Bona-fide, strong-variable galactic Luminous Blue Variable stars are
  fast rotators: detection of a high rotational velocity in HR Carinae
  We report optical observations of the Luminous Blue Variable (LBV) HR Carinae
which show that the star has reached a visual minimum phase in 2009. More
importantly, we detected absorptions due to Si IV 4088-4116 Angstroms. To match
their observed line profiles from 2009 May, a high rotational velocity of
vrot=150 +- 20 km/s is needed (assuming an inclination angle of 30 degrees),
implying that HR Car rotates at ~0.88 +- 0.2 of its critical velocity for
break-up (vcrit). Our results suggest that fast rotation is typical in all
strong-variable, bona-fide galactic LBVs, which present S Dor-type variability.
Strong-variable LBVs are located in a well-defined region of the HR diagram
during visual minimum (the "LBV minimum instability strip"). We suggest this
region corresponds to where vcrit is reached. To the left of this strip, a
forbidden zone with vrot/vcrit>1 is present, explaining why no LBVs are
detected in this zone. Since dormant/ex LBVs like P Cygni and HD 168625 have
low vrot, we propose that LBVs can be separated in two groups: fast-rotating,
strong-variable stars showing S-Dor cycles (such as AG Car and HR Car) and
slow-rotating stars with much less variability (such as P Cygni and HD 168625).
We speculate that SN progenitors which had S-Dor cycles before exploding (such
as in SN 2001ig, SN 2003bg, and SN 2005gj) could have been fast rotators. We
suggest that the potential difficulty of fast-rotating Galactic LBVs to lose
angular momentum is an additional evidence that such stars could explode during
the LBV phase.
