Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way (1910.03357v1)

Abstract: We compile and analyze ~200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars. These measurements strongly suggest that the Milky Way is a four-arm spiral. Fitting log-periodic spirals to the locations of the masers, allows us to significantly expand our view of the structure of the Milky Way. We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms. Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center, Ro = 8.15 +/- 0.15 kpc, the circular rotation speed at the Sun's position, To = 236 +/- 7 km/s, and the nature of the rotation curve. Our data strongly constrain the full circular velocity of the Sun, To + Vsun = 247 +/- 4 km/s, and its angular velocity, (To + Vsun)/Ro = 30.32 +/- 0.27 km/s/kpc. Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy, we find non-circular velocity components typically about 10 km/s. However, near the Galactic bar and in a portion of the Perseus arm, we find significantly larger non-circular motions. Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc and, thus, are well suited to define the Galactic plane. We find that the orientation of the plane is consistent with the IAU-defined plane to within +/-0.1 deg., and that the Sun is offset toward the north Galactic pole by Zsun = 5.5 +/- 5.8 pc. Accounting for this offset places the central supermassive black hole, Sgr A*, in the midplane of the Galaxy. Using our improved Galactic parameters, we predict the Hulse-Taylor binary pulsar to be at a distance of 6.54 +/- 0.24 kpc, assuming its orbital decay from gravitational radiation follows general relativity.

• The paper refines the Milky Way's spiral structure using ~200 trigonometric parallax measurements of high-mass star forming regions from VLBA and VERA.
• The paper determines key Galactic parameters, including a center distance of 8.15 kpc and a circular velocity of 247 km/s, through 3D maser kinematic modeling.
• The paper identifies significant non-circular motions near the Galactic bar and Perseus arm, indicating dynamic gravitational interactions that challenge simple rotation models.

Trigonometric Parallaxes of High-Mass Star Forming Regions: Our View of the Milky Way

This academic paper presents extensive research utilizing approximately 200 trigonometric parallaxes and proper motions of molecular masers linked to very young high-mass stars to enhance our understanding of the Milky Way's structure. The majority of these parallax measurements were obtained from the BeSSeL Survey using the Very Long Baseline Array (VLBA) and the Japanese VERA project. This research comprehensively models the Milky Way as a four-arm spiral galaxy, notably augmented with additional arm segments and spurs, thereby contradicting previous models that might have considered fewer primary arms.

Methodology and Key Findings

1. Spiral Structure Refinement: By fitting log-periodic spiral models to maser locations and incorporating well-documented arm tangencies in the 4th Galactic quadrant, the authors have significantly expanded the view of the Milky Way's spiral structure. The updated model provides a refined depiction of the spiral arms, incorporating kinks and varying pitch angles, a feature aligned with recent findings in extragalactic spiral arms.
2. Galactic Parameters: The paper presents an updated model that determines the Galactic center's distance $\Ro = 8.15 \pm 0.15$ kpc and the circular rotation speed at the Sun's location, $\Theta_0 = 236 \pm 7$ km/s. The full circular velocity of the Sun is tightly constrained to $247 \pm 4$ km/s, with an angular velocity calculated as $30.32 \pm 0.27$ km/s/kpc. These results are derived from the space motions modeled three-dimensionally using maser data, resulting in more precise Galactic parameters that clarify the dynamical motion of the Galaxy, particularly in regions affected by the Galactic bar and the Perseus arm.
3. Non-Circular Motions: The research identifies significant non-circular motions predominantly near the Galactic bar and a segment of the Perseus arm, indicating dynamic interactions in these areas likely driven by gravitational perturbations. These findings suggest complex kinematic behaviors beyond simple circular Galactic rotation models, emphasizing the need to consider these motions when addressing Galactic kinematics.
4. Galactic Plane and Solar Offset: It was found that young, high-mass stars, crucial to defining the Galactic plane, show a scale height of only 19 pc within 7 kpc of the Galactic center, aligning well with the International Astronomical Union (IAU) defined plane to within $\pm 0.1^\circ$. Remarkably, the Sun is slightly offset toward the north Galactic pole by $5.5 \pm 5.8$ pc.
5. Precession and Stability: The perpendicular motions measured limit potential precession of the Galactic plane to $\approx 4$ km/s at the Sun's radius, suggesting a relatively stable Galactic orientation over significant time scales.

Theoretical and Practical Implications

The paper provides a more cohesive understanding of the Milky Way, particularly relevant in the context of high-mass star formation located in the defined spiral structures. The robust model for the spiral arms enhances the accuracy of distance estimation methodologies for sources within these arms, refining theoretical models of Galactic evolution and dynamics. In practical terms, these findings update and potentially calibrate models employed in further studies, enhancing predictions of objects' positions, including the notable Hulse-Taylor binary pulsar, whose calculated distance aligns closely with general relativity's predictions.

Future Directions

The research points towards leveraging southern hemisphere VLBI arrays for further parallax measurements to fill gaps, particularly in less observed Galactic quadrants. Continued observations and the advent of next-generation radio facilities (such as the SKA and ngVLA) hold promise for extending these advancements, especially on the Galaxy's more distant regions, potentially leading to a dynamically integrated model of the Milky Way, encompassing more rotational and structural subtleties.

This detailed analysis not only refines fundamental parameters of our Galaxy but also stimulates further investigative paths in Galactic astronomy, underpinning our understanding of spiral structure formation and dynamics in the broader cosmic context.