NGC 1333: A Nearby Burst Of Star Formation

6Walawender et al.at these wavelengths and, together with SEDs compiled from the literature, classifiedthem according to their embedded nature.The NGC 1333 region was surveyed with the Spitzer Space Telescope, under anumber of programs including the “Cores to Disks” legacy project (as part of Perseus;see Jørgensen et al. 2005b and Rebull et al. 2007) and the GTO Spitzer Young Cluster Survey. From these combined data, Gutermuth et al. (2008) found 137 membersassociated with NGC 1333 itself, 39 protostars and 98 pre-main sequence stars withdisks. Class I objects were found to have a spatial distribution similar to that of densegas found in the region, as traced by Sandell & Knee (2001), while Class II objectsaccounted for the “double-cluster” morphology seen by Lada et al. (1996). The twodensity peaks of this morphology are anticorrelated with locations of dense gas. Despite this difference, the nearest-neighbor distance distributions of both classes eachpeak at 0.045 pc, suggesting the Class II population is both very young and has a lowoverall velocity distribution. They further argue that the cavities in the center of NGC1333 were thus dispersed by the Class II populations and not by outflows. Overall,NGC 1333 was revealed to have a roughly uniform density distribution within a 0.3 pcradius, and a steep decline at larger radii, possibly reflecting original cloud structure.Relative to the IC 348 cluster (also in Perseus but at the opposite end of the cloud),NGC 1333 shows a larger fraction of Class I objects vs. Class II objects, suggestinga significant age difference between these clusters. Indeed, NGC 1333 may be moresimilar in age to the smaller, very young groups in Perseus, e.g., L1448, L1455, or B1.Interestingly, before Spitzer, a small fraction of the deeply embedded protostarsin the region were known to have mid-infrared counterparts, but the high sensitivityand high angular resolution of the Spitzer observations identified a number of thesesources at wavelengths as short as 3.6 µm, making it possible to characterize them.To establish a relatively unbiased sample of such sources, Jørgensen et al. (2007b)combined surveys by Spitzer and the Submillimetre Common-User Bolometer Array(SCUBA) at the James Clerk Maxwell Telescope (JCMT) and studied the separationbetween the mid-IR sources and the SCUBA clumps and the characteristics of both(see Fig. 3). About half of the dust condensations in NGC 1333 were found to haveassociated mid-infrared sources and these cores were forming stars with an efficiencyof 10 15% (not including the efficiency of forming the cores in the first place), similarto what is observed in the larger scale cloud.3.Submillimeter, Millimeter & Radio Observations of NGC 1333NGC 1333 has long been known to harbor many embedded protostars. In particular,its southern half contains many bright Class 0/I objects, including SVS 13, IRAS 2,IRAS 4 and their individual components, which have been closely observed over thelast twenty years.In this section, we describe notable continuum observations made at submillimeterthrough centimeter wavelengths of these objects and their dense environments. Suchobservations are sensitive to high column density dust associated with embedded protostars. A general trend seen from such data has been that observations of increasedsensitivity, map extent or resolution have revealed new embedded objects in the region. Identifications of objects, especially at the lower brightness ends of samples, canbe biased depending on the method used (e.g., by eye or CLUMPFIND), leading todifferences between object lists found in different studies.

NGC 13337Figure 3. Comparison between Spitzer IRAC image (Jørgensen et al. 2005b;Gutermuth et al. 2008) and SCUBA 850 µm (contours; Sandell & Knee 2001; Kirket al. 2006). For further details about the comparison across these wavelengths seeJørgensen et al. (2007b) and Gutermuth et al. (2008).3.1.Single-Dish ObservationsEarly surveys with single element bolometers targeted individual known protostars inthe NGC 1333 region (e.g., Sandell et al. 1990, 1991, 1994), but with bolometer arrays in the late nineties it became possible to map larger (&10 square arcminute sized)regions.The higher angular resolution provided by large single-dish submillimeter telescopes made it possible to explore the dust envelopes of the IRAS objects and in somecases resolve them into multiple sources. For example, Sandell et al. (1990) and Sandellet al. (1994) observed SVS 13 and IRAS 2 with the UKT14 bolometer system on theJCMT at 1.1 mm, 800 µm, or 450 µm respectively (with respective resolutions of 19′′ , 14′′ , and 8.5′′ FWHM), revealing extended continuum emission associated with each

8Walawender et al.source, but no definitively identified new objects. Sandell et al. (1991), however, observed IRAS 4 and resolved it into two objects, 4A and 4B, with a 30′′ separationoriented NW-SE. A third object, 4C, was seen in these images at a position 50′′ NEof 4A, but its existence was only confirmed later by the VLA data of Rodrı́guez et al.(1999) and the SCUBA data of Smith et al. (2000).Lefloch et al. (1998) mapped the southern region of NGC 1333 at 1.3 mm usingthe IRAM 30 m telescope (11′′ resolution): they detected six dust peaks in the region,four associated with protostars, SVS 13, IRAS 2, IRAS 4A and IRAS 4B. They furthermore identified two cavities in the region, coinciding with similar cavities from lineobservations (e.g., Warin et al. 1996). They suggested that these cavities were relatedto the action of the outflows from the newly formed stars.Bolometric arrays at single-dish millimeter/submillimeter telescopes allowed widerfields in star forming regions to be surveyed at high sensitivity and resolution, reducingthe biases of earlier studies that forced observations toward previously identified objects. Figure 4 shows an 850 µm continuum map made from data obtained by Sandell& Knee (2001) with SCUBA on the JCMT, covering a 13′ 18′ region centered nearSVS 13. Sandell & Knee (2001) observed both 850 and 450 µm at 14′′ and 9′′ FWHMresolution respectively. These maps revealed further multiplicity in some sources, theyresolved IRAS 2 into three objects, 2A, 2B and 2C, with 30′′ separation orientedNW-SE. They identified 33 submillimeter sources associated with NGC 1333. Theyalso pointed toward the importance of outflows for the distribution of the matter inNGC 1333, suggesting that triggered star formation is a common mode there. IRAS4A was found to be the brightest object at 850 µm with a flux of 9 Jy, or 3 that ofthe next brightest objects, SVS 13, 4B, and 2A. Such maps revealed even more embedded objects within NGC 1333.Chini et al. (1997) resolved SVS 13 into three sources, MMS 1, MMS 2 and MMS3 (later identified as SVS 13, 13B and 13C2 ), with 15′′ separation oriented NE-SW. Inaddition, these maps revealed dust ridges or filaments bordering on possible cavities inthe central part of the region, e.g., see Lefloch et al. (1998). More recently, NGC 1333has been included in larger-scale continuum maps of the inner regions of the Perseuscloud, e.g., at 850 µm with the JCMT by Hatchell et al. (2005) and Kirk et al. (2006)(see also Ridge et al. 2006) and at 1.3 mm with the CSO by Enoch et al. 2006 at 31′′FWHM resolution.3.2.Interferometric ObservationsEmbedded objects in NGC 1333 identified in single-dish maps have been observed athigher resolution using various interferometers to improve understanding of the smallscale structure of their dust envelopes and circumstellar disks and to look for furthermultiplicity. A significant number of the deeply embedded objects were targeted byLooney et al. (2000) in their large subarcsecond millimeter wavelength interferometricsurvey, revealing the multiplicity of many of these deeply embedded sources on arcsecond scales. A number of the embedded sources in NGC 1333 have also been the targetof continuum surveys at longer (centimeter) wavelengths.Rodrı́guez et al. (1999) surveyed an 8′ 8′ region centered on SVS 13 using theVery Large Array (VLA) at 3.6 cm and 6 cm at 4′′ -5′′ FWHM resolution, detecting2SVS 13 is coincident with VLA 2, a radio continuum source identified by Haschick et al. (1980),Rodrı́guez & Canto (1983), and Snell & Bally (1986) as corresponding to the water maser H2 O(B).

NGC 13339Figure 4. A map of 850 µm continuum emission from NGC 1333, as observedwith SCUBA by Sandell & Knee (2001). The grey scale extends from 0.005 to 1.0Jy beam 1 . The resolution is 14′′ FWHM. Triangles denote the peak positions ofthe 33 objects identified by eye by Sandell & Knee. Stars denote the positions ofHH 12 IR (VLA 42), SVS 3, and BD 30 459, each associated with submillimetercontinuum emission.44 sources of which 26 were believed to be associated with young stellar objects inNGC 1333. Reipurth et al. (2002) followed up on those observations with deep, higherresolution images of a number of some of the deeply embedded outflow sources inthe region. These surveys identified most of the known far-infrared and submillimetercomponents of the embedded protostars at these long wavelengths. For most of thesources this was attributed to the action of thermal jets on small scales - although afew sources, including the two components in the IRAS 2 binary, showed evidence

10Walawender et al.for continuum from thermal dust emission extending from submillimeter to these longwavelengths (see also Fig. 5 of Jørgensen et al. 2005a).Detailed discussions of the interferometric observations of the SVS 13, IRAS 2and IRAS 4 source regions can be found in Sections 5.1., 5.2., and 5.3., respectively.4.Protostellar OutflowsThe large number and area covering factor of shock excited Herbig-Haro objects (seeFig. 5) and near-infrared H2 emission associated with dozens of overlapping outflowsdistinguishes NGC 1333 from other nearby star forming regions. Figure 6 shows theSpitzer IRAC 4.6 µm image of the NGC 1333 region. The cluster and the 1 degreelong ridge of molecular cloud extending south are covered with filaments, bow shocks,and irregular clumps of H2 emission. Near the central part of NGC 1333, the coveringfactor of shock-excited H2 emission and Herbig-Haro objects is greater than 50% atlow flux levels.Figure 5. Hα image from the survey of Walawender et al. (2005) of the southernhalf of NGC 1333.Bally et al. (1996) found over 30 groups of HH objects associated with over adozen and perhaps many more currently active outflows. They describe the region as a“microburst of star formation” of duration less than 1 Myr within a radius of less than1 pc.

NGC 133311Figure 6. NGC 1333 as imaged by Spitzer IRAC in band 2 at 4.5 µm showing thesame field as in Fig. 2 (Courtesy the c2dteam).First noted by Herbig (1974) on photographic plates, the objects HH 5, 6, 7-11,and 12 are the brightest HH objects in the region. HH 5 and 6 are relatively compactclusters of bright knots. HH 7-11 is a chain of very bright shocks propagating southwestfrom the general vicinity of SVS 13 (to be discussed in more detail in Section 5.1.).HH 12 (see Section 5.4.) is a complex shock structure located north of HH 7-11 at thenorthern end of a complex of H2 filaments which line the walls of a roughly 5′ longconical cavity in the NGC 1333 cloud. The HH 7-11 cavity is evident in deep I-bandand infrared images, and in sub-millimeter dust continuum and molecular line maps.At least two jets are aimed (in projection against the plane of the sky) towardsHH 12; the northern lobe of the IRAS 2 molecular jet, and the molecular jet emergingtowards the north from the cluster of sources associated with SVS 13 located at the baseof the HH 7-11 flow (e.g. Knee & Sandell 2000). The northern tip of HH 12 is furtherconfused by a low-excitation ([SII] and H2 dominated) outflow emerging from IRAS 6

12Walawender et al.towards PA 300 . Thus, the HH 12 shock complex may be driven by any of severaloutflows.Bally & Reipurth (2001) found several remarkable jets and bent Herbig-Haro outflows in the vicinity of NGC 1333. The several arcminute-long HH 333, located northof the brightest portion of the reflection nebula, is one of the most collimated HH flowsknown. This roughly east-west oriented jet emerges from an 18th magnitude (i-band)star whose circumstellar disk casts a faint shadow about 10′′ in extent on surroundingdust. This feature is oriented orthogonal to the jet axis. The jet consists of a chain ofknots connected by a faint filament of Hα and S II emission less than 2′′ wide. It isbipolar (visible on both sides of the source) and is over 3′ long. It exhibits a gentleS-shaped bend indicating that over the dynamical age of the most distant shocks, the jetorientation has changed by about 10 to 15 .The regions north and west of HH 12 contain some remarkable C-symmetric, bentjets emerging from visible stars (Fig. 7). C-symmetric flows include HH 334, 498, and499. While HH 334 shows only a mild bend, HH 498 and 499 show bends rangingfrom 40 to over 90 . While the C-symmetric outflows in the Orion nebula consistentlybend away from the nebular core, in NGC 1333 C-symmetric flows bend toward thecluster core. A relative motion of order 10 km s 1 is needed to explain the degree ofjet bending observed. Such a large velocity is not likely to be produced by gas infallingtoward the cluster core. Bally & Reipurth (2001) postulated that, in Orion, jets are bentby a wind flowing away from the nebular core while in NGC 1333 the bends are morereadily explained as being caused by the motion of the star with respect to the surrounding cloud. They postulated that the source stars of the bent jets in NGC 1333 have beenrecently expelled from the cluster core by dynamical interactions. This scenario is consistent with the relatively low obscuration toward the jet sources, as little circumstellarmaterial is expected to survive the acceleration of the star in such an interaction.An S-shaped outflow, HH 343, was discovered by Bally et al. (1996) in the southern part of NGC 1333, about 10′ south of the main cluster. Hodapp et al. (2005) examined this outflow and its source in detail using near-IR, optical, and submillimeterimaging and near-IR spectroscopy. Proper motions of the Hα and S II knots showedsome of the shocks moving at more than 100 km s 1 . The shape of the outflow indicates that the source has precessed by almost 90 over its 6000 yr dynamical age. Thespectroscopy and submillimeter data of Hodapp et al. (2005) indicated that the sourceis in a Class 0 or I evolutionary state.The Hα emission line star SVS 20, located about 10′ west of the south-easterntip of the HH 7-11 flow and east-southeast of SVS 13, drives an unusual outflow,HH 345/346, towards PA 220 . Although this flow is collimated on scales of severalarcminutes, it is only poorly collimated within the inner 10′′ of SVS 20. Hα images obtained between 1997 and 2001 show the emergence of a high velocity ( 300 km s 1 )bubble or bow shock with an opening angle of more than about 30 as seen from thestar. The Herbig-Haro object HH 15 lies on the axis of the SVS 20 outflow about 9′southwest of the star. However, HH 15 also lies on the suspected axis of the IRAS 1outflow which powers HH 338/339.An extensive complex of HH objects can be seen south and southwest of IRAS 2(see Section 5.2.) and IRAS 4A (see Section 5.3.). These shocks consist of HH 344,342, 341 and the various components of HH 13 (see Bally et al. 1996, Fig. 2a). Boththe IRAS 4A and IRAS 2B jets point in this general direction. As discussed below, bothground-based imaging and Spitzer reveal many additional shocks farther southwest.

NGC 133313Figure 7. An Hα S II image of the HH 334, 498, and 499 bent jets. Image fromWalawender et al. (2005).The infrared source IRAS 7 is located near the bright object HH 6 (displaced andelongated to the northeast of the IR source). Ground based and Spitzer images showa parsec-scale chain of shock-excited H2 features extending towards the northwest andsoutheast (PA 340/160 ). Evidently, IRAS 7 is the source of a quadrupolar outflowand may therefore be a multiple star system.Walawender et al. (2005) reported several new HH objects and jets well outsidethe cluster core, at the periphery of the NGC 1333 reflection nebula, and beyond theprojected edge of the molecular cloud where confusion is not as significant as in thecluster core. At least two large, parsec-scale flows erupt from the region. The firstcontains the faint shocks HH 752, 756, and 760 tracing a NW-SE flow located near thenorth end of the NGC 1333 reflection nebula. The second parsec-scale flow emergesfrom the NGC 1333 cloud a few arcminutes south of HH 7-11, propagates due east forat least 10′ into the degree-scale cavity between NGC 1333 and Barnard 1, and consistsof HH 348, 349, HH 766, and 767.Sandell & Knee (2001) found that outflows play an important role in the energybalance of NGC 1333. NGC 1333 has a filamentary cloud structure, consisting of manycavities, some of which can be traced to the action of current outflows and some whichmay be the remnants of past outflow activity. Knee & Sandell (2000) also estimate themomentum and energy injection by outflows in NGC 1333 (using an estimated age ofan outflow of 0.1 Myr determined by Bally et al. 1996) to be Ṗ 10 M km s 1and L 0.1 L , enough to disrupt the cloud if these rates hold for a typical cloudlifetime of 10 Myr. Sandell & Knee (2001) find evidence that star formation activitymay be triggered in density enhancements at the periphery of these cavities. Thus

14Walawender et al.star formation may be triggered by previous episodes of star formation and associatedoutflow activity.5.5.1.Individual Objects of Particular InterestThe SVS 13 Subcluster and HH 7-11 OutflowFirst discovered by Herbig (1974), the HH 7-11 group (Fig. 8) traces a collimatedflow emerging from a dense cloud core at roughly PA 125 and originates fromthe vicinity of an embedded near-IR source, SVS 13 (Strom et al. 1974, 1976), whichis also coincident with H2 O maser emission (Dickinson et al. 1974). Haschick et al.(1980) resolved the H2 O maser into 3 components (H2 O(A), H2 O(B), and H2 O(C)), ofwhich H2 O(A) coincides with the SVS 13 protostar. Grossman et al. (1987), using the3-element Owens Valley Radio Observatory (OVRO) millimeter array at 2.7 mm and 9′′ FWHM resolution, first resolved SVS 13 into SVS 13 and 13B (see also Woodyet al. 1989).Figure 8. Hα image of HH 7-11 from the survey of Walawend

NGC 1333 has been the target of numerous studies across the electromagnetic spectrum. In this chapter we summarize the infrared observations (Section 2.) and de-scribe notable observations in the radio (submillimeter through centimeter wavelengths; Section 3.) including high resolution interferometric observations. Protostellar outflows