An updated model for the Perseus Spiral Arm from Trigonometric Parallax and 3D kinematic distances of distant young stars

Abstract

We report trigonometric parallaxes and proper motions for three water masers and one methanol maser obtained with the VLBA as part of the BeSSeL Survey. Incorporating these parallaxes with 3-dimensional kinematic distances, we find the Perseus spiral arm in the 1$^{st}$ Galactic quadrant to lie between 0.5 and 1.0 kpc further from the Galactic Center than previously determined. Based on these results, the Perseus and Sagittarius arms intersect at a Galactocentric azimuth of ${189^\circ}^{+31}_{-19}$ and radius of $5.6^{+0.3}_{-0.4}$~kpc on the far side of the Galaxy.

Figures (4)

• Figure 1: Parallaxes and proper motion fits for G021.87$+$00.01, G037.82$+$00.41, G060.57--00.18, and G070.29$+$01.60. Filled and open symbols represent motion with respect to different reference quasars. Left panels: Total sky motion, the y-axis is North-South offsets, and the x-axis is East-West offsets. Center panels: Time--varying positions in North--South (triangles) and East--West (squares) directions vs. time. The x-axis is time in years, the y-axis is the offset in the corresponding direction. Right panels: Proper motion-subtracted time-varying positions in aforementioned directions. For clarity, RA and declination signatures for each reference quasar have been offset from one another.
• Figure 2: Example of the distance determination process for G070.29+01.60. The $v_\mathrm{lsr}$ PDF (dashed-red) and $l*,b$ (dashed-blue, cyan) proper motion PDFs are multiplied together to give the 3D kinematic distance PDF (magenta). This is then multiplied by the parallax PDF (green) to give the final PDF (black), the peak of which gives the most likely distance.
• Figure 3: Comparison of parallax distance (left, blue dots) with most likely distance (right, black dots) in the Galactic polar coordinate "plan view" frame. With no spiral structure/arm position a priori, maser positions converge to a much cleaner spiral shape. We have excluded G021.87 and G070.33 from the parallax distance plot as they do not have a well-measured parallax. The thin concentric rings centered on the Sun (red star at [0,8.15]) are spaced by 1 kpc, and the thick concentric circles are spaced by 5 kpc. The rays show Galactic longitude, spaced by 10 deg. The green square shows the location of the Galactic center at $[0,0]$ in both plots.
• Figure 4: Left: The log-periodic spiral arm fits. The shaded area shows the estimated Perseus arm width. Right: Plan view of the Milky Way. In both plots, the dashed lines are the spiral arm fits for the Outer (red), Perseus (black), Local (cyan), Sagittarian-Carina (magenta), and Scutum-Centaurus (blue) arms Reid2019ApJ...885..131R. The solid lines show the new models from this work (black), the Sagittarius fit Bian2024AJ....167..267B, and X1 bar orbitals Kumar2025ApJ...982..185K. The "K" shows the location of the Perseus arm kink, the green square shows the location of the Galactic center $[0,0]$, the Sun is shown with a red star at $[0,8.15]$, and new maser parallaxes from this work are highlighted with red circles.