1. School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES The chemical interaction of dust and gas in prestellar cores Paola Caselli

2. Collaborators Low-mass: Aikawa (Kobe), Bacmann (LAOG), Belloche (Bonn), Bizzocchi (Lisbon), Bourke (CfA), Ceccarelli (LAOG), Crapsi (Leiden), Di Francesco (Victoria), Emprechtinger (Caltech), Foster (BU), Friesen (NRAO), Goodman (Harvard), Jørgensen (Copenhagen), Keto (CfA), Myers (CfA), Pagani (LERMA), Pineda (Manchester), Schnee (NRAO), Tafalla (Madrid), Vastel (Toulouse), van der Tak (Groningen), Walmsley (Arcetri) Intermediate-mass: Alonso-Albi (Madrid), Ceccarelli (LAOG), Crimier (Grenoble), Fuente (Madrid), Johnstone (Victoria), Plume (Calgary) Massive: Bourke (CfA), Butler (Florida), Fontani (IRAM), Henshaw (Leeds), Hernandez (Florida), Jimenez-Serra (CfA), Pillai (Caltech), Tan (Florida), Zhang (CfA)

3. Main Uncertainties Cosmic-ray ionization rate Elemental abundance (metals) Oxygen chemistry PAHs abundance Surface chemistry Dust evolution H 2 ortho-to-para ratio (Herschel !) (e.g. Pagani et al. 2009; Troscompt et al. 2009) (e.g. Garrod et al. 2009; Semenov et al. 2010) Alves et al. 2001 (e.g. Keto & Caselli 2008) (e.g. Wakelam & Herbst 2008) (e.g. Ormel et al. 2009; Keto & Caselli 2010)

4. Outline 1.The formation of H 2 2.The chemistry of water 3.CO formation 4.Nitrogen chemistry 5.Molecular freeze-out 6.Deuterium fractionation 7.The Herschel view Bergin & Tafalla 2007 + Di Francesco et al. 2007 PDR layer Core center

5. H + H  H 2 on the surface of dust grains (Gould & Salpeter 1963; Hollenbach & Salpeter 1970; Jura 1974; Pirronello et al. 1999; Cazaux & Tielens 2002; Habart et al. 2003; Bergin et al. 2004; Cuppen & Herbst 2005; Cazaux et al. 2008; Cuppen et al. 2010) 1. The formation of H 2

6. Evidences of freeze-out: deuterium fractionation 2. The chemistry of water On the surface of dust grains (e.g. Tielens & Hagen 1982; Cuppen & Herbst 2007; Ioppolo et al. 2008; Cazaux et al. 2010): Cuppen & Herbst 2007 Tielens & Hagen 1982

7. Evidences of freeze-out: deuterium fractionation 2. The chemistry of water In the gas phase (e.g. Hollenbach et al. 2009): Desorption (d) from dust surfaces (Hollenbach et al. 2009; Garrod 2008; Cazaux et al. 2010):

8. Hollenbach et al. 2009 G 0 variationsGrain size variations Photodesorption yield variations 2. The chemistry of water Volume density variations SWAS + Odin upper limits: x(H 2 O) gas <10 -8 (Bergin & Snell 2002; Klotz et al. 2008) BUT Line trapping and absorption of the dust continuum challenge the measurement of x(H 2 O) gas (Poelman et al. 2007)

9. 3. CO formation CO t CO ~ n C /[  n(H 2 )] ~ 10 5 yr Sternberg & Dalgarno 1995

10. 4. Nitrogen chemistry Flower et al. 2006 Hily-Blant et al. 2010 N2N2 N + H 3 +  NH + + H 2  N + OH  NO + H N + CH  CN + H t N2 ~ 10 6 yr in UV-shielded clouds CO

11. 5. Molecular freeze-out Freeze-out versus Free-fall Walmsley 1991 van Dishoeck et al. 1993

12. 5. Molecular freeze-out C 17 O(1-0) emission (Caselli et al. 1999) CO hole dust peak Dust emission in a pre-stellar core (Ward-Thompson et al. 1999) 0.05 ly Molecules freeze out onto dust grains in the center of prestellar cores  Dust grain

13. D-fractionation increases towards the core center (~0.2; Caselli et al. 2002; Crapsi et al. 2004, 2005) N 2 D + (2-1) N 2 H + (1-0) Dust emission in the pre-stellar core L1544 (Ward-Thompson et al. 1999) See also Bacmann et al. 2002, 2003; Bergin et al. 2002; Lee, Evans et al. 2003 5. Molecular freeze-out

14. Friesen et al. 2010 850  m N 2 H + (1-0)

15. 5. Molecular freeze-out Crapsi, Caselli, Walmsley & Tafalla 2007 On size scale of  800 AU: No NH 3 freeze-out at n H ~ 10 6 cm -3 ! The gas temperature drops to ~6 K in the central 1000 AU (  4  larger dust emissivity; Keto & Caselli 2008) The deuterium fractionation is ~0.4 in the central 3000 AU (larger than in N 2 H + ; see also Pillai et al. 2006, Fontani et al. 2008; Busqet et al. 2010) Loss of specific angular momentum towards the small scales N(NH 3 ) @ VLA N(NH 2 D) @ PdBI 700 AU 1400 AU

16. H 3 + + HD  H 2 D + + H 2 + 230 K H 2 D + / H 3 + increases when T kin < 20 K + when the abundance of gas phase neutral species (in particular CO and O) decreases (Dalgarno & Lepp 1984; Roberts & Millar 2000). N 2  N 2 D + + H 2 H 2 D + + CO  DCO + + H 2 Watson 1974 Millar et al. 1989 6. Deuterium fractionation

17. Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation o- H 2 D + CSO N 2 D + (2-1) IRAM N 2 H + (1-0) IRAM Vastel et al. 2006 Caselli et al. 2003, 2008 The H 2 D + and N 2 D + lines trace the same region (size ~ 5000 AU)  Only models including all multiply deuterated forms of H 3 + can reproduce these data (Roberts et al. 2003; Walmsley et al. 2004; Aikawa et al. 2005)

18. Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation Flower, Pineau des Forêts, Walmsley 2004 L429 L694-2 L1544 L183 B68 16293E NGC2264G NGC1333 DCO + L1521F B1 OphD L1517B TMC-1C Caselli et al. 2008 See also Pagani et al. 2009 Sipilä et al. 2010 The ortho-to-para ratio:

19. Evidences of freeze-out: deuterium fractionation 7. The Herschel view 1.3 mm continuum map from Ward-Thompson et al. (1999) Caselli et al. 2010, submitted Caselli et al. 2010, in press

20. Evidences of freeze-out: deuterium fractionation 7. The Herschel view Caselli et al. 2010

21. Evidences of freeze-out: deuterium fractionation x(o-H 2 O) ~ 2  10 -10 within the central ~7000 AU ~ 5  10 -9 at larger radii Peak Using Keto & Caselli (2010) RT models : abundance ~ 10 -8 at ~0.1 pc from center 7. The Herschel view (very similar to what found by van der Tak et al. 2010 in high-mass star forming regions)

22. Evidences of freeze-out: deuterium fractionation 7. The Herschel view

23. Evidences of freeze-out: deuterium fractionation Summary 1.Prestellar cores (PCSs) are the earliest phases (initial conditions) of star/planet formation. Ideal laboratories. 2.Severe (> 90%) freeze-out of CO (CS, H 2 CO, CH 3 OH) at densities above a few  10 4 cm -3. 3.N 2 H + starts to freeze-out at n H > 10 5 cm -3. 4.No clear evidence of NH 3 freeze-out at large n H (Herschel needed!), as well as CN, HCN and HNC. 5.N 2 D + and deuterated ammonia peak toward the coldest and densest zones (t freeze < 1000 yr) (ALMA). 6.H 2 D + is spatially coincident with N 2 D + (i.e. it does not trace “molecular holes”). D 2 H + observations needed. 7.PSCs H 2 O abundances are low (~10 -10 ) within cores (steep gradients?). Chemical models need revision.

24. Evidences of freeze-out: deuterium fractionation What’s next? 1.WISH GT: Higher sensitivity spectrum toward L1544 2.Herschel OT1: High sensitivity water spectra toward TMC-1, L1689B, and L183 (30 h HIFI, Aikawa, Caselli, Tafalla et al.): Test intra-cloud variations of H 2 O abundance in the Taurus molecular cloud Compare the Taurus abundance with that of other clouds with different physical conditions. Link physical conditions with H 2 O observations to understand chemical processes.

25. Evidences of freeze-out: deuterium fractionation What’s next? PSC properties in cluster forming regions  Oph A seen in 850  m (color scale), N 2 H + (4-3) (green contours) and ortho-H 2 D + (1 01 -1 10 ) (blue contours). Di Francesco et al., in prep. With Herschel/HIFI (Di Francesco, Caselli, Jørgensen +): ortho-NH 3 (1 1 -0 0 ), N 2 H + (6-5) H 2 O(1 10 -1 01 ) para-D 2 H + (1 10 -1 11 ) ortho-D 2 H + (1 11 -0 00 )

26. Evidences of freeze-out: deuterium fractionation Spitzer IRAC 8μm MJy Sr -1 Mass Surface Density g cm -2 (Butler & Tan 2009; Peretto & Fuller 2009) 3´ What’s next? PSCs in high-mass star forming regions

27. Evidences of freeze-out: deuterium fractionation A.Dense cores and their molecular envelopes: HIFI simultaneous observations of ortho-H 2 O(1 10 -1 01 ) and ortho-NH 3 (1 1 -0 0 ) + N 2 H + (6-5). B.The atomic layer: [CII] 158  m maps with HIFI. C.The extinction low + [OI]63  m: PACS imaging spectroscopy between 51 and 73  m Multilayer spectroscopy of 8/24/70  m-dark pre-star-cluster clumps in IRDCs (31 h, HIFI+PACS; Caselli, Tan, Beltran et al.) What’s next? PSCs in high-mass star forming regions