J/ApJ/781/124 Granulation model for 508 KIC stars (Cranmer+, 2014)
Stellar granulation as the source of high-frequency flicker in Kepler light
curves.
Cranmer S.R., Bastien F.A., Stassun K.G., Saar S.H.
<Astrophys. J., 781, 124 (2014)>
=2014ApJ...781..124C 2014ApJ...781..124C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Stars, masses ;
Effective temperatures
Keywords: convection - stars: activity - stars: solar-type - starspots -
techniques: photometric
Abstract:
A large fraction of cool, low-mass stars exhibit brightness
fluctuations that arise from a combination of convective granulation,
acoustic oscillations, magnetic activity, and stellar rotation. Much
of the short-timescale variability takes the form of stochastic noise,
whose presence may limit the progress of extrasolar planet detection
and characterization. In order to lay the groundwork for extracting
useful information from these quasi-random signals, we focus on the
origin of the granulation-driven component of the variability. We
apply existing theoretical scaling relations to predict the
star-integrated variability amplitudes for 508 stars with photometric
light curves measured by the Kepler mission. We also derive an
empirical correction factor that aims to account for the suppression
of convection in F-dwarf stars with magnetic activity and shallow
convection zones. So that we can make predictions of specific
observational quantities, we performed Monte Carlo simulations of
granulation light curves using a Lorentzian power spectrum. These
simulations allowed us to reproduce the so-called flicker floor (i.e.,
a lower bound in the relationship between the full light-curve range
and power in short-timescale fluctuations) that was found in the
Kepler data. The Monte Carlo model also enabled us to convert the
modeled fluctuation variance into a flicker amplitude directly
comparable with observations. When the magnetic suppression factor
described above is applied, the model reproduces the observed
correlation between stellar surface gravity and flicker amplitude.
Observationally validated models like these provide new and
complementary evidence for a possible impact of magnetic activity on
the properties of near-surface convection.
Description:
A goal of this work is to find self-consistent and accurate ways to
predict the properties of stellar light-curve variability, and to use
this variability to calibrate against other methods of determining
their fundamental parameters. Thus, it may be possible to develop the
analysis of granular flicker measurements in a way that augments the
results of asteroseismology and improves the accuracy of, e.g.,
stellar mass and radius measurements.
To assist in this process, we provide tabulated data for 508 stars
with photometric light curves measured by the Kepler mission, which
also includes their derived masses and predicted values of the
turbulent Mach number (Ma), the root-mean-square (rms)
granulation intensity amplitude σ, and the flicker amplitude
F8. These data are also hosted, with updates as needed, on the first
author's Web site (http://www.cfa.harvard.edu/~scranmer/). With the
data is a short code written in the Interactive Data Language (IDL)
that reads the data and reproduces two of the three panels of Figure4
in the paper.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
stars.dat 95 508 Data of the paper's granulation model for 508
Kepler stars
figure4.pro 78 77 Example Interactive Data Language (IDL) routine
that reads the data in stars.dat file and
reproduces the results shown in panels (a) and
(c) of Figure4
readme 79 45 Description of input (stars.dat) and output
(figure4.pro) data
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See also:
V/133 : Kepler Input Catalog (Kepler Mission Team, 2009)
J/ApJS/210/1 : Asteroseismic study of solar-type stars (Chaplin+, 2014)
J/ApJS/199/30 : Effective temperature scale for KIC stars (Pinsonneault+, 2012)
http://www.cfa.harvard.edu/~scranmer/ : First author's Web site
Byte-by-byte Description of file: stars.dat
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Bytes Format Units Label Explanations
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1- 8 I8 --- KIC [1725815/12735580] KIC number of star (1)
10- 13 I4 K Teff [4544/6648] Effective temperature (2)
15- 18 F4.2 [cm/s2] logg [2.6/4.6] Log of stellar surface gravity (in cgs
units) (2)
20- 24 F5.3 Msun Mass [0.6/2.5] Mass of star (3)
26 I1 --- r_Mass [0/2] Integer flag describing source of mass
(O, 1, or 2) (4)
28- 39 E12.10 s tau [300/14000] Granulation timescale τeff (5)
41- 52 E12.10 --- Ma [0.16/0.37] Dimensionless turbulent Mach number
Ma (6)
54- 65 E12.10 10-3 sigma [0.01/0.32] Modeled root-mean-square granulation
intensity amplitude σ (in ppt=parts per
thousand) (7)
67- 78 E12.10 10-3 F8m [0.01/0.24] Modeled light-curve flicker amplitude
F8 (ppt) (8)
80- 85 F6.4 10-3 F8o [0.01/0.33] Observed light-curve flicker amplitude
F8 (ppt), from Bastien et al
(2013Natur.500..427B 2013Natur.500..427B) (8)
87- 91 F5.3 10-3 Rvar [0.04/2.4] Observed light-curve range Rvar
(ppt) (9)
93- 95 I3 10-3 Zc [7/263] Observed number of zero crossings ZC,
from Bastien et al. (2013Natur.500..427B 2013Natur.500..427B) (10)
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Note (1): The data of this table are hosted, with updates as needed, on the
first author's Web site (http://www.cfa.harvard.edu/~scranmer/).
Note (2): The 508 Kepler stars analyzed by Bastien et al. (2013Natur.500..427B 2013Natur.500..427B)
all have measured values of Teff and logg (see Chaplin et al.,
2011Sci...332..213C 2011Sci...332..213C; Pinsonneault et al. 2012, cat. J/ApJS/199/30).
Note (3): Definitive masses for the full set analyzed by Bastien et al.
(2013Natur.500..427B 2013Natur.500..427B) have not yet been determined.
Note (4): The codes for source of mass are defined as:
0 = Mass estimated by comparing their measured Teff and logg values against
evolutionary tracks computed by the Cambridge STARS code (Eggleton,
1971MNRAS.151..351E 1971MNRAS.151..351E; Eldridge et al., 2008MNRAS.384.1109E 2008MNRAS.384.1109E; Eldridge &
Stanway, 2009MNRAS.400.1019E 2009MNRAS.400.1019E).
1 = Mass analyzed in the earlier asteroseismology study of Chaplin et al.
(2011Sci...332..213C 2011Sci...332..213C);
2 = Mass computed from the recent ensemble asteroseismology of Chaplin et al.
2013 (cat. J/ApJS/210/1).
Note (5): Estimated using the scaling relation given by Samadi et al.
(2013A&A...559A..40S 2013A&A...559A..40S):
τeff=300(ν☉Ma☉/νmaxMa)0.98s (Eq.(7) in the
paper), where Ma☉=0.26.
Note (6): We use the Samadi et al. (2013A&A...559A..40S 2013A&A...559A..40S) scaling relation for
the Mach number: Ma=0.26(Teff/T☉)2.35(g☉/g)0.152
(Eq.(3) in the paper).
Note (7): Samadi et al. (2013A&A...559A..40S 2013A&A...559A..40S) derived the following scaling,
where σ is equal to (see Eq.(1) in the paper):
0.039[(Teff/T☉)3/4(M☉ν☉/M*νmax)1/2Φ(Ma)2]1.03
where σ is given in units of parts per thousand (ppt) and Φ is a
dimensionless temperature fluctuation amplitude that depends on the turbulent
Mach number Ma. The normalizing constants T☉=5777K, logg☉=4.438,
and ν☉=3.106mHz are taken from Samadi et al. (2013A&A...559A..40S 2013A&A...559A..40S).
The peak frequency νmax of p-mode oscillations is assumed to scale with
the acoustic cutoff frequency (e.g., Brown et al., 1991ApJ...368..599B 1991ApJ...368..599B;
Kjeldsen & Bedding, 1995A&A...293...87K 1995A&A...293...87K), with:
νmax=ν☉(g/g☉)(T☉/Teff)1/2 (Eq.(2) in the paper).
Note (8): Corresponds to fluctuations on timescales of 8hr or less.
Note (9): From Bastien et al (2013Natur.500..427B 2013Natur.500..427B). Defined as the difference
between the 5% and 95% percentile intensities.
Note (10): Experienced by the light curve, smoothed with a 10hr window, over the
full 90 days of the data set. ZC∼N/(4M)1/2 (Eq.(9) in the paper), where N
is the total number of data points and M is the width of the boxcar averaging
window used to smooth the data.
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History:
From electronic version of the journal
(End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 14-Jan-2016