Enrique Vázquez-Semadeni. Centro de Radioastronomía y Astrofísica, UNAM, México

Transcription

1 Enrique Vázquez-Semadeni Centro de Radioastronomía y Astrofísica, UNAM, México 1

2 Javier Ballesteros-Paredes Centro de Radioastronomía y Astrofísica, UNAM, México 2

3 Collaborators: Javier Ballesteros-Paredes Pedro Colín Gilberto Gómez Alan Watson Robi Banerjee Ralf Klessen STUDENTS: Manuel Zamora Avilés 3

4 Collaborators: Enrique Vázquez-Semadeni Pedro Colín Gilberto Gómez Alan Watson Robi Banerjee Ralf Klessen STUDENTS: Manuel Zamora Avilés 4

5 I. INTRODUCTION Within the turbulent model of molecular clouds and star formation (SF), there exist two alternative scenarios: Slow star formation (Norman & Silk 1980; Krumholz & McKee 2005; Li & Nakamura 2006; Krumholz & Tan 2007): Clouds are in near virial equilibrium, and last several times their free-fall time. Turbulence supports the clouds. Turbulence is replenished by stellar feedback. Turbulence maintains a low star formation rate (SFR). Rapid star formation (Ballesteros-Paredes et al. 1999; Elmegreen 2000; Klessen et al. 2000; Hartmann et al. 2001; Mac Low & Klessen 2004; VS et al. 2007, ): Clouds form by large-scale compressions and/or instabilities. SF occurs rapidly (at high SFR) and coherently. Stellar feedback disrupts the cloud and terminates SF. 5

6 Defining the star formation efficiency as where then: Slow SF large Δt, small SFR. Rapid SF short Δt, large SFR. 6

7 This work: Numerically investigate effect of massive-star feedback on parent cloud immersed in diffuse (WNM) medium: See Manuel Zamora s poster (this session) for approach to an analytical model. Cloud formed by transonic compression in WNM. Compression triggers phase transition to cold medium (Hennebelle & Pérault 1999). Cloud grows by incorporating material from WNM; Bounded by phase transition front. Able to freely interact with WNM environment. Accrete, return material; Disperse? Subject to ionization heating-like stellar feedback. 7

8 Numerical model: N-body + AMR hydrodynamics code (ART code, by Kravtsov et al. 1997; Kravtsov 2003). 256-pc box. 4 refinement levels. Equivalent resolution pc resolution. Stellar particle formation by density threshold criterion. n SF = 4 x 10 6 cm -3. M part ~ 120 M sun. Cooling function from Koyama & Inutsuka (2002). Vázquez-Semadeni et al

9 Numerical model (cont d): Initial velocity field consisting of oppositely-directed cylindrical streams with velocity 6 km s -1 (Mach # = M s,inf =0.8) in WNM. Superposed initial, low-amplitude turbulent velocity field of Mach # M s,rms to trigger instabilities in compressed layer. Scale: ~ cylinder radius. M s,inf : Mach number of inflow speed w.r.t. warm gas. Converging inflow setup M s,rms M s,rms : Mach number of background turbulence in WNM. M inf M s,inf L box M inf : Mass in colliding cylinders = 2 ρ π R inf 2 L inf R inf n WNM = 1 cm -3 L inflow T WNM = 5000 K c s = 7.4 km s -1 9

10 Four simulations: 10

11 Numerical model (cont d) OB-star ionization-like heating by stellar particles: Deposited in cell containing stellar particle during 10 Myr. Heating rate taken as free parameter, adjusted to achieve realistic HII regions: Density Temperature Velocity 11

12 Run LAF1 (Large-amplitude fluctuations with feedback) 12

13 Global cloud features and evolution: Clouds first appear as atomic, CNM structures (Vázquez- Semadeni et al. 2006). Gravitational contraction sets in globally, bringing column density up to molecular-cloud values (Vázquez-Semadeni et al. 2007, Heitsch & Hartmann 2008). Size and mass of clouds determined by scale of dominant compression mechanism: SA simulations: collapse of massive, pancake-shaped cloud of radius = R cyl. Clouds shaped by colliding inflows. LA simulations: collapse of amorphous, less massive, smaller clouds. Clouds shaped by turbulent fluctuations. 13

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15 20-pc measuring box Central Cloud in run SAF1 (small fluctuation amplitude with feedback). 15

16 20-pc measuring box Cloud 1 in run LAF1 (Large fluctuation amplitude with feedback). 16

17 20-pc measuring box Cloud 2 in run LAF1 (Large fluctuation amplitude with feedback). 17

18 Central Cloud, SA No feedback With feedback M(n>100 cm -3 ) [M sun ] Whole box 10-pc box M star [M sun ] 18

19 M(n>100 cm -3 ) [M sun ] No feedback Cloud 1, LA Whole box 30-pc box 20-pc box 10-pc box With feedback M star [M sun ] 19

20 Cloud 2, LA No feedback With feedback M(n>100 cm -3 ) [M sun ] Whole box 30-pc box 20-pc box 10-pc box M star [M sun ] 20

21 Suggests that, for massive clouds, Total mass (cloud + stars) mainly determined by accretion, consistent with Fuikui et al. (2009). SF inhibited by feedback. i.e, larger dense gas mass in case with feedback due to reduced rate of gas-to-stars conversion. Apparently due to focusing of feedback on gas closest to forming stars next. 21

22 3. SFE is reduced to realistic values SA, Central cloud (10-pc box) No feedback Feedback 22

23 No feedback LA runs With feedback Cloud 1 Cloud 1 stellar mass smaller in the presence of feedback. Cloud 2 Cloud 2 23

24 4. Factor by which SFE is reduced by feedback depends on the cloud mass (at roughly the same size) involved in coherent collapse. Apparently due to short-range effect of feedback vs. longrange nature of gravity. The more extended the infall motions, the less effective the feedback in disrupting them. 24

25 5. SFE correlates with SFR. Average low-mass cloud of Evans et al Orion A cloud 25

26 Caveats: 1. Only one kind of feedback star (~ early B star). Excessive for small clouds, weak for massive ones. May be have non-negligible role in dispersal of small clouds, permanence of large ones. Work in progress: consideration of a range of feedback-star masses. 2. No radiative transfer; just heat dumping. 3. No supernova-like feedback. May give the fatal blow to large clouds. 4. Large ambiguity in masses when one is not restricted to a certain tracer. 26

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28 Conclusions (cont d): 3. Turbulence does not seem to be able to hold up a large cloud: Initial turbulence (from cloud assembly) dissipates quickly. Stellar feedback seems too localized. Supersonic turbulent linewidths are indicative of global contraction. (Hartmann & Burkert 2007; Field et al. 2008; Vázquez- Semadeni et al. 2008; Heitsch et al. 2009). 28