+1 vote
130 views

Hello, I am finding the documentation for the lsst_sim.simdr2_binary table might be incorrect in some places. In particular, the orbital period of the binaries says it is in days, but a quick check using Kepler's law shows the period is probably in years instead (there is a factor of 365^2 needed to make the LHS and RHS of Kepler's law match). I attach below a python snippet that does this check:

from dl import queryClient as qc
import numpy as np
import pandas as pd

#Query binaries
sql = '''SELECT a, p, c1_mass, c2_mass FROM lsst_sim.simdr2_binary where random_id < 0.01'''
df  = qc.query(sql=sql,fmt='pandas')


a = df['a'].values * 6.957e+8 #From Rsun to metres
T = df['p'].values * 24 * 3600 #From days to seconds
G = 6.67e-11 #In SI units
M = df['c1_mass'].values * 1.989e30 #From Msun to Kg
m = df['c2_mass'].values * 1.989e30 #From Msun to Kg

#LHS and RHS of kepler's law
LHS = a**3/T**2
RHS = G*(M + m)/(4*np.pi**2)

#show ratios
print(LHS/RHS)

#If difference is entirely due to units in T:
print(np.sqrt(LHS/RHS))

#If difference is entirely due to units in a:
print(np.cbrt(LHS/RHS))

So either the period is off by factor of 365 or the semi-major axis is off by factor of 51. So to me it appeared the orbital period units were in years while the documentation says it is in days.

by dhayaa (130 points) | 130 views

1 Answer

+1 vote

Hi, thank you for bringing this to our attention! Yes, it does look like the orbital period is in units of years rather than days. We will fix the column description accordingly

by ajacques (860 points)

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