On the Mass Segregation of Cores and Stars

This is a repository copy of On the mass segregation of cores and stars. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/158837/ Version: Accepted Version Article: Alcock, H.L. and Parker, R.J. (2019) On the mass segregation of cores and stars. Monthly Notices of the Royal Astronomical Society, 490 (1). pp. 350-358. ISSN 0035-8711 https://doi.org/10.1093/mnras/stz2646 This is a pre-copyedited, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Hayley L Alcock, Richard J Parker, On the mass segregation of cores and stars, Monthly Notices of the Royal Astronomical Society, Volume 490, Issue 1, November 2019, Pages 350–358] is available online at: https://doi.org/10.1093/mnras/stz2646. Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ A MNRAS 000, 1–9 (2019) Preprint 19 September 2019 Compiled using MNRAS L TEX style file v3.0 On the mass segregation of cores and stars Hayley L. Alcock and Richard J. Parker⋆† Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK ABSTRACT Observations of pre-/proto-stellar cores in young star-forming regions show them to be mass segregated, i.e. the most massive cores are centrally concentrated, whereas pre-main sequence stars in the same star-forming regions (and older regions) are not. We test whether this apparent contradiction can be explained by the massive cores fragmenting into stars of much lower mass, thereby washing out any signature of mass segregation in pre-main sequence stars. Whilst our fragmentation model can reproduce the stellar initial mass function, we find that the resultant distribution of pre-main sequence stars is mass segregated to an even higher degree than that of the cores, because massive cores still produce massive stars if the number of fragments is reasonably low (between one and five). We therefore suggest that the reason cores are observed to be mass segregated and stars are not is likely due to dynamical evolution of the stars, which can move significant distances in star-forming regions after their formation. Key words: stars: protostars – formation – open clusters and associations: general 1 INTRODUCTION depending on their evolutionary stage (Andre et al. 2000; Alves et al. 2007; Andréet al. 2010; Arzoumanian et al. Understanding the process through which stars form from 2011). Not all of these observed cores are gravitationally the collapse and fragmentation of giant molecular clouds is bound, so only a subset are likely to form stars (e.g. one of the great unsolved puzzles of modern astrophysics. Caselli & Myers 1995). Studies of the core mass function Whilst it is an interesting topic of study in its own right, (CMF) show that it has a similar form to the stellar ini- the problem is fundamentally important for understanding tial mass function (IMF), but offset by a factor of three the early phases of planet formation (Bonnell et al. 2001; towards higher masses (Alves et al. 2007). This has led to Haisch et al. 2001; Adams et al. 2006), including our own considerable debate about whether the intrinsic properties Solar system (Adams 2010), as well as for understanding of stars are set by the cores, or whether the cores subse- the formation and evolution of galaxies (Kereˇset al. 2009; quently fragment into stars that have very little memory of Krumholz et al. 2018). the physics within the cores (e.g. Padoan & Nordlund 2002; Most stars appear to form in groups where the stel- Hennebelle & Chabrier 2008; Hopkins 2012; Holman et al. lar density exceeds that in the Galactic disc by up to sev- 2013; Offner et al. 2014, and references therein). eral orders of magnitude (Lada & Lada 2003; Bressert et al. 2010; Kruijssen 2012). These groups appear to form with Despite their relatively short lifetimes, pre/proto-stellar arXiv:1909.07982v1 [astro-ph.SR] 17 Sep 2019 structural (e.g. Larson 1995; Cartwright & Whitworth 2004; cores are now observed in significant numbers in nearby star- Gouliermis et al. 2014) and kinematic substructure (e.g. forming regions such that – in addition to measuring their Alfaro & González 2016; Wright et al. 2016). This substruc- mass distributions – we can also quantify the spatial distri- ture governs the long-term evolution of these star-forming butions of these objects, and compare these distributions to regions (Parker et al. 2014; Sills et al. 2018), but it is un- those of young (pre-main sequence) stars. clear how or why stars inherit these properties from their Several authors (Kirk et al. 2016; parent molecular clouds. Alfaro & Román-Zúñiga 2018; Parker 2018; Dib & Henning Advances in submillimetre observations in the past 2018; Plunkett et al. 2018) have shown that the spatial decade have shown that stars form from overdensities in distributions of cores in various star-forming regions tend 1 GMCs, often referred to as pre-stellar or protostellar cores , to be mass segregated, where the most massive cores are more centrally concentrated than would be expected from a ⋆ E-mail: R.Parker@sheffield.ac.uk random distribution. Mass segregation is one of the predic- † Royal Society Dorothy Hodgkin Fellow tions of the competitive accretion theory of star formation 1 In this paper, we will not distinguish between pre-stellar, pro- (Zinnecker 1982; Bonnell et al. 1997; Bonnell & Davies tostellar, or starless cores. 1998), though it is usually assumed that this arises from c 2019 The Authors 2 H. L. Alcock and R. J. Parker accretion of gas onto Jeans-mass fragments in the central We adopt a fractal dimension D = 1.6 for our core dis- regions of a forming star cluster, rather than as a direct tributions, which results in a highly substructured distri- consequence of the spatial distribution of cores. bution. However, we have repeated the analysis with a cen- Interestingly, many star-forming regions do not exhibit trally concentrated spherical distribution with a density pro- mass segregation of pre-main sequence stars (Parker et al. file n ∝ r−2.5 and our overall results are very similar. This is 2011, 2012; Parker & Alves de Oliveira 2017; Gennaro et al. because our method for quantifying mass segregation does 2017; Dib et al. 2018), though the Orion Nebula Clus- not assume, or depend on, anything about the underlying ter is a notable exception (Hillenbrand & Hartmann 1998; spatial distribution. The star-forming regions have radii of Allison et al. 2009). 1 pc, but this does not affect how mass segregation is quanti- Furthermore, Plunkett et al. (2018) find that whilst the fied using the dimensionless ΛMSR ratio (Allison et al. 2009). pre-stellar cores in Serpens South are mass segregated, the pre-main sequence stars are not. This raises the interesting question of why mass segregation appears in the earli- 2.2 Masses of cores and fragments est phases of star-formation, but appears to dissipate later Following the observations of Alves et al. (2007), we assume on (see also Elmegreen et al. 2014). There are four possible that the core mass function is the same as the stellar initial explanations for this. First, observations of cores may be bi- mass function, but shifted to higher masses by a factor of ∼ 3. ased to observing mass segregation, whereas observations of We therefore sample from the Maschberger (2013) Initial stars are not. Second, the cores that are forming today could Mass Function, which has a probability distribution of the have a different spatial distribution to those that formed the form pre-main sequence stars we observe today. Third, dynami- −α 1−α −β cal evolution of the pre-main sequence stars may have moved m m p(m) ∝ 1 + , (1) them away from their birth sites and erased any signature of µ ! µ ! mass segregation. Fourth, the fragmentation process within but we adopt µ = 0.6 M as the average core mass, and cores could produce stars randomly with no preferred spatial ⊙ sample masses from this function in the range 0.3 – 150 M . distribution. In this process, we might expect that massive ⊙ As the shape of this distribution is assumed to be the same as cores would fragment into many lower-mass objects, thus the stellar initial mass function, we adopt α = 2.3 (Salpeter erasing the mass segregation signal of the cores. 1955) and β = 1.4. In this paper, we investigate the fourth scenario, We sample N = 300 from Equation 1 and we then i.e. that the fragmentation of the pre-/proto-stellar cores cores allow the cores to fragment according to the following erases the primordial mass segregation of these objects. (We prescriptions: will also discuss the other scenarios in light of our results later in the paper.) The paper is organised as follows. In Random fragmentation: the cores are randomly fragmented Section 2 we describe the set-up and assumptions behind a into between one and five equal-mass pieces.

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