The formation of massive stars

Stellar physics is a broad field that touches on a range of phenomena from magnetic fields to radiative processes and thermonuclear fusion to plasmas. Stars form through the gravitational collapse of cold, dense, dusty protostellar cores, themselves embedded in thick molecular clouds or filaments. Massive stars, defined as those with a mass greater than 8 solar masses, are of key interest in star formation. Although they are extremely rare, comprising less than 1% of the total stellar population, they make their presence known by dominating the surrounding interstellar medium (ISM) with their powerful stellar winds as well as shocks from their eventual supernovae. Their formation is known to be impeded by several feedback mechanisms, including outflows, radiation pressure and magnetic fields. Today’s paper uses a series of radiative magnetohydrodynamic simulations to understand the overall impact that these combined mechanisms have on star formation.

The fact that massive stars are so rare is reflective of a more general problem with star formation: its inefficiency. Estimates of star formation efficiencies are as low as 33%. As massive stars begin to form, they launch powerful molecular outflows from their poles. These jets can interact with the surrounding molecular cloud and eject large quantities of material. This, in combination with other feedback mechanisms, limits the star’s ability to accrete material, ultimately limiting its final mass.

Knowing the upper limit of just how massive a star can be is incredibly valuable, for it allows us to set the upper boundary of the initial mass function. This function models the initial distribution of stellar masses for a given population of stars, and it is impossible to simulate the evolution of a stellar population without one. This is where massive stars are important, for they are the dominant source of radiative feedback and energy injection into the ISM through supernovae.

So, to help determine these upper mass limits, we must simulate the processes that inhibit star formation in as much detail as possible. The authors ran three main simulations: TurbRad (radiative feedback only), TurbRad+OF (adds collimated outflows), and TurbRad+OFB (adds magnetic fields).

After the stellar mass of the protostellar core exceeds 30 solar masses, we see several pressure-dominated bubbles expanding away from the star (this is most noticeable in the middle row TurbRad+OF simulation). This process is known as the “flashlight effect”, where thick material is beamed away from the poles, causing low-density bubbles to expand outwards.

Density plots for the authors’ three simulations, with the most massive star
shown in the center of each panel. CREDITS: Rosen & Krumholz 2020

Over time, strong entrained outflows begin to break through the protostellar core and eject large quantities of material. The outflows become steadier and more directed over time. Although the protostellar core is initially highly turbulent, as it accretes material its rotational axis stabilises over time.

One of the key results of these simulations is that the momentum feedback from these outflows is the dominant feedback mechanism (compared to radiation pressure) and helps to eject significant fractions of material, reducing the star formation efficiency. Outflows also help to act as conduit through which radiation can escape, weakening the feedback effects from radiation pressure.

Magnetic fields are known to affect star formation. Overall, the simulations that contained outflows resulted in lower efficiencies. So in order to reconcile observations that place overall star formation efficiency at around 33%, this work shows that it is necessary to account for the effects of outflows.

Source: “The Formation of Massive Stars” AASNOVA, 14 July 2020

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