ASTC22 Lecture 7

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ASTC22 - Lecture 7 Milky Way's kinematics and
structure
Parallax Luminosity and mass functions - a few
basic facts Kinematics of the solar
neighborhood Asymmetric drift Thin disk, thick
disk Open and globular clusters -
metallicity, age, distribution, motion Infrared
view Galactic bulge and center Differential
rotation
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Step number one measure the parallax
(Hipparcos satellite, 1989-1993) measure the
apparent magnitude m. This could be done for 0.12
mln bright stars, with positional accuracy
milliarcsec (1 milliarcsec 1/1000 seeing
disk) Step number two derive distance from
(d/ 1pc) (1 / parallax) derive the absolute
magnitude from distance modulus m - M 5 log
(d/ 10pc) 5 log (0.1 / parallax) This gave
accurate distances to few100 pc.
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Luminosity function --gt initial luminosity
function --gt ---gt initial mass function.

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The most numerous stars in the Galaxy are small,
0.3-0.5 Msun
M(Salpeter IMF)
Brown dwarfs
Frequency of stars with different masses a
power-law with exponent (index) -2.35
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Thin and thick disks of the Galaxy
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Vertical velocity w.r.t. sun (W) as a function of
stellar age stars are born in a thin disk with
small W old stars are in a thick disk.
-10 km/s
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The Bottlinger diagram for 200 main-sequence star
from the solar neighborhood U radial velocity
difference w.r.t. the sun V tangential
velocity diff.
- pop I objects similar to the sun
UU - Usun VV - Vsun gc Galactic
center in general (U,V,W)
V lt0 Ugt0 or lt0
Retrograde
prograde orbits
orbits
gc
V
Vsun
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Open clusters - e.g., Pleiades, Hyades
(Pop I)
16 Myr
100 Myr
Foreground gas nebulae
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47 Tucanae is the second brightest globular
cluster. It contains 1 mln star. It can only be
seen from the Southern Hemisphere. This image
is 34 arcmin across, 0.56 degrees (comparable
with Moon, Sun). The infrared colors of all
these stars are very similar.
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Globular clusters - e.g. omega Centauri, 47
Tucanae
(Pop II)
spherical system
thick disk
Connection between kinematics and geometry thick
disk of high-metallicity globular clusters
(left-hand panel) is made of objects on
low-inclination, nearly-circular orbits ltgt
the system has some prograde rotation. Spherical
system (right panel) has completely disorganized
motions, no rotation on average some clusters
have prograge, some retrograde motion, Orbits are
highly inclined.
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Age, distance, metallicity are varied in models
until the predicted H-R diagram (below) matches
the observations (above). For instance, 47 Tuc
has Fe/H -0.83 and age 12 Gyr M30 has
Fe/H -2.31 and age 14 Gyr One also uses
RR Lyr variables (pulsating low-mass stars
with L50 Lsun) as standard candles
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INFRARED RADIO VIEW of our GALAXY
SPECTRAL REGION WAVELENGTH TEMPERATURE
(microns) (Kelvin)
WHAT WE SEE Near-Infrared 0.8 to 5 740 to
5200 Cooler red stars, Red giants, Dust
is transparent Mid-Infrared 5 to 25 90 to
750 Planets, comets,
asteroids Dust warmed by starlight
Protoplanetary disks Far-Infrared 25
to 350 10 to 100 Cold dust Central
regions of galaxies Very cold molecular
clouds Sub-mm
and mm 850-2000 10 to 30 Larger
(mm), cold dust grains Radio e.g., 21 cm HI
line Global structure
of the Galaxy, hydrogen clouds
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Infrared view of the center of the
Galaxy optical view 2MASS (2
micron all-sky) survey
Picture made from star counts (not a direct
image) total of 250 mln stars measured in 2MASS.
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Infrared view of the Galaxy 2MASS (2 micron
all-sky) survey
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Infrared view of the Galaxy
hR 2 to 4 kpc, both for the thin (hz 0.3
kpc) and the thick disk (hz 1.5 kpc) Beyond
R15 kpc, the disk density is rapidly declining.
The brightness distributions of other galaxies
show similar downturns.
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Infrared view of the Galaxy 2MASS (2 micron
all-sky) survey
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kpc Stars few Gyr old, metal-rich unlike the
metal-poor stars of the galactic halo, the inner
halo is also more round and does not show
rotation (bulge rotates in the prograde sense,
like the sun, but slower ltVcgt 100 km/s) A
slight asymmetry of the bulge and additional
kinematic data show that the Milky Way has a
central bar extending to R2-3 kpc. It is a Sbc
galaxy or SABbc( r) - there can be no perfect
agreement when looking at multiwavelength
data! The center of the Galaxy (nucleus) is a
very exotic place, with the Sagittarius A radio
source, surrounded by a torus (R7 pc) of
molecular gas, which flows in at a rate of
0.001-0.01 Msun/yr and formed dozens of massive
stars within the last 3-7 Myr. Nucleus (right
panel, showing gas) is much smaller than the
black dot in the background picture. A fairly
dark and inactive, starved black hole (m
2-3e6 Msun) lurks in the center of Galactic
Nucleus (white dot).
Bulge
Galactic Nucleus
.
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Differential rotation of the Galaxy rotation
with shear, similar to Keplers laws
Differential rotation of the Galaxy was
discovered by Jan Oort in 1927 using proper
motions of stars at different galactic longitudes
l, because it varied as Vt const cos
2 l, (which was actually known in 1900).
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The position and velocity of the Local Standard
of Rest (solar neighb.) Ro sun-Galaxy center
dist. Ro 8.5 kpc (IAU), 8 kpc (recent) Vo 220
km/s (IAU), 200 km/s (recent)
IAUInternational Astron. Union
Rotation of the Galaxy
Assuming circular motions of S and P,
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Rotation of the Galaxy
Errata (CUP,on-line)!
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A B
Jan Oort (1900-1992)
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This section of the book shows you how A,B, Ro,
Vo, are fitted to the observed radial velocity
measurements..read it ! (other parts too!)
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Distribution of H and H2 in our Galaxy
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21 cm - line data are used to determine basic
Galactic parameters
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Rotation curve of Milky Way is approximately flat