Know Thy Planet, Know Thy Starspot

Starspots confound exoplanet transit spectroscopy and a concept for mitigation.
Includes content copied verbatim from white paper "Understanding Stellar Contamination in Exoplanet
Transmission Spectra as an Essential Step in
Small Planet Characterization" led by Daniel Apai, available at http://sites.nationalacademies.org/SSB/CurrentProjects/SSB_180659

Transcript

1. Starspots confound exoplanet transit spectroscopy, and a concept for mitigation Michael Gully-Santiago Kepler/K2 Guest Observer Office NASA Ames Research Center Bay Area Exoplanets Meeting March 2018
2. Know thy planet, know thy starspots Michael Gully-Santiago Kepler/K2 Guest Observer Office NASA Ames Research Center Bay Area Exoplanets Meeting March 2018
3. Star Planet Know thy planet, know thy star A conference in Pasadena last October.
4. Star Planet Know thy planet, know thy star A conference in Pasadena last October.
5. Star Planet Know thy planet, know thy star A conference in Pasadena last October.
6. Star Planet Know thy planet, know thy star A conference in Pasadena last October.
7. Star Planet Know thy planet, know thy star A conference in Pasadena last October. Starspots
8. Starspots Star Planet Know thy planet, know thy star A conference in Pasadena last October. Know thy starspot, know thy star
9. Starspots Star Planet Know thy planet, know thy star A conference in Pasadena last October. Know thy starspot, know thy star
10. Starspots Star Planet Know thy planet, know thy star A conference in Pasadena last October. Know thy starspot, know thy star knowthystarspot.github.io
11. Starspots Star Planet Know thy planet, know thy star A conference in Pasadena last October. Know thy starspot, know thy star A Splinter Session at #CoolStars20 in Boston Aug.2018
12. Starspots Star Planet Know thy planet, know thy star A conference in Pasadena last October. Know thy starspot, know thy star A Splinter Session at #CoolStars20 in Boston Aug.2018 Know thy planet, know thy starspots This talk
13. also: Rackham et al. 2017, 2018
14. What spectrum do you measure when a planet transits your star?
15. ⌦ = (R⇤/d)2 ⌦ = (R⇤/d)2 Solid angle (R⇤/d)2 stellar radius /d)2distance S emergent spectrum per solid angle f = ⌦ · S What spectrum do you measure when a planet transits your star? Out of transit:
16. f = (⌦⇤ ⌦p) · S ⌦ = (R⇤/d)2 ⌦ = (R⇤/d)2 Solid angle (R⇤/d)2 stellar radius /d)2distance S emergent spectrum per solid angle What spectrum do you measure when a planet transits your star? In transit: Rp
17. ⌦ = (R⇤/d)2 What spectrum do you measure when a planet transits your star? Rp f ,out f ,in f ,out = ⌦⇤·S (⌦⇤ ⌦p)·S ⌦⇤·S
18. ⌦ = (R⇤/d)2 What spectrum do you measure when a planet transits your star? Rp f ,out f ,in f ,out = ⌦p ⌦⇤ The fractional difference of flux in- and out- of transit is the ratio of areas. = Ap A⇤
19. ⌦ = (R⇤/d)2 What spectrum do you measure when a planet transits your star? Rp If your planet has an atmosphere, you can measure the wavelength-dependent atmospheric transmission. = Ap( ) A⇤ = R2 p ( ) R2 ⇤
20. What spectrum do you measure when a planet transits your ^star? spotted
21. What spectrum do you measure when a planet transits your ^star? Out of transit: spotted S ,spot Two distinct emission components: - ambient photosphere - starspot S ,amb f = ⌦amb · S ,amb + ⌦spot · S f = ⌦amb · S ,amb + ⌦spot · S ,amb f = ⌦amb · S ,amb + ⌦spot · S ,spot
22. What spectrum do you measure when a planet transits your ^star? In transit: spotted S ,spot Assume: Planet occults ambient photosphere, misses the starspot S ,amb f = ⌦amb · S ,amb + ⌦spot · S f = ⌦amb · S ,amb + ⌦spot · S ,amb f = (⌦amb ⌦p) · S ,amb + ⌦spot · S ,spot ⌦p
23. What spectrum do you measure when a planet transits your ^star? spotted S ,spot Assume: Planet occults ambient photosphere, misses the starspot S ,amb f = ⌦amb · S ,amb + ⌦spot · S f = ⌦amb · S ,amb + ⌦spot · S ,amb f ,out f ,in f ,out 6= ⌦p ⌦⇤ ! ⌦p
24. What spectrum do you measure when a planet transits your ^star? spotted S ,spot Assume: Planet occults ambient photosphere, misses the starspot S ,amb f = ⌦amb · S ,amb + ⌦spot · S f = ⌦amb · S ,amb + ⌦spot · S ,amb ⌦p f ,out f ,in f ,out = ⌦p ⌦amb + ⌦spot S ,spot S ,amb
25. What spectrum do you measure when a planet transits your ^star? spotted S ,spot Assume: Planet occults ambient photosphere, misses the starspot S ,amb f = ⌦amb · S ,amb + ⌦spot · S f = ⌦amb · S ,amb + ⌦spot · S ,amb ⌦p The planet transit spectrum cannot be trivially inverted without further assumptions about the starspot size and emergent spectrum.
26. Pre-transit Stellar Disk is the Assumed Light Source Actual Light Source is the Chord Defined by the Planet’s Projection The Transit Light Source Effect Spectral Difference due to Different Spot/Faculae Contributions Contaminates Transit Spectrum Observed Transit Spectrum True Planetary Spectrum
27. Simulation of TLSE with gray (featureless) spectra GJ1214-like photosphere assumptions, 3.2% coverage fraction of spots Custom modifications to Starfish- Czekala et al. 2015, Gully-Santiago et al. 2017 (Å)
28. Simulation of TLSE with gray (featureless) spectra Blocking the ambient photosphere blocks preferentially brighter flux, boosts fraction of starspot flux
29. Simulation of TLSE with gray (featureless) spectra The boosted starspot signal mimics the spectrum of water in an exoplanet atmosphere.
30. Expected spectrum from cloud free, 100x Solar metallicity, 1x Gj1214b insolation Morley et al. 2015
31. How large is a typical Transit Light Source Effect? Often larger than the planet signal itself. Rackham+ 2018
32. 1.Assume that starspots are small enough to ignore. 2.Assume the starspot spectrum is similar to the ambient spectrum. 3.Constrain the starspot area and temperature. 4.Measure the starspot area and temperature. 5.Design a point-and-stare mission with multi-epoch in- and out- of transit spectroscopy; model spots through rotation period. 6.Adapt Doppler tomography methods? What can be done to mitigate the effect of starspots on the inferred exoplanet transit spectrum?
33. Assume that starspots are small enough to ignore. f ,out f ,in f ,out = ⌦p ⌦amb + ⌦spot S ,spot S ,amb lim ⌦spot !0 f ,out f ,in f ,out = ⌦p ⌦⇤ What most folks do right now? How would you know the spots are small?
34. If your answer is "Kepler/K2/HAT/TESS" amplitude is small... many geometric effects confound Amplitude --> fspot Rackham+ 2018
35. Photometric amplitude If your answer is "Kepler/K2/HAT/TESS" amplitude is small... many geometric effects confound Amplitude --> fspot Rackham+ 2018
36. Circumpolar spots can be continuously in sight. Animation: Guo Zhen (KIAA)
37. Assume the starspot spectrum is similar to the ambient spectrum f ,out f ,in f ,out = ⌦p ⌦amb + ⌦spot S ,spot S ,amb f ,out f ,in f ,out = ⌦p ⌦⇤ Probably not justified based on expectations of starspot temperature contrasts. limS ,spot !S ,amb
38. Constrain the starspot area and temperature. We don't know the typical coverage fraction of spots as a function of stellar (SpT / Age / evolutionary state / rotation). It seems like we should know this, but geometrical factors confound many measurement techniques. Most starspot detection methods underestimate spot coverage.
39. Measure the starspot area and temperature (i.e. for the target star of interest). How?
40. How to measure starspot area and temperature: - Near-IR Interferometry (high fidelity, low scalability) Rottenbacher et al. 2016 - Zeeman Doppler Imaging (medium-high fidelity, low scalability) Donati et al. 2014 - Monochromatic lightcurve amplitudes (low fidelity, high scalability) Rebull et al. 2016ab, Douglas et al. 2017, #K2Clusters - Planet-transit spot modeling (medium fidelity, low scalability) Morris et al. 2017 - SED modeling (medium fidelity, medium scalability) Wolk and Walter 1996 - Lightcurve forward modeling (low fidelity, medium-low scalability) Notsu et al. 2013 - Polychromatic timeseries photometry (medium fidelity, high scalability) Grankin 1995, Grankin et al. 2007 - 2-component spectral modeling (medium fidelity, medium scalability) Neff, O'Neal, Saar 1995; Fang et al. 2016; Gully-Santiago et al. 2017 - 2-component time-resolved spectral modeling (high fidelity, low scalability) Gully-Santiago in progress w/ IGRINS, iSHELL - N-component spectral modeling, with N >2 (medium fidelity, low scalability) Not attempted AFAIK - Combinations of the above ( High fidelity, medium2 scalability) Gully-Santiago et al. 2017, this talk
41. How to measure starspot area and temperature? - Near-IR Interferometry (high fidelity, low scalability) Rottenbacher et al. 2016 - Zeeman Doppler Imaging (medium-high fidelity, low scalability) Donati et al. 2014 - Monochromatic lightcurve amplitudes (low fidelity, high scalability) Rebull et al. 2016ab, Douglas et al. 2017, #K2Clusters - Planet-transit spot modeling (medium fidelity, low scalability) Morris et al. 2017 - SED modeling (medium-low fidelity, medium scalability) Wolk and Walter 1996 - Lightcurve forward modeling (low fidelity, medium-low scalability) Notsu et al. 2013 - Polychromatic timeseries photometry (medium fidelity, high scalability) Grankin 1995, Grankin et al. 2007 - 2-component spectral modeling (medium fidelity, medium scalability) Neff, O'Neal, Saar 1995; Fang et al. 2016; Gully-Santiago et al. 2017 - 2-component time-resolved spectral modeling (high fidelity, low scalability) Gully-Santiago in progress w/ IGRINS, iSHELL - N-component spectral modeling, with N >2 (medium fidelity, low scalability) Not attempted AFAIK - Combinations of the above ( High fidelity, medium2 scalability) Gully-Santiago et al. 2017, this talk
42. How to measure starspot area and temperature? Gully-Santiago et al. 2017
43. Design a point-and-stare mission with multi-epoch in- and out- of transit spectroscopy; model spots through rotation period.
44. Adapt Doppler tomography methods? In principle disparate velocity components can isolate the origin of the signal. Difficult in practice for slow rotators.
45. Some level of stellar contamination should be expected for most, if not all stars. The stellar contamination can impact the spectral slope of the transmission spectrum (commonly used as a proxy of clouds and atmospheric particles) and may also introduce apparent atomic and molecular features The amplitude of the contamination is a complex function of the stellar heterogeneity and can range from negligible to levels that may overwhelm intrinsic planetary features. Summary
46. Some level of stellar contamination should be expected for most, if not all stars. The stellar contamination can impact the spectral slope of the transmission spectrum (commonly used as a proxy of clouds and atmospheric particles) and may also introduce apparent atomic and molecular features The amplitude of the contamination is a complex function of the stellar heterogeneity and can range from negligible to levels that may overwhelm intrinsic planetary features. Summary
47. Some level of stellar contamination should be expected for most, if not all stars. The stellar contamination can impact the spectral slope of the transmission spectrum (commonly used as a proxy of clouds and atmospheric particles) and may also introduce apparent atomic and molecular features The amplitude of the contamination is a complex function of the stellar heterogeneity and can range from negligible to levels that may overwhelm intrinsic planetary features. Summary
48. Correction methods based on stellar variability (photometric or spectroscopic) only probe the non-axisymmetric component of the heterogeneity. These methods possibly/likely underestimate stellar stellar contamination by factors of ∼2-10. Stellar contamination is expected to change on timescales of the stellar rotation and the starspot evolution. For rapidly rotating stars (e.g., TRAPPIST-1) stellar contamination will likely change even during a single transit
 The current understanding of the spatial distribution of the temperature/spectra over stellar disks is insufficient to provide a robust basis for correction methods Summary
49. Correction methods based on stellar variability (photometric or spectroscopic) only probe the non-axisymmetric component of the heterogeneity. These methods possibly/likely underestimate stellar stellar contamination by factors of ∼2-10. Stellar contamination is expected to change on timescales of the stellar rotation and the starspot evolution. For rapidly rotating stars (e.g., TRAPPIST-1) stellar contamination will likely change even during a single transit
 The current understanding of the spatial distribution of the temperature/spectra over stellar disks is insufficient to provide a robust basis for correction methods Summary
50. Correction methods based on stellar variability (photometric or spectroscopic) only probe the non-axisymmetric component of the heterogeneity. These methods possibly/likely underestimate stellar stellar contamination by factors of ∼2-10. Stellar contamination is expected to change on timescales of the stellar rotation and the starspot evolution. For rapidly rotating stars (e.g., TRAPPIST-1) stellar contamination will likely change even during a single transit
 The current understanding of the spatial distribution of the temperature/spectra over stellar disks is insufficient to provide a robust basis for correction methods Summary
51. K2 GO Cycle 6 Phase-2 proposals for C17, C18, C19 due on April 19, 2018. Must use targets already selected in Phase-1 https://keplerscience.arc.nasa.gov/k2-approved-programs.html Also: accepting applications for year-long K2 interns on a rolling basis Advert