PRISMAS PRISMAS PRobing InterStellar Molecules with Absorption line Studies M. Gerin, M. Ruaud, M....
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Transcript of PRISMAS PRISMAS PRobing InterStellar Molecules with Absorption line Studies M. Gerin, M. Ruaud, M....
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