star-formation

star-formation - n ouncemen’ s cumulative _ ial from the...

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Unformatted text preview: n ouncemen’ s cumulative _ ial from the course can be tested {30% of your grade 4;: indent will be allowed to bring in a single 8.5” x 11” sheet . ith whatever they want handwritten on it. (Yes, you ._ both sides.) t cover sections 14.5.2, 15.4, and 15.6.2- NASAI NOAO, ESA and The Hubble Heritage Team (STScl/ Star Formation (Kutner Ch. 15) ASTZDS (Spring 2011) Interstellar Molecules Interstellar clouds classification: H l clouds: mostly neutral hydrogen H II regions: ionized hydrogen (we'll see these in more detail when we look at star formation) Molecular clouds: H in molecularform (H2) We observe H l (neutral H) in 21 cm—this is the spin-flip transition We don't observe H2 directly, but instead observe CO at 2.6 mm —where CO forms we expect H2 to form as well. ASTZDS (Spring 2011) Star Formation (Carroll and Ostlie, Ch. 12) Can these dense clouds collapse under the influence of gravity? Thermal motions in the cloud resist gravitational collapse. Enough mass in a small region: gravity wins, cloud collapses. We can start with the virial theorem applied to the cloud: 2 (K) + (U> : 0 This is for a stable, gravitationally bound configuration. If the kinetic energy is too low, then the cloud will collapse. ASTZDS (Spring 2011) Jeans Length For a uniform cloud, we take 3GM2 (U) N? R We express the kinetic energy in terms of thermal energy: (K) : SNkT Assuming pure H: N = M/mp To collapse, we need 2 (K) < | (U)| so we take M M2 HR §G mp 5 R AST 203 (Spring 2011) Jeans Length Rearranging, we have M > 5kT R Gmp Using mass density: _ M we have '0 (4/3)7TR3 1/2 1/2 1/2 R > E [CT N [CT E RJ 47r Gmpp Gmpp This is called the Jeans length. We define the Jeans mass as 4 MJ : Asrzos (Spring 2011) Jeans Length If M>MJ orR>RJ —>collapse Note that as p increases, RJ decreases. For the conditions in a giant molecular cloud core, we have n N 105 CHI—3 and T N 50 K then R _ [CT 1/2_ [CT 1/2 J— GmPp _ Gmgn _ 1.38 X 10-16 erg K-1 50 K “2 T 6.67 x 10*8 dyn cm2 g’2 (1.67 x 10*24 g)2 105 cm’3 : 6 x 1017 cm: 0.2 pc AST 203 (Spring 2011i Jeans Length The mass corresponding to this is 4 . 4 r . MJ : gijpn : §(6.1 x 1017 cm)3 1.67 x 10"24 g 100 CHI—3 2 1.5 x 1035 g = 76M® These conditions are met in the cores of giant molecular clouds. The cores of molecular clouds can collapse, and this will lead to star formation—how long does it take to collapse? Asrzos (Spring 2011) Free-fall Time A particle a distance r from the center of the cloud experiences an acceleration of GM 7” CL(T) = T; ) Now, 4 3 M(r) : gm“ ,0 so 4 a(r) : gm'Gp Putting this together, we have the free-fall time 1 2 2r 1 r:§at :>tff: — a N x/Gp Note that the free-fall time is independent of the initial radius. AST 203 (Spring 2011) Free-fall Time and Jeans Length A disturbance in a gas cloud travels at the speed of sound The Jeans Length can also be found by setting the sound crossing time = free-fall time. ASTZDS (Spring 2011) Free-fall Time A real cloud has a higher density in its core, so the core will collapse faster than the surrounding material. For our example cloud, n = 105 cm—3 , and 1 1 tfi : : x/Gp \/6.67 x 10*8 dyn cmZ g72 . 105 . 1.67 x 10724 g : 3 x 105 yr This is very short compared to stellar lifetimes. In reality, rotation and magnetic fields affect the clouds collapse. AST 203 (Spring 2011) Rotation and Collapse Angular momentum is conserved in a rotating cloud. Hard to collapse perpendicular to rotation axis Can collapse parallel to the rotation axis As a result, a disk forms Smaller fragments of the cloud can collapse. Total angular momentum split between orbital I angular and rotational angular momentum of each cloud. Fragmentation allows these smaller clouds to more easily collapse. to i . _ T- (Kutner) Asrzos (Spring 2011) Outstanding Questions Do we need a trigger to initialize the collapse? Shock wave from nearby SNe? stellar wind from young star? How much of the mass of the cloud winds up in stars? What is the initial mass function of stars? OB associations: loose (unbound) collections of O and B stars Unbound stars form from a bound cloud—how? Mass loss needed. Do high and loss mass stars form in the same place? High mass stars may form in bursts. ASTZDS (Spring 2011) Molecular Clouds Molecular clouds are < 1% of the ISM, but they have high densities—likely sites for star formation. High density = easier to satisfy Jeans criteria Shorter free-fall time There are a number of different types of molecular clouds Bok globules are the smallest and can have clean, distinct shapes. R~1pc n~103—104cm'3 M~1O—1OOMO AV~1—1Omag ASTZDS (Spring 2011) (David Malin, Anglo Australian Observanory) Dark Clouds Moving up in size—dark clouds. Similar in density and T to globules Larger (R ~ 10 pc) More irregularly shaped M ~ 104 M 0 These clouds may contain low mass stars. Barnard 68 (FORS Team, 8.2-meter VLT Antu, ESO) ASTZDS (Spring 2011) Giant Molecular Clouds“ 3w 3, _ 4g; fiw ‘ A 1”; . l, .,__ Largest: giant molecular clouds. fife: _' K‘A‘wfi" 7‘ V a. r » : Elongated in shape R ~ 50-100 pc n ~ 300 cm'3 (smaller than globules) T ~ 15 K (hotterthan globules) M;:n;mmdc Detected by CO observations M ~105 MO, but they gather together in complexes that have M ~ 106 M . O CO emission (top) and intensity of ionized . gas (bottom) around Orion (B. A. Vlmson-T. M. Dame ASH” (8pm 2°11) - M. R. w. Masheder- P. Thaddeus, 2005) Dense Cores The densest regions of molecular clouds are called dense cores. These can be found in GMCs, dark clouds, or globules n ~105—106 cm'3 T ~ 50 K R < 1 pc This is where star formation is happening! Observations are challenging—small angular size. Ultimately, we'd like to see evidence for collapse of these cores (Doppler shifts of the CO lines). ASTZDS (Spring 2011) Protostars (Hester Ch. 15) After fragmentation, each core can still collapse. Cores become more dense, material falls onto them from further radii—the collapse goes from the inside-out. Protostar forms at the center. Some of the disk material accretes onto the protostar Some disk material goes into forming a solar system. Much higher luminosity than the Sun The protostar is very large Surface temperature ~ 1000s K } ASTZDS (Spring 2011) Protostars (Hester Ch. 15) Even though it is luminous, most of the radiation in in the infrared, and dust obscures our view. Gaseous Pillars - M16 ‘ HST - WFPC2 _ PHC95744a - ST Scl 0P0 - November 2, 1995 AST2°3 (SW19 2011) J. Hester and P. Scowen (AZ State Univ.), NASA Protostars (Hester Ch. 15) Protostar contracts —> radiates away gravitational energy 1/2 is radiated, 1/2 goes into thermal energy via the virial theorem. The core of the protostar heats up—H fusion begins. omina nl lluw within 10 D 6.000 3.000 surface temp-a lK-alvinl Asms (Spring 2011) «rem Bennett at an) H II Regions 0 stars produce lots of UV photons If A < 91.2 run, then E > 13.6 eV and we can ionize H 0 star inside an H cloud will ionize the surrounding region. This is called an H II region or a Stromgren sphere. H II region size is determined by balancing ionization and recombination of H. # of ionizations per second is just the number of UV photons per second from the central star Rion : NUV AST 203 (Spring 2011) H II Regions Recombination rate depends on the number density of protons, number density of electrons, and how likely they are to bind: Rrecomb : ans: an Here, oz ~ 3 x 10—13 cm3 5—1 is determined by considering all the possible energy levels the recombination can take place to. In equilibrium, we have: Rion : Rrecomb 4 . NUV : gwrjan; 3N 1/3 , rs—( UV) 1172/5 AST 203 (Spring 2011) H II Regions Hotter stars make more UV photons, and therefore have larger H II regions. Spectral type Nuv(x 1°“) 05 51.0000 06 17.4000 07 7.2000 08 3.9000 09 2.1000 BO 0.4300 B1 0.0033 ASTZDS (Spring 2011) H II Regions H II regions can be seen in radio, due to Bremsstrahlung ‘ .\ scattering of electrons off of protons (free- free radiation). Recombination to high energy levels in the . “ H atom produces photons as the electron ‘1 drops down into the ground state ‘1 Considerable H01 emission—appear red. E23, (Wikipedia) AST 203 (Spring 2011) H II Regions The Tarantula Nebula Spitzer' Space Telescope - IRAC NASA / JPL-Calt B. Brarldl [Corne‘l Un 'ty 8 Universwty 0| Leiden] 550200401 a ASTZDS (Spring 2011) H II Regions H II regions (red spots) in M51 (NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)) (Chris Schur) http://antwrp.gsfc.nasa.govlapodlap061123.htm| ...
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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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