Bona fide, strong-variable galactic luminous blue variable stars are fast rotators: Detection of a high rotational velocity in HR Carinae

• Autores: Groh, J. H.; Damineli, A.; Hillier, D. J.; Barbá, Rodolfo Héctor; Fernández Lajús, Eduardo; Gamen, Roberto Claudio; Moisés, A. P.; Solivella, Gladys Rebeca; Teodoro, M.
• Año de publicación: 2009
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Ciencias Astronómicas; Stars: atmospheres; Stars: individual (HR Carinae); Stars: mass loss; Stars: rotation; Stars: variables: other; Supergiants
• Institución: Universidad Nacional de La Plata

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 SiIV λλ4088-4116. To match their observed line profiles from 2009 May, a high rotational velocity of v rot ≃ 150 ∼ 20 km s^-1 is needed (assuming an inclination angle of 30°), implying that HR Car rotates at ≃0.88 ∼ 0.2 of its critical velocity for breakup (v_crit). 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 v crit is reached. To the left of this strip, a forbidden zone with v_rot/v_crit>1 is present, explaining why no LBVs are detected in this zone. Since dormant/ex LBVs like P Cygni and HD 168625 have low v rot, we propose that LBVs can be separated into 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 supernova (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 additional evidence that such stars could explode during the LBV phase.
Instituto de Astrofísica de La Plata
Facultad de Ciencias Astronómicas y Geofísicas