The K Stars: Orange Giants & Brighter Dwarfs

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

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 Luminosity cont ..

Spectra of two stars • Arcturus (K1 III) • Aldebaran (K5 III)

5.2 Spectroscopic Distances • L = 4 *  * D2 * 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

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 ~ Mv Distance indicator 5.3 Chromospheres

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 (o1 & o2) Cygni U Cephei & VV Cephei  Aurigae 5.4 Eclipsing Supergiants

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 5.5 Composition Variations

5.6 Towards 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

5.7 Parallaxes & Stellar Distances

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 Parallax

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 Mv vs spectral class Add nearby parallax stars also to diagram & use as standard Get better calibration for visual mag (luminosity) vs distance Distances farther out

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

THANKS TO ALL

The K Stars: Orange Giants & Brighter Dwarfs