J/ApJ/899/121 127 early type and pre-main-sequence stars in W4 (Lim+, 2020) ================================================================================ The Origin of a Distributed Stellar Population in the Star-forming Region W4. Lim B., Hong J., Yun H.-S., Hwang N., Kim J.S., Lee J.-E., Park B.-G., Park S. =2020ApJ...899..121L ================================================================================ ADC_Keywords: Clusters, open; Stars, early-type; Stars, pre-main sequence; Star Forming Region; Spectra, optical; Radial velocities; Proper motions Keywords: Star formation ; Stellar kinematics ; Stellar associations ; Stellar dynamics ; Open star clusters Abstract: Stellar kinematics provides the key to understanding the formation process and dynamical evolution of stellar systems. Here, we present a kinematic study of the massive star-forming region (SFR) W4 in the Cassiopeia OB6 association using the Gaia Data Release 2 and high-resolution optical spectra. This SFR is composed of a core cluster (IC1805) and a stellar population distributed over 20pc, which is a typical structural feature found in many OB associations. According to a classical model, this structural feature can be understood in the context of the dynamical evolution of a star cluster. The core-extended structure exhibits internally different kinematic properties. Stars in the core have an almost isotropic motion, and they appear to reach virial equilibrium given their velocity dispersion (0.9{+/-}0.3km/s) comparable to that in a virial state (~0.8km/s). On the other hand, the distributed population shows a clear pattern of radial expansion. From the N-body simulation for the dynamical evolution of a model cluster in subvirial state, we reproduce the observed structure and kinematics of stars. This model cluster experiences collapse for the first 2Myr. Some members begin to radially escape from the cluster after the initial collapse, eventually forming a distributed population. The internal structure and kinematics of the model cluster appear similar to those of W4. Our results support the idea that the stellar population distributed over 20pc in W4 originate from the dynamical evolution of IC1805. Description: We observed 198 low-mass pre-main-sequence (PMS) star candidates on 2018 November 29 using the high-resolution multi-object spectrograph Hectochelle on the 6.5m telescope of the Multi-Mirror Telescope (MMT) observatory. The spectra of these stars were taken with the RV31 filter in 2x2 binning mode to achieve good signal-to-noise ratios. Stars with proper motions within three times the standard deviations from the weighted mean values were selected as genuine members; a total of 127 members (55 early-type and 72 PMS stars) were selected. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 132 127 Catalogue of kinematic members -------------------------------------------------------------------------------- See also: I/345 : Gaia DR2 (Gaia Collaboration, 2018) J/AJ/105/499 : OB Associations and Spiral Structure in M33 (Regan+ 1993) J/ApJ/454/151 : OB Associations of the Northern Milky Way (Massey+ 1995) J/AJ/120/333 : UBVRI and H{alpha} in NGC 6530 (Sung+, 2000) J/A+A/371/497 : OB associations in spiral galaxy NGC 300 (Pietrzynski+, 2001) J/AJ/128/805 : Low-mass stars in Cepheus OB2 region (Sicilia-Aguilar+, 2004) J/AJ/134/1368 : Proper motions in NGC 2244 and NGC 6530 (Chen+, 2007) J/ApJ/688/1142 : Star formation in W5: Spitzer observations (Koenig+, 2008) J/A+A/502/1015 : OB associations in IC 1613 (Garcia+, 2009) J/ApJ/726/19 : Intermediate-mass stars in IC 1805 (Wolff+, 2011) J/ApJ/746/15 : Runaway massive stars from R136 (Banerjee+, 2012) J/A+A/537/A146 : Stellar models rotation. 0.8