1. Multi-Epoch Star Formation? The Curious Case of Cluster 1806-20 Stephen Eikenberry University of Florida 11 April 2007

2. SGR 1806-20 Soft Gamma-Ray Repeater – highly- magnetized (B ~10 15 G) neutron star Radio nebula (not SNR) Chandra X-ray position  IR-identified cluster of massive stars (Eikenberry et al., 2001; Kaplan et al., 2002) ISO images  still embedded in molecular cloud (Fuchs et al., 1999) Large LOS reddening (A V ~30 mag)

3. Cluster 1806-20 Several luminous OB supergiant stars Multiple Wolf-Rayet stars of various types Two WC9d stars (~10% of the known Galactic population) SGR is near edge of cluster core (“x”) Brightest star is Luminous Blue Variable (source of radio nebula) Projected image size ~3pc on a side (!!)

4. Distance CO & Galactic rotation  distance to molecular clouds A V, N H & CO suggest “far” distance for cluster Radio source shows NH 3 absorption from MC73  d = 15.1 (+1.8, -1.3) kpc (Corbel & Eikenberry, 2004)

5. LBV 1806-20 IR spectra give extinction, temperature (velocity consistent with MC) With distance  L > 4x10 6 L 0 (similar to Eta Car and Pistol Star) Implies mass > 150 M 0 (Eddington-based) Not a cluster; is it a binary?? Even if binary, minimum mass > 75 M 0 So … SGR = neutron star already; if same birthdate, progenitor must have been more massive than LBV But, stars > 75 M 0 don’t make neutron stars (??; max progenitor mass < 25 M 0 ) Could be multi-epoch SF?

6. Is LBV 1806-20 that big? Figer et al., 2004 find double-lined spectra  binary? (or wind structure??) Assume v sys = v mid Then, Galactic rotation implies d = 11.7 kpc Claim this is “strong difference” from Eikenberry et al. 2004 (but no error bars in Figer et al. 2004 …)

7. Reduced Distance Means … Lower luminosity, thus lower mass (130 M 0 ) Binary implies 65 M 0 lower limit on most massive star Figer et al. (2005)  near-IR spectra of several high- mass stars in cluster Claim consistent with single age = 3-4 Myr & SGR progenitor > 50 M 0 No need for multi-epoch SF (?? – still » 25M 0 ; plus, why did 50 M 0 star blow up before 65 M 0 star? …)

8. Is LBV 1806-20 that close? Figer et al. give no uncertainties (!);  d ~ 2.5  (only “Eikenberry” error bars) Figer used mismatched GC distance; correct that  d = 12.5 kpc (difference now <2  ) Also, v mid assumes that the binary mass ratio q = 1.000 (not necessarily true!) Model spectra  q  1-5 (Lavine, Eikenberry, Smith)

9. Is LBV 1806-20 that close? Also, Figer et al distance implies that both WC9d stars are least luminous in their class (anywhere!) Assume WC9d here has minimum luminosity of any other known WC9d  d> 15 kpc d = 10.7 – 18.1 kpc  consistent with (fully encompasses) original distance Center of range = 14.4 kpc (~0.5  of “Eikenberry-only” error bars)

10. Back to Multi-Epoch SF? Original distance more robust than others, but consistent with all (once you put in error bars!) M LBV > 150 M 0 total; >75 M 0 for binary More: we see major LBV line variability (factors of ~5-6 variation in EW in 1 year)  implies that one star is dominant source of ionizing radiation Thus, even if it is a binary, probable q>1 and mass limit >> 75 M 0 (And … something BIGGER made a neutron star ???) Single birthdate starting to stretch the imagination (if not smoking gun, at least “smoldering slingshot”)

11. The Smoking Gun (??) MIRLIN IRTF observations N1  LBV and WC9 star N4 & N5  central source Qs  13 Jy point source (!!); embedded protostar? Qs luminosity > L bol for 20 M 0 star  massive protostar NS progenitor born >2-3 Myr ago This object <1 Myr old

12. Conclusions Cluster 1806-20 is a rich/weird environment: SGRs, WRs, LBVs, etc., all within R<1 pc Best distance estimate (still) 15.1 kpc LBV 1806-20 is a very luminous/massive star(s?) Either a star > 75 (150) M 0 made a neutron star, or we have multi-epoch star formation here Apparent embedded massive protostar with much younger age independently suggests MESF One idea: NS progenitor forms, explodes near cloud edge; SN shock penetrates cloud and triggers burst of SF – particularly, unusually massive stars (???)