Paul Crowther (Sheffield), Atm Models Olivier Schnurr (AIP), Binaries Raphael Hirschi (Keele), Liza Yusof (Univ Malaya), Interior Richard Parker (ETH), Clusters Models Simon Goodwin (Sheffield), Hasan Kassim (Univ Malaya) Dynamics The most massive stars The most massive stars Stars For All, Lund Observatory
Lower stellar mass limit Lower stellar mass limit Firm lower limit 10 -3 Gyr to stellar mass sequence (~0.085 M ), 10 1 Gyr below which H- burning never commences (brown dwarfs). Burrows et al. 1993 Is there a comparable upper limit to stellar masses?
Upper stellar mass limit? Upper stellar mass limit? ?? M 8 M 0.1 M 1 M 1 M = 2 x 10 30 kg = 300,000 M Earth = 1,000 M Jupiter
Historical perspective Historical perspective R136a: Supermassive star?
30 Dor
74inch at Radcliffe Radcliffe 74inch at Observatory (South Africa) Observatory (South Africa) Feast et al.1960 SMC: R1-50 LMC: R51-158
Radcliffe #136 (=R136) #136 (=R136) Radcliffe Feast et al.: Central ‘star’ in 30 Dor but is probably composite. M R136a = 250-1,000 M ? (Feilzinger et al. 1980)
R136a: Supermassive Supermassive Star? Star? R136a: R136
R136a: Dense star cluster R136a: Dense star cluster ESO HST
Upper mass limit? Upper mass limit? ``The very concept of an `upper mass cut-off‘ has to be considered carefully; at what mass does the IMF predict one one star?’’ Massey & Hunter (1998)
Arches 30 Dor
Arches cluster Arches cluster VLT/NACO (Espinoza et al. 2009) http://www.eso.org/public/news/eso0921/
~150 M o upper mass limit? ~150 M o upper mass limit? Figer (2005), Nature
R136 m 3 =150 M o BD 30% High mass Low mass 0.5% 70% Intermediate mass 3%
Issues? Issues? o Eddington limit?
Mass-Luminosity Relation Mass-Luminosity Relation L ∝ M α α ~1.5 α ~2.5
Eddington parameter parameter Eddington Γ e = g e g = 3.10 − 5 q L / L o Limit M / M o
Issues? Issues? o Eddington limit? o Interpretation challenging for masses o Gold standard: Close spectroscopic binary with known inclination o Silver standard: Direct radius measurement + spectroscopic gravity o Bronze standard: Indirect radius from atmospheric models + spectroscopic gravity o `Sub-standard’: Mass from comparison between (Teff, logL) from atmospheric models with evolutionary model predictions
Issues? Issues? o Eddington limit? o Interpretation challenging for masses o Gold standard: Close spectroscopic binary with known inclination o Silver standard: Direct radius measurement + Conti (1986) IAU Symp 116 spectroscopic gravity o Bronze standard: Indirect radius from atmospheric models + spectroscopic gravity o Sub-standard: Mass from comparison between (Teff, logL) from atmospheric models with evolutionary model predictions
Recent 100M + contenders.. Recent 100M + contenders.. Pistol star η Carinae R136a1
Pistol star? Pistol star? 200-250 M (Figer et al. 1998) 100 M (Najarro et al.2009)
Carinae Carinae η η ESO 2.2m/WFI (BVR) Historically identified as the most massive star in the Milky Way..
Homunculus Homunculus η Car erupted in AAT 19th Century, becoming 2nd brightest star in sky, forming the Homunculus. η Car now identified as a ~120 + 90 M binary (~5.5 yr period, Damineli 1996).
R136a1 R136a1 M~136-155 M for R136a1 (T eff calibrations for OB stars!) + WFPC2 photometry (Massey & Hunter 1998)
Multi-Conjugate Adaptive Optics Multi-Conjugate Adaptive Optics Demonstrator (MAD) imaging Demonstrator (MAD) imaging 4” (1pc) Campbell et al. (2010)
VLT/MAD + SINFONI VLT/MAD + SINFONI a5 a5 Schnurr et et Schnurr al. (2009) al. (2009) a2 a2 a1 a1 0.8” ” 0.8 (0.2pc) (0.2pc) a3 a3 b b c c R136a1: m K =11.1 A K =0.2 M K =-7.6
Reassessment of brightest Reassessment of brightest stars in R136 stars in R136 No evidence for short period binaries from VLT/SINFONI (Schnurr et al. 2009). Stellar properties re-assessed (Crowther et al. 2010): (a) VLT/SINFONI spectroscopy + (b) VLT/MAD photometry + (c) Contemporary stellar atmosphere models (suited to emission line stars) + (d) Evolutionary models for very massive stars
Stellar temperatures.. Stellar temperatures.. Atmospheric model fits to UV (HST/FOS), optical (HST/FOS) & infrared (VLT/SINFONI) spectroscopy.
Stellar luminosities.. Stellar luminosities.. (a) Optical/IR photometry (HST/WFC3+VLT/MAD)+ (b) Large Magellanic Cloud distance (50kpc) + (c) Correction for interstellar dust (A V =1.7, A K =0.2).
Stellar masses? Stellar masses? Comparison of (Teff, logL) with evolutionary models ⇒ M current
Initial masses? Initial masses? M current = 135-265 M o , ages ~ 1.5-2 Myr Evolutionary models adopt theoretical rates of mass loss (several x 10 -5 Msun/yr*) M init =165-320M o *Spectroscopic dM/dt ~ 5 x 10 -5 Msun/yr
Vital statistics Vital statistics Mass Radius Luminosity Density Lifetime M R L H 2 O=1 Gyr 0.1 0.1 0.001 140 1,000 1 1 1 1.4 10 300 35 8,700,000 0.01 0.002 Density (VY CMa) = 0.000000003 Density (H 2 O) = 1.0 Density(NS) = 1,000,000,000,000,000
Sky & Telescope (Oct 2010)
Arches NGC 3603 30 Dor
Gold standard or sub-standard? Gold standard or sub-standard? NGC 3603 hosts an eclipsing binary A1a+b for which dynamical masses have been derived M dyn :116 ± 31M o + 89 ± 16M o (Schnurr et al. 2008) Spectral analysis + evolutionary models M current :120 M o + 92 M o
R136 R136 m 3 =150 M o m 3 =300 M o
Very Massive Stars vs vs OB stars OB stars Very Massive Stars Star/Sp Mass T eff N(LyC) R s Type M o K 10 49 s -1 pc* R136a1 265: 53,000 70 27 O3V 74 45,000 5 5.6 O5V 51 41,000 1.6 3.8 O7V 36 37,000 0.7 2.8 O9V 25 33,000 0.13 1.6 B0V 19 30,000 0.025 1.0 B1V 14 26,000 0.002 0.4 *R s for n e ~10 2 cm -3
High m m up from SDSS galaxies? from SDSS galaxies? High up Problems with population synthesis models for high surface brightness SDSS galaxies using M up =120 M o . g-r colours and EW(H α ) do not match predictions.. “ At the highest luminosities & surface brightnesses the [population synthesis] fit is improved by allowing even more massive stars to form .” (Hoversten & Glazebrook 2008, 2010)
Formation & death of Formation & death of very massive stars very massive stars
Star formation: low mass Star formation: low mass o Fragmentation of Molecular Clouds to low mass (~0.01 M o ) seeds. o Accretion, at M acc ~2x10 -7 M o /yr over ~5 Myr(!) required to build up Solar-type stars (Protostar ⇒ T Tauri ⇒ ZAMS)
Star formation: High mass Star formation: High mass o High mass stars ? W33A o Rapid formation (0.1 Myr) requires high accretion rate (M acc ~10 -4 M /yr), to build up 10 solar mass star. Accretion hindered by radiation pressure in star.. o Very high mass stars o V.high accretion rate (M acc ~10 -3 M /yr) to form a 100 solar mass star if radiation pressure overcome OR mergers of lower mass stars in dense Joshua Barnes protocluster cores..
Cycle 19 HST/STIS programme programme Cycle 19 HST/STIS Crowther (PI) Start date: Apr 2012
Formation of R136a? Formation of R136a? Simulation courtesy of Ian Bonnell
Thermonuclear Supernovae Supernovae Type Ia SN (low mass stars in close binary) Core-collapse Type II or Ib/c (high mass stars) Pair-instability (Very high mass) New Scientist (Feb 2010)
End state of very massive stars? End state of very massive stars? Metallicity Mass Heger et al. 2003
End state of very massive stars? End state of very massive stars? Core- collapse SN Metallicity (NS/BH) Pair instability SN Mass Heger et al. 2003
Super-supernovae Super-supernovae Gal-Yam et al. 2009 Several super-bright Type Ic SN have now been seen Young et al. 2010 (SN 2007bi; M R =-21.3 mag) L MaxSN 2007bi = 130 x L MaxSN 1987A = 30 billion x L Sun
Local PISN? Local PISN? Radioactive 56 Ni (several M ) & total ejected mass (100M +) from the light-curve evolution of SN 2007bi are consistent with pair instability SN models.
Pair instability SNe SNe & & Pair instability upper mass limit upper mass limit SN2007bi (*if* a PISN) should not exist if m up ~150M o . Upper mass limit required to be much higher for the possibility of PISN in local universe.. Langer (2009)
Death of Very Massive Stars Death of Very Massive Stars PISN restricted to SMC metallicities (or lower). Depends critically on post- MS mass-loss recipes Yusof & Hirschi (Priv. Comm.)
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