Low-mass stars: Their Protoplanetary Disc Lifetime Distribution

While most protoplanetary discs lose their gas within less than 10 Myr, individual disc lifetimes vary from < 1 Myr to >> 20 Myr, with some discs existing for > 40 Myr. Mean disc half lifetimes hide this diversity; only a so-far non-existing disc lifetime distribution could capture this fact. The benefit of a disc lifetime distribution would be twofold. First, it provides a stringent test on disc evolution theories. Second, it can function as input for planet formation models. Here, we derive such a disc lifetime distribution. We heuristically test different standard distribution forms for their ability to account for the observed disc fractions at certain ages. Here, we concentrate on the distribution for low-mass stars (spectral type M3.7 - M6, $M_s \approx $ 0.1 - 0.24 M$_{sun}$) because disc lifetimes depend on stellar mass. A Weibull-type distribution ($k$=1.78, $\lambda$=9.15) describes the observational data if all stars have a disc at a cluster age $t_c$=0. However, a better match exists for lower initial disc fractions. For f(t=0)= 0.65, a Weibull distribution (k=2.34, $\lambda$=11.22) and a Gauss distribution ($\sigma$=9.52, $\mu$=9.52) fit similarly well the data. All distributions have in common that they are wide, and most discs are dissipated at ages > 5 Myr. The next challenge is to quantitatively link the diversity of disc lifetimes to the diversity in planets.