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 2.8 - 5.2 Myr (68%)Naylor 2009, MNRAS, 399, 432

Low-mass isochrones 1 – 3 MyrModel dependent – precise, but inaccurateHillenbrand 1997, AJ, 113, 1733; Da Rio et al. 2010, ApJ, 722, 1092

Proplyd lifetimes <1.5 Myre.g. Clarke 2007, 376, 1350

Ejected runaways >2.5 MyrHoogerwerf et al. 2001, A&A, 365, 49But may not come from the ONC!

3 Myr 5 Myr

Absolute Age Constraints

ONC JHK Subaru

ONC: Da Rio et al. 2010, ApJ, 722, 1092

Siess isochrones

PM-selected1 Myr

310

ONC JHK Subaru

Mean = 6.42σ = 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?

t L-3/2 so (log t)=0.15-0.2 dex

(log L)Variability 0.030Distance 0.015Extinction 0.050Accretion 0.070Binarity 0.016/0.10

TOTAL 0.10/0.13

(log age)=0.43 dex

ONCPM members

(log age)=0.43

(log t)< (log age) - Uncertainties cannot explain spread

Reggiani et al. 2011, A&A, 534, A83

Estimated uncertaintiesFrom Reggiani et al. 2011

(unless they have been badly underestimated)

ONC observed age spread

t L-3/2 so (log t)=0.15-0.2 dex

(log L)Variability 0.030Distance 0.015Extinction 0.050Accretion 0.070Binarity 0.016/0.10

TOTAL 0.10/0.13

(log age)=0.43 dex

ONCPM members

(log age)=0.43

(log t)< (log age) - Uncertainties cannot explain spread

Reggiani et al. 2011, A&A, 534, A83

Estimated uncertaintiesFrom Reggiani et al. 2011

(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. 2011

10 Myr

1 Myr

[3.6]-[8.0]>0.7

“Old” stars, edge on disks? A few…

See Manara et al. 2013, arXiv: 1307.8118

Contamination by foreground? Unlikely

Av < 0.5

Jeffries, 2007, MNRAS 381, 1169ONC 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

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

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”

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 dex σ= 0.2 dex σ= 0.4 dexdiscs

no discs

Jeffries et al. 2011, MNRAS, 418, 1948

ONC: Megeath et al. 2011

10 Myr

1 Myr

[3.6]-[8.0]>0.7

Problem: Stars with and without discs have similar ages!

0.396.33No Discs

0.426.36Discs

(Log Age)

Mean Log Age (yr)

ONC

[3.6]-[8.0]

Null KS-Testp=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 Age Real Age Spread (dex)

Disk lifetime

Any age spread is limited to 0.14 dex at 99% confidenceDisc 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.

0.446.24No Discs

0.456.35Discs

(Log Age)

Mean Log Age (Myr)

Null KS-Testp=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 10-4 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

7Li + p 4He + 4He at 2.5x106 K

6Li destroyed at lower temperatures

Li gone in 10 Myr!

Siess isochrones

PM-selected

1310

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

Conclusions1. 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.