Multi-Epoch Star Formation?

The Curious Caseof

Cluster 1806-20

Stephen Eikenberry

University of Florida

11 April 2007

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)

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 (!!)

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)

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?

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 …)

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? …)

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)

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)

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”)

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

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 (???)