The Dispersal of Young Stars and the Greater Sco-Cen Association

We review topics related to the dispersal of young stars from their birth-sites, and focus in particular on the entourage of young stars related to the ongoing star-formation event in the Sco-Cen OB association. We conduct a follow-up kinematic study to that presented in Mamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby, isolated, young stars. In addition to the η Cha and TW Hya groups, we find several more intriguing Sco-Cen outlier candidates: most notably β Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connection between Sco-Cen and the southern “150 pc Conspiracy” molecular clouds, and in particular, Corona Australis. The kinematic evidence suggests that many of the nearby, isolated ∼10 Myr-old stars were born near Sco-Cen during the UCL and LCC starbursts 10–15 Myr ago. We hypothesize that these stars inherited 5–10 km s velocities moving away from Sco-Cen, either through molecular cloud turbulence, or through formation in molecular clouds associated with the expanding Sco-Cen superbubbles (e.g. Loop I). A question arose several times at the YStars conference: “Why are nearly all of the nearby, very young stars located in the southern, rather than northern, sky?” The “Gould Belt” is a general answer, however a more specific answer may involve the nearest, large component of the belt: Sco-Cen. We suggest that the Sco-Cen Association (Sco OB2) is largely responsible for this asymmetry through two mechanisms: the dispersal of stars that formed with the main ScoCen OB subgroups in the same giant molecular cloud (GMC) complex; and the concentration of gas and subsequent star-formation initiated by the Sco-Cen superbubbles. We present our heuristic argument in several steps. First, we provide an historical perspective (§1) and explore mechanisms of the dispersal of pre-main sequence (pre-MS) stars from their natal clouds (§2). In §3, we discuss observations (kinematics, ISM, etc.) linking young stars and groups to the Sco-Cen OB association. Finally, we discuss the possible relationship between the Sco-Cen superbubble and regions of current star formation (§4). Many of these thoughts are built on foundations laid in our earlier studies (Feigelson 1996; Mamajek, Lawson, & Feigelson 2000, MLF00).