Degree of stochastic asymmetry in the tidal tails of star clusters

Abstract

Context. Tidal tails of star clusters are commonly understood to be populated symmetrically. Recently, the analysis of Gaia data revealed large asymmetries between the leading and trailing tidal tail arms of the four open star clusters Hyades, Praesepe, Coma Berenices, and NGC 752. Aims. As the evaporation of stars from star clusters into the tidal tails is a stochastic process, the degree of stochastic asymmetry is quantified in this work. Methods. For each star cluster, 1000 configurations of test particles were integrated in the combined potential of a Plummer sphere and the Galactic tidal field over the lifetime of the particular star cluster. For each of the four star clusters, the distribution function of the stochastic asymmetry was determined and compared with the observed asymmetry. Results. The probabilities for a stochastic origin of the observed asymmetry of the four star clusters are ≈1.7σ for Praesepe, ≈2.4σ for Coma Berenices, ≈6.7σ for Hyades, and ≈1.6σ for NGC 752. Conclusions. In the case of Praesepe, Coma Berenices, and NGC 752, the observed asymmetry can be interpreted as a stochastic evaporation event. However, for the formation of the asymmetric tidal tails of the Hyades, additional dynamical processes beyond a pure statistical evaporation effect are required.

Asymmetrical tidal tails of open star clusters: stars crossing their cluster’s práh† challenge Newtonian gravitation

“…The analysis here based on the inner tidal tail data as obtained by the CP method shows the Hyades to be consistent with the null hypothesis while the Praesepe (a 4.9 sigma deviation) and COIN-Gaia 13 (an 8.39 sigma deviation) are not. Combining the available information: The extended tidal tails of the Hyades are in > 5 sigma tension with the null hypothesis (Kroupa et al 2022;Pflamm-Altenburg et al 2023), and the inner tidal tails of the Praesepe are in 4.9 sigma and of COIN-Gaia 13 in > 5 sigma tension with the null hypothesis. The asymmetry is the same in all three cases, namely, the leading tail contains significantly more stars than the trailing tail.…”

“…By applying Milgromian and Newtonian gravitational models to these data, Kroupa et al (2022) showed that while the full tails of the Praesepe, Coma Berenices and NGC 752 are consistent with Newtonian symmetry, the data are also consistent with Milgromian gravitation. The full-length tails of the Hyades cluster, however, are inconsistent with the Newtonian symmetry at more than 5 σ confidence (see also Pflamm-Altenburg et al 2023), while being consistent with Milgromian gravitation. The Galactic bar does not influence the evolution of star clusters orbiting at Galactocentric distances larger than ≈ 4 kpc (Rossi & Hurley 2015).…”

“…Newtonian dynamics is valid) the observed cluster data are stacked. Since the escape of stars under the null hypothesis can be very well described as a stochastic process (Pflamm-Altenburg et al 2023) we can add the leading and trailing tails in the observed clusters (Fig. 3), n l,sum = Σ 6 i=1 n l,i and n t,sum = Σ 6 i=1 n ti .…”

“…Owing to the linearity of Newtonian gravitation and near the Solar circle and at larger Galactocentric distances, for such star clusters the leading and trailing tails contain, within Poissonian fluctuations, the same number of stars. The symmetry of the tail populations has been quantified by Pflamm-Altenburg et al (2023), who show the evaporation process to be stochastic and describable as a Bernoulli process. This allows quantifying the number of stars in the leading, n l , and in the trailing tail, n t , and also how likely a certain degree of asymmetry is, given a total number of detected tail stars, n = n l + n t .…”

Back to the present: A general treatment for the tidal field from the wake of dynamical friction

“…Dynamical friction offers an alternative way to study the small-scale effects induced by hypothetical microphysical properties of DM (Hui et al 2017;Hartman et al 2021). Moreover, its applications are much broader than DM inference, as dynamical friction is the underlying phenomenon that influences a wide variety of astrophysical systems, from stellar streams and globular clusters to the orbits of satellite galaxies and galaxy merging times (Kipper et al 2019(Kipper et al , 2020Pflamm-Altenburg et al 2023;Correa Magnus & Vasiliev 2022).…”