[Paper Review] Local kinematics of K and M giants from Coravel/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters

Benoît Famaey, A. Jorissen+5 more|arXiv (Cornell University)|Sep 24, 2004

Stellar, planetary, and galactic studies49 references17 citations

TL;DR

This study presents a kinematic analysis of 5952 K and 739 M giants in the solar neighborhood using radial velocities from the CORAVEL survey, proper motions from Tycho-2, and parallaxes from Hipparcos, revealing that small-scale velocity structures—previously called 'superclusters'—are better explained as dynamical streams caused by transient spiral waves rather than remnants of star clusters. The key finding is that these streams, including the Hercules stream and Hyades-Pleiades group, span a wide age range and are not coeval, challenging the traditional supercluster concept and suggesting a dynamical origin for vertex deviation and kinematic biases in solar motion determinations.

ABSTRACT

The availability of the Hipparcos Catalogue triggered many kinematic and dynamical studies of the solar neighbourhood. Nevertheless, those studies generally lacked the third component of the space velocities, i.e., the radial velocities. This work presents the kinematic analysis of 5952 K and 739 M giants in the solar neighbourhood which includes for the first time radial velocity data from a large survey performed with the CORAVEL spectrovelocimeter. It also uses proper motions from the Tycho-2 catalogue, which are expected to be more accurate than the Hipparcos ones. The UV-plane constructed from these data for the stars with precise parallaxes reveals a rich small-scale structure, with several clumps corresponding to the Hercules stream, the Sirius moving group, and the Hyades and Pleiades superclusters. A maximum-likelihood method, based on a bayesian approach, has been applied to the data, in order to make full use of all the available stars and to derive the kinematic properties of these subgroups. Isochrones in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these groups. These groups are most probably related to the dynamical perturbation by transient spiral waves rather than to cluster remnants. A possible explanation for the presence of young clusters in the same area of the UV-plane is that they have been put there by the spiral wave associated with their formation, while the kinematics of the older stars of our sample has also been disturbed by the same wave. The term "dynamical stream" for the kinematic groups is thus more appropriate than the traditional term "supercluster" since it involves stars of different ages, not born at the same place nor at the same time.

Motivation & Objective

• To re-evaluate the concept of 'superclusters' in the solar neighborhood using high-precision kinematic data.
• To investigate whether observed velocity substructures in K and M giants are due to coeval cluster remnants or dynamical perturbations.
• To determine the true solar motion by accounting for biases introduced by kinematic streams in the velocity distribution.
• To assess the role of transient spiral waves in shaping the phase-space structure of the Galactic disk.
• To provide a refined kinematic model of the solar neighborhood using maximum-likelihood Bayesian methods on a large, unbiased sample.

Proposed method

• Combined radial velocities from the CORAVEL spectrovelocimeter survey (precision ~0.3 km s⁻¹) with proper motions from the Tycho-2 catalogue and parallaxes from Hipparcos.
• Used a maximum-likelihood Bayesian method to model the velocity distribution, including stars with uncertain parallaxes (σπ/π ≤ 20%) and accounting for observational biases.
• Constructed the UV-plane (U, V space velocities) to identify kinematic substructures such as the Hercules stream, Sirius moving group, and Hyades-Pleiades supercluster.
• Applied isochrone fitting in the Hertzsprung-Russell diagram to estimate ages of stars in each kinematic subgroup.
• Excluded spectroscopic binaries for which center-of-mass velocities could not be determined, reducing the sample to 5311 K giants and 719 M giants.
• Extracted the underlying velocity ellipsoid after removing stream components to assess the true solar motion relative to the local standard of rest.

Experimental results

Research questions

• RQ1 Are the observed kinematic substructures in the solar neighborhood, such as the Hyades-Pleiades group, remnants of coeval star clusters or the result of dynamical perturbations?
• RQ2 What is the true radial solar motion when accounting for biases introduced by kinematic streams in the velocity distribution?
• RQ3 Do the observed velocity structures in K and M giants arise from transient spiral density waves rather than stable cluster remnants?
• RQ4 Can the wide age range of stars in the same kinematic group be explained by a common dynamical origin rather than a common formation epoch?
• RQ5 Is the vertex deviation of 16.2° observed in the full sample due to the superposition of multiple dynamical streams rather than a true rotation anomaly?

Key findings

• The observed fraction of spectroscopic binaries is only 5.7% among M giants, compared to 13.7% among K giants, indicating lower binary frequency in cooler giants.
• After removing binaries, 5311 K giants and 719 M giants remain in the final sample with precise parallaxes (σπ/π ≤ 20%).
• The UV-plane reveals multiple kinematic clumps corresponding to the Hercules stream, Sirius moving group, and Hyades-Pleiades supercluster, indicating small-scale velocity structure.
• Isochrones in the Hertzsprung-Russell diagram show a very wide range of ages for stars in these subgroups, contradicting the idea of coeval cluster remnants.
• The underlying velocity ellipsoid, after removing streams, is centered on ⟨U⟩ = -2.78 ± 1.07 km s⁻¹, not the commonly assumed value, highlighting bias in standard solar motion determinations.
• The full sample yields the standard solar motion value of ⟨U⟩ = -10.25 ± 0.15 km s⁻¹ only when biases from streams are properly accounted for, suggesting that no single reference frame of zero net radial motion exists in the solar neighborhood.