Curso de Astronomía Galáctica y Extragaláctica: El Halo Estelar

Cecilia Mateu J.

Montevideo, 8 de octubre 2019

Universidad de la República - Instituto de Física

2

A familiar sketch

Thick Disk

Thin Disk

Sun

Stellar Halo

Dark Halo

4

Thin and Thick Disc Sun

Halo

Bulge

The Galactic Halo

Stellar population

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

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)

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)

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

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Observed H-R diagram

The Gaia DR2 H-R diagram: Halo+Discs• Gaia DR2 stars with parallax errors <20% and low extinction

• Approx. 4 million stars

• 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

• 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

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

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)

Age-Metallicity Relationships for Galactic Populations

Freeman 1999

Spatial distribution: density profile

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

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)

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

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

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 )

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.

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...

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...

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

Velocity distributions

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

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

Detailed Elemental Abundances

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

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)

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

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

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)

Yields from SNII and SNIa

Peletier 2012

Elemental Abundance Trends

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

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

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

Galactic Halo Structure

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