next up previous
Next: An Emerging Class of Up: Implications Previous: Implications

No $\eta $Car Analog Is Found

It is immediately obvious from Figure9 that none of the sources we identified closely resemble $\eta $Car. Their typical luminosities of $10^{5.7\pm0.2}L_\odot$ correspond to $\sim 40M_\odot$ stars (Maeder & Meynet1988; Maeder & Meynet1987; Meynet et al.1994; Maeder1981; Stothers & Chin1996) rather than the higher masses usually associated with LBV outbursts. Since we identify a significant population of fainter stars, this is unlikely to be a selection effect, and we conclude that these galaxies contain no analogs of $\eta $Car.

There are two ways we can interpret the result. First, we can ignore the existence of $\eta $Car, and set $N_c=0$. Alternatively, we can acknowledge the existence of $\eta $Car, in which case $N_c=1$, since $\eta $Car passes our selection criterion and mid-IR surveys of our Galaxy for objects as luminous as $\eta $Car are probably complete. For the first case, the 90% confidence upper limit is $F_e < 0.077 t_{d200}^{-1}$ where the period over which such systems can be detected is scaled to $t_d = 200 t_{d200}$ years. For the second case, where we include $\eta $Car, we find that $F_e = 0.046 t_{d200}^{-1}$ with $0.0083 < F_e t_{d200} < 0.19$ at 90% confidence. In either case, the rate of transients comparable to $\eta $Car is a small fraction of the supernova rate.

Stars as massive as $\eta $Car are also rare, representing only $f_e=0.02$ to $0.04$ of all massive stars for a mass range from $70$/$100M_\odot$ to $200M_\odot$. If every sufficiently massive star had one eruption, the results including $\eta $Car correspond to a minimum mass of $M_C = 65M_\odot$ ( $26 M_\odot < M_C < 138 M_\odot$). If every star has an average of two eruptions, the mass limits rise to $M_C = 94M_\odot$ ( $42 M_\odot < M_C < 162 M_\odot$). Similarly the upper limit from ignoring the existence of $\eta $Car corresponds to $M_C > 48M_\odot$ for an average of one eruption or $M_C > 72 M_\odot$ for an average of two. Kochanek (2011) estimated that the abundance of lower optical depth shells found at $24\mu$m around massive stars in the Galaxy was roughly consistent with all stars more massive than $M_C = 40M_\odot$ having an average of two eruptions, corresponding to $F_e \simeq 0.2$, which is consistent with the present results, but close to the upper limits.


next up previous
Next: An Emerging Class of Up: Implications Previous: Implications
Rubab Khan 2014-10-23