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Empirical Studies of the Secular Evolution of Pre-Main-Sequence Stars

Empirical Studies of the Secular Evolution of Pre-Main-Sequence Stars. nathan@astro.ex.ac.uk. Collaborators: Tim Naylor, Rob Jeffries, Stuart Littlefair and Ben Burningham. Papers: Mayne et al (2007), Mayne and Naylor (2008), Jeffries et al (2007), Naylor and Jeffries (2007).

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Empirical Studies of the Secular Evolution of Pre-Main-Sequence Stars

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  1. Empirical Studies of the Secular Evolution of Pre-Main-Sequence Stars nathan@astro.ex.ac.uk Collaborators: Tim Naylor, Rob Jeffries, Stuart Littlefair and Ben Burningham Papers: Mayne et al (2007), Mayne and Naylor (2008), Jeffries et al (2007), Naylor and Jeffries (2007) Background picture: http://universe.daylife.com/

  2. Structure • Problem • Theoretical, Timescales • Observations, ages • Isochrone fitting • Solution • 2 fitting • Empirical isochrones, age orders • Results • Local environment → Disc dissipation • Conclusions

  3. The Problem: Theory • Timescales: tform & tevolve • tform→ 2 Models, RSF and SSF (rapid- and slow-star-formation) • SSF, tform~10 Myrs, Shu (1977), Shu et al (1987). • RSF, tform~1-3 Myrs, Ballesteros-Paredes et al (1999), Hartmann (2001) • tevolve→ Jevolve (Gychronology, Barnes, 2003), ‘Skumanich’ winds • Winds insufficent, Herbst et al (2007) • Torque lock-disc? Camenzind (1990) and Edwards et al (1993), as pre-stellar collapse. • tevolve(J)~5 Myrs (Scholz et al, 2007), tevolve(disc)~5 Myrs (Haisch et al, 2000)….

  4. The Problem: Observables, assumptions? • Timescales → Ages • tform≈ Age spread • Constant SFR, one SF episode • Coeval ( Ori, Jeffries et al, 2006) • tevolve (disc), disc %=f(age) (Cieza & Baliber, 2007) • Initial distribution (e.g. IMF) constant • Environment • Heterogeneous foundation: • Models/Isochrones • Accretion effects (Tout et al, 1999, Siess et al, 1999) • Disc fractions (JHKL, Spitzer) • Local environment, O stars? (McCaughrean & O’Dell, 1996) (Hollenbach & Gorti, 2005)

  5. The Problem: Deriving Ages (isochrones) • Turn-on/contraction/Pre-MS • Model dependent • Age-Distance degenerate 1, 3, 5 &10 Myr isochrones and ZAMS. Blue=Pre-MS (Siess et al, 2000) . Red=MS (& post-MS) (Geneva)

  6. The Problem: Deriving Ages (isochrones) • MS? • ΔPos=slow F(age)

  7. The Problem: Deriving Ages (isochrones) • Thus, • dm=MS • Age=Pre-MS • Unsolved: • Goodness-of-fit? • 2, 2-D uncertainties & model • Extinction • Binaries • Spreads?

  8. The Solution: Distances, 2 2-D, generalised 2 Statistically robust uncertainties Models Binaries Minimise 2 Rigorous: Bolometric correction, colour-Teff, interpolate surface gravity, extinction vectors etc The ONC: 7.91<7.96<8.03, 391+12-9 pc

  9. Data + Theory → Evolution=Theory? Data + Data → Evolution=Theory?

  10. The Solution: Empirical isochrones, age orders Both: h and  Per (black), the ONC (red) & NGC2362 (green). NGC2264 (dashed, blue). Right hand: ZAMS subtract (minus colour of ZAMS).

  11. The Results: Rotation, tevolve(J) • Period dist (1 mass range) • IC348, NGC2264, NGC2362 and the ONC • ONC bi-modal • NGC2264 & NGC2362 uni-modal • Bi-modal → uni-modal (spin up) • IC348, bi-modal, older? Disc locking - Old SFR, more discs, less spin up → uni-modal - IC348 disc are destroyed later?

  12. The Results: tevolve(disc) • Disc Dissipation (add disc fractions) • IC348 disc %> NGC2362 & NGC2264 •  Ori,  Ori & NGC2264. Age=, disc % ≠ • Dissipation from O stars? • IC348 none. • NGC2362 13.

  13. Conclusions: • Parameters: • Precise distances (& E(B-V)). • Model dependency negligible (MS) • Pre-MS: • - Modeled pre-MS • Ages: • - Age ordered • - New ages for SFRs • Secular Evolution: • - Local environment effects? • - Disc-locking and tevolve(disc)

  14. Cep OB3b-younger 3 Myr (5.5Myr) IC348-older 4-5 Myr (3 Myr) The ONC-older 2 Myr (0.8 Myr) (distance)

  15. R-C gap: Observation Gap or terminus of Pre-MS - Noted by Stolte et al (2004) - Visible in CMDs e.g. Lyra et al (2006a)

  16. R-C gap: Theory • Phase change • Convective pre-MS → radiative core • Hayashi to Henyey track • Teff • CMD separation • Spreads → density, ‘gap’ • Size(gap)=F(age) Distance independent age indicator Theory from Siess and Dufour (2002) - Mass tracks (dotted lines) 0.8-7M☼ - Isochrones (blue lines) 1, 3, 4 and 13 Myrs and ZAMS - 1 and 3 M☼ (red dots, and lines)

  17. R-C gap: Overlap Lowest M on MS> highest M on pre-MS h and  Per (crosses) 13 Myr Geneva-Bessell isochrone, 13 and 23 Myr Siess and Dufour isochrone. The ONC (asterisks), Geneva-Bessell 1 Myr and Siess and Dufour 1, 3 and 10 Myrs. • Stars above turn-off (younger?) - Stars below turn-on (older?) • Isochronal age spread - Real (SSF) - Accretion history? (RSF)

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