1. High resolution (sub)millimetre studies of the chemistry of low-mass protostars Jes Jørgensen (CfA) Fredrik Schöier (Stockholm), Ewine van Dishoeck (Leiden), Michiel Hogerheijde (Leiden), Geoff Blake (Caltech) Tyler Bourke, David Wilner, Phil Myers (CfA) Cardiff, January 6th 2005...or “where did all the CO go?” ACP

2. Pictures from NASA/Astronomical picture of the day; Myers et al. (1998) Low-mass star formation

3. Protostellar properties Centrally condensed envelope of gas and dust Ongoing accretion through a circumstellar disk Densities ranging from 10 4 cm -3 to 10 7 -10 8 cm -3 (H 2 ) Temperatures ranging from  10 to a few hundred K.

4. What is the relation between the physical and chemical properties of low-mass protostars? What are the useful diagnostics of different protostellar components? Is it possible to use the chemistry to trace the protostellar evolution? This study Establish the physical and chemical structure of a sample of ~ 20 low-mass protostars (class 0/I); using single-dish obs. (JCMT), mm interferometry and detailed radiative transfer modeling.

5. Approach Dust continuum emission Physical structure Molecular excitation Chemical structure Interferometry: small scale structure Detailed chemical model SCUBA obs. + Rad. transfer model. Single-dish obs. + Monte Carlo model. CO CS, SO HCO +, N 2 H + HCN, HNC, CN DCN, DCO + H 2 CO, CH 3 OH SO 2, SiO, H 2 S, CH 3 CN (~ 40 transitions)

6. Today......very little about continuum observations and dust radiative transfer BUT: Continuum/Physical structures......describe star formation/core physical evolution...are crucial for molecular excitation calculations...establish reference scale (H 2 density) relative to which abundances are calculated...include significant simplifying assumptions (e.g., dust properties, dust-gas coupling...)

7. An example: CO depletion

8. Example: modeling of CO lines toward L723 Adopting n(r) and T(r) from continuum modeling: constrain abundances (and velocity field) from Monte Carlo line radiative transfer by comparison to observed line profiles.

9. CO freezes out at low temp. (  35 K) - as seen in pre- stellar cores (e.g., Caselli et al. (1999), Tafalla et al. (2002)) Objects with high envelope masses (younger?) show significantly higher degree of CO depletion CO depletion Jørgensen, Schöier & van Dishoeck 2002 A&A, 389, 981 “Canonical” CO abundance (Lacy et al. 1994)

10. Jørgensen, Schöier & van Dishoeck, 2005, A&A submitted Pre-stellar core: Low temperature Depletion toward center (high densities ~ time)...but not edge Protostellar core: Central heating ~ temperature gradient Thermal desorption toward center...outside (low T): depletion/no depletion regions as in pre-stellar stages Caselli et al. (1999), Tafalla et al. (2002), Bergin et al. (2002), Bacmann et al. (2002), Lee et al. (2003)... Abundance

11. “Drop” abundance model n de T ev

12. Constant abundance model “Drop” abundance model L723:

13. C 18 O 1-0 OVRO observations L483 (class 0 protostar @ 200 pc) Jørgensen, 2004, A&A, 424, 589

14. “Drop abundance structure” needed to account for both single-dish and interferometer observations Explains differences in CO abundances between YSOs with envelopes of different masses - but note: no trend between t de and “age” Potentially(!) a tracer of the “history” of the core - dense stage (where CO depletes) only 10 5 years? Depletion ~ Time ( 10 5 yrs)

15. Chemical effects of CO depletion (HCO + and N 2 H + )

16. HCN HC 3 N CN HNC HCO + CO CS SO Empirical chemical network Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

17. CO dust grains H3+H3+ N2N2 N2H+N2H+ HCO +

18. HCO + and N 2 H + abundances Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

19. L483:  450  m dust continuum  N 2 H + J = 1  0  C 18 O J = 1  0 Jørgensen, 2004, A&A, 424, 589 “Typical embedded pro- tostar (quite asymmetric, though) at a distance of approximately 200 pc.” CO desorption (T> 30 K) CO freeze-out  X(N 2 H + )  1” = 200 AU  3×10 15 cm

20. L483:  450  m dust continuum  N 2 H + J = 1  0  C 18 O J = 1  0 Jørgensen, 2004, A&A, 424, 589 1” = 200 AU  3×10 15 cm “Typical embedded pro- tostar (quite asymmetric, though) at a distance of approximately 200 pc.”

21. BIMA: N 2 H + 1-0* NGC 1333-IRAS2 SCUBA 850 µm Chemistry as a tool...

22. BIMA: N 2 H + 1-0* Chemistry as a tool... NGC 1333-IRAS2 Jørgensen, Hogerheijde, van Dishoeck et al., 2004, A&A, 413, 993 2C 2A 2B Dashed line: SCUBA continuum emission Solid line: Contrast N 2 H + /SCUBA emission

23. Depletion ~ Time ( 10 5 yrs)

24. BIMA: N 2 H + 1-0* Chemistry as a tool... NGC 1333-IRAS2 Jørgensen, Hogerheijde, van Dishoeck et al., 2004, A&A, 413, 993 2C 2A 2B Dashed line: SCUBA continuum emission Solid line: Contrast N 2 H + /SCUBA emission Time

25. CO dust grains H3+H3+ N2N2 N2H+N2H+ HCO + ? Previously N 2 assumed to freeze-out slower than CO (e.g., Bergin & Langer, 1997) – but recent observations show N 2 H + depleting towards the centers of pre- and protostellar cores (although slower than CO) (e.g., Bergin et al. (2002), Belloche & André (2004)) and lab. experiments show similar binding energies for CO and N 2 (Öberg et al.)

26. CO-HCO + -N 2 H + Chemistry of gas parcel at 10 6 cm -3 and 20 K after 10 4 years following model of Doty et al. (2004) with varying CO – and N 2 - depletion BLACK/BLUE: [CO] varying RED: [CO] & [N 2 ] varying

27. Conclusions Continuum emission; dust radiative transfer Physical structure of envelopes (down to 500 AU) (The presence or absence of disks) Molecular line studies Chemical evolution ~ thermal history (e.g., CO) Important link between high-resolution observations, single-dish surveys and detailed modeling A quantitative framework for the interpretation of the detailed physical and chemical structure of early protostellar sources has been established.