Ages and Age Spreads in The Orion Nebula Cluster Rob Jeffries: Keele University, UK Absolute Ages The HR diagram of the ONC Evidence for luminosity and age spreads

UMS evolution Myr (68%) Naylor 2009, MNRAS, 399, 432 Low-mass isochrones 1 – 3 Myr Model dependent – precise, but inaccurate Hillenbrand 1997, AJ, 113, 1733; Da Rio et al. 2010, ApJ, 722, 1092 Proplyd lifetimes <1.5 Myr e.g. Clarke 2007, 376, 1350 Ejected runaways >2.5 Myr Hoogerwerf et al. 2001, A&A, 365, 49 But may not come from the ONC! 3 Myr5 Myr Absolute Age Constraints

ONC JHK Subaru ONC: Da Rio et al. 2010, ApJ, 722, 1092 Siess isochrones PM-selected 1 Myr 3 10

ONC JHK Subaru ONC: Da Rio et al. 2010, ApJ, 722, 1092 Siess isochrones

ONC JHK Subaru Mean = 6.42 σ = 0.43 dex 5%95% 90% between 0.5 and 15 Myr Are these spreads in luminosity real? If so, do they imply large age spreads?

e.g. Huff & Stahler 2006, ApJ, 644, 355 Investigate star formation as a function of time, space, mass etc.

ONC JHK Subaru NGC 3603: Beccari et al. 2010, ApJ, 720, 1108 LH 95 in the LMC: Da Rio et al. 2010, 723, 166 Are these spreads in luminosity real? If so, do they imply large age spreads?

t  L -3/2 so  (log t)= dex  (log age)=0.43 dex ONC PM members  (log age)=0.43  (log t)<  (log age) - Uncertainties cannot explain spread Reggiani et al. 2011, A&A, 534, A83 Estimated uncertainties From Reggiani et al (unless they have been badly underestimated) ONC observed age spread

t  L -3/2 so  (log t)= dex  (log age)=0.43 dex ONC PM members  (log age)=0.43  (log t)<  (log age) - Uncertainties cannot explain spread Reggiani et al. 2011, A&A, 534, A83 Estimated uncertainties From Reggiani et al (unless they have been badly underestimated) ONC observed age spread

Slesnick et al. 2004, ApJ, 610, 1045 Extinction Problems? Move to the near-IR Same result – but perhaps some “old” objects are obscured by edge-on disks?

ONC: Megeath et al Myr 1 Myr [3.6]-[8.0]>0.7 “Old” stars, edge on disks? A few… See Manara et al. 2013, arXiv:

Contamination by foreground? Unlikely Av < 0.5

Jeffries, 2007, MNRAS 381, 1169 ONC JHK Subaru Sample is biased against “oldest” objects Find a sample with rotation period P and v sini R sini/R  = 0.02 (P/days) (v sini/km/s)

ONC JHK Subaru Rsini normalised to R at 3 Myr Excellent age discrimination at < 10 Myr

ONC JHK Subaru Excellent age discrimination at < 10 Myr

ONC JHK Subaru ONC: Jeffries 2007, MNRAS, 381, 1169 Spread in R of  2-3 FWHM: agrees with L spread dex

ONC Results Distribution of Rsin(i) is consistent with luminosity profile from H-R diagram for these stars. Radius at a given Teff varies by factors of 2-3 If interpreted as an age spread then the ONC is not coeval and the extent of age spreads broadly agrees with the HR diagram.

Are the luminosity spreads real…? Assessment of confounding uncertainties suggests so. Spread in stellar radii suggests so. Possible issues with some very low luminosity objects perhaps viewed in scattered light So overall…. Yes! But does this imply an age spread? We need independent clocks

Da Rio et al (2010)Cieza et al (2007) Older, smaller, No disks, fast Younger, larger, disks, slow Period (d) Young, (luminous) stars should rotate more slowly Rotation and disks as clocks

Rotation as a clock Henderson & Stassun 2012, ApJ, 747, 51 Age dependent slope in period- mass relation? 2 Myr 4-5 Myr 150 Myr Period Mass

Problem: Stars appear to spin down as they contract! Divide stars into “young” (luminous) and “old” (faint) subsets. Littlefair et al. 2011, MNRAS, 413, L56 Luminous /” Young”(?) Faint/”Old”(?) Period (days) ONC N2264 N2362 Cep OB3b

Stars with periods in Herbst et al. 2002, A&A, 396,513 1 Myr 10 Myr

No change in period-mass relationship with “age”

Period (d) No great change in period distribution with “age”

Disc presence as an independent clock? Hernandez et al. 2008, ApJ, 686, 1195 Based on Spitzer data

Disc presence as an independent clock e.g. Toy Model with Increasing real age spread Increasing REAL age spread within a cluster should bring differences in the age distributions of stars with and without discs Observed Log (Age/yr) σ= 0.0 dexσ= 0.2 dexσ= 0.4 dex discs no discs Jeffries et al. 2011, MNRAS, 418, 1948

ONC: Megeath et al Myr 1 Myr [3.6]-[8.0]>0.7 Problem: Stars with and without discs have similar ages! No Discs Discs  (Log Age) Mean Log Age (yr) ONC [3.6]-[8.0] Null KS-Test p=0.51 Jeffries et al. 2011, MNRAS, 418, 1948

Hernandez et al. 2008, ApJ, 686, 1195 Constant disk frequency with age? The Mean disk frequency, is as expected for a mean age of 2-3Myr… But why is it not age dependent?

Observed AgeReal Age Spread (dex) Disk lifetime Any age spread is limited to  0.14 dex at 99% confidence Disc lifetime is (6  1) Myr, constrained by mean age and fraction of stars with discs Model: Assume Gaussian dispersion in log Age and exponential disk decay Match: Disk fraction and “age” distributions of stars with and without disks Conclusion: Real age spread < Median disk lifetime Real age spread (dex) Age spread σ< 0.15 dex Jeffries et al. 2011, MNRAS, 418, 1948

[3.6]-[8.0]>0.7 Could this be affected by ONC foreground contamination? No No Discs Discs  (Log Age) Mean Log Age (Myr) Null KS-Test p=0.78 Only stars with Av >1

A possible solution – Early “Cold” Accretion? Baraffe et al. 2009, ApJ, 702, L27; 2012, ApJ, 756, 118 Class I stars accreting at M  /yr in short bursts. At 1 Myr, stars have much smaller radii and lower L than non- accreting model of same final mass. Hence APPEAR older than 10 Myr! (and may also have depleted Li) Position of stars at 1 Myr after episodic accretion 10Myr

ONC JHK Subaru Lithium depletion in PMS stars Halted by growing radiative core 7 Li + p 4 He + 4 He at 2.5x10 6 K 6 Li destroyed at lower temperatures Li gone in 10 Myr!

Siess isochrones PM-selected Palla et al. 2007, ApJ, 659, L41 Li depleted objects?

Little sign of strong Li depletion or correlation with “age” Sergison et al. 2013, MNRAS, 434, 966 Palla objects Model isochrones (veiling-corrected )

Sergison et al. 2013, MNRAS, 434, 966 Expected Li depletion Appears to rule out cold accretion as a major source of HRD scatter? Limit on observed depletion ONC Only models 4-8 result in big shifts in the HRD

Conclusions 1.Absolute ages uncertain by factors of 2 2.Luminosity and Radius spreads in young clusters are likely REAL. 3.However, they probably DON’T imply real age spreads of 10 Myr. 4.HR diagram uncertainties are hugely underestimated or something scrambles the HR diagram. 5.Either way, SFR histories and PMS ages (<10 Myr) seem UNRELIABLE. Conclusions 1.Absolute ages uncertain by factors of 2 2.Luminosity and Radius spreads in young clusters are likely REAL. 3.However, they probably DON’T imply real age spreads of 10 Myr. 4.HR diagram uncertainties are hugely underestimated or something scrambles the HR diagram. 5.Either way, SFR histories and PMS ages (<10 Myr) seem UNRELIABLE.