Star Formation

TL;DR

Star Formation chapter analyzes how stars form from giant molecular clouds by balancing gravity, turbulence, radiation, and magnetic fields. It connects turbulence-regulated frameworks, where the density PDF (log-normal) and the SFR per free-fall time $\mathrm{SFR}_{ff}$ depend on parameters such as the virial parameter $\alpha_{vir}$, Mach number $\mathcal{M}$, driving parameter $b$, and plasma beta $\beta$, to the observed initial mass function (IMF) of stars. It provides an observational perspective on protostellar envelopes, disks, jets/outflows, and accretion, focusing on low-mass stars and the observational classification into Class 0–III. It also discusses multiplicity and fragmentation—core, disk, and dynamical evolution—that shape the early stellar population and potential planet-forming environments, highlighting the link between theory, simulations, and infrared/submillimeter observations at redshift $z\simeq 0$.

Abstract

In this chapter, we will cover how stars form from the stellar nurseries that are giant molecular clouds. We will first review the physical processes that compete to regulate star formation. We then review star formation in turbulent, magnetized molecular clouds and the associated statistics giving rise to the star formation rate and the initial mass function of stars. We then present the protostellar stages in detail from an observational perspective. We will primarily discuss low-mass ($<1.5\msun$) stars. Finally, we examine how multiplicity complicates the single-star formation picture. This chapter will focus on star formation at redshift~0

Figures (8)

• Figure 1: Magnetic field morphology, side-on (left) and top-down (middle) in a simulation of protostar formation in an accretion disc. A three-dimensional rendering of this morphology is shown in the right panel. Adapted from figures 2, 5, and 11 of kuruwita_binary_2017.
• Figure 2: Schematic view of a young star accreting from a disk through the stellar magnetosphere. Jets are launched from the inner disk, while disk winds are launched at larger radii. Both mechanisms remove angular momentum, allowing the gas to move inwards through the disk. The protostellar magnetic field threads through the inner disk, allowing ionized material to be funneled along these lines onto the protostar. Figure 1 from hartmann_accretion_2016, used with permission.
• Figure 3: Top panels: gas column density in star-formation simulations with purely compressive (curl-free) turbulence driving (left) and purely solenoidal (divergence-free) turbulence driving (right), as defined in Sec. \ref{['sec:pdf']}. Young stars are shown as circles, forming in dense gas, primarily at the intersection of filamentary structures SchneiderEtAl2012. Bottom panel: comparison of various observational IMFs together with the IMF obtained in several sets of simulations using the driving modes of turbulence shown in the top panels: compressive driving (blue histogram) and solenoidal driving (red histogram). The curves are the system IMF models based on observational surveys by Salpeter1955 (dash-dotted), Chabrier2005 (short-dotted), ParravanoMcKeeHollenbach2011 (long-dotted), DaRioEtAl2012 (solid), KroupaEtAl2013 for brown dwarfs (long-dashed) and stars (short-dashed), and DamianEtAl2021 (dash-double-dotted). Adapted from figures 2 and 6 of MathewFederrathSeta2023.
• Figure 4: An illustrated overview of protostellar evolutionary classes. In the collapse stage, the infall motions create a dense central region in the prestellar core. Class 0/I stage (protostellar stage) is associated with powerful outflows and jets accompanied by the most vigorous accretion; this is also the stage for protoplanetary disk forms. In the Class II stage, the disk is cold and quiescent, associated with disk winds, this is also where embedded planets are detected. In Class III, the disk disperses and a residual dusty debris disk is present.
• Figure 5: Schematic of protostellar physical components with molecules and their associated emission/absorption lines that can be used to probe disk structure, chemistry, and dynamics through sub-millimeter spectroscopy. From Tychoniec.vanDishoeck.ea2021, Reproduced with permission from Astronomy & Astrophysics, © ESO.