Excitation Mechanisms:
The Irradiated and Stirred ISM
Marco Spaans (Groningen)
Rowin Meijerink (Leiden), Frank Israel (Leiden), Edo Loenen (Leiden),
Willem Baan (ASTRON), Juan-Pablo Perez-Beaupuits (Groningen)
Dominik Schleicher (Leiden/ESO), Ralf Klessen (Heidelberg),
Padelis Papadopoulos (Bonn), Paul van der Werf (Leiden)
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Concentrate on
irradiated turbulent gas
in star-forming regions
and close to AGN
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PDRs (UV/SB)
XDRs (X-ray/AGN)
MDRs (turbulence/flows)
CRDRs (CRs/SNe)
Photon excitation
(pumping/dust)
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Energetics
G0 = 1.6x10-3 erg cm s
is the Habing flux over 6-13.6
Orion Bar has 105 G0
-2
-1
eV
FX = 84 L44 r2-2 erg cm s
is the X-ray flux over 1-100 keV with a
power law E-0.9
Think of Seyfert nucleus at 100 pc or
TTauri star with 1032 erg/s at 20 AU
-2
-1
PDRs: 6 < E < 13.6 eV
Heating: Photo-electric emission from
grains and cosmic rays
 Cooling: Fine-structure lines like
[OI] 63, 145; [CII] 158 μm
and emission by H2, CO, H2O
 10 eV photon penetrates 0.5 mag of dust
 Heating efficiency ~ 0.1 – 1.0 %
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Orion Bar
(van der Werf 1993)
XDRs: E > 1 keV
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Heating: X-ray photo-ionization -->
fast electrons - Coulomb heating
H and H2vib excitation - UV
Cooling: [FeII] 1.26, 1.64; [OI] 63;
[CII] 158; [SiII] 35 μm;
thermal H2vib; gas-dust
1 keV photon penetrates 1022 cm-2 of NH
Heating efficiency ~ 10 – 50 %
30 Doradus (Brandl et al. 2007)
Maloney et al. (1996)
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PDR (left) with n=105 cm-3 and G=103.5
XDR with n=105 cm-3 and FX = 5.1 erg s-1 cm-3
Note NH dependence H2, C+, C, CO, OH, H2O:
FIR lines of species trace different regions
Fine-structure
lines
(Kaufman et al. 1999;
Meijerink et al. 2007
J=16-15 at 1841 GHz (0.16 mm)
redshifted into ALMA window
(Spaans & Meijerink 2008)
Mrk 231
SPIRE (see
Loenen
presentation;
van der Werf et al.
2010)
CO SLED has
not yet turned
down at
J=13-12
A comment on AGN:
Relative Size PDR/XDR
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107 M๏ BH at 3%
Eddington for
G0=100 and 1100 keV
powerlaw of
slope -1 (with
10% Lbol;
Schleicher et al. 2010)
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Check out poster
Pérez-Beaupuits!
Metallicity
and
Multi-Phase
ISM:
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Lower metallicity yields
smaller molecular
clouds
Atomic cooling
dominates by mass
X-factor:
Mihos
et al. (1999)
Bolatto et al.
(1999), Roellig et al.
(2006)
Multi-Phase Medium; atomic vs molecular
(Wolfire et al. 2003; Spaans & Norman 1997)
MDRs: how about kinetics?
Mechanically Dominated Regions
 Turbulent dissipation heats the gas,
which leads to IR emission
 UV only heats cloud surface
 Cosmic rays also heat deep inside
cloud, but strongly affect HCO+
 E.g., at T>100K: HNC + H  HCN + H
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Sources of Turbulence
YSOs
 SNe
 Sloshing motions (accretion)
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If 1-10% efficiency through a turbulent
cascade -> mechanical heating
competes with normal CR heating for
SF rates of 10 – 100 Mo/yr
M82, shock tracer SiO 2-1
+ 4.8 GHz radio
(García-Burillo et al.
2001, IRAM PdB)
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E.g., P cygni profiles in Arp220:
100 km/s outflow (100 pc scale;
Sakamoto et al. 2009)
g
Mrk 231,
SPIRE,
outflow in
OH upto
103 km/s
(Fischer et al.
2010)
Turbulent dissipation causes changes in
high density tracers (Loenen et al. 2008)
normal
mechanical
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Temperature increases
E.g., HNC, HCN, HCO+ affected
For some ULIRGs, dense gas tracers that
correlate with IR may trace more SN than
UV exposure
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How about CRs?
PDR model with CR rate = 5x10-15 s-1;
so SN rate for ~100 M0/yr
Note small changes in C, OH and H2O
CRs can dominate gas heating for SFR > 100
Mo/yr; think of Arp220 and IMF through MJeans
(Papadopoulos 2010)
CRs ≠ X-rays;
only very high CR
rates boost OH+
and H2O+
(fine-structure
lines little
affected by CRs)
Pumping of high density tracers
• HNC (HCN) rotational lines pumped by mid-IR photons at 21.5 (14) μm that are
absorbed by the degenerate bending mode (1st vibrational level) and decay via the P
branch (Aalto et al. 2007).
• A=5.2 (1.7) s-1 for HNC (HCN); requires
TIR~50 K (τ>1) and n<ncr
• Appears relevant for Arp 220,
Mrk 231, NGC 4418, where HNC
outshines HCN, and can
dominate upto 106 (103) cm-3 for
TIR=85 (55) K
Mrk 231; SPIRE,
IR pumping of
water lines by
dust emission
(Gonzalez-Alfonso 2010)
Summary
PDRs, XDRs, MDRs and CRDRs are
likely to be present simultaneously, but
are also distinguishable
 For the future, ALMA will be crucial to
provide spatial information to separate
the XDR and thus constrain properties
of accreting supermassive black holes
and feedback effects
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