Interstellar PAH variants IR Spectroscopy

Amit Pathak Banaras Hindu University

Varanasi

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Collaborators

Mridusmita Buragohain, Itsuki Sakon, Takashi Onaka (UoTokyo) Ekant Vats (BHU) N K Gour (Tezpur University) Peter Sarre (UoNottingham)

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Polycyclic Aromatic Hydrocarbons (PAHs)

They are stable, made mostly of carbon and are everywhere.

These sooty molecules make upto 10-15% of the total carbon.

They are carcinogenic.

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Why PAHs?

Aromatic Infrared Bands

UV bump at 217.5 nm.

3.4 µm absorption feature.

The Extended Red Emission (PAH clusters).

Diffuse Interstellar Bands.

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Infrared emission at 3030, 1610, 1300, 1160 & 890 cm-1 (3.3, 6.2, 7.7, 8.6 and 11.2 µm).

Weak features (ISO, SPITZER, AKARI, etc.) point to variations in the interstellar PAH population.

(Tielens 2008)

(Joblin 1992 & Clayton 2004)

(Salama et al., Ap. J. 1999, 526 : 265)

(Witt et al., Ap. J. 2006)

Aromatic Infrared Bands

UV bump at 217.5 nm.

3.4 µm absorption feature.

The Extended Red Emission (PAH clusters).

Diffuse Interstellar Bands.

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Why PAHs?

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PAH variants

Deuterated

Aliphatics

Dehydrogenated anions

Nitrogenated

ISO Observations Peeters et al. 2004

Deuterated and Deuteronated PAHs

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Deuterated and Deuteronated PAHs

AKARI Observations Onaka et al. 2014

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Deuterium abundance

Current Deuterium (D/H) in interstellar gas is less compared to the primordial abundance (7 to 22 ppm compared to 26 ppm).

This is not explained by astration (comparison of D/H with O/H).

Large variation in D/H is present (over a few hundred pc scale) compared to O/H.

No anti-correlation between D/H and O/H observed.

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IR spectra for coronene Buragohain et al. 2015, 2016

A peak at 4.4 µm (2275 cm-1) in

deuterated coronene C-D

stretching

In deuteronated PAHs,

frequency shifts towards longer

wavelength side - larger mass

peak at 4.7 µm (2130 cm-1) in

spectrum of deuteronated PAHs –

attributed to C – D stretching

Deuterated &

Deuteronated PAHs

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splits into 2 peaks

Protonated coronene H-C-H+ symmetric stretching

Deuteronated coronene

C-H stretching

C-D stretching

Protonated & Deuteronated PAHs

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The aliphatic C-D stretch in DPAH+

DPAH+ Wavelength µm

Relative Intensity

[D/H]num [D/H]int [D/H]sc

D-Pyrene 4.78 0.14 0.1 0.52 5.2

D-Perylene 4.79 0.06 0.083 0.25 3.0

D-Coronene 4.74 0.04 0.083 0.22 2.7

D-CorD+ 4.74 0.07 0.18 0.26 1.4

D-Circumcoronene 4.70 0.003 0.06 0.008 0.1

[D/H]num – no. of D atoms / no. of H atoms

[D/H]int – intensity of C-D stretch / intensity of C-H stretch

[D/H]sc – [D/H]num / [D/H]int

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The aliphatic C-D stretch in DPAH+

DPAH+ Wavelength µm

Relative Intensity

[D/H]num [D/H]int [D/H]sc

D-Pyrene 4.78 0.14 0.1 0.52 5.2

D-Perylene 4.79 0.06 0.083 0.25 3.0

D-Coronene 4.74 0.04 0.083 0.22 2.7

D-CorD+ 4.74 0.07 0.18 0.26 1.4

D-Circumcoronene 4.70 0.003 0.06 0.008 0.1

[D/H]num – no. of D atoms / no. of H atoms

[D/H]int – intensity of C-D stretch / intensity of C-H stretch

[D/H]sc – [D/H]num / [D/H]int

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Formation of Deuterated / Deuteronated PAHs

Deuterium is restricted to grain surface reactions (mostly in dense clouds).

PAHs may be the largest reservoir of materials enriched in D by ion-

molecule reactions (i) PAHs are more stable (ii) PAHs have peripheral H atoms (for substitution by D)

D atom addition to PAHs producing aliphatic C – D bonds.

D atom substituting the existing H atom on the PAH.

In harsh radiation field interstellar environments, since the rate of

unimolecular photodissociation will increase with decreasing molecular size, smaller PAHs are more likely to get deuterated.

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Aliphatic PAHs – C-H stretch

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PAH anions may be the principal carriers of negative charge in IS clouds

Dehydrogenated PAHs may lead to formation of Fullerenes

Carbon loss followed by pentagon formation initiates the curling of the molecule.

The graphene sheet curls up, forms a bowl-like structure, and zips up into a ball.

Dehydrogenation of PAHs

In less dense

environments

Chuvilin et al., Nature Chemistry 2010

Berne & Tielens, 2011, PNAS

Dehydrogenated PAH anions

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Dehydrogenated PAH anions

Partially dehydrogenated PAH anions

may contribute to the 3.3 μm band and

intense features in the 5–10 μm region.

Duo C–H group may convert into a solo

C–H unit producing 3.4 μm feature due to

C–H stretching.

5.2 μm feature (free C–C/C–C–C stretch)

identifies dehydrogenated PAH anions.

Absence of this feature suggests that

such PAHs may be dehydrogenated up to a

certain limit.

Buragohain et al. 2018

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Nitrogenated PAHs

Important to explain the

position of the 6.2 μm band.

Surprisingly the 11.2 μm is

strong even in ionized ones.

Observations of disks around

Herbig Ae/Be objects, Planetary

Nebulae, etc. have similar

charactersitics.

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Nitrogenated PAHs

Important to explain the

position of the 6.2 μm band.

Surprisingly the 11.2 μm is

strong even in ionized ones.

Observations of disks around

Herbig Ae/Be objects, Planetary

Nebulae, etc. have similar

charactersitics.

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COMICS N-band (7.5 – 13.5 μm) is appropriate for the observations of PAH bands.

Observations of MWC1080 (Herbig Ae/Be star) to estimate the ionization of PAHs.

Sakon et al. 2006

Summary

Detection of a single PAH still elusive (degeneracy is the culprit)

Dedicated observations in the mid-IR needed

COMICS fills in the gap due to unavailability of space instruments

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Acknowledgement

JSPS-DST collaboration

SERB-DST, New Delhi funding

UK India Education and Research Initiative

HPC@Nottingham

HPC@IUCAA

UGC-CSIR, New Delhi

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