Download Infrared Observations of Interstellar Ices Adwin Boogert NASA Herschel Science Center

Infrared Observations of
Interstellar Ices
Adwin Boogert
NASA Herschel Science Center
IPAC, Caltech
Pasadena, CA, USA
05 June 2012
Interstellar Dust School (Cuijk): Interstellar Ices (Boogert)
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Scope
Lecture 1 (Monday): What you need to know when planning, reducing,
or analyzing infrared spectroscopic observations of dust and ices.
●
Lecture 2 (Tuesday): Basic physical and chemical information derived
from interstellar ice observations. Not discussed: laboratory techniques
(see Palumbo lectures) and surface chemistry (see Cuppen lectures).
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Lecture 3 (Tuesday): Infrared spectroscopic databases. What's in them
and how (not) to use them.
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Drylabs (Tuesday): Using databases of interstellar infrared spectra and
of laboratory ices. Deriving ice abundances and analyzing ice band
profiles.
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NOTE: Please download all presentations and drylab tar file:
spider.ipac.caltech.edu/~aboogert/Cuijk/
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Topics
Basics
● Ice mantle formation
● Deriving ice column densities and abundances
●YSO and background source selection
●Continuum determination
●Vibrational modes
●The interstellar ice inventory
●Ice band profile analysis:
● Polar versus apolar ices
● Amorphous versus crystalline ices
● Segregation in the ices
● Grain size and shape effects
●Location of ices
●Processing of ices by YSOs
●Complex molecules in ices?
●
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Background Reading
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●
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For the basics: Dust in the galactic environment, 2nd ed.
by D.C.B. Whittet. Bristol: Institute of Physics (IOP)
Publishing, 2003 Series in Astronomy and Astrophysics,
ISBN 0750306246.
More advanced: Chapter 10 in “The Physics and
Chemistry of the Interstellar Medium”, A. G. G. M.
Tielens, ISBN 0521826349. Cambridge, UK: Cambridge
University Press, 2005.
Current status of observational ice studies: Oberg et al.
2011, ApJ 740, 109
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Basics: Ice Mantle Formation
Many molecules (H2, H2O) much
more easily formed on grain
surfaces. Freeze out <100 K.
Keywords: Physisorption,
chemisorption, tunneling (see
lectures by Cuppen)
Interstellar ‘ice’ or ‘dirty ice’: any
frozen volatile, e.g. H2O, H2O
mixtures, pure CO, but NOT H2.
More realistic grain:
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Basics: Ice Mantle Formation
For gas at number density n, mean speed <v>, mean particle
mass <m>, gas-to-grain sticking coefficient S, grain radius a,
and grain density 
●Grain mantle thickness:
Mass growth rate:
dm/dt=S**a2*n*<v>*<m>
Radius growth rate: da/dt=(dm/dt)/(4**a2*)
 da/dt=S*n*<v>*<m>/(4*)
Mantle thickness independent of grain radius
● Dense clouds can have mantles as thick as 0.1 um, and in
deeply embedded protostars even more.
● Mantle thicker than most grain cores according to MRN grain
size distribution
n(a)~a-3.5, amin=0.005 μm, amax=0.25 μm
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Basics: Ice Mantle Formation
●Grain temperature and interstellar radiation field inhibit ice
Column Density
formation at low visual extinction (AV): the ice formation threshold
●Taurus cloud: H2O ices absent below visual extinction AV~3 and
CO ices below AV~7.
●Difference due to lower Tsub of CO.
●Variation between clouds due to different conditions
H2O
CO
Extinction (AV)
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Extinction (AV)
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Basics: Ice Column Densities and
Abundances
Ice column densities:
– N=peak*FWHM/Alab
–
Alab integrated band strength measured in laboratory
–
A[H2O 3 m]=2.0x10-16 cm/molecule
Order of magnitude in quiescent dense clouds:
– N(H2O-ice)=1018 cm-2 along absorption 'pencil beam'.
–
This is ice layer of 0.3 m at 1 g/cm3 in laboratory.
Order of magnitude estimate of NH (for ice abundances):
– AV=9.7*18.5 mag (Roche & Aitken 1984)
–
NH=AV*2.0*1021 cm-2 (Bohlin et al. 1977)
Ice abundance:
– X(H2O-ice)=N(H2O-ice)/NH~10-4
–
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This is comparable to X(CO-gas)
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Observing Ices
H2O
H2O
NH4+
Ices form anywhere T<90 K and
Av>few magn. Visible against
Foreground continuum YSO or background star.
cloud(s)
Star-forming dense core
silicates
CO2
Background
star
H2O NH4+
H2O
envelope
silicates CO2
star
disk
outflow
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Source Selection
Taking advantage of large scale infrared imaging surveys, YSOs
and background stars can be selected using broad-band 2-25 μm
colors. Extinction determined for many background stars,
assuming average, intrinsic stellar colors (“NICE” method).
L 1014; AV=2-35; 20” resol; Huard et al. (ApJ 640, 391, 2006)
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Spectra of Spitzer-selected YSO
and Background Star
Isolated core L 1014; AV=2-35; Huard
et al. 2006, ApJ 640, 391, 2006
YSO
Background
Star
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Continuum Determination
Ice features are studies on optical depth scale
flux 
=−ln 

continuum
Critical step in the analysis of ice bands is continuum determination.
This can be done:
Locally, on a limited wavelength range (for single or few features),
usually done with polynomial fit
●
●
Globally:
● Physical model of source (can be done for background stars,
rarely feasible for YSOs)
● Polynomial or spline. Relatively subjective. Fits can be guided by
taking into account models and laboratory spectra of dominant
absorbers (H2O and silicates).
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Continuum Determination
Background Stars
Red: M1 III model and featureless extinction curve at AK=1.5 magn
Green: H2O ice and silicate model added
H2O
H2O
?
2 minimization includes:
●
Spectral type (CO and SiO bands)
●
Stellar models (MARC; Decin et al.)
●
Extinction laws
●
silicates
Silicates model
●
L-band spectra (H2O ice)
●
H2O ice model
●
H2O
CO2
1-25 m photometry
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Continuum Determination YSOs
Continuum
determination
YSOs much
harder because
models have
many poorly
constrained
degrees of
freedom.
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Observing Solid State Molecules
●H2O
ice has many broad
absorption bands:
● Symmetric stretch
●
Asymmetric stretch
●
Bending mode
●
Libration mode
●
Combination modes
●
●
Lattice mode (can be in
emission)
etc...
●CO:one
vibrational mode
●No features for species
without permanent dipole
moment (O2, N2, H2).
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Ice Inventory
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Ice Inventory
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Ice Inventory
[H2O and silicate subtracted!] 05 June 2012
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Ice Inventory
[H2O and silicate subtracted!] 05 June 2012
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Ice Inventory
NH3/CH3OH=4
(SVS 4­5)
NH3/CH3OH<0.5
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Low Mass vs High Mass Protostar
Protostellar luminosity apparently not a dominant factor in
ice formation and processing
Noriega­Crespo et al. ApJS 154, 352 (2004) 05 June 2012
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Ice Inventory
CO, incl 13CO
CO2, incl. 13CO2
few-50%
15-35%
CH4
2-4%
CH3OH
[HCOOH]
NH3
<8, 30%
H2CO
[HCOO-]
OCS
[SO2]
<2, 7%
[NH4+]
3-12%
[OCN-]
<0.2, 7%
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3-8%
<10, 40%
'Typical'
abundances
with respect to H2O
ice. Species in
brackets somewhat
disputed.
0.3%
<0.05, 0.2%
<=3%
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Ice Versus
Gas Phase
Inventory
Gas phase molecules
detected in space (123 listed
here). Currently up to ~150?
www.cv.nrao.edu/~awootten/allmols.html
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Far less ices than gas phase
species detected because
ices can only be detected by
absorption spectroscopy:
weakest features (1%)
represent column density
0.01*4 [cm-1]/1e-17
[cm/molecule]=4e15 cm-2,
orders of magnitude higher
than gas phase detections!
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Ice Band Profiles Analysis
Ice band profiles contain wealth of information because they
depend on dipole interactions (bond lengths are modified by
attractive or repulsive electric forces). Strong effects caused
by:
Ice composition, pure ices versus mixtures
●Matrix structure: amorphous versus crystalline. Temperature.
●Grain size and shape: surface charge induced by external light
(polarizability)
●
Comparison with laboratory analogs powerful tool, but at same
time fitting is subtle and solutions not unique.
Instead of fitting 1000 lab spectra to the interstellar spectra,
best to use trends in peak position versus width to draw
general conclusions.
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Ice Band Profiles
Polar vs Apolar Ices
Peak position and
width of CO ice
band depends
strongly on what
other molecules are
present in the ice.
(also note strong
dependence on
thermal history and
grain shape)
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Ice Band Profiles
Polar vs Apolar Ices
•
Interstellar CO ice band
consists of 3 components,
explained by laboratory
simulations as originating
from CO in 3 distinct
mixtures:
–
'polar'
H2O:CO
–
'apolar'
CO2:CO
–
'apolar'
pure CO
(Boogert, Hogerheijde & Blake, ApJ 568,761, 2002)
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Ice Band Profiles
Polar vs Apolar Ices
●CO
ice profiles vary in different sight-lines, as a result of different
sublimation temperatures: ~90 K for H2O-rich and ~18 K for COrich. Profile good indicator of thermal processing.
●Polar/non-polar distinction relevant for outgassing behavior of
comets. 'Pockets' of apolar CO may result in sudden sublimation.
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Interstellar Dust School (Cuijk): Interstellar Ices (Boogert)
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Ice Band Profiles: Amorphous vs.
Crystalline
Interstellar H2O ices formed
in amorphous phase, as
evidenced by great width.
Crystallization by YSO heat.
Crystallization temperature
~120 K in laboratory, but
~70 K in space due to longer
time scales: ~1 hour in lab
and 104-105 yr in space:
Time scale ~exp(Ebarrier/T)
[For same reason
sublimation temperature in
lab (~180 K) higher than in
space (~90 K)]
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Ice Band Profiles
Segregation
●If
average bond strength in
pure ice is stronger than in a
mixture, mixture will
segregate into clusters of
pure ice as the ice is heated.
●This
is how 'pure CO2' ice is
formed in space: CO2, H2O,
and CH3OH formation are
linked through OH. The
characteristic double peak of
the 15 m CO2 bending mode
appears when the mixture of
CO2 with H2O and/or CH3OH
Oberg et al., 2009, A&A 505, 183
is heated.
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Ice Band Profiles
Grain Shape and Size Effects
Laboratory and interstellar absorption spectra cannot always be compared directly:
●Scattering on large (micron sized) grains leads to 3 μm red wing (often observed)
●Surface modes in small grains may lead to large absorption profile variations
●For ice refractive index m=n+ik, absorption cross section ellipsoidal grain
proportional to (Mie theory) (2nk/L2)/[(1/L-1+n2-k2)2+(2nk)2]
●Resonance for sphere (L=1/3) occurs at k2-n2=2, so at large k (=strong transitions)
●Important for pure CO, but not for CO diluted in H2O and also not for 13CO.
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Ice Band Profiles
Grain Shape Effects
•
•
•
First detection of solid
13CO
High spectral resolution
required!
New insights into nature
apolar ices:
–
–
(Boogert, Blake & Tielens, ApJ 577, 271 (2002))
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13CO
well fitted with
pure CO, but 12CO
requires ellipsoidal
grains
Most CO not mixed with
CO2
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Processing of YSO Ices
Ice processing
'hot topic' in
astrobiology.
Which
processing
mechanism
(heating, UV
light, cosmic
rays) is relevant
where? Do
processed ices
survive and
how?
Bill Saxton (NRAO/AUI/NSF)
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Where ARE the Ices?
Ices formed anywhere T<90 K and extinction large enough.
Proper interpretation of ice features, such as the causes of
processing history, requires knowledge of location of ices
along the line of sight. This can be achieved by
complementary gas phase observations, by high spatial
resolution spectroscopy, and/or physical models.
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Where ARE the Ices?
Physical conditions vary with
radius from the star and
vertically in disk. Understanding
ice processing of protoplanetary environment requires
disk modelling.
-1.5
ρ(R)~R
Spatial distribution CO2 in circumstellar envelope.
(Pontoppidan et al. 2008)
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Where ARE the Ices?
Ices on small scales can nowadays be studied using infrared
spectrometers behind adaptive optics systems on large (>=8
m) telescopes, approaching diffraction limit (<0.1''). E.g., IRCS
at Subaru and CONICA at VLT.
slit
1X1'' FOV
Edge on disk HV Tau C can be
Crystalline H2O ice from
separated by binary component and slit
can be oriented at desired angle w.r.t.
upper disk layers YLW16A
detected by Schegerer et al. disk. H2O ice band depth varies on few
(2010)
year time scale (Terada et al. 2007).
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Processing Ices YSO Envelopes
Observational evidence for
thermal processing of ices near
YSOs:
●Solid
CO2 band profile varies
toward different protostars…
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Processing Ices YSO Envelopes
Observational evidence for thermal
processing of ices near YSOs:
Solid 13CO2 band profile varies
toward different protostars…
● …and laboratory simulated
spectra show this is due to
CO2:H2O mixture progressively
heated by young star (Boogert et
al. 2000; Gerakines et al. 1999)
●
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Processing Ices YSO Envelopes
Observational evidence for thermal
processing of ices near YSOs:
●Solid
CO2 band profile varies
toward different protostars…
●…and laboratory simulated spectra
show this is due to CO2:H2O mixture
progressively heated by young star
(Boogert et al. 2000; Gerakines et al. 1999)
●H2O crystallization (Smith et al. 1989)
●gas/solid ratio increases (van Dishoeck et al. 1997)
●Detailed modelling gas phase mm­
wave observations (van der Tak et al. 2000)
Little evidence for energetic
processing of ices, however......
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Complex Molecules?
Complex species formed, some are of
biological interest:
•POM (polyoxymethylene, -(CH2-O)n•HMT (hexamethylenetetramine,
C6H12N4)
•Amino acids (glycine)
•Urea (H2NCONH2)
•PAHs (polycyclic aromatic
hydrocarbons)
Greenberg et al. ApJ 455, L177 (1995): launched
processed ice sample in earth orbit exposing
directly to solar radiation (EURECA experiment).
Yellow stuff turned brown: highly carbonaceous
residue, also including PAH.
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Complex Molecules?
Observational evidence for complex products of UV/CR
bombardment of simple ices is weak at best:
●
Triple peak 3.4 m band seen only in diffuse medium and
it is not polarized as opposed to silicates/ices: not in
processed mantle but separate grains
●
Perhaps organic residue in 5-8 m region?
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Molecular Evolution: Hot Cores
Formic acid
Methyl
formate
Formic acid
Dimethyl
ether
Ceccarelli et al. A&A 521, L22 (2010)
High spectral and spatial resolution observations of
Hot cores. Trace the ices indirectly, after sublimation.
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Future of Astrochemistry is Bright....
Atacama Large MM Array
Thirty Meter Telescope
James Webb Space Telescope
….plus a lot more……
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