Stellar Evolution & Planetary System Formation: Lecture Notes for ASTR-1020 at ETSU - Prof, Study notes of Astronomy

Download Stellar Evolution & Planetary System Formation: Lecture Notes for ASTR-1020 at ETSU - Prof and more Study notes Astronomy in PDF only on Docsity! ASTR-1020: Astronomy II Course Lecture Notes Section V Dr. Donald G. Luttermoser East Tennessee State University Edition 4.0 Abstract These class notes are designed for use of the instructor and students of the course ASTR-1020: Astronomy II at East Tennessee State University. Donald G. Luttermoser, ETSU V–3 4. As a large portion of the GMC collapses, many internal eddies and turbulent motions can exist within the cloud. As a re- sult, when fragmentation to stellar-mass sizes occur, each little cloudlet has a rotation associated with it that was induced from one of these eddies as shown below. axi s Cloudlet 5. As the cloudlet contracts, it spins faster due to the conservation of angular momentum L = M R2 ω, where ω is the angular ve- locity of the protostellar cloudlet =⇒ since M is constant, as R gets smaller, ω gets larger. This increased spin causes the equa- torial region to bulge outward which flattens the cloudlet. This continues until a central bulge with an equatorial disk forms. axi s Bulge Disk V–4 ASTR-1020: Astronomy II a) The equatorial disk flattens rather quickly (hundreds of years) during this stage. At this point, the disk is now referred to as a proplyd or protoplanetary disk. In the case of the Solar System, we call this stage the solar nebula. b) Numerous such disks have been seen in stellar nurseries with the Hubble Space Telescope. They are especially easy to see at IR wavelengths. c) In this protoplanetary disk, dust grains begin to stick to- gether from condensation and accretion, building in size to form planetesimals. These planetesimals conglomer- ate further into protoplanets. Protostar IR Light Protoplanet 6. As this contraction continues, the temperature and the pressure at the center of the cloud rises. There comes a time when this center gets so hot, it starts emitting visible photons. The sur- rounding gas and dust in the cloudlet (i.e., the cocoon) absorbs Donald G. Luttermoser, ETSU V–5 the visible light and re-emits it as infrared light. The contract- ing cloudlet now is called a protostar. 7. The visible light now reaches the surface of the protostar (still being power by gravitational contraction) and the pressure from this light starts to push out the unused material in the planetary disk. This spring cleaning phase is called the T-Tauri stage of the star. Bipolar Outflow (no disk at poles to restrict wind) T-Tauri Model Herbig-Haro (HH) Object HH Object (bowshock of jet) Planetary Disk Protostar 8. At the center, the temperature and pressure build so high that nuclear reactions start =⇒ A STAR IS BORN. The star (e.g., the Sun) is now a main sequence star and only planets and asteroids remain in the inner solar system. V–8 ASTR-1020: Astronomy II 0 1 2 3 Orbital Semimajor Axis (AU) 0.25 MJ 0.54 MJ 4.1 MJ 0.48 MJ 0.46 MJ 0.63 MJ 0.46 MJ 0.68 MJ 1.3 MJ 1.1 MJ 0.93 MJ 4.2 MJ 3.5 MJ 0.81 MJ 0.99 MJ 8.1 MJ 2.1 MJ 0.22 MJ 11.0 MJ 7.4 MJ 1.1 MJ 3.5 MJ 1.1 MJ 7.2 MJ 1.6 MJ 2.8 MJ 3.0 MJ 1.2 MJ 1.3 MJ 5.2 MJ 1.7 MJ 6.1 MJ 2.6 MJ 5.4 MJ HD 46375 HD 187123 Tau Boo BD-10 3166 HD 75289 HD 209458 51 Peg Ups And HD 217107 HD 130322 55 Cnc GJ 86 HD 195019 HD 192263 Rho CrB HD 168443 GL 876 HD 16141 HD 114762 70 Vir HD 52265 HD 1237 HD 37124 HD 89744 HD 134987 HD 12661 Iota Hor HD 177830 HD 210277 HD 222582 16 Cyg B HD 10697 47 UMa 14 Her 2.0 MJ 4.3 MJ 4. There are a variety of techniques used in determining whether or not a star has a planetary system around it. a) Direct imaging: Getting pictures of the actual plan- ets. This is virtually impossible to accomplish due to the large distances of the stars and the relative small size of planetary systems with respect to these distances. A planet like Jupiter, 5 AU from the brightest star in the sky, Sirius, only would be 2.0 arcsecs from the star and Donald G. Luttermoser, ETSU V–9 the brightness of the star would hide such a planet in its glare. b) Detection of a “wobble” in the proper motion of a star: As stars orbit the center of the Milky Way Galaxy, stars change their relative positions to each other =⇒ stars have both a radial (line-of-sight ) velocity com- ponent and a velocity component in the plane of the sky perpendicular to the radial velocity called the star’s proper motion (see §III.A). i) Large planets in orbit about a star would cause the star to wobble along its proper motion path across the sky as the star and the large planet both orbit about the common center-of-mass. ii) Such a wobble would be a small scale effect and no planetary systems have yet to be discovered using this technique. For instance, the center-of- mass of the Sun and Jupiter is just outside the surface of the Sun some 4.6 × 107 m (0.066 R ) above the Sun’s photosphere. From the distance of the nearest star α Cen, this corresponds to a total wobble deviation of 0.0038 arcseconds! This would be extremely hard to detect. iii) Faint (unseen) stars (typically M dwarfs) have been detected in this manner. In the diagram be- low, each cycle of the wobble (i.e., the time that passes between each maximum of the wavy line) corresponds to one orbit of the unseen companion about the visible star. If the picture below corre- sponds to a change of position of the visible star V–10 ASTR-1020: Astronomy II over 100 years, the orbital period of the unseen companion would be 16.7 years since there are a total of 6 cycles over this time period. Proper Motion of Nearby Star (wobble indicates an unseen companion is present) c) Doppler shifts in spectral lines as the star or- bits the center-of-mass: The figure below shows the physics of the situation. As the star and planet orbit a common center-of-mass, the spectral lines of the star will shift back and forth due to the changing orbital velocity. i) The velocity shifts of a planet star interaction would be very small — on the order of a few me- ters/second for a large Jupiter-like planet orbiting close to the star. ii) This is the type of technique that has been used to detect these recently detected extrasolar plan- ets.