Were the First Stars Actually Hypermassive?

Paper title: Protostellar Feedback Halts the Growth of the First Stars in the Universe
Authors: Takashi Hosokawa, Kazuyuki Omukai, Naoki Yoshida, Harold W. Yorke
Author’s Affiliation: Jet Propulsion Laboratory; Department of Physics, Kyoto University

The problem of understanding the formation and evolution of the first to form stars in the universe lies at the intersection of many fields of astrophysics. Since the first stars could only have formed once their host dark matter halos had begun to collapse, one must understand the formation of these stars in a cosmological context, tracking gas from extremely low intergalactic densities (~10-27 g cm-3), to extremely high, stellar densities (~1 g cm-3). Since all the metal content of the universe had not yet been synthesized in the cores of stars, the gas that collapsed to form the first stars would have been metal-free and thus possessed very different thermal properties compared to the interstellar and intergalactic gas in the local universe that can cool via metal line emission.

Many studies of first star formation have focused on the cosmological piece of the puzzle: starting with a simulation of cold dark matter and gas in a ΛCDM cosmology, they look for the first ~106 solar mass dark matter halo to collapse, and then follow the collapse of first the dark matter and then the gas to very high densities. Due to the extremely large dynamic range in these simulations, it becomes prohobitively expensive to reach stellar densities and impossible to directly model the evolution of the first star. For reference, a recent simulation of the formation of the first stars followed the collapse to densities just above 10-8 g cm-3, far below typical stellar densities. The primary finding of these sorts of simulations is that around a thousand solar masses of material should have collapsed onto the first star, implying that that the first stars were probably hypermassive behemoths totally unlike any stars that have formed since.

The study we will be be discussing for today’s astrobite takes a different approach to simulating the first star. After pointing out that conservation of angular momentum prohibits the direct collapse of a primordial dense core into a first star, the authors suggest a more likely initial configuration would be a relatively low-mass first star surrounded by a massive accretion disk. While the entire collapsing cloud would eventually fall onto the protostar, it will spend some time being processed through the accretion disk. If some process could disrupt the disk and halt collapse, then the first stars need not be hypermassive after all. Several physical effects could do the trick, but in this paper, the authors focus on ionizing radiation from the still-accreting protostar as the likely culprit.

The gas in the collapsing protostellar core when the central star is (a) 20 solar masses, (b) 25 solar masses, (c) 35 solar masses, and (d) 42 solar masses. Radiative feedback acts on a collapsing protostellar core, driving an outflow, ionizing gas, and halting accretion onto the protostar.

This simulations discussed in this paper take the results of a cosmological SPH simulation of the formation of a first star as the initial conditions. Once the gas in the cosmological simulation reached densities of about 10-15 g cm-3 the simulation volume was extracted and averaged in the azimuthal direction relative to the rotation axis of the collapsing core, producing the initial conditions for a 2D simulation of the protostellar collapse and subsequent evolution. This 2D approach allows the authors to follow the protostellar accretion process in detail, however, any nonaxisymmetric effects will be missed. The authors follow the collapse of the gas in their 2D simulation to densities of 10-9 g cm-3 and replace the gas in the central collapsing grid cell with a ‘sink’ cell that removes matter off of the computational grid. The sink cell is coupled with a 1D zero-metallicity protostellar evolution model which accretes mass at a rate prescribed by the accretion rate onto the sink cell. The protostellar evolution model predicts the ionizing and bolometric luminosity of the central protostar and feeds back into the hydrodynamical simulation as a source of radiation.

Radiative feedback shuts off accretion once the star reaches approximately 42 solar masses.

The evolution of the collapsing protostellar core is depicted graphically above. This is a bit of a busy figure, so let me walk you through it. The four quadrants correspond to four different times in the simulation, when the star is 20 solar masses (top left), 25 solar masses (top right), 35 solar masses (bottom left), and 42 solar masses (bottom right). In each quadrant, the left half of the image depicts the gas temperature. Cold, molecular material is in purple and hot ionized material is in green. The other half of the image depicts the gas density. High density molecular gas is in dark blue and low density ionized gas is white or light blue. Once the protostar has accreted approximately 20 solar masses of material, its ionizing luminosity becomes so great that it begins to ionize the relatively low-density material above and below the disk. This two-lobed ionized bubble very quickly breaks out of the collapsing core and expands to fill the working angle above and below the protostar. The ionized material is accelerated to high velocities, tens of kilometers per second, more than sufficient to escape the collapsing mini dark matter halo (with a mass of ~106 solar masses). As the ionized region expands, a large fraction of the accretion disk becomes exposed to direct irradiation by the central protostar, and the accretion disk begins to photoevaporate. Shortly after the last frame, the disk dissipates, and accretion halts. The accretion history of two simulations, one with radiative feedback, and one without radiative feedback is depicted at right. We see the two simulations begin to look apprreciably different once the ionized bubble starts to appear.

This paper may shed light on the lack of observational evidence of the abundance patterns expected from pair instability supernovae, the likely end result of hypermassive stellar evolution.  This finding is also consistent with high resolution 3D calculations of first star formation, which indicate that fragmentation may prevent the formation of hypermassive first stars.

About Nathan Goldbaum

Nathan is a third year graduate student in Astrophysics at UCSC. His interests include the local ISM, molecular clouds, and the role of star formation in galactic evolution.

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