curso de astronomía galáctica y extragaláctica: el halo estelar...curso de astronomía galáctica...
TRANSCRIPT
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Curso de Astronomía Galáctica y Extragaláctica: El Halo Estelar
Cecilia Mateu J.
Montevideo, 8 de octubre 2019
Universidad de la República - Instituto de Física
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A familiar sketch
Thick Disk
Thin Disk
Sun
Stellar Halo
Dark Halo
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4
Thin and Thick Disc Sun
Halo
Bulge
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The Galactic Halo
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Stellar population
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The Halo: Metallicity Distribution Function
• Stars in the stellar Halo are metal-poor, the majority having [Fe/H]~-1.7 (e.g. Carollo et al. 2010, Prantzos et al. 2009)
• The most metal-poor stars in the Galaxy are found in the stellar Halo. The metallicity spans the range
-3.5 ≤ [Fe/H] ≤ -0.6
Halo
Carney et al. 1994
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Metallicity Distributions for Galactic Populations
Thick Disk
Halo
Thick Disk
Thin Disk
Abundances from Gilmore et al. (1995), Carney et al. (1994), Wyse & Gilmore (1995) Fullbright et al. (2000)
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Resumen de Propiedades del Disco Grueso (DG) Galáctico
Halo Thick Disk
• Stars in the stellar Halo are metal-poor, the majority having [Fe/H]~-1.7 (e.g. Carollo et al. 2010)
• Stars in the Galactic Halo are old, with ages ~12-13 Gyr
• Again, its difficult to know whether the younger Halo stars are older than the Thick Disk or not
•Wyse (2009) use kinematically selected MS turn-off (TO) stars to study the (g-r) color as a function of [Fe/H]
• Remember that for a given metallicity, bluer TO means younger age
Kinematically selected MS turn-off stars (Wyse 2009)
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The Halo: Stellar Population
• The Stellar Halo has an old stellar population, with an age of ~12-13 Gyr (almost as old as the Universe)
• There is no current star formation and no gas or dust in the Halo
Theoretical H-R diagram
Newberg et al. 2002 From
Bru
zual
& Ch
arlo
t 200
3 m
odels
Observed H-R diagram
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The Gaia DR2 H-R diagram: Halo+Discs• Gaia DR2 stars with parallax errors <20% and low extinction
• Approx. 4 million stars
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• Gaia DR2 stars with parallax errors <20% and low extinction, + kinematic selection:
• selected stars with large total velocity with respect to the Sun (>200 km/s)
• ~116000 stars
The Gaia DR2 H-R diagram: Halo
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• Gaia DR2 stars with parallax errors <20% and low extinction, + kinematic selection:
• selected stars with large tangential velocity with respect to the Sun (>200 km/s)
• ~64000 stars
•Halo population: old and metal poor
The Gaia DR2 H-R diagram: Halo
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The Gaia DR2 H-R diagram: Halo• Gaia DR2 stars with parallax errors <20% and low extinction, + kinematic selection:
• selected stars with large tangential velocity with respect to the Sun (>200 km/s)
• ~64000 stars
•Halo population: overall old and metal poor
•clearly two Halo populations:
• first isochrone fits:
•13 Gyr, [M/H]=-1.3
•11Gyr, [M/H]=-0.7
•More details on the interpreation of this later
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The Halo: The Globular Cluster System
• Globular clusters are ~spherically distributed around the Galactic center, out to a radius of about 50 kpc
• Metal-poor globular clusters ([Fe/H]<-0.8) are kinematically associated with the Halo
• Metal-rich globular clusters ([Fe/H]>-0.8) lie close to the Galactic center and are kinematically associated with the Bulge
Zinn (1985)
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Age-Metallicity Relationships for Galactic Populations
Freeman 1999
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Spatial distribution: density profile
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Coordinate systems
• Heliocentric galactic coordinates:
• (l,b) = longitude, latitude (spherical)
• XYZ = cartesian coordinates:
• Z=perpendicular to Galactic Plane • X= Sun-Galactic Center (points
towards GC, or away) • Y= direction of Galactic rotation
• UVW = cartesian velocities
• U = vel. in X direction • V = vel. in Y direction • W = vel. in W direction
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Reference frames
• Local Standard of Rest:
• reference frame of stars in the Solar Neighbourhood (D<~100pc-1kpc (?)
• defined by the mean motion of stars in the vicinity of the Sun
• the LSR does not coincide with the Sun, so the Sun has a velocity w.r.t the LSR, this is called peculiar motion of the Sun with respect to the LSR :
• (U⊙,V⊙,W⊙)LSR=(11.1,12.2,7.3) km/s (Katz et al. 2018, Gaia DR2)
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Reference frames
• Galactic Standard of Rest (GSR):
• reference frame of stars at rest with respect to the center of the Galaxy
• (U,V,W)LSR=(0.,240.,0.) km/s. -> the LSR rotates around the Galactic Center (GC) at 240 km/s
• peculiar motion of the Sun with respect to the LSR:
• (ULSR,VLSR,WLSR)GSR=(0,240.,0) km/s (Katz et al. 2018, Gaia DR2)
• in this frame UVW are also sometimes called Vx,Vy,Vz to differentiate from UVW
• VLSR is the circular velocity at the “solar radius”(=distance from the Sun to GC)
• TAREA: calcular el período orbital del Sol alrededor de centro galáctico
GSR
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Coordinate systems
• Galactocentric coordinates:
• Cylindrical Coordinates
• Also used:
• Spherical coordinates
• Cartesian coordinates = same as heliocentric but with origin at Galactic Center • Z=perpendicular to disc • X= +/-Sun-GC • Y= +/-galactic rotation • note: Xsun= +/-8.35kpc
(R, z, ϕ)
(r, ϕ, θ)
Sparke & Gallagher, Galaxies in the Universe
Xsun
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Density profile
• The density profile describes how the true number density of stars changes with position across the Galaxy
• = # of stars / kpc3
• This is (usually) different than the observed number of stars because any given survey is rarely 100% complete
• The selection function or completeness function describes the fraction of objects a survey observes in a given line of sight
• different for each survey, depends on survey design
• it typically depends on the magnitude and color of the stars
ρ = ρ( ⃗r )
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The Halo Density Profile
Average Halo RR Lyrae density profile from Vivas & Zinn (2006)
✦ Halo density profile
with q=constant up to R~20kpc
or variable according to the expression
(Preston, 1991)
With different tracers, in particular RR Lyrae stars, but also BHB, MSTO, etc.
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Substructure in the Galactic Halo
Halo RR Lyrae density profile in different lines of sight. (Vivas & Zinn 2006)Watkins et al. (2009)
• On average the Halo is well described by a power-law density profile
• However, a lot of substructure has been observed with different tracers: the Sgr dSph tidal tails, the Virgo Overdensity, Pisces Overdensity, G-D streams, Monoceros Stream, etc, etc...
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Substructure in the Galactic Halo
Majewski (2003)
Belokurov (2006)
• A lot of substructure has been observed with different tracers: the Sgr dSph tidal tails, the Virgo Overdensity, Pisces Overdensity, G-D streams, Monoceros Stream, etc, etc...
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The Thin + Thick Disks: Structure
• The number density profile for stars in the Galactic disk can be described by a double exponential
Z (pc)
From the thick disk discovery paper of Gilmore & Reid (1983)
• Gilmore & Reid (1983) find the density profile follows an exponential with hz~300 pc up to z~1 kpc
• Recent studies (Cabrera-Lavers et al. 2005, López-Corredoira et al. 2002)using Red Clump stars find:
Thick disk: hz=0.9 kpc, hR=3.6 kpcThin disk: hz=300 pc, hR=2.6 kpc
Thin Disk ---- Thick Disk ----
The Thick Disk dominates at 2<z<6 kpc
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Velocity distributions
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Halo and Disk Kinematic Decomp.: Toomre Diagram
Halo ● Retrograde ● Thick Disk ● Thin Disk ●
• Thick disk stars rotate slower (V~180km/s) than Thin disk stars (~240 km/s). Their orbits are slightly non-circular.
• The Galactic Halo (as a whole) does not rotate on average, there’s a large velocity dispersion ~120 km/s
Venn et al. 2004
Galactocentric V (km/s)
T = U2 + W2
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Halo and Thick Disk Kinematics
Halo ● Retrograde ● Thick Disk ● Thin Disk ●
Venn et al. 2004Galactocentric V (km/s) Galactocentric V (km/s)T
=U
2+
W2
• Thick disk stars rotate slower (V~180km/s) than Thin disk stars (~240 km/s). Their orbits are slightly non-circular.
• The Galactic Halo (as a whole) does not rotate on average, there’s a large velocity dispersion ~120 km/s
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Detailed Elemental Abundances
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The creation of heavy elements
• Elements heavier than Fe cannot be produced by fusion (curve of binding energy)
• Coulomb barrier is too great
• Nevertheless, heavy elements do exist, so how are they produced?
• α-particle capture
• Slow and rapid neutron captures
• n-captures do not suffer from the issues due to the coulomb barrier since neutrons are, well, neutral!
• These processes occur in different astrophysical sites, therefore there are different timescales for the chemical enrichment in elements produced by different processes
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α-particle captures
• α-elements
• α-elements are those produced by the capture of an α particle (He core).
• The α-capture process is limited by the Coulomb barrier, so these captures have to happen in an energetic environment with high number density of α-particles
• α-elements are produced in the explosions of SN II (core-collapse). The typical time scale of α enrichment is ~100 Myr.
• This mechanism produces relatively light elements
• Some α-elements are:
• C, N, O, S, Si, Ca, Mg, Ti
Matteucci (2001)
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Neutron captures (Burbidge, Burbidge, Fowler and Hoyle 1957)
Cowan & Thielemann, 2004)
• n-capture processes go like this:
• An atom (Z,A) with atomic number Z and mass number A captures a neutron n, increasing the mass number and releasing a photon γ
• the new isotope (Z,A+1) can
capture another n
eventually will β-decay
r-process
β-decay, increasing the atomic number Z and emiting an e- and a νe
or
s-process
This is all very nice, but there’s a minor issue.....
free neutrons are not β-stable. Their half-life
is ~15 min !!!!!
• n-captures are not limited by the Coulomb barrier
• The heaviest elements in the Universe are synthesized via n-capture processes
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Slow and Rapid Neutron Captures
• The process is called slow (s-process) if τn>> τβ, i.e the n-captures occur in a typical timescale longer than τβ, the β-decay timescale
• The s-process occurs under moderate neutron flows ~108 neutrons/cm3 (Rauscher 2004)
• The process is called rapid (r-process) if τn<< τβ , i.e the n-captures occur in a typical timescale shorter than τβ, the β-decay timescale
r-processs-process
• The r-process occurs under intense neutron flows ~1022-1024 neutrons/cm3 (Rauscher 2004)
• s-process elements are synthesized mostly on the H- and He- burning shells during the RGB stars and AGB phase
• r-process elements are synthesized during SN II explosions or neutron star mergers
Therefore the typical time-scale for s-process
enrichment is long, ~1-2 Gyr
Therefore the typical time-scale for r-process enrichment is
short, ~few x 107 yrs
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Brief Summary of α, s and r process elements
• Some α-elements are:
• C, N, O, S, Si, Ca, Mg, Ti
• Some s-process elements are:
• Sr, Ba, La, Pb, Y, Ce
• Some r-process elements are:
• Se, Y, Tc, Eu, Au, Pt, U, Th
Matteucci (2001)
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Yields from SNII and SNIa
Peletier 2012
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Elemental Abundance Trends
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Elemental Abundance Trends
• The ratio [alpha/Fe] is set by the relative yields of massive stars w.r.t low mass stars
onset of SNIa
• The increase in SFR will shift the “knee” towards higher metallicities since SNII will increase the Fe abundance at constant [alpha/Fe]
• At the onset of the SNIa contributions (~1Gyr) the iron abundance increases at a faster rate than the alpha abundance, therefore Fe/H increases while [α/Fe] diminishes
McWilliam 1997
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Elemental Abundance Trends
• The ratio [alpha/Fe] is set by the relative yields of massive stars w.r.t low mass stars
onset of SNIa
• The increase in SFR will shift the “knee” towards higher metallicities since SNII will increase the Fe abundance at constant [alpha/Fe]
• At the onset of SNIa events (~1Gyr) the iron abundance increases at a faster rate than the alpha abundance, therefore Fe/H increases while [α/Fe] diminishes
McWilliam 1997
Winds
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Elemental Abundance Trends
• Halo and Thick Disk stars are alpha-enhanced, with [α/Fe]~+0.2
• Thin Disk stars have ~solar alpha abundances, [α/Fe]~+0.0
• Bulge stars are also alpha enhanced. The enhanced stars are associated with the metal-poor bulge population, while the solar-like stars are associated with metal-rich Bar population
Navarro et al. 2011
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Galactic Halo Structure
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The Halo Dichotomy
• Note the break at <30 kpc
• At distances >~30kpc all globular clusters are metal-poor
• At distances <30kpc there is a large spread in the globular cluster metallicity distribution
Geisler et al. (2003)
• The same break is observed when analyzing HB morphology, i.e. Halo globular clusters have blue, extended HBs; while Bulge and Thick Disk clusters have red HBs tending towards Red Clumps
47 Tuc (Vazdekis et al. 2001)
Halo GC
Bulge GC
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