An Observational Perspective of Low-Mass Dense Cores I:Internal Physical and Chemical PropertiesJames Di FrancescoNational Research Council of CanadaNeal J. Evans IIThe University of Texas at AustinPaola CaselliArcetri ObservatoryPhilip C. MyersHarvard-Smithsonian Center for AstrophysicsYancy ShirleyUniversity of ArizonaYuri AikawaKobe UniversityMario TafallaNational Astronomical Observatory of SpainLow-mass dense cores represent the state of molecular gas associated with the earliestphases of low-mass star formation. Such cores are called “protostellar” or “starless,” dependingon whether they do or do not contain compact sources of luminosity. In this chapter, the firsthalf of the review of low-mass dense cores, we describe the numerous inferences made about thenature of starless cores as a result of recent observations, since these reveal the initial conditionsof star formation. We focus on the identification of isolated starless cores and their internalphysical and chemical properties, including morphologies, densities, temperatures, kinematics,and molecular abundances. These objects display a wide range of properties since they areeach at different points on evolutionary paths from ambient molecular cloud material to cold,contracting, and centrally concentrated configurations with significant molecular depletionsand, in rare cases, enhancements.1. INTRODUCTION AND SCOPE OF REVIEWOver the last decade, dedicated observations have re-vealed much about the earliest phases of low-mass star for-mation, primarily through studies of low-mass dense coreswith and without internal protostellar sources, e.g., proto-stellar and starless cores. Such cores are dense zones ofmolecular gas of relatively high density, and represent thephysical and chemical conditions of interstellar gas just af-ter or prior to localized gravitational collapse. These ob-jects are essentially the seeds from which young stars mayspring, and define the fundamental starting point of stellarevolution. Since PPIV (see Langer et al. 2000; Andr´e etal. 2000), enormous strides have been made in the observa-tional characterization of such cores, thanks to the increasedinterest stemming from the promising early results reportedat PPIV and to new instrumentation.This review summarizes the advances made since PPIV,and, given the large number, it has been divided into twoparts. In Part I (i.e., this chapter), we summarize major re-cent observational studies of the initial conditions of starformation, i.e., the individual physical and chemical prop-erties of starless cores, with emphasis on cores not foundin crowded regions. This subject includes their identifi-cation, morphologies, densities, temperatures, molecularabundances, and kinematics. In recent years, the internaldensity and temperature structures of starless cores havebeen resolved, regions of chemical depletion (or enhance-ment) have been studied, and inward motions have beenmeasured. Part II of this review (see next chapter by Ward-Thompson et al.) summarizes what observed individual andgroup characteristics, including magnetic field properties,1mass distributions and apparent lifetimes, have taught usabout the evolution of low-mass dense cores through pro-tostellar formation. Despite significant progress, the obser-vational picture of the earliest stages of star formation re-mains incomplete, and new observational and experimentaldata and theoretical work will be necessary to make furtheradvances.2. STARLESS CORES2.1 DefinitionStars form within molecular gas behind large amounts ofextinction from dust. This extinction has been used to locatemolecular clouds (Lynds, 1962), isolated smaller clouds(Clemens and Barvainis, 1988), and particularly opaque re-gions within larger clouds (Myers et al., 1983; Lee and My-ers 1999). Locations within these clouds of relatively highdensity or column density, though identifiable from opti-cal or infrared absorption (e.g., see chapter by Alves et al.),can also be detected using submillimeter, millimeter, or ra-dio emission. For instance, spectroscopic studies of cloudsfound via extinction studies further selected those with ev-idence for dense gas (e.g., see Myers et al., 1983; My-ers and Benson 1983). These became known collectivelyas “dense cores,” based primarily on whether or not theyshowed emission from NH3, indicative of densities aboveabout 104cm−3(Benson and Myers, 1989; see also Jijina etal., 1999). Using IRAS data, Beichman et al. (1986) foundthat roughly half the known dense cores in clouds containedIRAS sources, while Yun and Clemens (1990) found thatabout 23% of the isolated smaller clouds contained infraredsources within; the remainder became known as “starlesscores.” Follow-up studies of clouds with sensitive submil-limeter continuum detection systems allowed further dis-crimination within the class of starless cores. Those withdetected emission, indicating relatively high densities of105−6cm−3(Ward-Thompson et al., 1994), were called“pre-protostellar” cores or later “prestellar cores.” Molec-ular line studies showed that prestellar cores were indeedmore likely to show evidence for inflowing material thanwere the merely starless cores (e.g., see Gregersen andEvans, 2000).In this review, we describe the recent observational char-acterization of “starless cores,” low-mass dense cores with-out compact luminous sources of any mass. (We call suchsources “young stellar objects,” regardless of whether or notthey are stellar or substellar in mass.) By “low-mass,” wemean cores with masses M < 10 M, such as those foundin the nearby Taurus or Ophiuchus clouds. Starless coresare important because they represent best the physical con-ditions of dense gas prior to star formation. Conceptually,we distinguish starless cores that are gravitationally boundas “prestellar cores” though it is very difficult to determineobservationally at present whether a given core is boundor not (see below). In addition, we consider such coresas either “isolated” or “clustered” depending on the localsurface density of other nearby cores and young stellar ob-jects. Observations of isolated cores (e.g., Clemens andBarvainis, 1988) are easier to interpret since they generallyinhabit simpler, less confused environments, i.e., withoutnearby cores or young stellar objects. Observations of clus-tered cores (e.g., Motte et al., 1998) are important also be-cause they are more representative of star
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