ism - Interstellar Medium(Kutner Ch 14 also Shu Ch 11...

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Unformatted text preview: Interstellar Medium (Kutner, Ch. 14; also Shu, Ch. 11, Carroll and Ostlie Ch. 12) ASTZDS (Spring 2011) Distance Measures - Trigonometric Parallax ~500 pc Galactic - Moving Cluster Method - Spectroscopic Parallax ~30 kpc - Main-Sequence Fitting ~30 kpc Extra- galactic - Cepheid Variables ~30 Mpc - Type la Supernovae ~1000 Mpc ASTZDS (Spring 2011) Absorption (see also Bennett Ch. 16) In some regions of the sky, we see fewer stars Actually, dust is blocking the starlight from reaching us. Dust is usually concentrated into interstellar clouds. Spectra reveal the presence of cool gas. Next generation of stars form here. Barnard 68 (FORS Team, 8.2-meterVLT Antu, ESO) Note that the stars at the edge of the cloud appear red ASTZDS (Spring 2011) Extinction We don't see this dust directly in visible light—it's too cool. What wavelengths do you think it emits in? We do see the extinction due to scattering and absorption. Dust grains are slightly smaller than visible wavelengths of light Scattering preferentially affects shorter wavelengths The result is interstellar reddening of stars. blue starlight (Shu) . red starlight my ® . . interstellar Observer scattering Chm of starlight Figure 11.3. The mechanism of interstellar reddening. ASTZDS (Spring 2011) Reflection Nebula We can also see reflection nebula. Here the starlight is scattered off of a nearby dust cloud into our line of sight. These appear blue since it is the blue light that is scattered more. (Shu) Scattere “blue fight v reddened 51am Figure 11.7. Why a reflection nebula looks blucr than its illumi- ’ _ mating star. Compare this diagram with Color Plate 4. The PleladeS cluster (Robert Gendler) ASTZDS (Spring 2011) Optical Depth Reminder Optical depth: 7 = nal optically thin: 7' << 1 r = fraction of radiation absorbed only in this limit!). Io 11 I2 In In general, split the domain into thin slabs, such that air << 1 Change in radiation as we pass through one such slab is d] = 71 d7 Integrating, we find I : [De—T Ali d7 Optical depth can be thought of as the number of mean free paths a photon must move until it reaches the surface of the star. (Carrouandoaue) Mean free path: L = 1/(n0) AST 203 (Spring 2011) Extinction We can measure extinction: compare a star's apparent magnitude with what we expect from its spectral type (which tells us L) and distance: observed extinction expected We can express A in terms of the optical depth of the dust. We know that I = lee—T and m’ — m : 2.5log Flux is proportional to the intensity, so the extinction is I A : m’ — m : 2.5 : 2.5log(eT) : 2.57'loge : 1.086T Extinction of one magnitude means an optical depth of 1. AST 203 (Spring 2011) Star Counting Image two fields of stars down to a limiting magnitude mo: the field with extinction will, on average, show fewer stars. If there is extinction, only those stars with undimmed magnitudes m0 — A will appear Allows for an estimate of extinction, A N’(m)dm is the number of stars per unit area with magnitudes between m and E m + dm 80 (m) AST 203 (Spring 2011) Exflncflon In different wavelengths we see different amounts of extinction Dust preferentially removes the blue light. spectroscopic parallax Determine its spectral type (via spectra) —> luminosity Measure apparent magnitude to get distance Note that A DC 7' oc nl oc ND —extinction is proportional to the column density of dust along the line of sight. AST 203 lSpring 2011 l Exflncfion Ifwe measure the magnitude in the B and V bands, we have: my : My —— 510g(d/10 pc) —— AV m3 : MB —— 5log(d/10 pc) —— AB Subtracting, we have (7713 — WV) : (MB — Mv) -l- (AB — AV) w a This is observed This is B-V and is determined by the spectral type Spectral type is based on the presence/absence of lines—not sensitive to extinction. Measuring the magnitude in two bands tells us the difference in the extinction along the line of sight to that star in those bands. ASTZDS (Spring 2011) Stellar Colors Table 9.2. Spectral type, color, and effective temperature.“ Main sequence Giants Spectral type B — V TE (K) B — V Ti, (K) 05 *0.45 35.000 7 — B0 *031 21,000 — — BS i017 13.500 — —~ A0 0.00 9.700 — —- A5 0.16 8.100 — i F0 0.80 7.200 — — F5 0.45 6.500 — — G0 0.57 6,000 0.65 5.400 G5 0.70 5,400 0.84 4,700 K0 0.84 4,700 1.06 4,100 K5 1.11 4,000 1.40 3,500 M0 1.24 3,300 1.65 2.900 M5 1.61 2.600 — — ‘ Adapted from C. W., Allen. Astrophysical Quantities “— (Shu) ASTZDS (Spring 2011) Extinction We can use this behavior to do spectroscopic parallax to tell us the distance to a star—we just need to correct for the dust. Consider a BS star: My : —0.9, B — V : —0.17 We observe mB : 11, my : 10 Starting AB — AV : (mB — my) — (MB — MV) AB — AV : (11 — 10) — (—0.17) : 1.17 What do we do now? ASTZDS (Spring 2011) Exflncflon A useful quantity is the ratio of total-to-selective absorption: AV R—ABZAV Since both AV and A3 are oc N, the column density drops out. This is independent of the amount of dust along the line of sight. We might expect R to be roughly the same all over the sky It doesn't matter how much dust we are looking through. Observations tell us thatR N 3,1 R is determined by the nature of the dust. AST 203 (Spring 2011) Exflncfion Using Av -R:thz; we have AV Z — AV) Z ' Z Then my : My —l— 5 log(d/10 pc) —l— AV d : 10 pc - 10(mV—Mv—AVV5 : 10 pc . 10(10—(—0.9)—3453)/5 : 285 pc What would happen if we did not account for the extinction? ASTZDS (Spring 2011) Without Extinction Using mv = Mv + 5log(d/10 p0) d : pc _ 10(TTLV—JVIV)/5 : pc . 10(10_(_0'9))/5 = 1510 pc ASTZDS (Spring 2011) ...
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This note was uploaded on 05/04/2011 for the course AST 203 taught by Professor Simon,m during the Spring '08 term at SUNY Stony Brook.

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