Jordy BouwmanSackler Laboratory for Astrophysics, Leiden Observatory

Shining light on PAHs in interstellar ices

IR absorption

PDR

Bauschlicher et al., ApJ 697, 311, 2009

Wealth of frozen species

E.L. Gibb et al., ApJ 536, 347 (2000)

IR spectrum towards a high mass protostar and laboratory spectrum

H2Ostretch

H2Obend

H2OLib.

CO

Bouwman et al., A&A, 476, 995 (2007)

IR Emission bands

PDR

3.3, 6.2, 7.7, 8.6, and 11.3 m

PAHs in ices

• PAHs are likely to freeze out on cold grains and are incorporated in ices…

1.How can we study these mixtures spectroscopically?

2.How do PAHs trapped in ices behaveupon VUV irradiation?

IR spectroscopy

• Anthracene:H2O ice 1:70 mixture

Bernstein et al., ApJ, 664, 1264 (2007)

PAHs in ices

E.L. Gibb et al., ApJ 536, 347 (2000)

Bernstein et al., ApJ, 664, 1264 (2007)

Other options?

• PAHs are known to be strong UV/VIS absorbers

• Common interstellar ice constituents are transparent for > 240 nm

Near UV/VIS spectroscopy!

Sensitive Solid-State Spectrometer

J. Bouwman, D.M. Paardekooper, H.M. Cuppen, H. Linnartz, L.J. Allamandola, ApJ, in press (2009)

Pyrene:H2O ice spectrum

J. Bouwman et al., Astronomy and Astrophysics, in prep.

Assignment 400 nm band

• Not a direct photon product!

• Evidence for HCO. radicals

Time and temperature evolution

Ionization is most efficient at low temperatures

Photoproduct band growth faster at high temperatures

PyH. nm band more prominent at high temperaturesJ. Bouwman et al., Astronomy and Astrophysics, in prep.

T=11 K T=100 K

Chemical evolution• VUV irradiation while measuring spectra

every 10 s permits real time tracking

J. Bouwman et al., Astronomy and Astrophysics, in prep.

T=25 K

Reaction rates

Py = Pyrene

Py.+ = Pyrene cation

PyH. = unknownPi = Product ikjj = reaction rate jj

Reaction scheme:

Pyrene H2O ice photochemistry

• Evidence for PAH-ion mediated (photo-) chemistry

• Two different temperature regimesReaction scheme:

low T regimeHigh T regime

Py = PyrenePy+ = Pyrene cation

PyH. = unknownPi = Product iKjj = reaction rate jj

Astrochemical picture

Bernstein, Sandford, Allamandola , Sci. Am. 7, p26 (1999)

Conclusions

• The setup shows great potential for both spectroscopy and kinetics

• PAH reaction paths are temperature dependent

• PAHs in H2O ice are readily ionized• Ions are stabilized in water ice and play a

previously neglected role in ice chemistry• VLT UV/VIS observations of embedded

objects are awaiting data reduction (help)

AcknowledgementsThanks to the Sackler laboratory group:

€€€

• Harold Linnartz• Herma Cuppen• Lou Allamandola• Claire Romanzin• Arthur Bakker• Daniël Paardekooper

• Nadine Wehres• Sergio Ioppolo• Karoliina Isokoski• Karin Öberg• Edith Fayolle• Ankan Das

Trapping of cations• Irradiate to obtain maximum cation signal

J. Bouwman et al., Astronomy and Astrophysics, in prep.

T=25 K

Normalized pyrene cation decay

• Double exponential decay at 10 K

1 = 2 hrs

2 = 351 hrs

J. Bouwman et al., Astrophysical Journal, in press (2009)