SISSI Project: Supernovae in a Stratified ISM

• SISSI Project is a simulation suite that models supernova remnants evolving in a stratified, shearing interstellar medium shaped by galactic rotation, gravity, and turbulence.
• It employs a multiscale numerical framework to capture remnant geometry and dynamics with detailed comparisons to analytic models and Local Bubble observations.
• The project reveals that realistic ISM structures rapidly deform SNRs, making geometric diagnostics a powerful tool for inferring galactic dynamics and supernova history.

The SISSI Project—“Supernovae In a Stratified, Shearing Interstellar medium”—is a simulation program devoted to the evolution of supernova remnants (SNRs) in a galactic interstellar medium shaped by galactic rotation, gravity, turbulence, and vertical stratification, rather than in an idealized homogeneous background. Its central purpose is to quantify how realistic Galactic environments alter remnant morphology, dynamics, and interpretation, with particular emphasis on geometric diagnostics and on comparison with nearby structures such as the Local Bubble (Romano et al., 17 Mar 2025, Romano, 4 Sep 2025).

1. Scientific rationale and project scope

SISSI was introduced in response to a mismatch between classical SNR theory and the observational reality of evolved Galactic cavities. Standard analytic and numerical treatments are highly successful for spherical blast waves in uniform media, but they are poorly matched to remnants and superbubbles embedded in a live, anisotropic ISM. The first SISSI paper states this explicitly by targeting SNRs evolving in a complex interstellar medium structured under the influence of galactic rotation, gravity and turbulence, and by framing geometry as a primary observable rather than a secondary by-product (Romano et al., 17 Mar 2025).

This framing matters because nearby cavities are now observed with genuine three-dimensional information. In the SISSI interpretation, structures such as the Local Bubble are not merely energy-driven shells whose size can be read off from one-zone models; they are objects whose axis ratios, orientations, and deviations from spherical symmetry encode the effects of stratification, shear, low-density channels, and ambient substructure. The second SISSI paper extends that program from general remnant morphology to a specific local application, using the Local Bubble as a constraint on recent supernova activity and on the recent evolution of the solar-neighborhood ISM (Romano, 4 Sep 2025).

A plausible implication is that SISSI is best understood not only as a simulation suite, but also as a methodological program: it treats remnant geometry as a way to infer the physical processes shaping the Galactic ISM. That emphasis distinguishes it from work focused only on global energy or momentum budgets.

2. Numerical framework and simulation suite

SISSI uses a deliberately multiscale design. The parent environment is an isolated Milky-Way-like disk galaxy from the AVALON project, evolved with RAMSES in a 48 kpc cubic box with outflow boundaries and a $256^3$ coarse grid. The galaxy-scale ISM reaches a finest resolution of approximately $11.7\,\mathrm{pc}$, while the zoomed SNR calculations reach approximately $0.18\,\mathrm{pc}$ near the explosion site. The galaxy includes gas self-gravity, a static axisymmetric background potential, radiative cooling and heating, and star formation under the conditions $n_{\rm H}>100\,\mathrm{cm^{-3}}$ and $T<150\,\mathrm{K}$ with $\epsilon_{\rm ff}=1\%$ (Romano et al., 17 Mar 2025).

The ISM is first relaxed for less than $500\,\mathrm{Myr}$, during which star particles deposit $2\times 10^{52}\,\mathrm{erg}$ and $200\,M_\odot$ after $8\,\mathrm{Myr}$, with cooling disabled for approximately $11.7\,\mathrm{pc}$0 in affected cells. SISSI then selects 30 future explosion sites at galactocentric radii $11.7\,\mathrm{pc}$1, close to the midplane and roughly equally spaced in azimuth. Around each site, cells within

$11.7\,\mathrm{pc}$2

are refined up to $11.7\,\mathrm{pc}$3, corresponding to $11.7\,\mathrm{pc}$4. The local region is then relaxed for less than $11.7\,\mathrm{pc}$5 before the supernova is injected (Romano et al., 17 Mar 2025).

The suite contains control runs without new supernovae and three explosion prescriptions that define the main SISSI comparison set:

Model	$11.7\,\mathrm{pc}$6	Interpretation
N1	$11.7\,\mathrm{pc}$7	single-SN remnant
N10	$11.7\,\mathrm{pc}$8	10 SNe exploding all at once
N1x10	$11.7\,\mathrm{pc}$9	10 SNe exploding sequentially

Each SN injects $0.18\,\mathrm{pc}$0 of thermal energy and $0.18\,\mathrm{pc}$1 of ejecta inside a sphere of radius $0.18\,\mathrm{pc}$2. Cooling remains active during the zoom-in SNR runs. Refinement is also tied to the shell-formation scale through

$0.18\,\mathrm{pc}$3

and the Sedov-Taylor phase is resolved up to

$0.18\,\mathrm{pc}$4

The first paper is also explicit about what is absent: it does not include magnetic fields, cosmic rays, thermal conduction, or non-equilibrium chemistry (Romano et al., 17 Mar 2025).

3. Geometric formalism and analytic baselines

The first SISSI paper is fundamentally a geometry paper, and it defines a formal shape analysis for each remnant. SNR material is identified with passive ejecta scalars, and the geometry is summarized by the volume-weighted shape tensor

$0.18\,\mathrm{pc}$5

If the remnant is approximated as an ellipsoid, the three equivalent ellipsoidal radii follow from the eigenvalues $0.18\,\mathrm{pc}$6 as

$0.18\,\mathrm{pc}$7

with minor, intermediate, and major axes denoted by $0.18\,\mathrm{pc}$8, $0.18\,\mathrm{pc}$9, and $n_{\rm H}>100\,\mathrm{cm^{-3}}$0, and an effective radius

$n_{\rm H}>100\,\mathrm{cm^{-3}}$1

Orientation is tracked by a pitch angle $n_{\rm H}>100\,\mathrm{cm^{-3}}$2 relative to Galactic rotation and by $n_{\rm H}>100\,\mathrm{cm^{-3}}$3, where $n_{\rm H}>100\,\mathrm{cm^{-3}}$4 is in-plane and $n_{\rm H}>100\,\mathrm{cm^{-3}}$5 is perpendicular to the disk (Romano et al., 17 Mar 2025).

SISSI compares these geometric diagnostics to analytic expectations for isolated remnants in uniform media. The paper explicitly quotes the standard shell-formation and snowplow scalings, including

$n_{\rm H}>100\,\mathrm{cm^{-3}}$6

$n_{\rm H}>100\,\mathrm{cm^{-3}}$7

and

$n_{\rm H}>100\,\mathrm{cm^{-3}}$8

For the sequential-supernova case, it also uses the momentum-driven superbubble scaling

$n_{\rm H}>100\,\mathrm{cm^{-3}}$9

To isolate the role of differential rotation, SISSI introduces a simple shearing-sphere model, whose characteristic deformation time is

$T<150\,\mathrm{K}$0

This model becomes the reference against which the simulated loss of sphericity is measured (Romano et al., 17 Mar 2025).

The methodological point is important: SISSI does not use geometry descriptively. It treats geometric quantities as dynamical observables that can be compared quantitatively with both analytic theory and observational reconstructions.

4. Principal findings on remnant evolution and morphology

One of the clearest SISSI results is that young remnants remain well described by standard theory even in a complex ISM. The first paper reports that shell-formation time, pressure-driven snowplow duration, characteristic radii, and momentum per SN all agree well with isolated-uniform-medium expectations at early stages, especially for moderate and high ambient densities. This means the realistic galactic environment does not erase the validity of classical SNR theory; rather, it limits where that theory remains applicable (Romano et al., 17 Mar 2025).

The main departure occurs at later times. SISSI finds that remnants begin to lose spherical symmetry after roughly a percent of the local orbital timescale, and often even earlier. The paper defines the deformation time through the condition

$T<150\,\mathrm{K}$1

and reports that most remnants satisfy it within $T<150\,\mathrm{K}$2 of an orbit, with some as early as $T<150\,\mathrm{K}$3 of an orbit. By $T<150\,\mathrm{K}$4, only 4 of the 30 simulated remnants remain spherical. Most have become distinctly prolate or oblate, with dense-environment cases reaching approximately

$T<150\,\mathrm{K}$5

In the N1 sample, single-SN remnants tend toward lower $T<150\,\mathrm{K}$6, whereas the sequential model N1x10 is periodically “re-sphericalized” by later explosions and remains closer to the simple shear trajectory (Romano et al., 17 Mar 2025).

This deformation is too rapid to be explained by direct galactic shear acting on an initially spherical cavity. SISSI therefore concludes that galactic shear alone cannot explain the observed timing and degree of asymmetry. Yet the preferred orientations remain consistent with a shear-related origin: oblate remnants tend to have minor axes pointing out of the disk, while prolate remnants tend to have major axes lying in or near the disk plane with pitch angles clustered around values expected from the shearing-sphere model. The paper’s interpretation is that remnants are expanding into ambient density structures that were themselves pre-shaped by galactic shear. That reading is reinforced by the reported correlation

$T<150\,\mathrm{K}$7

which ties earlier deformation to larger ambient density anisotropy (Romano et al., 17 Mar 2025).

The result is conceptually significant. In SISSI, late-time remnant geometry is not dominated by the remnant’s own internal symmetry; it is dominated by the anisotropic structure of the surrounding ISM.

5. The Local Bubble program

The second SISSI paper applies the suite to the Local Bubble by reconstructing its geometry from the public 3D dust maps of Edenhofer et al. (2024). It uses the 12 sample dust maps from that release, follows the shell-finding strategy of O’Neill et al. (2024) with modifications, and smooths the density field with $T<150\,\mathrm{K}$8 using a mass-conserving kernel. The shell geometry is then summarized with the same shape-tensor formalism used in the simulations (Romano, 4 Sep 2025).

The reconstructed Local Bubble properties are highly specific. SISSI reports an effective radius

$T<150\,\mathrm{K}$9

a mass

$\epsilon_{\rm ff}=1\%$0

and a mean hydrogen number density

$\epsilon_{\rm ff}=1\%$1

Its axis ratios are minor-to-major $\epsilon_{\rm ff}=1\%$2 and semi-major-to-major $\epsilon_{\rm ff}=1\%$3, with a major-axis pitch angle $\epsilon_{\rm ff}=1\%$4 and a minor-axis pitch angle $\epsilon_{\rm ff}=1\%$5. Relative to earlier reconstructions, the paper states that this Local Bubble is larger and embedded in a much lower mean ambient density (Romano, 4 Sep 2025).

SISSI then combines this geometry with a momentum estimator. Assuming homologous expansion with $\epsilon_{\rm ff}=1\%$6,

$\epsilon_{\rm ff}=1\%$7

After calibrating this estimator against the SISSI sample, the corrected Local Bubble momentum becomes

$\epsilon_{\rm ff}=1\%$8

Using the SISSI momentum-per-supernova relation

$\epsilon_{\rm ff}=1\%$9

the paper obtains, for the Local Bubble density,

$500\,\mathrm{Myr}$0

and therefore

$500\,\mathrm{Myr}$1

The central conclusion is that, in contrast to previous estimates, the Local Bubble requires $500\,\mathrm{Myr}$2 SNe over approximately $500\,\mathrm{Myr}$3 to explain both its size and momentum (Romano, 4 Sep 2025).

In the paper’s extrapolated simultaneous solution, the Local Bubble age is

$500\,\mathrm{Myr}$4

with

$500\,\mathrm{Myr}$5

a mean SN interval

$500\,\mathrm{Myr}$6

and an expansion speed

$500\,\mathrm{Myr}$7

This substantially revises the older picture of a $500\,\mathrm{Myr}$8-old bubble powered by roughly 15–20 supernovae (Romano, 4 Sep 2025).

6. Interpretation, limitations, and significance

The revised Local Bubble chronology has direct astrophysical consequences. The second SISSI paper argues that the new age and SN count are in tension with the assumption that the Local Bubble was powered solely by supernovae from the nearby Scorpius-Centaurus OB association. It instead points to a broader and more recent episode of star formation and supernova activity in the $500\,\mathrm{Myr}$9-Persei family. On that basis, the paper casts serious doubt on the claim that star formation in the solar neighborhood was triggered by Local Bubble expansion and argues that the expansion may instead have quenched nearby star formation (Romano, 4 Sep 2025).

At the same time, SISSI is careful about its own limits. The Local Bubble reconstruction depends on identifying the shell as the first prominent density peak along each line of sight, which cannot perfectly separate a true shell from unrelated or entrained structures. The central $2\times 10^{52}\,\mathrm{erg}$0 of the dust map lacks differential extinction information and must be filled by assumption. More importantly, the preferred Local Bubble solution lies partly outside the parameter space covered directly by the simulations, so the age estimate relies on extrapolation from the simulated tracks. The first paper also notes that the underlying SNR calculations omit magnetic fields, cosmic rays, thermal conduction, and non-equilibrium chemistry, all of which could affect late-time structure (Romano et al., 17 Mar 2025, Romano, 4 Sep 2025).

Even with those caveats, the project establishes a coherent result: early SNR evolution remains close to classical theory, but late-time morphology is strongly controlled by the anisotropic ISM assembled by galactic dynamics. That conclusion makes geometry an astrophysical diagnostic in its own right. The first paper explicitly states that future studies targeting the geometry of Galactic SNRs may use this insight to obtain a clearer picture of the processes shaping the Galactic ISM (Romano et al., 17 Mar 2025).

The project also emphasizes reproducibility. The second paper makes the Local Bubble analysis code public in Julia at https://doi.org/10.5281/zenodo.17054923, which suggests that SISSI is intended as an open comparison framework as well as a simulation suite (Romano, 4 Sep 2025). In that sense, SISSI’s significance is twofold: it provides a specific high-resolution sample of supernova remnants in a stratified, shearing ISM, and it advances a broader claim that realistic remnant geometry can be used to infer the recent dynamical history of the Galactic environment.