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Takashi ShimonishiUniversity of Tokyo

T. Onaka, D. Kato, I. Sakon, Y. Ita, A. Kawamura, H. Kaneda

Feb 3, 2011 @ IAS, France

Infrared Observation of Ices around Extragalactic Young Stellar Objects

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Fig.1 Formation of a star and chemical evolution of molecules [van Dishoeck and Blake, 1998]

Molecular Cloud Protostar Proplyd

Planetary System

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3.05umH2O 4.27um

CO2

4.67umCO

Fig.3 ISO spectrum of a embedded YSO [Gibb et al. 2004]

Fig.2 Formation of ices in the dense region of Mol. C [Boogert & Ehrenfreund, 2004]

Unknown

Reaction pathway of molecules

Dominant factor controlling the chemical composition of ices

Ice properties as a function of evolutionary stages of a YSO

Ices around extragalactic YSOs

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Known

Kinds of solid molecules

Typical abundance of each molecule

Almost molecules are formed on a dust surface

Similar molecular abundance toward comets

z

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HIGH Metallicity LOW

MW

LMC

SMC

Fig.16 YSO Chemistry as a Function of Metallicity

How does a chemical property of ices around a YSO vary in other galaxies?

Are there any correlation between a property of ices and a physical environment of a YSO?

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Proximity and well-determined distance

― LMC, ~50kpc ( 1”~0.25pc)

― SMC, ~60kpc

Nearly face-on (i ~35°)

Low metallicity

(LMC, ~1/3; SMC, ~1/10 of solar neighborhood)

Ideal galaxy for the extragalactic YSO study

Fig.5 Optical images of the LMC and SMC

LMC

SMC

Japan + Korea + ESA project

Launched in 2006

Operation is currently stopped

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Fig.6 AKARI in space Infrared Camera (IRC) on board AKARI

－ 2.5—5um, R~80-100

－ Detection limit: ~1mJy

－Spatial resl.: ~6” (~1.5pc at LMC)

3.05umH2O 4.27um

CO2

4.67umCO

Fig.7 ISO-SWS spectrum of a embedded YSO: AFGL2136 [from Gibb et al. 2004]

IRC/AKARI covers major absorption features of dominant ices

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yellow：AKARI / IRC( NIR Prism, 3, 7, 11,15,24 mm)green：Spitzer SAGE

(IRAC 1—4, MIPS 1--3)purple：IRSF / SIRIUS

(J, H, Ks)Fig. 8 AKARI LMC spectroscopic survey area

IRC multi-object prism spectoscopy(2.5－5mm, R～20)Detection Limit: 1mJy @3umPlus, 5 imaging bands

~10deg2 survey area

Large samples of NIR spectra

H2O ice(3.05um)

CO2 ice(4.27um)

2005, van Loon et al.

― First spectroscopic detection of ices toward an extragalactic YSO

2008, Shimonishi et al.

― Discovery of 7 YSOs in the LMC with strong ice features, and systematic comparison of the CO2/H2O ratio in the LMC and MW

2009-2010

― NIR follow-up observations of ~15 YSOs with AKARI (Shimonishi+ 2010, 2011 in prep.)

― MIR observations of ~40 LMC’s YSOs with Spitzer (Seale+ 2009,2010, Oliveira+ 2009)

― NIR and MIR observations toward 5 SMC’s YSO with AKARI (Shimonishi+ 2011 in prep.) and Spitzer + VLT (Oliveira+ 2009)

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15 high-mass YSOs in the LMC (10~36 Msun, Class Ⅰ, derived by the YSO model of Robbitaile et al. 2006)

Almost objects are for the first time spectroscopically observed in the NIR wavelength

LMC (3 micron)

Fig.9 Spitzer SAGE survey [Meixner+ 2006]

AKARI image & spectrum

2mm

5mm

1’

8 deg

From Shimonishi et al. 2010

ST3

H2O ice(3.05um)

CO2 ice(4.27um)

CO ice(4.67um)

ST3

Fig. 11Observed Spectra of Embedded YSOs in the LMC

How does a chemical property of ices around a YSO vary in other galaxy?

Are there any correlation between a property of ices and a physical environment of a YSO

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Species Galactic YSO[%]

Comets [%]

ExtragalacticYSO

H2O (water) --- --- ?

CO2 (carbon dioxide) 17 3-6 ?

CO (carbon monoxide) 1－50 7-20 ?

CH3OH (methanol) 2－5 ～2 ?

Table.1 Abundances of ices toward various objects

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□：LMC (Shimonishi+2008, 2010)

■：Galactic massive YSO(Gibb+ 2004)

Fig.12 H2O ice vs. CO2 ice column density

CO2/H2O~36%

(LMC)

17% (Galactic massive YSO, Gerakines et al. 1999)

YSOs in other galaxy have chemically different nature from Milky Way’s YSOs.

*1017 [molecule * cm-2]

CO2/H2O ~ 70% !!!

Elemental abundance of the

galaxy (e.g. C/O ratio, ref. Das+ 2010)

Cosmic ray density

― UV photon inside a dense cloud is induced by cosmic ray (e.g. Shen+ 2004)

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Observation

Abundant detection of CO2 ice in Mol.C (Whittet et al.1998)

Experiment

High activation barrier in the CO2 ice formation reaction:

Need UV radiation field (e.g. Allamandolla et al. 1988)

CO + O →CO2 ？

Theory

Alternative mechanism“Diffusive Grain Surface Chemistry”

CO + OH →CO2 + H ？O + HCO →CO2 + H ？

No need for UV photons?

Dust temperature produce sufficient CO2 ice? (Ruffle & Herbst, 2001)

Others

C/O ratio in the LMC

― is lower or nearly same with our Galaxy

(Dufour+ 1982, Andrievsky+ 2001, Rollenston+ 2002)

CR density in the LMC

― is 25~50% smaller than that of typical Galactic value

(Abdo+ 2009, 2010 based on gamma-ray observations by FERMI)

UV radiation field in the LMC

― is 10—100 times stronger than typical Galactic value (Israel & Graauw, 1986, Tumlinson+ 2002)

Dust temperature in the LMC (diffuse region)

― is higher than our Galaxy (e.g., LMC:~22K, Sakon+ 2006, MW: ~17K,Boulanger+ 1996 )

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Typical abundance of major solid molecules (H2O and CO2) is different in the LMC, due to different interstellar radiation environment

Milky Way

H2O

CO2

ice

Interstellar Radiation

CO2/H2O ~36%(LMC)

17% (Galactic massive YSO, Gerakines et al. 1999)

LMC

H2O

CO2

ice

Interstellar Radiation

Spitzer archive/ Ground-based observations

― Dust and other minor ice features

Herschel

― FIR dust and ice features with high spectral resolution

― Dust temperature of individual YSOs

ALMA

― Gas-phase chemistry of extragalactic YSOs

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