[Paper Review] A Galactic Bar to Beyond the Solar Circle and its Relevance for Microlensing

M. W. Feast, P. A. Whitelock|Apr 7, 2000

Stellar, planetary, and galactic studies56 citations

TL;DR

This paper presents kinematic evidence from Mira variables with periods of 145–200 days indicating a bar-like structure extending from the Galactic bulge beyond the solar circle. Using infrared photometry, Hipparcos astrometry, and radial velocities, the authors find that the major axes of these stars' orbits are aligned with the Galactic bar, supporting its extended nature and relevance for microlensing studies.

ABSTRACT

The Galactic kinematics of Mira variables have been studied using infrared photometry, radial velocities, and Hipparcos parallaxes and proper motions. For Miras in the period range 145 to 200 days (probably corresponding to [Fe/H] in the range -0.8 to -1.3) the major axes of the stellar orbits are concentrated in the first quadrant of Galactic longitude. This is interpreted as a continuation of the bar-like structure of the Galactic Bulge out to the solar circle and beyond.

Motivation & Objective

• To investigate the kinematics of Mira variables in the period range 145–200 days to probe Galactic structure beyond the solar circle.
• To determine whether the observed orbital alignment of Mira variables indicates a continuation of the Galactic bar into the outer disk.
• To assess the implications of these kinematic results for microlensing studies, particularly in light of bar-induced lensing effects.
• To clarify the nature of the anomalous radial velocity and asymmetric drift observed in intermediate-period Miras.
• To test whether the period–luminosity relation and metallicity correlations in Miras can be used to trace distinct stellar populations in the Galactic disk and bulge.

Proposed method

• Combined near-infrared JHKL photometry from SAAO with Hipparcos astrometric data (parallaxes and proper motions) for Galactic Mira variables.
• Used the (Hp−K)0–log P plane to distinguish two sequences: a blue main sequence and a redder, less populated sequence at shorter periods.
• Applied Hipparcos parallaxes to determine absolute magnitudes and confirm that red-sequence stars are brighter than blue-sequence stars at the same period.
• Analyzed radial and rotational velocities (VR and Vθ) to infer orbital geometry and identify preferred alignment of orbital major axes.
• Modelled the expected Vθ–VR relation under a simple bar model with a 17° angle to the Sun–Galactic center line.
• Excluded highly eccentric stars (e.g., S Car) from the main analysis to isolate coherent kinematic trends.

Experimental results

Research questions

• RQ1 Do Mira variables with periods of 145–200 days trace a bar-like structure extending beyond the solar circle?
• RQ2 What causes the anomalously high outward radial velocity (VR ≈ +67 km s⁻¹) observed in the 145–200 day period group?
• RQ3 How do the orbital major axes of these Miras relate to the known orientation of the Galactic bar?
• RQ4 Can the kinematic alignment of Miras in this period range be explained by a coherent, extended bar structure rather than local interlopers?
• RQ5 What is the significance of the red sequence in the (Hp−K)0–log P plane for understanding the evolutionary state and metallicity of these stars?

Key findings

• The mean orbital major axis of Mira variables with periods 145–200 days is aligned at 17° ± 11°/−4° from the Sun–Galactic center line, consistent with the Galactic bar's orientation.
• The rotational velocity (Vθ) of this group is 176 ± 14 km s⁻¹, close to the circular velocity inferred from Cepheids (231 km s⁻¹), indicating a dynamically coherent population.
• The radial velocity (VR) is +67 ± 17 km s⁻¹, indicating a net outward motion, which is inconsistent with axial symmetry and suggests a deficit of incoming orbits.
• The red sequence in the (Hp−K)0–log P plane contains stars brighter than the blue sequence at the same period, suggesting higher luminosity and possibly higher evolutionary state or pulsation mode.
• The orbital alignment and kinematics are best explained by a bar-like structure extending beyond the solar circle, not by local interlopers or non-axisymmetric features.
• The results imply that the Galactic bar may extend further into the disk than previously thought, with implications for microlensing event modeling and bar dynamics.