The initial luminosity and mass functions of the Galactic open clusters

TL;DR

This study analyzes the luminosity and initial mass functions of Galactic open clusters, revealing their structure, formation rates, and significant contribution to the field star population in the Milky Way.

Contribution

It provides the first detailed characterization of the initial mass function of open clusters and quantifies their role in forming the Galactic field star population.

Findings

The luminosity function slope matches that in nearby galaxies.

The initial mass function has a two-segment power-law structure.

Approximately 40% of Galactic field stars originated from dissolved open clusters.

Abstract

(... abridged) The observed luminosity function can be constructed in a range of absolute integrated magnitudes $I_{M_V}= [-10, -0.5]$ mag, i.e. about 5 magnitudes deeper than in the most nearby galaxies. It increases linearly from the brightest limit to a turnover at about $I_{M_V}\approx-2.5$. The slope of this linear portion is $a=0.41\pm0.01$, which agrees perfectly with the slope deduced for star cluster observations in nearby galaxies. (...) We find that the initial mass function of open clusters (CIMF) has a two-segment structure with the slopes $\alpha=1.66\pm0.14$ in the range $\log M_c/M_\odot=3.37...4.93$ and $\alpha=0.82\pm0.14$ in the range $\log M_c/M_\odot=1.7...3.37$. The average mass of open clusters at birth is $4.5\cdot 10^3 M_\odot$, which should be compared to the average observed mass of about $700 M_\odot$. The average cluster formation rate derived from the comparison of initial and observed mass functions is $\bar{\upsilon}=0.4 \mathrm{kpc}^{-2}\mathrm{Myr}^{-1}$. Multiplying by the age of the Galactic disc (T = 13 Gyr) the predicted surface density of Galactic disc field stars originating from dissolved open clusters amounts to $22 M_\odot \mathrm{pc}^{-2}$ which is about 40% of the total surface density of the Galactic disc in the solar neighbourhood. Thus, we conclude that almost half of all field stars were born in open clusters, a much higher fraction than previously thought.