Birth location of an object changes with different time ranges

[sivdur]

I initialized the orbit of an object with the following code:

cl = SkyCoord(ra=259.103*u.deg,
              dec=-39.316*u.deg,
              distance=2.53*u.kpc,
              pm_ra_cosdec=-0.455*u.mas/u.yr,
              pm_dec=-1.439*u.mas/u.yr,
              radial_velocity=-14.423*u.km/u.s, 
              frame='icrs')
o = Orbit(cl)
# ob = Orbit(cl)
logAge = 7.0
clage = 10**(logage-9)#*u.Gyr

ts = np.linspace(-200.0, 200.0, 10001)#*u.Gyr
o.integrate(ts, MWPotential2014)

and I calculate the birth location in the (X, Y, Z) space as follows:

o.x(-clage), o.y(-clage), o.z(-clage)

which gives the following plot:

when I change the integration time as follows:

ts = np.linspace(-100.0, 100.0, 10001)#*u.Gyr

the plots look like this:

the green stars indicates the birth location, whereas the red dot is the current location of the object.
When I multiply the integration time by u.Gyr, it doesn't change the result. The birth location changes everytime.
The version of galpy is 1.8.0

How can I find the birth location of an object?

Best,
Sivan.

[jobovy]

Hi!

The first entry in the time array is the time of the initial condition (that is, the initial condition isn't assumed to be at t=0, which I think you may have thought). So because you change the initial time, your clage will be a different time in the past. If you keep the initial times the same, e.g., in your second run use

ts = np.linspace(-200.0, 100.0, 10001)#*u.Gyr

then you'll see that you get a consistent result.

A few more comments:

• You do have to add *u.Gyr if you want your times to be interpreted as being in Gyr. Without this, the times are assumed to be in internal units. Given how you define clage, I think this is supposed to be in Gyr.
• I think you want to integrate backwards in time, but you are integrating forwards in time. For your purposes, the best thing to do would be to use

  ts = np.linspace(0,-clage, 10001)
  

  So you go from the present (defined to be t=0 here) to the time you want to integrate backwards for.

I hope this helps!