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Star Formation at the Galactic Center

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Star Formation at the Galactic Center Star Formation at the Galactic Center Mark Morris, UCLA collaborators: Don Figer Sungsoo Kim Gene Serabyn Ian McLean Alain Omont Mike Rich Andrea Ghez Eric Becklin Outline • Introduction and the KAO legacy • Peculiar conditions affecting star formation • 3 remarkably massive, young star clusters: – fate – IMF – formation mechanisms • The central parsec • SOFIA’s promise The Galactic Center: a favorite KAO target: • extremely luminous in the infrared • most of that luminosity owed directly to recent star formation. • MSX images Egan et al. 1998 [not yet exploited] • KAO accomplishments: – Discovery of the circumnuclear disk. – Determination of luminosity of central cluster. – Polarization measurements revealing highly ordered magnetic fields in clouds. – Far-IR spectroscopy demonstrating that unusual HII regions are photoionized. – IR counterparts of thermal radio structures, showing tight correlation of luminosity with ionization. 32 and 37 µm observations with KWIC (Stacey et al.) Conditions Affecting Star Formation Favorable: High surface density of molecular gas -2 (200 Mo pc within the central molecular zone, compared to ~25 in the Galaxy’s molecular ring) High frequency of compressional events vCloud collisions vencounters with the bar shock vsupernovae vexplosive release of accretion energy Conditions Affecting Star Formation Unfavorable: DLarge cloud velocity dispersions, > 10 km/s DLarge cloud temperatures: ~ 50 K DTidal forces prevent collapse & destroy clouds unless n > 104 cm-3 (75 pc/R)2 H2 DStrong magnetic fields (milligauss), both in and out of clouds Ô Net result: high pressure environment, with high mass stars favored, relative to IMF in disk Conditions Affecting Star Formation in the Galactic Center Unclear effect: Metallicity (a factor of 2 or 3 enhanced). á Stronger cooling, but á Higher opacities, and á Enhanced winds Massive Young Clusters in and near the Galactic Center 1. The central cluster (coextensive with the central stellar core of older stars), age ~ 5 Myr 2. The Quintuplet Cluster (AFGL2004), age ~ 4 Myr 3. The Arches Cluster (G0.121+0.017), age ~ 2 Myr All three: 1-2 x 104 M o Quintuplet Cluster HST/NICMOS Figer et al. 1999 JHK color composite 3pc x 3pc Pistol star (LBV) --> Pistol Nebula HST/NICMOS Figer et al. 1999b Paschen-a Quintuplet Census: LBV’s ………………………. 2 log(L/Lo) > 6.7 (Pistol Star) WC’s …………………………4 log(L/Lo) ~ 5.7 - 6.5 WN’s ………...…….………... 4 log(L/Lo) ~ 5.7 - 6 DWCL’s (?) ………………….5 log(L/Lo) ~ 4.5 - 5.2 (original quintuplet members, with cool, featureless spectra) WN9/Ofpe …………………... 2 log(L/Lo) ~ 6.5 MIa ………………………… 1 log(L/Lo) ~ 4.9 B ………………………….. 10 incomplete OBI ……………………….. 4 Arches Cluster HST/NICMOS Figer et al. 1999 JHK color composite 1.5pc x 1.5pc (38.4” x 38.4”) 150 O Stars Cluster Parameters -1 mass(Mo) log[NLyc(s )] log(L/Lo) Quintuplet ~1.7 x 104 50.9 7.5 Arches 1.2 x 104 51.3 8.0 Central pc ~ 104 7.3 Fate of Galactic Center Clusters: Cluster disintegration near a galactic nucleus Thesis work of Sungsoo Kim (UCLA & Pusan Univ.) Multiple-mass Fokker-Planck models with: G Stellar evolution and mass loss G Heating by binaries G Evaporation beyond the tidal limit* *key point: tidal radius ~ cluster radius Models Follow: G Core collapse, and consequent envelope expansion G Mass segregation G Cluster disintegration Generalities drawn from this work: nUnless we live at a rather special moment, formation of massive, luminous clusters should be frequent near the Galactic Center nThey last < 107 years nThe scattered stars of recently demised clusters should be sought. nSuch cluster disintegration is common to all galactic nuclei. IMF Determinations Figer, Kim, Morris, Serabyn, Rich,& McLean 1999 Adopt Geneva models for mass-magnitude relation (Meynet et al. 1994) Done only for the Arches cluster so far, because at the age of the Quintuplet, the mass-luminosity relationship is double-valued. G = d(logN)/d(logM) = -0.6 (overall, Arches cluster) -0.2 (2.5” < r < 4.5”) -0.8 (4.5” < r < 7.5”) -1.35 (Salpeter) è Massive stars relatively favored in GC clusters è Even a 2 x 106-year old cluster has undergone mass segregation, as anticipated in dynamical models What about the central cluster? It can’t be destroyed like the others, because it lies at the bottom of the gravitational potential well. Krabbe et al. 1995 But how are stars formed within a few tenths of a 6 parsec of a 2.6 x 10 Mo black hole? The limiting Roche density at the distance of the IRS16 stars (~2” = 0.1 pc) is 1013 cm-3 True: gas migrating inward can only collect at the center of the potential well until it either becomes dense enough to collapse or is expelled in some violent event. But then, accretion onto the central black hole will occur at the Eddington rate long before the density gets high enough for stars to form. So, the formation of stars in the central parsec is probably linked to episodes of violent AGN activity, and may require the shocks and compression accompanying such activity. --> limit cycle of activity/star formation (Morris et al. 1999). Opportunities with SOFIA Polarization: Essential for understanding the geometry and thus the role of the magnetic field in the central few hundred parsecs. Spectroscopy: HII regions and PDR’s: stellar radiation fields, physical gas conditions, abundances, kinematics. Superbubbles. Circumnuclear disk: reservoir of material for next activity cycle. What is its dynamical, thermal, density and chemical structure? Can stars form there? Photometric far-IR mapping: compressed dust ridges where star formation is taking place..
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