On the Origin of Kinematic Structure in the Young Association Serpens OB2

TL;DR

This study characterizes the origin of kinematic structure in Serpens OB2 by integrating Gaia DR3 proper motions with SPICY YSOs, ALS III OB stars, and MWISP $^{13}$CO data. Using parallax refinement, clustering analysis, and structure-function–style pairwise velocity statistics (VSAT), the authors uncover a multi-scale velocity field: strong small-scale velocity alignment, increasing velocity differences with separation, and a prominent global expansion gradient perpendicular to the Galactic plane of $0.10 \pm 0.02$ km s$^{-1}$ pc$^{-1}$. They interpret these patterns as a fossil imprint of a turbulent natal cloud, modified by feedback from the region’s OB stars, notably through the Sh 2-54 H II region’s expansion, potentially accelerating the molecular cloud away from the plane. The results imply that OB associations can begin large-scale expansion while star formation is still ongoing, linking local clustering, turbulence, and feedback to the observed kinematic architecture.

Abstract

The Serpens OB2 association (l ~ 18.5 deg, b ~ 1.9 deg, d = 1950 +/- 30 pc) is a large star-forming complex ~65 pc above the Galactic midplane, with a clumpy, elongated structure extending ~50 pc parallel to the plane. We analyse a sample of probable association members, including OB stars and low-to-intermediate-mass young stellar objects (YSOs) from the SPICY catalogue. While both populations are found throughout the association, the OB stars lie preferentially on the side nearest the Galactic plane, while the YSOs are generally younger and more strongly clustered around molecular-cloud clumps detected in 13CO MWISP data. Using Gaia DR3 proper motions to probe the association's internal kinematics, we find aligned stellar velocities on length scales <2 pc, two-point velocity statistics that show increasing velocity differences and predominantly divergent motions at larger separations, and distinct velocities for star clusters within the association. Finally, the association exhibits gradual, but statistically significant global expansion perpendicular to the Galactic plane, with a spatial gradient of 0.10 +/- 0.02 km/s/pc. The expansion of the H II region Sh 2-54, powered by the association's OB stars, may be accelerating the star-forming cloud away from the plane given the system's geometry, plausibly inducing the vertical stellar velocity gradient. The clumpy stellar distribution, correlated velocities on small scales, and increasingly divergent motions on larger scales are consistent with an initial velocity field inherited from a turbulent molecular cloud modified by stellar feedback. Ser OB2 demonstrates that the multi-scale expansion of an OB association can begin even while star formation is still ongoing throughout the complex.

Figures (10)

• Figure 1: Spitzer/IRAC mosaic of the Ser OB2 region in the 3.6 $\mu$m (blue), 5.8 $\mu$m (green), and 8.0 $\mu$m (red) bands. The yellow contours indicate the surface-density distribution of cluster members (Section \ref{['sec:clust']}), with the $60\times60$ pc box used for this analysis indicated (green rectangle). Several subregions are labelled.
• Figure 2: Parallaxes for OB stars (blue asterisks) and YSOs (red circles) in the Ser OB2 field. Left: Corrected Gaia parallax ($\varpi$) versus formal parallax uncertainty ($\sigma_\varpi$). The diagonal lines show the $\pm2\sigma$ envelope around the association mean $\varpi_0=0.512$ mas, using the quadrature sum of statistical and systematic uncertainties. Sources whose parallaxes differ from $\varpi_0$ by more than $2\sigma$ are indicated by open circles (YSOs) or fainter symbols (OB stars). Right: Parallax versus Galactic longitude ($\ell$) using the same symbols as the left panel. The $1\sigma$ error bars are indicated for YSOs (OB-star parallax uncertainties are smaller than their symbols). The horizontal black line marks $\varpi_0$. The larger parallax spread of the YSOs reflects their larger formal uncertainties.
• Figure 3: Near-infrared colour--magnitude diagram of probable Ser OB2 members (OB stars and YSOs). Stars are colour-coded by the availability of high-quality Gaia astrometry. The YSO sample reaches $\sim$0.4 $M_\odot$, but the subset with Gaia astrometry is sensitive only above $\sim$1 $M_\odot$. Stars with discrepant parallaxes are excluded from the diagram. PARSEC isochrones 2012MNRAS.427..127B are shown at the Ser OB2 distance, reddened to extinctions typical of members.
• Figure 4: Left: Smoothed surface density distribution for members of Ser OB2, in units of sample members per square parsec (greyscale image and yellow contour lines). The membership list is comprised of YSOs from the SPICY catalogue (red circles) and OB stars from the ALS III catalogue (blue asterisks), excluding those with inconsistent parallaxes. Right: Mixture-model cluster solution with 8 cluster components. The black ellipses trace the isodensity contours of each component at 2 times the core radius. The assignments of stars to groups are indicated by the symbol shapes and colours. Distinct clusters are labelled. The two larger ellipses near the centre of the region identify the core (stellar members of the Ser OB2 ridge W and E indicated by asterisks and diamonds, respectively). Ser OB2 halo members are indicated by the open circles.
• Figure 5: Left: The 2D projected velocity distribution for Ser OB2, plotted with the same symbols as Fig. \ref{['fig:plx']} and including only stars whose parallaxes are consistent with membership. We use $| \mathbf v | \leq 10$ km s$^{-1}$ (dashed circle) as an approximate cut-off to separate the slow-moving cluster members from higher velocity stars, including walkaway and runaway association members and contaminants. Right: Positions and velocities of OB stars (blue) and YSOs (red) from the $| \mathbf v | \leq 10$ km s$^{-1}$ sample. The arrow lengths and directions indicate the projected velocities of these stars in the $(\xi, \eta)$ plane.