PPRRIISSMMAASSPRobing InterStellar Molecules

with Absorption line Studies

M. Gerin, M. Ruaud, M. de LucaJ. Cernicharo, E. Falgarone, B. Godard, J. Goicoechea, C Gry, A. Gusdorf, D. Lis, K Menten, F

Viallefond

Sounding the diffuse ISM phases by C+ absorption spectroscopy with Herschel

FIR AbsorptionSensitive technique : direct determination of opacities, easier computation of column densities

Some Previous studies : KAO (Poglitsch et al 96,

DR21) ISO high spectral

resolution (FP) : Absorption in CII and OI-63, limited velocity resolution. Difficulty in accounting the absorption by HI/CNM + H2/CO. Contribution from warm phases ? (Baluteau, Vastel, Keene ...)

Audit & Henebelle

2010

3D, MHD with AMR and cooling

Max densityN ~1000 cm-3

min densityn ~ few cm-3

Fine structure lines (CII, CI , NII)PRISMAS + OT1 project

Strategy : single pointings in DBS (continuum) and Load chop (extended emission) map.Reference position for load chop : 1.5° to 2° away from the Galactic Plane

CII OTF Map in LC , ~ 50'' x 50''.

CI 492 & 809 GHz →→ comparison with Spire NII 1460 GHz → comparison with SpireCII 1900 GHz → comparison with PACS

M. Ruaud (master student) & M. de Luca

Observing geometryObserving geometry

NASA/SSC R. Monje

Massive star forming regions as background sources for absorptionspectroscopy

Examples of spectra towards W49N and W51

Gerin et al 12

NII (205 μm(HIFI/SPIRE) & 122 μm (PACS))

Weak extended emission in Spire/PACS, peaking outside the central pixel

W31C

CI (3P1 – 3P0) 492 GHz & (3P2 – 3P1) 809 GHz

Strong lines from background source. Outflow wings more pronounced in the excited line. Weak features from foreground gas (0.2 – 1 K) comparable with SWAS data but higher spectral resolution and sensitivity Check contamination from OFF beam in LC : ~ OK except few chanels) W51 : the 6 km/s cloud is not detected in CI (?)N (CI) < 2e16 (W/1.1 Kkm/s) W49N The 40 km/s feature is thermalized with the backgroundThe 60 km/s emission feature : T = 100 – 200K, n ~ 150 – 300 cm3→ Diffuse gas , higher pressure / ISM average (~ 3500 Kcm-3)

W51

W49N

G34.3

CII maps Strong absorption from foreground gas Complex profiles towards the star forming regions (outflows, self-absorption) Spatial variations of the CII emission across the mapped area.

CII maps

W33A : limb brightening with no CII at the central pixel.

W51  : extended emission, self reversals

CII – structure

Comparison with PACS continuum : extended absorption from the foreground gas. The depth of the absorption scales with the continuum → constant opacity.

Comparison with OFF & GotC+

CII – excitation

Comparison of the DBS and LC data → average spectrum of the OFF position of the DBS observation (3' from source)Comparison with GotC+ (~ 20' from source)

Typical level of the diffuse CII emission ~ 0.5K Typical opacity from absorption : τ ~ 1 → Tex ~ 20K 

Most of the C+ ions are in the ground state (~ 98%) Excitation conditions consistent with diffuse gas.

CII – excitation

Tex ~ 20K and τ ~ 1 Excitation model for the gas density.

Kinetic temperature Tk from the HI spin temperature

With typical Tk ~ 100K, (CNM)

n_H ~ 40 cm-3 and p = nT ~ 4000 Kcm-3

CII – Phases and filling factor

The C+ absorption  samples all ISM phases where C+ is the main carrier of carbon : Diffuse molecular gas : sampled by CH and HF absorption, up to the point where CO becomes dominant over C+?

Atomic gas in the cold neutral medium : sampled by HI absorption

Atomic gas in the warm neutral medium or in the unstable regime ? (T ~ 8000 K, n ~ 0.4 cm-3 for WNM ) ?→ could appear as an excess absorption when the contributions of the other phases have been taken into account.

CII – Phases and filling factor

Path length ~ 5kpc

Line width (CNM and H2 ~ 3 km/s, WNM > 10 km/s)

Extinction : 1 mag/kpc → 5 mag

N(HI + 2 H2) ~ 1022 cm-2

H2 : 2.5x1021 CNM : 2.5x1021 WNM : 2.5x1021

Using C/H = 1.4 x10-4, and all carbon in C+

N(C+/CNM) = N(C+/WNM) = 0.5N(C+/H2) = 3.5x1017cm-2

τ(C+/CNM) ~ 0.5τ(C+/H2) ~ 0.8 τ(C+/WNM) <~ 0.1 – 0.2

Comparison with HI (Menten et al in prep)

Excellent correspondence of

line profilesCII (HIFI)HI (EVLA)

Different opacity ratios in different velocity ranges

Excess of CII at low HI column densities : detection of the warm neutral medium ?At high column densities : presence of diffuse molecular gas. => All neutral phases are present and contribute to the extinction

Goicoechea, Cernicharo et al. (2010)

OI and OH line absorption/emission towards W51

3D structure : local variations of diffuse ISM properties

Lallement and collabInversion of extinction data and line absorptionRaimond et al 2012

Summary and conclusions

C+ absorption is widespread towards strong continuum sources C+ emission profiles are complex in massive star forming regions C+ absorption traces all neutral diffuse ISM phases.Needs HI data and H2 tracers like CH or HF