The chemical interaction of dust and gas in prestellar cores

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 • H2 ortho-to-para ratio (e.g. Keto & Caselli 2008) (Herschel !) (e.g. Wakelam & Herbst 2008) (e.g. Garrod et al. 2009; Semenov et al. 2010) (e.g. Ormel et al. 2009; Keto & Caselli 2010) (e.g. Pagani et al. 2009; Troscompt et al. 2009) Alves et al. 2001

4. Outline • The formation of H2 • The chemistry of water • CO formation • Nitrogen chemistry • Molecular freeze-out • Deuterium fractionation • The Herschel view PDR layer Core center Bergin & Tafalla 2007 + Di Francesco et al. 2007

5. 1. The formation of H2 H + H  H2 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)

6. Tielens & Hagen 1982 Cuppen & Herbst 2007 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):

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. 2. The chemistry of water G0 variations Grain size variations Volume density variations Photodesorption yield variations SWAS + Odin upper limits: x(H2O)gas <10-8 (Bergin & Snell 2002; Klotz et al. 2008) BUT Line trapping and absorption of the dust continuum challenge the measurement of x(H2O)gas (Poelman et al. 2007) Hollenbach et al. 2009

9. 3. CO formation Sternberg & Dalgarno 1995 CO tCO ~ nC/[n(H2)] ~ 105yr

10. 4. Nitrogen chemistry CO Flower et al. 2006 Hily-Blant et al. 2010 N2 N + H3+ NH+ + H2  N + OH  NO + H N + CH  CN + H tN2~ 106 yr in UV-shielded clouds

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

12. 5. Molecular freeze-out C17O(1-0) emission (Caselli et al. 1999) CO hole Dust grain Molecules freeze out onto dust grains in the center of prestellar cores  dust peak 0.05 ly Dust emission in a pre-stellar core (Ward-Thompson et al. 1999)

13. 5. Molecular freeze-out N2D+(2-1) N2H+(1-0) D-fractionation increases towards the core center (~0.2; Caselli et al. 2002; Crapsi et al. 2004, 2005) 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

14. 5. Molecular freeze-out 850 m N2H+(1-0) Friesen et al. 2010

15. 5. Molecular freeze-out • • On size scale of 800 AU: • No NH3 freeze-out at nH ~ 106 cm-3 ! • • The gastemperaturedrops to ~6 K in the central 1000 AU ( 4  larger dust emissivity; Keto & Caselli 2008) • • The deuterium fractionationis ~0.4 in the central 3000 AU (larger than in N2H+; see also Pillai et al. 2006, Fontani et al. 2008; Busqet et al. 2010) • Loss of specific angular • momentum towards the small scales N(NH3) @ VLA 1400 AU N(NH2D) @ PdBI 700 AU Crapsi, Caselli, Walmsley & Tafalla 2007

16. N2  N2D+ + H2 H2D+ + CO  DCO+ + H2 6. Deuterium fractionation Watson 1974 Millar et al. 1989 H3+ + HD  H2D+ + H2 + 230 K H2D+ / H3+increases when Tkin < 20 K + when the abundance of gas phase neutral species (in particular CO and O) decreases (Dalgarno & Lepp 1984; Roberts & Millar 2000).

17. Caselli et al. 2003, 2008 Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation The H2D+ and N2D+ lines trace the same region (size ~ 5000 AU)  Only models including all multiply deuterated forms of H3+ can reproduce these data (Roberts et al. 2003; Walmsley et al. 2004; Aikawa et al. 2005) Vastel et al. 2006 o-H2D+ CSO N2H+(1-0) IRAM N2D+(2-1) IRAM

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

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

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

21. Evidences of freeze-out: deuterium fractionation 7. The Herschel view x(o-H2O) ~ 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 (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 Prestellar cores (PCSs) are the earliest phases (initial conditions) of star/planet formation. Ideal laboratories. Severe (> 90%) freeze-out of CO (CS, H2CO, CH3OH) at densities above a few  104 cm-3. N2H+ starts to freeze-out at nH> 105 cm-3. No clear evidence of NH3 freeze-out at large nH (Herschel needed!), as well as CN, HCN and HNC. N2D+ and deuterated ammonia peak toward the coldest and densest zones (tfreeze < 1000 yr) (ALMA). H2D+ is spatially coincident with N2D+ (i.e. it does not trace “molecular holes”). D2H+ observations needed. PSCs H2O abundances are low (~10-10) within cores (steep gradients?). Chemical models need revision.

24. Evidences of freeze-out: deuterium fractionation What’s next? • WISH GT: Higher sensitivity spectrum toward L1544 • 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 H2O abundance in the Taurus molecular cloud • Compare the Taurus abundance with that of other clouds with different physical conditions. • Link physical conditions with H2O observations to understand chemical processes.

25. Evidences of freeze-out: deuterium fractionation What’s next? PSC properties incluster forming regions  Oph A seen in 850 m (color scale), N2H+(4-3) (green contours) and ortho-H2D+(101-110) (blue contours). With Herschel/HIFI (Di Francesco, Caselli, Jørgensen +): ortho-NH3(11-00), N2H+(6-5) H2O(110-101) para-D2H+(110-111) ortho-D2H+(111-000) Di Francesco et al., in prep.

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

27. Evidences of freeze-out: deuterium fractionation What’s next? PSCs in high-mass star forming regions Multilayer spectroscopy of 8/24/70m-dark pre-star-cluster clumps in IRDCs (31 h, HIFI+PACS;Caselli, Tan, Beltran et al.) Dense cores and their molecular envelopes: HIFI simultaneous observations of ortho-H2O(110-101) and ortho-NH3(11-00) + N2H+(6-5). The atomic layer: [CII] 158m maps with HIFI. The extinction low + [OI]63m: PACS imaging spectroscopy between 51 and 73m