The chemical interaction of dust and gas in prestellar cores

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

QuickTime™ and a decompressor

are needed to see this picture.

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):

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


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



Friesen et al. 2010

850 m N2H+(1-0)


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






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)



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



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)


Spitzer IRAC 8μmMJy Sr-1

Mass Surface Density g cm-2

(Butler & Tan 2009; Peretto & Fuller 2009)

3´

QuickTime™ and aTIFF (Uncompressed) decompressor


What’s next?PSCs in high-mass star forming regions


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

Documents

The chemical interaction of dust and gas in prestellar cores