Letter to the Editor

Turbulent fragmentation as the primary driver of core formation in Polaris Flare and Lupus I^⋆

¹ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
² Department of Astronomical Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁴ Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan

^⋆⋆ Corresponding author; kousuke.ishihara@grad.nao.ac.jp

Received: 30 September 2024
Accepted: 23 February 2025

Abstract

Context. Stars form from dense cores in turbulent molecular clouds. According to the standard scenario of star formation, dense cores are created by cloud fragmentation. However, the physical mechanisms driving this process are still not fully understood from an observational standpoint.

Aims. Our goal is to investigate the process of cloud fragmentation using observational data from nearby clouds. Specifically, we aim to examine the role of self-gravity and turbulence, both of which are key to the dynamical evolution of clouds.

Methods. We applied astrodendro to the Herschel H₂ column density maps to identify dense cores and determine their mass and separation in two nearby low-mass clouds: the Polaris Flare and Lupus I clouds. We then compared the observed core masses and separations with predictions from models of gravitational and turbulent fragmentation. In the gravitational fragmentation model, the characteristic length and mass are determined by the Jeans length and Jeans mass. For turbulent fragmentation, the key scales are the cloud’s sonic scale and its corresponding mass.

Results. The average core masses are estimated to be 0.242 M_⊙ for Lupus I and 0.276 M_⊙ for the Polaris Flare. The core separations peak at about 2 − 4 × 10⁴ au (≈0.1–0.2 pc) in both clouds. These separations are significantly smaller than the Jeans length but agree well with the cloud sonic scale. Additionally, the density probability distribution functions of the dense cores follow log-normal distributions, which is consistent with the predictions of turbulent fragmentation.

Conclusions. These findings suggest that the primary process driving core formation in the observed low-mass star-forming regions is not gravitational fragmentation but rather turbulent fragmentation. We found no evidence that filament fragmentation plays a significant role in the formation of dense cores.