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Publications of the Astronomical Society of the Pacific 101:229-243, March 1989

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Publications of the Astronomical Society of the Pacific 101:229-243, March 1989 Publications of the Astronomical Society of the Pacific 101:229-243, March 1989 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. 101 March 1989 No. 637 THE FORMATION OF LOW-MASS STARS* BRUCE A. WILKING Department of Physics, University of Missouri, St. Louis, Missouri 63121 Received 1988 December 24 ABSTRACT The global and individual aspects of low-mass (SK < 3 SKq) star formation which have been revealed by visible to millimeter wavelength observations will be reviewed. Optical studies have been able to infer many of these global properties which include the fact that most low-mass stars originate in clouds which produce gravitationally unbound Τ associations. However, direct study of the formation and evolution of low-mass stars necessitates infrared and millimeter-wave techniques which can probe the optically opaque dust in the cloud and circumstellar environment. These techniques have revealed large collections of dust-embedded young stellar objects associated with the densest regions of molecular clouds. More recently, the IRAS survey has enabled several comprehensive infrared studies of these low-mass populations in nearby clouds; the results of studies in the Taurus-Auriga and ρ Ophiuchi molecular cloud complexes will be discussed. The individual properties of young stellar objects, such as their bolometric luminosities and evolution- ary states, can be inferred by modeling their 1-100 μιη spectral energy distributions, A proposed evolutionary sequence for the various classes of spectral energy distributions observed for low-mass stars will be described. Direct study of the distribution of circumstellar gas and dust demands high-resolution techniques. Several of these techniques and their contributions to our understand- ing of low-mass star formation will be discussed with particular attention to recent results from millimeter-wave interferometry. Key words: star: evolution-interstellar matter-infrared observations-millimeter-wave astronomy 1. Introduction cussion begins with the results of optical studies of young The observational study of the formation of stars with stars followed by a review of near-infrared and millime- masses comparable to that of the Sun is not a recent ter-wave observations of star-forming molecular clouds. undertaking but began in the 1940s with the recognition Recent investigations of far-infrared emission toward by Joy of the Τ Tauri class of stars. By studying collections nearby regions of star formation will be discussed. Fi- of young visible stars, astronomers have been able to nally, high-resolution infrared and millimeter-wave tech- make inferences about their formation history. In the last niques which have detected circumstellar structures to- 15 years, however, technological advances have given ward young stellar objects will be briefly reviewed. The astronomers the opportunity to study more directly the sequence of topics is almost chronological in nature and early stages in the formation of low-mass stars as they lie moves us toward progressively earlier stages in the forma- embedded in molecular clouds. These advances include tion of low-mass stars. the opening of new far-infrared and millimeter-wave win- 2. Optical Studies of Low-Mass Stars dows of the electromagnetic spectrum for astronomy. Gravitationally bound star clusters and unbound asso- In this paper I will review both the global and individ- ciations of Τ Tauri stars constitute the "fossil record" of the ual properties of low-mass star formation as revealed by formation history of low-mass stars. Early Ha surveys visible to millimeter wavelength observations. The dis- recognized aggregates of emission-line variable stars asso- *One in a series of invited review papers currently appearing in these ciated with dark nebulae in Taurus, Ophiuchus, and Publications. Orion (e.g., Joy 1946; Struve and Rudkj0bing 1949; Haro 229 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 230 BRUCE A. WILKING 1949; Herbig 1950); these low stellar density aggregates formation of low-mass stars is consistent with the age provided a low-mass analog to OB associations (Kholopov spread of Κ stars in the Pleiades derived from rotational 1959a,fo). The intimacy of these collections of Τ Tauri data (Stauffer et al. 1984). Additional evidence for the stars, or Τ associations, with the dark cloud material and, continuous formation of low-mass stars comes from the in some cases, O and Β stars and reflection nebulosity, fact that age estimates for Τ Tauri stars in a given cloud hinted at their youth and prompted the idea they were range from 106-107 years (e.g., Cohen and Kuhi 1979). pre-main-sequence objects (Ambartsumian 1947). This It is not clear to date whether the formation of a disrup- was confirmed by the fact that when placed in an H-R tive, massive star is a random occurrence within a cloud diagram, Τ Tauri stars fell well above the zero-age main or if there is a correlation between the mass and age of the sequence (see Herbig 1962α for review). While OB or R cluster members. In the NGC 2264 cloud, it appears that associations appear always to be associated with Τ associa- stars with > 0.1formed sequentially in mass over a tions, the Τ associations can form in isolation from massive period of 107 years while producing a stellar mass spec- stars. This has led to the suggestion that low-mass star trum consistent with the IMF (Iben and Talbot 1966; formation often precedes massive star formation (Gras- Adams, Strom, and Strom 1983). Stahler (1985) suggests dalen et al. 1975; Lada 1987). that this effect could be mimicked for the more-massive A second inference from the "fossil record" is that stars if PMS ages were erroneously assigned to main-se- low-mass stars form with much greater frequency than quence stars. A correlation of stellar mass and age is a those of high mass. This was examined in detail by possible explanation for the luminosity function of the ρ Salpeter (1955) who determined the Initial Mass Func- Ophiuchi infrared cluster (Wilking, Lada, and Young tion (IMF) for field stars in the solar neighborhood. Sub- 1989, see Section 4). sequent studies of the mass functions in open clusters and associations have shown remarkably little variation from 3. Near-Infrared Observations of Low-Mass the field star IMF for 33? > 3 Sí© and imply that, in this Stars in Molecular Clouds mass regime, the IMF can be approximated by a power With the advent of infrared detectors, several groups law N(m) oc m25 (Miller and Scalo 1979; Scalo 1986). The pioneered searches for embedded infrared sources in similarities of the IMFs in these diverse regions can dark clouds associated with emission-line stars (e.g., perhaps be traced to the fragmentation process within Grasdalen, Strom, and Strom 1973; Gatley et al. 1974; molecular clouds (Elmegreen and Mathieu 1983; Zin- Strom, Strom, and Vrba 1976). They found there was a necker 1984). large population of low-luminosity objects associated with Despite the dominance of low-mass stars in the field these clouds which were rendered invisible by the obscu- population, their place of origin can only be inferred ration from dust both in the cloud and in the circumstellar indirectly. Only about 10% of these stars can have their environment. At about the same time, millimeter-wave origins in bound open clusters owing to the stability of telescopes began extensive mapping of the molecular these clusters against disruption (Roberts 1957; Miller component long supposed to pervade the dark clouds and Scalo 1978). Therefore, the majority of low-mass stars (e.g., Penzias etal. 1972; Tucker, Kutner, and Thaddens in the field must result from the dispersal of gravitation- 1973; Loren 1975; Encrenaz, Falgarone, and Lucas 1975). ally unbound associations. Due to their low stellar densi- The narrow molecular linewidths observed in the dark ties, these associations are unstable to disruption by clouds (1-3 km s-1) were comparable to the velocity dis- galactic tides in < 107 years (e,g., Bok 1934). Miller and persions of stars in clusters and associations (Jones and Scalo (1978) estimate that known OB, R, and Τ associa- Herbig 1979; Hartmann et al. 1986). A clearer picture was tions can probably account for the formation of all stars emerging, namely that stars are born gravitationally with S)î > 2-5 Φι© ηο^ produced in open clusters. In fact, bound in the denser regions of dark clouds whose binding they estimate that associations may be the birthplace of all mass was predominantly molecular gas. stars not produced in clusters if the association and field star IMFs are similar below 2 3.1 The Physical Conditions of Low-Mass Star Formation From his analysis of the H-R diagram of the Pleiades CO mapping of molecular clouds has revealed dis- cluster, Herbig (1962¾) proposed that low-mass stars tinctly different physical conditions for regions forming form continuously within clouds until a massive star is massive stars and those forming exclusively low-mass produced which disperses the cloud and abruptly halts objects. As summarized in Table 1, OB stars have been the formation process. Herbig and others have shown that found to form in association with giant molecular clouds the main sequence of the Pleiades extends to lower (GMCs) characterized by large masses (SR = 105-6 SJΩ), masses than expected from the nuclear age of the cluster. high gas temperatures (10 K-50 K), and large velocity Apparently, low-mass stars began to form about 3 X 107 dispersions (~ 10 km s-1). In contrast, the dark cloud years before the appearance of massive stars in the complexes are the domain of low-mass star formation and Pleiades (Stauffer 1984). This extended period for the are typified by lower masses, temperatures, and velocity © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System THE FORMATION OF LOW-MASS STARS 231 TABLE 1 striking differences in the distributions of their molecular Properties of Star-Forming Molecular Clouds gas and in the densities of their embedded stellar popula- tions.
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