🔭 astrophysics

What is the reason for the lack of cool evolved stars near the center of the Galaxy?

This paper proposes that repeated passages of red giants through past jets from Sgr A* can strip their envelopes and heat their surfaces, potentially transforming cool M-type giants into hot A-type sources and thereby explaining the observed deficit of cool evolved stars near the Galactic center.

Original authors: Petr Kurfürst, Michal Zajaček, Norbert Werner, Jiří Krtička, Matúš Labaj

Published 2026-08-27

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CC0 1.0

Original authors: Petr Kurfürst, Michal Zajaček, Norbert Werner, Jiří Krtička, Matúš Labaj

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the heart of our Milky Way galaxy lies a region of intense gravity and crowded stars, centered on a supermassive black hole known as Sgr A*. This central area, spanning just a few light-years, is home to a dense cluster of stars. Astronomers have long mapped this neighborhood and found a peculiar imbalance. While faint, older stars are scattered throughout the core, the bright, cool, and bloated red giants that should be common there are strangely missing. These giants are stars that have exhausted their core fuel and expanded into massive, puffy spheres, yet they seem to vanish as one looks closer to the black hole. Scientists have proposed several reasons for this disappearance, suggesting that the extreme gravity might rip them apart, or that collisions with other stars or disks of gas could destroy them. However, a new study explores a different possibility: that these stars are not destroyed, but rather transformed by a powerful, invisible wind from the black hole's past.

Researchers Petr Kurfüst and his team at Masaryk University in the Czech Republic investigated whether repeated encounters with a jet of high-speed gas, shot out by the black hole in the past, could alter these stars. They focused on red giants, which have large, loosely held outer layers of gas. Using powerful computer simulations, the team modeled a typical red giant, about the mass of our Sun but with a radius one hundred times larger, as it traveled through a jet of gas moving at a significant fraction of the speed of light. They watched what happened when this star crossed the jet's path, not just once, but up to ten times over a period of one hundred thousand years. The simulations showed that each time the star entered the jet, the collision created a shock wave that stripped away the star's outer atmosphere and heated it up.

The results of these simulations reveal a process of gradual change rather than sudden destruction. As the star repeatedly plowed through the jet, the friction and pressure shaved off small amounts of its outer gas, creating a long, trailing tail of debris behind the star. While the total amount of mass lost over this long period was relatively small—comparable to what the star would lose naturally through its own slow wind—the effect on the star's appearance was dramatic. The constant bombardment of the jet heated the star's surface layers significantly. In the models, the surface temperature of the star rose from a cool three thousand six hundred degrees to a much hotter eight thousand five hundred degrees. This shift in temperature is enough to change the star's color and spectral classification from a cool, red giant to a hot, blue-white star.

This transformation offers a compelling explanation for the missing giants. The stars may still be there, but they no longer look like the cool, red giants astronomers expect to find. Instead, the jet's repeated strikes have cooked their surfaces, making them appear as the hot, young-looking stars that are actually observed in excess near the galactic center. The study suggests that this mechanism, combined with other factors like collisions, could account for the observed gap in the population of cool giants. The researchers note that while their simulations show this heating and stripping effect clearly, the exact amount of mass lost depends on the strength of the jet and the specific path of the star. They conclude that the jet acts as a cosmic oven, altering the visible identity of these ancient stars without necessarily destroying them entirely, leaving behind a record of the black hole's past activity in the very stars that orbit it.

Technical Summary: Traversing the Galactic Center in Space and Time

Problem Statement
The central parsec of the Milky Way exhibits a well-documented anomaly: while faint late-type stars follow a cusp-like distribution around the supermassive black hole (Sgr A*), bright red giants (RGs) and supergiants show a pronounced central depletion. This "missing cool giants" problem remains unresolved. Proposed mechanisms include tidal stripping, star–disk collisions, star–star collisions, and interactions with a nuclear jet. This paper investigates the hypothesis that repeated passages of red giants through a past jet from Sgr A* can modify their envelopes and apparent spectral types, potentially explaining the observed deficit.

Methodology
The authors employ three-dimensional hydrodynamical simulations to model the interaction between a red giant and a relativistic jet.

Stellar Model: A $1 M_\odot$ red giant with an initial radius of $100 R_\odot$ is modeled. The structure is derived from a one-dimensional MESA model mapped to a polytropic envelope (index $n \approx 3.68$ ) with a softened central gravitational potential representing the dense core.
Numerical Setup: Simulations are performed using the CASTRO code in a cubic domain ( $2 \text{ au}$ side) with adaptive mesh refinement (AMR). The star is fixed at the domain center, while jet motion and orbital dynamics are imposed via time-dependent inflow boundaries.
Jet Parameters: The study explores jet kinetic luminosities ( $L_j$ ) of $10^{42}$ , $10^{44}$ , and $10^{48} \text{ erg s}^{-1}$ with velocities ( $v_j$ ) of $0.33c$ and $0.66c$ . The primary models place the star at $r = 10^{-3} \text{ pc}$ , simulating up to ten jet crossings (five orbital periods). A secondary case at $10^{-2} \text{ pc}$ is followed for two crossings in two dimensions.
Physical Approximations: The simulations solve compressible hydrodynamic equations with self-gravity and an ideal-gas equation of state. Radiative cooling, thermal conduction, magnetic fields, and stellar rotation are excluded; thus, temperature evolution represents the adiabatic hydrodynamic response.
Stabilization: To prevent spurious motions from contaminating weakly bound surface layers during the mapping of 1D models to a Cartesian mesh, a velocity-damping prescription and a sponge term are applied only to the innermost region ( $r < 0.05 R_\star$ ).

Key Results

Hydrodynamical Evolution: Upon jet entry, the supersonic relative motion generates a bow shock in the jet material and a secondary shock propagating into the stellar envelope. The contact discontinuity wraps around the upstream hemisphere, creating an asymmetric downstream tail deflected opposite to the orbital motion. The star retains thermal and dynamical memory of previous passages because the interval between crossings ( $\sim 240$ days) is significantly shorter than the stellar Kelvin–Helmholtz time ( $\sim 190$ years).
Mass Ablation:
- For $L_j \le 10^{44} \text{ erg s}^{-1}$ , the cumulative ablated mass follows a scaling law of $\Delta M \propto t^{1/2}$ . Over a $10^5$ -year active phase, the total mass loss reaches approximately $10^{-4} M_\odot$ .
- At the highest luminosity ( $10^{48} \text{ erg s}^{-1}$ ), the mass-loss rate flattens. Dense jet gas temporarily accumulates within the original stellar volume, and the heated surface develops increased thermal pressure that resists further stripping. Consequently, the extrapolated loss is only $\sim 10^{-5} M_\odot$ , indicating a non-monotonic relationship between jet power and ablation efficiency.
- The total mass removed is insufficient to destroy the star but is comparable to mass lost via ordinary stellar winds over the same interval.
Surface Heating and Spectral Transformation: The most significant finding is the rapid heating of the stellar surface. Repeated exposure to shocks raises the characteristic temperature of the outer layers from $\sim 3600 \text{ K}$ to $\sim 8500 \text{ K}$ within the first ten passages. This thermal shift can alter the apparent spectral type from M-type (cool giant) to A-type (hot star).

Significance and Claims
The paper posits that jet-induced ablation contributes to the apparent depletion of cool giants and the excess of hot stars in galactic nuclei. While the total mass loss is modest and does not destroy the star, the continuous modification of the thermodynamic state and observable appearance of the outer atmosphere is sufficient to render an evolved star unidentifiable as a cool giant.

The authors conclude that this mechanism offers a plausible explanation for the missing cool giants near Sgr A*, operating concurrently with other mechanisms like tidal stripping and collisions. Furthermore, this process broadens the concept of AGN feedback, suggesting that nuclear jets can fundamentally alter the observable properties of stellar populations. The paper notes that after the jet switches off, the heated surface would cool on a Kelvin–Helmholtz timescale of roughly 10–100 years. Future work is identified as necessary to directly calculate gravitationally bound mass and include radiative cooling, magnetic fields, and rotation.

Technical Summary: Traversing the Galactic Center in Space and Time

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