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 nonmembers. The members are subdivided into at least six 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 freefall timescales for the natal clouds. Thus, we suggest the nearly freefall collapse of a turbulent molecular cloud as the most likely scenario for star formation in this complex.

The First Extensive Spectroscopic Study of Young Stars in the North America and Pelican Nebulae

“…NAP members (white pluses) identified in this work overplotted on a three-color image of the North America Nebula created in the same way as in Figure 1. The color-coded open circles are members of the individual kinematic groups identified by Kuhn et al (2020). The color-coded large ellipses show the boundaries of the kinematic groups depicted in this work to group the YSOs without reliable astrometric measurements.…”

“…The white dashed circle indicates the approximate boundary of the H II region (Wendker et al 1983). All the YSO candidates in the field are shown as open circles, magenta for those with infrared excess (Rebull et al 2011) and green for those identified with Gaia (Kuhn et al 2020). Our spectroscopically confirmed YSOs are shown as yellow filled circles.…”

“…Besides the YSOs that are already grouped, we have 316 members in our sample without assigned groups from Kuhn et al (2020). We tentatively assign these stars to groups based on their spatial locations.…”

“…Groups C and D overlap, and it is hard to assign a group without reliable kinematic data. To avoid potential contamination, we only use the kinematic members identified by Kuhn et al (2020) in the case of Group C and include only the ungrouped YSOs within the boundary of Group D but outside of the boundary of Group C as members of Group D. In this way, we have 43, 383, 89, and 15 YSOs in GroupsC, D, E, and F, respectively.…”

“…This H II region is ionized mainly 8 by 2MASSJ20555125+4352246 (Comerón & Pasquali 2005), also called the Bajamar Star, which has spectral type O3.5-O5 and sits behind the Lynds Dark Nebula LDN935 (Lynds 1962). The dark region that separates the North America Nebula from the Pelican Nebula is the foreground molecular cloud associated with W80 and has a total mass of about (2-5)×10 4 M e (Bally & Scoville 1980;Zhang et al 2014;Kuhn et al 2020). A distance of 795 pc based on Gaia Data Release 2 (DR2) astrometry (Gaia Collaboration et al 2016 is calculated by Kuhn et al (2020), placing the region somewhat further away than previous estimates (see Reipurth & Schneider 2008 for a review).…”