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Daniele GalliOsservatorio Astrofisico di Arcetri
Firenze
The First Hydrides
The Hydride ToolboxParis, 12-15 December 2016
A. Loeb (2006)
• Francesco Palla (Arcetri)• Carla Maria Coppola, Savino Longo (Univ. Bari)• Stefano Bovino (Univ. Hamburg)• Dominik Schleicher (Univ. Concepciòn)• Franco A. Gianturco (Univ. Innsbruck)
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Francesco Palla (1954-2016)
SOC chairs: Steve Stahler & Hans Zinnecker
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importance of three-body reactions:
Palla, Salpeter & Stahler (1983), ApJ, 271, 632
no 3-bodyH2
HMJ
T no 3-body
no 3-body
50 M¤
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Omukai et al. (2010)
primordial cloud
present-day cloud
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Turk et al. (2009)
density H2 fraction temperature
timeseparation 800 AUmass 10 M¤
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The Dawn of the HydridesThe Early Universe: a hostile environment for chemistry:• rapid expansion (low density and temperature)• strong radiation field (CMB + H, He recombination photons)• chemically poor (H=0.924, He=0.076, D=2x10-5, Li=4x10-10)• no solid particles (catalyzers)→ few molecules, mostly hydrides
The first hydrides are formed after recombination (z<1000):• Hydrogen subsystem: H2, H2
+, H3+, H-
• Deuterium “ “: HD, HD+, H2D+
• Helium “ “: HeH+
• Lithium “ “: LiH, LiH+
• In non-standard Big Bang nucleosynthesis, possible formation of CH, NH, OH, HF (Puy et al. 2007, Vonlanthen et al. 2009)
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Few species, relatively small chemical network:
• Lepp & Shull (1983) ~ 25 reaction, ~ 10 species
• Galli & Palla (1998) ~ 90 reactions, ~ 20 species
• Gay et al. (2011) ~ 250 reactions, ~ 30 species
Why are the first hydrides important?
• Theoretical models: control the thermodynamics of metal-free gas determining the mass of the first stars (well established)
• Observations: produce spectral/spatial signatures in the CMB that may allow to observationally probe the Dark Ages (challenging)
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400,000 yr after BB 400 Myr after BB
Galli & Palla (1998)
Galli & Palla (2013)
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Formation:He + H+→ HeH+ + g
Destruction: HeH+ + g → He + H+
HeH+ + H → He + H2+
Galli & Palla (1998), Bovino et al. (2011)
Helium chemistry
the first hydride!
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1. Molecular Hydrogen H2
• Cooling agent• Crucial ingredient in collapse models
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H2+ channel
H+H+ → H2+ + g
H2+ + H → H2 + H+
H- channel H + e- → H- + gH-+H → H2 + e-
Galli & Palla (1998), Coppola et al. (2011, 2012, 2013)
Hydrogen chemistry
NO state-resolved H2, H2+
NO H He recombination
photons
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Schematic picture of primordial star formation
baryons
dark matter
baryons
Hydrodynamics
Gravity
Chemistrymolecular cooling
from Yoshida (2003)
overdense regions
fragmentation
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Evolution of an overdense region
Tegmark et al. (1997), Galli & Palla (2002)
HD
H2 and HD cooling
turnaround
virializationadiabatic collapse
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virialization
H2 cooling
H2 cooling saturates (LTE)
3-body reactions
Yoshida et al. (2006)
first-star formation
H2
frac
tion
adiabatic collapse T ~ n2/3
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H-H2 cooling rate
Galli & Palla (1998)
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Low-density H-H2 cooling rate, present status
• Forrey, Balakrishnan, Dalgarno & Lepp (1997) → GP98 H-H2 cooling rate• Wrathmall, Gusdorf & Flower (2007) → GA08 H-H2 cooling rate• Lique, Honvault & Faure (2012, 2014)
→
→
old
new
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2. Hydrogen Deuteride HD
• Cooling agent• Important for cooling of post-shock gas
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Formation of HD:H2 + D+ → HD + H+
Destruction of HD: HD + H+ → D+ + H2
Stancil et al. (1998), Galli & Palla (2002), Coppola et al. (2011, 2012, 2013)
Deuterium chemistry
NO state-resolved H2, H2+
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• H2 cooling: DE (J=2-0)= 510 K, DE (J=3-1)=845 K → Tgas ~ 100 K• HD cooling: DE (J=1-0) = 128 K → Tgas ~ 10 K→ HD line cooling allows strongly shocked primordial gas to cool to the temperature of the CMB (the minimum value attainable)
• HD cooling functions computed by:Roueff & Flower (1999), Flower et al. (2000), Lipovka et al. (2005), Coppola et al. (2011) [LTE]
Johnson & Bromm (2006), Yoshida et al. (2007), McGreer & Bryan (2008), Glover & Abel (2008)
TCMB
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3. Lithium Hydride LiH
Rise and Fall of Primordial LiH
• LiH ≈ 10-12 – 10-11 (Lepp & Shull 1983)• LiH ≈ 10-14 – 10-13 (Palla, Galli & Silk 1995)• LiH ≈ 10-19 (Galli & Palla 1998)• LiH ≈ 10-17 (Bovino et al. 2011)
• The highest dipole moment• But primordial Li ≈ 10-10
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Li(2S)
Li(2P)
Dalgarno, Kirby & Stancil (1996)Gianturco & Gori Giorgi (1996)Bacchus-Montabonel & Talbi (1999)Bennett et al. (2003, 2008)
Lepp & Shull (1983)
Khersonskii & Lipovka (1993)
LiH formation
in the Early Universe
Li + H → LiH + hn
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Bovino et al. (2011)
incomplete Li recombination
with H, He recombination photons
Switzer & Hirata (2005)Hirata & Padmanabhan (2006)
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4. Hydride Ion H-
• Optical depth of the Universe
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CMBz~1000
400,000 yr after BB
First starsz~10
400 Myr after BB
Reionization completed
z~7
WMAPPLANCK
LOFARSKA
optical depth of the Dark Ages
t (H-, HeH+, LiH, HD+,…)
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Redshift integrated optical depth of H-
Black (2006), Schleicher et al. (2008)
H + e + g → H + e (free-free)
H- + g → H + e (bound-free)
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Redshift-integrated optical depth
Schleicher et al. (2008)
• IRAM 220 GHz (de Bernardis et al. 1993)• RATAN-600 6.2 cm (Gosachinkij et al. 2002)• ODIN 500 GHz (Persson et al. 2010)
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Molecular signals from the Dark Ages• The first hydrides induce spectral/spatial features in absorption or
emission against the CMB. Effects of order ~ t(Dubrovich 1977, Zeldovich 1978, Maoli et al. 1994, 1996, Puy et al. 2002)
Current limits:• DTr/Tr (spectral) ≈ 10-4 (COBE) • DTr/Tr (spatial) ≈ 10-6 (WMAP, PLANCK)Future improvement by 3-4 orders of magnitude (PIXIE, PRISM)
• Phase of expansionspectral distortions (absorption)
• Phase of linear growth : spatial anisotropies (resonant scattering)
• Phase of gravitational collapse: line emission
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Single star formation event not detectable. Protogalactic cloud with SFR ≈ 103-104 M¤ yr-1 detectable by JWST if z < 10. If z>20 ALMA?
M = 105 M¤
n = 102 cm-3 T = 400 K f(H2) = 10-4
M = 105 M¤
n = 106 cm-3 T = 400 K f(H2) = 10-3
H2
H2
HD
HD
Kamaya & Silk (2002, 2003)Mizusawa, Omukai & Nishi (2005)K. Walker, PhD Thesis (2016)
H2 and HD emission from primordial clouds
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• H2 and HD (and possibly H3+) control the cooling of metal-free gas
and determine the mass of the first stars.
• H-, HeH+ (and Li) dominate the optical depth of the Dark Ages.
• LiH plays a marginal role (if Li ≈ 10-10).
• Detection of the first hydrides would provide a powerful tool to constrain the evolution of the Universe from z ≈ 1000 to z ≈ 10.
Conclusions:The role of the first hydrides
ARAA vol. 51 (2013)