Curso de Astronomía Galáctica y Extragaláctica: Cinemática y Abundancias Químicas

Cecilia Mateu J.

Montevideo, 9 de octubre 2019

Universidad de la República - Instituto de Física

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 with radius such that it starts at an initial value q0 and increases smoothly reaching q=1 at ~20 kpc

RRLs: (Preston, 1991; Vivas & Zinn 206, Mateu & Vivas 2018, Sesar et al. 2009)

this is measured with various tracers, in particular RR Lyrae stars, but also BHB, MSTO, etc.

n ~ -2.8 and q~0.8 for the inner halo (r<20kpc) n~-3.5 and q=1 for the outer halo

The Thin + Thick Disks: Density Profile

• 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)

Thin Disk ---- Thick Disk ----

The Thick Disk dominates at 2<z<6 kpc

• Gilmore & Reid (1983) find the density profile follows an exponential with hz~300 pc up to z~1 kpc, another exponential needed at higher z

• Recent studies using Red Clump and RRLyrae stars find:

Thick disk: hz=750 pc, hR=2.1 kpcThin disk: hz=300 pc, hR=2.6 kpc

(Bovy et al. 2012, Mackereth et al. 2018, Mateu & Vivas 2018)

Halo and Discs total mass and luminosity

MW Luminous Mass ~2x1011 M☉ Robin et al. (2003)

Thin Disc Luminous mass ~1011 M☉ Robin et al. (2003)

Thick Disc Luminous mass ~1010 M☉ Robin et al. (2003)

Halo Disc Luminous mass ~109 M☉ Robin et al. (2003)

Dark Mass 0.8-1.2x 1012 M☉ Battaglia et al. (2005)

R☉ 8.34 kpc Katz et al. 2018

(U,V,W)☉ (w.r.t LSR) (10,7,12) km/s Robin et al. (2003)

VLSR 240 km/s Shoenrich et al. 2016

Velocity distributions

Age determination for field stars

Bensby et al. 2013

Thin disc Age-Metallicity relation

Holmberg et al. 2009 - Geneva-Copenhagen survey (Stromgren photometry +Hipparcos)

Thin Disc kinematics vs Age

Holmberg et al. 2009 - Geneva-Copenhagen survey (Stromgren photometry +Hipparcos)

Thin disk velocity dispersion

Holmberg et al. 2009 - Geneva-Copenhagen survey (Stromgren photometry +Hipparcos)

Thin/Thick Discs: Velocity dispersion as a function of age

• Solar-neighbourhood F-G star sample from the Edvardsson et al. (1993) catalogue (which uses Hipparcos data)

• Quillen & Garnet (2001) find that velocity dispersions increase with age, saturating at ~2-3Gyr

• Note the discontinuous jump at 10 Gyr. This is associated to the thick disk

• The fact that the jump is discontinuous supports the idea of the thick disk being a separate component, not just an old extension of the thin disk

Quillen & Garnet (2001)

Thick Disk

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

Tangential velocity distributions of Galactic Components

Lam et al. 2018 (separation in Galactic components based on chemical abundances)

14

Bulge Thin Disk Thick Disk Stellar Halo ReferenceAge range 11 Gyr <9 Gyr 10-12 Gyr 12-13 Gyr Wyse 2009

Mean [Fe/H] +0.0 -0.6 to +0.2 -0.7 -1.5 Zoccalli et al. 2003, Matteucci 2003, Carollo et al. 2010

[Fe/H] range -1.0 to +0.5 -0.5 -1.0 to +0.2 -4.0 to -1.0 Zoccalli et al. 2003, Matteucci 2003, Carollo et al. 2010

[α/Fe] +0.1 to +0.3 +0.0 +0.25 +0.25 Reddy et al. ’03 (TnD), ‘06 (TkD), ‘08 (H), Fulbright et al. 2007 (B)

Vrot (km/s) (w.r.t GSR)

175 240 180 0 Vieira et al. ‘07 (B), Robin ’03 (TnD), Carollo et al. ’10 (TkD), Chiba &

Beers ’00 (H) mean UσU (km/s) 113 ~40 ~70 131 Robin et al. 2012

mean VσV (km/s) 115 ~25 ~50 106 Robin et al. 2012

mean WσW (km/s) 100 18 ~40 85 Robin et al. 2012

Mass/MTnD 0.1 1 0.1 0.01 Robin et al. 2003

hR (kpc) ··· 2.6 2.0 ··· Juric et al. 2008, Mateu & Vivas 2018

hZ (kpc) ··· 0.3 0.65 ··· Juric et al. 2008, Mateu & Vivas 2018

n -2.2 ··· ··· ~ -3 Vivas & Zinn 2006, Carollo et al. 2010 (H) ??? (B)

rellenar usando el Modelo de Besançon (Robin et al. 2003)

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

How to define the thin/thick disc?

• Chemically: • high/low [α/Fe]

• Mono-abundance populations (MAPs) = bins in [α/Fe]-[Fe/H] plane as proxies for age

high-α

low-α

Clar

ke e

t al.

2019

APOGEE RGBs

[α/F

e]

[Fe/H] -0.5 +0.50.0 -0.1

+0.3

+0.0

+0.2

+0.1

Tangential velocity distributions of Galactic Components

Lam et al. 2018 (separation in Galactic components based on chemical abundances)

The Besançon Galactic Model

• http://model.obs-besancon.fr

The Bensaçon Galactic model

• http://model.obs-besancon.fr

• The Milky Way is modelled assuming density