RR Lyrae Stars

By: Mike Lundquist

What Are They

• Old Stars, Over 10 Billion Years Old

• Helium Burning, Horizontal Branch Stars

• Variable Stars With Periods of 0.2 to 1.1 days

• ~0.7 Solar Masses

• ~4-6 Solar Radii

• Metal Poor Stars

RR Lyrae Stars In M3

Evolution

• Ascend RGB

• Helium Flash onto HB

• Enter Instability Strip

• Ascend AGB

• Forms Planetary Nebula

• Becomes a White Dwarf

Types of RR Lyrae Stars

RRc -- Overtone Mode -- Increase in the Light Curve Nearly Equal to the Decrease

RRd – Pulsate in both the RRab and RRc modes.

RRab -- Fundamental Mode --Rapid Increase to the Peak of the Light Curve, with a Relatively Slow Decrease

RRab Type Stars Have Longer Periods

RRab Type Stars Are Easier to Find than RRc

Most RR Lyrae stars found are RRab Type

RR Lyrae Stars are Very Common in Globular Clusters

PulsationIonized Helium Layers Absorb Heat

Pressure and Luminosity Increase

Star Expands and Cools

Pressure and Luminosity Decrease

Star Compresses

Ionized Helium Absorbs Heat

Cycle Continues

The Blazhko Effect

The Blazhko Effect

• Periodic Variations in Light Curve• Only Mostly in RRab Type.

Leading Theories

• Result of Resonance• Result of Magnetic Field and

Rotation

Other Important Uses

• Standard Candles

Accurate to ~10% Distance

• Real Time Stellar Evolution

Standard Candles

• Absolute magnitudes: MV ~ 0.6

• Accurate Distance Measurements

Metallicity Relationship

Real Time Stellar Evolution?

• RR Lyrae Star V79

• Between 1962 and 1996, Pulsations in the First Overtone Increased Significantly, while the Period has Decreased.

• Decrease in Period Suggests Blueward Stellar Evolution

Summary

Period 0.2-1.1 days

Mv 0.6 ± 0.2

Te 7400 K – 6100 K

Log g 2.5-3.0

[Fe/H] 0.0 – -2.5

Mass ~0.7 Solar Masses

Radius ~4-6 Solar Radii

•Good For Calculating Distances

•Good For Testing Pulsation Models

•Insight into Stellar Evolution of Low Mass Stars

RR Lyrae Properties

For More Information• Principles of Stellar Evolution and Nucleosynthesis By Donald D. Clayton• RR Lyrae Stars By Horace A. Smith• http://www.aavso.org/• http://www.journals.uchicago.edu/doi/pdf/10.1086/316352• http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes16.html• http://www.pa.msu.edu/people/smith/• http://spiff.rit.edu/classes/phys230/lectures/mw_size/mw_size.html• http://antwrp.gsfc.nasa.gov/apod/ap041012.html• http://www.aanda.org/index.php?option=article&access=bibcode&bibcode=2

006A%2526A...459..577KPDF• http://www.journals.uchicago.edu/doi/pdf/10.1086/506198• http://www.aanda.org/index.php?

option=article&access=bibcode&bibcode=2007A%2526A...474..557MPDF• http://adsabs.harvard.edu/abs/2004rrls.book.....S• http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?

1998ASPC..135..385F&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf