Stellar Objects: Star Formation and Protostars 1 Star Formation and Protostars 1 Preliminaries Objects on the way to become stars, but extract energy primarily from gravitational contraction are called Young stellar objects here. Young stellar objects (YSO): the entire stellar system throughout all premain sequence (MS) evolutionary phases. Protostar: the optically thick stellar core that forms during the adiabatic contraction phase and grows during the accretion phase. The YSO is fully convective and evolves along the so-called Hayash track in the HR diagram. PMS (pre-main sequence) star: the premature star that becomes visible once the natal envelope has been fully accreted and which still contracts towards the main sequence (MS), where the hydrogen burning dominates. In this evolutionary stage, the YSO has formed a radiative core, though still growing with time. Theoretically, the formation of a star may be divided into four stages (as illustrated in Fig. 2.1: (a) proto-star core formation; (b) the proto-star star builds up from the inside out, forming a disk around (core still contracts and is optically thick); (c) bipolar outflows; (d) the surrounding nebula swept away. The energy source is gravitational potential energy. While the total luminosity is comparable to the solar value, the proto-star stages have the KH time scale [ 2 10 7 (M/M ) 2 (L/L ) 1 (R/R ) 1 ]. They are also fully convective and is thus homogeneous chemically. Observationally, the proto-stars are classified into classes 0, 1, and 2, according to the ratio of infrared to optical, amount of molecular gas around, inflow/outflow, etc. Class 0 proto-stars are highly obscured and have short time scales (corresponding to the stage b?); a few are known. The class 1 and 2 proto-stars are already living partly on nuclear energy (c and d); but the total luminosity is still dominated by gravitational energy. The low-mass
Stellar Objects: Star Formation and Protostars 2 YSO prototype is T Tauri. We still know little about high-mass YSOs, which evolve very fast and interact strongly with their environments. Signatures of YSOs: variability on hours and days due to temperature irregularities on both the stellar surface and disk emission lines from the disk and/or outflow more inferred luminosity due to dust emission high level of magnetic field triggered activities (flares, spots, corona ejection, etc) due to fast rotation and convection Strong X-ray emission from hot corona. 2 The birth of stars From the virial theorem, 2E = Ω, we have 3kTM M = µm A 0 GM r dm r (1) r for the hydrostatic equilibrium of a gas sphere with a total mass M. Assuming that the density is constant, the right side of the equation is 3/5(GM 2 /R). If the left side is smaller than the right side, the cloud would collapse. For the given chemical composition, ρ and T, this criterion gives the minimum mass (called Jeans mass) of the cloud to undergo a gravitational collapse: M > M J ( ) 1/2 ( ) 3/2 3 5kT. (2) 4πρ Gµm A For typical temperatures and densities of large molecular clouds, M J 10 5 M with a collapse time scale of t ff (Gρ) 1/2. Such mass clouds may be formed in spiral density waves and other density perturbations (e.g., caused by the expansion of a supernova remnant or superbubble).
Stellar Objects: Star Formation and Protostars 3 What exactly happens during the collapse depends very much on the temperature evolution of the cloud. Initially, the cooling processes (due to molecular and dust radiation) are very efficient. If the cooling time scale t cool is much shorter than t ff, the collapse is approximately isothermal. As M J ρ 1/2 decreases, inhomogeneities with mass larger than the actual M J will collapse by themselves with their local t ff, different from the initial t ff of the whole cloud. This fragmentation process will continue as long as the local t col is shorter than the local t ff, producing increasingly smaller collapsing subunits. Eventually the density of subunits becomes so large that they become optically thick and the evolution becomes adiabatic (i.e., T ρ 2/3 for an ideal gas), then M J ρ 1/2. As the density has to increase, the evolution will always reach a point when M = M J, when we assume that a star is born. From this moment on the cloud would start to evolve in hydrostatic equilibrium. This way a giant molecular cloud can form a group of stars with their mass distribution being determined by the fragmentation process. The process depends on the physical and chemical properties of the cloud (ambient pressure, magnetic field, rotation, composition, dust fraction, stellar feedback, etc.). Much of the process is yet to be understood. We cannot yet theoretically determine the distribution of the initial mass function (IMF) of stars. The classic expression for the IMF, determined empirically is the so-called Salpeter s law dn/dm = CM x (3) where x = 2.35 for M/M 0.5 and x = 1.3 for 0.1 M/M 0.5 in the solar neighborhood. 3 Evolution of YSOs Still incomplete!!! The actually formation and evolution of a YSO is much more complicated, but roughly can be divided into the following stages: During the collapse the density increases inwards. The optically thick phase is reached first in the central region, which leads to the formation of a more-orless hydrostatic core with free falling gas surrounding it. The energy released
Stellar Objects: Star Formation and Protostars 4 by the core (now obeying the virial theorem) is absorbed by the envelope and radiated away as infrared radiation. Because of the heavy obscuration by the surrounding dusty gas, stars in this stage cannot be directly observed in optical and probably in near-ir. When the accretion is essentially completed, the YSO increases the temperature as the core continue to contract. The steady increase of the central temperature causes the dissociation of the H 2, then the ionization of H, and the first and second ionization of He. The sum of the energy involved in all these processes has to be at most equal to the energy available to the star through the virial theorem. The simple estimate gives the maximum initial radius R i max of a YSO has to be Therefore, the luminosity can be very large. R i max R 50 M M (4) 3.1 Evolution along the Hayashi Track Starting from the surface inward, we have encountered the simple atmosphere model in the heating transfer section. The temperature and pressure at the photosphere are T(τ P ) = T eff and P(τ P ) 2 g s, where g 3 s = GM/R 2 is the κ P local gravity and κ P is the opacity κ at the photosphere. Note that we have assumed that convection plays no role in hear transport between the true and photosphere surface, consistent with our notion of a radiating, static, and visible surface (although this is not true for the sun). Next we consider the stellar envelope, which consists of the portion of a star that starts at the photosphere, and continues inward, but contains negligible mass, has no thermonuclear or gravitational energy sources, and is in hydrostatic equilibrium. We want to see how deep the radiative envelope may be until the convection takes over for heat transfer. In such a radiative envelope, = rad with = dlnt dlnp = 3κL 16πacGM P T 4. (5)
Stellar Objects: Star Formation and Protostars 5 Have the opacity written in the interpolation form for an idea gas, κ = κ g P n T n s, the above equation can be re-written as where P n dp = 16πacGM 3κ g L T n+s+3 dt. (6) P n+1 P n+1 P = (K ) n+1 (T n+s+4 T n+s+4 P ) (7) ( ) 1/(n+1) K 1 16πacGM = (8) 1 + n eff 3κ g L where n eff = (s + 3)/(n + 1). It is easy to prove that this can be written as (r) = ( ) n+s+4 ( 1 Teff + P 1 + n eff T(r) where P is evaluated at the photosphere: 1 1 + n eff ) (9) P = 3L 16πacGMκ P P P T 4 P = 1/8 (10) where P p = 2g s /3κ P, g s = GM/R 2, and L = 4πR 2 σr 4 eff are used. For H opacity, n = 1/2 and s = 9, we have n eff = 4 and (r) = 1 3 + 11 24 ( ) 9/2 T(r) (11) T eff Note that since temperature increases with depth, so does. Eventually, when > ad, the stellar material becomes convective. For simplicity, we assume that the convection is adiabatic. Thus at depths deeper than the critical depth, = ad = 0.4 for ideal gas. The transition to convection occurs at T f = (8/5) 2/9 T eff = 1.11T eff. The corresponding pressure P f at the top of the convective interiors can be found ( P P P ) n+1 = 1 + 1 1 1 + n eff P [ (T(r) ) n+s+4 1] T eff (12) which yield P f = 2 2/3 P P.
Stellar Objects: Star Formation and Protostars 6 For the assumed convection, the implying polytrope of index is 3/2 and (K here is different from the previously defined??). P = K n=3/2t 5/2 (13) For a completely convective star, K n=3/2 can be related to the the mass and radius of the star as defined by the E-solution polytrope K n=3/2 1 µ 5/2 M 1/2 R 3/2 (14) Using P f = K n=3/2 T 5/2 f and L = 4πσR 2 T 4 eff to replace R, we get T eff µ 13/51 (M/M ) 7/51 (L/L ) 1/102 (15) Therefore, such a completely convective star has a nearly constant effective temperature, independent of the luminosity. A radiative core will form at the center and grow in mass as the star evolves. This evolutionary phase is the PMS, When the radiative core appears, the star is no longer fully convective, and it has to depart from its Hayashi track, which forms the rightmost boundary to the evolution of stars in the HRD. As the center temperature increases due to the virial theorem, the path is almost horizontal on the HRD. When the temperature reaches the order of 10 6 K, deuterium is transformed into 3 He by proton captures. The exact location when this happens depends on the stellar mass. In any case, the energy generation of this burning is comparably low and does not significantly change the evolution track. The location of the Hayashi track is sensitive to the chemical composition of the stars. Does T eff increase or decrease with the increase of the metallicity or the decrease of the He abundance? Brown dwarfs, which are only able to burn deuterium (at T 10 6 K with masses 0.05 0.1M ) may still be called stars.
Stellar Objects: Star Formation and Protostars 7 4 Review Key concepts: Jeans mass, initial mass function, Salpeter s law, Hayashi track, Brown dwarfs What are the basic signatures of low-mass YSOs? What is the basic reason for the mass fragmentation of a collapsing cloud? When is the fragmentation expected to stop? Qualitatively what is the basic structure of a proto-star? What may be the structure change of a YSO that could end its evolution along the Hayashi track?