New star formation scenarios in stellar

systems

Paolo Ventura

INAF - Osservatorio Astronomico di Roma - Italy

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) phaseThese 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

GalaxiesGilmozzi et al. (1994) identified the first

population ofPMS 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

significantdifferences between the star formation

processes inthe Galaxy and the LMC

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

PMS

A more efficient treatment of convection favours smaller radii, thus hotter PMS tracks

The shift of thetrack in the HRdiagram is massdependent!

To fit the radiusof the Sun we need=1.9

Convection is traditionally described by means of a parametric approach, with the introduction of a free parameter () associated to the efficiency of convection

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

Hillenbrand & White (2004, ApJ 604, 741) compared themasses (determined from measured orbital dynamics andspectral and photometric information) of 150 stars withthe theoretical values obtained by using tracks fromvarious groups…

* Good agreement for M > 1.2Msun

* Systematic underestimate of the mass for M < Msun

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 massesconsidered!

The slope of the expected sequence on the HR diagramvaries according to the set of tracks adopted (Hillenbrand et al. 2007) …

Predictionsregarding thefraction of binaries expectedcompletely different

A further observational test for the PMS tracks istheir capability to reproduce the surface lithiumabundances patterns observed in open cluster stars,that show a strong increase of surface lithium depletionwhen the effective temperature decreases

M

Pleaides

Hyades

The extent of the depletion of surface lithium is stronglylinked with the location of the PMS track on the HRdiagram (D’Antona & Montalban 2003)

The PMS models in which convection is modelled with thesame efficiency required to fit the radius of the Sundeplete too much lithium (D’Antona & Montalban 2003)

Only low-efficiencyconvective models(=1) can reproducethe observations!

Is it a metallicityeffect?

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

* 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!

Warnings

Massive stars of solar metallicity develop an angular velocity gradient as angularmomentum is lost from thesurface (Meynet & Mader2000, A&A 361, 101)

The meridional circulationvelocity becomes more andmore negative due to theGratton-Opik term

U(r)~(E+E)-2/2g

Meynet & Maeder (2002, A&A, 390, 561)

a) At lower metallicities much less angular momentum is removed from the starb) 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 thestar, and a much steeper gradient from the core to thesurface enhanced mixing !!

Stellar rotation in low Z models leads to the productionof primary nitrogen in agreement with those observed

Core He-burning phase Boost After boost

phase

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

The meridional circulation is expected to be very slow inpopIII 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 almostconserved, so that the star spins down as a consequenceof the expansion (Ekstrom et al. 2008)

The trend of primary 14N production with metallicity isreversed 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

Spite et al. (2005, A&A, 430,655)

Israelian et al. (2004,A&A, 421, 649)

Spite et al. 2006, A&A, 455, 291

In post-main sequence stars, magnetic torques couplethe rapidly-rotatong core and the slowly-rotating envelope,producing a spin-down of the core (Suijs et al. 2008, A&A481, 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!

Asteroseismic measurementsfrom ZZ Ceti stars

Neutron stars

Magnetic WDs

Including the effects of magnetic torques leads to aconsiderable lowering of the angular momentum of the stars

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)

?

Bedin et al. (2000, A&A 363, 159)

Blue tails and lack of stars in the RR Lyr gap of NGC

2808...another cluster showing Na-O anticorrelation(Carretta et al. 2004,2005)

The photometric analysis of the Bologna & Padova groups lead to the discovery of multiple main sequences in massive

The detailed analysisof MS stars of NGC2808 evidentiated thepresence of at least three MS, differingin the helium content(Piotto et al. 2007,ApJ 661, L53)

Globular Clusters

How do we explain the existence of multiplepopulations in Globular Clusters?

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.

Winds from massivestars during the coreH-burning phase (Decressin et al. 2007, A&A 574, 859)

Winds from intermediatemass stars during theAGB phase (D’Ercole et al.2008, MNRAS, 391, 825)

Decressin et al. (2007, A&A 574, 859)

Break-up velocities duringthe core H-burning, to allow strong rotational mixing& injection into the interstellarmedium of low-velocity gas(disc?)

Angular momentum loss favoursrotation to slow down, so thatduring the core He-burningphase matter is ejected viahigh-velocity winds, that escapefrom the cluster

When convection (again!) is modelled efficiently, massiveAGBs achieve an advanced nucleosynthesis at the bottomof their envelope, in agreement with the chemical patterns

observed in GC stars

This gas is ejected intothe interstellar mediumvia low-velocity winds,that remain inside thecluster. From here newstars form, with the“appropriate chemistry”

How do we explain thehigh percentage of second generation stars?

Age

Radiative cooling: gasmoves to the centre ofthe GC

Second generation stars form in the central regions

Star formation endsafter ~ 50Myr: onlymassive AGBs winds

Y=0.38Y=0.30

Y=.24

to explain at the same time the HB, we require THREE different populations:

AND... Carretta et al. 2005 find three different classes of objects in O-Na anticorr.