A&A 440, 151–161 (2005) Astronomy DOI: 10.1051/0004-6361:20041836 & c ESO 2005 Astrophysics
Star formation in Perseus
Clusters, filaments and the conditions for star formation
J. Hatchell1,2,J.S.Richer3,G.A.Fuller4, C. J. Qualtrough3,E.F.Ladd5, and C. J. Chandler6
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [email protected] 2 School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK 3 Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK 4 Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK 5 Department of Physics, Bucknell University, Lewisburg, PA 17837, USA 6 National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
Received 12 August 2004 / Accepted 4 May 2005
Abstract. We present a complete survey of current star formation in the Perseus molecular cloud, made at 850 and 450 µm with SCUBA at the JCMT. Covering 3 deg2, this submillimetre continuum survey for protostellar activity is second in size only to that of ρ Ophiuchus (Johnstone et al. 2004, ApJ, 611, L45). Complete above 0.4 M (5σ detection in a 14 beam), we detect a total of 91 protostars and prestellar cores. Of these, 80% lie in clusters, representative of star formation across the Galaxy. Two of the groups of cores are associated with the young stellar clusters IC 348 and NGC 1333, and are consistent with a steady or reduced star formation rate in the last 0.5 Myr, but not an increasing one. In Perseus, 40–60% of cores are in small clusters (<50 M ) and isolated objects, much more than the 10% suggested from infrared studies. Complementing the dust continuum, we present a C18O map of the whole cloud at 1 resolution. The gas and dust show filamentary structure of the dense gas on large and small scales, with the high column density filaments breaking up into clusters of cores. The filament mass per unit length is 5–11 M per 0.1 pc. Given these filament masses, there is no requirement for substantial large scale flows along or onto the filaments in order to gather sufficient material for star formation. We find that the probability of finding a submillimetre core is a strongly increasing function of column density, as measured by C18O integrated intensity, P(core) ∝ I 3.0.Thispower law relation holds down to low column density, suggesting that there is no Av threshold for star formation in Perseus, unless all the low-Av submm cores can be demonstrated to be older protostars which have begun to lose their natal molecular cloud.
Key words. stars: formation – submillimeter – dust, extinction – ISM: molecules – ISM: clouds – ISM: structure
1. Introduction no longer exist. Therefore we are driven to look for stars at the point of formation or shortly after, at protostellar cores and Recent advances in submillimetre (submm) detector technol- deeply embedded Class 0 objects, with ages less than or of or- ogy now make it possible to image entire star forming regions der 104 years. With submm surveys we can relate the positions and detect all the protostellar and starless cores within. For the of the young protostars to the properties of the dust and gas first time, we can gather statistically significant samples of stars in the molecular clouds in which they form. This has led other at the earliest stages of their evolution. At the same time, theo- authors (Onishi et al. 1998; Johnstone et al. 2004) to conclude ries of star formation are moving on from the formation of indi- that there is an extinction or column density threshold for star vidual, isolated objects (e.g., Shu et al. 1987) to the formation formation, that is, that stars only form in the densest parts of of clusters (e.g., Mac Low & Klessen 2004). It is thus hugely molecular clouds. We investigate this conclusion further in this important at this time to make large surveys of star formation paper. regions to compare with the theories. An important question which we can only address with We have carried out a complete survey of star formation submm surveys is where in molecular clouds stars form. This above 0.4 M in the Perseus molecular cloud. Perseus was cannot be answered by looking at older populations of pre- selected as it is known to be forming clusters of stars and main-sequence stars detectable in the infrared and optical be- more massive stars than, e.g., Taurus. The Perseus molecular cause the typical velocities of the stars lead them to wander cloud contains a total of 17 000 M of gas, estimated from by1pc/Myr, and the environment in which they formed may visual extinction (Bachiller & Cernicharo 1986b). Our sur- Appendix is only available in electronic form at vey area of ∼3deg2 was selected to include all the moder- http://www.edpsciences.org ate to high extinction material (Av > 4) where star formation
Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20041836 152 J. Hatchell et al.: SCUBA Perseus survey might occur. Imaging dust emission at 850 and 450 µm with (Churchwell et al. 1990). Our beam mass sensitivity to a 12 K the Submillimetre Common User Bolometer Array (SCUBA) core is then 0.4 M at 850 µm, although we also detect warmer, at the James Clerk Maxwell Telescope (JCMT), we detect less massive objects, and our beam-averaged column density 22 −2 all the protostars and starless cores above 0.4 M /14 beam. sensitivity is 6 × 10 cm (both limits given for 5σ detec- Complementing the dust continuum images, we present molec- tions). ular line (C18O J = 1–0) data at 1 resolution. Thus, we can re- Maps of the molecular cloud in C18O J = 1–0 were made late the star formation activity to the underlying physical prop- in January and December 2000 using the 4×4 array SEQUOIA erties of the molecular cloud and to the kinematics. By imaging on the FCRAO 14 m telescope. The beamsize was 46 , corre- a single molecular cloud we minimise the uncertainties in de- sponding to 0.07 pc at the distance of Perseus. The 92 500 spec- rived masses, sizes, etc., due to distance. tra were baselined, calibrated to TMB with an efficiency of 0.45, The Perseus molecular cloud is a well known star forming smoothed to 0.25 km s−1 resolution, gridded and integrated in cloud in the nearby Galaxy. It is associated with two clusters velocity over 0–12 km s−1 using the GILDAS software, to pro- containing pre-main-sequence stars: IC 348, with an estimated duce the integrated intensity map shown in Fig. 1. Visualisation age of 2 Myr (and spread of ±1.5 Myr; Luhman et al. 2003); and further analysis of the C18O were carried out using the NGC 1333, which is less than 1 Myr in age (Lada et al. 1996; Karma visualisation suite (Gooch 1996). Maps of 13CO(1–0) Wilking et al. 2004); and the Per 0B2 association, which con- were also obtained during this same observing period, the re- tains a B0.5 star (Steenbrugge et al. 2003) and therefore must sults from which will be presented at a later date. 18 14 −2 be less than 13 Myr in age (Meynet & Maeder 2000). These We take N(C O) = 3.4 × 10 (Av − 1.6) cm (Bachiller 18 clusters therefore all show evidence of star formation activity & Cernicharo 1986b), or alternatively, I(C O) = (Av − within the last ∼107 years. The molecular cloud itself contains 1.6)/3.6Kkms−1, assuming LTE at 12 K. We also assume 20 a number of previously known protoclusters and isolated pro- N(H+2H2) = 0.9 × 10 Av (Bohlin et al. 1978). The tempera- tostars but until now there has been no complete census of the ture of the molecular gas is typically 10–12 K from NH3 obser- current star formation activity. vations (Bachiller & Cernicharo 1986a), although protostellar We assume the distance to the Perseus molecular cloud to cores are warmer (30–50 K: Motte & André 2001; Jennings be the same as the Hipparcos distance to the embedded clusters, et al. 1987). and use 320 pc throughout (de Zeeuw et al. 1999). Our definition of a core for the purposes of this paper is a flux peak with closed contours at the 500, 250 or 150 mJy (∼4σ) levels. Elongated regions at those levels were divided 2. Observations and data reduction into subpeaks at subsidiary maxima. It should be borne in mind Images of Perseus were obtained at 850 and 450 µmus- that it is not necessarily straightforward to convert our obser- / ing SCUBA on the JCMT during 20 nights between 1999 vational sensitivity of 0.4 M beam to the minimum total mass and 2003. The region selected corresponds roughly to the of a core identifiable in our survey as this limit depends on the T ∗ dV = 4Kkms−1 contour of the Bell labs 7m 13CO core density distribution and size: massive but extended cores A could easily remain undetected. As a consequence it is likely map1, although not all of this area was mapped with both we are least sensitive to the youngest and hence least centrally SCUBA and C18O, while some regions outside this contour condensed prestellar cores. In addition if the pressure external were mapped with each. For completeness and calibration con- to a core determines its size for a given mass, our core mass sistency we included the NGC 1333 region although it was sensitivity may vary from region to region in the cloud giving a mapped previously with SCUBA by Sandell & Knee (2001). higher limit in the lower pressure regions away from the clus- The whole region covered contains more than 10 000 M of ters (Johnstone et al. 2004). We are also insensitive to weak gas, estimated from 13CO. Fields of size 10 × 10 were scan- sources in the emission wings of strong ones and therefore un- mapped 6 times with each of 3 chop throws (30 ,44 and derestimate the number of weak sources in clusters, an effect 68 ) and 2 chop directions (RA and Dec). The data were which is discussed further in the next section. reduced using SURF (Jenness & Lightfoot 1998). The typi- cal rms noise level is 35 mJy/beam at 850 µm. The weather conditions for the 450 µm observations were more variable: 3. Clusters for the clusters (observed in the best conditions as we ex- The complete 850 µm image in Fig. 1 shows emission from pected to find sources here) the noise level is ∼200 mJy/beam, dust cores and high column density gas. In many cases the cores but increases to >1Jy/beam in other regions. The 450 and already show central concentration indicative of gravitational 850 µm beam sizes are 8 and 14 respectively or 0.012 and collapse, and must be associated with the earliest stages of star 0.022 pc at the distance of Perseus. We assume a dust emis- − formation. sivity of κ = 0.012 cm2 g 1 (gas+dust), from Ossenkopf & 850 We detect 91 dust cores with an 850 µm flux above Henning (1994) model 5 for icy coagulated grains and a gas- − 150 mJy/beam (>4σ detections). These are listed in Table A.1. to-dust mass ratio of 161 (1.5 × 10 26 g of dust per H atom; About half of these are new submm detections. We see from Draine & Lee 1984), assuming gas is 0.89% H by number Fig. 1 that most of the cores lie in five main groups: from 1 The 13CO map is available from John Bally, at east to west in the cloud, the HH211 region, B1, NGC 1333, http://casa.Colorado.EDU/∼bally/ftp/7mdata/ L1455 and L1448. Additionally, there are a few scattered iso- Perseus_13co.fits lated cores and two cores in B5. These groups are shown in J. Hatchell et al.: SCUBA Perseus survey 153
Fig. 1. a) The eastern part of the Perseus molecular cloud complex. Top: SCUBA 850 µm map. The colourscale (greyscale) is from 0 to 2281 mJy/beam on a square root (flux density) scale to bring out the low intensity features. Bottom: contours of C18O 1–0 integrated intensity at 1 resolution overlaid on 850 µm colourscale (greyscale). Contours are 1, 2, 4, 8, 16 K km s−1. In both maps the black boundary marks the area mapped with SCUBA and the grey boundary marks the area mapped in C18O (note, B5 was not mapped in C18O). Some well-known clusters and IRAS sources are marked. 154 J. Hatchell et al.: SCUBA Perseus survey
Fig. 1. b) The western part of the Perseus molecular cloud complex. Top: SCUBA 850 µm map. The colourscale (greyscale) is from 0 to 2281 mJy/beam on a square root (flux density) scale to bring out the low intensity features. Bottom: contours of C18O 1–0 integrated intensity at 1 resolution overlaid on 850 µm colourscale (greyscale). Contours are 1, 2, 4, 8, 16 K km s−1. In both maps the black boundary marks the area mapped with SCUBA and the grey boundary marks the area mapped in C18O (note, B5 was not mapped in C18O). Some well-known clusters and IRAS sources are marked. J. Hatchell et al.: SCUBA Perseus survey 155 m 100, 200, 400, (f)L1455 µ (e)NGC1333 . 1 − 3213 regions. Colourscale + 1Kkms = IRAS 03271 i) 3035 and + O integrated intensity 18 IRAS 03282 h) (i) 3039, + (h) (b)HH211 IRAS 03292 (g) g) L1455, f) NGC 1333, (d)L1448 e) beam. Dark green contours (thick grey contours) are where the C L1448, / d) beam (black) on a square root (flux density) scale to bring out the low intensity features. Black contours are 850 / B1, c) m2,4,8Jy µ HH211, b) (c)B1 B5, a) Right Ascension (J2000) (a)B5 mmapsofthe beam. White contours are 450 / µ m continuum from 0 (white) to 2281 mJy µ
SCUBA 850 Declination (J2000) Declination Fig. 2. (greyscale) is 850 800, 1600, 3200 mJy 156 J. Hatchell et al.: SCUBA Perseus survey more detail in Fig. 2, where the individual cores and the In IC 348, the infrared population is consistent with con- filamentary structure connecting them are more evident. A full tinuous star formation at a rate of 50 stars with masses above inventory of the pre/protostellar population of Perseus includ- 0.4 M per Myr from 3.5 Myr to 0.5 Myr ago (Muench ing estimates of individual core masses will form the basis of et al. 2003, based on an estimate that 2/5 of the total of 348 a later publication, but it is clear from the submm fluxes that stars shown on the IMF in their Fig. 16, have masses above these groups contain up to a few solar masses of stars. 0.4 M ), This value and a constant star formation rate, suggests Lower flux sources appear slightly less clustered than ∼25 embedded protostars would be expected, if the embedded stronger sources (78% of sources with F850 µm < 250 mJy phase has a lifetime of 0.5 Myr. This is consistent with the are in clusters compared to 88% of sources with F850 µm > 18 submm cores, containing an estimated 18–35 protostars, we 500 mJy), but this may be due to our core selection tech- detect in the HH211 region. nique, which is less sensitive to weak cores in clusters (see Taking an age spread and population for NGC 1333 of Sect. 2). While there may be additional physical reasons why 0.5–1.5 Myr and 143 pre-main-sequence stars (Wilking et al. higher mass (and therefore brighter submm) sources preferen- 2004), a constant star formation rate would predict ∼70 proto- tially form in clusters, we are not yet in a position to test this. stars in the last 0.5 Myr, whereas we observe 36 cores contain- Making a count of cores, we find that more than 80% of ing an estimated 36–70 protostars. Here the submm population the cores fall into five main clusters. We define a core to be appears consistent or a little less than that expected from a con- in a cluster if it has 2 or more neighbours within 0.5 pc. With stant star formation rate and an embedded phase lifetime of our sensitivity to all cores above 0.4 M , this definition guaran- 0.5 Myr. tees a mass surface density of these regions of at least 1.2 M An embedded phase lifetime of 0.5 Myr is consistent within 1 pc2. This definition is then consistent with the Spitzer with the youngest T Tauri sources in IC 348 and NGC 1333, −3 tidal stability criterion that a stellar density >1.0 M pc is re- but is longer than estimates for the embedded phase lifetime quired in order for a cluster not to be disrupted on a timescale in ρ Ophiuchus and Taurus-Auriga, 0.1–0.4 Myr (Wilking et al. ∼108 yr (Spitzer 1958), although during star formation clusters 1989; Kenyon et al. 1990). The age spread of T Tauri pop- almost certainly rely on the gravitational potential of the molec- ulations is notoriously difficult to estimate from isochrones ular gas for binding. The remaining few isolated cores are scat- (Hartmann 2001), and the youngest IC 348 and NGC 1333 in- tered throughout the molecular cloud. 80% is probably a lower frared members may well be less than 0.5 Myr old. However, limit on the fraction of star formation in clusters in Perseus an embedded phase lifetime as short as 0.1 Myr would require because we underestimate the number of cores in clusters due either a significant correction to the T Tauri ages or that we to confusion (see below). Infrared studies of embedded clus- happen to be looking at a star formation burst after a signifi- ters of pre-main-sequence stars estimate the fraction of stars in cant gap in both IC 348 and NGC 1333, which seems unlikely. clusters as 60–90% (L1630; Lada et al. 1991) and 50–100% In conclusion, in IC 348 and NGC 1333 our observations (4 nearby clouds; Carpenter 2000), which are consistent with are consistent with a steady or reduced star formation rate over our findings. the last 0.5 Myr, but not an increasing one. This clustered mode of star formation contrasts with the The small population of cores in many of the clusters in Taurus region (Andre & Montmerle 1994), where isolated star Perseus raises the question of the cluster IMF – how many stars formation dominates. With the majority of stars in the Galaxy form in large/massive clusters compared to small ones? The forming in clusters (Lada & Lada 2003), Perseus, like Orion B massive NGC 1333 cluster, which has a stellar population of (Mitchell et al. 2001) is more likely representative of typical 143 stars with a total stellar mass of 79 M (Lada et al. 1996) star formation in our Galaxy. contains 40% of our pre/protostellar cores. A further 15% lie in the HH211 region bordering IC 348, which has >300 stellar members, and 160 M (Luhman et al. 2003). So, in Perseus, 3.1. Core clusters and stellar clusters 40–60% of cores are in massive clusters having >50 M .Onthe other hand small clusters and isolated objects also contribute Two of the groups of cores are spatially associated with 40–60%, much more than the 10% suggested from the infrared young stellar clusters: the HH211 group can be associated with studies: Lada & Lada (2003) suggest that 20–50 M clusters IC 348, and the NGC 1333 cores with the NGC 1333 cluster, contribute no more than 10% of all stars. Unlike our statistics and may be evidence for continuing star formation in these on cores, our statistics on clusters are derived from a very small clusters. We now consider the evolution of star formation in sample of 5, so are less certain. Nevertheless, although the in- these regions by comparing the number of stars in the clusters frared and submillimetre observations are sensitive to similar with the likely stellar yield of the current protostellar cores. rangesofmassesofstars(>0.1–0.3M ; Carpenter 2000), the As our pre/protostellar core sample is incomplete below difference in age between the submillimetre sources and the 0.4 M , we take this as the lower mass limit for the count. The infrared visible stars may explain this discrepancy. It is quite yield (in stars above 0.4 M ) of the protostellar cores can only likely that the small protostellar clusters are not stable and be a rough estimate because of the unknown multiple star frac- therefore do not classify as clusters by the Lada & Lada criteria tion and the unknown final masses of the accreting stars. We (>35 members obeying the Spitzer tidal stability criterion) by assume each core yields between 1 and 2 stars above 0.4 M ; the time they have become visible at near infrared wavelengths. this takes into account complex cores forming multiple stars. At a typical velocity of 1 km s−1 astarmoves1pcin1Myr J. Hatchell et al.: SCUBA Perseus survey 157 or 10 at the distance of Perseus. A small group of <10 prestel- the cloud (Bachiller & Cernicharo 1986a). SCUBA detects lar cores which evolved to form a 1 Myr pre-main-sequence 850 µm emission at the 5-σ levelatH2 column densities of 22 −2 18 population scattered over a volume 20 in diameter could well ∼6 × 10 cm (Av 60), as the C O starts to become remain unidentified as a cluster in the infrared. Therefore in- optically thick. This column density is rarely reached and the frared estimates of the amount of star formation in small groups SCUBA map is mostly empty. Some extended structure is also may be biassed low. On the other hand, in the submm, by tak- lost in the reconstruction of the SCUBA maps because of the ing a snapshot at the earliest stages of star formation, we un- limited range of chop throws. derestimate the count of objects in clusters because future star The filaments are strongly reminiscent of structures formed formation is not counted. The smaller clusters may yet go on to in turbulent flows (Mac Low & Klessen 2004). What we see form many more stars. Specifically, B1 has a large reservoir of in the dust continuum is that the filaments are observed down molecular gas and no known population later than Class I, and to the small scales and high densities associated with cluster so may be at the earliest stages of cluster formation. formation. This suggests that not only do molecular clouds on The total mass of gas above a column density threshold of the parsec scales of the whole Perseus molecular cloud form 6 × 1022 cm−2 in the clusters and cores is estimated from the by turbulence, but that the role of the filaments persists down dust emission to be ∼2600 M . This assumes isothermal 12 K to the cluster formation scale. A more detailed investigation of dust: the mass will be lower if the temperature is higher, as the role of turbulence, including kinematical information from is the case around the protostars. Lowering the flux threshold molecular lines, is left to a future paper. from the 5σ level to include more low column density mate- The mass in the filaments is substantial with a mean mass rial would result in a higher mass. A 5σ cutoff is used to avoid per unit length of between 4.7 and 11.5 M per 0.1 pc (∼1 ), contributions from artefacts on large spatial scales due to the assuming the filaments are isothermal at 12 K, and taking a reconstruction of the chopped map. With the 5σ cutoff,this sample of 43 filament cross-sections. The uncertainty is due to contribution is limited to be less than 30% of the total: the mass the subtraction of background flux, some of which is extended estimate for the clusters alone (NGC 1333, HH211, B1, L1448, flux belonging the filaments and some of which is an artefact L1455) is 1800 M . The mass fraction of the molecular cloud of the reconstruction process. An estimate of the artificial back- actively involved in star formation at this time is less than 20% ground in regions of blank sky suggests that the true filament of the total gas mass of 17 000 M estimated from the visual mass lies roughly midway between these values. extinction (Bachiller & Cernicharo 1986b). The mass of dense The filament mass per unit length of 5–11 M per 0.1 pc molecular gas traced by C18O 1–0, which has a column den- (1 ) is 2–4 times the traditional thermal Jeans mass for a Jeans 21 −2 sity threshold for detection of 1.6 × 10 cm (Av 2) and a length of 0.1 pc but this does not imply that the filaments −3 critical density of 400 cm ,is∼6000 M , lower than the mass have to be unstable. The criteria for a filament are different for estimate from extinction as it excludes low density and atomic those of a uniform medium (Larson 1985). Theoretical mod- gas. Thus a greater fraction of high column density gas than els for filaments range from a simple non-magnetic isother- low column density gas is in the active star formation regions, mal filament (Ostriker 1964) to magnetised filaments with he- up to as much as half of the gas above Av 2. lical fields which can be toroidal or poloidal or both (Tilley & Pudritz 2003; Fiege & Pudritz 2000; Fiege et al. 2004). A key prediction from these models is the ratio of the mass per 4. Filaments unit length m to the cylindrical virial mass per unit length mvir, which is given by The SCUBA 850 µm image shows filamentary structure σ2 throughout the cloud (Fig. 2). The active protostellar clusters 2 tot mvir = , are linked to dense filaments: the horseshoes of HH211 and G B1, the arcs of L1455 and L1448, and the zigzag of dense gas where σtot is the 1-dimensional total (thermal plus non- in NGC 1333. Weaker filaments imaged by SCUBA to the east thermal) velocity dispersion of the average gas molecule. For of HH211, the south of B1, the south of NGC 1333 and around filaments with or without magnetic field, L1448 each reach up to 1 pc in length, with the upper limit here presumably set by our sensitivity limit. The dust filaments have m M PS 1 − = 1 − , typical full width half maxima of 1 . Filaments to the east of the mvir |W| P HH211 cluster and to the southwest of B1 do not appear to be /| | connected to current protostellar activity but may be fragment- where M W is the ratio of magnetic energy to gravitational en- ing into cores to form the next round of stars. All the filaments ergy per unit length. Following Fiege & Pudritz (2000) Table 2, 4.5 −3 show intensity fluctuations along their length, with the most we assume the external pressure to be 10 Kcm , and esti- fragmented filaments already identified as strings of cores. mate the internal pressure from 18 On much larger scales, ∼20 pc, the C O map (Fig. 1), 2 ρσ − which traces material down to lower column densities, also P/k = tot = 2 × 106 Kcm 3 k shows filamentary structure. The dust filaments detected by SCUBA are found in regions of high C18O column density. using an average density in the filaments of 105 cm−3. The exact In fact the entire Perseus cloud can be described as consist- values of these quantities are unimportant; what is important is ing of one filament with a continuous velocity structure along that the ratio of external to internal pressure PS/P 0.01, 158 J. Hatchell et al.: SCUBA Perseus survey which is much less than 1, so there are three regimes: m/mvir ∼ is reached, and most of the filament mass may ultimately go 1, the filament can exist without magnetic support, m/mvir 1 into cores. This potentially high conversion efficiency of dense a supporting poloidal field is required, and m/mvir 1, when molecular gas into stars has also been noticed for NH3 cores a binding toroidal field is required (see Fiege & Pudritz 2000; (Fuller & Myers 1987). Alternatively, stellar clusters may be Fiege et al. 2004, for details). built up by a series of filaments that form a few stars ineffi- From a sample of 7 C18O 1–0 spectra towards filaments, we ciently, disperse, and are replaced, requiring ongoing filament measure velocity FWHM of 0.6–1.0 km s−1. The conversion production in the same location. 18 from velocity FWHM ∆vobs measured in C Otoσtot,taking into account that the C18O molecule is more massive than the 5. Column density condition for star formation predominant H2,is We are interested in the conditions under which stars form and 1 kT 1 1 σ2 = ∆v2 + 8ln2 − , it is clear that the density of the parent molecular cloud is an tot obs m m m 8ln2 P ¯ C18O important factor. Density itself is not so easy to measure di- 18 where mP is the proton mass,m ¯ = 2.33 assuming 1 He for every rectly from dust continuum or C O emission, but what we can 5H2 molecules, T = 10 K, and mC18O = 30. consider is how the numbers of stars formed varies with the The measured C18O linewidths correspond to virial masses line-of-sight column density (or equivalently visual extinction). −1 of 45–100 M pc , compared to the measured mass per unit An extinction threshold for star formation at Av = 7–9 was −1 length of 47–115 M pc . This simple analysis suggests that found in both Taurus and Ophiuchus (Johnstone et al. 2004; the Perseus star forming filaments are consistent with being Onishi et al. 1998), which could indicate that a certain amount non-magnetic filaments (Ostriker 1964). of shielding from the interstellar radiation field is required for This last result disagrees with the Fiege & Pudritz star formation to occur. Onishi et al. (1998) measured the col- (2000) studies of star-forming filaments, which typically used umn density of C18O associated with sources in Taurus, and 13 CO linewidths, and found m/mvir < 1. From a sam- found that the cold IRAS sources (assumed to be the younger ple of 8 13CO 1−0 spectra, we measure velocity FWHM protostars) and starless H13CO+ cores had a average column −1 21 −2 of 1.4−2.0 km s , typically a factor of 2 greater than density of at least 8 × 10 cm or Av = 9. Johnstone et al. C18O linewidths. If instead these 13CO linewidths are repre- (2004) compared 850 µmcoresinρ Ophiuchus with an opti- sentative of the velocity field within the filaments then ours too cal/IR extinction map and found no submm cores below an Av require additional binding by a toroidal field. However, it seems of 7. In both cases the interpretation is that star formation is 18 morelikelytousthattheC O linewidths are more appropri- inhibited below a certain Av. ate for the thermal support within the dense filaments, as C18O The C18O integrated intensity is a good tracer of total gas profiles are less affected by optical depth, and that the correct column density over the typical range of column densities in solution is one where the total toroidal field is small. molecular clouds, but is less reliable at extreme column den- The mass derived from the dust emission, which increases sities. At low column densities the C18O abundance falls due if either the dust temperature or the 850 µm emissivity is lower to photodissociation. On the other hand at high densities the than those assumed, both of which are possibilities, adds an- line becomes optically thick and in addition in low tempera- other uncertainty. An examination of the transverse column ture, high density regions the C18O depletes onto dust grains density structure and the dust polarisation would provide a fur- (e.g. Tafalla et al. 2004) and so ceases to trace the gas. But 18 ther test of the filament support but this is beyond the scope of between these extremes, a linear C O–Av relationship holds the current paper. above Av ∼ 4 (Frerking et al. 1982). The alternative conclusion that could be reached from the We see in Fig. 1 that the C18O is relatively smooth on scales high virial mass is that the filaments are not stable but tran- of a few arcminutes across the cloud, rather than peaking at the sient, dispersing on the crossing time of 0.1 Myr (assuming submm cores. C18O integrated intensity at 1 resolution is a a filament width of 0.1 pc and velocity dispersion from C18O poor tracer of submm cores, tracing instead the environment −1 of σtot = 0.4kms ). If this is true, then the star formation on larger scales. This is probably largely due to beam dilution which is clearly occurring in the filaments must also happen on –theC18Oeffective beam area is (1 /14 )2 = 18 times greater a crossing time. than the 850 µm beam, so a core of as much as 0.7 M could −1 18 Given filament masses of 5–11 M per 0.1 pc, there is produce less than 1 K km s in C O integrated intensity if no requirement for substantial large scale flows along or onto it had no extended envelope. Depletion also helps reduce the the filaments in order to gather sufficient material for further contribution of core emission to the C18O. C18O depletion has star formation. Typically, in regions where cores have already been measured in prestellar cores on scales of 0.1 pc (Tafalla formed, one or two cores are found within 0.1 pc, and lo- et al. 2004), with the C18O abundance substantially reduced in cal fragmentation will suffice to explain their masses. On the the dense parts of the cores. Finally, the C18O 1–0 line saturates other hand, each filament does not hold a vast reservoir of at about the same column density at which the SCUBA 850 µm mass – a few tens of M . Therefore the filaments are not re- emission becomes visible, which further reduces the contribu- quired to disperse large fractions of their mass in order to re- tion from the highest column density lines-of-sight. The result main consistent with the observed stellar densities in IC 348 or is that the C18O traces the environment of the cores, and thus, NGC 1333. In other words the conversion of dense molecular if we look at young enough cores, the environment in which gasintostarsmaybefairlyefficient once the filament stage cores form. J. Hatchell et al.: SCUBA Perseus survey 159
Table 1. Fraction of cloud by area and number of cores in each C18O integrated intensity range.
18 1 I(C O) Av %area N(cores) % of cores Kkms−1 <1 <5.2 83 10 11.4 1–2 5.2–8.8 13 14 15.9 2–3 8.8–12.4 3 32 36.4 3–4 12.4–15.9 0.5 17 19.3 4–5 15.9–19.5 0.2 13 15.9 >5 >19.50.041 1.1 1 Fraction of the area of the cloud mapped in C18O.