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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. Absolute Age Constraints. 3 Myr. 5 Myr. UMS evolution 2.8 - 5.2 Myr (68%) Naylor 2009, MNRAS, 399, 432 - PowerPoint PPT Presentation
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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.