multi-epoch star formation? the curious case of cluster 1806-20 stephen eikenberry university of...
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Multi-Epoch Star Formation?
The Curious Caseof
Cluster 1806-20
Stephen Eikenberry
University of Florida
11 April 2007
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SGR 1806-20• 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)
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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 (!!)
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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)
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LBV 1806-20• 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?
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Is LBV 1806-20 that big?• 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 …)
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Reduced Distance Means …• 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? …)
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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, vmid assumes that the binary mass ratio q = 1.000 (not necessarily true!)
• Model spectra q 1-5 (Lavine, Eikenberry, Smith)
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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)
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Back to Multi-Epoch SF?• 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”)
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The Smoking Gun (??)• 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
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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) 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 (???)