Probing the formation mechanism of prestellar cores and the origin of the IMF: First results from Herschel

Transcription

1 Probing the formation mechanism of prestellar cores and the origin of the IMF: First results from Herschel Philippe André, CEA/SAp Saclay Herschel GB survey Ophiuchus 70/250/500 µm composite With: A. Menshchikov, V. Könyves, N. Schneider, D. Arzoumanian, S. Bontemps, F. Motte,P.Didelon, N. Peretto, D. Ward-Thompson, J. Di Francesco, P. Martin, P. Saraceno, P. Palmeirim, J. Kirk & the Herschel Gould Belt KP Consortium Ph. André - The millimeter and submillimeter sky in the Planck mission era - Paris 12 Jan 2011

2 The Herschel Gould Belt Survey SPIRE/PACS µm imaging of the bulk of nearby (d < 0.5 kpc) molecular clouds (~ 160 deg 2 ), mostly located in Gould s Belt. Complete census of prestellar cores and Class 0 protostars. Taurus Perseus Cepheus Serpens Aquila Ophiuchus Lupus Gould Belt Chamaeleon CrA Orion IRAS 100 µm ~ 15 resolution at λ ~ 200 µm ~ 0.02 pc < Jeans d = 300 pc Motivation: Key issues on the early stages of star formation Nature of the relationship between the CMF and the IMF? What generates prestellar cores and what governs their evolution to protostars and proto-brown dwarfs?

3 Outline: First images from the Herschel Gould Belt survey Preliminary results on cores (e.g. CMF vs. IMF) The role of filaments in the core formation process Implications/Speculations Herschel GB survey L /250/500 µm composite Ph. André - Planck 2011 Conference - Paris 12 Jan 2011 PACS

4 First images from the Gould Belt Survey PACS/SPIRE // mode 70/160/250/350/500 µm 1) Aquila Rift star-forming cloud (d ~ 260 pc) cf. Red : SPIRE 500 µm Green : PACS 160 µm Blue : PACS 70 µm ~ 3.3 o x 3.3 o field André et al Könyves et al Bontemps et al Men shchikov et al A&A special issue (vol. 518)

5 First images from the Gould Belt Survey PACS/SPIRE // mode SPIRE 250 µm image 2) Polaris flare translucent cloud (d ~ 150 pc) ~ 5500 M (CO+HI) Heithausen & Thaddeus 90 ~ 13 deg 2 field Miville-Deschênes et al Ward-Thompson et al Men shchikov et al André et al A&A special issue

6 Revealing the structure of one of the nearest infrared dark clouds (Aquila Main: d ~ 260 pc) Herschel (SPIRE+PACS) Dust temperature map (K) Herschel (SPIRE+PACS) Column density map (H 2 /cm 2 ) W40 1 deg ~ 4.5 pc

7 Dense cores form primarily in filaments Cores Morphological Component Analysis: Wavelet component (H 2 /cm 2 ) Herschel Column density map = + (P. Didelon based on Starck et al. 2003) Filaments Curvelet component (H 2 /cm 2 )

8 Aquila: `Compact Source Extraction (using getsources A. Menshchikov et al. 2010) Herschel (SPIRE+PACS) Aquila entire field: N H2 (cm -2 ) Spatial distribution of extracted cores 541 starless 201 YSOs Starless = no PACS 70 µ emission (F 70µ < > L proto cf. Dunham, Crapsi, Evans e.a c2d) Könyves et al deg ~ 14 pc 5σ level: ~ cm -2 70/160/500 µm composite image

9 Core: Examples of starless cores in Aquila-East local column density peak simple (convex) shape no substructure at Herschel resol. potential single star-forming entity Herschel N H2 map (cm -2 ) Radial intensity profiles Including background Background subtracted Ellipses: FWHM sizes of 24 starless cores at 250 µm Könyves et al. 2010, A&A special issue

10 Most of the Herschel starless cores in Aquila are bound Motte et al. (1998,2001) Mass, M (M ) T = 20 K T = 7 K Cirrus noise limit in Aquila > 60% are likely prestellar in nature Könyves et al Deconvolved FWHM size, R (pc) (at 250 µm) Positions in mass vs. size diagram, consistent with ~ critical Bonnor-Ebert spheroids: M BE = 2.4 R BE c s 2 /G for T ~ 7-20 K

11 Most of the ~300 Polaris starless cores are unbound Motte et al. (1998,2001) Mass, M (M ) T = 20 K T = 7 K Cirrus noise limit in Polaris not (yet?) prestellar André et al Ward-Thompson et al Deconvolved FWHM size, R (pc) (at 250 µm) Locations in mass vs. size diagram: 2 orders of magnitude below the density of self-gravitating Bonnor-Ebert isothermal spheres

12 Confirming the link between the prestellar CMF & the IMF Könyves et al André et al A&A special issue prestellar cores in Aquila Factor ~ 2-9 better statistics than earlier CMF studies: e.g. Motte, André, Neri 1998; Johnstone et al. 2000; Stanke et al. 2006; Enoch et al. 2006; Alves et al. 2007; Nutter & Ward-Thompson 07 Number of objects per mass bin: ΔN/ΔlogM Core Mass Function (CMF) in Aquila Complex Good (~ one-to-one) correspondence between core mass and stellar system mass: M * = ε M core with ε ~ in Aquila The IMF is at least partly determined by pre-collapse cloud fragmentation (cf. models by Padoan & Nordlund 2002, Hennebelle & Chabrier 2008) IMF CMF 0.3 M Mass (M ) CO clumps (Kramer et al. 1998) ε Salpeter s IMF

13 Prestellar cores form out of a filamentary background : Class 0 protostars : Aquila N H2 map (cm -2 ) Prestellar cores Aquila curvelet N H2 map (cm -2 )

14 Herschel reveals a network of filaments in every cloud Polaris A characteristic width ~ 0.1 pc? (deconvolved FWHM) Distribution of widths for 90 filaments Number of filaments per bin 0.1 pc Distribution of Jeans lengths [λ J ~ c s2 /(GΣ)] Aquila Polaris IC5146 IC5146 Filament width (FWHM) [pc] Using the skeleton or DisPerSE algorithm (Sousbie, Pichon et al. 2008, 2010) to trace the ridge of each filament D. Arzoumanian et al. 2011

15 Prestellar cores form out of a filamentary background : Class 0 protostars : Prestellar cores - 90% found at A v (back) > 7 Aquila N H2 map (cm -2 ) Aquila curvelet N H2 map (cm -2 ) Unstable 0.1Mline/Mline,crit Stable

16 Number of cores per extinction bin: ΔN/ΔA V Confirmation of an extinction threshold for the formation of prestellar cores Distribution of background extinctions for the Aquila prestellar cores A v ~ 7 Background cloud extinction, A V In Aquila, ~ 90% of the prestellar cores identified with Herschel are found above A v ~ 7 Σ ~ 150 M pc -2 cf. Onishi et al (Taurus) Johnstone, Di Francesco, Kirk 04 (Ophiuchus) See also (for YSOs): Heiderman, Evans et al. 2010

17 Only the densest filaments are gravitationally unstable and contain prestellar cores ( ) Aquila curvelet N H2 map (cm -2 ) André et al. 2010, A&A Special issue 1 Unstable Mline/M line,crit 0.1 Stable The gravitational instability of filaments is controlled by the mass per unit length M line (cf. Ostriker 1964, Inutsuka & Miyama 1997): unstable if M line > M line, crit unbound if M line < M line, crit M line, crit = 2 c s 2 /G ~ 15 M /pc for T ~ 10K A V threshold Simple estimate: M line N H2 x Width (~ 0.1 pc) Unstable filaments highlighted in white in the N H2 map

18 Importance of the star formation threshold on (extra)galactic scales Star formation rate vs. Gas surface density Σ SFR Σ gas for Σ gas > Σ threshold Σ th ~ 150 M pc -2 A V ~ 7-8 Heiderman et al Lada et al See also Gao & Solomon 2004 for external galaxies Heiderman, Evans et al. 2010

19 Origin of the filaments: Large-scale turbulence? Numerical simulations including large-scale turbulence: Padoan, Juvela, Goodman, Nordlund (2001) Klessen & Burkert (2000) Li & Nakamura (2004) Also: Bate, Bonnell, Bromm (2003) Basu, Ciolek et al. (2009) Hennebelle, Banerjee, Vazquez-Semadeni et al. (2008) Duffin, Pudritz et al. 2010

20 Turbulence dissipation and filament formation In Polaris, one of the most tenuous filaments detected by SPIRE coincides with a CO(2-1) structure of intense velocity shear (~ 40 km/s/pc) found at IRAM 30m (Hily-Blant & Falgarone 2009) SPIRE 250 µm 0.8 Jy/b SPIRE 250 µm + CO(2-1) CVI 0.1 Jy/b 1 deg ~ 2.6 pc 10 ~ 0.45 pc

21 Filaments permeate the ISM on all scales Herschel SPIRE 500 µm + PACS 160/70 µm (from ~0.1 pc to > 50 pc) Planck HFI 540/350 µm + IRAS 100 µm Aquila ESA and the Gould Belt KP ESA and the HFI Consortium

22 Conclusions First results from Herschel on prestellar cores are very promising: Confirm the close link between the prestellar CMF and the IMF, although the whole survey will be required to fully characterize the nature of this link. Suggest that prestellar core formation occurs in 2 main steps: 1) Filaments form first in the cold ISM, probably as a result of the dissipation of MHD turbulence; 2) The densest filaments then fragment into prestellar cores via gravitational instability above a critical column density threshold Σ th ~ 150 M pc -2 Spectroscopic and polarimetric observations required to clarify the roles of turbulence, B fields, gravity in forming the filaments.