PH222 - Astrophysical Concepts PDF

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Last time We left off thinking about the impact the ISM has on light In particular we saw how we could take advantage of a star's known spectr...

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Last time We left off thinking about the impact the ISM has on light In particular we saw how we could take advantage of a star's known spectral class a star's expected spectral energy distribution a star's observed spectral energy distribution these are neatly bundled into colour indices If we have a means of quantifying the ISM's extinction effect we can infer the distance between us an the star spectroscopic parallax (which we continue today...) 2 ( - ) is also expressed as & =( - )-( - ) 3 By convention, the extinction estimate is based around the and bands. The more widely used expresson is = 4 Whiteboard time... 5 F0V star, = +3.0, - = +0.30, = +11.0, = +10.1 & = 3.1 ? ? From before we were presented with expressions for and And we also saw that there was a relationship between , and - so we're going to solve this by taking = and solving for having calculated and knowing. 6 F0V star, = +3.0, - = +0.30, = +11.0, = +10.1 & = 3.1 ? ? From (3) before =( - )-( - ) = (11.0 - 10.1) - (0.30) = 0.60 From (4) before = ( ) = (3.1)(0.60) = 1.86 7 F0V star, = +3.0, - = +0.30, = +11.0, = +10.1 & = 3.1 ? ? From (3) before =( - )-( - ) = (11.0 - 10.1) - (0.30) = 0.60 From (2) before 5 = - +5- = 10.1 - 3.0 + 5 - 1.86 = 10.24 = 10 = 112 pc 8 Interstellar Extinction Curves The actual value(s) of and so the overall effects of extinction vary from galaxy environments Note the 'bump' around 2175 Å - believed to be a consequence of graphite or similar carbonaceous dust. So what about dust?? 9 Recall Barnard 68 10 Polarimetric image stack of comet 2I/Borisov with from it's December 2019 apparition - false-color top panel, isophote map lower panel. 11 How dust polarises light... 12 Mapping the large scale magnetic field of the galaxy using dust polarisation measurements 13 Studying the 'weather' on brown dwarfs using polarimetry After spectral class M comes L & T... L-dwarfs' atmospheres are just the right temperature/pressure for dust grains to nucleate out - their polarimetric signature is detectable, and can be used to study their meteorology. 14 Detecting exoplanetary oceans... 15 16 Dust in the Cas A Supernova Remnant Cassiopeia A (Cas A) located 3.4 kpc away, believed to have exploded - from our perspective - in the 1690s Brightest radio source in the sky - used as a calibrator 17 Dust closer to home.. 18 OSIRIS-REx 'grabbing' a sample of 101955 Bennu (20/10/20) 19 DIY Dust Formation 20 Dust plume from NASA's DART impact into asteroid moon Dimorphos on 26/09/2022 The ATLAS project captured stills of the DART mission's impact on Dimorphos. Images: ATLAS Project (University of Hawai`i / NASA); Image processing: Emily Lakdawalla 21 V. important for subsequent coalesence into larger aggregates.. also chemistry 22 Whiteboard time... 23 =....? 24 Show that the temperature of a dust grain at a distance from a star of size and temperature is = Let's think about this problem... We have a dust grain orbiting a star... It's going to be in thermal equilibrium... So we expect the energy emitting by the grain = energy absorbed at a distance from the star... the energy emitted will follow a blackbody distribution function based on the energy absorbed will follow a blackbody distribution function based on The trick is to derive expressions for the energy emitted & absorbed and solve for 25 Show that the temperature of a dust grain at a distance from a star of size and temperature is = The luminosity of a star with and with a temperature can be defined as =4 At a distance , the fraction (or ratio) of the spherical surface area 'radiated' to this point against the area of a dust grain of size is = The fraction actually absorbed is defined as =( ) where is the albedo of the dust grain. albedo is a measure of reflectance 26 Show that the temperature of a dust grain at a distance from a star of size and temperature is = So, the power absorbed by the grain fraction absorbed fraction at initial power at the star or or =( ) 4 = 27 Show that the temperature of a dust grain at a distance from a star of size and temperature is = What about the power radiated by a grain of size and temperature ? Normally we could just apply the basic BB luminosity relation but we have to correct for the albedo effect of the dust grain itself, so = 4 28 Show that the temperature of a dust grain at a distance from a star of size and temperature is = Well, the grain has to be thermal equilibrium so = = 4 Lots of things cancel out 29 Show that the temperature of a dust grain at a distance from a star of size and temperature is = = = = 30 What is the temperature of a dust grain 0.01 pc distant from the Sun? We already calculated = So we need = 5772 K = 6.96 10 m = 0.01 pc = 10 3.086 10 m = 5772 = 6.13 K Using Wein's Law, this corresponds to a peak emission wavelength of 0.47 mm (or 360 GHz)... 31 Wrap up we reviewed all the 'stuff' between our Solar system and the Centauri system captures what we'd expect to see happening everywhere else on in the galaxy what's there is a strong function of the era when the environment was processed to form gravitationally bound systems in particular we looked at how we can factor in the effects of dust extinction to estimate distances more accurately discussed the important role dust plays as the basis for solid objects, and also as a substrate for complex chemistry to occur. 32