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I am looking at several galactic sources in the $I$, $R$ and $V$ bands and I want to calculate their absolute magnitudes. I can get the apparent magnitude and I know the distance so I just need to account for extinction to acquire the absolute magnitude. I first used the NED Coordinate Transformation & Galactic Extinction Calculator but the calculated extinction values are too high. I figured that this is probably due to the fact that my sources are relatively close ($\sim 100\,\text{pc}$) and I am thus only looking through a fraction of the dust that these coefficients were calculated for.

If I lower the extinction coefficients to a certain fraction of NED's value (say 60% in a particular case) then I get the expected answer but I'm looking for an accurate way to choose this fraction. I've looked for a method to calculate the extinction coefficients as a function of distance but haven't been able to find anything. Does there exist a map of extinction as a function of distance (and galactic coordinates) in the galaxy from which I can extrapolate the information required? Is there a way to calculate the extinction coefficients ($A_I$, $A_R$ and $A_V$) for an object in the galaxy a known distance away? I found IPHAS which seems to be the right idea but I'm not sure if I can use it to get what I want (or how).

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If your sources are within 100pc then the best thing to do is assume the extinction is zero, unless you are looking for absolute magnitudes that are a lot more precise than say +/- 0.1 mag. I think you would be very unfortunate if the extinction was any more than about ~0.1 mag in the V-band (and less than that at R and I).

Beyond that you are really struggling because the structure of the ISM is very non-uniform, so you would have to go with the kinds of maps that are produced (for example) using IPHAS - but these are only in the galactic plane. But even if you could find some general map covering all positions it would be unlikely to give you an accurate answer out to 100pc, because there isn't enough extinction to give an effect that is big enough to be measured with any precision (i.e. the uncertainty might be bigger than the value).

If you have spectral types for your objects then you could always compare the V-I colour with what you expect for a star of that spectral type. This gives you the reddening $E(V-I)$ and then (roughly speaking) $A_V \simeq 2E(V-I)$.

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  • $\begingroup$ Yeah, the reason I asked this is because I know the absolute magnitude of the star and there is a large discrepancy between this value and my calculated value; I originally put this down to extinction but since asking this question I have concluded that it couldn't be that. I'm not sure what could be the source of this discrepancy though. Thank you for the response. Sorry that I can't upvote your answer until I get more rep. $\endgroup$ – davly Apr 2 '15 at 23:11
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You might want to have a look to the GALExtin models. The official site is still not finished (not even sure if it's still being developed), but you can access the original article here, and download the models here.

Here's a poster that provides a quick introduction.

Basically this is composed of two models of the Galaxy (one with spiral arms and one without) to which you give a (l, b) direction and a distance, and it gives you back the extinction.

It's a little bit old but perhaps it can be useful to you.

Also, the advice given by Rob is a good one: for such small distances perhaps the best thing to do is to assume zero extinction.

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