the chemical interaction of dust and gas in prestellar cores
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School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES. The chemical interaction of dust and gas in prestellar cores. Paola Caselli. Collaborators. - PowerPoint PPT PresentationTRANSCRIPT
School of Physics and AstronomyFACULTY OF MATHEMATICS & PHYSICAL SCIENCES
The chemical
interaction of
dust and gas
in prestellar
cores Paola Caselli
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)
Main Uncertainties
• Cosmic-ray ionization rate
• Elemental abundance (metals)
• Oxygen chemistry
• PAHs abundance
• Surface chemistry
• Dust evolution
• H2 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)
Outline
1. The formation of H2
2. The chemistry of water 3. CO formation4. Nitrogen chemistry
5. Molecular freeze-out 6. Deuterium fractionation7. The Herschel view
Bergin & Tafalla 2007 + Di Francesco et al. 2007
PD
R la
yer
Core center
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)
€
RH2=
1
2nHvHAngSHγ
≅ 10−17 cm-3s-1
1. The formation of H2
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
Evidences of freeze-out: deuterium fractionation 2. The chemistry of water
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In the gas phase (e.g. Hollenbach et al. 2009):
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Desorption (d) from dust surfaces (Hollenbach et al. 2009; Garrod 2008; Cazaux et al. 2010):
Hollenbach et al. 2009
G0 variations Grain size variationsPhotodesorption yield variations
2. The chemistry of water
Volume density variations
SWAS + Odin upper limits: x(H2O)gas <10-8 (Bergin & Snell 2002; Klotz et al. 2008)
BUTLine trapping and absorption of the dust continuum challenge
the measurement of x(H2O)gas (Poelman et al. 2007)
3. CO formation
CO
tCO ~ nC/[n(H2)] ~ 105yr
Sternberg & Dalgarno 1995
4. Nitrogen chemistry
Flower et al. 2006Hily-Blant et al. 2010
N2
N + H3+ NH+ + H2
N + OH NO + HN + CH CN + H
tN2 ~ 106 yr in UV-shielded clouds
CO
5. Molecular freeze-out
Freeze-out versus Free-fall
Walmsley 1991van Dishoeck et al. 1993
€
tdep =1
αndπad2v t
≈109 mX /T (nHα )−1 yr
€
t ff =3π
32Gρ
⎛
⎝ ⎜
⎞
⎠ ⎟
−1/ 2
= 4 ×107 (nH )−1/ 2 yr
5. Molecular freeze-out
C17O(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
D-fractionation increases towards the core center (~0.2; Caselli et al. 2002; Crapsi et al. 2004, 2005)
N2D+(2-1)
N2H+(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
5. Molecular freeze-out
Friesen et al. 2010
850 m N2H+(1-0)
5. Molecular freeze-out
Crapsi, Caselli, Walmsley & Tafalla 2007
• On size scale of 800 AU:No NH3 freeze-out at nH ~ 106 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 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
N(NH2D) @ PdBI
700 AU
1400 AU
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).
N2 N2D+ + H2
H2D+ + CO DCO+ + H2
Watson 1974Millar et al. 1989
6. Deuterium fractionation
Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation
o-H2D+
CSO
N2D+(2-1)IRAM
N2H+(1-0)IRAM
Vastel et al. 2006
Caselli et al. 2003, 2008
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)
Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation
Flower, Pineau des Forêts, Walmsley 2004
L429
L694-2L1544
L183
B68
16293E
NGC2264GNGC1333 DCO+
L1521FB1
OphD
L1517B
TMC-1C
Caselli et al. 2008
See also Pagani et al. 2009
Sipilä et al. 2010
The ortho-to-para ratio:
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, submittedCaselli et al. 2010, in press
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Evidences of freeze-out: deuterium fractionation 7. The Herschel view
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Caselli et al. 2010
Evidences of freeze-out: deuterium fractionation
x(o-H2O) ~ 210-10 within the central ~7000 AU ~ 510-9 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)
Evidences of freeze-out: deuterium fractionation 7. The Herschel view
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, H2CO, CH3OH) at densities above a few 104 cm-3.
3. N2H+ starts to freeze-out at nH > 105 cm-3. 4. No clear evidence of NH3 freeze-out at large nH
(Herschel needed!), as well as CN, HCN and HNC.5. N2D+ and deuterated ammonia peak toward the coldest
and densest zones (tfreeze < 1000 yr) (ALMA).6. H2D+ is spatially coincident with N2D+ (i.e. it does not
trace “molecular holes”). D2H+ observations needed.7. PSCs H2O abundances are low (~10-10) within cores
(steep gradients?). Chemical models need revision.
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 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.
Evidences of freeze-out: deuterium fractionation What’s next?PSC properties in cluster forming regions
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Oph A seen in 850 m (color scale), N2H+(4-3) (green contours) and ortho-H2D+(101-110) (blue contours).
Di Francesco et al., in prep.
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)
Evidences of freeze-out: deuterium fractionation
Spitzer IRAC 8μmMJy Sr-1
Mass Surface Density g cm-2
(Butler & Tan 2009; Peretto & Fuller 2009)
3´
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What’s next?PSCs in high-mass star forming regions
Evidences of freeze-out: deuterium fractionation
A. Dense cores and their molecular envelopes: HIFI simultaneous observations of ortho-H2O(110-101) and ortho-NH3(11-00) + N2H+(6-5).
B. The atomic layer: [CII] 158m maps with HIFI.
C. The extinction low + [OI]63m: PACS imaging spectroscopy between 51 and 73m
Multilayer spectroscopy of 8/24/70m-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