The uncertainty in Galactic parameters

Abstract: We reanalyse the measurements of parallax, proper motion, and line-of-sight velocity for 18 masers in high mass star-forming regions presented by Reid et al. (2009). We use a likelihood analysis to investigate the distance of the Sun from the Galactic centre, R_0, the rotational speed of the local standard of rest, v_0, and the peculiar velocity of the Sun, vsol, for various models of the rotation curve, and models which allow for a typical peculiar motion of the high mass star-forming regions. We find that these data are best fit by models with non-standard values for vsol or a net peculiar motion of the high mass star-forming regions. We argue that a correction to vsol is much more likely, and that these data support the conclusion of Binney (2009) that V_sol should be revised upwards from 5.2 km/s to 11 km/s. We find that the values of R_0 and v_0 that we determine are heavily dependent on the model we use for the rotation curve, with model-dependent estimates of R_0 ranging from 6.7 \pm 0.5kpc to 8.9 \pm 0.9kpc, and those of v_0 ranging from 200 \pm 20 km/s to 279 \pm 33 km/s. We argue that these data cannot be thought of as implying any particular values of R_0 or v_0. However, we find that v_0/R_0 is better constrained, lying in the range 29.9-31.6 km/s/kpc for all models but one.

An Examination of Uncertainty in Galactic Parameters

The paper "The uncertainty in Galactic parameters" by P. J. McMillan and J. J. Binney presents an in-depth analysis of the solar position and velocity parameters within the Milky Way, emphasizing the persistent uncertainty in these fundamental aspects. The authors employ measurements from 18 masers within high mass star-forming regions to investigate these uncertainties, with specific attention to the solar distance from the Galactic center ( R_0 ), the local standard of rest (LSR) rotational speed ( v_0 ), and the peculiar velocity of the Sun.

Utilizing a likelihood analysis framework, the study explores various rotation curve models and accounts for potential peculiar motions of the maser sources. Notably, a central conclusion is that the data under consideration support non-traditional values for the peculiar velocity of the Sun, ( v_\odot ). The paper aligns with findings from Binney (2009), suggesting a significant upward revision of ( V_\odot ) from 5.2 km/s to 11 km/s. However, the determination of ( R_0 ) and ( v_0 ) is highly model-dependent, with estimated values ranging from 6.7 kpc to 8.9 kpc for ( R_0 ) and from 200 km/s to 279 km/s for ( v_0 ), as constrained by the chosen model for the Galaxy’s rotation curve. Despite these variations, the analysis constrains the ratio ( v_0/R_0 ) within a tighter range of 29.9–31.6 km/s/kpc.

The paper also critiques the assumption, seen in previous studies, that the high-mass star-forming regions move on circular orbits in a flat rotation curve. The inclusion of non-zero peculiar motions, particularly a revised value of ( V_\odot ), presents a superior fit to the maser data, challenging the widely accepted Dehnen and Binney (1998 - DB98) parameters. By adopting an evidence-based Bayesian approach, McMillan and Binney demonstrate that the adoption of the revised ( V_\odot ) significantly increases the model's likelihood.

This research has profound implications for our understanding of Galactic dynamics. The evidence suggesting a revision of ( V_\odot ) implies potential reconsiderations for dynamical modeling within the Milky Way. Given the persistent model-dependence of ( R_0 ) and ( v_0 ), and the implications this has for Galactic rotation models, further observational data, possibly involving additional masers or alternative kinematic tracers, will be vital. A robust derivation of Galactic parameters remains crucial for developing accurate models of spiral structure and other dynamic processes within the Milky Way.

Moving forward, as computational methods and observational techniques continue to improve, further refinements in the solar parameters and greater constraints on the Galactic rotation curve can be anticipated. This will undoubtedly enhance our ability to interpret the kinematics and dynamics not only within our Galaxy but also compared with extragalactic systems. The paper underscores the importance of re-evaluating longstanding assumptions in the field, especially when leveraging new data sets and methods, as they hold the potential to significantly impact our comprehension of Galactic structure and motion.