Modelling Brightness Variability of Sun-Like Stars

Transcription

1 Modelling Brightness Variability of Sun-Like Stars V. Witzke, A. I. Shapiro, S. K. Solanki, N. A. Krivova Cool Stars 20 Fundamental Properties of Cool Stars August 1st, 2018 Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

2 Time-scales < a day: convection and oscillations [Seleznyov et al. 2011] Solar Irradiance Variations; Mechanism Time-scales > a day: caused by surface magnetic fields [Ermolli et al. 2013, Solanki et al. 2013] Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

3 How to Model Stellar Brightness Variability What do we need for modelling stellar brightness 1 Dimensional Approach changes? 3 Dimensional Approach - Radiative Equilibrium (Umbra, Penumbra, Quiet Sun) - Semi-Empirical: Faculae Atmospheric 1D Structures: ATLAS9 Code 3D Magneto-Convection Cubes using the MURaM Code: 1D Rays, from magnetic and non-magnetic cases Emergent Spectra: - different viewing angle - different magnetic features Brightness change on the magnetic activity time-scale Emergent Spectra Magnetic Feature Distribution (SATIRE) Brightness Evolution 1.5 D approach: Many emergent spectra from 1D rays Computationally expensive! -Averaged emergent spectra - Find relation between magnetic field and emergent spectra for magnetic features Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

4 Observed Long-Term Brightness Variations of Sun-like Stars Mount Wilson Observatory [Wilson 1968, Wilson 1978]; Lowell Observatory Lockwood et al. 1992], Fairborn Observatory [Hall et al. 2009]; Kepler mission [Borucki et al. 2010] 72 primarily main-sequence Sun-like stars; Figure from Radick et al. (2018). Solar photometric variability calculated with SATIRE model Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

5 Delicate balance between faculae and spot contribution Spot: smooth not much contribution from Fraunhofer lines Stellar Brightness Variability using SATIRE model Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

6 Effect of Metallicity on Stellar Brightness Change (1D model) Brightness change [mmag] Strömgren b filter Strömgren y filter Kepler pass-band Dependence of brightness change integrated over Strömgren b and y filters and Kepler pass-band Observed activity and greater brightness change of the solar analoge HD can be explained (Karoff et al. 2018) M/H Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

7 Effect of Small Effective Temperature Deviations (1D model) Small T eff changes of ±100 K, which is of the order of measurment accuracy (Pinsonneault et al. 2012) Drop in effective temperature - spot dominated brightness changes Note, 1-D models do not capture geometric effects, e.g. from hot faculae walls More detailed investigation including the effect of effective temperature and inclination will be soon available (Witzke, V. et al., submitted to A & A) Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

8 Placing a hypothetical Sun with M/H = 0.4 The solar fundamental parameters are close to a local minimum for the brightness changes on the magnetic activity time-scale 72 primarily main-sequence Sun-like stars; Figure from Radick et al. (2018). Solar photometric variability calculated with SATIRE model Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

9 1D atmospheric structure versus 3D MURaM Cubes 9000 Quiet Sun 8000 Faculae Umbra Penumbra 7000 T [K] Column mass [ g cm -2 ] Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

10 Relation between the vertical magnetic field and Intensity Contrast dependence on viewing angle: Flux [erg s -1 cm -2 ] Contrast Kepler, = 1.0 Contrast Kepler, = # points in bin, =1.0 # points in bin, = B [Gauss] Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

11 Effect of Fraunhofer lines on the contrast: Relation between the vertical magnetic field and Intensity Flux [erg s -1 cm -2 ] Contrast Kepler, = 1.0, M/H = 0.3 Contrast Kepler, = 1.0, M/H = 0.0 Contrast Kepler, = 1.0, M/H = B [Gauss] Contrasts in Kepler passband increase gradually with metallicity Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

12 Conclusion and Outlook Main results: The solar fundamental parameters are close to a local minimum for the brightness changes on the magnetic activity time-scale (Witzke, V. et al., submitted to A & A) First preliminary results using 1.5D approach confirm a higher faculae contrast in the Kepler passband for M/H = 0.3 and lower contrasts for M/H = -0.3 Future steps: Finalising 3D MHD investigations for a more realistic modelling Comparison of stars for which extended and detailed measurements exist, so far only for the solar analoge HD (Karoff et al. 2018). Large sample of stars, for example from Kepler full-frame images Thank you for your attention! Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

13 Stellar Brightness Variability using SATIRE model 11-years solar cycle driven by magnetic activity; phenomena are spots and faculae F(λ) = F Q (λ) + F m(λ), irradiance [W/m 2 ] Solar magnetic feature s distribution (1th of July, 2000 & 1st of Dec, 2008) Corresponding to the amplitude of solar cycle 23 Uses solar magnetic feature distribution time [yr] Solar brightness in Strömgren b filter; blue lines are the time point of the two upper plots Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12

14 Stars of Solar Magnetic Activity HD P rot = 38 d 10-3 HD P = 33 or 15 d rot T = 5400 eff Spot -dominated T eff = 4000 K HD P rot = > 18.7 d T = 5770 eff spot-dominated HD P rot = T = 5850 eff Spot-dominated HD 6920 P rot = 14 d T eff = 5900 K HD P rot = d T eff = 5800 K HD P rot = --- HD P = d rot T eff = 5750 K T eff = 5700 b + y / Sun P rot = 24.5 d T eff = 5770 K M/H Veronika Witzke (MPS) 2018 Cool Stars 20 August 1st, / 12