the composition of interstellar icesaboogert/talk_kauai_aboogert_feb2015.pdf · 23 feb 2015 lab...

23 Feb 2015 Lab Astro Kauai: Composition of Ices 1

The Composition of Interstellar

Ices

Adwin Boogert SOFIA/USRA NASA Ames

Moffett Field, CA, USA


Contents

1 Introduction/Motivation:

1.1 Securely/Likely/Possibly Detected Ice Species

1.2 A Decades-Old Problem: Unidentified Ice Features

1.3 Future Observations

2 The Way Forward:

2.1 Basic Chemistry

2.2 Cometary Abundances

2.3 Elemental Budget-Carbon in the Ices

2.4 (Gas Phase Observations)

3 Conclusions and Future Work

Reference: Boogert, Gerakines, & Whittet, ARAA 53 (2015-in press)


Secure, Likely, Possibly detected ice species: abundances, relative to H2O ice.

●~48 ice features detected, attributed to at least 14 molecules. Identification many ice features disputed!

●Any identification has major impact on interstellar chemistry studies as it must represent abundant species: ‒ peak=0.01:

● N=0.01*4 [cm-1]/1e-17 [cm/molecule]=4e15 cm-2

● ...orders of magnitude higher than gas phase detections!

CO, incl. 13CO few-60% CO2, incl. 13CO2 15-60%

CH4 2-4%

CH3OH 1-30%

NH3 2-15%

H2CO ~6%

OCN- <0.2, 2%

OCS <0.05, 0.2%

NH4+ 4-30%

SO2 <=3%

HCOO-/CH3CHO ~0.3/3%

HCOOH/CH3CH2OH 1-5%

PAH (50 C) ~8%

1.1 Introduction: Detected Ice Species


1.2 Introduction: Ice Feature Identification-3.47 m Band

●3.47 m band originally attributed to nano-diamonds (Allamandola et al. 1992).●Carrier is, however, as volatile as H2O ice (Brooke et al. 1999-see Figure)●Ammonia hydrates (NH3.H2O) more likely explanation (Dartois et al. 2002)


1.2 Introduction: Ice Feature Identification-3.25 m Band

●3.25 m band (appears to) correlate better with silicates than ices (Brooke et al. 1999-see Figure)●Carrier less volatile than H2O ice: C-H mode of PAH (Sellgren et al. 1994) or NH4+ salt (Schutte & Khanna 2003)●More data points needed, complicated by poor atmospheric transmission.


1.2 Introduction: 6 m AbsorptionAn 'old' problem: fair amount absorption in 5-8 m not explained by known species?

●Are we missing fundamental aspect?

―Not bulk H2O ice? χ CO2:H2O insufficient (Knez et al.

2005)―Scattering off large grains (e.g.,

Dartois et al. 2001)?χ Not seen in other ice bands

●Result of yet to be confirmed species:

―Most absorption due to volatiles. Some (6.2, 6.9 m) less volatile than H2O, but not like refractory dust.


1.3 Introduction: Future Ice Observations●JWST:

— NIRSpec: 1-5 m at R=100-3000

— MIRI: 5-28 m at R=100-3000

— Sensitive: S/N~100 on <50 mJy continuum on routine basis

— High spatial resolution

●SOFIA:— FORCAST: 5-50 m at R=100-1,000

— EXES: 5-28 m R=1,000-100,000

— suited for bright (few Jy) targets

— Upgradable!

— Much smaller beam than ISO!

●Instruments to be determined on SPICA satellite, large ground based telescopes TMT, ELT


1.3 Introduction: Future Ice Observations: Disks

1X1'' FOV

slit

Terada et al. (2007, 2012)

Boogert et al. (2015)

●High spatial resolution spectroscopy has already revealed ice processing in circumstellar disks.●These observations will become routine with new telescopes.


Coronagraphic spectrophotometric imaging of debris disk shows evidence for heavily processed simple species ('tholins'; Debes et al. 2008). 1-5 m optical constants needed!

1.3 Introduction: Future Ice Observations: Disks


2.1 The Way Forward: Basic Chemistry

Grain surface chemistry dominates molecule formation.

At T<20 K, n>=105 cm-3, a CO-rich chemistry takes place.


Ice formation threshold:

●H2O+CO2 form at AV=3.2/2 ●CO at AV=6/2 ●CH3OH at AV=18/2

●CH3OH formation relies on sufficient CO freeze-out




Thermal processing of ices commonly observed toward YSOs (distillation, crystallization, segregation). Chemical processing expected at this stage.

Occurence of energetic processing not observationally proven.


CO2 segregates out of H2O:CO2 ice


CO2 distills out of CO:CO2 ice

Ice tem

pera

ture

CO2 formed in CO ice

CO2 formed in H2O ice


2.1 Basic Chemistry: Generation 1 Species

Heating of ices creates “generation 1” species:

●Diffusion radicals creates new species (Herbst & Van Dishoeck 2009)

●Purely thermal reactions among generation 0 species formed by grain surface chemistry (Theule et al. 2013):

Acid-base reactions Nucleophilic additions H2O+HNCO H3O+OCN- CO2+NH3 NH2COOH NH3+HCOOH NH4+HCOO- CO2+CH3NH2 CH3NHCOOH

NH3+HNCO NH4+OCN- H2CO+H2O HOCH2OH NH3+HCN NH4+CN- H2CO+NH3 NH2CH2OH H2CO+CH3NH2 CH3NHCH2OH CH3CHO+NH3 NH2CH(CH3)OH



Theule et al. (2013)

Ice

tem

pe

ratu

re




Ice

tem

per

atu

re


2.1 Generation 1 Species: NH4+?✔acid/base mixture NH3+HNCO forms a salt (T~10 K; Raunier et al. 2004)

✗band too broad and shallow in H2O mixtures (Galvez et al., 2010), but not in salt clusters.

✗identification based on only one feature (6.85 m).

✗Interstellar OCN- abundance much less than NH4+

✔heating shifts band to longer

✔high Tsub of salt agrees with high 6.85/H2O warmer sightlines.

HEATING


2.1 Generation 1 Species: Aminomethanol?


Ice

tem

per

atu

re


2.1 Basic Chemistry: Laboratory versus Observations

Aminomethanol (NH2CH2OH) compared to the observations (Bossa et al. 2009):

Unresolvedemissionline!

Noise!

Need to analyze full-wavelength spectraNeed to make laboratory spectra available to the community


CO2/H2O ratio lower by ~50% in comets relative to envelopes low mass YSOs:

●The distributions do overlap, however! Conclusions on 'differences' are limited.

●“Cometary abundances might be underestimated due to opacity effects?”

●Regardless.......

2.2 The Way Forward: Cometary Abundances

(Boogert et al. 2015, Ootsubo et al. 2012)



● The same conclusion was drawn for CH3OH, CO, and NH3 (Oberg et al. 2011, Kawakita & Mumma 2011)

●Cometary ices appear to be carbon and nitrogen-poor, compared to envelopes of low mass YSOs.

● Can be used as guideline to search for species detected in comets, but not yet in YSOs



Lower abundance C-bearing species related to formation solar system in high radiation environment?

IC 1396 cloud (Reach et al. 2009)


2.3 The Way Forward: Elemental Abundances

●Elemental abundances in dense clouds and YSO envelopes give some clues in search for new species

●Areas in black are for elements locked up in silicate and graphite dust.


2.3 The Way Forward: Elemental Abundances

●Some 30% of O is 'missing'. How about 'invisible' very large (>several m) grains?

●'Only' 20% of C is missing, and half of that is most likely in frozen PAHs.

●Much N is missing, but a lot could be in N2 ice, which is hard to verify.

●The location of S in dense clouds is a mystery.


2.3 Ices and the Role of Carbon Dust~50% of C is in form of graphitic dust. What is its role w.r.t. ices?

1.H2O ice on top of or mixed with graphite dust could lead to CO2 formation in the presence of energetic particles. Effect small for typical ISM conditions (Ioppolo et al. 2013).

2.Graphite grains more abundant than silicate grains for sizes <0.01 m

● Observations of UV (electronic) bands of H2O could constrain ice mantles on small (graphite) grains (Goebel et al. 1983, more work needed)

● Grains smaller than ~20 Å do not form ice mantles due to stochastic heating,

● Graphite grains could freeze out on the larger grains, mixed with H2O. Abundance too small to disturb H2O hydrogen bonding network, in agreement with shape of 3 m ice band.


2.3 Carbon Budget

Sightline with very high CO freeze-out, where high CO2 and CH3OH abundance goes at the cost of CO:

●Well-identified species explain ~50% of 'volatile' C

PAHs big reservoir of C (~20% of non-dust C).


2.3 Carbon Budget: Frozen PAHs

Hardegree-Ullman, Gudipati, Boogert, et al., ApJ (2014)

●Experiments of pyrene in H2O and in D2O, simultaneously in UV+IR

●Improvements:—Absolute IR band strengths—Includes C-H stretch

●Results:—PAHs can explain ~10% of 5-8

m non-H2O absorption.

●Analysis complicated by: —PAH size distribution—PAHs easily ionize

PAH molecules universally present in the gas phase; how about ices?

Pyrene/H2O


2.3 Carbon Budget: Frozen PAHs●While individual PAHs show distinct peaks, they would smear out ―in a size distribution―in ion-neutral mixtures

●Ionized PAHs have weaker C-H band and stronger C-C modes, affecting abundance determinations.

●To Be Continued....

C14

C15

C22

Bouwman et al. (2011)


3. Conclusions and Future Work●Almost 50 ice features detected, attributed to at least 14 ice species

●Considerable identification issues remain.

●New telescope facilities with wide IR coverage and high sensitivity will give new constraints, but continued laboratory work essential.

●Thermal processing interstellar ices observationally well established. Molecule formation in the bulk ice at higher temperatures must be considered.

●Comets are C and N-poor compared to low mass YSO envelopes. Gives some constraints in search for carrier interstellar features.

●Search for N and S-bearing species especially needed

●Frozen PAHs reasonable candidate for 'unidentified' IR features.


Backup Slides


2.3 Carbon Budget: Frozen PAHs

Reality check: could all PAH embedded in the ices affect the profile of the 3 m H2O ice band? Answer: no

―Typical PAH molecule contains 50 C atoms in space, so molecular concentration in H2O ice <10%.

―Size of 50 C PAH species is ~14 Å, small compared to ~50 Å ice mantle thickness.

the composition of interstellar icesaboogert/talk_kauai_aboogert_feb2015.pdf · 23 feb 2015 lab...

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