J/A+A/598/A62 KIC red giants showing depressed mixed modes (Mosser+, 2017)
Dipole modes with depressed amplitudes in red giants are mixed modes.
Mosser B., Belkacem K., Pincon C., Takata M., Vrard M., Barban C.,
Goupil M.-J., Kallinger T., Samadi R.
<Astron. Astrophys., 598, A62-62 (2017)>
=2017A&A...598A..62M 2017A&A...598A..62M (SIMBAD/NED BibCode)
ADC_Keywords: Stars, giant ; Spectroscopy ; Magnetic fields
Keywords: stars: oscillations - stars: evolution - stars: magnetic field -
stars: interiors
Abstract:
Seismic observations with the space-borne Kepler mission have shown
that a number of evolved stars exhibit low-amplitude dipole modes,
which is referred to as depressed modes. Recently, these low
amplitudes have been attributed to the presence of a strong magnetic
field in the stellar core of those stars. Subsequently, and based on
this scenario, the prevalence of high magnetic fields in evolved stars
has been inferred. It should be noted, however, that this conclusion
remains indirect.
We intend to study the properties of mode depression in evolved stars,
which is a necessary condition before reaching conclusions about the
physical nature of the mechanism responsible for the reduction of the
dipole mode amplitudes.
We perform a thorough characterization of the global seismic
parameters of depressed dipole modes and show that these modes have a
mixed character. The observation of stars showing dipole mixed modes
that are depressed is especially useful for deriving model-independent
conclusions on the dipole mode damping. We use a simple model to
explain how mode visibilities are connected to the extra damping seen
in depressed modes. Results. Observations prove that depressed dipole
modes in red giants are not pure pressure modes but mixed modes. This
result, observed in more than 90% of the bright stars (mV≤11),
invalidates the hypothesis that depressed dipole modes result from the
suppression of the oscillation in the radiative core of the stars.
Observations also show that, except for visibility, seismic properties
of the stars with depressed modes are equivalent to those of normal
stars. The measurement of the extra damping that is responsible for
the reduction of mode amplitudes, without any prior on its physical
nature, potentially provides an efficient tool for elucidating the
mechanism responsible for the mode depression.
The mixed nature of the depressed modes in red giants and their
unperturbed global seismic parameters carry strong constraints on the
physical mechanism responsible for the damping of the oscillation in
the core. This mechanism is able to damp the oscillation in the core
but cannot fully suppress it. Moreover, it cannot modify the radiative
cavity probed by the gravity component of the mixed modes. The recent
mechanism involving high magnetic fields proposed for explaining
depressed modes is not compliant with the observations and cannot be
used to infer the strength and prevalence of high magnetic fields in
red giants.
Description:
We performed a thorough study of red giants showing low dipole-mode
visibility, based on the identification of their dipole mode pattern
and the characterization of their global seismic properties. We have
shown that these stars share the same global seismic parameters as
other stars, regardless of the value of the dipole mode visibilities.
This analysis sustains the fact that the mechanism responsible for the
damping does not significantly impact the stellar structure and does
not change the property of the cavity where gravity waves propagate.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 67 71 Seismic properties of 71 stars showing depressed
mixed modes
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See also:
V/133 : Kepler Input Catalog (Kepler Mission Team, 2009)
J/A+A/525/A131 : Solar-like oscillations in Kepler red giants (Hekker+, 2011)
J/A+A/540/A143 : Oscillations of red giants observed by Kepler (Mosser+, 2012)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 8 I8 --- KIC KIC identification number
10- 12 A3 --- Evol Evolutionary stage (1)
15- 19 F5.1 uHz numax Maximum frequency
21- 25 F5.2 uHz Dnu Frequency separation
27- 31 F5.1 s DPI1 Period spacing ({DELTA}{PI}1)
33- 36 F4.2 --- q Coupling factor (2)
38- 41 F4.2 Msun Mass Mass
43- 46 F4.2 uHz Gamma0 Radial mode width (defined as full width as
half maximum) (3)
48- 50 I3 nHz dnurot Rotational splitting (4)
52- 55 F4.2 --- V1 Total dipole visibility
57- 61 F5.3 --- Vlow Low visibility
63- 67 F5.3 --- Vstrong Strong visibility
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Note (1): Evolutionary stages as follows:
RGB: red giant branch
RC: red clump
RC2: secondary red clump.
Note (2): Uncertainties on q are of about
±0.027 for stars on the RGB and ±0.057 in the red clump.
Note (3): Uncertainties on Gamma0 are of about 30%.
Note (4): A null value for dnurot indicates that the rotational splitting could
not be measured because the star is seen pole-on.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 17-May-2017