DGalanDieguez

SPAMMS

Spectroscopic PAtch Model for Massive Stars:
spectral synthesis for stars that are not spheres.

Code on GitHub

Why SPAMMS?

Massive stars shape the mechanical and chemical evolution of galaxies, and many of them are far from spherical. Rapid rotation flattens them at the equator, and in close binaries tides stretch both stars and can even make them share a common envelope. These distortions change the surface gravity and temperature from one point of the surface to another, and with them the spectrum we observe.

Yet the tools used to analyse these spectra are almost always one-dimensional: they treat the star as a sphere with a single temperature and gravity, and add rotation afterwards by convolving the lines with a broadening kernel.

SPAMMS (Spectroscopic PAtch Model for Massive Stars) takes the three-dimensional geometry of the system into account. It builds the distorted surface as a mesh of small triangles, with PANORAMA (for rotating single stars) and with the PHOEBE 2 code (for binaries), and combines it with model atmospheres that give the emergent line profile of each triangle — according to its own temperature, gravity and viewing angle. Integrating the light of the visible surface, with the radial velocity of every element, returns the spectrum an observer would see at a given orbital phase and orientation.

Because each element is Doppler-shifted individually, rotational broadening comes out of the geometry itself instead of being applied afterwards as a convolution. That reproduces asymmetric, inclination-dependent line shapes that a single v sin i parameter cannot.

From model to spectrum

From a 3D surface mesh to the line profile an observer would see.

Four panels: a triangulated mesh of a rotating star; the same mesh coloured by temperature; spectra assigned to two surface elements; the integrated line profile.
The four stages of SPAMMS for a rotating star: the surface mesh, its temperature map,
the local line profiles of two surface elements, and the integrated spectrum.
  1. Build the mesh. PANORAMA generates a triangulated surface that follows the rotational distortion of a single star; for binaries, PHOEBE 2 builds the Roche geometry of both components.
  2. Assign local physics. Each triangle gets its own effective temperature and surface gravity from the Roche model and a gravity-darkening law (von Zeipel or Espinosa Lara & Rieutord).
  3. Attach a spectrum. Each triangle receives the specific intensity for its conditions and viewing angle, taken from a grid of model atmospheres (FASTWIND, TLUSTY or Kurucz).
  4. Integrate. SPAMMS sums the visible surface, accounting for projected area, Doppler shift and limb darkening, to produce the line profile. Wind and photosphere can both contribute.

Modelling capabilities

What sets SPAMMS apart: from physical modelling to practical spectral analysis.

Single stars and binaries

Rotating single stars are modelled with PANORAMA, and detached, semi-detached or contact binaries with PHOEBE 2, all within the same framework.

No limb-darkening law

Angle-dependent specific intensities make limb darkening intrinsic, line by line, instead of relying on a parametric law.

From O to K stars

LTE and non-LTE grids from 3 500 to 55 000 K and log g 0–5, at 101 emergent angles, computed with PRISMAS.

Model grids on multiple cores

Series of models over parameter ranges run in parallel and can be compared directly with observed spectra.

Spectroscopy + interferometry

With PANORAMA, SPAMMS and interferometry, observables are derived from the same stellar surface, enabling a complete analysis of the stellar properties of rotating stars.

MCMC fitting (in development)

Bayesian parameter estimation with full posteriors, being implemented in collaboration with Newcastle University.

Modelling with SPAMMS

Contact binaries, rapid rotators and planetary transits with the same machinery.

Animation: temperature map of the contact binary VFTS 352 and its helium lines changing over the orbit.
Binary

Overcontact binary VFTS 352

Surface temperature, with contrasts above 10 000 K, and the He I and He II lines through the orbit. Both stars and the overcontact surface are modelled together.

Animation: a star rotating at 90% of critical, coloured by temperature, and its helium lines as the inclination changes.
Single star

Star at 90% of critical rotation

Hot poles and a cool equator. Seen pole-on, He II strengthens and He I weakens, so a spherical fit would overestimate the temperature.

Animation: a planet crossing a rotating star and the distortion it produces in a spectral line.
Exoplanet

Rossiter–McLaughlin effect

A planet crossing a rotating star blocks part of its surface, and the missing light travels through the line profile during the transit.