Gas and dust in the ISM towards the Center of the Galaxy through X-ray Spectroscopy Ciro Pinto(1) J. S. Kaastra(1,2), E. Costantini(1), F. Verbunt(1,2) (1) SRON – Netherlands Institute for Space Research (2) Astronomical Institute, Utrecht University

Interstellar medium (ISM) and Galactic life cycle Crucial role in the evolution of the entire Galaxy Stellar evolution Heavy elements ISM Metallicity gradient towards the G. center Evolution of the Galaxy

Interstellar medium: 3-phase structure Chemical composition close to Solar abundances

X-ray Spectroscopy: a powerful tool Abundances + Constituents Absorption edges and lines Column densities Recent results: Ne/O ratio higher than Solar value Metals depleted: ≥ 70% into dust Oxygen depleted: 10-40% into dust Dust grains mostly made of Silicates Galactic Ionized Gas

Best Targets Best Instrument The Gratings 1) XRB GS 1826-238, 4U 1820-303, 4U 1636-536, GX 339-4, 2) Nebula: Crab 3) Nova: V2491 Cyg Best Targets Best Instrument The Gratings on board: XMM-Newton + Chandra Ne, O, Mg & Fe edges

GS 1826-238: neutral gas Absorption: Cold neutral gas → Ne, O, Mg & Fe Fitting package SPEX (Kaastra et al. 1996) Absorption: Cold neutral gas → Ne, O, Mg & Fe Continuum: (Thompson 2008) Residuals: 17.5 Å → dust 22.9 Å → dust 21.6 Å → O VII 23.3 Å → O II

GS 1826-238: ionized gas Model 2: 3-phase Gas 3 distinct phases Cold, warm and hot phases Abundances same for all phases 3 distinct phases Pure-gas not fully satisfactory Oxygen Edge COLD GAS: v ~ 10 km/s WARM GAS: v ~ 60 km/s HOT GAS: v ~ 150 km/s 103 104 105 106 T (K) NH (1021 atoms cm-2)

GS 1826-238: dust and molecules Iron Edge GS 1826-238: dust and molecules Pure-gas model doesn't work → need gas + dust !!! Our current models: - Shielding by dust grains - Structure by molecules Models in development !!! Oxygen Edge

GS 1826-238: dust and molecules Fe → shielding by grains O → CO, H20 ice, silicates .. Models in development !!! > 90 % of iron in dust 10-20 % oxygen in dust

Contributions to interstellar oxygen

Analysis extended: several sight-lines Crab nebula → Kaastra et al. (2009) Cyg X-2 → Yao et al. (2009) 4U 1820-303 → Costantini et al. (2010) GS 1826-238 → Pinto et al. (2010) I All sources → Pinto et al. (2010) II in prep. Analysis extended: several sight-lines

How is oxygen distributed ? Crab nebula → Kaastra et al. (2009) Cyg X-2 → Yao et al. (2009) 4U 1820-303 → Costantini et al. (2010) GS 1826-238 → Pinto et al. (2010) I All sources → Pinto et al. (2010) II in prep. How is oxygen distributed ? Ionized gas provides < 10 % of O-column Dust silicates provide ~ 10 – 15 %

Abundances confirm the metallicity gradient GS 1826-238 → 6-7 kpc vs. G. center 4U 1820-303 → 7-8 kpc vs. G. center All elements are over-abundant with respect to Solar composition Over-abundances almost agree Metallicity gradient

Abundances confirm the metallicity gradient Crab nebula → Kaastra (2009) Cyg X-2 → Yao (2009) 4U 1820-303 → Costantini (2010) GS 1826-238 → Pinto et al. (2010) I All sources → Pinto et al. (2010) II Average result: Abundances increase towards the center of the Galaxy But Ne doesn't show a clear change …

Conclusion and future Chemical analysis of ISM Evolutionary effects, e.g. metallicity gradient Study compounds like dust Future: ASTRO-H, IXO, Lab. data → distinguish several compounds → mapping ISM → know better the evolution of our Galaxy