The K Stars: Orange Giants & Brighter Dwarfs Presentation by Paramita Barai Astr 8600, Stellar Spectroscopy

K stars Bright orange giants Arcturus Pollux Aldebaran K dwarfs 61 Cygni Fainter member of  Centauri Subgiants & White Dwarfs

General Features T ~ (3500 – 5500)K Spectra: Strong metal lines Weak CH & CN molecular bands H line weakens

K stars in HR diagram

5.1 Classification

Distinguishing spectral features 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 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.. Use of single criterion to get L  Errors To be more accurate  Account for all defined criterion 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 L = 4 *  * D 2 * f M = m + 5 – 5 log(D) – A MKK class  L  D But !! Giant stars have spread about average Use several stars / whole main sequence Calibrate L/M vs. apparent brightness

5.3 Chromospheres Low density, hot gas Strong emission lines : Fraunhofer H & K lines of Ca II Self absorbed emission Ca K abs (photosphere) : K1 Narrow bright emission (chromosphere) : K2 Narrower abs : K3 Wilson-Bappu effect Width of K2 ~ M v Distance indicator

5.4 Eclipsing Supergiants Eclipse analysis: 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  Aurigae – faintest of ‘kids’ of Cappella 31 & 32 (o 1 & o 2 ) Cygni U Cephei & VV Cephei  Aurigae

5.5 Composition Variations C overabundance - R stars Counterpart in class M -> N stars S stars – C & O abundance, s process elements Underabundance CN break – sensitive to L Not applicable for metal deficient, Pop II stars in galactic halo CN anomaly CH stars

5.6Towards Dwarfs K giants  Subgiants (class IV)  Dwarfs Above main sequence, T Tauri stars 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

5.7Parallaxes & Stellar Distances

Parallax Semi annual shift of star’s position in sky caused by earth’s rotation around sun D = 1 /  D = distance (pc)  = parallax (arc sec) 61 Cygni 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 Hyades Radiant position: where star will be after millions of yrs Angle between radiant & present position  Transverse velocity  Distance Calibrate HR diagram for Hyades with M v 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 Subdwarfs 1 mag below MS – metal poor, too early / blue by several subtypes Classification metal lines weak – get earlier subclass White dwarfs Several mag below Tiny, v high density

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