New star formation scenarios in stellar systems

1. New star formation scenarios in stellar systems Paolo Ventura INAF - Osservatorio Astronomico di Roma - Italy

2. An essential tool to understand how stars form and evolve is the study of young stellar systems, populated by stars still in the Pre-Main-Sequence (PMS) phase These studies have been traditionally limited to the Milky Way, but the advent of HST made it possible to tackle the open questions about star formation in outer Galaxies Gilmozzi et al. (1994) identified the first population of PMS in NGC 1850 (LMC), followed by Romaniello et al (2006, SN 1987A field), and Romaniello et al. (2006, 30 Doradus region) These preliminary investigations suggest significant differences between the star formation processes in the Galaxy and the LMC

3. Cignoni et al. (2009, AJ, 137, 3668) tried to understand the possible star formation history in NGC 602 (SMC) PMS

5. Convection is traditionally described by means of a parametric approach, with the introduction of a free parameter (a) associated to the efficiency of convection To fit the radius of the Sun we need a=1.9 A more efficient treatment of convection favours smaller radii, thus hotter PMS tracks The shift of the track in the HR diagram is mass dependent!

6. Landin et al. (2006, A&A, 456, 269) found the interpretation of the observed locii of the HR diagram of the Orion Nebula to be extremely dependent on the description of convection

7. Hillenbrand & White (2004, ApJ 604, 741) compared the masses (determined from measured orbital dynamics and spectral and photometric information) of 150 stars with the theoretical values obtained by using tracks from various groups… * Good agreement for M > 1.2Msun * Systematic underestimate of the mass for M < Msun

8. Hillenbrand et al. (2007): not only the various sets of tracks predict different ages for the young stellar associations… The ages assigned depend on the ranges of masses considered!

9. The slope of the expected sequence on the HR diagram varies according to the set of tracks adopted (Hillenbrand et al. 2007) … Predictions regarding the fraction of binaries expected completely different

10. A further observational test for the PMS tracks is their capability to reproduce the surface lithium abundances patterns observed in open cluster stars, that show a strong increase of surface lithium depletion when the effective temperature decreases Pleaides Hyades M

11. The extent of the depletion of surface lithium is strongly linked with the location of the PMS track on the HR diagram (D’Antona & Montalban 2003)

12. The PMS models in which convection is modelled with the same efficiency required to fit the radius of the Sun deplete too much lithium (D’Antona & Montalban 2003) Only low-efficiency convective models (a=1) can reproduce the observations! Is it a metallicity effect?

13. Sestito et al. (2006, A&A 454, 311) studied the effects of changing the metallicity of the models and the individual C, N and O abundances on the extent of the depletion of lithium, and found …

14. Warnings * The comparison of the masses of PMS stars derived observationally and from the tracks indicate that in the cool part of the HR diagram a smaller efficiency of convection is needed, compared to the average efficiency demanded by the solar fit * The lithium vs. Teff trend support this conclusion * Other physical processes (rotation, magnetic fields) seem to affect the efficiency of convection in late-type stars * PMS tracks must be used with particular care when intepreting the popolation of young stellar systems!

15. The meridional circulation velocity becomes more and more negative due to the Gratton-Opik term -W2/2pgr U(r)~(EW+En) Massive stars of solar metallicity develop an angular velocity gradient as angular momentum is lost from the surface (Meynet & Mader 2000, A&A 361, 101)

16. Meynet & Maeder (2002, A&A, 390, 561) a) At lower metallicities much less angular momentum is removed from the star b) The meridional circulation velocity is much smaller, because low Z models are more compact, thus the impact of the Gratton-Opik term is reduced Points (a) and (b) favours a higher angular velocity in the star, and a much steeper gradient from the core to the surface  enhanced mixing !!

19. Stellar rotation in low Z models leads to the production of primary nitrogen in agreement with those observed

20. In popIII stars, with Z=0, the transition from H- to He- burning is smooth: rotational mixing makes carbon produced in the core to diffuse outwards into the H-burning shell, triggering a “boost” (Ekstrom et al. 2008, A&A, 489, 685) Core He-burning phase After boost phase Boost

21. The meridional circulation is expected to be very slow in popIII models, even more than in low-Z stars (Gratton- Opik term in the expression for U(r)) The U(r) is so small that angular momentum is almost conserved, so that the star spins down as a consequence of the expansion (Ekstrom et al. 2008)

22. The trend of primary 14N production with metallicity is reversed at very small Z PopIII stars, with no CNO, burn helium in the blue part of the HR diagram, when the gradient of the angular velocity is shallower, thus triggering much less mixing

23. Spite et al. (2005, A&A, 430,655) Israelian et al. (2004, A&A, 421, 649) Spite et al. 2006, A&A, 455, 291

24. In post-main sequence stars, magnetic torques couple the rapidly-rotatong core and the slowly-rotating envelope, producing a spin-down of the core (Suijs et al. 2008, A&A 481, L87) ZAMS XC=0.25 Start He-burn YC=0.4 Start AGB 5th TP The main drain of angular momentum occurs between core H exhaustion and core-He ignition: the total loss is more than a factor of 100!

25. Including the effects of magnetic torques leads to a considerable lowering of the angular momentum of the stars Neutron stars Asteroseismic measurements from ZZ Ceti stars Magnetic WDs

26. Oxygen vs. Sodium(Carretta et al. 2006 A&A 450, 523) RGB stars (many clusters) NGC 2808 RGB stars TO & SGB stars (Gratton et al. 2001) ?

27. Blue tails and lack of stars in the RR Lyr gap of NGC 2808 Bedin et al. (2000, A&A 363, 159) ...another cluster showing Na-O anticorrelation (Carretta et al. 2004,2005)

28. The photometric analysis of the Bologna & Padova groups lead to the discovery of multiple main sequences in massive Globular Clusters The detailed analysis of MS stars of NGC 2808 evidentiated the presence of at least three MS, differing in the helium content (Piotto et al. 2007, ApJ 661, L53)

29. The observations indicate the presence in some Globular Clusters of a stellar population enriched in helium, and with a chemistry showing the signature of CNO processing. How do we explain the existence of multiple populations in Globular Clusters? Winds from massive stars during the core H-burning phase (Decressin et al. 2007, A&A 574, 859) Winds from intermediate mass stars during the AGB phase (D’Ercole et al. 2008, MNRAS, 391, 825)

30. Decressin et al. (2007, A&A 574, 859) Break-up velocities during the core H-burning, to allow strong rotational mixing & injection into the interstellar medium of low-velocity gas (disc?) Angular momentum loss favours rotation to slow down, so that during the core He-burning phase matter is ejected via high-velocity winds, that escape from the cluster

31. When convection (again!) is modelled efficiently, massive AGBs achieve an advanced nucleosynthesis at the bottom of their envelope, in agreement with the chemical patterns observed in GC stars This gas is ejected into the interstellar medium via low-velocity winds, that remain inside the cluster. From here new stars form, with the “appropriate chemistry” How do we explain the high percentage of second generation stars? Age

32. Second generation stars form in the central regions Radiative cooling: gas moves to the centre of the GC Star formation ends after ~ 50Myr: only massive AGBs winds

33. to explain at the same time the HB, we require THREE different populations: Y=.24 Y=0.30 Y=0.38 AND... Carretta et al. 2005 find three different classes of objects in O-Na anticorr.