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Published May 21, 2004 | public
Journal Article

The Cradle of the Solar System

Abstract

What kind of environment gave birth to the Sun and planets? Most astronomers who study star formation would probably say that the solar system originated in a region much like the well-studied Taurus-Auriga molecular cloud (1)—a region in which low-mass, Sun-like stars form in relative isolation—but this conventional wisdom is almost certainly incorrect. Recent studies of meteorites confirm the presence of live ⁶⁰Fe in the early solar system (2). No known mechanism could have formed this short-lived (half-life = 1.5 million years) radionuclide locally within the young solar system. However, ⁶⁰Fe is produced in supernova explosions, along with ²⁶Al, ⁴¹Ca, and other radioisotopes (3). Material from nearby supernovae must have rapidly mixed with the material from which the meteorites formed. The implications of this are clear. The Sun did not form in a region like Taurus-Auriga. Rather, like most low-mass stars (4), the Sun formed in a high-mass star-forming region where one or more stars went supernova. Understanding our origins means understanding the process of low-mass star formation in environments that are shaped by the presence of massive stars. The intense ultraviolet (UV) radiation from massive stars carves out ionized cavities and blisters in the dense molecular clouds within which the stars formed. Examples of these regions of ionized gas, called HII regions, include such well-known objects as the Orion Nebula and the Eagle Nebula. There is growing evidence that most low-mass star formation in such environments is triggered by shocks driven in advance of the HII region ionization front as it expands into its dense surroundings (5). Stars seen in the ionized volumes of HII regions were formed in this way, and then subsequently were uncovered by the advance of the ionization front itself. Low-mass stars that form around an HII region should pass through a well-defined sequence: (i) A shock driven in advance of an ionization front compresses molecular gas around the periphery of an HII region, compressing dense cores and causing them to become unstable to gravitational collapse (6). (ii) These cores are overrun by the advancing ionization front within ∼10⁵ years. As cores emerge into the HII region interior, they go through a short-lived (∼10⁴ year) phase during which the dense core itself photoevaporates. This is the "evaporating gaseous globule" or EGG phase best seen in Hubble Space Telescope (HST) images of the Eagle Nebula (7). (iii) EGGs that do not contain stars are dispersed, but when a star-bearing EGG evaporates, the circumstellar disk inside is exposed directly to UV radiation from the massive stars. The object transitions into an "evaporating disk" phase, best seen in HST images of "proplyds" in the Orion Nebula (8). (iv) The evaporating disk phase is also short-lived (9). Within a few tens of thousands of years, photoevaporation erodes the gaseous disk to within a few tens of astronomical units of the central young stellar object (YSO) (10). (v) The young star and its truncated disk then reside within the ionized, low-density interior of the HII region for the remainder of the few-million-year lifetime of the region. This is the environment in which planetary systems such as our own form. (vi) When the massive stars exciting the region go through a high mass-loss "Wolf-Rayet" phase and/or go supernova, the protoplanetary disks surrounding nearby low-mass YSOs are pelted with ejecta. Such events are responsible for the short-lived radionuclides found in meteorites in our own solar system.

Additional details

Created:
August 22, 2023
Modified:
October 25, 2023