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Lecture Star Formation 2. Pptx.Pptx Lecture Star Formaon History 2 Prof. George F. Smoot Star Formaon 2 1 Star Formaon, H II Regions, and the Interstellar Medium Star Formaon 2 2 Introduc-on • Baryonic maer: Maer composed of protons and neutrons • Interstellar Medium (ISM): Consists of molecular, atomic, and ionized gas with large ranges of densi-es and temperatures, as well as dust • New Stars form from molecular, gas-phase maer • ISM releases stellar energy by means of physical processes that are dis-nct from star and stellar formaon Star Formaon 2 3 What to Expect • Focus on the ISM in star-forming regions • Briefly survey star forming environments • Proper-es of young stellar populaons • Physical processes that operate in the ISM around newly formed stars and the observable conseQuences of those processes Star Formaon 2 4 5.1 Cloud Collapse and Star Formaon • Stars form in molecular clouds, which are one component of ISM • Clouds are randomly shaped agglomeraons composed of molecular hydrogen 2 5 • Typical mass of 10 -10 MSun • Sizes of hundreds of parsecs • Densies of 108-1010 m-3 (mostly hydrogen) • Temperature between ~ 10–100 K Star Formaon 2 5 Molecular Clouds • Clouds are largest, most massive, gravitaonally bound objects in ISM • Densi-es of clouds are among highest found in ISM • Nearest giant molecular clouds are in the Orion star- forming region, ~500 pc away Star Formaon 2 6 7 Cloud in Orion Molecular hp://www.outerspaceuniverse.org/media/orion-molecular-cloud.jpg Star Formaon 2 Molecular Clouds • Mean mass density in stars are ~103 kg/m3 and par-cle density of ~1030 m-3 • To form new stars, regions of molecular cloud must compress by many orders of magnitude • Star formaon processes are not fully understood yet • Cloud contracts under gravitaonal pressure and lead to condi-ons reQuired for star formaon Star Formaon 2 8 Jeans Instability • Consider a spherical gas cloud of constant density ρ and temperature T, composed of par-cles with mean mass <m> • The gas is ideal, classical, and nonrelavis-c • The cloud’s mass is M, its radius is r, and its gravitaonal energy is GM 2 Egr ≈ r Star Formaon 2 9 Jeans Instability 2 • If the cloud undergoes a radial compression dr, the gravitaonal energy will change by GM 2 dEgr = dr r2 The Volume will decrease by dV = 4πr2 dr • The thermal energy will therefore grow by 2 M dr dEth = PdV =nkT 4πr dr =...=3 kT < m > r Star Formaon 2 10 Jeans Instability 3 • The cloud will be unstable to gravitaonal collapse if the change in gravitaonal energy is greater than the rise in thermal energy |dEgr| > dEth • For a cloud of given radius r and temperature T, collapse will occur if the mass is greater than the Jeans Mass 3kT M J = r G < m > Star Formaon 2 11 Jeans Instability 4 • A cloud of mass M will collapse only if its radius is smaller than the Jeans Radius G < m > r = M J 3kT • A cloud must have a density greater than Jeans Density 3 M 3 ⎛ 3kT ⎞ ρJ = 4 3 = 2 ⎜ ⎟ 3 πr 4πM ⎝ G < m >⎠ Star Formaon 2 12 Jeans Instability 5 • At low temperatures (T ~ 20K), there are few free electrons and most of the hydrogen is bound in H2 molecules, so <m> ≈ 2 mH. For a cloud of M ~ 1000 MSun -21 3 ρr = 3·10 kg/m • This corresponds to a number density of ρ n (H ) = J ~ 1×106 m−3 J 2 < m > • Thus, typical observed density of molecular clouds, 108-1010 m-3, is two to four orders of magnitude higher than the Jeans density Star Formaon 2 13 Jeans Instability 6 • Thus, typical observed density of molecular clouds, 108-1010 m-3, is several orders of magnitude higher than the Jeans density, (~106 m-3) • Clouds should be unstable to gravitaonal collapse • Since clouds exist and are long-lived, there must be another source of pressure present • We currently believe the dominant pressure is provided by turbulence, magne-c fields, or both Star Formaon 2 14 Jeans Instability 7 • If we consider a one-solar mass clump of gas inside 8 10 -3 a 1000MSun cloud (T=20 K, ρ = 10 -10 m ), the Jeans Density for this clump is ~103 -mes larger ~ 1012 m-3 • This is much higher than the actual density of the clump and the clump is thus stable against collapse • Therefore we expect the collapse of a molecular cloud to begin with larger masses that easily pass Jeans’ criterion Star Formaon 2 15 Jeans Instability 8 • As collapse proceeds, the cloud’s fragmentaon into smaller and denser masses con-nues un-l the creaon of a cluster of individual protostars • Protostar – objects that will evolve into stars on the main seQuence • The Jeans Radius of a molecule cloud fragment of Mass 1MSun, which will collapse into a protostar is 14 rJ ~ 5·10 m ~ 3,300 A.U. Star Formaon 2 16 Fragmentaon A -me seQuence showing the collapse and fragmentaon of a molecular cloud core to form a bound system of three stars Star Formaon 2 17 From: hBp://www.astro.ex.ac.uk/people/mbate/Research/96-04.html Protostar: Herbig-Haro Star Formaon 2 18 From: hBp://upload.wikimedia.org/wikipedia/commons/6/6e/Protostar_Herbig-Haro_46_47.jpg Cloud Collapse • Condi-ons can be present for collapse, but can only proceed with an avenue for energy release • If Egr is converted to Eth, T and P rise and the collapse will stop • Two avenues for energy “loss”: – Dissociaon of H2 using 4.5 eV per molecule – Ionizaon of hydrogen using 13.6 eV per atom Star Formaon 2 19 Energy Release • If ionizaon and dissociaon take place, the density -15 3 will reach Jeans Density, ρJ = 3x10 kg/m , where a free fall collapse will occur: 1 ⎛ 3π ⎞ 2 τ ff ≈ ⎜ ⎟ = 40,000 yrs ⎝ 32Gρ ⎠ • Free fall collapse stops aer dissociaon and ionizaon are over because of rapidly increased T and P • Adiabac approximaon gas does not radiate away any energy 1 ⎛ M M ⎞ E 4.5eV 13.6eV gr = −⎜ + ⎟ 2 ⎝ 2mH mH ⎠ Star Formaon 2 20 • 2 8eV T = = 60,000 K 3 0.86x10 −4 eV ⋅ K −1 • GMmH r ≈ = 0.3 A.U. 6kT – Dissociaon and ionizaon of hydrogen can consume enough energy to bring a solar mass protostar down to dimensions of 100x that of the Sun – Processes do occur that radiate energy considerably lowering T – Thus, adiabac approximaon gives only an upper limit – Angular momentum conservaon restrains the collapse and forms an accre-on disk structure Star Formaon 2 21 Protostar in LMC From: http://www.spacetelescope.org/static/archives/images/screen/heic0411a.jpg Star Formaon 2 22 Final Stage • Loss of thermal energy is controlled by radiave efficiency and opacity of gas • Protostar at center of disk grows in mass and decreases its radius • Arer ~107-108 yrs, T~107 K and hydrogen igni-on occurs in core of protostar, which then moves to main seQuence in H-R diagram • None of this has been observed, but it is a plausible path for star formaon Star Formaon 2 23 Planetary System Formaon • Disks of gas and dust have been discovered around many young stellar objects • These protoplanetary disks are considered to be poten-al planetary systems in the making From: hBp://www.nasa.gov/mul-media/imagegallery/image_feature_560.html Star Formaon 2 24 Planetary System Formaon • One theory… – Mutual adhesion due to collision of small fragments in the disk followed by gravitaonal accre-on of addi-onal solid and gas phase materials form the rocky cores of planets – The formaon of icy planets and gas giants are explained by this theory; having their formaon occurring in the lower temperature outer regions of the disk (beyond the ice forma0on radius) Star Formaon 2 25 Planetary System Formaon • During the final stages of a pre-main seQuence contrac-on, a star undergoes a T-Tauri phase – Characterized by strong stellar wind • It is believed that the stellar wind cleans out any remaining gas or dust not bound to the planets while mainly the rocky cores remain • Observaons of gas giants in small-radius orbits suggest a process of “migraon” of the giant planets inward (process not well known) Star Formaon 2 26 Ini-al Mass Func-on • Stars form with a range of masses between ~10 Mʘ (hydrogen burning limit) and ~100 Mʘ (most massive known stars) • It is suggested that the Number Distribu-on of stars can be portrayed as a power law known as the Ini-al Mass Funcon: dN −α ∝ m dm • There is some evidence that this is a “universal” func-on but this is not accepted by all astrophysicists Star Formaon 2 27 Ini-al Mass Func-on • α is considered to be 2.35 over most of the mass range, known as the “Salpeter ini-al mass func-on” • Thus, low-mass stars are much more common than high- mass stars • However, in the 0.1 Mʘ to 1 Mʘ range, the slope most likely flaens out and there may be a maximum to the distribu-on around 0.1 Mʘ to 0.5 Mʘ • Also, the faintness of low mass stars and the rarity and short lives of high mass stars make the calculaon of the ini-al mass Quite difficult, thus introducing uncertainty to the already men-oned theore-cal uncertainty regarding the details of star formaon Star Formaon 2 28 Star Clusters • Stars are formed in star clusters • Two main types: – Open Clusters • Collec-ons of 50 – 1000 stars; generally observed to be young due to the presence of massive star clusters • Not bound by their self-gravity • Disperse on a crossing mescale – -me it takes for a star to cross the cluster, given the typical random internal veloci-es Star Formaon 2 29 Star Clusters – Globular Clusters • Bound systems of 104 – 106 stars within a spherical distribu-on of a radius of a few parsecs • Populaon consists of main seQuence stars and giants of masses lower than about a solar mass • Also contain stellar remnants (white dwarfs, neutron stars) of the more massive stars that existed in the past • Stars contain low metallici-es, indicang that these are ancient systems that were formed from relavely unprocessed
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