Physical properties of starspots

My talk at #KeplerSciCon at NASA Ames Research Center in Mountain View, CA, USA on Thursday, June 22, 2017 at 2:15 pm.

Transcript

1. Physical properties of starspots and their effects on young stellar ages Michael Gully-Santiago Kepler/K2 Guest Observer Office Thursday, June 22, 2017 at NASA Ames Kepler/K2 Science Conference Collaborators: Greg Herczeg (KIAA), Ian Czekala (CfA/KICAP), Garrett Somers (OSU/Vanderbilt), J.F. Donati (CNRS), Konstantin Grankin (CrAO), Kevin Covey (WWU), G. Mace (UTexas), MATYSSE team, ASASSN team, ++
2. Disk fraction as a function of age gives us the disk lifetime. Haisch et al. 2001
3. Disk fraction as a function of age gives us the disk lifetime. The disk lifetime limits the planet formation timescale. Haisch et al. 2001, Alexander & Armitage 2009, Kraus et al. 2012
4. The disk lifetime limits the planet formation timescale. Haisch et al. 2001, Alexander & Armitage 2009, Kraus et al. 2012 How accurate are cluster ages?
5. How accurate are cluster ages? ? ? ? ?
6. We want to know the ages and masses of young stars so that we can understand how star and planet formation proceed in time.
7. We want to know the ages and masses of young stars so that we can understand how star and planet formation proceed in time. Age is not a direct observable. Mass is sometimes a direct observable, but only in rare eclipsing binary systems or resolved gas disk systems.
8. We want to know the ages and masses of young stars so that we can understand how star and planet formation proceed in time. Age is not a direct observable. Mass is sometimes a direct observable, but only in rare eclipsing binary systems or resolved gas disk systems.
9. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
10. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
11. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
12. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
13. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
14. Ages are estimated by placing a young star on a pre-main sequence HR diagram.
15. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why?
16. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots
17. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots Herczeg & Hillenbrand 2014 Spectral types map imprecisely to Teff
18. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots APOGEE spectra of thousands of young stars show large disagreement Cottaar et al. 2014
19. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots
20. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots 290 300 310 320 330 340 350 360 l 10 5 0 5 10 15 20 25 30 b ⇢ Oph Lup I Lup II Lup III Lup IV F/G/K/M Pre-MS Accretors Turnoff B-type Other B1-B5 24 Myr 20 Myr 16 Myr 13 Myr 11 Myr 9 Myr 7 Myr Age Pecaut & Mamajek 2016
21. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots
22. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots
23. Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why? - Observational uncertainties - True age spreads - Episodic accretion - Physics beyond the standard evolutionary models - Magnetic fields - Starspots
26. Conspicuous starspot transits have been relatively rare. Little/no enhanced in-transit variance has been observed. - B. Morris & E. Krause from discussions at this conference
27. Starspots imbue periodic flux diminution as they enter-and-exit the unresolved stellar disk. Spots have been taken for granted for measuring rotation periods. Spot amplitudes encode information about longitudinally asymmetric spots. Notsu et al. 2013
28. Starspots have both theoretical and observational consequences.
29. Starspots inhibit convective efficiency. Somers & Pinnsoneault 2015 Less efficient energy transport means stars get larger (R increases) but cooler (T decreases) Postdoc at Vanderbilt
30. Starspots confound measurements of L and Teff. Tamb=Teff 0th order assumption No starspots Tamb
31. Starspots confound measurements of L and Teff. Tamb=Teff 0th order assumption No starspots 1st order correction Non-emitting starspots Tamb Tspot = 0 K fspot Tamb
32. Starspots confound measurements of L and Teff. Tamb=Teff 0th order assumption No starspots 1st order correction Non-emitting starspots Tamb Tspot > 0 K fspot Tamb Tspot = 0 K fspot Tamb 2nd order correction Emitting starspots
33. Starspots confound measurements of L and Teff. Tamb=Teff 0th order assumption No starspots 1st order correction Non-emitting starspots Tamb Tspot > 0 K fspot Tamb Tspot = 0 K fspot Tamb 2nd order correction Emitting starspots Teff < Tamb
34. Photometric modulation probes longitudinally asymmetric spots. ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2 LkCa 4 light curve
35. You can find a minimum coverage of starspots for LkCa 4 ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2 Spot-free Spotted, non-emitting spots
36. You can find a minimum coverage of starspots for LkCa 4 ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2 Spot-free Spotted, emitting spots Tspot
37. Expected range of starspot properties
38. "any  photometric  modula2ons  due  to  starspots  is  the  asymmetric   component  of  the  starspot  coverage"     Neff  et  al.  1995,  Harrison  et  al  2011 Smith  1994 Uniformly distributed spots; pole-on star spots circumpolar spots Differential rotation confounding periods Geometrical effects hinder interpretation of lightcurve amplitudes.
39. K2 Campaign 2 Observed the ~few Myr Oph and Upper Sco. What can be learned from periodic lightcurve amplitudes in K2?
40. Young stars have larger amplitude of periodic variability than typical stars (we already knew that). K2  Cycle  2  light  curves  for     1658  candidate  or  confirmed  young   stars  towards  Oph/Sco.   compared  to     everything  else  in  that  Cycle. Uncorrected Vanderburg and Johnson lightcurves
41. Caveats: non-members non-periodic structures Vanderburg and Johnson lightcurves Periods < 10 days K2 C2 lightcurve amplitudes: ~0.5-5% peak to valley
42. How to resolve geometric degeneracies to assess total starspot coverage fraction?
43. Directly detect the spectrum arising from starspots. Tamb Tspot fspot 1-fspot My Research - This has historically been challenging due to low flux ratio of spots. - My strategy leverages new high resolution near-IR spectrographs.
44. Starspot emission Tamb Tspot fspot 1-fspot Tspot = 2800 K Tamb = 4100 K fspot = 0.5 (!) Example Key insight: - In the visible, starspot flux is 5-20x weaker than the ambient photosphere. - In the near-IR, starspot flux is only 2.5-4x weaker than ambient
45. Starspot emission Tamb Tspot fspot 1-fspot Tspot = 2800 K Tamb = 4100 K fspot = 0.5 (!) Example Key insight: - In the visible, starspot flux is 5-20x weaker than the ambient photosphere. - In the near-IR, starspot flux is only 2.5-4x weaker than ambient IGRINS ESPaDOnS
46. How to figure out which lines are attributable to starspots or ambient photosphere? portion of LkCa 4 IGRINS spectrum from November 2015
47. We forward  model the IGRINS spectra. Starfish is an open source spectral inference framework for stellar spectra. Czekala et al. 2015 github.com/iancze/Starfish Starfish parameters: 1. Tamb 2. logg 3. [Fe/H] 4. v sini 5. vz 6. Ω 7-9. c0, c1, c2... 10. GP scale 11. GP amplitude 12. σ scale 13. Tspot 14. fspot Intrinsic Starspots = + Composite Ambient Starspot Tspot = 2800 K Tamb = 4100 K **Lots  of  assump,ons  embedded  here
48. The spectrum has features from both ambient photosphere and starspots. λ (Angstrom) The constraint on filling factor comes from the range of flux ratios.
50. Expected range of starspot properties
52. LkCa 4 looks younger and less massive in *unspotted* tracks.
53. Unspotted tracks from G. Somers
54. Spotted tracks from G. Somers
55. Take aways 1. Stars are probably more spotted than we previously thought 2. Polar spots would have evaded most conventional methods of detecting and characterizing starspots, since they induce zero photometric modulation. 3. If LkCa 4 is representative of other young stars, the masses and ages of all young stars are considerably biased. 4. The stellar age biases change timescale available for planet formation. 5. What matters is the starspot coverage history, which is generally unobservable. 6. Teff measurement is hindered for highly inclined young spotted stars.
56. 1. Stars are probably more spotted than we previously thought 2. Polar spots would have evaded most conventional methods of detecting and characterizing starspots, since they induce zero photometric modulation. 3. If LkCa 4 is representative of other young stars, the masses and ages of all young stars are considerably biased. 4. The stellar age biases change timescale available for planet formation. 5. What matters is the starspot coverage history, which is generally unobservable. 6. Teff measurement is hindered for highly inclined young spotted stars. Take aways
57. 1. Stars are probably more spotted than we previously thought 2. Polar spots would have evaded most conventional methods of detecting and characterizing starspots, since they induce zero photometric modulation. 3. If LkCa 4 is representative of other young stars, the masses and ages of all young stars are considerably biased. 4. The stellar age biases change timescale available for planet formation. 5. What matters is the starspot coverage history, which is generally unobservable. 6. Teff measurement is hindered for highly inclined young spotted stars. Take aways
58. LkCa 4 has photometric monitoring going back 31 years. May 6, 1985 June 22, 2017
59. 1. Stars are probably more spotted than we previously thought 2. Polar spots would have evaded most conventional methods of detecting and characterizing starspots, since they induce zero photometric modulation. 3. If LkCa 4 is representative of other young stars, the masses and ages of all young stars are considerably biased. 4. The stellar age biases change timescale available for planet formation. 5. What matters is the starspot coverage history, which is generally unobservable. 6. Need spectroscopy to assess full spot coverage fraction. Take aways
60. 1. Stars are probably more spotted than we previously thought 2. Polar spots would have evaded most conventional methods of detecting and characterizing starspots, since they induce zero photometric modulation. 3. If LkCa 4 is representative of other young stars, the masses and ages of all young stars are considerably biased. 4. The stellar age biases change timescale available for planet formation. 5. What matters is the starspot coverage history, which is generally unobservable. 6. Need spectroscopy to assess full spot coverage fraction. Take aways
61. Recent evidence for large spot coverage in Pleiades 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2600 3000 3500 4000 4500 5000 5500 6000 6500 TiO2n Teff (K) Inactive dwarfs PHOENIX(4.5) PHOENIX(5.0) Cubic splines fits Estimate of Tamb, Tspot, fspot in 304 LAMOST spectra 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 3000 3500 3800 4000 4500 5000 5500 6000 6500 fs Teff (K) Pleiades? Pleiades
62. Thanks!