# Gaia DR2 white dwarfs in the Hercules stream

## TL;DR

This study identifies and characterizes the Hercules stellar stream within the Gaia white dwarf population near 100 pc, revealing substructures and age distributions using kinematic clustering and white dwarf cosmochronology.

## Contribution

It introduces a novel kinematic clustering approach to detect the Hercules stream in white dwarfs and derives their age distribution, providing new insights into the stream's composition and history.

## Key findings

- Hercules stream appears as an overdensity in the thick-disk white dwarf velocity space.
- Three substreams within Hercules were identified, with distinct age distributions.
- The age of the Hercules a and b substreams peaks around 4 Gyr, while Hercules c is younger than 10 Gyr.

## Abstract

We analyzed the velocity space of the thin and thick-disk Gaia white dwarf population within 100 pc looking for signatures of the Hercules stellar stream. We aimed to identify those objects belonging to the Hercules stream and, by taking advantage of white dwarf stars as reliable cosmochronometers, to derive a first age distribution. We applied a kernel density estimation to the $UV$ velocity space of white dwarfs. For the region where a clear overdensity of stars was found, we created a 5-D space of dynamic variables. We applied a hierarchichal clustering method, HDBSCAN, to this 5-D space, identifying those white dwarfs that share similar kinematic characteristics. Finally, under general assumptions and from their photometric properties, we derived an age estimate for each object. The Hercules stream was firstly revealed as an overdensity in the $UV$ velocity space of the thick-disk white dwarf population. Three substreams were then found: Hercules $a$ and Hercules $b$, formed by thick-disk stars with an age distribution peaked $4\,$Gyr in the past and extended to very old ages; and Hercules $c$, with a ratio of 65:35 thin:thick stars and a more uniform age distribution younger than 10 Gyr.