Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

Abstract

Context. Rotational evolution in young stars is described by pre-main sequence evolutionary tracks including non-gray boundary conditions, rotation, conservation of angular momentum, and simulations of disk-locking. Aims. By assuming that disk-locking is the regulation mechanism for the stellar angular velocity during the early stages of pre-main sequence evolution, we use our rotating models and observational data to constrain disk lifetimes (T disk ) of a representative sample of low-mass stars in two young clusters, the Orion Nebula cluster (ONC) and NGC 2264, and to better understand their rotational evolution. Methods. The period distributions of the ONC and NGC 2264 are known to be bimodal and to depend on the stellar mass. To follow the rotational evolution of these two clusters' stars, we generated sets of evolutionary tracks from a fully convective configuration with low central temperatures (before D-and Li-burning). We assumed that the evolution of fast rotators can be represented by models considering conservation of angular momentum during all stages and of moderate rotators by models considering conservation of angular velocity during the first stages of evolution. With these models we estimate a mass and an age for all stars. Results. The resulting mass distribution for the bulk of the cluster population is in the ranges of 0.2−0.4 M and 0.1−0.6 M for the ONC and NGC 2264, respectively. For the ONC, we assume that the secondary peak in the period distribution is due to high-mass objects still locked in their disks, with a locking period (P lock ) of ∼8 days. For NGC 2264 we make two hypotheses: (1) the stars in the secondary peak are still locked with P lock = 5 days, and (2) NGC 2264 is in a later stage in the rotational evolution. Hypothesis 2 implies in a disk-locking scenario with P lock = 8 days, a disk lifetime of 1 Myr and, after that, constant angular momentum evolution. We then simulated the period distribution of NGC 2264 when the mean age of the cluster was 1 Myr. Dichotomy and bimodality appear in the simulated distribution, presenting one peak at 2 days and another one at 5−7 days, indicating that the assumption of P lock = 8 days is plausible. Our hypotheses are compared with observational disk diagnoses available in the literature for the ONC and NGC 2264, such as near-infrared excess, Hα emission, and spectral energy distribution slope in the mid-infrared. Conclusions. Disk-locking models with P lock = 8 days and 0.2 Myr ≤ T disk ≤ 3 Myr are consistent with observed periods of moderate rotators of the ONC. For NGC 2264, the more promising explanation for the observed period distribution is an evolution with disk-locking (with P lock near 8 days) during the first 1 Myr, approximately, but after this, the evolution continued with constant angular momentum.

Multicolor Variability of Young Stars in the Lagoon Nebula: Driving Causes and Intrinsic Timescales

“…If the same connection can be assumed to be true for all stars, the observed trend in typical τ knee values suggests that stars whose dynamics are dominated by star-disk interaction phenomena (notably 'B', 'S', 'QPD', 'APD') tend to rotate more slowly than stars dominated by geometric modulation by uneven surface features (spots; 'P', 'QPS', 'MP'). This is consistent with the distinct distributions of rotation rates that have been reconstructed for several disked, as opposed to non-disked, young stellar populations (e.g., Venuti et al 2017;Roquette et al 2017;Rebull et al 2018), and it can be understood in the presence of a disk-locking mechanism that prevents the star from spinning up until the inner disk has evolved (e.g., Herbst & Mundt 2005;Vasconcelos & Bouvier 2015;Landin et al 2016). In any case, our analysis indicates that: i) for both disk-bearing and disk-free young stars, the most prominent timescale of variability (τ knee ) is on the order of days, which corresponds to the rotational timescales;…”

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

“…Moreover, accreting CTTs seem to rotate slower than non-accreting WTTs, which can be indirectly explained from MA if the stars are initially locked to the inner disk Keplerian rotation through the magnetic channels [29,30]. Although some controversy still remains concerning the "disk-locking" view (see e.g., the related discussion in [31]), the main lines of evidence summarized above are supported by many independent works that reached a consensus about the validity of MA against BL. A similar discussion about MA in CTTs as observed at short UV and X-ray wavelengths can be found in the review by Schneider et al (2020) [32] for this same special issue of the journal.…”

The influence of the environment on the spin evolution of low-mass stars. I. External photoevaporation of circumstellar disks

“…The disk-locking hypothesis has been widely adopted by previous models in the literature (e.g., Bouvier et al 1997;Gallet & Bouvier 2013Vasconcelos & Bouvier 2015Landin et al 2016;Amard et al 2016;Johnstone et al 2021) and has the observational support discussed in the Section 1. In this scenario, as a consequence of the magnetic SDI, stars that are still accreting are exchanging angular momentum with their disks, and the net result of this interaction is a torque that counteracts the stellar contraction and stellar winds.…”