The Formation of a Stellar Association in the NGC 7000/IC 5070 Complex: Results from Kinematic Analysis of Stars and Gas

Abstract

We examine the clustering and kinematics of young stellar objects (YSOs) in the North America/Pelican Nebulae, as revealed by Gaia astrometry, in relation to the structure and motions of the molecular gas, as indicated in molecular line maps. The Gaia parallaxes and proper motions allow us to significantly refine previously published lists of YSOs, demonstrating that many of the objects previously thought to form a distributed population turn out to be non-members. The members are subdivided into at least 6 spatio-kinematic groups, each of which is associated with its own molecular cloud component or components. Three of the groups are expanding, with velocity gradients of 0.3-0.5 km s −1 pc −1 , up to maximum velocities of ∼8 km s −1 away from the groups' centers. The two known O-type stars associated with the region, 2MASS J20555125+4352246 and HD 199579, are rapidly escaping one of these groups, following the same position-velocity relation as the low-mass stars. We calculate that a combination of gas expulsion and tidal forces from the clumpy distribution of molecular gas could impart the observed velocity gradients within the groups. However, on a global scale, the relative motions of the groups do not appear either divergent or convergent. The velocity dispersion of the whole system is consistent with the kinetic energy gained due to gravitational collapse of the complex. Most of the stellar population has ages similar to the free-fall timescales for the natal clouds. Thus, we suggest the nearly free-fall collapse of a turbulent molecular cloud as the most likely scenario for star formation in this complex.1. INTRODUCTION The way that stars and gas are distributed in a starforming region can provide useful constraints on the conditions in which the stars were formed (e.g., Elmegreen 2002;Parker et al. 2014;Gouliermis 2018;Feigelson 2018). For the nearest star-forming regions, astrometric measurements by ESA's Gaia spacecraft (Gaia Collaboration et al. 2016) can provide a multi-dimensional picture of how the young stars are clustered. Spatial and kinematic clustering has already been examined in several of the major star-forming regions within 1 kpc,

The Villafranca catalog of Galactic OB groups: I. Systems with O2-O3.5 stars

“…After this paper was submitted to the journal, an independent analysis of the North America and Pelican nebulae using Gaia DR2 data was published and the referee dutifully pointed it to us (Kuhn et al 2020). Most of the results in that paper agree with what we present here but there are some small differences that we discuss now.…”

“…We are currently working on a paper with an orbit for the system. Regarding the distance, Kuhn et al (2020) find a mean parallax for their group E (objects in the Gulf of Mexico) of 1.27±0.02 mas, which can be compared to our non-zero-pointcorrected value of 1.354±0.029 mas. Their value for the dispersion apparently does not include the spatial covariance term (their section 7.4) but including it would leave the difference between our two results at a two-sigma level.…”

“…That scenario is plausible but we note that it is far below the standard threshold for runaway stars and that some refer to such objects as "walkaway stars" (Renzo et al 2019). Note that Kuhn et al (2020) place their group D beyond their group E and that the parallax for the Bajamar star places it even closer albeit with a relatively large uncertainty. Hopefully, future Gaia releases will shed some light into these (relatively small) distance discrepancies.…”

“…In this paper, however, we have presented the case of the Bajamar star, which is located almost two orders of magnitude closer than 30 Doradus and, furthermore, with a primary of earliest type than any of the Bressert et al (2012) objects. It is located near the center of the molecular cloud from where it appears to have been born, shows only a small relative velocity with respect to its natal cloud (Kuhn et al 2020), and there are no other ejected O stars in the vicinity seen in the Gaia DR2 data. It is a short-period spectroscopic binary with approximate masses (as estimated from their spectral types) of 50 M and ∼25 M , there is no cluster around it, and the few relatively nearby association members are at most of intermediate mass.…”