Astrophys Space Sci (2008) 313: 29–34DOI 10.1007/s10509-007-9601-x

O R I G I NA L A RT I C L E

Early stages of star formation

The ALMA promise

Philippe André · Patrick Hennebelle · Nicolas Peretto

Received: 14 May 2007 / Accepted: 9 July 2007 / Published online: 1 September 2007© Springer Science+Business Media B.V. 2007

Abstract The study of the earliest stages of star formationin molecular clouds is one of the fields that should bene-fit most from ALMA. Improving our understanding of thesedeeply embedded stages is crucial to gain insight into theorigin of stellar masses and binary systems. While the useof large single-dish (sub)millimeter radiotelescopes and ex-isting interferometers has led to good progress on the over-all density structure of isolated prestellar cores and youngprotostars, many questions remain open concerning, e.g.,their fragmentation properties and detailed kinematics. Fur-thermore, the classical paradigm for the formation of sin-gle low-mass stars in well-separated, magnetized prestellarcores has been challenged on the grounds that most youngstars actually belong to multiple systems and/or coherentclusters. A new paradigm based on supersonic turbulencehas emerged which emphasizes the role of dynamical in-teractions between individual (proto)stars in cluster-formingclumps. The debate is far from settled and ALMA willgreatly help to discriminate between these two paradigms.

Keywords Stars: formation · Stars: low-mass · Browndwarfs · Stars: binaries · ISM: structure

P. André (�)CEA Saclay—Service d’Astrophysique, 91191 Gif-sur-Yvette,Francee-mail: pandre@cea.fr

P. HennebelleENS/LERMA, Paris, France

N. PerettoDepartment of Physics & Astronomy, University of Manchester,Manchester, UK

1 Introduction: formation and fragmentation ofprestellar cores

Two major limitations in our present understanding of thestar formation process are that we do not know well (a) howprestellar dense cores are generated in molecular clouds, and(b) how such prestellar cores subsequently collapse into pro-tostars or, more likely, protostellar systems. The problem ofthe formation of cloud cores is currently the subject of alively debate between two schools of thought: The classicalpicture of slow, quasi-static core formation by ambipolar dif-fusion in magnetically-supported clouds (e.g. Mouschoviasand Ciolek 1999; Shu et al. 1987, 2004) has been seri-ously challenged by a new, more dynamic picture, whichemphasizes the role of supersonic turbulence in supportingclouds on large scales and generating density fluctuations onsmall scales (e.g. Padoan and Nordlund 2002; Mac Low andKlessen 2004). In the first picture, prestellar cores are quasi-equilibrium structures which evolve over several dynamicaltimescales toward higher degrees of central condensationbefore collapsing dynamically. By contrast, in the secondpicture, cloud cores are always far from hydrostatic equilib-rium and form by collision of large-scale, converging super-sonic flows in molecular clouds (e.g. Hartmann et al. 2001;Klessen et al. 2005).

The detailed manner in which individual prestellar coresfragment (or not) during collapse to form multiple (or sin-gle) stars is another major open problem (see Goodwin et al.2007 for a recent review). In particular, the typical outcomeof cloud core collapse (e.g., single star, binary, or higher-order multiple system) remains unclear (cf. Lada 2006), andthe influence of magnetic fields on core fragmentation is stillpoorly understood (see Sect. 3 below).

Improving our understanding of the formation and frag-mentation of prestellar cores is of fundamental importance

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Fig. 1 Cumulative mass distribution of a sample of 57 prestellar con-densations, complete down to ∼0.1M�, in the ρ Oph protocluster (his-togram with error bars—from Motte et al. 1998 MAN98). For compar-ison, the solid curve shows the shape of the field star IMF (e.g. Kroupa2001), while the dashed curve corresponds to the IMF of multiple sys-tems (e.g. Chabrier 2005). The star markers represent the mass func-tion of ρ Oph (primary) pre-main sequence objects as derived from amid-IR survey with ISOCAM (Bontemps et al. 2001). The thin dottedline shows a N(>M) ∝ M−0.6 power-law distribution correspondingto the typical mass spectrum found for CO clumps (see Blitz 1993)

for at least two reasons: (a) there is good evidence that densecores represent the basic units of star formation on galac-tic scales (e.g. Gao and Solomon 2004), and (b) the stel-lar Initial Mass Function (IMF) appears to be largely deter-mined at the prestellar core stage (e.g. Motte et al. 1998).Several (sub)-millimeter dust continuum surveys of nearby,compact cluster-forming clouds such as ρ Ophiuchi, Ser-pens, and Orion B have indeed uncovered ‘complete’ (butsmall) samples of self-gravitating prestellar condensationswhose associated mass distributions resemble the stellarIMF from ∼0.1M� to ∼5M� (e.g. Motte et al. 1998 andFig. 1; see also Testi and Sargent 1998; Motte et al. 2001;Stanke et al. 2006). These findings strongly support sce-narios according to which there is a one-to-one (or one-to-two) correspondence between prestellar core mass and stel-lar mass, and the bulk of the IMF is at least partly deter-mined by pre-collapse, gravo-turbulent cloud fragmentation(cf. Larson 1985; Klessen and Burkert 2000; Padoan andNordlund 2002). Subsequent, additional processes such asrotationally-driven core fragmentation during collapse (e.g.Goodwin et al. 2007) are however required to account for theformation of binary systems and fully explain the low-massend of the IMF.

Observationally, the best way to make progress on theseissues is through detailed submillimeter line and continuumstudies of prestellar cores and Class 0 protostars (see Andréet al. 2000; Di Francesco et al. 2007; Ward-Thompson et al.2007 for reviews). Herschel, the Far InfraRed and Submil-limeter Telescope to be launched by ESA in 2008, will make

possible complete surveys for prestellar cores and Class 0objects down to the proto-brown dwarf regime in the nearbycloud complexes of the Gould Belt (cf. André and Saraceno2005). These Herschel surveys will provide rich databases,including in the Southern hemisphere, for follow-up high-resolution studies of statistically representative samples ofprestellar cores and protostars with ALMA. ALMA will bea powerful tool to carry out detailed studies of the struc-ture and kinematics of the sources found with Herschel,which will greatly help to discriminate between competingmodels of core formation and evolution. Here, we discussthe unique potential of ALMA for identifying proto-browndwarfs (Sect. 2), constraining the formation process of bi-nary (proto)stars (Sect. 3), and investigating the structureand dynamics of protoclusters (Sect. 4).

2 Searching for proto-brown dwarfs

Accounting for the ultra-low-mass end of the IMF (5MJup �m < 0.075M�) is a particularly severe test for any de-tailed theory of star/core formation. In the turbulent frag-mentation scenario (Padoan and Nordlund 2002), browndwarfs form essentially in the same way as normal stars,from self-gravitating starless gas condensations of substel-lar mass or (pre)proto-brown dwarfs. In this theoreticalscenario, gas condensations with a wide range of massesare formed from material compressed by shocks result-ing from supersonic turbulence. At low masses, this in-cludes both gravitationally bound and unbound conden-sations. A fraction of the low-mass condensations pro-duced by turbulence are dense enough to be gravitationallyunstable to collapse: their masses exceed the local Jeansor Bonnor–Ebert mass. Turbulence can indeed generatelarge enough pressure or density enhancements (δn > nBD)for the corresponding critical Bonnor–Ebert mass, mBE =0.1M� (T /10 K)3/2(δn/106 cm−3)−1/2 to become smallerthan the maximum brown-dwarf mass mBD = 0.075M�.Padoan and Nordlund (2004) have shown that the abundanceof proto-brown dwarfs produced by turbulent fragmenta-tion is consistent with the observed abundance of browndwarfs, assuming initial cloud conditions typical of densecluster-forming regions such as the ρ Oph protocluster. Tosummarize, the Padoan and Nordlund scenario generatesa large number of proto-brown dwarfs, which can be ap-proximately described as critical Bonnor–Ebert isothermalspheres of mass mBE < mBD. As illustrated in Fig. 2, theseproto-brown dwarfs are expected to be dense (n � nBD ≈2 × 106 cm−3), compact (R ∼ RBE � RBD ≈ 800 AU),to exhibit small, thermally-dominated velocity dispersions,and to be associated with large velocity jumps tracing strongshock compressions.

Astrophys Space Sci (2008) 313: 29–34 31

Fig. 2 Density and velocity profiles through a proto-brown dwarfdense core forming from a convergent flow in a model of turbulentcloud fragmentation (from Whitworth et al. 2007). Note the large den-sity and velocity jumps at the location of the shock (∼300 AU) nearthe core boundary. ALMA will have the resolution and sensitivity toobserve these proto-brown dwarf signatures up to a few kpc

A major alternative theoretical picture has been pro-posed, however, in which there is no direct correspon-dence between prestellar core mass and stellar mass, andthe IMF results entirely from dynamical interactions andcompetitive accretion at the protostellar/pre-main sequencestages (e.g. Bate et al. 2002, 2003). Here, each prestel-lar core typically fragments into an unstable small-N bodysystem (with N � 3–5) during protostellar collapse, whichthen disintegrates into binaries and single stars in less than∼105 yr. In this alternative picture, proto-brown dwarfs donot exist and brown dwarfs are formed as aborted stel-lar embryos, dynamically ejected from such unstable mul-tiple systems before they can accrete enough mass to be-come true stars (Reipurth and Clarke 2001; Bate et al. 2002;Goodwin et al. 2004).

Because proto-brown dwarfs are expected to be verycompact and to be characterized by large velocity con-trasts over their parent clouds (cf. Fig. 2), they will beeasy dust continuum targets for ALMA in nearby cluster-forming clouds, provided of course that they exist. Futuresingle-dish submillimeter continuum surveys with Herscheland SCUBA2 will have the sensitivity to identify candidateproto-brown dwarfs (see Greaves et al. 2003), but only a sen-sitive millimeter interferometer can unambiguously confirmthe nature of such compact objects. Alternatively, if browndwarfs are formed as aborted stellar embryos ejected duringthe collapse phase, a large number of substellar companionsshould be detectable with ALMA in the outer envelopes ofsolar-type Class 0 protostars (cf. Reipurth and Clarke 2001).

3 Testing binary fragmentation models

It is generally believed that multiple systems form by dy-namical fragmentation at the end of the first collapse phaseof prestellar cores when the central H2 density reaches val-ues close to ncrit ∼ 3 × 1010 cm−3 and the equation ofstate of the gas switches from isothermality to adiabac-ity (cf. Goodwin et al. 2007). Observationally, this corre-sponds to the very beginning of the Class 0 phase (cf. An-dré et al. 2000), suggesting that the fragmentation physicsmay still be imprinted in the structure of young Class 0 ob-jects.

Purely hydrodynamic SPH simulations of rotating cloudcore collapse show that a low level of initial core tur-bulence (e.g. Eturb/Egrav ∼ 5%) is sufficient to lead tothe formation of a multiple system (Goodwin et al. 2004;Hennebelle et al. 2004—see Fig. 3a). In such SPH simu-lations, fragmentation occurs in large (�100 AU) disk-likestructures or “circumstellar accretion regions” (CARs—cf.Goodwin et al. 2007). These CARs are not rotationally sup-ported and are highly susceptible to spiral instabilities whichfragment them into small-N systems with typical valuesof N ∼ 3–5 (Goodwin et al. 2004; Delgado-Donate et al.2003—see Fig. 3a).

On the other hand, a very different outcome is found insimulations of magnetized cloud collapse, as shown by theresults recently obtained by (Fromang et al. 2006) with anMHD version of the RAMSES code using the AMR tech-nique (cf. Fig. 3b). The simulations illustrated in Fig. 3a andFig. 3b were both obtained using this code and started fromthe same initial conditions (rotating core of radius 0.015 pcand uniform density nH2 = 1.2 × 106 cm−3), except for theinitial value of the magnetic field (B = 0 in Fig. 3a; moder-ate field ∼1/2 of the value required to hold the core againstgravitational collapse in Fig. 3b). The presence of a non-zeromagnetic field strongly modifies angular momentum trans-port during collapse and at least partly suppresses core frag-mentation, leading to the formation of a single (as opposedto quadruple) object in the example shown in Fig. 3.

High-resolution imaging with ALMA will provide crit-ical tests of such binary fragmentation models. The 0.2′′-resolution continuum images shown in Fig. 3 were obtainedwith the ALMA simulator available in the IRAM GILDASsoftware (cf. Pety et al. 2001) and used the above-describedRAMSES simulations as synthetic input models assuminga distance of 140 pc. ALMA will clearly have the capa-bility to discriminate between these two models (B = 0vs. B = Bcrit/2). This example illustrates the potential ofALMA for gaining insight into the physics of core fragmen-tation and multiple star formation.

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Fig. 3 Synthetic 0.2′′-resolution images representing snapshot viewsof the column density distribution during the collapse and fragmen-tation of a model prestellar core observed at d = 140 pc. Both mapswere obtained by processing model images resulting from MHD sim-ulations of protostellar collapse (Fromang et al. 2006) with the ALMAsimulator included in the GILDAS software (Pety et al. 2001). Themagnetic field was set to 0 in the (purely hydrodynamic) simulation

shown on the left, while its initial strength was half the value requiredto prevent collapse in the simulation shown on the right. In both cases,the snapshot was taken ∼104 yr after the start of core collapse, cor-responding to the early Class 0 phase of protostellar evolution. Notethe presence of a quadruple system when B = 0, contrasting with thesingle object and disk-like structure obtained when B = Bcrit/2

4 Structure and dynamics of protoclusters

Another exciting prospect with ALMA is that it can shedlight on the demography and dynamical structure of em-bedded protoclusters throughout the Galaxy, while presentstudies are limited to the nearest cluster-forming clouds (cf.Motte et al. 1998; Testi and Sargent 1998; Peretto et al.2006, 2007; André et al. 2007).

As illustrated by our recent study (Peretto et al. 2006,2007) of the NGC 2264-C clump in the Mon OB1 giantmolecular cloud complex (d ∼ 800 pc), combining com-prehensive, high-resolution millimeter continuum/line ob-servations of embedded protoclusters with numerical sim-ulations (cf. Fig. 4) can be used to set strong constraintson cluster formation models. Dust continuum mapping ofNGC 2264-C at 1.2 mm with MAMBO on the IRAM 30 mtelescope (e.g. Fig. 4a) resolved the column density struc-ture of the region, uncovering a total of 12 compact prestel-lar/protostellar cores, at least 8 of which are Class 0-like ob-jects with associated near-IR H2 jets (Wang et al. 2002). Thethree most massive cores of the complex (C-MM2, C-MM3,C-MM4, with masses of ∼8M�, 30M�, and 25M�, respec-tively) are located near the center of NGC 2264-C. Map-ping in the optically thick HCO+(3–2) and CS(3–2) linetracers combined with radiative transfer modelling (Perettoet al. 2006) established the presence of large-scale collapsemotions (see also Williams and Garland 2002), converg-ing onto the central, most massive core, C-MM3. A totalmass inflow rate ∼10−3M� yr−1 was inferred onto C-MM3,which is comparable to the accretion rate of the McKee andTan (McKee and Tan 2003) model for high-mass protostars.Mapping of the same region in the low-optical-depth trac-

ers N2H+(1–0) and H13CO+(1–0) revealed a sharp veloc-ity discontinuity (∼2 km s−1 in amplitude) at the locationof C-MM3, tracing of a strong dynamical interaction in thecentral part of NGC 2264-C. This led us (Peretto et al. 2006)to propose a simple model, according to which the Class 0-like protostellar cores C-MM2 and C-MM4 (cf. Fig. 4a)were moving toward each other and dynamically interact-ing with C-MM3. We speculated that a massive, ultra-denseprotostellar core was in the making in the central part of theNGC 2264-C clump as a result of the gravitational merger oftwo or more lower-mass Class 0 objects. High-resolution ob-servations of the inner part of NGC 2264-C with the IRAMPlateau de Bure interferometer (PdBI) allowed us to resolvethe central velocity discontinuity (cf. Fig. 4b). Detailed com-parison of the 30 m/PdBI observations with numerical SPHsimulations of the evolution of a 1000M� Jeans-unstable,elongated clump (Peretto et al. 2007—see Figs. 4c, d) thenconfirmed the view that NGC2264-C is an elongated clumpcollapsing and fragmenting along its long axis. Further-more, our SPH simulations (Peretto et al. 2007) indicatethat NGC 2264-C is observed at a very early stage of globalclump collapse, typically �105 yr after the start of dynami-cal contraction. Surprisingly, a very low level of initial tur-bulence (i.e., an initial turbulent to gravitational energy ratioαturb ∼ 5%) was required in the simulations to get a goodmatch to the observations (cf. Fig. 4). This suggests that theNGC 2264-C cluster-forming clump is structured primar-ily by gravity rather than by interstellar turbulence, which,taken at face value, is at variance with the purely turbu-lent fragmentation scenario of (Padoan and Nordlund 2002).A significant shortcoming of our SPH simulations, however,is that they only produced the observed level of clump frag-mentation when the total mass of dense (>104 cm−3) gas

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Fig. 4 Comparison between millimeter observations of theNGC 2264-C cluster-forming clump (top row) and numerical SPHsimulations of the collapse and fragmentation of a Jeans-unstable,elongated clump (bottom row). a 1.2 mm dust continuum map ob-tained with MAMBO at the IRAM 30 m telescope, to be comparedwith the synthetic column density map shown in (c). b N2H+(101–012) position-velocity diagram taken along the long axis of the

NGC 2264-C clump, resulting from the combination of Plateau deBure and 30 m data (effective synthesized beam ∼5′′). d Syntheticposition-velocity diagram for an optically thin line tracer, convolvedto the same resolution as in (b). The SPH simulations shown in (c) and(d) have an initial turbulent to gravitational energy ratio αturb ∼ 5%and best fit the observations at a time step ∼105 yr after the start oflarge-scale, dynamical clump contraction. (From Peretto et al. 2007.)

in the model was a factor of ∼10 lower than in the actualNGC 2264-C clump. This points to the need for extra sup-port against gravity, not included in the current simulations,such as support provided by magnetic fields or feedbackfrom protostellar outflows.

Overall, our results in NGC 2264-C suggest a pictureof clustered star formation intermediate between the twoextreme scenarios described in Sect. 2 for brown dwarfs(Padoan and Nordlund 2002 vs. Bate et al. 2003), wherebyself-gravity, (protostellar) turbulence, magnetic fields, anddynamical interactions all play some role in shaping thestructure and emergent distribution of stellar masses in aprotocluster.

The great advantage of ALMA in this area is that itwill allow systematic studies of more distant, more mas-sive protoclusters, including in the extreme environment ofthe Galactic center region which is possibly reminiscent ofthe star-forming conditions found in starburst galaxies. Innearby protoclusters such as NGC 2264-C, ALMA will alsomake it possible to probe (ultra-)low-mass star formationand intermediate- to high-mass star formation simultane-

ously. Since clustered star formation appears to be the domi-nant star formation mode in the Milky Way and other galax-ies (e.g. Lada and Lada 2003), such studies are of funda-mental importance.

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5 Conclusions

ALMA will undoubtedly revolutionize our understandingof the early stages of star formation, which likely hold thekey to unlocking the crucial issue of the origin and possi-ble universality of the stellar IMF. In this short paper, wediscussed three specific key problems (proto-brown dwarfs,binary fragmentation, and protocluster dynamics) for whichthe potential of ALMA seems particularly promising. Ofcourse, these were only examples and ALMA will be usedto make progress in many other areas of star formation re-search (see, e.g., papers by van Dishoeck, Cesaroni, Shep-herd, Aikawa, Crutcher, and Tafalla in this volume).

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