SPYGLASS. III. The Fornax–Horologium Association and Its Traceback History within the Austral Complex

Abstract

The study of young associations is essential for building a complete record of local star formation processes. The Fornax–Horologium association (FH), including the χ 1 Fornacis cluster, represents one of the nearest young stellar populations to the Sun. This association has recently been linked to the Tuc-Hor, Carina, and Columba associations, building an extensive “Austral Complex” almost entirely within 150 pc. Using Gaia astrometry and photometry in addition to new spectroscopic observations, we perform the deepest survey of FH to date, identifying over 300 candidate members, nearly doubling the known population. By combining this sample with literature surveys of the other constituent populations, we produce a contiguous stellar population covering the entire Austral Complex, allowing the definitions of subpopulations to be reassessed along with connections to external populations. This analysis recovers new definitions for FH, Tuc-Hor, Columba, and Carina, while also revealing a connection between the Austral complex and the Sco-Cen-affiliated Platais 8 cluster. This suggests that the Austral complex may be just a small component of a much larger and more diverse star formation event. Computing ages and tracing stellar populations back to formation reveals two distinct nodes of cospatial and continuous formation in the Austral Complex, one containing Tuc-Hor, and the other containing FH, Carina, and Columba. This mirrors recent work showing similar structure elsewhere, suggesting that these nodes, which only emerge through the use of traceback, may represent the clearest discrete unit of local star formation, and a key building block needed to reconstruct larger star-forming events.

Kinematic Age of the β-Pictoris Moving Group

“…We attempted to find the time when the average pairwise distance is at its minimum. The approach employed in our calculation shares similarities with the one utilized in Kerr et al (2022), although there are differences in details. In Kerr et al (2022), they computed a median distance for each star to all other stars and determined the time at which the median distance was minimal.…”

“…The approach employed in our calculation shares similarities with the one utilized in Kerr et al (2022), although there are differences in details. In Kerr et al (2022), they computed a median distance for each star to all other stars and determined the time at which the median distance was minimal. The minimal distance time (i.e., age) is obtained for all stars, and the final age is obtained from the distribution of ages.…”

A Quick Guide to Nearby Young Associations

“…The advent of the Hipparcos mission allowed to begin uncovering such "moving groups" of stars in the immediate neighborhood of the Sun (e.g., see Kastner et al 1997;Zuckerman & Webb 2000;Zuckerman et al 2001Torres et al 2008 for relevant reviews on the topic), but it is only with the Gaia mission (Gaia Collaboration et al 2016) that it became possible possible to catalog loose associations at distances much beyond 100 pc (e.g., see Oh et al 2017;Kounkel & Covey 2019;Tang et al 2019;Meingast et al 2021) or with particularly low densities and intermediate ages in the immediate solar neighborhood (Moranta et al 2022;Gagné et al 2023). The rich data provided by Gaia also made it possible to unveil the full complexity of the velocity distribution and sub-populations in young OB associations as well as younger star-forming regions (see e.g., Kerr et al 2021;Liu et al 2021;Kerr et al 2022aKerr et al , 2022bKerr et al , 2023.…”

SPYGLASS. IV. New Stellar Survey of Recent Star Formation within 1 kpc

“…By preserving elements of the dynamics and history of their starforming environments, these populations provide a powerful record of recent star formation, which can be detected for tens of millions of years after their formation Krause et al 2020). Detailed studies of young populations therefore have the unique ability to reconstruct entire star formation histories for recent events, using dynamics to trace the locations of stars back in time, and ages to determine the location of stars and their subsequent environments at the time of formation (e.g., Galli et al 2021;Kerr et al 2022aKerr et al , 2022bMiret-Roig et al 2022). The results can reveal star formation patterns at a range of scales, from molecular clouds to spiral arms (e.g., Pecaut & Mamajek 2016;Pang et al 2021;Pantaleoni González et al 2021;Zucker et al 2022a).…”