1. PRISMAS PRISMAS PRobing InterStellar Molecules with Absorption line Studies M. Gerin, M. Ruaud, M. de Luca J. 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

2. FIR Absorption Sensitive 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...)

3. Audit & Henebelle 2010 3D, MHD with AMR and cooling Max density N ~1000 cm -3 min density n ~ few cm -3

4. 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 Spire CII 1900 GHz → comparison with PACS M. Ruaud (master student) & M. de Luca

5. Observing geometry NASA/SSC R. Monje Massive star forming regions as background sources for absorption spectroscopy

6. Examples of spectra towards W49N and W51 Gerin et al 12

7. NII (205 μm(HIFI/SPIRE) & 122 μm (PACS)) Weak extended emission in Spire/PACS, peaking outside the central pixel W31C

8. CI ( 3 P 1 – 3 P 0 ) 492 GHz & ( 3 P 2 – 3 P 1 ) 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 background The 60 km/s emission feature : T = 100 – 200K, n ~ 150 – 300 cm3 → Diffuse gas, higher pressure / ISM average (~ 3500 Kcm- 3)

9. W51 W49N G34.3

10. 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.

11. CII maps W33A : limb brightening with no CII at the central pixel. W51 : extended emission, self reversals

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

13. Comparison with OFF & GotC+

14. 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.

16. 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

17. 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.

18. CII – Phases and filling factor Path length ~ 5kpc Line width (CNM and H 2 ~ 3 km/s, WNM > 10 km/s) Extinction : 1 mag/kpc → 5 mag N(HI + 2 H 2 ) ~ 10 22 cm -2 H 2 : 2.5x10 21 CNM : 2.5x10 21 WNM : 2.5x10 21 Using C/H = 1.4 x10 -4, and all carbon in C+ N(C + /CNM) = N(C + /WNM) = 0.5N(C + /H 2 ) = 3.5x10 17 cm -2 τ(C + /CNM) ~ 0.5τ(C + /H 2 ) ~ 0.8 τ(C + /WNM) <~ 0.1 – 0.2

19. Comparison with HI (Menten et al in prep) Excellent correspondence of line profiles CII (HIFI) HI (EVLA) Different opacity ratios in different velocity ranges

20. 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

21. Goicoechea, Cernicharo et al. (2010)‏ OI and OH line absorption/emission towards W51

22. 3D structure : local variations of diffuse ISM properties Lallement and collab Inversion of extinction data and line absorption Raimond et al 2012

23. 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