Interstellar extinction

The presence of dust in the interstellar medium was first recognized by its reddening effect on the light from distant stars. The apparent magnitude, m, of a star is given by m(A) = M(A) + 5log[d] + Aa, (5.85)

with M the absolute magnitude, d the distance, and Aa the extinction due to dust. Now, the extinction can be derived by comparing a reddened star with a nearby star with the same spectral type, resulting in a magnitude difference given by d1 d2.

The color excess between two wavelengths, A1 and A2, can then be defined as E(A1 - A2) = Am(A1) - Am(A^ = Aa1 - AA2 . (5.87)

Because extinction generally increases with decreasing wavelength through the visible, extinction is often called reddening. Color differences between different stars can readily be compared after normalization on a common color difference. Often, the B—V color in the Johnson color system is used for this; viz.

Measurement of extinction in terms of reddening is, thus, rather straightforward. However, it is physically of more interest to consider the normalized extinction ratio, Aa/Av. Define the total-to-selective extinction ratio, RV, as

The extinction ratio, Aa/Av can then be expressed in terms of the color excess as,

Because extinction rapidly decreases with increasing wavelength into the infrared, we can write,

Thus, the total-to-selective extinction ratio can be determined by extrapolating measured color excesses into the infrared and hence the extinction ratio Aa/Av can be determined.

While RV is introduced here to connect reddening to extinction, this parameter is often used in a different capacity as well. In particular, extensive observations have shown that the observed interstellar extinction - from the mid-IR through the visible and near- and far-UV - can be characterized by one free parameter, which is then chosen to be the total-to-selective extinction ratio. Figure 5.7 shows the mean extinction curve in the solar neighborhood for three different values of RV. In each case, the extinction is characterized by a gradual increase from the infrared through the visible to the near-ultraviolet. In the near-infrared part of the spectrum the "continuum" extinction is approximately proportional to A—17, while the visible extinction is somewhat less steep (A ~ A—1). The near-ultraviolet is characterized by a distinct knee around A—1 = 2.5 1. The ultraviolet portion of the extinction curve is dominated by a pronounced bump centered at 2175 A (or 4.6 1) and a steep rise to short wavelengths.

Thus, the interstellar extinction curve separates out into four distinct parts -the infrared, the visible, the 2175 A bump, and the far-UV rise. Extinction curves vary from region to region but they can (almost) all be parameterized by the total-to-selective extinction ratio, RV, over the full wavelength range 0.1-2 ^m. The value of RV depends on the environment traversed by the line of sight. The diffuse ISM is characterized by RV = 3.1, while dense molecular clouds have values

Colour Excess Astronomy

Figure 5.7 Three observed extinction curves are shown as a function of A-1. These curves show the range in wavelength behavior of the extinction laws in the interstellar medium. The solid lines show, for comparison, the computed parameterized extinction. The insert shows the deviations. Figure courtesy of J.S. Mathis; reprinted with permission from Ann. Rev. Astron. Astrophys., 28, p. 37, ©1990 by Ann. Rev. (www.annualreviews.org).

Figure 5.7 Three observed extinction curves are shown as a function of A-1. These curves show the range in wavelength behavior of the extinction laws in the interstellar medium. The solid lines show, for comparison, the computed parameterized extinction. The insert shows the deviations. Figure courtesy of J.S. Mathis; reprinted with permission from Ann. Rev. Astron. Astrophys., 28, p. 37, ©1990 by Ann. Rev. (www.annualreviews.org).

in the range 4-6. Figure 5.7 compares observed extinction curves in different environments with the RV -parameterized curve.

Structure in the extinction curve can provide important clues to the nature of the absorbing materials. The ultraviolet bump is the most prominent feature of the interstellar extinction curve. Its peak position is very constant (Ap = 2175 A with a mean deviation of 9 A). The width of the bump varies much more widely with a typical value of 480 A but extremes of 360 and 770 A. After subtraction of an underlying linear continuum extinction, the 2175 A feature is well represented by a so-called Drude profile, characteristic for absorption associated with a resonance in a conductor.

The visible extinction curve shows weak fine structures, called the diffuse interstellar bands (DIBs), with typical widths of 1-20 A and strengths < 0.01 AV. Some 200 bands have been discovered, spanning the wavelength range of the near-UV (4300 A) to the far-red (~1 ^m). These bands are now generally thought to be due to absorption by large molecules rather than dust (cf. Section 6.7.4).

Several absorption features are present in the infrared (cf. Fig. 5.8). The 9.7 ^m feature is the strongest one - t9 7/Av = 18.5 in the solar neighborhood. This band

Wavelength (|lm)

Figure 5.8 The spectrum of sources in the galactic center show strong absorption features due to dust grains along the line of sight. These are shown here on an optical depth scale. Identifications are indicated on top. Some of these features originate in the diffuse interstellar medium (e.g., the hydrocarbon [HAC] bands), while others are due to material in molecular clouds (e.g., H2O, NH3, and CH4 ice). Some bands have probable contributions from both media. Figure courtesy of J.E. Chiar. Data from J.E. Chiar, et al., 2000, Ap. J., 537, p. 749.

Wavelength (|lm)

Figure 5.8 The spectrum of sources in the galactic center show strong absorption features due to dust grains along the line of sight. These are shown here on an optical depth scale. Identifications are indicated on top. Some of these features originate in the diffuse interstellar medium (e.g., the hydrocarbon [HAC] bands), while others are due to material in molecular clouds (e.g., H2O, NH3, and CH4 ice). Some bands have probable contributions from both media. Figure courtesy of J.E. Chiar. Data from J.E. Chiar, et al., 2000, Ap. J., 537, p. 749.

is very broad and structureless, AA ~ 2.5 ^m. It is accompanied by a slightly weaker - t18/t9 7 = 0.6 - and even broader (~5 ^m) feature at about 18 ^m. There is also a weak feature at 3.4 ^m, which shows characteristic substructure. For sources in or behind dense molecular clouds, a plethora of absorption features appear. These are attributed to interstellar ice mantles growing on the dust grains in the shielded environment of interstellar clouds and these are discussed in detail in Section 10.7.5.

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