21 st International Conference on General relativity and Gravitation, Columbia University, New York 10–15 July 2016 Accret Ac etion ion of of gas aseou eous clumps ps fr from om the e Gal alac actic ic Cen entre e Min Mini-sp spiral on onto o Mi Milky Wa Way’s s su superma massi ssive black ck hole Vl Vlad adimir KA KARAS Astronomical Institute, Prague, Czech Republic De Devaky KUNNER ERIATH The North American ALMA Science Center, NRAO, Charlottesville, Virginia (VA) Boze Bo zena CZER ERNY, Ag Agata RO ROZANSKA, & & Te Tek P. P. ADHIKARI Nicolaus Copernicus Astronomical Centre, Warsaw, Poland
Ar Artis ist’s s pict cture of a galact ctic c nu nucl cleus: s: st stars s passi ssing ng through gase seous s env nvironme nment nt gas gas an and d du dust st SM SMBH NS NSC 3
Sg Sgr A* A* past ast ac activity from fr om its pr present X-ra ray e emis issio ion Koyama et al. (1996) Sunyaev et al. (1993) 4
Ponti et al. (2010) 5
Pa Past st a activity o y of S Sgr A* A* – de decay of the X-ra ray ligh ghtcurve urve Ca Capelli et t al. . (2012) 2012) Ry Ryu et al. l. (2013) 6
Region on of of interest: Th The Mi Mini-sp spiral X-ray: NASA/CXC/UCLA (Li et al. 2013) Radio: NRAO/VLA 7 T. Krichbaum 1998
Cold phase (gas 3-21x10 4 cm -3 , 5000- Region on of of interest: Co 13000 K, and dust ~300 K) Th The Mi Mini-sp spiral Ho Hot phase (fully ionised gas, n e =18 cm -3 , T e =3.5 keV at 1.5’’) Left: radio 1.3mm (Kunneriath et al. 2012) Right: MIR 8.6 µm (Sabha et al. 2012) 8
El Elementary accretion on events from om in infallin ing clou ouds ‘Standard’ thin disk accretion: d M /d t |( R ) ~ 6 p R 1/2 d (R 1/2 nS ) / d R (Lynden-Bell & Pringle 1974) Superposition of elementary events: d M /d t |(0) ~ t - 5/3 / t visc (Zdziarski et al. 2009) 9
Multiple accretion on events Past activity of of Sg Sgr A* A* (Czerny et al. 2013) 10
…to study accretion driven by the thermal instability (TI) 11
Triggering the accretion on events: the hermal instability in in a two-ph phase mediu dium • Fi Field (1965) – Radiatively heated plasma can be thermally unstable (cooler, dense matter coexists with hot medium in pressure equilibrium) l F = mean thermal conduction / total heating per volum e à (10 -7 T 7/2 / H tot ) 1/2 Clumps of size < l F ev evaporate • Ba Barai et al (2012 12) – Two-phase medium forms spontaneously. Ionization parameter: Ξ = P rad / P gas à ( L central + L stars ) / ( P gas cR 2 ) 3): filaments break into cloudlets • Mo Moscibrodzka et al l (201 2013) Cooling time: t cool ~ E / L tot , L tot ~ n 2 T 1 /2 Colder clumps form filaments à they become accreted faster à en enhanced ed accret retion ra rate 12
Method od and parameters Cloudy photoionisation code to model the ISM in the GC • calculate instability curves log( T ) vs. log( Ξ ) for different luminosity states of Sgr A*, • include the influence of dust grains on the onset of TI • include the influence of stellar radiation and winds . We consider • clouds: different density and different distance • accretion mode: M dot ~ m x 10 -9 M sun yr -1 13
Th Thermal l inst stabili lity – different states of of luminos osity (distance R =0.008pc from the center) ß Stable à ß Unstable à ß Stable à log( L bol [erg/s]) = 37.57, 38.33, 39.06, 39.76, 40.61, 41.33 14
Thermal instability for or different states of of luminos osity ( R =0 =0.008pc) log( L bol [erg/s]) = 37.57, 38.33, 39.06, 39.76, 40.61, 41.33 15
Influence of of dust: suppression on of of instability The Mini-spiral contains dust (200–300K; Cotera et al. 1999). (Treated by an option in Cloudy.) Without dust With dust: suppresing the instability 16
Size of of clou ouds Thermal conduction limits the smallest size of the clouds (Rozanska et al. 2014). Four different curves: bolometric luminosity L bol • Radius of the yellow circles: Field length l F • Mass of clouds M c in units of Earth mass • 17
Instability strips for or two-ph phase mediu dium wit ithin in Bon ondi flow ow n out = 18 cm -3 n out = 1 cm -3 18
The rol ole of of stellar radiation on and wind (mechanical heating) Sg Sgr A* A* NSC M6 M60-UC UCD1 19
S-cur curves of f Thermal Instability: Te Tempera rature re vs vs. . Ionisat sation pa parameter On Only th the centr tral + Including t + I the + I + Including t the sourc so rce of f st stellar r ra radiation by by stellar st r mechanical à à irradia ir iatio ion 6 My 6 Myr SED ED he heating by by winds 20
Co Concl nclusi sions ns • Current level of luminosity of Sgr A* not enough to drive the thermal instability • For luminosity > 10 39 erg/s at distance ~ 0.008–0.2 pc, the thermal instability operates, cold clumps can accrete • Cooling time-scales are long (~ hundreds years) • Typical cloud size of 10 14-15 cm, mass of ~10 M Earth • Influence of dust is small Th Thank y you! 21
Discussion on slides: Tw Two-ph phase mediu dium in in Bon ondi flow ow arou ound Sg Sgr A* A* Bondi accretion operates up to ~ 0.1pc from Sgr A* 22
Thermal instability for or different states of of luminos osity ( R =0 =0.2pc) log( L bol [erg/s]) = 37.57, 38.33, 39.06, 39.76, 40.61, 41.33 23
Past activity of Sgr A* – predictions for X-ray polarimetry Molinari et al. (2011), Marin et al. (2014) 24
Past activity of Sgr A* – predictions for X-ray polarimetry Marin et al. (2015) 25
Past activity of Sgr A* – predictions for X-ray polarimetry Marin et al. (2015) 26
Introduction HKL composite NIR image 27 GC group, University of Cologne
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