Local Kinematics and the Local Standard of Rest

Abstract: We re-examine the stellar kinematics of the Solar neighbourhood in terms of the velocity of the Sun with respect to the local standard of rest. We show that the classical determination of its component V_sun in the direction of Galactic rotation via Stroemberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U,V,W)_sun = (11.1 +/- 0.74, 12.24 +/- 0.47, 7.25 +/-0.37) km/s, with additional systematic uncertainties of ~ (1,2,0.5) km/s. In particular, V_sun is 7 km/s larger than previously estimated. The new values of solar motion are extremely insensitive to the metallicity gradient within the disc.

• The paper critiques the traditional Strömberg method for underestimating the Sun's rotational velocity by overlooking metallicity gradients.
• It employs a chemodynamical model to update solar motion values, revealing that the solar V component is approximately 7 km/s greater than earlier estimates.
• The study demonstrates that its findings remain robust against metallicity variations, urging a revision of current kinematic interpretations in Galactic dynamics.

Local Kinematics and the Local Standard of Rest

The paper "Local Kinematics and the Local Standard of Rest" by R. Schönrich, J. Binney, and W. Dehnen explores a reevaluation of the Sun's velocity relative to the Local Standard of Rest (LSR). This is an essential inquiry in Galactic astronomy as it enables the interpretation of heliocentric velocities in the context of Galactic structure. The authors critique the conventional method of determining the solar velocity component in the direction of Galactic rotation, $\Vsun$, that leverages Strömberg's relation, and demonstrate that this approach is flawed due to the presence of a metallicity gradient in the Galactic disc.

Key Findings

1. Critique of Conventional Methodology: The paper highlights that classical determinations of $\Vsun$ underestimate the actual value due to unconsidered correlations caused by the disc's metallicity gradient. This gradient affects the kinematics of stars in the Solar neighborhood, leading to a more complex relationship between mean rotational velocity and velocity dispersion than previously assumed.
2. New Determination of Solar Motion: Utilizing a chemodynamical model of the Galaxy, the authors calculate the solar motion as $(U,V,W)_\odot=(11.1_{-0.75}^{+0.69},12.24_{-0.47}^{+0.47},7.25_{-0.36}^{+0.37})\,\kms$, while acknowledging systematic uncertainties of $\sim(1,2,0.5)\,\kms$. Notably, $\Vsun$ is concluded to be $7\,\kms$ larger than the widely used estimates by previous studies.
3. Implication of Metallicity Gradient: The study effectively illustrates how metallicity gradients induce systematic shifts in kinematics around the main-sequence turnoff region, resulting in significant deviations from expected kinematic behavior when analyzed through Strömberg's equation. This misrepresentation previously led to an underestimation of $\Vsun$.
4. Robustness Against Metallicity Variations: The authors assert that their new values for solar motion are remarkably robust to changes in the metallicity gradient assumed within the model. This finding is crucial for the accurate determination of solar motion in related contexts.

Implications and Future Directions

The findings bear significant theoretical and practical implications. Theoretical models of Galactic dynamics and evolution now have a revised basis for understanding the Sun's motion relative to its surroundings. Practically, the study suggests that prevailing methods for correcting observed stellar velocities relative to the solar motion need revision. Additionally, future research might delve further into the complexities introduced by gradients and other systematic variations within the galactic disc, refining both model assumptions and observational methodologies.

This paper encourages a critical reexamination of how kinematic data are interpreted concerning the Galactic frame. More sophisticated models that incorporate chemical evolution, stellar formation, and dynamical features like spiral arms might provide deeper insights into the kinematic structures encountered in the Galaxy. The study also underscores the need for caution when extrapolating velocity parameters and hints at the potential for uncovering further discrepancies across various stellar populations and regions.

In conclusion, the reevaluation of $\Vsun$ through a comprehensive chemodynamical lens represents an important step toward refining our understanding of local stellar kinematics and highlights the ongoing need to align theoretical predictions with observational data intricacies.