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Winds in Star Clusters Drive Kolmogorov Turbulence Draft version June 29, 2020 Typeset using LATEX twocolumn style in AASTeX61 WINDS IN STAR CLUSTERS DRIVE KOLMOGOROV TURBULENCE Monica Gallegos-Garcia,1, 2 Blakesley Burkhart,3, 4 Anna Rosen,5, 6, 7 Jill P. Naiman,8 and Enrico Ramirez-Ruiz9, 10 1Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA 2Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),1800 Sherman, Evanston, IL 60201, USA 3Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA 4Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd, Piscataway, NJ 08854, USA 5Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 6Einstein Fellow 7ITC Fellow 8School of Information Sciences, University of Illinois, Urbana-Champaign, IL, 61820 9Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA 10Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark ABSTRACT Intermediate and massive stars drive fast and powerful isotropic winds that interact with the winds of nearby stars in star clusters and the surrounding interstellar medium (ISM). Wind-ISM collisions generate astrospheres around these stars that contain hot T 107 K gas that adiabatically expands. As individual bubbles expand and collide they ∼ become unstable, potentially driving turbulence in star clusters. In this paper we use hydrodynamic simulations to model a densely populated young star cluster within a homogeneous cloud to study stellar wind collisions with the surrounding ISM. We model a mass-segregated cluster of 20 B-type young main sequence stars with masses ranging from 3{17 M . We evolve the winds for 11 kyrs and show that wind-ISM collisions and over-lapping wind-blown ∼ bubbles around B-stars mixes the hot gas and ISM material generating Kolmogorov-like turbulence on small scales early in its evolution. We discuss how turbulence driven by stellar winds may impact the subsequent generation of star formation in the cluster. Keywords: ISM: Turbulence { stars: winds arXiv:2006.14626v1 [astro-ph.GA] 25 Jun 2020 Corresponding author: Monica Gallegos-Garcia [email protected] 2 1. INTRODUCTION spectra of density and momentum but do impact the Feedback from stellar winds play an important role in Fourier velocity spectrum. They conclude that stel- shaping the structure of the interstellar medium (ISM, lar winds with high mass-loss rates can contribute to Krumholz 2014). Massive stars produce powerful winds turbulence in molecular clouds. since the mass loss rates and wind velocities are de- A natural extension in studying how wind-blown bub- termined by the star's radiation output (Castor et al. bles interact with the ISM and contribute to large-scale 1975b). Intermediate- and low-mass stars also con- turbulence in molecular clouds is to study how these tribute to producing ionized bubbles in the ISM, i.e. bubbles interact with one another in clustered environ- so-called astrospheres (Wood 2004; Mackey et al. 2016), ments. Expanding shells have been observed around which are a potential source of local ISM turbulence small star clusters like the ρ-Oph cluster, which con- (Burkhart & Loeb 2017), cosmic rays (del Valle et al. tains five B-stars located in the Ophiuchus molecular 2015), dust processing (Katushkina et al. 2017), and can cloud (Lada & Lada 2003; Chen et al. 2020, in prep). In be used to identify runaway stars (Peri et al. 2012). this scenario, fast winds ejected from stars collide with In regards to massive stars, early theoretical models winds from neighboring stars causing the bubbles to by Castor et al.(1975a) and Weaver et al.(1977) demon- overlap and form a collective \cluster wind" (Cant´oet al. strated that the interaction between fast, isotropic stel- 2000). The resulting \super-bubble," which is filled with lar winds and the surrounding ISM produces a large cav- hot and diffuse gas, eventually expands beyond the star ity or \bubble" surrounded by a thin shell of dense, cold cluster itself (Bruhweiler et al. 1980; Stevens & Hartwell material. In agreement with these models, parsec-scale 2003; Rodr´ıguez-Gonz´alezet al. 2007, 2008). Similar to circular cavities are regularly found in regions of high- the single wind-blown shell, where Rayleigh-Taylor and mass star formation (Churchwell et al. 2006, 2007; Beau- Kelvin-Helmholtz instabilities lead to turbulent mixing mont & Williams 2010; Deharveng et al. 2010). Such (McKee et al. 1984; Nakamura et al. 2006), wind-wind features likely contribute to parsec-scale turbulence in collisions in a multiple star system may also lead to in- these environments and drive density fluctuations that stabilities within the cluster wind and produce small- influence subsequent generations of stars (Offner & Arce scale turbulence within the ISM. This turbulent motion 2015; Burkhart 2018). may act in the same way as the single star case, intro- Although it was previously thought that only winds ducing energy and turbulence into its environment as from O or early B-type stars could drive bubbles in the super-bubble grows. molecular clouds, numerous shells have been found in Motivated by this, in this Letter we perform hydrody- low- and intermediate-mass star forming regions (Arce namic simulations to model the collective cluster wind et al. 2011; Li et al. 2015). These studies concluded from a dense star cluster of young B-type stars em- that these bubbles are likely driven by stellar winds bedded in a uniform molecular cloud to determine how from intermediate-mass stars and the energetics of these wind-wind collisions and overlapping bubbles can drive bubbles may help sustain turbulence in the Perseus and turbulence in star clusters. This is in contrast to Cant´o Taurus star-forming regions, which may explain the ob- et al.(2000) in which only a single mass of star was served density and velocity power spectrum in Perseus used in the cluster simulations. Offner & Arce(2015) (Pingel et al. 2018; Padoan et al. 2006). use an isothermal equation of state and therefore only To study the development and expansion of wind- follow the momentum injection by winds of young inter- blown bubbles around intermediate-mass stars and mediate mass stars. Here we use an adiabatic equation their contribution to sustaining turbulence in molecular of state and calculate the energy losses using a realistic clouds, Offner & Arce(2015) performed isothermal mag- cooling function, which allows us to fully capture the netohydrodynamic (MHD) simulations that modeled kinetic energy and momentum injection from the fast stellar wind momentum feedback from intermediate- stellar winds and to follow the expansion of the result- mass main sequence stars embedded in a turbulent ing super bubble. We investigate on what time scales molecular cloud. Similar to Arce et al.(2011), they turbulence can be effectively generated within a cluster find that for a random distribution of stars whose by these intermediate- and high-mass stars. individual winds do not interact, a mass-loss rate of This Letter is organized as follows: in Section2 we 7 1 1 describe the stellar wind properties, the initial condi- 10− M yr− and a wind velocity of 200 km s− is ≥ required to drive the shells observed in a Perseus-like tions, and the corresponding physics of our simulation. molecular cloud. Their study also showed that the In Section 3.1, we describe the bulk properties of our stellar winds that produce and drive the expansion of simulations and show how overlapping wind bubbles can these shells do not produce clear features in the Fourier drive turbulence in young star clusters. In Section 3.2 we 3 show the evolution of the density-weighted power spec- from Dalgarno & McCray(1972) for 10 K T 104 K. ≤ ≤ trum and PDFs of physical properties of interest such Metallicity is fixed at solar. as the temperature, density and Mach numbers. In Sec- The star cluster modeled is embedded in a non- tion 3.3 we show the cooling efficiently of the collective turbulent background so that we can self-consistently cluster wind. Finally, in Section4 we summarize our follow the driving of turbulence generated only by winds. findings and discuss their implications. While our initial condition of a uniform background den- sity is certainly idealized, it is likely that turbulence is significantly damped on the scales of a few tenths of a 2. METHODS parsec due to various viscous and MHD damping mech- anisms (Li et al. 2008; Burkhart et al. 2015a; Xu et al. We assume a star cluster mass of 400 M with indi- 2016; Qian et al. 2018). As we are interested to study vidual star masses chosen by stochastically sampling the the direct impact of turbulence produced by the star Kroupa initial mass function (Kroupa 2001). We only cluster we restrict ourselves to a case in which the am- model the 20 most massive stars in the cluster (masses bient medium is uniform. The ambient medium has a ranging between 3:2 { 17 M ) because the energy and 3 3 density of namb = 10 cm− and a cloud temperature momentum injected by their winds dominate over the of 10 K. The box size is (1.24 pc)3 with a finest spatial total momentum and energy of the entire stellar popu- resolution of 120AU. For reference, the shell radii in lation in the cluster (Rosen et al. 2014). The 20 stars in ∼ Perseus identified by Arce et al.(2011) range within 0.14 our cluster are mass-segregated, with a cluster radius of 4 3 { 2.79 pc. They also use a cloud density 10 cm− to r = 0:14 pc and a stellar density profile resembling the ∼ calculate mass-loss rates of the stars embedded within Orion Nebula Cluster (Da Rio et al.
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