Goal: To understand binary stars Objectives: 1)To review why we get binary star systems 2)To learn about the 2 different binary star types and how we find them 3)To learn about what we can learn about stars by studying binary stars 4)To explore the weird stars that get created as a result of binary systems

Star formation As we learned earlier in semester: Stars for in groups Furthermore somehow they seem to most often form in systems of multiple stars Most stars are in multi-star systems

Two main types Visual Binary –A telescope will resolve what appears to be one star into multiple stars Spectroscopic Binary –A telescope will NOT resolve the one star into multiple stars as they are either too close and/or too far away

Doppler shifts As the stars orbit each other (around their center of mass) there are times they move towards us and away from us. As move towards us, blue shift. As away, red shift. So, can measure velocity as function of time

From Barlow et al, 2011, RADIAL VELOCITY CONFIRMATION OF A BINARY DETECTED FROM PULSE TIMINGS, ApJ Letters 737 (2011) L2

Another Result: Eclipsing Binaries

Another one

What we can find: Period, how long it takes to do a full cycle is the orbital period Orbital velocity, magnitude of the sine wave

Orbital distance: You make a circle once per period which will have a circumference of 2 pi r Distance = 2 pi r = velocity * time(aka period) So, the orbital radius for the star is therefore: R = velocity * period / (2 pi) However, this is the orbital distance from the CENTER OF MASS of the system

Minimum value to mass: Vorbit = (GM/r) 1/2 So, Mass = r * v 2 / G Oh, and since period is time to do a full circle –Circumference = 2 pi r = velocity * period –So, r = v * period / (2 pi) So, minimum mass = period * v 3 / (2 pi * G)

Oh no, that is too hard! Make it easy! If you use cleaver units it gets easier… Make the period in years and the velocity as an Earth velocity (30 km/s) and we know the mass is 1 solar mass. So, Mass (in solar masses) = Period (in years) * (v/30km/s) 3 We call this the “Mass function”

Period = 15 days, V = 17 km/s M = 15/365 * (15/30) 3 = solar masses

Oops, what just happened?! 2 things 1) This measures the combined mass of the 2 stars 2) we only measure the radial velocity. What happens if the orbit is tilted? (so we find Mass * sin(inclination)) Actual stars probably closer to 0.4 and 0.1 solar masses.

What oddities can we see: 1) black holes Have a bright star and a HIGHER mass companion… 2) red dwarfs and brown dwarfs Normally too dim to see These are called Astrometric Binaries

Interactions They interact in a few different ways 1) Primary hits giant phase Here secondary takes mass making it larger than the primary (in some cases it can become bigger) Secondary gets more metals

2) Both in giant phase at the same time Contact binary – the 2 stars actually touch You get a hot spot that rotates with the stars and makes a very strange chart

3) Primary dead, secondary giant Sirius is one example of that (3 solar mass star orbited by a white dwarf). This is when you get periodic novas (or x- ray bursts) on the dead star as it accretes matter. This is the case where you can find solar mass type black holes.

Conclusion We refreshed our memories for why we get binary stars We have examined how we discover binary star systems We can learn about masses of stars through binary interactions We can find dim objects we could not find otherwise They have interesting interactions between binary stars which can influence the evolution or perceived evolution of the stars.