Astronomical Observational Techniques and Instrumentation Professor Don Figer Energy sources of astronomical objects

Aims and outline for this lecture describe energy sources of astronomical objects stars: nuclear reactions protostars: gravitational energy nebulae/clouds: stellar heating and ionizing radiation galaxy clusters: shocks give case studies of using multiwavelength data to analyze two star clusters

Stellar Structure

Solar Atomic Abundances Note that N_He is 10^(-1.07)N_H=0.08N_H, or ~25% by mass. Li and Be are depleted because they burn easily, but they are much more depleted than in some other Sun-like stars. Note that meteorites DO have more Lithium. See: http://adsabs.harvard.edu/abs/2009Natur.462..189I Enhanced lithium depletion in Sun-like stars with orbiting planets The surface abundance of lithium on the Sun is 140 times less than the protosolar value, yet the temperature at the base of the surface convective zone is not hot enough to burn-and hence deplete-Li (refs 2, 3). A large range of Li abundances is observed in solar-type stars of the same age, mass and metallicity as the Sun, but such a range is theoretically difficult to understand. An earlier suggestion that Li is more depleted in stars with planets was weakened by the lack of a proper comparison sample of stars without detected planets. Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than one per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average ten times more Li. The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned. The Primordial Lithium Problem Brian D. Fields (Submitted on 15 Mar 2012) Big-bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic baryon density, makes tight predictions for the abundances of the lightest elements. Deuterium and 4He measurements agree well with expectations, but 7Li observations lie a factor 3-4 below the BBN+WMAP prediction. This 4-5\sigma\ mismatch constitutes the cosmic "lithium problem," with disparate solutions possible. (1) Astrophysical systematics in the observations could exist but are increasingly constrained. (2) Nuclear physics experiments provide a wealth of well-measured cross-section data, but 7Be destruction could be enhanced by unknown or poorly-measured resonances, such as 7Be + 3He -> 10C^* -> p + 9B. (3) Physics beyond the Standard Model can alter the 7Li abundance, though D and 4He must remain unperturbed; we discuss such scenarios, highlighting decaying Supersymmetric particles and time-varying fundamental constants. Present and planned experiments could reveal which (if any) of these is the solution to the problem.

Solar System Atomic Abundances The horizontal scale is the atomic number, from 1 for Hydrogen (H) to 92 for Uranium (U). The vertical axis is on a logarithmic scale, each interval represents a 10-fold increase in abundance. The scale has been normalised to an abundance of 106 for Silicon (Si). The graph shows that Hydrogen is more than 1,000,000,000,000 times as abundant as Uranium. It also shows that elements with even atomic numbers tend to be more abundant than those with odd atomic numbers. This is believed to be due to differences in the stability of atomic nuclei with even and odd numbers of protons. Reference Stephen Mason. Chemical Evolution (Clarendon Press, 1992), pp. 40-45.

Stars: energy source: proton-proton chain The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium 2H, releasing a positron and a neutrino as one proton changes into a neutron. 1H + 1H → 2H + e+ + νe with the neutrinos released in this step carrying energies up to 0.42 MeV. This first step is extremely slow, because it depends on an endoergic beta positive decay, which requires energy to be absorbed, to convert one proton into a neutron. In fact this is the limiting step, with a proton waiting an average of 109 years before fusing into deuterium[citation needed]. The positron immediately annihilates with an electron, and their mass energy is carried off by two gamma ray photons. e+ + e− → 2γ + 1.02 MeV After this, the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3He: 2H + 1H → 3He + γ + 5.49 MeV From here there are three possible paths to generate helium isotope 4He. In pp1 helium-4 comes from fusing two of the helium-3 nuclei produced; the pp2 and pp3 branches fuse 3He with a pre-existing 4He to make Beryllium. In the Sun, branch pp1 takes place with a frequency of 86%, pp2 with 14% and pp3 with 0.11%. There is also an extremely rare pp4 branch.

Stars: energy source: proton-proton chain PPI (85% for Sun): H1 + H1 -> D2 + e+ + nu(1) (1.442 MeV) D2 + H1 -> He3 + gamma (5.493 MeV) He3 + He3 -> He4 + 2H1 (12.859 MeV) PPII (15%): He3 + He4 -> Be7 + gamma (1.586 MeV) Be7 + e- -> Li7 + nu(2) (0.861 MeV) Li7 + H1 -> He4 + He4 (17.347 MeV) PPIII (0.01%): Be7 + H1 -> B8 + gamma (0.135 MeV) B8 -> Be8 + e+ + nu(3) (followed by spontaneous decay...) Be8 -> 2He4 (18.074 MeV) The pp I branch 3He +3He → 4He + 1H + 1H + 12.86 MeV The complete pp I chain reaction releases a net energy of 26.7 MeV when one considers that the first two steps must be done twice to obtain the two He3 nuclei. Also, note that the energies in parentheses are total yields and could be apportioned to any of the products. For instance, in the first step, the 1.442 MeV comes from 0.42 MeV from the neutrino and 1.02 MeV from the positron (when it immediately annihilates with an electron). The pp I branch is dominant at temperatures of 10 to 14 megakelvins (MK). Below 10 MK, the PP chain does not produce much 4He. The pp II branch       3He + 4He→7Be + γ      7Be + e−→7Li + νe      7Li + 1H→4He + 4He The pp II branch is dominant at temperatures of 14 to 23 MK. 90% of the neutrinos produced in the reaction 7Be(e−,νe)7Li* carry an energy of 0.861 MeV, while the remaining 10% carry 0.383 MeV (depending on whether lithium-7 is in the ground state or an excited state, respectively). The pp III branch       3He + 4He→7Be + γ      7Be + 1H→8B + γ      8B→8Be + e+ + νe      8Be↔4He + 4He The pp III chain is dominant if the temperature exceeds 23 MK. The pp III chain is not a major source of energy in the Sun (only 0.11%), but was very important in the solar neutrino problem because it generates very high energy neutrinos (up to 14.06 MeV). The pp IV or Hep This reaction is predicted but has never been observed due to its great rarity (about 0.3 parts per million in the Sun). In this reaction, Helium-3 reacts directly with a proton to give helium-4, with an even higher possible neutrino energy (up to 18.8 MeV). 3He + 1H → 4He + νe + e+

Stars: energy source: pp chain: Gamow Peak Protons in center of star have high energies have the same charge (they repel each other) At sufficiently high energy, particles will fuse. The Gamow peak is the product of the Maxwell-Boltzmann distribution with the tunnelling probability of the nuclei through their Coulomb barrier. This is the energy region where the reaction is more likely to take place: at higher energies, the number of particles becomes insignificant while at lower energies the tunnelling through the Coulomb barrier makes the reaction improbable. The dimension of the Maxwell-Boltzmann distribution and of the Gamow peak is keV-1, while the tunnelling probability is dimensionless.

Stars: energy source: pp chain timescales Rates for the PP Chain The average time required for a nucleus to undergo each step of this sequence in a typical stellar interior is indicated in the figure shown above. Thus, for example, a hydrogen nucleus waits on the average 1 billion years before it undergoes an interaction with another hydrogen nucleus to initiate the sequence! Since all other steps require much less time than this, it is this initial step that controls the rate of the reaction. This incredibly small rate nevertheless accounts for the luminosities of normal stars because there are so many hydrogen atoms in the core of a star that at any one instant many are undergoing the reactions of the PP chain.

Stars: energy source: CNO cycle The CNO cycle (for carbon-nitrogen-oxygen), or sometimes Bethe-Weizsäcker-cycle, is one of two fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. The proton-proton chain is more important in stars the mass of the sun or less. Only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. However theoretical models show that the CNO cycle is the dominant source of energy in heavier stars. The CNO process was proposed by Carl von Weizsäcker[1] and Hans Bethe[2] independently in 1938 and 1939, respectively.

Stars: energy source: CNO cycle 12C + 1H → 13N + γ +1.95 MeV 13N 13C + e+ + νe +2.22 MeV 13C + 1H 14N + γ +7.54 MeV 14N + 1H 15O + γ +7.35 MeV 15O 15N + e+ + νe +2.75 MeV 15N + 1H 12C + 4He +4.96 MeV The net result of the cycle is to fuse four protons into an alpha particle plus two positrons (annihilating with electrons and releasing energy in the form of gamma rays) plus two neutrinos which escape from the star carrying away some energy. The carbon, oxygen, and nitrogen nuclei serve as catalysts and are regenerated. While the total number of "catalytic" CNO nuclei is conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the 12C/13C nuclei is driven to 3.5, and 14N becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower 12C/13C and 12C/14N ratios than main sequence stars, which is considered to be proof of nuclear energy generation in stars by hydrogen fusion.

Stars: energy source: CNO cycle The CNO cycle has several branches that are favored based on temperature. The CNO cycle has several branches that are favored based on temperature.

Stars: energy source: CNO vs PP The CNO cycle produces more energy than the PP chain at higher temperatures. The coulombic repulsion of a Carbon nucleus is much greater than that in a hydrogen nucleus, so it takes higher temperatures to start the CNO cycle. The mass at which stellar burning begins to favor the CNO cycle is ~1.5 Msun.

Betelguese and Rigel in Orion Betelgeuse: 3,500 K (a red supergiant) Consider two stars with very different temperatures. These stars have similar luminosities, ~10^5 Lsun. (L_betelguese is actually 40% higher than this value). Their effective radii are very different, ~71 Rsun for Rigel and 1180 Rsun for Betelguese. Rigel: 11,000 K (a blue supergiant)

Blackbody curves for hot and cool stars

Two stars Hotter Star emits MUCH more light per unit area  much brighter at short wavelengths.

Stars: energy source: Protostars Interstellar Cloud Evolution Artist’s conception of the changes in an interstellar cloud during the early evolutionary stages outlined in Table 19.1. Shown are a stage 1 interstellar cloud; a stage 2 fragment; a smaller, hotter stage 3 fragment; and a stage 4/stage 5 protostar. (Not drawn to scale.) The duration of each stage, in years, is also indicated.

Stars: energy source: Gravitational Energy As molecular cloud contracts, gravitational potential energy of particles is converted into kinetic energy. With higher kinetic energies, the collision rate between particles increases, i.e. temperature and thermal radiation increase. At sufficiently high density, the gas becomes opaque to escaping radiation at shorter wavelengths, making it difficult to observe the star formation process. The radiation generated by gravitational energy cannot counterbalance the force of gravity of the overlying material. Temperature increase until nuclear fusion turns on.

Star Formation: Hayashi Track hydrostatic equilibrium gravitational energy nuclear fusion Protostar on the H–R Diagram Diagram of the approximate evolutionary track followed by an interstellar cloud fragment before reaching the end of the Kelvin–Helmholtz contraction phase as a stage 4 protostar. Newborn Star on the H–R Diagram The changes in a protostar’s observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6, often called the Hayashi track. At stage 7, the newborn star has arrived on the main sequence. This is a good link: http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT319/HTML/AT31902.HTM 100,000 years from 4 to 6 10 million years from 6 to 7 timescales depend heavily on mass

Stages of Star Formation on the H-R Diagram

Arrival on the Main Sequence The mass of the protostar determines: how long the protostar phase will last where the new-born star will land on the MS i.e., what spectral type the star will have while on the main sequence

Protostar Luminosity Derivation

Star Formation: Gravitational Energy: B68 B68 is thought to be in hydrostatic equilibrium, such that the outward pressure balances the inward force of gravity. The cloud should contract as it cools/radiates gravitational energy converted into kinetic energy. Optical Near-Infrared 1.2 mm Dust Continuum C18O N2H+ B68 is thought to be in hydrostatic equilibrium, such that the outward radiation pressure balances the inward force of gravity. The cloud should contract as it radiates gravitational energy converted into kinetic energy. The green contours are measurements of Av estimated by NIR color excesses (Alves et al. 2001). The molecular line widths are very small, ~0.1 km/s, suggesting T~10 K. For the molecular maps, the color scale corresponds to thermal emission, and the contours correspond to molecular emission. https://www.cfa.harvard.edu/~clada/pubs_html/b68_vel.html

Disks & infrared emission 1 104 0.1 10 100 1000 Wavelength (mm) Vega b Pic x 0.1 9700 K RY Tau x 10 DL Tau x 2 GM Aur / 20 102 10-4 10-2 nFn (10-12 W m-2) Beckwith & Sargent 1996, Nature, 383, 139-144.

Spectrum of Protoostar McCaughrean et al. 1996

Circumstellar Dust Vega Disk Detection l Flux* Contrast (m) (Jy) Star/Disk 11m 2.4 1.5x107 22m 400 2x104 33m 1300 3x103 Reflected & emitted light detected with a simple coronograph. *per Airy disk

Star Formation: Debris Disks BD+31643

Dust Clouds: energy source Dust clouds usually emit radiation that they absorb from stars (internal or external). Young stars are often the internal heat source for star forming dust clouds, e.g. Sgr B2, W49, W51.

Dust Clouds: energy source: Sgr B2

Dust Clouds: energy source: Sgr B2

Dust Clouds: energy source

HCHII Regions in Sgr B2 Gaume et al. 1995 There are ~100 HCHII regions in Sgr B2.

HCHII Regions in Sgr B2 De Pree et al. 1998 The clumps break up into even smaller clumps with sizes ~100 AU and densities >107 cm-3. Each clump contains an OB star.

Dust Clouds: energy source: external heating M0.20-0.033 molecular cloud is warm (molecular emission in contours) Notice that its surface is ionized (free-free emission in greyscale). Pistol nebula is also ionized and heated.

Dust Clouds: energy source: external heating M0.20-0.033 is externally heated by nearby Quintuplet cluster of massive stars. Notice that its surface is ionized by the nearby hot stars. Pistol is ejecta that is ionized/heated by Pistol star.

Dust Clouds: energy source: external heating: Pa-a

Nebulae: energy source: stars 3 um The Pistol nebula is heated by the Pistol star that resides at its center. Note in the figure that the dust thermal emission peaks in the mid-infrared, indicating temperature of a few 100 K. The starlight fades in relative intensity at longer wavelengths. Ionized gas emission suggest an external energy source (other hot stars in Quintuplet). Luminosity of dust is around a few 10^6 Lsun, comparable to Pistol star. However, the ionizing rate is far higher than what the Pistol star contributes. Note that the morphology is assymetric with more intensity to the north. This is in the direction of the hot Quintuplet stars, and they ionize the surface of the expanding ejecta. 17 um

Galaxy Clusters: energy source: Shock Heating http://www-astro.physics.ox.ac.uk/~garret/teaching/lecture2.pdf

Galaxy Clusters: energy source: Shock Heating Over last 10 Billion years there have been many galaxy collisions in galaxy clusters. When two galaxies pass through each other stars will continue on their original path – more or less. Interstellar gas clouds collide and cannot pass through each other. They get stripped and pass into the gravitational well of the cluster. This fills with very hot shocked gas over time. So hot it emits x-rays. Shows matter distribution. (Mostly dark matter again.)

Galaxy Clusters: energy source: Shock Heating blue=x-ray Stephan's Quintet is a picturesque but clearly troubled grouping of galaxies about 300 million light-years away toward the high-flying constellation Pegasus. Spanning over 200,000 light-years at that distance, this composite false-color image illustrates the powerful nature of this multiple galaxy collision, showing x-ray data from the Chandra Observatory in blue superposed on optical data in yellow. The x-rays from the central blue cloud running vertically through the image are produced by gas heated to millions of degrees by an energetic shock on a cosmic scale. The shock was likely the result of the interstellar gas in the large spiral galaxy, seen immediately to the right of the cloud, colliding with the quintet's tenuous intergalactic gas as this galaxy plunged through group's central regions. In fact, over billions of years, repeated passages of the group galaxies through the hot intergalactic gas should progressively strip them of their own star forming material. In this view, the large spiral galaxy just seen peeking above the bottom edge is an unrelated foreground galaxy a mere 35 million light-years distant.

Multiwavelength View of Energy Sources red=8um green=6 cm blue=20 cm red=8um green=5.8um blue=3.6um

Multi-wavelength analysis of star clusters: the cases of GLIMPSE9 and Cl1813-178 (left) K-band image of the cluster from 2MASS, with overlaid contours of 90 cm data (White et al. 2005). Contour levels, in mJy beam1, are 40, 60, 80, 100, 120, 140, 160, 200. The beam size is 24 # 18, FWHM, and the position angle of the major axis is along the north-south direction. 90 cm

Cl 1813-175: Multiwavelength Image Messineo et al. (2008) ApJL, 683, 155 SNR G12.82-0.02 HESS J1813-178 SNR G12.72-0.0 W33 2MASS Two non-thermal SNRs appear in the 90 cm data. The star formation region near the bottom is W33. 3.6 um 8 um 90 cm 44

Cl 1813-175: Multiwavelength Plot 74 Chandra point sources from Helfand et al. (2007) 45

Cl 1813-175: NIR Spectroscopy Red supergiant Blue supergiants Keck/NIRSPEC high– and low–resolution spectroscopy 46

Chandra data from Helfand et al. (2006) Cl 1813-175: CMD 4.7 kpc 6-8 Myr Ak=0.8 mag 2000-6000 Msun Chandra data from Helfand et al. (2006) 47

Cl 1813-175: distance From the radial velocity of star #1, we derive a kinematic heliocentric distance of 4.7±0.4 kpc by using the rotation curve of Brand & Blitz (1993). We conclude from the CMDs and distance estimates, that the RSG, the WR star, and the BSGs are all part of the same stellar cluster. The average spectrophotometric distance of 3.7 ± 1.7 kpc is consistent with the kinematic distance 4.7±0.4 kpc within uncertainties. We assume the kinematic distance. 48

Cl 1813-175: age and mass We assume coevality of the evolved objects – 1 WR, 1 RSG, 2 BSGs, and several X–ray emitters. We conclude that the cluster is 6 − 8 Myr old since this age allows for the coexistence of both WR and RSG stars. Assuming that the other eight X–ray emitters associated with the cluster, other than the WR star, are BSGs with masses larger than 20 Msun, and by assuming a Salpeter IMF down to 1.0 Msun, we derive a total initial cluster mass of 2000 Msun. Messineo et al. (2008, ApJ 683-155) 49

24 additional massive stars in CL 11813-178 (Messineo et al.)

GLIMPSE9: location (l,b)=(22.76°, -0.40°) HST/NICMOS f.o.v. = 51.5”x51.5”; pixel scale = 0.2”; filters = F160W, F222M exptime = 19.94s, 55.94s 51

Age = 6-30 Myr (presence of RSGs) Ak = 1.6 ± 0.3 mag #3 4.2 kpc #4 4.7 kpc 52

Cluster surroundings Blue = 3.6 um Green = 90 cm Red = 24 um Giant Molecular cloud – from CO 10^6 Msun -- 4 SNR remnants 53

Ongoing ESO observations with SINFONI to observe the brightest stars REG1 REG3 REG4 REG6 Ongoing ESO observations with SINFONI to observe the brightest stars of REG1, REG3, REG4 and REG6 54

GLIMPSE9 and CL1813-178 Summary GLIMPSE9 and CL1813-178 are two young clusters. The combination of radio and infrared data allowed us to detect their parental clouds, which appear rich in HII regions and SNRs. With similar studies of other clusters and giant HII regions we will be able to shed light on the initial masses of the supernova progenitors, and therefore on the fate of massive stars. 55