at ν_{max}, the fit allows to correctly identify the multiplets. 

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Fig. A.3
Same as Fig. A.1, for the clump star KIC 4770846. The apparent low quality of the fit for pm modes at large frequency is due to their short lifetimes (Baudin et al. 2011). 

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Fig. A.4
Same as Fig. A.1, for the RGB stars KIC 9267654 and KIC 10866415, where the total splitting 2δν_{rot} is nearly equal to the mixedmode spacing at ν_{max}. Apparent narrow multiplets are artifacts due to close combinations between components of different mixedmodes radial orders. 

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A.2. Rotational splittings
Great care must be taken to disentangle the splittings from the mixed mode spacings. Three major cases have to be considered for fitting the rotational splittings.

If splittings are small and almost uniform with frequency, exceptthe modulation depicted by ℛ (Eq. (4)), then the estimate is straightforward. The unknown stellar inclination can be derived from the mode visibility, which depends on the azimuthal order m. According to the probability of having an inclination i proportional to sini, in most cases doublets with m = ± 1 are observed. Note that, even if the components m = −1 and + 1 have the same visibility, they may in practice present different heights, due to the stochastic excitation of the modes. Such splittings smaller than the mixedmode spacings are seen in the lower stages of the RGB and in the clump (Fig. A.1).

If apparent splittings at ν_{max} seem to increase with increasing frequency, then the most plausible solution is that δν_{rot} is close to half the mixedmode spacing at ν_{max}. These apparent splittings result in fact from a mixing of the splittings embedded with the spacings. Such a situation occurs when the apparent splittings are composed of the m = ± 1 component of the mixed mode order n_{m} and of the m = ∓ 1 component of the adjacent orders n_{m} ± 1. The true splittings, significantly larger than the apparent splittings, are almost uniform for gm modes. This uniformity is used for iterating the solution. Such cases occur most often for RGB stars with Δν in the range [9–12 μHz] (Fig. A.4).

If apparent splittings seem very irregular, then the most plausible solution is that δν_{rot} is much larger than half the mixedmode spacing at ν_{max}. In fact, the apparent splittings are complex structures resulting from a mixing of components of two or three different mixedmode orders. A careful visual inspection is necessary to disentangle them. The mixedmode asymptotic expression and the empirical expression of the rotational splitting are accurate enough for resolving complex cases that occur for RGB stars with Δν ≤ 9 μHz (Fig. A.5).
We have used gravity échelle diagrams to represent the mixed modes (Bedding et al. 2011; Mosser et al. 2012c). Due to the complexity of the features caused by embedded splittings and mixed modes spacings, the échelle diagrams cannot be used to identify the rotational splittings, but are useful for improving the accuracy of the fit. In the examples shown (Fig. A.6), a 10s shift between the periods of the observed and modeled peaks correspond to an accuracy in frequency of about Δν/100.
A.3. A dipole mode forest?
The complete fit of the rotational splittings is based on three parameters: the maximum splitting δν_{rot} and the two parameters λ and β entering the definition of ℛ. The best fit is provided by correlating the observed multiplets with synthetic multiplets.
Since the parameters λ and β are found to vary in narrow ranges, the solution for inferring δν_{rot} (and simultaneously ΔΠ_{1} on the RGB with low Δν) is based on considering them as constants. As a result, five free parameters are enough for fitting the whole red giant oscillation spectrum. Variation of λ and β allows a better fit. The stellar inclination can be derived from the ratio of the visibility of the m = ± 1 components compared to the central component.
In a typical spectrum, more than 30 mixedmode orders, representing about 60 to 120 individual modes with a height larger than eight times the background are simultaneously fitted. The typical accuracy of the fit, of about Δν/200 or better, is enough for avoiding any confusion in almost all cases, except for the most evolved RGB stars.
Finally, with the identification of the mixed mode spacings and of the rotational splittings, the dipole mode forest becomes a wellorganized garden à la française.
Fig. A.5
Same as Fig. A.4, for the RGB star KIC 11550492. The nonnegligible amplitudes of the m = 0 components complicate the analysis. 

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Fig. A.6
Gravity échelle diagrams of the two RGB stars KIC 5858947 and 11550492. The xaxis is the period 1/ν modulo the gravity spacing ΔΠ_{1}; for clarity, the range has been extended from − 0.5 to 1.5 ΔΠ_{1}. The size of the selected observed mixed modes (red diamonds) indicates their height. Plusses give the expected location of the mixed modes, with m = −1 in light blue, m = 0 in green and m = +1 in dark blue. 

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Appendix B: Twolayer model
B.1. Core and surface contributions
In order to estimate the contribution of the core and surface rotation, we simplify the stellar stratification to a 2layer model. We denote by δν_{c} and δν_{s} the rotational frequency of the core and at the surface, respectively, and δν_{g}/2 and δν_{p} the measured splitting on g and p modes, respectively. The factor 1/2 in δν_{g}/2 accounts for the Ledoux coefficient. The contributions of the surface and of the core are written (B.1)The coefficient x_{p} and x_{g} are derived from the rotational kernels. From the solution (B.2)and from the observation of the splitting of pm modes indicating δν_{p} ≃ δν_{g}/4 (a factor of about 1/2 comes from 1 − λ in Eq. (4), an another factor of 1/2 comes from the Ledoux coefficient), one derives that the measure of δν_{g} is an indicator of the core rotation (B.3)For an RGB star at the bump with Δν = 5 μHz, the values x_{p} and x_{g} derived from the kernels give η = 1.06 ± 0.04, very close to unity. A less evolved star, as considered by Beck et al. (2012), whose mixed modes correspond to much smaller radial gravity orders, has . Deheuvels et al. (2012a) derived a similar result for a giant with Δν ≃ 29 μHz at the bottom of the RGB. This shows that δν_{rot} is less dominated by the core rotation for early RGB stars. One also derives that, in all cases, the surface rotation δν_{s} is small, and that measuring it precisely from the gm modesplitting is not possible.
B.2. Link to the eigenfunction properties
The value of the coefficients x_{g} and x_{p} introduced in Eq. (B.1) can be approximated by the expression of the rotational splitting (Eqs. (8) and (9)). Basically, the integration of the wave function has a contribution varying as the number of nodes in the core and in the envelope, respectively. As a consequence, x_{g} ∝ n_{g} and x_{p} ∝ n_{p}. In order to more precisely account for the complex form of the wave function, we suppose: (B.4)\newpage\noindentwith γ_{g} < 0 to account for the negative value of n_{g}. The validity of this development implicitly assumes that γ_{p} and  γ_{g}  are constant not so far from unity. Hence, neglecting δν_{s} in Eq. (B.1), we derive: (B.5)The radial order n_{p} and n_{g} have to be estimated at the frequency ν_{max} where the oscillation amplitude is maximum. Then, we can derive that η = δν_{c}/δν_{g} is related to the global seismic parameters Δν and ΔΠ_{1}, where ΔΠ_{1} is the period spacing of gravity modes: (B.6)The fit of the integrated kernels calculated at different evolutionary stages gives γ ≃ 0.65. This phenomenological result based on a simple twolayer model is to be considered as a proxy only.
© ESO, 2012
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