ASTR2050 Spring 2005 Lecture 10am 29 March 2005 Please turn in your homework now! In this class we will discuss the Interstellar Medium: Introduction: Dust and Gas Extinction and Reddening Physics of Dust Grains Atomic and Molecular Gas 1
2 Sure it s beautiful but I can t help thinking about all that interstellar dust out there.
Introduction: Dust and Gas A Prelude to Star Formation (next week) The space between stars is not empty: Dust: Relatively large grains of relatively complex materials. Although only about 1% of the mass of a cloud, they are the main reason the cloud appears dark. Gas: Atoms or molecules, making up 99% of the mass of the cloud. Observed by narrow emission or absorption lines. We see gas in the direction of the dark clouds, so we are pretty sure that they are part of the same thing. 3
Absorption and Scattering 4 Light scatters more effectively at short wavelengths, so the reflected light is blue. Therefore, the light that passes through appears reddish. Different effect: H-II regions are hot gas, and glow red from Balmer H-α emission.
Example: The Horsehead Nebula See Textbook Figure 14.3 H-II Region (Red!) Reflection Nebulae (Blue!) 5 Dark Cloud
Example: The Trifid Nebula More clear examples of reflection nebulae, hot H-II regions, and dust lanes. 6
Example: The Black Cloud Barnard 68 This is an extreme example of reddening. Blue, Visible, Infrared Blue, Infrared, K-band 1/2 Light Year 7
Using filters at different wavelengths: 8
Extinction and Reddening We can quantify the amount of light absorbed by a cloud. Light intensity is exponentially attenuated: I = I 0 e! This results in a difference in apparent magnitude: A = m m = 2.5log 10 (I/I 0 ) = 2.5!log 10 (e) = 1.086! i.e. one optical depth gives one magnitude of extinction So, we can talk about the true apparent magnitude: m = m no extinction + A = M + 5log(r/10 pc) + A The distance r to the star may or may not be known. 9
Ratio of Total-to-Selective Extinction Red light is absorbed less than blue light, so the extinction for visible light A V should be less than A B for blue light. Note that we can measure A B -A V as follows: m V =M V + 5log(r/10 pc) + A V m B =M B + 5log(r/10 pc) + A B (m B m V )=(M B M V ) + (A B A V ) The left side is what we observe directly, and the first term on the right (the color of the star) can be deduced by using spectral classification. 10
Both A V and A B are proportional to the amount of dust in between the star and the observer. Therefore, the quantity R A V A B A V > 0 is independent of the amount of dust. R is called the ratio of total-to-selective extinction. Observations find R 3.1 for almost all regions. This is the standard value we will use. Therefore if we measure A B -A V then we can infer A V! (This is a way to determine stellar distances. See Homework.) 11
Extinction Curves Carry through this same exercise but as a function of the wavelength λ. The hump near λ= 200nm shows up all the time! More on this later. 12 See also Kutner Fig.14.6
Physics of Dust Grains Physical properties of dust grains include... Size and shape Alignment mechanism and polarization Composition Temperature Electric charge Formation and evolution We are not going to study these in any detail, but your textbook has many details. 13
Infrared spectra of a late stage red giant (HR 4049) and planetary nebulae. Emission from 20-100 C atom polycyclic aromatic hydrocarbons (PAH s) are shown in red. This tells us about the composition, size, and temperature of grains. 14 See also Kutner Fig.14.7
15
Dust grain concepts Key point: The grain size, relative to the wavelength, determines extinction. 16 See also Kutner Fig.14.8
Blocking light with dust grains Dust is only 1% of the mass of the ISM, but remember our old discussion about 0! l=0.5 c 1yr =0.5 10 18 cm n=1/10 6 m 3 = 10 12 /cm 3! (3µm) 2 = 10 7 cm 2!1/2 Light Year 17 nl! 0.05 In other words, for these values, 5% of the light with wavelength below 3 microns is blocked.
Atomic and Molecular Gas Free atoms and (small) molecules Some important components: Hot (ionized) hydrogen gas ( H-II ) Cold (atomic) hydrogen gas ( H-1 ) Cold molecular hydrogen (H 2 ) Other cold molecules (e.g. HD, CN, CO,...) 18
HII Regions: Hot Ionized Hydrogen Example: Rosette Nebula Red glow is from Balmer Hα (3 2) emission Familiar sight in regions of massive, hot, young stars. In fact, this one shows a Stromgen sphere But this is not a large component of the interstellar medium. 19
Cold Atomic Hydrogen Gas Key to detection: Very low energy spin flip transition Wavelength (Frequency) of radiation is 21cm (1.4GHz) See also Kutner Figure 14.11 20
Application: Spiral Arms of our Galaxy Locate clumps of HI gas by observing Doppler shift from differential rotation of arms. Necessary velocity resolution is easy with radio waves (HW) -50 0 +50 +100 Radial Velocity (km/sec) 21
Cold molecular gas Rotational Energy Levels: E = h2 [J(J + 1)] 2I Energy (ev) Have J=0,1,2,3,... unless the molecule is diatomic (eg H 2 ) Important interplay with level population with temperature: n j = g [ j exp E ] j E i n i g i kt (Kutner Equation 3.9) Example: CO 22 So, the molecular species can have a lot to with the local environment of the ISM.
(Optical image) Example: CO Map of a distant galaxy viewed edge-on 23