Showing extended objects on astronomical images with Aladin & Simbad

Page 4

New October 2016


 


Opening your VOTABLE in Aladin and showing the extended objects

 

Having generated a VOTABLE containing the extended ISM objects we now need to open this in Aladin. The steps are quite simple and are similar to those used in page 1.

 

  1. First open Aladin as normal

  2. Then enter the object or co-ordinates at the centre of the area you used to generate your VOTABLE catalogue, Aladin will open a DSS image of the area. You might need to zoom out as you are interested in extended objects.

  3. Then open your VOTABLE using the 'File - Open Local File' command. You need to navigate to the folder you saved the VOTABLE in. Aladin will understand that this is a catalogue rather than an image.

  4. Having loaded the catalogue in your VOTABLE the centre position of the various ISM objects should be shown by markers, in my case this is an orange cross.

  5. The final thing is to load the filter we generated previously. The size section is redundant as we have used a similar size in the query to generate the VOTABLE but there is no harm in leaving it there. It allows the filter to be easily edited if you want to eliminate the smaller areas of ISM. This is loaded in the same way - open the filter dialog box, select advanced and then 'Load Filter'

  6. The areas of ISM should be drawn as ellipses as previously.

I used the following query to generate a VOTABLE for the area around Gamma Cygni:

 

otypes = ISM & dimmajor > 0.1 & region(box,gamma cygni,20d 20d)

 

 

This gives something similar to the image below:

 

 

Extended ISM areas around Gamma Cygni
Extended ISM areas around Gamma Cygni

 

 

You should be able to zoom in and out and pan the image and the ISM should be shown as long as the centre is within the 20 degree box defined in the VOTABLE query.

 

There is one obvious and rather strange feature in the image above  that I have zoomed into below. There is what appears to be a conspicuous region that does not have an ISM identifier - it is not clear if it is a nebula or bubble. It is unlikely to be a SN remnant as there is still a star at the centre.
Zoom in to image above
ISM bubble or nebula without ISM identifier.

 

As the bubble / nebula does not have an ISM identity and we have no star labels in out list how can we find out what the star in the centre is allowing us to investigate further ?

 

There is a useful feature in Aladin that will help us - if you have the mode set to 'select' the pointer will normally be an arrow. If the tip of the pointer is placed over an object such as the star in the centre of the bubble / nebula a pop-up should appear with the identifier and brief that are stored in Simbad.

 

This should show the star is HD 192163 -- a Wolf-Rayet Star, it is probable the hot winds from this star are creating a bubble in the ISM

 

Why is the nebula not shown ?

 

The nebula around the star is NGC 6888, the crescent nebula. Opening the relevant Simbad page here shows that there are no measurements for the major / minor diameters of the ellipse so our filter has eliminated this object both from the VOTABLE catalogue and from being displayed !

 

This all goes to prove that any astronomical data must be used with caution.

 

Conclusion

 

The methods described on these pages can be used as a basis to generate catalogues and filters to show other types of objects for instance you could show only high proper motion stars, those within a certain radius or those of a particular spectral type as well as showing quasars only.

 

If you have any comments on these pages or require assistance in the use of The Virtual Observatory please contact me via the mail link on my home page.

 

John Murrell 10th October 2016

 

(c) John Murrell 2016

  


 

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