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PostPosted: Sun, 10-07-11, 14:57 GMT 
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I am having a problem finding an answer to this.
I read that EPS Hya A and B are seperated by 2.7 arcsec.
How do I convert that to either km or au?

Thanks,
Tim


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PostPosted: Sun, 10-07-11, 16:30 GMT 
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Tim,

the star separation expressed in angular units [= arcsecs] is dimensionally different from a separation e.g. in km, AU , ly,..[ = length]. Hence you are missing another quantity with dimension of a length => the star distance d <=, in order to calculate the desired separation in km, AU, etc. Since distances d are usually somewhat uncertain, astronomers prefer to quote star separations in angular units that may be directly measured (without knowing d).

The desired conversion formula is simple as long as the distances d1, d2 of the two stars are about equal and the separation is small compared to the distance... [For ultra precise calculations you need to know some (spherical) trigonometry, though.]

separation in km = PI / 180 * separation in arcsec/3600 * distance in km
separation in AU = PI / 180 * separation in arcsec/3600 * distance in AU
separation in ly = PI / 180 * separation in arcsec/3600 * distance in ly

PI= 3.14159...

The factor 1/3600 converts arcsecs into degrees (1 degree = 3600 arcsecs), while the factor PI/180 converts degrees into the natural angular units of radian, such that the dimensional check now reads correctly

[length] = PI/180 [arcsecs]/3600 * [length] = [radians] * [length]

Hope this helps,

Fridger


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PostPosted: Mon, 11-07-11, 20:47 GMT 
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Well I guess thats's the problem. I can't find a distance except one- http://en.wikipedia.org/wiki/Epsilon_Hydrae

So I followed the referance links at the bottom of that page to here- http://simbad.u-strasbg.fr/simbad/sim-id?Ident=epsilon+hydrae&NbIdent=1&Radius=10&Radius.unit=arcmin&jsessionid=A5BECA1D6164C4C897B78DA25683842E

I can't really see if this info helps me or not. So I found this page- http://stars.astro.illinois.edu/sow/epshya.html
But there are no distances to help here either.

I was going to try to map the whole system, but maybe I should just leave it a single star, like it is in Celestia.

I do thank you for your explination though. :)

Tim

edit-I forgot to explain the reason I needed a distance. (must be age) The first link gives a seperation of 2.7 arcseconds, while the last link gives .2 seconds of arc and a distance of 10.57 au average.
Could there be that big of a mistake or am I reading it wrong?


Last edited by WilsonTC on Mon, 11-07-11, 20:53 GMT, edited 1 time in total.

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PostPosted: Mon, 11-07-11, 20:52 GMT 
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WilsonTC wrote:
Well I guess thats's the problem. I can't find a distance except one- http://en.wikipedia.org/wiki/Epsilon_Hydrae

So I followed the referance links at the bottom of that page to here- http://simbad.u-strasbg.fr/simbad/sim-id?Ident=epsilon+hydrae&NbIdent=1&Radius=10&Radius.unit=arcmin&jsessionid=A5BECA1D6164C4C897B78DA25683842E

I can't really see if this info helps me or not. So I found this page- http://stars.astro.illinois.edu/sow/epshya.html
But there are no distances to help here either.

I was going to try to map the whole system, but maybe I should just leave it a single star, like it is in Celestia.

I do thank you for your explination though. :)

Tim


But Wiki quotes the distance as 135 ly. So what's missing? The distances of binary components are usually close such that you may use 135 ly as the common one (barycenter).

The simple reason why I was unable to place a lot more binary orbits into Celestia was precisely that some distance values and|or some orbital parameters were lacking.

Fridger


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PostPosted: Mon, 11-07-11, 21:34 GMT 
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Thank you.
I think I have it now.

Code:
separation in ly = PI / 180 * separation in arcsec/3600 * distance in ly
                  0.0174533 *          0.00075          *    135         =0.001767 ly
                                                                                                        or 3.16 au


Tim


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PostPosted: Mon, 11-07-11, 22:23 GMT 
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I agree with your separation distance in ly, but not with its conversion to AU, since

1 ly = 63241.978609626 AU,

hence 0.001767 ly = 0.001767 * 63241.978609626 AU = 111.75 AU

So you got to imagine the two stars to orbit on average at 112 Earth-Sun distances...
Fridger


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PostPosted: Tue, 12-07-11, 0:09 GMT 
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Well, that's the last time I use an online converter.

Thank you very much for your help.

Tim


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PostPosted: Thu, 14-07-11, 22:55 GMT 
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Well here is what I put together with info from the links above.
Fridger, I would like your opinion. I know it's not scientifically correct, what with all the missing information. But, this is the best I could come up with.

Thanks again,
Tim

Code:
Barycenter 43109 "Eps Hya:HIP 43109:Epsilon Hydrae:Epsilon Hydra"

   {
   RA      131.694358900
   Dec        6.418908030
   Distance        129.277447
   }


Star "Eps Hya A"


   {
   OrbitBarycenter    "Eps Hya"
   SpectralType    "G0III"
   AbsMag       0.39
   
   EllipticalOrbit
      {
                      Period      0.1
                      SemiMajorAxis         0.01
                      Eccentricity          0
       
      }
   RotationPeriod    24
   }


Star "Eps Hya B"

   {
   OrbitBarycenter    "Eps Hya"
   SpectralType    "A5III"
   AppMag       4.7
   
   EllipticalOrbit
      {
                      Period      15.05
                      SemiMajorAxis         10.5
                      Eccentricity          0
     
      }
   RotationPeriod    24
   }

Barycenter "Eps Hya CD"

{
   OrbitBarycenter "Eps Hya"

   EllipticalOrbit {
      Period          900
      SemiMajorAxis   190
      
   }
}

Star "Eps Hya C"

   {
   OrbitBarycenter    "Eps Hya CD"
   SpectralType    "F5V"
   AbsMag       3.96
   
   EllipticalOrbit
      {
                      Period      0.136
                      SemiMajorAxis         0.045
                      Eccentricity          0
                      MeanLongitude    90
     
      }
   RotationPeriod    24
   }

Star "Eps Hya D"

   {
   OrbitBarycenter    "Eps Hya CD"
   SpectralType    "F5V"
   AbsMag       3.96
   
   EllipticalOrbit
      {
                      Period      0.136
                      SemiMajorAxis         0.045
                      Eccentricity          0
                      MeanLongitude    270
     
      }
   RotationPeriod    24
   }

Star "Eps Hya E"

   {
   OrbitBarycenter    "Eps Hya"
   SpectralType    "M3.5V"
   AbsMag       10.73
   
   EllipticalOrbit
      {
                      Period      9500
                      SemiMajorAxis         800
                      Eccentricity          0
     
      }
   RotationPeriod    24
   }


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PostPosted: Fri, 15-07-11, 16:33 GMT 
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Tim,

the 6 fold eps HYA system looks nice indeed! It's a pity that the scientifically known orbital elements of at least 3 were not implemented. Here are the sources I would start with:

The most interesting paper on

Multiple star catalogue (MSC) (Tokovinin 1997-1999)
http://cds.aanda.org/index.php?option=c ... .124...75T

along with the CDS catalog of 612 multiple stars with 3 to 7 components!

http://cdsarc.u-strasbg.fr/cgi-bin/myqc ... +AS/124/75

The IDS number of eps HYA is 08415+0647 you will easily find the respective catalog data along with the orbital elements, remarks etc.

Try and check whether your elements are conform with the MSC catalog data!?

It is an old interesting project on my waiting list of tasks, to implement that MSC catalog for Celestia.Sci. Also Andrew (aka ajtribick ) has plenty of interest in that matter.

Keep me informed &
good luck,
Fridger


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PostPosted: Sun, 17-07-11, 16:26 GMT 
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Very interesting reading, of course most of it is above my head :?
I have a few questions about some of the parameters.

1- 10 A10 ---IDS IDS(1900)
12- 15 I4 ---Level= Level in hierarchy
17- 26 F10.4 ---Period = Period (see X_period for the units)---(OK)
27 A1 --- x_Period = [dy] Units of period (d,y)---(OK)
29- 38 F10.3 --- T= Periastron epoch (y or JD-2400000)---(closest approach in its orbit, correct?)
40- 45 F6.3 --- e= Eccentricity---(OK)
47- 53 F7.3 --- a= Semi-major axis (see x_a fir the units)---(Longest point of orbit, correct?)
54 A1 --- x_a = ["m] Units of axis (",m)---(is this minutes,meters?)
56- 61 F6.1 deg Node = P.A. of the node---(could not find an explination for this)
63- 68 F6.1 deg LongP = Longitude of periastron---(OK)
69 A1 --- n_Node= 'A' if primary node, 'B' otherwise---(could not find what this means)
71- 76 F6.1 deg Incl = Inclination---(OK)
78- 83 F6.1 km/s K1 = Primary semi-amplitude---(could not find what this means)
85- 90 F6.1 km/s K2 = Secondary semi-amplitude---(could not find what this means)
92- 98 F7.1 km/s V0 = Center of mass velocity---(is this needed? is this for getting the period)
100-130 A31 --- Comm = Comment, reference

Thanks,
Tim


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PostPosted: Sun, 17-07-11, 18:28 GMT 
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WilsonTC wrote:
Very interesting reading, of course most of it is above my head :?
I have a few questions about some of the parameters.

1- 10 A10 ---IDS IDS(1900)
12- 15 I4 ---Level= Level in hierarchy
17- 26 F10.4 ---Period = Period (see X_period for the units)---(OK)
27 A1 --- x_Period = [dy] Units of period (d,y)---(OK)
29- 38 F10.3 --- T= Periastron epoch (y or JD-2400000)---(closest approach in its orbit, correct?)
40- 45 F6.3 --- e= Eccentricity---(OK)
47- 53 F7.3 --- a= Semi-major axis (see x_a fir the units)---(Longest point of orbit, correct?)
54 A1 --- x_a = ["m] Units of axis (",m)---(is this minutes,meters?)
56- 61 F6.1 deg Node = P.A. of the node---(could not find an explination for this)
63- 68 F6.1 deg LongP = Longitude of periastron---(OK)
69 A1 --- n_Node= 'A' if primary node, 'B' otherwise---(could not find what this means)
71- 76 F6.1 deg Incl = Inclination---(OK)
78- 83 F6.1 km/s K1 = Primary semi-amplitude---(could not find what this means)
85- 90 F6.1 km/s K2 = Secondary semi-amplitude---(could not find what this means)
92- 98 F7.1 km/s V0 = Center of mass velocity---(is this needed? is this for getting the period)
100-130 A31 --- Comm = Comment, reference

Thanks,
Tim


Tim,

Periastron epoch (y or JD-2400000)---(closest approach in its orbit, correct?) YES.

Semi-major axis (see x_a fir the units)---(Longest point of orbit, correct?) YES, assuming orbits are ellipses that are characterized by a minor and major half-axis in direct generalization of the radius of a circular orbit.
units: " = arcsecs, ' = arcmins, m=ambiguous.

PA = position angle [deg]. Could be e.g. of a galaxy, whence it is counted eastward from North. In case of a node it's similar.


Fridger


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PostPosted: Sat, 23-07-11, 19:22 GMT 
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Well, I'm stuck again.
I am not sure where to begin putting in the numbers from the tables.
Under "12- 15 I4 ---Level= Level in hierarchy", which star is which? (A, B, C, ...)
The tables you pointed me too, the first list under 08415+0647, is 11. But then the Period(x) is 990y. So is that the farthest star from the barycenter? I am assuming so. It would be star C.
The next one is Level 111 with a period of 15.06y, I would assume that is star B.
The last is Level 112 with a period of 9.9047d (days), so this star must be star A.

Now the T= Periastron epoch (y or JD-2400000)---(closest approach in its orbit) is really throwing me off. Closest approach to what? Here are the numbers-for 11=1920 / for 111=1991.180 / for 112=23800.007. Now if I assumed that Level 112 is star A, then why is its "T" so big?

Thanks,
Tim

ps found some more stuff an eps hya--http://www.dibonsmith.com/orbits.htm

Image

Image

No idea how to read the drawings :?


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PostPosted: Mon, 25-07-11, 19:06 GMT 
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Tim,

sorry I am kind of busy for a while. But what you see in these plots is a skyplane projection of the orbit of the secondary around the primary star. The latter is centered in the observation instrument (at all times). The numbers refer to the observed positions of the secondary star at the displayed years. Usually, the position angles are counted from North (zero degrees). I am planning to implement such a plot for all my binary stars in Celestia.Sci.

Fridger


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PostPosted: Mon, 29-08-11, 20:10 GMT 
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Not to bother you if you're still busy, I kinda got busy myself, almost forgot about this.
Do you have anymore pointers for me so I can plot this system as accurate as is possible?
Thank you for your help so far.

Tim


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PostPosted: Tue, 30-08-11, 13:01 GMT 
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WilsonTC wrote:
Not to bother you if you're still busy, I kinda got busy myself, almost forgot about this.
Do you have anymore pointers for me so I can plot this system as accurate as is possible?
Thank you for your help so far.

Tim


Just returned from a 5 weeks trip yesterday. Hence, sorry, no further pointers, yet.

Fridger


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