Breakfast Talk: Views on Star Formation. Current Views on Star Formation Hamburg, 7 July, 2004...
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Transcript of Breakfast Talk: Views on Star Formation. Current Views on Star Formation Hamburg, 7 July, 2004...
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