The value of the interestellar extinction is necessary to deredden the observed photometry before analyzing it. If the extinction is not negligible, the shape of the real SED can be very different from the real one and any physical property estimated using the SED, if not properly unreddened, can be erroneous.
For instance, see the difference the observed SED (gray line) and the dereddened one (red points) for an object with Av=3.
You can provide a value for the visual extinction Av for each object in your Input file. But, if you don't have those values, VOSA also offers the posibility to search VO catalogs for extinction properties.
And, finally, you can also give a range of values for Av so that the model fits (chi2 and bayes) fits together the model physical parameters and the value for Av.
The extinction law.
For dereddening the SEDs we make use of the extinction law by Fitzpatrick (1999) improved by Indebetouw et al (2005) in the infrared. Take a look to the corresponding Credits Page for more information.
(You can download the tabulated data for the extinction law).
The extinction at each wavelength is calculated as: Aλ = AV * kλ/kV, where kλ is the opacity for a given λ and kV=211.4
VO extinction properties.
The tool offers the possibility of finding extinction properties of the objects in the user file.
In order to do this, the object coordinates are used to query some VO services to find AV or RV and E(B-V) for each object.
Then you can choose to incorporate the found values (if any) into the final data or not. In fact, if it happens that diffferent catalogues give different information about the relevant quantities, you can choose which data to use to build the final AV value.
Remember that, if you decide to save new values for AV, the original data will have to be deredden again using the new values. This will change the final SED and, thus, if any other analysis has been done for the corresponding SED (for instance, a model fit) this analysis will have to be done again.
The first time that you enter this section for a given input file, the tool shows the AV values given in the input file (if any) and a button to search into VO services. When a search has been done, the tool will show the user values together with the found values for each relevant quantity so that you can choose which ones should be used (checking the corresponding box).
In fact, this form has several options that can be combined. Take into account that
- Some VO catalogues provide AV directly, but other catalogues give information about RV or E(B-V).
Using RV and E(B-V), AV can be calculated as AV = RV * E(B-V).
- VOSA can combine the data given by you in your input file and the information found in VO catalogues.
- You can write default values for AV, RV, E(B-V) or the AV range to be used in the fits.
If you do so, click the 'Add user values' button to fill the corresponding User columns with those values (only if there is not a previously saved value).
- Click the 'Search for extinction properties' button for seaching several VO catalogues.
- For each object, if you want to save some information, check the corresponding checkbox. If you mark values for RV and E(B-V) they will be used to calculate AV. If you want to do so, mark the ticks with the values that you want to save, and click the 'Save extinction properties button' and the AV values will be saved and shown in the 'Final' column (if there was enough information).
- When you have a large list of objects, it is difficult to do this object by object. You can also give some general criteria about what catalog information you trust better, and let VOSA try to build Av values for all the objects in the file using the available information. See below for a detailed example on how to use the different forms.
Take into account that the tool queries VO services using the object coordinates and returns the closer object to those coordinates for each catalogue in a given search radius. It could happen that the obtained information corresponds to a different object if the desired one is not in the catalogue. In that case, the obtained data could be erroneus, as it corresponds to a different object. So, please, check the coordinates given by the catalogue for each object to see if they seem to be the appropiate ones (within the catalogue precision) before using the obtained values.
Take a look to the corresponding Credits Page for more information about the VO catalogues used by VOSA.
We have uploaded a file with some objects and their coordinates, but we don't have information about the extinction for each object.
Thus, when we enter the "Objects:Extinction" tab in VOSA we see the list of objects and no extinction properties. We also see some forms:
- A button to seach the VO for extinction properties.
- A box, on the left, where we can add default values for AV, RV, E(B-V) or the AV range to be used later in the fits.
- A box, on the right, where we can give some criteria about our preferences among catalogs (this is of no use yet because we still don't have any information).
- A pagination form to see all the objects page by page.
- A "Save extinction properties" button that would be useful to save particular values for only the objects that we can see in the page.
We will see all these options with some detail below.
But, given that we don't have any information, our first step is searching for these objects in VO catalogs. And, thus, we click the "Search for extinction properties" button.
We get a list of all the catalogs that VOSA can use to search for VO properties. You can leave it as it is and just click the "Search" button. But you also could unmark some of them if you know, for some reason, that they are not going to be useful. You also can change the default Search Radius for some catalogs if you are aware that a differrent radius is more adequeate for your case.
We just click "Search". When we 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.
When the search is finished, VOSA shows you, on the right, all the information that has been found for each object. In some cases, we don't get any information at all (for instance, for objects 'test01' and 'test02'). In other cases we only get information from one catalog. But in some cases (for instance, objects 'test03' and 'test04') we get heterogeneus information from more than one catalog.
It happens very often that catalogs give values for E(B-V) but not for Av (like the Savage one in this example) and we need a value of RV to calculate AV using the expression AV=RV * E(B-V).
Thus, our first action is going to be adding 'Default user values' for some quantities. We write a value RV=3.1 in the "Default User Values" form and also a default fit range of (0-1) for Av. Then we click in the "Add user values" button (we could write the RV in the "User" column, object by object, but it's easier to do it this way).
Now we have values for RV so that VOSA can use them if they are necessary to build a AV value for some object.
Next, we use the form on the right to let VOSA try to build values for Av for all the objects. We mark the tick correspoding to "Select any combination of values that permits that a value for Av can be built" and click the "Save values" button.
As you can see:
- For objects 'test01' and 'test02' nothing can be done, because there isn't enough information available, and nothing has been changed.
- For objects 'test07' to 'test10' it has been easy. There is only information from one catalog for each object. And in every case the catalg gives a value for Av. Thus, this is set as the final value of Av for those objects.
- For objects 'test03' and 'test04' there are two different options that can be used to build a value for Av. VOSA always chooses the first combination of values that allows for calculating Av. Thus:
- For 'test03', VOSA first tries E(B-V)=0.61 from Savage and, given that we have a default value Rv=3.1, a value Av=3.1*0.61=1.891 is calculated and saved.
- For 'test04', VOSA first tries E(B-V)=0.69 from Savage and, given that we have a default value Rv=3.1, a value Av=3.1*0.69=2.139 is calculated and saved.
But we decide that we prefer Av=1.8 (from Morales) for the object 'test03' instead of the 1.891 value calculated before. And we want to make that particular change only.
Thus, we go to the list and:
- We mark the tick corresponding to the Morales catalog for 'test03' so that it is the value saved as final.
- We then click the 'Save Extinction properties' button.
and the 1.8 value is set as the final one for 'test03'.
But then we notice that, given that for objects 'test03' and 'test04' we have Av values 1.8 and 2.139 it does not make sense that, later, when performing model fits, we try an Av range between 0 and 1. We set that default range before, when we didn't have any information, but now we should change that range, at least, for these two objects.
Thus, we go to the list and make these changes one by one.
- We set Av range = 0-2 for 'test03' and mark the tick on its right.
- We set Av range = 0-3 for 'test04' and mark the tick on its right.
- We then click the 'Save Extinction properties' button.
And the Av fit ranges are changed only for these two objects.