Dynamical masses of YSOs with the VLBA: DYNAMO VLBA: Trigonometric parallaxes and proper motions of YSOs in Orion

Abstract

We present results from a multi-epoch Very Long Baseline Array (VLBA) survey of compact radio sources in the Orion complex, conducted within both the DYNAMO-VLBA and the GOBELINS projects. Our observations detected 216 compact radio sources, of which 58 yielded reliable multi-epoch astrometric solutions. For these sources, we derived trigonometric parallaxes and proper motions with typical precisions of about 0.05 mas and 0.10 mas yr$^{-1}$, respectively. The measured parallaxes range between 2.26 and 2.65 mas, corresponding to distances of 380 - 440 pc, and delineate the depth of the Orion star-forming complex. We determine mean distances of $405\pm16$ pc for NGC 2068, $403\pm5$ pc for NGC 2024, $407\pm12$ pc for the $σ$ Orionis region, $388.5\pm1.7$ pc for the Orion Nebula Cluster (ONC), and $438\pm12$ pc for L1641. A comparison with Gaia DR3 astrometry for 28 common sources reveals negligible mean parallax offsets ($Δ\varpi=-0.02\pm0.01$ mas) and small systematic differences in proper motions ($\sim$0.07 mas yr$^{-1}$), likely due to residual rotation of the Gaia reference frame. Our results demonstrate the capability of high-precision radio astrometry to map embedded stellar populations and to provide an independent calibration of the Gaia reference system in obscured regions.

Figures (5)

• Figure 1: Astrometric motion of V* V1399 Ori. Left: Filled circles indicate the measured positions, with $1\sigma$ uncertainties. The blue-dotted curve shows the best-fit linear model to the motion, with filled squares indicating the model positions at the observed epochs. Measured and model positions are color-coded by observing epoch, and line segments connect the corresponding epochs. The red dotted line represents the motion after subtracting the trigonometric parallax, with red squares indicating the corresponding model positions. Central panels: position offsets in declination (top) and right ascension (second panel) as a function of time, obtained after subtracting the best-fit linear proper motion, together with the best-fit parallax model (dashed lines). The two lower panels show the post-fit residuals in declination and right ascension. Symbols are color-coded by observing epoch, as in the left panel. Right: Parameters derived from the linear motion fit.
• Figure 2: Histograms showing the separation of sources through epochs.
• Figure 3: Histograms showing the pairwise separation of background sources through epochs.
• Figure 4: Distribution of the differences of Gaia and VLBA positions. The central panel shows the offsets in right ascension and declination for three categories of sources: single stars (black circles), sources with RUWE $>1.4$ (red stars), and sources identified as multiple systems (green crosses). The red ellipse represents a 3$\sigma$ contour derived from a 2D Gaussian fit to the single-star and RUWE $\leq1.4$ distributions. The top and right panels display the corresponding histograms of offsets in right ascension and declination, respectively, for single stars, and RUWE $\leq1.4$. Dashed red lines show the best-fit Gaussian models, with the fitted mean and standard deviation indicated in each subplot.
• Figure 5: VLBA-Gaia parallax (left-panel) and proper motion ($\mu^*_\alpha$ in the center, and $\mu_\delta$ to the right) comparisons. Top-panels: Scatter plots of measured values. Red symbols indicate sources related to multiple systems, and black symbols are likely single sources. The dashed line is the equality line, which is included for eye guidance. Determined correlation coefficient (r) and its statistical significance are included in each case. Lower panels: Histograms showing the differences between VLBI and Gaia EDR3 astrometric parameters for matched likely single sources.