Slide1: Massive stars in the SMC Danny Lennon (ING, La Palma) Stellar Evolution at Low Metallicity,Tartu, Estonia


Why is the SMC important?: Why is the SMC important? Z ~ 1/5thZsolar -> significant difference in mass-loss rate wrt nearby OB stars (Mdot ~ Z1/2) and mass-loss is a major factor in influencing massive star evolution Since [N/H] ~ 1/30th Zsolar -> easy to detect evolutionary effects from different processes (mass-loss, mixing, mass transfer etc) Known distance -> accurate radii and spectroscopic masses Low extinction -> access to NUV/FUV wind diagnostics Review some recent observational work in the SMC - NOT comprehensive. (Apologies to those left out!) Review spectral signatures of mass-loss, especially terminal velocities (concentrate on one particular survey) Look at problem of “weak’’ winds in O-dwarfs Stellar rotation in the SMC Surface composition of massive stars – N enhancements Be stars + fast rotators


A survey of ‘normal’ OB stars in the SMCO stars: Walborn, Lennon, Heap, Lindler, Smith, Evans, Parker (2000), PASP, 112, 1243B stars: Evans, Lennon, Walborn, Trundle, Rix (2004), PASP, 116, 909: A survey of ‘normal’ OB stars in the SMC O stars: Walborn, Lennon, Heap, Lindler, Smith, Evans, Parker (2000), PASP, 112, 1243 B stars: Evans, Lennon, Walborn, Trundle, Rix (2004), PASP, 116, 909 HST/STIS high resolution UV survey of OB stars in SMC (GO7437,9116). High resolution optical data and FUSE FUV data Sample ‘upper main sequence’ of HRD but try to get close to ZAMS Low vsini (one exception) One pair of O7 stars with (almost) identical positions in HRD but different luminosity classes NGC346 AV60 & AV83 A major driver was spectrum synthesis of low metallicity star-forming galaxies (Max Pettini)


Slide4: UV spectra of O-stars: Weak winds of dwarfs N V C IV He II


UV spectra of B-stars: UV spectra of B-stars Milky Way SMC


A direct observational test of radiation driven winds:Dependence of v∞/vesc on Z: A direct observational test of radiation driven winds: Dependence of v∞/vesc on Z Radiation driven wind theory predicts that : Theoretically the value of â is only weakly dependent on Z down to about 1/10th solar, but is dependent on Teff The terminal velocity is measured from the saturated edge of P-Cygni wind lines in the UV For dwarfs v∞ is NOT observed! For supergiants the Z dependence is confirmed by Evans, Lennon,Trundle, Heap, Lindler (2004), ApJ, 607, 451 Crowther, Lennon, Walborn (2005): bistability ‘jump’ not confirmed for normal stars


Mass-loss rates for dwarfs in NGC346Bouret, Lanz, Hillier, Heap, Hubeny, Lennon, Smith, Evans (2003), ApJ, 595, 1182: Mass-loss rates for dwarfs in NGC346 Bouret, Lanz, Hillier, Heap, Hubeny, Lennon, Smith, Evans (2003), ApJ, 595, 1182 The three most luminous dwarfs are consistent with theory Three lowest luminosity dwarfs have UV mass-loss rates which are much weaker than theory Fits to OV λ1371 imply volume clumping factors of f∞~0.1 which would make discrepancy worse (see talks of Alex de Koter and Carrie Trundle). PROBLEM: The spectra of these stars do not show any strong wind signatures to provide an accurate estimate of mass-loss rate. (e.g. the Hα line is purely photospheric.) Q: How does one measure mass-loss rates for weak-wind stars?


Similar results for 4 SMC dwarfs from Martins, Schaerer, Hillier, Heydari-Malayeri (2004), A&A, 420, 1087: Similar results for 4 SMC dwarfs from Martins, Schaerer, Hillier, Heydari-Malayeri (2004), A&A, 420, 1087 Mdot=10-6.4 β=0.8 Mdot=10-7.2 β=0.8 Mdot=10-7.0 β=1.7 Milky Way LMC SMC


A tale of two stars: AV69 and AV83Hillier, Lanz, Heap, Hubeny, Smith, Evans, Lennon, Bouret (2003), ApJ, 588, 1039: A tale of two stars: AV69 and AV83 Hillier, Lanz, Heap, Hubeny, Smith, Evans, Lennon, Bouret (2003), ApJ, 588, 1039 AV 69 OC7.5 III((f)) vsini = 70 km/s logg = 3.50 M ~ 40 M Mdot = 9.2x10-7 Myr-1 (f = 1.0) [N/N] = 0.02 AV 83 O7 Iaf+ vsini = 80 km/s logg =3.25 M ~ 22 M Mdot = 7.3x10-7 Myr-1 (f = 0.1) [N/N] = 1.8 AV 83 was initially a fast rotator but has undergone rotationally enhanced mass-loss and mixing resulting in a lower current mass and higher surface nitrogen abundance than AV 69 AV 69 really is a slow rotator since it has evolved away from the ZAMS but has pristine surface composition for the SMC. Two O7 stars occupying the same position in the HR diagram:


Stellar Rotation and Metallicity: Stellar Rotation and Metallicity The idea that stellar rotation depends on Z stems from high Be star fraction in the SMC cluster NGC330, ~40%. However for the Galactic cluster NGC7419 the fraction is 36+/-7%. (Maeder, Grebel, Mermilliod 1999; Pigulski & Kopacki 2000) Penny et al (2004, ApJ, 617, 1316) found no dependence of O-star rotational velocities on Z - but the study is flawed! Keller (2004, PASA, 21, 310) found ~100 cluster and field B-stars in the LMC to be rotating faster than counterparts in the Galaxy. Location vsini (km/s) Galactic field 85 Galactic clusters 116 LMC field 112 LMC clusters 146 Larger unbiased samples are needed!


Massive stars in the SMC: Surface composition : Massive stars in the SMC: Surface composition Surface composition modified by mass-loss, rotation, mass-transfer, magnetic fields… Most obvious consequence is modified Nitrogen abundance at the surface (quantify!) At Solar composition it is very difficult to detect factor of 2 change in surface N abundance ..for a massive star The ‘pristine’ N abundance of the SMC is 1/30th solar, i.e. nitrogen is almost undetectable in massive stars with this composition Therefore the same enhancement (in absolute terms) for an SMC star produces a star with Solar N abundance. Concentrate on the SMC!


Massive stars in the SMC: Nitrogen Abundance: B-giants & Supergiants Lennon, Dufton, Crowley (2003) Trundle, Lennon, Puls, Dufton (2004) Trundle & Lennon (2005) Dufton, Ryans, Trundle, Lennon, Hubeny, Lanz, Allende Prieto (2005) O-stars Hillier, Lanz, Heap, Hubeny, Smith, Evans, Lennon, Bouret (2003) Bouret, Lanz, Hillier, Heap, Hubeny, Lennon, Smith, Evans (2003) Evans, Crowther, Fullerton, Hillier (2004) Heap, Lanz, Hubeny (2005) Evolutionary Tracks Maeder & Meynet (2001): initial rotational velocity 300 km/s and masses 60, 40, 25, 20, 15, 12 and 9 solar masses. Massive stars in the SMC: Nitrogen Abundance Solar SMC B-stars = 65 km/s O-stars = 67 km/s vTAMS ~ 200 km/s ?


Slide13: There doesn’t appear to be a strong correlation between N line strength and rotation! One extreme pole-on Be star in the SMC cluster NGC330 is not N-enhanced plus one other pole-on Be star is apparently normal (Lennon, Lee, Dufton, Ryans, 2005, A&A, 438, 265) Rotational mixing is not effective – low (~10M) mass? - or magnetic fields inhibit mixing? - binarity? VLT/FLAMES survey of massive stars (talk by Chris Evans) indicates picture is complicated


A shopping list:: A shopping list: Larger unbiased samples are needed! Distribution of stellar rotational velocities Weak wind stars need reliable diagnostic for mass-loss rate. (Br alpha?) Clumping issue needs to be resolved (zero point) Need more precise [N/H] for O-stars. Need to study ‘fast’ rotators! Take binaries into account. Place [N/H] in context Need to look at massive stars in lower Z environments (large galacto-centric distances in M33/M31, other metal poor Local Group galaxies like GR8).