The distances to the objects are used by VOSA to transform the total fluxes given by the 'model fit' into bolumetric luminosities as:
Lbol = 4πD2 Ftot
If you don't give a value for the distance, VOSA will assume it to be 10pc to calculate the Luminosity.
If you don't care about the final luminosities and you don't intend to make an HR diagram, you can forget about distances and write them as "---" in your input file.
You can also provide a value for the error in the distance in your input file. In order to do that write D+-ΔD (for instance: 100+-20), without spaces, in the fourth column of your input file. See below for an example. (Remember to write both symbols, + and -, together, not a ± symbol or something else; otherwise vosa will not understand the value).
ΔD will be propagated as a component of ΔL as follows:
ΔLbol (from D) = Ftot * 2ΔD/D
If you don't give a value for ΔD or you don't find one in the VO, it will be zero. This will imply very small errors in ΔL as only errors coming from the observed fluxes will be considered.
VOSA offers the possibility of searching for the distance of the objects in VO catalogs.
In order to do this, the object coordinates are used to query some VO services (like Hipparcos catalog) to find observed parallaxes. Thus, object coordinates must be known (either provided in your input file or obtained in the Objects:Coordinates tab) if you want to search the VO for information about distances,
Take into account that the tool queries VO services using the object coordinates and returns the closer object to those coordinates, within the search radius, for each catalog. It could happen that the obtained information corresponds to a different object if the desired one is not in the catalog. In that case, the obtained distance could be erroneous because it corresponds to a different object. So, please, check the coordinates given by the catalog for each object to see if they seem to be the appropriate ones (within the catalog precision) before using the obtained values.
VOSA marks as "doubtful" those values found in catalogs so that the observed error is bigger than 10% of the parallax. It has been shown that for bigger errors the estimation of the distance from the parallax is biased (See Brown et al 1997). These values will be shown in red so that you are easily aware of large uncertainties.
The user can choose to incorporate the found distance (if any) into the final data or not. This decision can be taken in two different ways:
- Object by object. You can give user values for the distance (and error) for some particular objects, choose the value (user or VO catalogs) object by object, and just save those values.
- Specifying global criteria that will be applied to all the objects in the user file (which is specially useful for large files with many objects). Criteria are based on two main ideas: (1) what catalog you prefer to use and (2) the uncertainties for the distance values. See below for details.
Take a look to the corresponding Credits Page for more information about the VO catalogs used by VOSA.
We have uploaded a file with information about the distance to some of the objects (in some cases we have included errors for the distance too). As you see we have values for distance and error for 4 objects, only the distance for HD004307 and no information about 7 objects.
We want to check the VO to search for more information, so we enter the Objects:Distances subtab to try to find something.
At this stage, we see three main functionalities:
- First, the possibility to search the VO.
- A pagination form (when there are more objects in the file than the number that you choose to visualize in each page).
- A form with the values of the distance for each object.
In this last form there are several groups of columns:
- Object information (object name, RA and DEC)
- The Final values. These are the values for the distance that will be used by VOSA outside this tab. In principle, the values here will be those values provided by you in your input file. But now you can change them.
- The User values. In principle, the values here will be those values provided by you in your input file. But now you can change them. And you can decide to used those new values as the final ones (or not).
- Information about the distance values found in VO catalogs for each object. At first this information is "unknown" because you haven't done a VO search yet.
The first thing that you can do is editing the User values as you wish. For instance, you can give a value of 350±50 pc for the HD002665. You just need to write those values in the User column, mark the "tick" to its right and click the "Save Obj. Distances".
And you see that the final value for this object has been changed accordingly. If you leave this tab now, whenever a distance value is needed, VOSA will use a distance of 350±50 pc for this object.
The next natural step is searching the VO for distance values. In order to to this you can just click the "Search for Obj. Distances".
When you click this button, VOSA starts the operation of querying VO catalogs.
This search is performed asynchronously so that you don't need to stay in front of the computer waiting for the search results. You can close your browser and come back later. If the search is not finished, VOSA will give you some estimation of the status of the operation and the remaining time.
And, when everything is ready, you will see the values found in the VO catalogs for the distance to these objects.
Values with large relative errors are shown in red so that you are easily aware of large uncertainties.
At this point you still can choose to edit User values one by one and save them with the "Save Obj. Distances" button (as explained above). Or you also can decide to mark individually what value you prefer for each object among those available and click the "Save Obj. Distances" button to save those values as the final ones.
But you also can see a new form that offers you some options to choose the final values for all the objects in the file with just one click.
The form has two main parts:
- On the left, you have three different ways to prefer one catalog (or user values) over the other.
- Select first user value if available. And then, if not, always the VO value with the smallest uncertainty.
If you choose this, for those objects with user values, these will be chosen as the final ones. For those objects that don't have a user value for the distance, the one in VO catalogs with a smaller relative error will be saved as the final one.
- Select always the value with the smallest uncertainty.
This means, quite obviously, that the value with a smaller relative error will be chosen. Take into account that if there is not a value for ΔDis, this will be considered as the largest uncertainty.
- Select values by ranking.
Here you can specify the option that you prefer if there is a value available. If not, your second option will be used, and so on.
- On the right, you can decide if the above conditions are applied always or only when the relative error in the distance is below a certain limit. For instance, if you say that "Only when ΔDis/Dis < 0.5", if for one object, in one catalog, the relative error is larger, that will be considered as "there is not a value in this catalog".
When the "Make all changes" button is pressed, VOSA makes the selection adequate for your criteria and the corresponding values are saved as final.
For instance, if you mark the first option on the left, for those objects where is a value of the user distance, it will be the selected one, for the other objects, the van Leuween values are selected because they have smaller relative errors that the Kharcheko's ones.
Then, we select the third option on the left. And we set our preferences as: (1) user, (2) Kharchenko, (3) van Leuween. When we press the "Make all changes" button, Kharchenko's values for the distance is selected for HD002796 and HD003567, because there is not a user value for those objects.
Then, we change the order of preference to: (1) user, (2) van Leuween (3), Kharchenko. And we also set a limit ΔDis/Dis < 0.2 to make changes. In this case, for HD003567 there is not user value, then the van Leuween one is considered and ΔDis/Dis = 0.116 so it is selected and saved. But for HD002796 ΔDis/Dis = 0.92 in van Leuween and ΔDis/Dis = 10,4 in Kharchenko. So none of the values is selected and no change is made: the final value is kept as it previously was.