Breakfast Talk:

Views on Star Formation

Current Views on Star Formation

Hamburg, 7 July, 2004

Francesco PallaINAF-Osservatorio di Arcetri, Firenze

1. From clouds to stars: a unified scenario2. Star Formation in clusters & associations: SF histories and accelerating SF3. Physical origin: slow vs fast mode of SF critical tests on age spreads4. Conclusions

Star formation withinStar formation withinmolecular cloud complexesmolecular cloud complexes

CO(1-0)CO(1-0)Taurus

13CO(1-0)

5 pc

distribution of moleculardistribution of moleculargas in the galactic planegas in the galactic plane

NN22HH++(1-0)(1-0)

M ~ 5 MM ~ 5 M

dense coredense core

Dame et al. 2001

dense coresdense cores

1pc

gravitationalgravitationalcollapsecollapse

10 000 AU

embeddedembeddedyoung staryoung star

t =104-10

5 yr

T Tauri starT Tauri star

jet

Accretion disk

100 AU

t =106-10

7 yr

main sequencemain sequencestarstar

planetarysystem

50 AU

t >107yr

L1551 binary jet

Star Formation: current views

Low & Intermediate mass stars followthe same route whether in clusters or isolated. Problems with initial conditions

Next step: from individual to global properties IMF & SFH

Empirical information from study of the distribution of gas & stars in nearby SFRs

SFH solar neighborhood

IC 348 - PerseusChamaeleon

SFH solar neighborhood

Rho Ophiuchi

interior regions

exterior

Λ Orionisassociation

interior: SF 0external: SF accel

Taurus-Auriga:SF in space & time

Distribution of stars

Distribution ofdense gas infilaments C18O

13CO

Turbolenza

Taurus-Auriga

accelerating

decelerat

SFH of clusters & associations

• SF starts at low rates and increases in time• SF accelerates with e-folding times t~1-3x106 yr both in clusters and associations

SF only occurs above threshold NH>N(HIH2)• Lack of stars with ages >107 yr: post-TTS are rare due to limited SF activity• Acceleration followed by rapid dissipation

(outflows, UV, X-rays…): OK for massive clusters, but problematic for T associations

Interpretation of SFH: different views on core formation/evolution

Case 1: Magnetically controlled quasi-static evolution

Cores form as a result of gradual loss of magnetic & turbulent support due to ambipolar diffusion: long timesscales (e.g. Ciolek & Basu 2003)

Magnetic Field Diffusion

tad

()in

<()in

vdrift

3-10 Myr

Interpretation of SFH: different views on core formation/evolution

Case 2: Turbulence controlled & dynamic

Cores are transient fluctuations induced by shock dissipation at flowintersections (e.g. Mac Low & Klessen 2003)

decay time <~ 1 Myr fresh turbulence SF active only for few crossing times

Turbulence

filaments & cores

Klessen 2003

Understanding SFH: how to decide?

Empirical information from:Dense core lifetimes HI in H2

indicates t≥1-2 Myr (Goldmsith et al.2004)

Dynamics of dense cores: velocitygradients inconsistent with shockdissipation (Galli et al. 2004)

Age spreads in clusters: Li-depletion as age diagnostic (Palla et al. 2004)

Ages of densecores in Taurus

1 Myr

Predictions on core shape and kinematics

Fiedler & Mouschovias (1993) Ballesteros-Paredes et al. (2003)

magnetically controlled turbulence driven

nz

nx= ny

vx

vy

vz

nz

ny

nx

vz

vx= vy

+0.2 km s-1

-0.2 km s-1

Age spreads: the lithium test

• SFRs are not the best sites: SF is taking place and will continue in the future age spreads are lower limits

• Test of the Lithium Depletion Boundary (LDB) in

young gas-free clusters, e.g. Orion Cluster, Upp Sco

intermediate age open clusters (~10-30 Myr): SF is

finished, isochrones are not too crowded, LDB in

stellar regime (not BDs as in Pleiades)

Orion Nebula Cluster: HRD & Lithium Depletion Region

full depletion

How to test the age spread? Using the lithium line at 6708 Ǻ as a diagnostic of depletion history

1 Myr

20 Myr

GTO VLT Flames/Giraffe: Pallavicini, Palla, Randich, Flaccomio

Derived [Li] abundances, veiling corrected

M=0.4 Msun

LX vs mass & age for Li-sample:drop at lowest mass & for older stars

1 Myr 10 Myr

The Upper Scorpius Association:evidence for lithium depletion

Preibisch & Zinnecker 2002

HR diagram

Low-masssample

Lithiumdepletion

Conclusions

• Route from molecular clouds to young stars & disk is ~ understood: main phases for low-mass SF have been identified

• Importance of initial conditions: rapid vs slow core formation and collapse

• In clusters & associations SF accelerates in time threshold f(AV, NH, xe)

• Duration of SF: age spreads & independent tests (lithium…) long lived

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

Log Mass (solar masses)

100.0

101.0

102.0

2

3

4

56789

2

3

4

56789

Log

N +

Con

stan

t

Trapezium Cluster Initial Mass Function

HBL

Brown Dwarfs

At stellar birth, the IMF is Salpeter from ~0.5 Msun

Orion Nebula ClusterK-band image

Sun

IC 348

Taurus

Chamaeleon I

Trapezium

Bouvier et al. 1998 Hambly et al. 1999 Moraux et al. 2003

Pleiades (100 Myr)

Luhman 2004

The IMF in Star Forming Regions & the Pleiades

0.1 0.3 1

0.3

0.1

1

los (km s-1)

vir = [GM/5R]1/2

(km s-1)

Taurus cores

Klessen et al. (2003)

• Modo 1: SF lenta… le nubi molecolari sono in equilibrio di forze e autogravitanti:

evoluzione quasi-statica tcross~tff & tnube~10 Myr>> tff

nascita di stelle localizzata nello spazio & tempo: fenomeno di soglia (NH, G0, xe…)

SFR è basso inizialmente e accelera nel tempo SFE è bassa a causa della rapida decelerazione

• Modo 2: SF rapida… le nubi molecolari sono entità dinamiche: frammentazione

turbolenta & dissipazione, tnube~ tff ~2-3 Myr SF avviene in shells create da flussi turbolenti su larga scala

SF è rapida, tSF~1-2 tcross

SFR è basso a causa della bassa efficienza nelle shells

• stelle giovani

• stelle evolute

Turbolenza

Distribuzione di età Tau-Aur

accelerazione

deceleraz

Evolution of the central density

Desch & Mouschovias (2001)

t0 t1 t2

The years 2000:Modes of Star Formation

Isolated: Taurus

Cluster: Orion

Rich Cluster:NGC 3603

Dissipation time of energy

Magnetic energy

Kinetic energy (vertical)

Kinetic energy (lateral)

The sum of the all

The time we stop driving force

Dissipation timeyear100.28 6

0 ttd

dtteE /

Note that the energy in transversemodes remains much greater thanthat in generated longitudinal modes.

HR diagramof the OrionCluster

most starsformed ~2 Myr ago

notice thelarge age spread: realor not?

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

Log Mass (solar masses)

100.0

101.0

102.0

2

3

4

56789

2

3

4

56789

Lo

g N

+ C

on

sta

nt

Trapezium Cluster Initial Mass Function

HBLSun

Brown Dwarfs

At stellar

birth, IMF

really is

given by

Salpeter

(1955) IMF.

Hillenbrand 1997