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Find stellar activity lifetimes

Compute how long a star (or cluster of stars) stays above an activity threshold, such as an X-ray luminosity limit, or how long it remains in the saturated regime.

Prerequisites

Make sure the package and stellar evolution data are installed: 

pip install fwl-mors
mors download all

Units

Thresholds must match the units of the chosen quantity (e.g., Lx in erg s⁻¹). Ages returned are in Myr.

Option 1: Standalone helper on an explicit track

mors.ActivityLifetime takes an age array and a track array directly and returns the age when the track finally drops below the threshold:

import mors

star = mors.Star(Mstar=1.0, Omega=1.0)

age_active = mors.ActivityLifetime(
    Age=star.AgeTrack,
    Track=star.LxTrack,
    Threshold=1.0e28
)
print(age_active)  # Myr

Behaviour:

  • If the track crosses the threshold multiple times, the final crossing age is returned.
  • If the track never drops below the threshold, the final age in the track is returned.
  • If the track is always below the threshold, 0.0 is returned.

To limit the search to ages below a maximum:

age_active = mors.ActivityLifetime(
    Age=star.AgeTrack,
    Track=star.LxTrack,
    Threshold=1.0e28,
    AgeMax=2000.0
)

This selects the track internally — you only provide the quantity name and threshold:

age_active = star.ActivityLifetime(Quantity='Lx', Threshold=1.0e28)
print(age_active)  # Myr

To limit the search to ages below a maximum:

age_active = star.ActivityLifetime(Quantity='Lx', Threshold=1.0e28, AgeMax=1000.0)

The Quantity argument must be one of the following supported activity quantities:

Band Valid quantities
X-ray Lx, Fx, Rx, FxHZ
EUV1 (10–36 nm) Leuv1, Feuv1, Reuv1, Feuv1HZ
EUV2 (36–92 nm) Leuv2, Feuv2, Reuv2, Feuv2HZ
EUV (10–92 nm) Leuv, Feuv, Reuv, FeuvHZ
Ly-\(\alpha\) Lly, Fly, Rly, FlyHZ

Cluster activity lifetimes

For a mors.Cluster, the same method returns one value per star as a NumPy array:

import numpy as np
import mors

Mstar = np.array([0.5, 0.8, 1.0, 1.2])
Omega = np.array([1.0, 1.0, 1.0, 1.0])
cluster = mors.Cluster(Mstar=Mstar, Omega=Omega)

ages_active = cluster.ActivityLifetime(Quantity='Lx', Threshold=1.0e28)
print(ages_active)  # array of Myr, one per star

Saturation lifetime

To find how long a star remains in the saturated X-ray regime, pass Threshold='sat'. This normalises the track to \(\Omega_\mathrm{env}/\Omega_\mathrm{sat}\) and finds when it drops below 1, so the result is independent of which valid quantity you choose:

age_saturated = star.ActivityLifetime(Quantity='Lx', Threshold='sat')
print(age_saturated)  # Myr

Common pitfalls

  • The threshold must be in the same units as the chosen quantity (Lx in erg s⁻¹, Fx in erg s⁻¹ cm⁻², Rx dimensionless, etc.).
  • The function always returns the final crossing age when there are multiple crossings.
  • Threshold='sat' is available on Star.ActivityLifetime and Cluster.ActivityLifetime, but not on the standalone mors.ActivityLifetime.