J/ApJ/765/126 Star spot models for M-dwarfs in NGC 2516 (MacDonald+, 2013)
Magnetic effects and oversized M dwarfs in the young open cluster NGC 2516.
MacDonald J., Mullan D.J.
<Astrophys. J., 765, 126 (2013)>
=2013ApJ...765..126M 2013ApJ...765..126M
ADC_Keywords: Models ; Magnetic fields ; Stars, M-type ; Clusters, open ;
Stars, diameters ; Stars, masses
Keywords: open clusters and associations: individual: NGC 2516;
stars: low-mass; stars: magnetic field
Abstract:
By combining rotation periods with spectroscopic determinations of
projected rotation velocity, Jackson et al. (2009MNRAS.399L..89J 2009MNRAS.399L..89J) have
found that the mean radii for low-mass M-dwarfs in the young, open
cluster NGC 2516 are larger than model predictions at a given absolute
I magnitude or I-K color and also larger than measured radii of
magnetically inactive M-dwarfs. The relative radius difference is
correlated with magnitude, increasing from a few percent at MI=7 to
greater than 50% for the lowest luminosity stars in their sample at
MI∼9.5. Jackson et al. have suggested that a two-temperature star
spot model is capable of explaining the observations, but their model
requires spot coverage fractions of at least 50% in rapidly rotating
M-dwarfs. Here we examine these results in terms of stellar models
that include the inhibiting effects of magnetic fields on convective
energy transport, with and without the effects of star spots. We find
that a pure spot model is inconsistent with the color-magnitude
diagram. The observations of radii versus color and radii versus
absolute magnitude in NGC 2516 are consistent with models which
include only magnetic inhibition or a combination of magnetic
inhibition and spots. At a given mass we find a large dispersion in
the strength of the vertical component of the magnetic field in the
stellar photosphere but the general trend is that the vertical field
increases with decreasing mass from a few hundred Gauss at
0.65M☉ to 600-900G, depending on spot coverage, in the lowest
mass stars in the sample at 0.25M☉.
Objects:
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RA (ICRS) DE Designation(s)
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07 58 04 -60 45.2 NGC 2516 = OCl 776.0
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 120 234 Different 160Myr isochrone model results
table3.dat 76 855 Details of the stellar parameters determined from
the different model 160Myr isochrones
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See also:
J/MNRAS/423/2966 : Low-mass stars spectroscopy in NGC 2516 (Jackson+, 2012)
J/other/A+ARV/18.67 : Accurate masses and radii of normal stars (Torres+, 2010)
J/MNRAS/407/2269 : Polarisation of a sample of late M dwarfs (Morin+, 2010)
J/MNRAS/407/465 : Fast-rotating M dwarfs in NGC2516 (Jackson+, 2010)
J/MNRAS/394/1338 : Stellar effective magnetic fields. II. (Bychkov+, 2009)
J/A+A/468/151 : Nearby open clusters radii and masses (Piskunov+, 2007)
J/MNRAS/377/741 : VIc photometry of NGC 2516 (Irwin+, 2007)
J/A+A/456/977 : XMM observations of NGC 2516 stars (Marino+, 2006)
J/A+A/453/101 : UBV(RI)c photometry of 4 open clusters (Lyra+, 2006)
J/ApJ/588/1009 : Chandra X-ray observations of NGC 2516 (Damiani+, 2003)
J/AJ/123/290 : UBVI CCD photometry of NGC 2516 (Sung+, 2002)
J/A+A/375/863 : Photometry in NGC 2516 (Jeffries+, 2001)
J/A+A/337/403 : Low-mass stars evolutionary models (Baraffe+ 1998)
J/A+A/333/231 : O-M stars model atmospheres (Bessell+ 1998)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Type Model type (G1)
10- 13 F4.2 --- delta [0/0.09] Magnetic inhibition parameter (G2)
15- 18 F4.2 --- fs [0/1] Equivalent surface fraction covered
by dark spots fs (G2)
20- 29 E10.4 Msun Mass [0.1/0.7] Stellar mass
31- 41 E11.4 [Lsun] logL [-2.8/-0.7] Log stellar luminosity
43- 53 E11.4 [Rsun] logR [-0.8/0.07] Log stellar radius
55- 64 E10.4 K Teff [1981/4327] Effective temperature
66- 76 E11.4 [-] ALi [-8.4/2.8] Lithium abundance parameter,
as 12+log(N(Li)/N(H))
78- 87 E10.4 mag IMag1 [6/12.8] BCP Absolute Cousins I magnitude (G3)
89- 98 E10.4 mag (I-K)1 BCP (I-K) color index (G3)
100-109 E10.4 mag IMag2 NextGen Absolute Cousins I magnitude (G3)
111-120 E10.4 mag (I-K)2 NextGen (I-K) color index (G3)
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- BC Bolometric correction used (BCP or NG) (G3)
5- 12 A8 --- Type Model type (G1)
14- 18 F5.3 mag (I-K) [1.7/3] The (I-K) color index (1)
20- 25 F6.3 mag Imag [14.5/18.1] Apparent Cousins I band magnitude
27- 31 F5.3 mag (I-K)0 Dereddenned (I-K) color index
33- 37 F5.3 mag IMag [6.3/10] Absolute Cousins I band magnitude
39- 43 F5.3 Msun Mass [0.1/0.7] Stellar mass
45- 49 F5.3 --- delta [0/0.09] Magnetic inhibition parameter (G2)
51- 55 F5.3 --- fs [0/1] Equivalent surface fraction covered by
dark spots fs (G2)
57- 61 F5.3 Rsun Rad [0.2/0.9] Stellar radius
63- 69 F7.5 Lsun Lum [0.005/0.2] Stellar luminosity
71- 76 F6.1 K Teff [2401/4213] Effective temperature
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Note (1): From Jackson et al. 2009MNRAS.399L..89J 2009MNRAS.399L..89J
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Global notes:
Note (G1): Model types are:
Spots = The spots only model 160Myr isochrones (used to produce Figure 3).
Magnetic = The magnetic inhibition only model 160Myr isochrones (used to
produce Figure 4).
Hybrid = The hybrid model 160Myr isochrones (used to produce figure 5).
Note (G2): the magnetic inhibition parameter δ is defined in cgs as
δ= Bv2/(Bv2+4πγPgas), if Bv is the vertical
magnetic field, γ the first adiabatic exponent, and Pgas the
gas pressure.
* in the "Spots" rows δ is set to 0.00 as a placeholder.
* similiarly in the "Magnetic" rows fs is also set to 0.00.
* to determine the magnitude and color, it has been assumed that the
dark spots have a temperature 70% of the unspotted surface in the
"Spots" and "Hybrid" models.
Note (G3): Bolometric corrections to relate the observed magnitude and color
to the stellar parameters are:
BCP = Bessell, Castelli and Plez 1998 (J/A+A/333/231)
NG = NextGen atmosphere models, Hauschildt et al. 1999ApJ...512..377H 1999ApJ...512..377H),
as given at http://phoenix.ens-lyon.fr/Grids/NextGen/.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 03-Nov-2014