1116What kind of environment gavebirth to the Sun and planets?Most astronomers who study starformation would probably say that the solarsystem originated in a region much like thewell-studied Taurus-Auriga molecularcloud (1)—a region in which low-mass,Sun-like stars form in relative isolation—but this conventional wisdom is almost cer-tainly incorrect. Recent studies of mete-orites confirm the presence of live 60Fe inthe early solar system (2). No known mech-anism could have formed this short-lived(half-life = 1.5 million years) radionuclidelocally within the young solar system.However, 60Fe is produced in supernova ex-plosions, along with 26Al, 41Ca, and otherradioisotopes (3). Material from nearby su-pernovae must have rapidly mixed with thematerial from which the meteorites formed.The implications of this are clear. The Sundid not form in a region like Taurus-Auriga.Rather, like most low-mass stars (4), theSun formed in a high-mass star–formingregion where one or more stars went super-nova. Understanding our origins means un-derstanding the process of low-mass starformation in environments that are shapedby the presence of massive stars.The intense ultraviolet (UV) radiationfrom massive stars carves out ionized cav-ities and blisters in the dense molecularclouds 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 theEagle Nebula. There is growing evidencethat most low-mass star formation in suchenvironments is triggered by shocks drivenin advance of the HII region ionizationfront as it expands into its dense surround-ings (5). Stars seen in the ionized volumesof HII regions were formed in this way, andthen subsequently were uncovered by theadvance of the ionization front itself.Low-mass stars that form around an HIIregion should pass through a well-definedsequence: (i) A shock driven in advance ofan ionization front compresses moleculargas around the periphery of an HII region,compressing dense cores and causing themto become unstable to gravitational col-lapse (6). (ii) These cores are overrun bythe advancing ionization front within ~105years. As cores emerge into the HII regioninterior, they go through a short-lived(~104year) phase during which the densecore itself photoevaporates. This is the“evaporating gaseous globule” or EGGphase best seen in Hubble Space Telescope(HST) images of the Eagle Nebula (7). (iii)EGGs that do not contain stars are dis-persed, but when a star-bearing EGG evap-orates, the circumstellar disk inside is ex-posed directly to UV radiation from themassive stars. The object transitions into an“evaporating disk” phase, best seen in HSTimages of “proplyds” in the Orion Nebula(8). (iv) The evaporating disk phase is alsoshort-lived (9). Within a few tens of thou-sands of years, photoevaporation erodesthe gaseous disk to within a few tens of as-tronomical units of the central young stel-lar object (YSO) (10). (v) The young starand its truncated disk then reside withinthe ionized, low-density interior of the HIIregion for the remainder of the few-million-year lifetime of the region. This isthe environment in which planetary systemssuch as our own form. (vi) When the mas-sive stars exciting the region go through ahigh mass-loss “Wolf-Rayet” phase and/orgo supernova, the protoplanetary disks surrounding nearby low-mass YSOs arepelted with ejecta. Such events are responsi-ble for the short-lived radionuclides foundin meteorites in our own solar system.ASTRONOMYThe Cradle of the Solar SystemJ. Jeff Hester, Steven J. Desch, Kevin R. Healy, Laurie A. LeshinPERSPECTIVESJ. J. Hester, S. J. Desch, and K. R. Healy are in theDepartment of Physics and Astronomy, ArizonaState University, Tempe, AZ 85287–1504, USA. L. A.Leshin is in the Department of Geological Sciencesand Center for Meteorite Studies, Arizona StateUniversity, Tempe, AZ 85387–1404, USA. E-mail:[email protected] molecular columnClusters of YSOs left behindby the advancing ionization front0.5-Jy water maserAn EGG evolving into a “proplyd”Ionization front inphotoevaporativeflow from EGGand diskJetContinuumfrom protostarand disk“Shadow” fingerconnecting EGGto wall of HII regionPlanetary system nursery. Hubble Space Telescope wide-field camera observation of a field in thesouthern portion of the Trifid Nebula illustrating several of the observational consequences of thestar-formation scenario discussed. The inset (an enlargement of the region indicated by the smallyellow box) shows a YSO-bearing EGG seen as it is evolving into a “proplyd.” Evidence for triggeredstar formation in the region includes the HH399 jet, which arises from an embedded source im-mediately interior to the ionization front, and the presence of a 0.5-Jy water maser. Clustering ofYSOs, especially around the remains of a largely evaporated column in the upper left of the field,is evidence of pockets of triggered star formation that have been overrun by the ionization front.CREDIT: JEFF HESTER/ARIZONA STATE UNIVERSITY; NASA21 MAY 2004 VOL 304 SCIENCE www.sciencemag.org1117This scenario for star formation makesmany testable predictions that are support-ed by observations already in the literature.For example, fingerprints of the star-formation process discussed here are clearin the HST image of a region in the TrifidNebula (11) shown in the figure. In this re-gion, intense UV radiation from a massivestar (located well above the field of view)is incident on the surface of dense molecu-lar gas that fills most of the field of view.Sharply defined orange and yellow fea-tures mark the current location of the ion-ization front. The HH399 jet originatesfrom an unseen protostar located a shortdistance from the ionization front. A watermaser is also seen in projection a short dis-tance behind an ionization front. Jet andmaser activity are both evidence of contin-uing accretion onto these two very youngprotostars. In 10,000 years or so, both ofthese objects will be cut off from their ac-cretion reservoirs when they are overrun bythe advancing ionization front. When thishappens, these objects will be seen asEGGs, much like the prominent EGGshown in the inset. The EGG seen in the figure is itself aremarkable demonstration of the evolution-ary tie between EGGs and proplyds. Fromthe bottom down, this feature is a classicalEGG, of the sort seen in the Eagle Nebula.But at the tip of this EGG we see a star, asmall
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