Fine structure in the luminosity function in young stellar populations
  with Gaia DR2
  A pioneering study showed that the fine structure in the luminosity function
(LF) of young star clusters contains information about the evolutionary stage
(age) and composition of the stellar population. The notable features include
the H-peak, which is the result of the onset of hydrogen burning turning
pre-main sequence stars into main sequence stars. The feature moves toward the
faint end of the LF, and eventually disappears as the population evolves.
Another detectable feature is the Wielen dip, a dip at M_V ~ 7 mag in the LF
first identified in 1974 for stars in the solar environment. Later studies also
identified this feature in the LF of star clusters. The Wielen dip is caused by
the increased importance of H- opacity in a certain range of low-mass stars. We
studied the detailed structure in the luminosity function using the data from
Gaia DR2 and PARSEC stellar evolution models with the aim to further our
understanding of young stellar populations. We analyzed the astrometric
properties of stars in the solar neighborhood (< 20 pc) and in various
relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB
associations, and compare the features in the luminosity function with those
generated by PARSEC models. The Wielen dip is confirmed in the LF of all the
populations, including the solar neighborhood, at M_G ~7 mag. The H-peak is
present in the LF of the field stars in the solar neighborhood. It likely
signals that the population is mixed with a significant number of stars younger
than 100 Myr. The H-peak is found in the LF of young open clusters and OB
associations, and its location varies with age. Our observations with Gaia DR2
confirm the evolution of the H-peak from 5 Myr up to 47 Myr. The fine structure
in the luminosity function in young stellar populations can be used to estimate
their age.
