Using model percentiles for initial rotation

Pick an initial rotation rate from the built-in model rotation distribution ¹, inspect a star's inferred percentile, and convert between rotation rate and percentile for a given stellar mass.

Prerequisites

Install MORS and download stellar evolution data:

pip install fwl-mors
mors download all

Step 1: Get the built-in model cluster distribution

This returns arrays of stellar masses and their 1 Myr rotation rates, derived by evolving observed cluster rotation distributions back to 1 Myr:

import mors

Mstar_dist, Omega_dist = mors.ModelCluster()
print(Mstar_dist.shape, Omega_dist.shape)

Step 2: Create a star by percentile

Set initial rotation using a numeric percentile (0–100) or the string shortcuts 'slow', 'medium', 'fast', which map to the 5th, 50th, and 95th percentiles respectively:

import mors

star_slow   = mors.Star(Mstar=1.0, percentile='slow')    # 5th percentile
star_medium = mors.Star(Mstar=1.0, percentile='medium')  # 50th percentile
star_fast   = mors.Star(Mstar=1.0, percentile='fast')    # 95th percentile

# or with a numeric value
star = mors.Star(Mstar=1.0, percentile=10.0)

Step 3: Read back the star's inferred percentile

Regardless of how the star is created (Omega, Prot, or percentile), MORS stores the inferred percentile in the 1 Myr distribution after computing the tracks:

print(star_slow.percentile)

Step 4: Convert a rotation rate to a percentile

import mors

# from Omega (Ω☉)
p = mors.Percentile(Mstar=1.0, Omega=10.0)
print(p)

# from Prot (days)
p = mors.Percentile(Mstar=1.0, Prot=1.0)
print(p)

Step 5: Convert a percentile to a rotation rate

import mors

Omega = mors.Percentile(Mstar=1.0, percentile=10.0)  # returns Ω in Ω☉
print(Omega)

Step 6: Control the mass window used for percentiles

Percentiles are computed from stars within a mass window around Mstar. The half-width is controlled by dMstarPer (default: \(0.1\,M_\odot\)). To change it, create a custom parameter dictionary:

import mors

my_params = mors.NewParams(dMstarPer=0.05)
star = mors.Star(Mstar=1.0, percentile='medium', params=my_params)

Step 7: Use a custom rotation distribution (optional)

Pass your own mass and rotation arrays into Percentile instead of using the built-in 1 Myr distribution:

import numpy as np
import mors

MstarDist = np.array([1.0, 1.0, 1.0, 0.9, 1.1])
OmegaDist = np.array([2.0, 5.0, 10.0, 3.0, 7.0])

p = mors.Percentile(Mstar=1.0, Omega=6.0, MstarDist=MstarDist, OmegaDist=OmegaDist)
print(p)

You can also supply a period distribution instead:

MstarDist = np.array([1.0, 1.0, 0.9, 1.1])
ProtDist  = np.array([12.0, 6.0, 9.0, 7.0])  # days

p = mors.Percentile(Mstar=1.0, Prot=8.0, MstarDist=MstarDist, ProtDist=ProtDist)
print(p)

Common pitfalls

• Percentiles refer to the initial (~1 Myr) rotation distribution used by the model cluster.
• Omega is in Ω☉, not rad s\(^{-1}\). Prot is in days.
• If the custom distribution is sparse or covers a narrow mass range, at least two stars must fall within the dMstarPer window, otherwise Percentile will raise an exception.