Seeking Kinematic Association of Known FU Orionis Stars with Young Clusters in Cygnus

TL;DR

The paper uses Gaia DR3 astrometry to test whether five known FU Orionis stars in Cygnus are physically linked to nearby young clusters. It develops four adaptive neighbor-selection criteria (M1–M4) and employs vector-point-diagram analysis, Gaussian fits, and Gaussian-smoothed histograms to assess kinematic consistency in 2D proper motion and parallax, due to sparse radial velocity data. The authors identify plausible associations for four FU Ori stars with nearby structures: HBC 722 and V1057 Cygni with the North America Nebula, V1515 Cygni with NGC 6914, V1735 Cygni with the IC 5146 streamer, and V2494 Cygni with LDN 1003/Braid Nebula, with colour-magnitude diagrams showing pre-main-sequence populations in these groups. The findings support physical connections between these young FU Ori stars and their local clusters, highlighting the value of Gaia-based kinematics while underscoring the need for radial velocity measurements to complete the 3D kinematic picture.

Abstract

Kinematic studies of young stars and star clusters have increased our understanding of the process of star formation and evolution in the Milky Way. FU Orionis objects are a specific class of young stellar object notable for their extremely high disk-to-star accretion rates. We use parallax and proper motion information from the Gaia astrometric survey to study five known FU Ori stars towards the Cygnus clouds, in the distance range ~500-900 parsecs, and seek evidence of their kinematic association with proximal stellar groups or clusters. We develop multiple search criteria within the Gaia datasets to look for nearby stellar aggregates and to reliably isolate their likely member stars. We show that V1057 Cygni and HBC 722 are kinematically consistent with the 3D locations as well as the inferred proper motion fields of the North America Nebula cluster. We show a similar association of V1515 Cygni with NGC 6914 in the Cygnus-X region, and of V2494 Cygni with stars in the dark cloud LDN 1003 and Braid Nebula. Further, we find that V1735 Cygni is consistent in both position and proper motion with the streamer structure of IC 5146, and we trace the streamer's similar proper motions to the main cluster. Color-magnitude diagrams of all identified clusters show the presence of pre-main-sequence populations, strengthening the likelihood of a physical association between the young FU Ori stars and their respective nearby clusters.

Figures (15)

• Figure 1: Left: Vector point diagram for the sample of stars selected using method $M_1$ (25 pc around centre of NAP) with colorbars indicating the corresponding distance in pc. Two kinematic components are marked with boxes (the larger corresponds to NAP, and the smaller corresponds to NGC 6997). The proper motion values of HBC 722 and V1057 Cygni are also marked, and both sources are consistent with the more populated kinematic component. Middle: Distance histograms for stars within the two kinematic boxes; there are no clear peaks in the distribution. Right: Image of the NAP region in infrared WISE bands 22, 12 and 3.4 $\mu$m mapped to R,G,B. Stars belonging to the two kinematic components are marked in blue (larger) and orange (smaller), with V1057 Cygni and HBC 722 indicated by pink diamonds. Some marginal spatial clustering can be seen for both components, which appear to have different spatial peaks.
• Figure 2: Top panels: VPD and histogram of distances for the sample obtained by $M_2$ with a parallax range of 0.86--1.61 (taken from Kuhn_2019). Three kinematic components are clearly seen as overdensities on the VPD (marked in boxes, from left to right: NGC 6997, NAP, LISC 1908); each of these has a distinct peak in their distance histogram. Bottom panels: WISE images of the three kinematic groups of NAP, NGC 6997 and LISC 1908 in blue, light orange and purple respectively (HBC 722 and V1057 Cygni are marked with pink diamonds). Spatial clustering is evident for all three groups, which can be identified with NAP, NGC 6997 and LISC 1908.
• Figure 3: Top panels: VPD of stars obtained using $M_4$ ($1^\circ$ search around NAP and NGC 6997, including $1\sigma$ distance consistency) with the kinematic overdensities corresponding to NAP and NGC 6997 marked in the larger and smaller boxes respectively. $1^\circ$ translates to $13.88$ pc at NAP distance. Distance histograms for both components show clear peaks. The right panel shows WISE 22, 12 and 3.4 $\mu$m images, mapped to R,G,B, with blue circles marking the members of the kinematic concentrations of NAP, with the white dashed boundary showing our visual cut on position to separate the cluster. Bottom panels: 2D histogram and fitted bivariate Gaussian to the NAP kinematic component in the left and middle, and the proper motions shown for the same on the right. The thick brown arrow shows the mean proper motion of the cleaned sample. Mean-subtracted proper motions of individual stars are shown in pink for cleaned sample. The diamonds show V1057 Cygni (green) and HBC 722 (purple). The lighter arrows show their mean-subtracted proper motion, and the darker ones show the overall proper motion which can be seen to be broadly consistent with the mean proper motion of the cluster itself.
• Figure 4: Left: VPD of stars obtained using $M_3$ from the 50 parsec region around V1515 Cygni, with possible kinematic overdensities marked. One of these overdensities is consistent with the proper motions of V1515 Cygni Middle: Distance histogram for stars belonging to the kinematic component containing V1515 Cygni. No clear peaks emerge. Right: Stars in the kinematic concentration containing V1515 Cygni are marked in the WISE image (22, 12 and 3.4 $\mu$m mapped to R,G,B), and can clearly be seen as clumping in two distinct groups. The three other overdensities marked in the left panel do not show any positional clustering and are thus likely not real clusters. V1515 Cygni is marked with a white diamond at the centre of the image. The NGC 6914 region is marked in an orange dashed circle.
• Figure 5: Top panels: VPD of stars obtained using $M_4$ from the $0.5^\circ$ (7.88 parsec) region around V1515 Cygni. The kinematic overdensity consistent with V1515 Cygni and corresponding to NGC 6914 is marked, and can be seen more clearly relative to Fig \ref{['fig:50pc-cygx']}. In the distance histogram for this component a peak is clearly discernible. The VPD clump stars (blue dots) and V1515 Cygni (white diamond) are overlayed on a WISE color image (22, 12 and 3.4 $\mu$m mapped to R,G,B). Bottom panels: The bivariate Gaussian fit to the kinematic concentration on the VPD, with the proper motion arrows in the right panel. Brown denotes the mean motion of the cleaned sample, the green diamond V1515 Cygni, with the darker arrow showing the full proper motion and the lighter showing the residual motion w.r.t. the cluster mean. The pink arrows show the residual proper motions of other member stars with respect to the mean.