Transcript Link to pps show of talk on "K Stars"

The K Stars: Orange Giants
& Brighter Dwarfs
Presentation by
Paramita Barai
Astr 8600, Stellar Spectroscopy
K stars
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Bright orange giants
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K dwarfs
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Arcturus
Pollux
Aldebaran
61 Cygni
Fainter member of  Centauri
Subgiants & White Dwarfs
General Features
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T ~ (3500 – 5500)K
Spectra:
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Strong metal lines
Weak CH & CN
molecular bands
H line weakens
K stars in HR diagram
5.1 Classification
Distinguishing spectral
features
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TiO bands disappear, H
appears in absorption
Ca I weakens, H & K Ca
II emerges
Na D weakens
Tightly bound
molecules, G band of
CH
Optical CN & infra red
CO in giants
Comparison of star’s spectra
Luminosity determination
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H & Ca I lines (later subtypes) , show
negative & positive luminosity effects
Metallic abs ratio: Ionized Strontium &
Titanium / Fe I - positive luminosity
effect
Molecular CN band – 4215 A – CN break
Luminosity cont ..
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Use of single
criterion to get L 
Errors
To be more accurate
Account for all
defined criterion
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Use temperature
sensitive features to
get Draper class
From density
criterion get
Luminosity
Once know star is
giant / dwarf ->
refine T class
Spectra of two stars
• Arcturus (K1 III)
• Aldebaran (K5 III)
5.2 Spectroscopic Distances
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L = 4 *  * D2 * f
M = m + 5 – 5 log(D) – A
MKK class  L  D
But !!
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Giant stars have spread about average
Use several stars / whole main sequence
Calibrate L/M vs. apparent brightness
5.3 Chromospheres
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Low density, hot gas
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Strong emission
lines :
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Fraunhofer H & K
lines of Ca II
Self absorbed
emission
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Ca K abs
(photosphere) : K1
Narrow bright
emission
(chromosphere) : K2
Narrower abs : K3
Wilson-Bappu effect
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Width of K2 ~ Mv
Distance indicator
5.4 Eclipsing Supergiants
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Eclipse analysis:
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Time to cover each other
 Diameter
Drop in intensity 
Relative surface
brightness  T
Doppler Shift of spectra
 Orbital velocity
Mass of star
Inclination of orbit
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 Aurigae – faintest
of ‘kids’ of Cappella
31 & 32 (o1 & o2)
Cygni
U Cephei & VV
Cephei
 Aurigae
5.5 Composition Variations
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C overabundance R stars
Counterpart in class
M -> N stars
S stars –
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Underabundance
CN break – sensitive
to L
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C & O abundance, s
process elements
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Not applicable for
metal deficient, Pop
II stars in galactic
halo
CN anomaly
CH stars
5.6 Towards Dwarfs
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K giants  Subgiants (class IV)  Dwarfs
Above main sequence, T Tauri stars
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Odd : emission, irregular variation
T associations, with ISM clouds
Newly formed, youngest
K dwarfs (6-8 mag) – less than M dwarfs,
larger mass, higher MS luminosity
5.7 Parallaxes & Stellar
Distances
Parallax
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Semi annual shift of
star’s position in sky
caused by earth’s
rotation around sun
D=1/
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D = distance (pc)
 = parallax (arc
sec)
61 Cygni
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D < 50 pc
Distances to Sirius,
Vega, Arcturus,
Pollux
For classes V & III
using parallax, can
get distances &
hence M, can
calibrate HR diagram
Distances farther out
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Hyades
Radiant position:
where star will be
after millions of yrs
Angle between
radiant & present
position 
Transverse velocity
 Distance
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Calibrate HR diagram
for Hyades with Mv vs
spectral class
Add nearby parallax
stars also to diagram &
use as standard
Get better calibration
for visual mag
(luminosity) vs distance
5.8 Subdwarfs & White Dwarfs
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Subdwarfs
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1 mag below MS – metal poor, too early /
blue by several subtypes
Classification metal lines weak – get earlier
subclass
White dwarfs
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Several mag below
Tiny, v high density
THANKS TO ALL