Molecular Differentiation in Pre-stellar Cores

1. Molecular Differentiation in Pre-stellar Cores Mario Tafalla Observatorio Astronómico Nacional (Spain) Outline * Basic physical properties of cores * Evidence for chemical differentiation * How to use chemical composition to trace core evolution

2. Dense cores: simplest star-forming sites L1544-Ward-Thompson et al. (1999) L1498-Tafalla et al. (2002) B68- Alves et al. (2001) • Variety of sizes, shapes • embedded cores, exposed globules • observed in multiple tracers • Simplest laboratories to study ISM chemistry • Need to characterize their physical properties

3. Pysical properies: core sizes • In Taurus: relatively narrow range • <r0> = 0.03 pc (= 7,000 AU) • Coincides with break point in the distribution of stars • cluster vs. binary regimes 44 Cores in Taurus (Tafalla et al. 2007) • Change in the gas physics • Transition • r > r0: supersonic/turbulent (cloud) • r < r0: subsonic/gravity (core) • How the transition occurs? • turbulence dissipation • magnetic field dissipation binaries 0.04 pc clustering Larson (1995)

4. Density structure of starless cores & globules • Recent agreement between different techniques • Radial profiles: • central flattening (r0) • central density 105-106 cm-3 • In some cases • fit very close to expectation from isothermal (BE) sphere • center/edge contrast close to maximal (14) Alves et al. (2001) • Caveats • dust properties are not well known: factor of 2 uncertainty • most cores are not spheres (axial ratio close to 2:1). Additional source of stability (magnetic field?)

5. Temperature: dust and gas • Dust and gas can have different temperatures • temperature set by heating = cooling • for densities > 105 cm-3, dust and gas couple thermally Galli et al. (2001)

6. Dust temperature gradient • Cloud-wide dust temperature gradient • cloud is warm (15-20 K) • core is cold (10 K) • Consistent with extinction of ISRF Ward-Thompson et al. (2002) IC 5146 core cloud cloud Kramer et al. (1998) • Dust emissivity increases • evidence for grain coagulation: fluffy grains • small grains disappear

7. Gas temperature • From NH3 observations • (J,K)=(1,1) & (2,2) sensitive to temperature • NH3 survives in gas phase inside core (enhanced abundance) • Low resolution data: constant temperature ≈ 10 K • High resolution observations: central drop in L1544 Crapsi et al. (2007) Tafalla et al. (2004)

8. Kinematics. Internal motions • Derived from linewidth • N2H+ & NH3 • In contrast with clouds • internal subsonic motions • Thermal pressure support dominates turbulent pressure • Break-down of Larson’s law • velocity coherent cores (Goodman et al 1998) Taurus Tafalla et al. (2007) L1498 Tafalla et al. (2004)

9. Molecular differentiation • Accurate determination of physical properties provides • reliable estimate of chemical composition • solution of old discrepancies between tracers • Most species do not survive in gas phase if n > few 104 cm-3 L1517B Tafalla et al. (2002)

10. Molecular freeze out • At low temperatures: molecules stick on grains • Equilibrium situation • molecules on grains if Tgrain < Tfreezeout • molecules in gas phase if Tgrain > Tfreezeout • Time for equilibrium is ≈ 5 109 /n(H2) yr • if n(H2) = 105 cm-3, tfrz ≈ 5 104 yr (<< core lifetime) Leger (1983) Scott Sandford

11. Why do N2H+ and NH3 stay in the gas phase? • Initial models (e.g., Bergin & Langer 1997) • N2binding energy assumed lower than CO b.e. • CO, etc freezes at low densities, N2 at high densities, N2H+ and NH3 are formed from N2 • Laboratory work (Öberg et al. 2005, Bisschop et al. 2006) • binding energies are almost equal • sticking probability S=1 • New understanding of differential freeze out • both CO and N2 freeze out simultaneously • disappearance of CO from gas phase changes chemistry • CO is highly abundant (X≈10-5) • major destroyer of N2H+ • freeze out of CO dominates chemistry (over N2 freeze out) • N2H+ is enhanced (NH3 is formed from N2H+)

12. Molecular survey of L1498 and L1517B • Motivation: • study behavior of as many species as possible • determine a set of “well determined” abundances • Sample • L1498 and L1517B: close to round, isolated (simple) • CH3OH, SO, C3H2, H2CO, HC3N, DCO+, HCO+, C2S, CO, CS, N2H+, NH3 • Two or more transitions, optically thin isotopologues, IRAM 30m L1498 L1517B

13. Sample maps for L1498 Tafalla et al. (2006)

14. Abundance results (L1498)

15. Comparison with models • Models reproduce pattern of selective depletion • CO, CS, CCS, etc deplete at center • N2H+ and NH3 survive or are even enhanced Aikawa et al. (2005)

16. Comparison with Aikawa et al. (2005) • Despite qualitative agreement with models • order of magnitude discrepancies in some abundances (e.g., CH3OH) L1498 andL1517B Tafalla et al. (2006)

17. Using freeze out to trace core history • Freeze out is a time-dependent process • it can be used as a clock of core contraction • In the simplest form: • old core: CO depleted, N2H+ abundant, high deuteration • young core: CO abundant, N2H+ weak, low deuteration • Define ratio R • Old core: small R • Young core: large R (measured at core center)

18. L1521E: a core with negligible depletion

19. Comparison with other cores: depletion-sensitive species • L1521E inner abundances: within factor 2 of outer abundances for L1498 and L1517B • Little chemical evolution except for depletion • cores originate from cloud material of similar composition • Use L1498 and L1517 as “standards”

20. N2H+ and NH3: unusually rare • N2H+: 8 times lower than average • NH3: 20 times lower than average • Most extreme core in sample

21. L1521E still a mystery • Chemical composition • consistent with lack of depletion in CO, etc. • unusually young core • Physical composition • central density and concentration typical of mature cores • unusually fast contraction? • We need to identify more cases like L1521E

22. How to search for chemically young cores? • Compare effect of depletion in models (L1498 & L1517B) • Depletion decreases central intensity by factor ≈ 2 • Some species more sensitive to depletion • combination of excitation and hole size • SO and C2S appear as the most sensitive to depletion L1498 &L1517B constant/depletion

23. Searching for the youngest cores in L1517 N2H+

24. Searching for the youngest cores in L1517 DSS2 red and SO(23-12) SO

25. Summary • Dense cores • first coherent structures that form in clouds • separate from the self-similar, turbulent component • dominated by self-gravity and subsonic motions • Physical structure • flattened density profile (reminiscent BE) • temperature gradient (dust and possibly gas) • Chemical structure • inhomogeneous, differentiated, dominated by freeze out • qualitative but not quantitative agreement with models • new generation of models still needed to extract ages • Using chemistry as timer of core evolution • L1521E is consistent with being chemically young (but physics) • we have tools to search for new candidates: first fruits