New insights into low-mass star formation

1. New insights into low-mass star formation - through high angular resolution (sub)mm observations Jes Jørgensen (CfA)Fredrik Schöier (Stockholm), Ewine van Dishoeck (Leiden), Michiel Hogerheijde (Leiden), Geoff Blake (Caltech)Tyler Bourke, David Wilner, Phil Myers (CfA) Submm meeting Copenhagen, December 20th 2004

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

3. Important questions about low-mass star formation Dark Cloud Cores • How do we get from a pre-stellar “dark cloud core” to a protostellar object? • What is triggering the process? • What are the important parameters defining the “end product”? • And on what timescales does this happen? t = 0Gravitational collapse t ~ 104– 105 yrs Protostar embedded in ~10,000 AU envelope

4. Protostellar properties • Centrally condensed envelope of gas and dust • Ongoing accretion through a circumstellar disk • Densities ranging from 104 cm-3 to 107-108 cm-3 (H2) • Temperatures ranging from 10 to a few hundred K.

5. 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. • 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?

6. Approach Dust continuum emission Physical structure(temperature, density) Molecular excitation Chemical structure(abundance)

7. This study: single-dish; continuum • Physical models based on SCUBA 450 and 850 micron data for ~ 20 pre- and protostellar objectsJCMT/SCUBA beam: 9-15” (1500 – 5000 AU) • Physical structure from 1D radiative transfer modeling, DUSTY (Ivesi et al., 1999) See also studies by Hogerheijde & Sandell (2000), Motte & André (2001), Shirley et al., Evans et al., Young et al. (2000-2003)

8. This study...2: single-dish; line • Molecular line survey of ~40 lines (3.5–0.8 mm) using JCMT and Onsala 20 m telescopesBeam sizes: 14-44” (2000 – 14000 AU) • CO • CS, SO • HCO+, N2H+ • HCN, HNC, CN • DCN, DCO+ • H2CO, CH3OH • SO2, SiO, H2S, CH3CN(~ 40 transitions) • Chemical structure from Monte Carlo line radiative transfer (Hogerheijde & van der Tak, 2000, Schöier et al. 2002)

9. This study...3: interferometer • Further millimeter interferometer (BIMA and OVRO) observations of selected objects. Synth. beam size: 3-7” (400 – 2000 AU) • Extrapolation of models from single-dish continuum and line obs. to small scales.

10. Single-dish telescope Interferometer Protostellar properties ~ a few  104 AU (50”) ~ a few  102 AU (0.5”)

11. BIMA 3 mm Continuum: interferometry Example: NGC 1333-IRAS2 SCUBA 850 µm Jørgensen, Hogerheijde, van Dishoeck et al., 2004, A&A, 413, 993

12. Continuum: interferometry

13. 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 (H2 density) relative to which abundances are calculated

14. Approach Dust continuum emission Physical structure(temperature, density) Molecular excitation Chemical structure(abundance)

15. An example: CO depletion

16. 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.

17. “Canonical” CO abundance (Lacy et al. 1994) CO depletion CO freezes out at low temp. ( 35 K) Objects with high envelope masses (younger?) show significantly higher degree of CO depletion Jørgensen, Schöier & van Dishoeck 2002 A&A, 389, 981

18. Abundance • 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 Jørgensen, Schöier & van Dishoeck, 2005, A&A submitted

19. “Drop” abundance model nde Tev

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

21. C18O 1-0 OVRO observations L483 (class 0 protostar @ 200 pc) Jørgensen, 2004, A&A, 424, 589

22. Depletion ~ Time ( 105 yrs) • “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 tde and “age” • Potentially(!) a tracer of the “history” of the core- dense stage (where CO depletes) only 105 years?

23. Chemical effects of CO depletion (HCO+ and N2H+)

24. Empirical chemical network CO CS H2CO HCO+ SO CN HNC HCN HC3N Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

25. dust grains CO H3+ HCO+ N2 N2H+

26. HCO+ and N2H+ abundances Jørgensen, Schöier & van Dishoeck 2004, A&A, 416, 603

27. L483:  450 m cont.  N2H+  C18O Understanding the chemistry is important for constraining the physical/dynamical structure from line observations - but of course the chemistry also depends on the core physics/dynamics. Jørgensen, 2004, A&A, 424, 589

28. Outlook: The inner envelope

29. Protostellar core: In the innermost region temperature may increase to 90+ K –> H2O rich ice mantles evaporate completely

30. Hot cores • High-mass protostars: separate evolutionary stage with high temperatures (> 100 K) and high densities (107 cm-3); chemically characterized by high abundances of organic, S, Si, ... molecules. • In centrally heated low-mass protostellar envelopes temperatures may increase above 100 K at radii smaller than ~100 AU • Ice mantles are liberated. Abundant organic molecules? Need: - High excitation lines (probing high densities and temperatures). - Molecules that are not too sensitive to the chemistry of the outer envelope (H2CO, e.g., follows CO abundance structure in the outer envelope). - High angular resolution (beam dilution/contribution from outflows)

31. Then what do we do...?

32. Possibilities for mm interferometry outside France Submillimeter Array, Mauna Kea, Hawaii (SAO + ASIAA, Taiwan) 180-356 GHz (690 GHz), 8 x 6 mExcellent site: 1mm observations routine, sub mm (300+ GHz) science about half nightsAbout 10% time available to outside PI’s Australia Telescope Compact Array: Narrabri, Australia 83.5-106 GHz, 5 x 22 mAccess to Southern sky, large collecting area, (kangaroos)Limited millimeter expertise, poor site CARMA: California, US 85-115 GHz, 200-250 GHz (6 x 10.4 m, 10 x 6.1 m) Combination of old BIMA and OVRO antennaeBetter site, (3, 1 mm), excellent (u,v) coverage, good supportRunning again late 2005+, oversubscribed?

33. Then what do we do...? ...go (back) to Hawaii!

34. IRAS 16293-2422 with the SMA (Kuan et al., 2004)

35. NGC13333-IRAS2A with SMA@345 GHz (Jørgensen et al., in prep.) Finally: organic molecules?

36. Conclusions • The chemical structure of protostellar envelopes provide interesting insights and constraints on star formation processes. • The abundance of CO non-constant and can be used as aprobe of pre- and protostellar age. It has important implications for abundances of other species. • Organic molecules may be abundant in the innermost regions of low-mass protostars. • The link between high-resolution observations, single-dish surveys and detailed modeling is important.