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Multi-Epoch Star Formation? The Curious Case of Cluster 1806-20

This study investigates the peculiar cluster 1806-20, which contains a highly-magnetized neutron star and a radio nebula. It suggests the possibility of multi-epoch star formation due to the presence of massive stars and a massive protostar. The findings challenge conventional theories and raise questions about the nature of this cluster.

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Multi-Epoch Star Formation? The Curious Case of Cluster 1806-20

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

  2. Soft Gamma-Ray Repeater – highly-magnetized (B ~1015 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 (AV ~30 mag) SGR 1806-20

  3. 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 (!!) Cluster 1806-20

  4. Distance • CO & Galactic rotation  distance to molecular clouds • AV, NH & CO suggest “far” distance for cluster • Radio source shows NH3 absorption from MC73  d = 15.1 (+1.8, -1.3) kpc(Corbel & Eikenberry, 2004)

  5. IR spectra give extinction, temperature (velocity consistent with MC) • With distance  L > 4x106 L0(similar to Eta Car and Pistol Star) • Implies mass > 150 M0 (Eddington-based) • Not a cluster; is it a binary?? Even if binary, minimum mass > 75 M0 • So … • SGR = neutron star already; if same birthdate, progenitor must have been more massive than LBV • But, stars > 75 M0 don’t make neutron stars (??; max progenitor mass < 25 M0) • Could be multi-epoch SF? LBV 1806-20

  6. Figer et al., 2004 find double-lined spectra  binary? (or wind structure??) • Assume vsys = vmid • 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 …) Is LBV 1806-20 that big?

  7. Lower luminosity, thus lower mass (130 M0) • Binary implies 65 M0 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 M0 • No need for multi-epoch SF (?? – still » 25M0 ; plus, why did 50 M0 star blow up before 65 M0 star? …) Reduced Distance Means …

  8. 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, vmidassumes that the binary mass ratio q = 1.000 (not necessarily true!) • Model spectra  q  1-5(Lavine, Eikenberry, Smith) Is LBV 1806-20 that close?

  9. 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) 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

  10. Original distance more robust than others, but consistent with all (once you put in error bars!) • MLBV > 150 M0 total; >75 M0 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 M0 • (And … something BIGGER made a neutron star ???) • Single birthdate starting to stretch the imagination (if not smoking gun, at least “smoldering slingshot”) Back to Multi-Epoch SF?

  11. MIRLIN IRTF observations • N1  LBV and WC9 star • N4 & N5 central source • Qs  13 Jy point source (!!); embedded protostar? • Qs luminosity > Lbol for 20 M0 star  massive protostar • NS progenitor born >2-3 Myr ago • This object <1 Myr old The Smoking Gun (??)

  12. 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) M0 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 (???) Conclusions

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