Measuring fundamental properties of young stars

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Measuring fundamental properties of young stars Michael Gully-Santiago postdoctoral work at Kavli Institute for Astronomy & Astrophysics Friday, November 18, 2016 at Columbia University 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, ++

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We want to know the ages and masses of young stars so that we can understand how star and planet formation proceed in time.

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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.

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Ages are estimated by placing a young star on a pre-main sequence HR diagram.

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Ages are estimated by placing a young star on a pre-main sequence HR diagram.

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Observed star clusters (~1 Myr) show large age spreads of 1-10 Myr. Why?

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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

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Starspots inhibit convective efficiency. Somers & Pinnsoneault 2015 Less efficient energy transport means stars get larger (R increases) but cooler (T decreases) Postdoc at Vanderbilt

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Starspots confound measurements of L and Teff. Tamb=Teff 0th order assumption No starspots Tamb

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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

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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

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Starspots confound measurements of L and Teff. Tamb Tspot fspot 1-fspot My Research - We directly detect the spectrum arising from starspots. - That should probably surprise you... T4 is steep! - Not only that, but starspots are on Wien side of BB curve. - We benefit from moving to the infrared and high res. - Still: You need large covering fraction of spots to make up for T4

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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

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IGRINS: Immersion Grating Infrared Spectrograph Park et al. 2014 - R = λ/δλ = 45,000 - Δλ = 1.4 - 2.4 μm - 2.7 m HJST at McDonald Observatory (*now 4.3 m DCT at Lowell Observatory) - Single slit echelle spectrograph: ~28 H-band orders and 25 K-band orders Silicon Immersion Grating (diffraction grating) Gully-Santiago et al. 2012

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~28 H-band orders and 25 K-band orders

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LkCa 4 is an ideal target for detecting starspot emission. Vrba et al. 1993 1. Associated with nearby (~140 pc) Taurus young (~1 Myr) star cluster 2. No mid-IR to sub-mm excess that would indicate a circumstellar disk 3. Weak-lined T-Tauri Star (no ongoing accretion based on UV excess). 4. No evidence for a nearby companion from AO imaging, and spec. monitoring 5. Large amplitude of photometric variability 6. Availability of >20 years of polychromatic photometric monitoring 7. Recent spectropolarimetric tomography Hartigan+ 1995, Andrews & Williams 2005, Edwards+ 2006, Kraus+ 2011, Nguyen+ 2012, Donati+ 2014, Grankin+ 2008

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LkCa 4 is an ideal target for detecting starspot emission. Vrba et al. 1993 1. Associated with nearby (~140 pc) Taurus young (~1 Myr) 2. No mid-IR to sub-mm excess that would indicate a circumstellar disk 3. Weak-lined T-Tauri Star (no ongoing accretion based on UV excess). 4. No evidence for a nearby companion from AO imaging, and spec. monitoring 5. Large amplitude of photometric variability 6. Availability of >20 years of polychromatic photometric monitoring 7. Recent spectropolarimetric tomography Hartigan+ 1995, Andrews & Williams 2005, Edwards+ 2006, Kraus+ 2011, Nguyen+ 2012, Donati+ 2014, Grankin+ 2008 LkCa 4 spectrum should be devoid of complicating factors, and should have a large starspot signal in its spectrum.

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LkCa 4 spectrum should be devoid of complicating factors, and should have a large starspot signal in its spectrum. How to figure out which lines are attributable to starspots or ambient photosphere? portion of LkCa 4 IGRINS spectrum from November 2015

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We forward model the IGRINS spectra. 0.0 0.2 0.4 0.6 0.8 1.0 raw 5164 5165 5166 5167 5168 5169 5170 [˚ A] 0.0 0.2 0.4 0.6 0.8 1.0 convolved and resampled f ⇥ 107 [erg cm 2 s 1 ˚ A 1 ] Synthetic spectra from pre-computed PHOENIX model grids in Teff, logg, [Fe/H] rameter space of the grid. Variable Range Step size Teﬀ [K] 2300–7000 100 7000–12 000 200 log g 0.0–+6.0 0.5 [Fe/H] −4.0−−2.0 1.0 –2.0–+1.0 0.5 [α/Fe] –0.2–+1.2 0.2 ha element abundances [α/Fe] 0 are only available for eﬀ ≤ 8000 K and −3 ≤ [Fe/H] ≤ 0. mpling of the spectra in the grid. Range [Å] Sampling 500–3000 ∆λ = 0.1Å 3000–25 000 R ≈ 500 000 25 000–55 000 R ≈ 100 000 Husser et al. 2013 Czekala et al. 2015

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We forward model the IGRINS spectra. Starfish takes into account the uncertainty introduced by discrete models. Czekala et al. 2015 Emulator Eigenspectra modified by extrinsic parameters emulator covariance matrix Gaussian process models eigenspectra weights as function of reconstruction of mean model spectrum delivers probability distribution of weights as function of

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We forward model the IGRINS spectra. Starfish is an open source spectral inference framework for stellar spectra. github.com/iancze/Starfish

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We forward model the IGRINS spectra. Czekala et al. 2015 0.6 1.2 1.8 2.4 data model 5140 5150 5160 5170 5180 5190 5200 [˚ A] 0.5 0.0 0.5 residuals f ⇥ 10 13 [erg cm 2 s 1 ˚ A 1 ] Starfish parameters: 1. Teff 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 fits for all stellar and nuisance parameters simultaneously.

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We forward model the IGRINS spectra. Czekala et al. 2015 0.6 1.2 1.8 2.4 data model 5140 5150 5160 5170 5180 5190 5200 [˚ A] 0.5 0.0 0.5 residuals f ⇥ 10 13 [erg cm 2 s 1 ˚ A 1 ] 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 fits for all stellar and nuisance parameters simultaneously. Intrinsic Extrinsic Nuisance Starspots

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We forward model the IGRINS spectra. 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 Tspot = 2800 K Tamb = 4100 K Ambient Starspot

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We forward model the IGRINS spectra. Starfish is an open source spectral inference framework for stellar spectra. 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

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The spectrum has features from both ambient photosphere and starspots. λ (Angstrom) The constraint on filling factor comes from the range of flux ratios.

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Each spectral order yields an estimate for Tamb, Tspot, fspot The models provide a range of credibility, with some orders more informative than others.

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The data are most consistent with Tamb = 4100 K, Tspot = 2750 K, fspot = 0.8

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fspot = 80%?! That means 1-fspot = 0.2! Wait, isn't that actually a hotspot? All previous optical measurements of LkCa 4 mark the 4100 K component as photosphere.

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Photometric modulation only probes longitudinally asymmetric spots. ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2 LkCa 4 light curve

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You can find a minimum coverage of starspots for LkCa 4 ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2

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You can find a minimum coverage of starspots for LkCa 4 ΔV LkCa 4 ΔV 2015: 0.5 2004: 0.8 1986: 0.2

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LkCa 4 in the HR diagram - Teff 4100 K --> ~3300 - 3500 K depending on adopted parameters - Inferred LkCa 4 mass decreases by 2-3x, assuming same tracks** - Inferred LkCa 4 age decreases by ~2x, assuming same tracks** - **assuming same stellar evolutionary tracks does not make sense-- we have just shown that this source has a much larger opacity source than what is assumed in the Baraffe et al. 2015 tracks.

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Big Picture / Why does this matter? 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.

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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

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Conclusions - We have measured a large covering fraction of starspots on the surface of the large-amplitude variable WTTS LkCa 4. - Our technique employs forward modeling IGRINS spectra. - Recent results (Fang+2016, Roettenbacher+2016, Covey+ 2016) suggest that large / polar starspots could be common. - Estimates of masses and ages of stars have probably been systematically biased, but more work is needed

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Extras

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Single component Two components Whole spectrum fitting Czekala et al. 2015. + probabilistic - slow mixing Sampling issue. Chunking order- by-order What I originally did. Robust against systematics, but heuristic. Amount of spectrum fit at once. Sampling method. I have altered the Czekala et al. 2015 spectroscopic framework.

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Sampling issue: How to sample with strongly correlated parameters in many dimensions, without emcee?

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A fit to a single IGRINS spectral order: m = 85 + nuisances Before: 50,000 samples After: 40 x 4,000 samples

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A fit to a single IGRINS spectral order: m = 85 + nuisances Before: 50,000 samples After: 40 x 4,000 samples

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A fit to a single IGRINS spectral order: m = 85 + nuisances Before: 50,000 samples After: 40 x 4,000 samples

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A fit to a single IGRINS spectral order: m = 85 Before: 50,000 samples After: 40 x 4,000 samples + nuisances

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Starspots confound measurements of L and Teff. Tamb Tspot fspot 1-fspot Tspot > 0 K

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IGRINS has high throughput. VPH Immersion grating KECK+ NIRSPEC, S/N 80 in 16 min HJST + IGRINS, S/N 140 in 40 min G. Mace Gully-Santiago et al. 2012 Gully-Santiago et al. unpublished

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We could look for known, clean, temperature-sensitive lines. O'Neal & Neff 1997

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We could look for known, clean, temperature-sensitive lines. O'Neal & Neff 1997

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All of the so6ware development is done in the open.

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The spectrum has features from both ambient photosphere and starspots. But - The gross appearance is dominated by temperature variation - Large bandwidth offers resilience We expect the model fits will be imperfect: - Bad oscillator strengths - Zeeman splitting - Assumptions about shared extrinsic properties. - Starspots will probe higher pressure regions, mimicking logg effects

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LkCa 4 has photometric monitoring going back 31 years. May 6, 1985 Nov. 16, 2016

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P = 3.375 days The LkCa 4 IGRINS spectrum was acquired somewhere near the middle of its variability.

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P = 3.375 days There is multi-epoch spectropolarimetry data from ESPaDOnS: High resolution optical echelle spectrograph on CFHT.

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P = 3.375 days We examined the spectral energy distribution (SED) at the 2MASS, DBLSpec, and TripleSpec epochs

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LkCa 4 varies between ~74-86% coverage fraction of cool spots. - We can scale V magnitude to spot coverage, assuming the spot temperature is constant. Some starspots on the stellar surface always face the observer. This geometry can arise from polar starspots. - Safe to assume all of the V-band flux comes from the ambient photosphere.

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Spectral Energy Distribution assuming Tamb = 4100 K, Tspot = 2750 K, fspot = f2MASS Consistent with large coverage fraction of starspots

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Flux-calibrated, near-contemporaneous low-res optical and near-IR data from DoubleSpec and TripleSpec. Consistent with large coverage fraction of starspots

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ESPaDOnS tomographic modeling provides a surface brightness map. Donati et al. 2014 There is evidence for polar spots. But tomography is only sensitive to large features; small features can "hide", biasing the coverage fraction estimates.

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Observed TiO lines are consistent with large coverage fraction of starspots. 12 12.5 13 13.5 1.25 1.3 1.35 1.4 1.45 1.5 1.55 Vmag V−R (mag) Observed V-R is consistent with large coverage fraction of starspots. Further evidence for large coverage fraction of spots on LkCa 4

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Is LkCa 4 merely an extreme source? K2 Cycle 2 light curves for 1658 candidate or conﬁrmed young stars towards Oph/Sco. compared to everything else in that Cycle. (Young stars are usually more variable everything else.) -‐ InterquarKle Range (IQR: Q3-‐Q1) -‐ Standard DeviaKon (σ). (IQR vs. σ separates bursty and smooth lightcurve morphologies.) LkCa 4

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Recent evidence for large spot coverage in Pleiades 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 - Tamb fixed from V-I - TiO band index scale from many inactive dwarfs - Tspot taken as value that minimizes fspot - Evidence for trends with Rossby number, Tamb and Tspot Fang et al. 2016 arXiv:1608.05452 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2500 3000 3500 4000 4500 fs Ts (K) 5118 K 4722 K 4224 K Tq = 3609 K + 50 K + 100 K - 50 K - 100 K 3399 K PELS 162 HII 1883 HII 335 HCG 101 HCG 219

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APOGEE spectra of thousands of young stars show large disagreement Cottaar et al. 2014 Un-accounted for starspots are probably responsible for systematic differences in stellar properties derived between the optical and near-IR