How to make a locked tidal bulge ?

[Cham]

I need to make a star deformed by a black hole in close range. So I have few questions :

How do I make an oblate star ? For a planet, it's just the command "Oblateness". But what for a star ?

I know that I can enter a negative value for the oblateness, so it gives two bulges on the planet. I may set the RotationPeriod to a very long period, so the planet appears locked, relative to the stars. But the bulges are fixed too relative to the stars and I want them to rotate, so they are always in the orientation of the black hole. I guess I need to use the command PrecessionRate, but how ?

[Cham]

I've found that this code could work, but I still need to know how to set a star oblateness.

"Test Planet" "Black Hole"
{
Texture "any.jpg" # arbitrary
Radius 600 # arbitrary

EllipticalOrbit {
Period 1 # arbitrary
SemiMajorAxis 0.0005 # arbitrary
Eccentricity 0
Inclination 0
AscendingNode 0
ArgOfPericenter 0
MeanAnomaly 0
}

RotationPeriod 1E12 # arbitrary ?
Oblateness -0.2 # between -0.1 and -0.2 appears good
Albedo 0.001 # arbitrary
Obliquity 87 # not sure on this, should be 90, isn't ?
Orientation [90 0 0 1] # so the bulge are oriented to the Black hole.
PrecessionRate 0.9856090133 # 360/(Period * 365.2564 ??)
}

Can you confirm the value used for Earth year in Celestia ?

And how to set up a star oblateness ?

[Guest]

Look at Altair in nearstars.stc to see the use of the SemiAxes parameter for stars.

Grant

[Cham]

Thanks Grant. But I'm unable to lock the bulges toward the Black hole. How am I supposed to do that ? I gave the star RotationPeriod the same as the Black hole, but the bulges appears to be rotated by about 40 degrees or so. I'm unable to put them in front of the Black hole.

[Cham]

I've found a partial solution to my problem, but there's a catch. I need some help on this. Using PrecessionRate, the star's bulges are almost aligned with the black hole defined below. But after some time, they get non-aligned anymore and I don't know how to correct this. Here's the stc and ssc codes :

Barycenter "Ogre's navel"
{
RA 6.02788
Dec -72.0221
Distance 13371
}

805000 "Banquet"
{
OrbitBarycenter "Ogre's navel"
SpectralType "L"
AbsMag 27.5

EllipticalOrbit {
Period -0.000004
SemiMajorAxis 0.0001
AscendingNode 265.145
ArgOfPericenter 281.050
MeanAnomaly 102.385
}

#RotationPeriod -0.03510
#SemiAxes [ 0.9 0.9 1 ] # I'M UNABLE TO GET THE BULGES TOWARD BLACK HOLE WITH THIS.

RotationPeriod 9E-12 # CAN I SET THIS TO 0 ?
SemiAxes [ 0.9 1 0.9 ]
Obliquity 89 # CAN I SET THIS TO 90 ?
PrecessionRate -246575.342465753 #NEED HELP HERE

}

Here's a part of the ssc code for the Black Hole :

"Ogre" "Ogre's navel"
{
Texture "Black Hole.jpg"
Radius 4.05 # 1
Emissive true

EllipticalOrbit
{
Period 1E-12
SemiMajorAxis 1E-12
Inclination 0
AscendingNode 265.145
ArgOfPericenter 101.050
MeanAnomaly 102.385
}

RotationPeriod 0.00045
Oblateness 0.1
Albedo 0.001
Obliquity 7
}

PLEASE help !

[Guest]

Cham,

Shouldn't Ogre and Banquet have the same orbital period? If so, it should be easy to keep the bulge pointing toward the black hole with just a constant RotationPeriod.

--Chris

[Cham]

It doesn't work ! If I accelerate time a bit, after just one year, the bulges aren't oriented toward the black hole anymore. Here's my latest stc and ssc :

Barycenter "Ogre's navel"
{
RA 6.02788
Dec -72.0221
Distance 13371
}

805000 "Banquet"
{
OrbitBarycenter "Ogre's navel"
SpectralType "L"
AbsMag 27.5

EllipticalOrbit {
Period -0.000004
SemiMajorAxis 0.0001
Inclination 0
Epoch 2453168.3159722
AscendingNode 0
ArgOfPericenter 180
MeanAnomaly 180
}

#RotationPeriod -2 # -1E-12
#SemiAxes [ 0.9 1 0.9 ]
#Obliquity -90
#EquatorAscendingNode 90
#PrecessionRate -246372.844237613


RotationPeriod -0.0350688
SemiAxes [ 1 0.86 0.86 ]
EquatorAscendingNode 0
}


Black Hole ssc :

"Ogre" "Ogre's navel"
{
Texture "Black Hole.jpg"
Radius 4.05
Emissive true

EllipticalOrbit
{
Period -0.000004 # 1E-12
SemiMajorAxis 1E-12
Inclination 0
Epoch 2453168.3159722
AscendingNode 0
ArgOfPericenter 0
MeanAnomaly 180
}

RotationPeriod 0.00045
Oblateness 0.1
Albedo 0.001
Obliquity 7
}

[Cham]

Cham,

Shouldn't Ogre and Banquet have the same orbital period? If so, it should be easy to keep the bulge pointing toward the black hole with just a constant RotationPeriod.

--Chris

Both objects have exactly the same orbital Period. But how can I turn the bulges toward the Black hole ? I tried RotationOffset and EquatorAscendingNode, but how do I choose the parameter value ? I always get (at Epoch) the bulges rotated with an apparently arbitrary angle relative to the black hole.

[Guest]

You're a victim of the very short orbital period of your star around the black hole - hundreds of thousands of revolutions per day will very quickly push the necessary accuracy of the PrecessionRate far down the decimal places.
Here's a definition for Banquet that places the tidal bulges in the correct place at epoch J2000:

Code: Select all

"Banquet"
{
OrbitBarycenter "Ogre's navel"
SpectralType "L"
AbsMag 27.5

EllipticalOrbit {
Period 0.000004
SemiMajorAxis 0.0001
}

RotationPeriod 9E12              # very slow indeed
SemiAxes [ 0.9 1 0.9 ]           # prolate
Obliquity 90                     # 90 degree tilt
EquatorAscendingNode 90          # rotate to face black hole
PrecessionRate 246411.8399736995 # 360/(365.2421897*Period)
}

Go to 12:00 1 Jan 2000, and you'll see it behaving correctly. I've simplified things by eliminating the various random orbital parameters you'd inserted. If you reinsert these, you can compensate for them now by adding corresponding RotationOffsets to Banquet.
Notice, too, that if you want the star not to rotate, you set RotationPeriod high, not low.
Trouble is, drift sets in quickly, because of a mismatch in the late decimals between PrecessionRate and Period. I've used the length of the tropical year, which is what Celestia should be using for conversion. I haven't the time to trawl through the code looking for the exact value Celestia uses, sorry; but someone else may be able to provide that figure, which would then tighten down the match between precession and revolution as much as is possible.

Grant

[Guest]

I've simplified things by eliminating the various random orbital parameters you'd inserted. If you reinsert these, you can compensate for them now by adding corresponding RotationOffsets to Banquet.

Oops. Actually, no you can't. RotationOffset is useless to you in this particular set-up, because it adjusts the object's rotation around its axis, not in its orbital plane. You would need to add appropriate increments to EquatorAscendingNode, instead.

Grant

[Cham]

Thanks a lot for the reply, Grant.

After many experimentations, I've found those settings, which are pretty accurate. What do you think of those parameters ?

805000 "Banquet"
{
OrbitBarycenter "Ogre's navel"
SpectralType "L"
AbsMag 27.5

EllipticalOrbit {
Period -0.000004
SemiMajorAxis 0.0001
Inclination 0
Epoch 2453168.3159722 # 2004 June 11 19:35
AscendingNode 0
ArgOfPericenter 180
MeanAnomaly 180
}

RotationPeriod -1E6
SemiAxes [ 0.86 1 0.86 ]
Obliquity -90
EquatorAscendingNode -10 # approximate value found with the eye !
PrecessionRate -246406.55 # trial and error approximate value

# The next parameters are another approximate way
# to get the bulges in the right orientation,
# but the result is much less accurate in time.

#SemiAxes [ 1 0.86 0.86 ]
#RotationOffset 62 # approximate value found with the eye !
#RotationPeriod -0.03506399877375 # trial and error approximate value
}

# With this way, you'll see the bulges drifting away from the black hole
# if you accelerate time by a 1E7 factor. The parameters are accurate only
# for dates between 2004 and 2007, approximately.
# I didn't found the right way to permanently lock the bulges.

[chris]

Cham,

I was able to get the bulge to stay aligned with the black hole without using precession. I think that you calculated the rotation period incorrectly. You used -0.0350688 hours when it should be -0.035064 hours. When converting from years (units for the orbital period of planets) to hours (for rotation period), multiply by 365.25 * 24.

--Chris

[Cham]

Chris, do the bulges stay aligned if you accelerate time by a 1E5 factor ? With your RotationPeriod, it doesn't stay properly aligned on my system. They tend to drift away from the black hole.

However, I clearly solved my problem using the PrecessionRate and a fine tuned value found by a pain in the butt trial and error !

I'll make an annoucement soon, in the addon forum. I'll upload my black holes to the Motherlode very soon now.

I'm just doing some checks to see if there's something which could be polished.

[Guest]

You should be able to get the bulges precisely aligned at epoch without having to eyeball them. Setting RotationEpoch equal to Epoch should allow you to initialize orbital and rotational parameters together, without having to compensate for rotation that has taken place between J2000.0 and your orbital epoch. EquatorAscendingNode will then be some exact multiple of 90, given that your ArgOfPericenter and MeanAnomaly values effectively take Banquet right around the black hole and back to to zero position.

Grant

[Cham]

I just tried it, Grant. Well then, there's something I really don't understand here. I used those commands for "Banquet" :

RotationEpoch 2453168.3159722 # June 11 2004
RotationPeriod -1E6
SemiAxes [ 0.86 1 0.86 ]
Obliquity -90
EquatorAscendingNode 0
PrecessionRate -246406.55

And it doesn't work ! The bulges are apparently rotated by about 10 degrees relative to the black hole, which is using the same Epoch as the star.

I uploaded my addon to the Motherlode yesterday night. So when it will be available, please try it and tell me what you think.

[Guest]

Well, I was actually thinking of the simple, RotationPeriod solution:

Code: Select all

"Banquet"
{
OrbitBarycenter "Ogre's navel"
SpectralType "L"
AbsMag 27.5

EllipticalOrbit {
   Period -0.000004
   SemiMajorAxis 0.0001
   Epoch 2453168.3159722
   ArgOfPericenter 180
   MeanAnomaly 180
}

RotationEpoch 2453168.3159722
RotationPeriod -0.035064
SemiAxes [ 1 0.86 0.86 ]
}

Which certainly does work for me, with the bulges neatly aligned at epoch without any eyeballing required.
Just out of curiosity, why are you using all those negative numbers for your parameters?

Grant

[Cham]

Good question, Grant.

That's because I started with the black hole alone. I added the star later. And because of the accretion disks, I had to make one object rotate in the reverse sense of the other object. Well, maybe it wasn't the best way to do it, but the end result is satisfying to me. I'll probably make more addons like this one, later.