The influence of reddening

(You should have found this page from SED plots or Color-Magnitude and Color-Color plots.)

Introduction

By now, you may have realized that:

• Color-magnitude diagrams (CMDs) or for that matter color-color diagrams are a good way of looking at the ensemble distribution of points for a set of objects, e.g., looking at the properties of many objects at once in 2 to 4 bands at once.
• Spectral energy distributions (SEDs) are a good way of looking at the ensemble distribution of the set of detections for a single object, e.g., looking at the properties of one object in many bands (usually all the bands you have) at once.

So, they are two different facets of the same information -- you could have a suite of CMDs looking at a set of objects in several different color-mag (and color-color) spaces, and a suite of SEDs looking at several different objects in a set of several SEDs. Both ways of looking at the information are useful, and we will use them both.

In some cases, the influence of reddening can be important, and that is what this page is about. (For still more information, consult your local college astro textbook, or try Wikipedia entry on reddening/extinction)

Reddening is the same phenomenon we observe here on Earth when watching a sunrise or sunset -- the Sun appears redder than usual because the blue (higher energy) light is preferentially scattered by the atmosphere out of the line of sight, leaving mostly red light to reach your eye. In that case, the more junk in the atmosphere (like from a volcano), the more dramatic the reddening. Same thing happens in space, just due to junk inbetween us and the source we're observing.

As you might imagine, this happens rather a lot in star forming regions, because the stars form in cocoons, in clouds of gas and dust. So we do often have to worry about this, formally. However, the influence of reddening is strongest at bluer bands (e.g., U band is very sensitive to it) and becomes less strong the farther into the IR you go -- JHK can suffer some effects, and you kind of have to be behind a brick wall before it matters for IRAC bands. But it can happen.

Reddening is commonly abbreviated Ax or A_x or A subscript(x), and means the amount of reddening at band x, e.g., Av is reddening at V band. When expressed in this way, the units of the reddening are magnitudes, e.g., "This object has 4.3 magnitudes of reddening at V, or Av=4.3."

Effect of Av on SEDs

Now let's try and understand how Av affects SEDs.

Remember this YouTube video which has a bunch of blackbody curves for a variety of temperatures corresponding to the effective temperatures (Teff) of a variety of spectral types. It has vertical lines indicating the position of various bandpasses, and the idea is that you can see how the shape of the underlying approximate stellar photosphere changes as the Teff changes, and you can then infer how the ratio between measured fluxes in the various bandpasses might change.

Now, see this YouTube video which now holds the underlying blackbody temperature constant at T~5800K, but now varies the Av. The bandpasses are indicated, though I think the vertical lines are different between this and the prior movie, sorry. Look at the upper right of the plot -- it tells you both the Aj and the Av. Most people think in Av, but my code happens to be written in Aj for no fantastically great reason other than the paper from which I was copying the reddening law (Mathis et al 1990, I am pretty sure) was working in Aj. One thing to note is that Av gets pretty large pretty fast, and Aj doesn't get as large as fast. This is because V-band is more sensitive to reddening than J-band (since Jband is further into the IR).

So, run this through a few times and watch how the SED changes. More reddening pulls down the left (blue) side of the SED really fast. The bluer the band, the faster and stronger the influence of Av; the redder the band, the slower and weaker the influence of Av. Note too that even though I'm still stepping through Aj with the same stepsize (0.2 mags, I think), for large Aj, the rate of change of the SED shape slows. The deep absorption feature that appears comes from the 10 micron silicate absorption - the interstellar medium has a lot of silicates (e.g., beach sand) that have a prominent absorption line at 10 microns, so the more junk there is intercepting the light, the more there is to absorb the light at 10 microns.

If you look at this second movie in comparison to the first movie, you can see that if we just have photometric points for a given object, we can't a priori tell if it is a cooler star, or a hotter star with lots of Av. This is one reason (of many) that we need a follow-up spectrum of our YSO candidates to determine if they are real YSOs, and if so, what spectral type they are, e.g., what is the temperature of the underlying photosphere. Then we can get a better handle on the amount of Av.

When looking at the second movie where Av changes, as for the first movie where Teff changes, you can look by eye at the locations of your two favorite bandpasses, and start to get an intuitive sense of how the colors must change as Av changes (or Teff changes in the first movie). Let's look at this next.

Effect of Av on color-mag and color-color diagrams

In papers like this, you can often see Av vectors indicated in the plots.