Populasi bintang di Galaksi

Populasi dan Distribusi
Bintang di Galaksi
{
22 Februari 2011
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Tujuan : mengerti konsep tentang populasi dan
distribusi bintang Galaksi dan berbagai
karakteristik yang membedakannya
Prasyarat : mengerti berbagai besaran
astrofisika bintang seperti umur, massa,
komposisi kimia, temperatur efektif, kelas
spektrum, tahap-tahap evolusi bintang
Properti yang paling penting dari sebuah bintang
adalah massa
Semakin masif sebuah bintang akan semakin kuat
gaya gravitasinya, hingga menyebabkan
meningkatnya tekanan dan temperatur di pusat
Review Astrofisika
L  4  R    T
2
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Massa dan suhu yang lebih tinggi  luminositas tinggi
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4
Massa yang lebih tinggi juga memerlukan dukungan lebih melawan
gravitasi (kesetimbangan hidrostatik)
Dukungan ini berasal dari generasi peningkatan energi dari reaksi
fusi di pusat bintang
Ini adalah cara lain untuk melihat mengapa bintang-bintang besar
lebih bercahaya
Semakin “luminous” bintang akan menghabiskan energi lebih
banyak dalam waktu yang lebih singkat
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Meskipun mereka memiliki lebih banyak bahan bakar, mereka
menggunakannya dengan sangat cepat sehingga bintang-bintang
raksasa tidak hidup yang sangat panjang.
Review Astrofisika
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Magnitudo
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F1
m1  2.5  log
Fref
Kebiasaan lama yang masih digunakan
Menggunakan sistem logaritmik
M1 disebut magnitudo semu
Review Astrofisika
Fotometri bintang
m  M  5  log( d )  5
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Magnitudo semu bergantung pada pengamatan kita, tetapi tidak
menjelaskan tetapi tidak memberi tahu kami tentang sifat sejati
bintang
Untuk itu kita gunakan magnitudo mutlak (M)
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Komposisi atom dari bintang
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70% Hydrogen 28% Helium 2% lainnya
Bagaimana kita tahu ?
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Garis-garis pada spektrum
Review Astrofisika
Spektroskopi Bintang
Comparing Spectra
Diagram Hertzsprung-Russel
Radius
Bima sakti
Bima sakti pada berbagai panjang gelombang
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Populasi Bintang : kumpulan bintang dengan
properti (karakteristik) yang sama
Parameter penting yang menyatakan properti yang
sama adalah umur (bukan massa bintang ).
Beberapa parameter lain yang menunjukkan
properti yang sama adalah :
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Komposisi kimia awal (metalisitas)
Fungsi massa awal (IMF), fraksi bintang ganda
Kinematika
Jarak
Distribusi ruang
Asal-usul, sejarah pembentukan bintang
Sebuah populasi dimana semua bintangnya memiliki
umur dan metalisitas yang sama disebut : simple
stellar population (SSP). Contoh : open cluster
Populasi Bintang
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Sebuah galaksi terdiri dari berbagai populasi (bintang
dengan berbagai umur dan metalisitas, gas dan molekul
antar bintang)
Galaksi = Ni populasii = gabungan (composite) populasi
Populasi = Ni SSPi = superposisi dari berbagai SSP
Contoh Bima Sakti :
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Komponen Galaksi dengan berbagai populasi yang terpisah
seperti bulge, disk dan halo
Setiap komponen terdiri dari gabungan berbagai macam SSP
Asumsi
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Parameter-parameter yang digunakan untuk menjelaskan
properti dari populasi bintang :
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Fungsi massa awal (Initial Mass Function – IMF) : IMF=IMF(x,t)
Kelimpahan spesies atom Xj : X=X(x,t,X1,X2,X3,...)
Laju pembentukan bintang (Star Formation Rate – SFR):
SFR=SFR(x,t)
Distrubusi bintang (dan gas) pada ruang fase: f = f (x,v,t)
Evolusi terhadap waktu : chemo-dynamical models
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Beberapa contoh gabungan populasi komponen utama dalam
Bima Sakti kita adalah : halo, disk dan bulge.
Masing-masing kelompok di atas merupakan kompleks
bintang-bintang dan materi antar bintang, tapi dengan sifat
global yang berbeda / distribusi kimia / umur / kinematika dari
satu sama lain.
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Perbedaan ini mungkin berhubungan dengan campuran yang
berbeda dari SSP.
Semakin kecil basis set (n, mn), semakin mudah adalah populasi
komposit (dan, akhirnya, sejarah Galactic) untuk diungkap
Mengidentifikasi SSPS individu mungkin sulit di galaksi yang
kompleks, tapi, mungkin untuk SSP yang dirangkai dalam pola
agak sederhana. .
Komponen Utama
(misalkan pembentukan bintang pada piringan dengan memperkaya
serangkaian pembentukan SSP disertai meningkatnya kecepatan rotasi
terhadap pusat)
. . . yang membentuk populasi komponen utama dari sebuah galaksi
Ini adalah satu tujuan dari studi populasi bintang.
Kita berharap untuk menyederhanakan apa yang mungkin menjadi
masalah yang rumit untuk menemukan salah satu pola dalam
Populasi Komponen Utama.
Lebih spesifik lagi:
Kita mencari: Korelasi antara bebrbagai parameter seperti:
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DISTRUBUSI RUANG, e.g., stellar density laws, phase space density
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KINEMATIKA, kecepatan, dispersi kecepatan (i.e. Fitur dinamika sistem
yang teramati)
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KIMIAWI, misal: metalisitas rata-rata (mean [Fe/H]), pola kelimpahan
kimiawi ([O/Fe], [Ca/Fe], [Zn/Fe], ...)
UMUR, direfleksikanoleh berbagai tipe spektrum bintang (keadaan
evolusinya)
UNTUK MENGIDENTIFIKASI DAN MENDEFINISIKAN: Komponen
populasi utama, yang akan memungkinkan kita
UNTUK MEREKONSTRUKSI: Sebuah model yang lengkap secara fisikal,
evolusi kimiawi dan dinamik dari Galaksi Bima Sakti (atau sistem galaksi
lainnya)
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The Ultimate Chemodynamical Model untuk evolusi
sebuah galaksi dapat memasukan berbagai variabel
(bergantung pada waktu) seperti :
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: evolusi dari distribusi ruang ruang fase
bintang, gas dan materi gelap
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: evolusi dari spesies atom Xi dari
pengayaan gas antar bintang tempat bintang
terbentuk
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: Laju pembentukan bintang (SFR)
: IMF, bagaimana bintang baru
terdistribusi terhadap massa (yang menjelaskan
bagaimana populasi berevolusi secara kimiawi dan
apa saja jenis sisa (spesies atom) yang dihasilkan
{
W. Baade
 CMD types  structural
components
First sweeping
collectivization of “stellar
population” properties
The Andromeda system M31, M32 and N205.
Baade's famous plate, reproduced from Majewski (ed.), Galaxy Evolution: The
Milky Way Perspective, ASP Conf. Ser. 49.
High contrast zoom of previous image to show the incipient resolution of the "Baade
sheet". Baade's famous plate, reproduced from Majewski (ed.), Galaxy Evolution: The
Milky Way Perspective, ASP Conf. Ser. 49.
Baade's definition of populations based on CMD type.
A modern HR diagram of
the solar neighborhood.
From Wikipedia.
Bingelli's famous diagram, taken from Sparke & Gallagher, Galaxies in the Universe
Spheroidal/elliptical characteristics by Kormendy, taken from Sparke & Gallagher,
Galaxies in the Universe
Baade's Population II:
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K giants brighter than Pop I (now
known to be an abundance
effect).
No red and blue supergiants
(now known to be an age effect).
Has short period, cluster
Cepheids (i.e. RR Lyrae stars -now known to be an
age/metallicity effect).
"high velocity stars (w.r.t. Sun)"
(kinematics).
subdwarfs (now known to be an
abundance effect).
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weak-lined stars (now known to
be an abundance effect).
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globular clusters
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dE, Sa galaxies (central parts
anyway; location).
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outer Milky Way and bulge
(location).
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"Pop II can be found without
associated Pop I".
Baade's Population I:
•
Open clusters (already known to be connected to slow
moving stars).
•
OB stars (now known as an age effect).
•
solar neighborhood stars (location).
•
"slow moving stars (w.r.t. Sun)": (kinematics).
•
strong lines stars (abundance).
•
"only seen with Population II stars associated" (e.g., Milky
Way, Spirals, Magellanic Cloud clusters).
halo
disk
bulge
Spiral Galaxy
Disk Component:
Bintang dengan berbagai
umur dan banyak awan gas
Spheroidal Component:
bulge & halo, bintang-bintang tua,
dan sedikit awan gas
Disk
Component:
Bintang
dengan
berbagai
umur dan
banyak awan
gas
Spheroidal
Component:
bulge & halo,
bintangbintang tua,
dan sedikit
awan gas
Disk
Component:
Bintang
dengan
berbagai
umur dan
banyak awan
gas
Spheroidal
Component:
bulge & halo,
bintangbintang tua,
dan sedikit
awan gas
Warna biru-putih
mengindikasikan
adanya proses
pembentukan
bintang
Warna merahkuning
mengindikasikan
bintang-bintang tua
Disk
Component:
Bintang
dengan
berbagai
umur dan
banyak awan
gas
Spheroidal
Component:
bulge & halo,
bintangbintang tua,
dan sedikit
awan gas
Warna biru-putih
mengindikasikan
adanya proses
pembentukan
bintang
Warna merahkuning
mengindikasikan
bintang-bintang tua
Subdivide/refine Baade's broad groupings:
Summary tables from the 1957 Vatican
Conference proceedings. This book
makes great reading, because all of the
conversations of participants have
been preserved and recorded in the
proceedings. Note that the ages listed
in the table are based on well
outdated stellar evolution models, and
are too small by about a factor of two.
F. A “conventional, modern view of the primary Galactic stellar populations
and their spatial (density law), chemical, and kinematical properties.
Though it should be kept in mind that this conventional picture is still debated.
Note the difference between
the luminous stellar halo,
and the dark matter halo
postulated to exist and in
which the luminous matter
is embedded.
Another view of the Milky Way and its populations. From Buser (2000, Science, 287, 5450, 69).
His caption: Schematic view of the major components that make up the Galaxy's overall
structure, shown in a cross section perpendicular to the plane of rotation and going through
the sun and the Galactic center. From the observer's vantage point at the sun's position, the
directions to the North (NGP) and South (SGP) Galactic Poles are particularly suitable for
studying the layered structure and other properties of the stellar disks and halo, whereas the
concentration of gas and dust in the extreme disk severely obstructs observations of the distant
bulge at visual-optical wavelengths. The central parts of the Galaxy are better accessible
through longer wavelength infrared and radio observations.
Cartoon (left) and modeled (right) illustration of the Galactic dark matter halo. In
right figure the plot is only of the density of dark matter in a simulated Milky Way
halo, with light on a logarithmic scale and 600,000 light years on a side.
From http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/RotationsReckon.html and
http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2003-12/hl200312-en.html.
Galactic Structure
Flat disk:
•1011 stars (Pop.I)
• ISM (gas, dust)
• 5% of the Galaxy mass, 90% of
the visible light
• Active star formation since 10
Gyr.
Central bulge:
• moderately old stars with low
specific angular momentum.
• Wide range of metallicity
• Triaxial shape (central bar)
• Central supermassive BH
Stellar Halo
• 109 old and metal poor stars
(Pop.II)
• 150 globular clusters (13 Gyr)
• <0.2% Galaxy mass, 2% of the
light
•Dark Halo
Thin disk
The galactic disk is a complex system including stars, dust and gas
clouds, active star forming regions, spiral arm structures, spurs, ring, ...
However, most of disk stars belong to an “axisymmetric” structure, the
Thin disk, which is usually represented by an exponential density law:
 ( R, z ) 0 e
 z  z0 / hz
e
 ( R  R0 ) / hR
• hz  250 pc vertical scale height  W = 20 km/s
• hR  3.5 kpc radial scale-lenght
• z0  20 pc Sun position above the plane
• R0  8.5 kpc Solar galactocentric distance
Thin disk: kinematics
(a) Local Standard of Rest (LSR)
Definition: Ideal point rotating along a circular
orbit with radius R
VLSR 220 km/s (Vz=0,Vr=0)
 T  250 Myr
VRot (r) = - [Kr (r,z=0) r]1/2
GC
R
LSR
NGP
(b) Galactic velocities:
G.C.
U
Rot.
V
W
(U,V,W) components with respect to the LSR
In particular, (U,V,W) = (+10.0, +5.2, +7.2) km/s
(Dehnen & Binney 1998)
Thin disk: kinematics
lv
G.C.
(c) Velocity Ellipsoid
Definition: Ellipsoid of velocity dispersions for a
Schwarzchild stellar population (1907) with
multivariate gaussian velocities, defined by:
• the dispersions (1 , 2 , 3 ) along the (v1 ,v2 ,v3 )
principal axis
• lv = vertex deviation, with respect to (U,V,W)
v1
U
v2
V
  v12
v 22
v 32 

Pr( v1 , v 2 , v 3 ) 
exp   


2/3
2
2
2 
(2 )  1 2 3
  2 1 2 2 2 3 
1
Thin disk: kinematics
(d) Asymmetric drift
N.ro of stars
Definition: systematic lag of the rotation
velocity with respect to the LSR of a given stellar
population
va = vLSR - v
-va
Generally, old stars show larger velocity
dispersion and asymmetric drift, but
smaller vertex deviation, than young stars
V
Local kinematics
from Hipparcos data
(Dehnen & Binney
1998)
Thin disk: kinematics
Velocity ellipsoid of the “old” thin disk
(U , V , W ;va ) = (34, 21, 18; +6 ) km/s
from Binney & Merrifield (1998) “Galactic Astronomy”
For an isotherm population:
 ( z)  e
hz 
|z|/ hz
W
2G   ( z  0)
1/ 2
where,  (M/pc²) = galactic surface density
Thin disk: metallicity
Range of Metallicity:
0.008 < Z < 0.03 (Z = 0.02)
No apparent age-metallicity
relation is present in the Thin disk
(Edvardsson et al 1993, Feltzing et al.
2001)
Age-metallicity distribution of
5828 stars with /<0.5 and
Mv<4.4
Galactic Halo
• Spatial density.
Axisymmetric, flattened (~0.7-0.9), power law (n~2.5 - 4) function.
For instance:
 2 z 
 ( R, z )  0   R  2 
 

2
•halo(z=0)/0 ~ 1/600
• Age: 12-13 Gyr
• Metallicity: [Fe/H] ~ (-1, -3)
-
[Fe/H] ~ -1.5
n/2
Galactic Halo: kinematics
Velocity ellipsoid of the “halo”
(U , V , W ;va ) =
(160, 89, 94; +217 ) km/s
from Casertano, Ratnatunga & Bahcall (1990, AJ, 357,
435)
Rotation velocity. Halo - Thick Disk
distributions from Chiba & Beers
(2001)
T h i c k disk
Basic parameters:
• hz  1000 pc
• W  40-60 km/s
• Pop. II Intermediate
• [Fe/H]  -0.6 dex
with low metallicity tail
down to -1.5
• Age: 10-12 Gyr
• thick(z=0)/0  4-6 %
Thick disk
A matter of debate
Thick disk
A matter of debate
Velocity ellipsoid of the “thick” disk
(U , V , W ;va ) = (61, 58, 39; +36 ) km/s
from Binney & Merrifield (1998) “Galactic Astronomy”
The various measurements of the velocity ellipsoid are quite consistent,
but a controversy concerning the presence of a vertical gradient is still
unresolved:
•  va/  z =  i /  z = 0
according to several authors
•  va/  z = -14 ± 5 km/s per kpc Majewski et al. (1992, AJ)
Thick disk: Formation Process
• Bottom-up. Dynamical heating of the old disk because of an ancient
major merger
V
m
M
m 2
  VSat
M
2
W
V  200 km/s , m/M  0.10  W  60 km/s
• Top-down. Halo-disk intermediate component. Hypothesis:
dissipative phase of the protogalactic clouds at the end of the halo
collapse (Jones & Wise 1983)
Heating of a galactic disk by a
merger of a high density small
satellite. N-body simulations by
Quinn et al. (1993, ApJ)
Actually, more recently,
Huang & Calberg (1997) found
that low density satellites
with mass < 20% seem to
generate tilted disks instead
of thick disks.
Thick disk: Signature of the
Formation Process
FORMATION PROCESS
PHYSICAL PROPERTIES
Dynamical heating of an ancient
thin disk
Discrete component: No vertical
chemical and kinematic gradients
expected in the Thick Disk
Intermediate phase Halo-Disk
Continuity of the velocity ellipsoids
and asymmetric drift
Thick disk: Signature of the
Formation Process
Proper motion survey towards the NGP (GSC2 material)
Types of surveys suitable for Galactic studies:
•Selective surveys. For examples, stellar samples selected on the
basis of the chemical or kinematic properties (e.g. low metallicity and high
proper motion stars  Pop. II halo stars. Warning: “biased” results)
• Surveys with tracers. High luminosity objects which can be
observed up to great distances, easy to identify and to measure their
distance (e.g. globular clusters, giants, variable RR Lyrae, … ) . It is
assumed that tracers are representative of the whole population.
• In situ surveys.
These measure directly the bulk of the objects
which constitute the target populations (e.g. dwarfs of the galactic Pop.I
and Pop.II). These should guarantee “unbiased” results if systematic
effects due to the magnitude threshold, photometric accuracy, angular
resolution, etc. are properly taken into account.
Fundamental Equation of the Stellar
Statistics
(von Seeliger 1989)

A(m)    ( Mr, r )  D( r )  r dr
2
0
(M)=Luminosity function
D(x,y,z)=density distribution
M  m  5  5 log r  a ( r )
(Integral Fredholm’s equation of
the first kind).


Problem: inversion of the
integral equation!
Galaxy models
An alternative approach: integrate the Eqn of stellar statistics
assuming some prior information concerning the stellar population.
In practice,
•(1) They assume discrete galactic components, each parametrized
by specific spatial density, (R,z; p), velocity ellipsoid and by a well
defined LF/CMD consistent with the age/metallicity of each
component.
•(2) Predicted starcounts (i.e. N.ro of stars vs. magnitude, color,
proper motion, radial velocity, etc.) are derived by means of the
fundamental Eqn. of the stellar Statistics.
•(3) Comparisons against observations are used to confute or
validate and improve the model parameters.
Galaxy models
Models:
Bahcall&Soneira IASG - Besancon Gilmore-Reid Majewski - GM Barcelona - Mendez Sky - HDR-GST - … …
Galaxy models: LF & CMD
Synthetic HR diagram for
thin, thick disk and halo
from IASG model
(Ratnatunga, Casertano &
Bahcall)
Galaxy models: simulated catalogs
All components
Old thin disk
Young thin disk
thick disk
Intermediate
thin disk
halo
GSC 2.2 starcounts vs. Mendez’s Galaxy model
Halo Luminosity Function(s)
Gizis & Reid
(1999)
Gould et al
(1998)
Gizis & Reid (1999, ApJ,
117, 508)
Galaxy models:
No unique solutions!
The controversy regarding the
scale height of the thick disk can
be partially explained by means of
the (anti)correlations between hz
and 0 of the thin and thick disks.
Similarly, the estimation of the
halo flatness is correlated to the
power-index, and it is also
sensitive to the separation between
halo and thick disk stars.
Galaxy models
What are the “optimal” line of sights to avoid model degeneracy?
Answer: use all-sky directions + multiparameters
(photometry+astrometry) + multidimensional best-fitting methods
Kinematic deconvolution of the local
luminosity function
Recently, Pichon, Siebert & Bienaymè (2001) presented a new method for
inverting a generalized Eqn of Stellar Statistics including proper motions.
Multidimensional starcounts N(l,b,lcosb, b) are used with supplementary
constraints required by dynamical consistency* in order to derive both (1) the
luminosity function and (2) kinematics
_________________________________
* Based on general dynamical models (stationary, axisymmetric and fixed
kinematic radial gradients), such as in (a) the Schwatzchild model (velocity
ellipsoid anisotropy ,and (b) Epicyclic model (density gradients)
Kinematic deconvolution of the local
luminosity function