University of Naples Federico II, Academic Year 2011-2012 Istituzioni di Astrofisica, read by prof. Massimo Capaccioli Lecture 16 Stellar populations Walter Baade (1893-1960)
Learning outcomes The student will see: xxx
Stellar clusters Simple stellar populations: stars were probably all born at nearly the same time; thus they have the same age and composition. Open clusters: contain 10-1000 stars loose structure Globular clusters: 1000 1 million stars centrally concentrated ~ 5 pc
Galaxies contain billions of stars. stars generally have a variety of ages, compositions. ~ 20,000 pc
Globular Clusters Mostly found in the halo of the Milky Way. Concentrated around the Galactic centre In fact their spatial distribution was first used to identify the centre of the Galaxy. Clusters NGC 5466 in Bootes Globular clusters: Open clusters Open clusters Mostly found in the disk of the Milky Way.
Stellar systems Galaxy groups: A few tens of galaxies in orbit about one another. Galaxy clusters: Thousands of galaxies, trillions of stars. The largest bound structures in the Universe. ~5 10 5 pc ~2 10 6 pc
Review: stellar evolution Main Sequence: core hydrogen burning. Red Giant Branch: shell-hydrogen burning. Horizontal Branch: core helium burning. Asymptotic Red Giant Branch: shell helium (and hydrogen) burning, around a CO, electron degenerate core.
Isochrones and evolutionary tracks For a given mass, we can model how it will evolve with time. For a collection of stars with a range of masses, we can plot where they will be at a given time: these are isochrones. Models for different masses Models for different ages
Single-aged populations Nearby stars of all ages Cluster of stars all formed at the same time.
Star clusters The colour-magnitude diagram of a cluster contains information about the age and composition of a cluster. Evolutionary tracks for stars of different masses HB RGB MS
Star clusters The colour-magnitude diagram of a cluster contains information about the age and composition of a cluster. Isochrones for stars of a fixed age ZAMS in Myr 10 Gyr 16 Gyr 25 Gyr
Theoretical Isochrones The Main Sequence Turnoff is a good indicator of cluster age. 1.58 Gyr 1.82 Gyr 1.69 Gyr
Theoretical isochrones Stars with more heavy elements (metal-rich) tend to be redder. The magnitude of the turnoff depends on distance. The color depends on metallicity. Metallicity Distance
Stars with more heavy elements (metal-rich) tend to be redder. Theoretical isochrones Metallicity Z increasing Distance distance increasing The magnitude of the turnoff depends on distance. Oxygen abundance O increasing Age age increasing The Main Sequence turnoff is a good indicator of cluster age.
Color-magnitude diagrams A young cluster: the Main Sequence is the most prominent structure; there has not been enough time for stars to leave the Main Sequence.
Open clusters Example: the Hyades cluster in Taurus Spectral type B V [mag] Age [Gyr] O 0.4 < 0.001 B 0.2 0.03 A 0.2 0.4 F 0.5 4 G 0.7 10 K 1.0 60 M 1.6 > 100 The colour of the brightest Main Sequence stars is (B V) ~ 0.1 This corresponds to an A0 star.
Open clusters Typically young, and metal-rich < 1 Gyr old. Mostly found in the disk of the Milky Way. Skyview map of IRAS dust emission at 100 µm in the regions going from (l = 222, b = 28 ) to (l = 278, b = +28). All the known open clusters in the Lyngå catalogue are shown as small crosses (+). Name The ten nearest known open clusters Age [Myr] Distance [pc] [Fe/H] Collinder 285 199 25 0 Melotte n25 787 45 +0.17 Melotte 111 449 96 0 Mamajek 1 7.9 97 0 Melotte 227 135 120 0 Platais 8 60.2 132 0 Melotte 22 135.2 150 0 IC 2602 32.1 161 0.09 Platais 3 398 161 0 Platais 9 100 174 0
Globular clusters 47 Tucanae Old clusters: Only the faintest (low-mass) stars are still on the Main Sequence. Most of the stars on the CMD are in post-main sequence phases of evolution.
NGC2419in Lynx In old clusters, the bright blue stars are horizontal branch stars, while the yellow-red stars are giants
For a given composition and distance, find the model age that gives the best fit to the data. Here, isochrones are shown for ages of 8, 10, 12, 14, 16, 18 Gyr. Globular clusters
Globular clusters Example: M92 Best fit model: Age = 14 Gyr [Fe/H] = 2.31
8 Gyr 18 Gyr Globular clusters In each panel isochrones for 8,10,12,14,16,18 Gyr ages, shown for different compositions and distances.
Cluster ages Model isochrone fits to various different open and globular clusters. It shows: 1. the range of ages and 2. HR-diagram morphologies spanned by these objects.
Observational Difficulties Resolution and crowding
Observational difficulties Finite width of the Main Sequence and turn-off. Presence of blue-stragglers. Probably binary mergers.
Other galaxies The Milky Way and Andromeda are the largest members of the Local Group of Galaxies. There are ~ 30 smaller galaxies, with distances of up to ~ 1 Mpc away.
Local Group galaxies For some galaxies in the Local Group, it is possible to measure the colours and magnitudes of individual stars Consider an intermediate age stellar population, 4 Gyr old. Assuming a solar metallicity, what is the absolute magnitude of the Main-Sequence Turnoff? What would be the apparent magnitude of the Turnoff, in the Andromeda galaxy (~800 kpc away)?
Local Group galaxies Most Main Sequence stars are too faint to be seen, so the colour-magnitude diagrams are dominated by evolved stars. It is not usually a good approximation that all stars formed at the same time.
Need a range of model ages & metallicities to match the width of the Main Sequence Turnoff. Composite stellar populations
Outside the Local Group For more distant galaxies, we can only measure the integrated luminosity and colour of all stars. How will the colour and luminosity of a single burst of star formation changes with time? Isochrones for stars of a fixed age ZAM S in Myr 10 Gyr 16 Gyr 25 Gyr
Outside the Local Group For more distant galaxies, we can only measure the integrated luminosity and colour of all stars. How will the colour and luminosity of a single burst of star formation changes with time?
Elliptical galaxies The easiest ones to model. Pretty well modeled by single age, metallicity. Models which use high-resolution spectra of stars do a good job of reproducing features in the galaxy spectrum. These models show elliptical galaxies tend to be old. Have formed most of their stars at least ~10 Gyr ago. Metallicities are about solar or a bit less.
Spiral galaxies Generally have stars with a wide range of ages and metallicites. Usually modeled with continuous star formation (the rate may increase or decrease with time). Different components (bulge, disk, halo) have different stellar populations.