An Observational Perspective of Low Mass Dense Cores II:Evolution towards the Initial Mass FunctionDerek Ward-ThompsonCardiff UniversityPhilippe Andr´eService d’Astrophysique de SaclayRichard CrutcherUniversity of IllinoisDoug JohnstoneNational Research Council of CanadaToshikazu OnishiNagoya UniversityChristine WilsonMcMaster UniversityWe review the properties of low mass dense molecular cloud cores, including starless,prestellar, and Class 0 protostellar cores, as derived from observations. In particular we discussthem in the context of the current debate surrounding the formation and evolution of cores.There exist several families of model scenarios to explain this evolution (with many variationsof each) that can be thought of as a continuum of models lying between two extreme paradigmsfor the star and core formation process. At one extreme there is the dynamic, turbulent picture,while at the other extreme there is a slow, quasi-static vision of core evolution. In the latterview the magnetic field plays a dominant role, and it may also play some role in the formerpicture. Polarization and Zeeman measurements indicate that some, if not all, cores contain asignificant magnetic field. Wide-field surveys constrain the timescales of the core formationand evolution processes, as well as the statistical distribution of core masses. The formerindicates that prestellar cores typically live for 2–5 free-fall times, while the latter seems todetermine the stellar initial mass function. In addition, multiple surveys allow one to comparecore properties in different regions. From this it appears that aspects of different models maybe relevant to different star-forming regions, depending on the environment. Prestellar coresin cluster-forming regions are smaller in radius and have higher column densities, by up toan order of magnitude, than isolated prestellar cores. This is probably due to the fact thatin cluster-forming regions the prestellar cores are formed by fragmentation of larger, moreturbulent cluster-forming cores, which in turn form as a result of strong external compression.It is then the fragmentation of the cluster-forming core (or cores) that forms a stellar cluster.In more isolated, more quiescent, star-forming regions the lower ambient pressure can onlysupport lower density cores, which go on to form only a single star or a binary/multiple starsystem. Hence the evolution of cluster-forming cores appears to differ from the evolution ofmore isolated cores. Furthermore, for the isolated prestellar cores studied in detail, the magneticfield and turbulence appear to be playing a roughly equal role.1. INTRODUCTIONA great deal is now known about dense cores in molec-ular clouds that are the progenitors of protostars – see theprevious chapter by Di Francesco et al., which details manyof the observational constraints that have been placed upontheir physical parameters (this chapter and the precedingchapter should be read in conjunction). What is less clear isthe manner in which the cores are formed and subsequentlyevolve. In this chapter we discuss what the observations cantell us about the formation and evolution of cores. Clearlythe evolution depends heavily upon the formation mech-128:00 30 16:27:00 30 26:00152025−24:303540Right ascensionDeclinationFig. 1.— SCUBA image of the ρ Oph molecular cloud region seen in dust continuum at 850 µm (adapted from Johnstoneet al., 2000). Prestellar, protostellar and cluster-forming cores can all be seen in this molecular cloud region. For example,the cluster-forming core ρ Oph A (extended region in the upper right of this image) contains within it (inside the whitecontour) the prestellar core SM1 and Class 0 protostellar core VLA1623 (cf. Andr´e et al., 1993). Note also that large areasof the cloud contain no dense cores, leading to the idea of a threshold criterion discussed in Section 6.2anism, and upon the dominant physics of that formation.Several model scenarios have been proposed for this mech-anism.These models can be thought of as a small number offamilies of models, each of which contains many variations,representing a continuum lying between two extremes. Atone extreme there is a school of thought that proposes aslow, quasi-static evolution, in which a core gradually be-comes more centrally condensed. This evolution may bemoderated by the magnetic field (e.g., Mouschovias andCiolek, 1999) or else by the gradual dissipation of low-levelturbulent velocity fields (e.g., Myers, 1998, 2000). At theother extreme is a very dynamic picture (e.g., Ballesteros-Paredes et al., 2003), in which highly turbulent gas cre-ates large density inhomogeneities, some of which becomegravitationally unstable and collapse to form stars (for a re-view, see: Ward-Thompson, 2002). Once again the mag-netic field may play a role in this latter picture, in whichmagneto-hydrodynamic (MHD) waves may be responsiblefor carrying away excess turbulent energy (e.g., Ostriker etal., 1999).What we find from the observations is that some aspectsof each of these different model scenarios may be relevantin differentregions of star formation, dependingon the localenvironment. No two regions are the same, and the effectsof local density, pressure and magnetic field strength, andthe presence or absence of other nearby stars and protostarsall play an important role in determining what dominatesthe formation and evolution of dense molecular cloud cores.Throughoutthis chapter we define a dense core as any re-gion in a molecular cloud that is observed to be significantlyover-dense relative to its surroundings. We define a starlesscore as any dense core that does not contain any evidencethat it harbours a protostar, young stellar object or youngstar (Beichman et al., 1986). Such evidence would includean embedded infra-red source, centimetre radio source orbipolar outflow, for example (cf. Andr´e et al., 1993, 2000).Any core that does contain such evidence we define asa protostellar core. This might be a Class 0 protostellarcore (Andr´e et al., 1993, 2000) or a Class I protostellar core(Lada, 1987; Wilking et al., 1989) depending upon its evo-lutionary status.We here define prestellar cores (formerly pre-protostellarcores – Ward-Thompson et al., 1994) as that subset of star-less cores which are gravitationally bound and hence areexpected to participate in the star formation process. Wefurther define cluster-forming cores as those cores that havesignificant observed
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