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 evolutionary 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 understanding 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 SJÎ©), 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

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 populations. It has been suggested that shocks or magnetic fields A. Dark Cloud Complexes Giant Molecular Clouds may play an important role in the efficiency with which low-mass stars are formed (Vrba 1977; Shu, Adams, and 3 4 5 6 Mass ΙΟ " M0 ΙΟ " Mq Lizano 1987). Β y way of example, we consider the Tau- Size 10-20 pc 20-80 pc Temperature 10-25 Κ 10-50 Κ rus-Auriga and ρ Oph complexes. Δν 1-3 km s-1 10 km s"1 Stellar Pop. low mass, Τ Tauri OB, Τ Tauri Both the Taurus-Auriga and Ophiuchus dark clouds are Examples Taurus-Auriga, ρ Oph Orion, W3, Ml7 nearby (~ 160 pc) filamentary complexes which extend for tens of parsecs. Both clouds are comprised of low-density molecular gas interspersed with dense cores and B. Molecular Cores associated concentrations of young stellar objects (YSOs). Figure 1 presents a schematic of a 170 square degree area Taurus-Auriga ρ Oph GMC 3 of the Taurus-Auriga complex from Myers (1987) indicat- Mass 0.3-10 M0 4-44 Mq 10-10 M@ Size 0.05-0.24 5 pc3 0.05-0.254 5 5 pc3 0.1-34 6 pc 3 ing the distribution of low-density molecular gas (as Density ΙΟ " cm" ΙΟ " · cm" ΙΟ " cm" 12 Temperature 9-12 Κ 10-20 Κ 30-100 Κ traced by CO emission lines), dense cores (from NH3 Δν 0.2-0.4 km s-1 0.4-1.3 km s-1 1-3 km s-1 Tracer NHj (1) DCO+ (2) NH3 (3) observations), highly obscured YSOs (as revealed by the IRAS survey), and Τ Tauri stars (from optical studies of (1) NHjaK) = (1,1) and (2,2) from Myers (1985b). emission-line stars). This region of the complex displays (2) DCO+ J=l-0, 2-1, and 3-2 from Loren, Wootten, and Wilking (1989). (3) NH3(J,K) = (1,1) and (2,2) from Ho, Martin, and Barrett (1981). several distinct centers of star formation including TMC1, TMC2, L 1551, and L 1495. A duration of star formation dispersions in the molecular gas. A more detailed of ~ 107 years in the Taurus-Auriga clouds is inferred by overview of molecular cloud properties can be found in placing Τ Tauri stars in the H-R diagram and adopting a Goldsmith (1987). set of evolutionary tracks (Cohen and Kuhi 1979). Esti- More recently, observations of density-sensitive mates for the efficiency of star formation over the complex molecular transitions such as NH3, H2CO, and DCO+ are about 2% (Jones and Herbig 1979) but range from have revealed that molecular clouds are interspersed with 2%-9% in the vicinity of dense cores (Cohen and Kuhi dense cores and that the youngest stellar objects are in 1979). The low star-formation efficiency suggests that, in close proximity to these cores (Ho, Martin, and Barrett the absence of the binding mass of molecular gas, the 1981; Loren, Sandqvist, and Wootten 1983; Myers and population of YSOs in Taurus-Auriga will be gravitation- Benson 1983; Loren, Wootten, and Wilking 1989). As in ally unbound and the Τ association will diffuse into the the case of GMCs and dark cloud complexes, there is a general field population. dichotomy between the physical conditions of molecular In contrast to the Taurus-Auriga clouds is the ρ Oph cores associated with high-mass star formation and those dark cloud complex. Figures 2a and 2b show the distribution of molecular gas over a 25 square degree region of the associated with low-mass star formation. As summarized 13 in Table 1, low-mass stars are associated with low-mass Ophiuchus complex as delineated by CO emission lines (0.3-10 3Wq) cores such as those found in the Taurus-Au- (Loren 1989). Embedded throughout the low-density molecular gas are cold, dense cores as revealed by H2CO riga complex which are colder and have lower velocity + dispersions than their more massive counterparts found and DCO emission (Loren et al. 1983; Loren et al. 1989). in GMCs. The low-mass cores appear to have all but The Ophiuchus complex, unlike the Taurus-Auriga region, has a large centrally condensed core defining the dissipated their nonthermal support; it is estimated that 5 western edge of the complex. This high column density they will collapse to form stars within 10 years (Myers core is comprised of a 1 pc X 2 pc ridge of molecular gas 1985a). This is in contrast to high-mass cores which dis- containing about 500 SR© (Wilking and Lada 1983). As play supersonic linewidths. It is interesting to note that shown in Figure 2c, this ridge contains several distinct while the cold cores observed in the Ophiuchus complex high-density cores. Near-infrared and IRAS observations resemble those in Taurus-Auriga, there are a number of the cloud have revealed a high concentration of YSOs which are intermediate in mass and temperature to the associated with these cores (see Fig. 2c; Vrba et al. 1975; Taurus-Auriga and GMC cores. Like the cores in Taurus- Elias 1978; Wilking and Lada 1983). A duration of star Auriga, it has been argued that the Ophiuchus cores are formation of only 1.5-3.5 million years is derived for the on the verge of collapse but that they may form stars of Ophiuchus core based on the relative numbers of visible larger mass (Loren et al. 1983). to heavily obscured YSOs in the cloud (see Section 4.1). 3.2 The Diversity Among Low-Mass Star-Forming Regions Estimates of the star-formation efficiency in the high Despite the basic similarities of the physical condition- column density region are significantly higher than those samong low-mass star-forming regions, there are often in Taurus-Auriga (and in dark clouds in general); a lower

RIGHT ASCENSION (1950) Fig. 1-A sketch of the molecular and stellar components in the Taurus-Auriga dark cloud from Myers (1987). The contours trace the integrated intensity of the low-density molecular gas in Κ km s-1 (Ungerechts and Thaddens 1987) and the large filled circles denote the locations of dense molecular cores (Myers and Benson 1983). Τ Tauri stars are marked by the small filled circles (Cohen and Kuhi 1979; Jones and Herbig 1979) and highly obscured YSOs by crosses (Myers et al. 1987). limit of 22% is obtained through consideration of both 3.3 Ε nergetic M ass Loss from YSOs near-infrared and IRAS observations (see Appendix B, A more recent contribution of millimeter-wave astron- Wilking et al. 1989). Evidence for a molecular compres- omy has been the discovery of large-scale molecular out- sion front in the core region has led Loren and Wootten flows driven by strong stellar winds from YSOs. The (1986) to propose that shocks are responsible for the classic example is the bipolar outflow found associated enhanced star-formation efficiency. The relatively high with the low-mass YSO L 1551-IRS 5 (Snell, Loren, and efficiency, coupled with the youth of the infrared cluster, Plambeck 1980). Subsequent molecular-line surveys suggest that in contrast with the Taurus-Auriga region, have revealed that outflows are common among YSOs, stars in the Ophiuchus core have formed in an efficient characterized by high-velocity molecular gas (5-100 km burst of activity. In the absence of massive stars which s-1) and dynamical time scales of 103-105 years (Bally and could suddenly strip away the molecular gas, the cloud Lada 1983; Lada 1985; Levreault 1988). It now appears core will continue to convert its mass into stars and ulti- that most, if not all, stars undergo a phase of energetic mately produce a gravitationally bound open cluster (Wil- mass loss during their pre-main-sequence evolution. One king and Lada 1983; Lada, Margulis, and Dearborn 1984). interesting feature of molecular outflows is their tendency

α (1950) Fig. 2-The distribution of molecular gas and YSOs in the Ophiuchus complex. Figures 2a and 2b are maps of 13CO (l-*0) emission from Loren (1989). The contours are in units of Γκ*; the lowest (dashed) contours represent values of 2 Κ and 3 Κ followed by solid contours of 4, 5, 6, 7, 8, 10 (bold), 12, 14, 16, 18, and 20 Κ (bold). Also shown for reference are the locations of several early Β stars in the region. to be bipolar, i.e., the red and blueshifted gas often distributed about the YS O. Other manifestations of this occurs in two distinct lobes of emission symmetrically mass-loss activity include Herbig-Haro objects and highly

10"

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8(1950)

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Fig. 2 (Continued)-Figure 2c shows the distribution of dense molecular gas (solid contours) in the Ophiuchus core as mapped by DCO+ (2->l) emission (from Loren et al. 1989). The dense cores are labeled A-F. In addition, the locations of YSOs, radio continuum sources, and IRAS sources with no known near-infrared counterparts are shown. The dashed contour outlines the boundary of 13CO emission.

collimated optical and radio jets (see Schwartz (1983) and infrared. Examples include low-mass YSOs such as HH Mündt (1985) for reviews). At the base of some molecular exciting stars (Harvey, Wilking, and Cohen 1982; Cohen outflows, highly focused jets are observed which originate et al. 19S4a,b), and B335 (Keene et al. 1983). Advancing within several hundred AU of the YSO. This implies that the study of low-mass YSOs were the high-sensitivity the stellar wind is collimated initially by an anisotropic survey and Pointed Observations performed by the In- distribution of circumstellar dust and gas. The observa- frared Astronomical Satellite in 1983 (IRAS 1985). The tional evidence for such circumstellar disks will be re- IRAS survey gave unprecedented spatial coverage and viewed in Section 5. sensitivity in low-mass star-forming clouds, with the ability to resolve the dust emission associated with individual 4. Far-Infrared Observations of YSOs in the 12 μηι and 25 μιη bands. Pointed Observa- Low-Luminosity Stars tions utilizing the edge detectors in the IRAS focal plane An important breakthrough in investigating the earliest gave an angular resolution of about 0.75 arc min X 1.2 arc stages in the formation of low-mass stars was the opening min at 12 μιη and made far-infrared studies of normally of the far-infrared window of the electromagnetic spec- confused regions of YSOs possible with IRAS (e.g.. trum. The youngest low-mass objects are enshrouded by Young, Lada, and Wilking 1986). dust which can completely absorb visible to near-infrared The combination of optical/near-infrared data with radiation from the central object and reradiate it in the IRAS observations has proved a powerful tool with which far-infrared. Observations with the Kuiper Airborne Ob- to study regions of low-mass star formation (e.g., Myers servatory have demonstrated that there are large num- et al. 1987; Wilking, Lada, and Young 1989). This combi- bers of cold infrared sources in molecular clouds with nation provides the most complete census of embedded steeply rising spectral energy distributions; these objects YSOs ranging from heavily obscured protostellar objects radiate 50% or more of their luminosity in the far- to Τ Tauri stars. The energy distributions of individual

YSOs over the 1-100 μιη spectral region can be con- hence, its evolutionary state. Theoretical models have structed and compared with theoretical models to infer been developed to describe the emergent flux from YSOs their evolutionary states (Adams, Lada, and Shu 1987; for varying distributions of circumstellar dust and gas Myers et al. 1987). (Adams and Shu 1986; Adams et al. 1987; Myers et al. 1987). These models have led to the suggestion that the 4.1 Spectral Energy Distributions different SED shapes form a quasi-continuous evolution- The spectral energy distribution (SED) of a YS O can be ary sequence for low-mass stars (Adams et al. 1987; Lada characterized by its slope between 2.2 μιη and 25 μιη: 1987). In this scenario, shown schematically in Figure 4, a = d log(XFj/d log λ , Class I SEDs are modeled as rotating protostars which derive most of their energy from accretion. An infalling, where α = 0 for a SED which is flat when plotted in units spherical dust envelope emits the far-infrared radiation of XFx. Following Lada (1987) we can empirically classify while the warmer mid-infrared emission arises from a each SED by its spectral index. A Class I SED has α > 0 dusty disk. The disk resides within a cavity several hun- and is much broader than that of a single-temperature dreds of AU in diameter. A gas photosphere, defined by blackbody. They are associated with the coldest and most the accretion shock, produces the near-infrared portion of heavily obscured YSOs which display little or no emission the SED which is heavily reddened by the dust envelope. from a stellar photosphere. Class I objects are found in the At some point during the accretion phase, a strong stellar closest proximity to dense molecular cores and are often wind is generated by the protostar which begins to re- associated with high-velocity molecular gas (Myers et al. verse the infall and dissipate the spherical envelope. The 1987). Simple considerations concerning the dust density wind, which may begin early in the accretion phase (e.g., distributions and visual extinctions toward Class I objects Shu et al. 1988), eventually dissipates enough of the suggest the stellar-like core must lie within a dust-free envelope to reduce dramatically the extinction to the cavity 10-100 AU in diameter. Yet the Class I SED central object. The resulting Class HD SED displays a requires that there are large quantities of hot dust (Td = strong peak in the near-infrared from the central object 300 K-1000 K) within this cavity which radiate between and a second peak due to far-infrared emitting dust in the 3.4 μιη and 12 μιη. Thus, the distribution of dust within residual infalling dust envelope. At the end of the accre- this cavity must be highly anisotropic, providing little tion/outflow phase, the outer envelope is completely dis- visual extinction along most lines of sight to the central persed. As the YSO begins the convective-radiative phase object (Myers et al. 1987). A subset of Class I SEDs for of its pre-main-sequence evolution, only the gas photo- YSOs in the ρ Oph infrared cluster is shown in Figure 3a. sphere and a dusty disk remain. These combine to pro- A Class II SED has —2 < α < 0 and is characteristic of Τ duce a flat spectrum SED if the disk is flared or undergo- Tauri stars (e.g., Rucinski 1985). While these SEDs are ing nonviscous accretion (Kenyon and Hartmann 1987; still broader than a single-temperature blackbody due to a Adams, Lada, and Shu 1988), or a classic Τ Tauri (Class II) small quantity of hot dust, the large component of cool, SED if the disk is passive or undergoing viscous accre- far-infrared emitting dust evident in Class I SEDs is tion. Finally, as the YSO approaches the main sequence, absent. A sampling of Class II SEDs from the ρ Oph the disk is dissipated giving rise to a Class III SED. infrared cluster is shown in Figure 3b. An SED with a < Perhaps this evolutionary scheme is too simplistic since —2 has little or no infrared excess and is classified as Class it does not account for the naked Τ Tauri stars which have III. The Class III SED resembles that of a reddened Class III SEDs but appear to be similar in age to the blackbody. Post-T Tauri stars, naked Τ Tauri stars (e.g., classic Τ Tauri stars (Walter 1986; Walter et al. 1988). Walter 1986), and field stars display Class III SEDs. However, this scenario can be used as a starting point for Unfortunately, using the IRAS database selects against investigations of the age and formation history of infrared the detection of these types of objects. The subclasses of clusters. For example, in the ρ Oph infrared cluster, the Class IID and HID have been added to describe double- relative number of Class I to Class II objects (24 to 27) can peaked SEDs (e.g., Adams et al. 1987); the first peak be used to estimate the duration of the Class I phase arises in the far-visible/near-infrared from a reddened assuming an average age for Τ Tauri stars in the cloud photosphere (akin to either a Class II or III SED) and a (Wilking et al. 1989). A duration of 1-4 X 105 years is distinct second peak due to cooler dust occurs in the obtained for the accretion (Class I) phase, the lower limit far-infrared (Wilking et al. 1989). Class HID SEDs are permitting a population of naked Τ Tauri stars equal in typical of luminous stars associated with reflection nebu- number to the observed Class II population. This infers a losity. Examples of double-peaked SEDs in the ρ Oph mass accretion rate in ρ Oph of 2.5-10 X 10-6 STÎ© year-1 cluster are shown in Figure 3c. for a 1 SWq star. By estimating the number of Class III The SED shape gives us clues to the amount and objects in the cloud, an upper limit to the age of the ρ Oph distribution of circumstellar dust toward a YSO and. cluster of 3.5 million years is derived.

12 ¿¿m 60 ¿¿m ΙΟΟμπΊ

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Fig. 3-Spectral energy distributions, grouped by morphology, for selected YSOs in Ophiuchus displaying IRAS emission (from Wilking et al. 1989). The power often used to scale the SED is given in parentheses below the source name. Figure 3a presents Class I SEDs with spectral index α > 1. Class II SEDs (—2 < a<0) are displayed in Figure 3b which include three YSOs classified as Τ Tauri stars. The SED for a 2300 Κ blackbody is shown for comparison. Double-peaked Class IID and HID SEDs are shown in Figure 3c along with that of a 300 Κ blackbody.

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Cb) -24.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOG λ (μπη) Fig. 3(b)-See caption to Figure 3 on page 236.

4.2 Βolometric Luminosities By simply integrating the spectral energy distribution may overestimate the true bolometric luminosity for of a YS O over visible to far-infrared wavelengths, an Class II objects by as much as a factor of two if the disk, estimate for the bolometric luminosity is obtained: which does not radiate isotropically, is viewed nearly /•00 /-00 face-on (Strom et al. 1988; Wilking et al. 1989). In a 2 2 L(obs) = 4mí I FkdX = (9.2)ιτί/ I \Fxd(\og10K) . sample of 59 Class II objects, Strom et al. (1988) found Jo Jo that 80% showed evidence for a modest amount of ob- This observed luminosity is a good approximation to the served luminosity (< 60%) in excess of the stellar lumi- bolometric luminosity provided (1) the luminosity of the nosity derived from the spectral type. This small excess source is radiated isotropically and either (2) there is no could be attributed to the presence of a passive disk or extinction toward the source or (3) all the extinction to- chromospheric emission. The remaining 20% of their ward the source is produced by circumstellar dust which sample displayed much larger excesses perhaps indicat- completely surrounds the source and reradiates the ab- ing the presence of active disks in these Class II objects sorbed light in the near- to far-infrared spectral region. (see, also, Cohen, Emerson, and Beichman 1989). For Class I objects SED modeling suggests that L(bol) ~ The distribution of YS O luminosities within a star- L(obs) since much of the energy emitted by the YS O forming region, or luminosity function, can also give us either from the central object or an active disk would be insight into the star-forming history of a cloud. The lumi- completely absorbed and reradiated by dust in the spheri- nosity function for 74 members of the ρ Oph infrared cal outer envelope. However, the observed luminosity cluster is shown in Figure 5; the YSOs within each lumi-

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-23.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOG λ (/¿m) Fig. 3(c)-See caption to Figure 3 on page 236. nosity bin are grouped according to their SED class. One has no statistically significant deviations from that ex- remarkable feature of this luminosity function is the seg- pected from an IMF (i. e., the Initial Luminosity Function regation of luminosities by their SED class. Class I ob- or ILF). However, the cloud has formed stars sequen- jects dominate Class II objects by 9 to 1 in the 5.6 to 56 Lq tially in mass. If the latter suggestion is borne out, then range while at lower luminosities Class II objects domi- the 1 ïïî© Class I objects will undergo luminosity evolu- nate by a 2 to 1 margin. The large number of Class I tion as they approach the main sequence relative to their objects at intermediate luminosities represents either (1) Class II counterparts. This will produce a cluster luminos- a population of intermediate-mass objects (2-4 HTÍq), ity function (when corrected for incomplete sampling) which are the most recent YSOs to form in the cloud, or which appears deficient in intermediate mass objects rel- (2) 1SPÎ© YSOs with an additional source of luminosity over ative to the ILF. In this case, future episodes of star formation would have to favor more massive stars to their Class II counterparts (i.e., accretion). We are cur- produce ultimately an ILF. rently unable to distinguish between these two possibili- ties but both have important implications for star forma- 5. The Resolution of Circumstellar tion in ρ Oph. If the former explanation is correct, then Gas and Dust the cloud has formed a cluster whose luminosity iunction As discussed above, the anisotropic distribution of cir-

1.0 λ (μίτι)

I Class HD

λ (μιτι)

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\W/

1.0 2.0 10.0 100.0 λ (μ m) Fig. 4-The quasi-continuous evolutionary sequence of SEDs for low-mass stars proposed by Adams et al. (1987). On the left, spectral energy distributions typical of the four morphological classes are shown. The corresponding stages in the pre-main-sequence evolution of a low-mass star are shown schematically on the right. These stages include (1) an accreting protostar (Class I SED), (2) an accreting protostar with a well-developed outflow (Class IID), (3) the Τ Tauri phase (Class II), and (4) the naked Τ Tauri or post-T Tauri phase (Class III). Figure adapted from Lada (1987) and Shu et al. (1987). cumstellar gas and dust within several hundred AU of the these circumstellar structures are observed to have major stellar core has been inferred from studies of mass loss axes aligned perpendicular to the axes of mass outflow and from YSOs and from modeling of their SEDs. However, to the local magnetic-field direction. only recently have high-resolution observational tech- 5.1 High-Resolution Infrared Techniques niques begun to resolve these structures. In most cases Near-infrared scattering disks of circumstellar dust

techniques at 7.8 μιη and 12.5 μιη, Lester et al. (1985) have resolved the IRc2/KL source in Orion into a Γ/1 Χ Γ.'9 ellipsoid (530 X 910 AU at a distance of480 pc). High-resolution scanning at 50 μιη and 100 μιη using the Kuiper Airborne Observatory coupled with MEM reconstruction has yielded spatial information on 10" scales for strong sources and resolved a toroidal distribution of circumstellar dust toward S106 (Lester et al. 1986; Harvey, Lester, and Joy 1987). 5.2 Millimeter-Wave Interferometry Interferometric observations at millimeter wavelengths are not only capable of resolving circumstellar structures of cold dust toward nearby objects but also can 1 2 reveal the distribution and dynamics of the circumstellar Log [ L Qjjg / Lq ] molecular gas. An added advantage afforded by interfer- Fig. 5-The luminosity function for 74 members of the ρ Oph infrared ometry is the ability to resolve out extended emission cluster (Wilking et al. 1989). For comparison, the luminosity function components from the cloud and cloud core and to focus on derived from the IMF is shown, normalized to the number of sources with well-determined luminosities in the —0.25 < log L < 0.75 range. compact structures. Using multiple configurations of the The YSOs in each luminosity bin are grouped according to the spectral three-element millimeter-wave interferometer at Owens indices of their SEDs. Luminosity estimates for sources with no de- Valley Radio Observatory (OVRO), angular resolutions of tectable IRAS flux are shown as upper limits but not included in the 3''5-6" have been obtained on dust and gas associated normalization of the ILF. with the low-mass YSOs IRAS 16293-2422, HL Tau, and have been observed toward several low-mass YSOs. Us- L 1551-IRS 5. Disk-like structures have been observed ing speckle interferometry at 2.2 μιη, Beckwith et al. toward these objects in the millimeter-wave continuum (1984) have detected elongated structures toward HL and the l->0 transitions of 13CO or C180. Tauri and R Monocerotis (a YSO of intermediate mass). IRAS 16293-2422 is an extremely cold source in the The Γ/2 X 2"1 structure toward HL Tau corresponds to L 1689 cloud of the Ophiuchus complex. To date, no dimensions of 200 AU X 340 AU at a distance of 160 pc. emission shortward of 25 μιη has been detected from this Speckle interferometry at 3.4 μιη of GSS30 (= EL21) in 27 Lq source. Mundy, Wilking, and Myers (1986) have Ophiuchus by Zinnecker, Perrier, and Chelli (1988) sug- detected and resolved strong 2.7 mm continuum emis- gests an asymmetry in the 32 AU core which may corre- sion (550 mjy) from cold dust from IRAS 16293-2422. The spond to a disk proposed for the source (Castelaz et al. continuum emission arises from an elongated region 1800 1985). A second technique to detect scattering disks uti- AU X < 800 AU in size and containing 1-6 Sft© of gas and lizes maximum-entropy reconstruction methods (MEM) dust. Subsequent observations of C180 emission lines applied to high-resolution, oversampled 1.6-μιη and 2.2- have revealed a structure similar in extent to the contin- μιη maps of YSOs. Using this technique, evidence for a uum source (see Fig. 6a). The data strongly suggest that scattering disk toward HL Tau with a diameter of270 AU this structure is rotating about its minor axis with a full and a thick disk toward L 1551-IRS 5 of diameter 1000 AU velocity shift of 2.4 km s-1 ±6" from the disk centroid have been obtained (Grasdalen et al. 1984; Strom et al. (Mundy, Wootten, and Wilking 1989). Assuming the gas 1985). Observations of lunar occultations at 2.2 μιη are a is in circular orbit and that the disk is intrinsically circular, potentially powerful technique to detect near-infrared the included mass would be 1.5-1.7 S)?©. The discovery of scattering disks on the scale of AUs toward YSOs in a double radio continuum source within the disk-like Taurus and Ophiuchus (Simon et al. 1987). Future structure opens the possibility that IRAS 16293-2422 is a searches for scattering disks toward low-mass stars will be circumbinary disk or a combination of two disks in a greatly facilitated by the advent of near-infrared array binary star system (Wootten 1989). cameras (e.g., Zinnecker 1988). In Taurus, initial interferometric observations of CO There are several promising high-resolution tech- (l->0) toward HL Tau revealed an unresolved concentra- niques which have resolved the thermal emission from tion of molecular gas; the low-velocity dispersion of the circumstellar dust, but these have yet to be applied to gas suggested it was bound to the star (Beckwith et al. low-mass stars. Observations of lunar occultations of 1986). Subsequent 13CO observations have resolved the M8E-IR by Simon et al. (1985) at 3.5 μιη and 10 μιη molecular gas into an elongated structure about 4000 AU suggest the source is associated with a 3 X 36 milliarc (25") in extent containing about 0.1 Slí© (see Fig. 6b). The second structure (5.4 AU X 65 AU at a distance of 1.8 kpc) rotation curve deduced for the gas suggests it is in Kep- which they interpret as a flared disk. Using scanning lerian rotation about the star (Sargent and Beckwith

IRAS 16293-2422

Fig. 6-Interferometric observations of molecular-line emission toward low-mass YSOs IRAS 16293-2422, HL Tau, and L 1551-IRS 5. Figure 6a shows the C180 (l->0) integrated intensity and 2.7 mm continuum emission observed from IRAS 16293-2422 (Mundy et al. 1989). The synthesized beam for the maps is 6'.'3 X 4"5 P.A. = 0°. Crosses mark the positions of two radio continuum sources found by Wootten (1989). Contour levels for the C180 map are 1, 2, 3, 4, and5 Jy beam-1 Κ km s"1. The continuum map contour levels are (-2, 2, 4, 6, 8, 12, 16, 20, and 24)* 12.5 mjy beam-1. The peak in the continuum map is 315 mjy.

velocity shift across the source, the resolution is not suffi- cient to establish if the rotation is Keplerian.

6. Future Prospects Over the past 15 years we have seen dramatic progress in our understanding of low-mass star formation which has accompanied new technological advances in observational astronomy. Each new advance has brought us closer to observing the birth of a star in a molecular cloud core. This trend will no doubt continue in the future. CCDs, used with ground-based or space-borne telescopes, will be important in new optical studies with the sensitivity to investigate in detail possible deviations of the mass functions of clusters and associations below 1 SKq· CCDs and near-infrared array cameras will enable us to extend studies of collections of low-mass YSOs to more distant regions and allow us to investigate low-mass star formation in giant molecular clouds. Large single-dish millimeter and submillimeter-wave telescopes will per- mit detailed studies of the distribution of dense molecular 13 Fig. 6-Figure 6b shows a map of CO integrated intensity from cores within a cloud. It will be of great importance to HL Tau (Sargent and Beckwith 1987). The synthesized beam for this determine the mass function of dense molecular frag- map is 5"8 X 1078, P.A. = —2°. The position of the star is indicated by the cross and the contour levels are 1, 2, and 3 Jy s-1 per beam. ments within a cloud and to begin relating cloud fragmentation to the resulting stellar mass spectrum. We have seen that far-infrared observations are critical in the study 1987). More recently, an elongated distribution of C180 of evolution and luminosity functions of infrared clusters, emission with a major axis of about 1500 AU has been but these observations are often confusion-limited even observed toward L 1551-IRS 5 (Fig. 6c; Sargent et al. in the nearest star-forming regions. The high angular 1988). The disk-like feature is at nearly the same position resolution and sensitivity in the far-infrared provided by angle proposed for the dust-scattering disk and perpen- SOFIA and SIRTF will be crucial in advancing studies of dicular to the axis of mass outflow. About 0.1 SPÎq is low-luminosity infrared clusters. contained in this feature. While there is evidence for a Direct observation of the actual birth of a star has

04h 28rn41.0s 40.5s 40.0s 39.5s Right Ascension (1950) Fig. 6-The integrated intensity of C180 emission from L 1551-IRS 5 at 3"5 resolution is presented in Figure 6c (Sargent et al. 1988). The grey scale levels represent 200, 400, 600, and 800 mjy beam-1 Κ km s_1. The peak is about 1 Jy beam-1 Κ km s_1. The cross marks the position of compact radio continuum sources.

remained elusive. Already, millimeter-wave interferom- Anneila Sargent generously supplied figures from their eters have detected structures which appear to be rotat- work, both published and unpublished, for reproduction ing circumstellar disks but which are somewhat larger in in this manuscript. Partial support under the IRAS Data size than near-infrared scattering disks and disks ex- Analysis Program of NASA funded through the Jet pected from theoretical considerations. Further study Propulsion Laboratory is gratefully acknowledged. and identification of near-infrared scattering disks will be REFERENCES greatly enhanced with the new infrared array cameras, particularly when used in conjunction with the Hubble Adams, F. C., and Shu, F. H. 1986, Ap. /., 308, 836. Space Telescope. New submillimeter and millimeter- Adams, F. C., Lada, C. J., and Shu F. H. 1987, Ap. /., 312, 788. 1988, Ap./., 326, 865. wave instrumentation will aid in the identification of addi- Adams, M. T., Strom, S. E., and Strom Κ. M. 1983, Ap. /. Suppl., 53, tional examples of rotating circumstellar disks and lead to 893. a better understanding of their role in accretion, mass Ambartsumian, V. A. 1947, Stellar Evolution and Astrophysics {Erevan: loss, and the formation of planetary systems. The high Academy of Sciences of the Armenian S. S. R.). angular resolution and high sensitivity available with sub- Bally, J., and Lada, C. J. 1983, Ap. /., 265, 824. millimeter telescopes and expanded millimeter-wave ar- Beckwith, S., Sargent, A. I., Scoville, N. Z., Masson, C. R., Zucker- man, B., and Phillips, T. G. 1986, Ap. /., 309, 755. rays may soon reveal the dynamical motions within the Beckwith, S., Zuckerman, B., Skrutskie, M. F., and Dyck, M. 1984, circumstellar gas which can be associated with the accre- Αρ./., 287, 793. tion process. Bok, B. J. 1934, Harvard Cire., No. 384. Castelaz, M. W., Hackwell, J. Α., Grasdalen, G. L., Gehrz, R. D., and I would like to thank Charles Lada, Lee Mundy, Gullixon, C. 1985, Ap. /., 290, 261. Richard D. Schwartz, and Angie Schultz for helpful com- Cohen, M., and Kuhi, L. V. 1979, Ap. /. Suppl., 41, 743. ments on this manuscript. Bob Loren, Phil Myers, and Cohen, M., Emerson, J. P., andBeichman, C. A. 1989, Ap./., in press.

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