Nature of the system Epsilon Aurigae

bright class f star , companion class b star surrounded dusty disk (artist impression)

the nature of epsilon aurigae system unclear. has long been known consist of @ least 2 components undergo periodic eclipses unusual flat-bottomed dimming every 27 years. explanations exceptionally large diffuse stars, black holes, , odd doughnut-shaped discs no longer accepted. there 2 main explanations can account known observed characteristics: high mass model primary yellow supergiant of around 15 m☉; , low mass model primary 2 m☉ , less luminous evolved star.

variations on high mass model have been popular, since primary star appearances large supergiant star. spectroscopically f or late luminosity class ia or iab. distance estimates consistently lead luminosities expected bright supergiant. 1 exception hipparcos parallax measurement, margin of error large value , derived distance 355 4167 parsecs. main problem model nature of secondary, required known mass function have mass comparable primary, @ odds observations appears b-type main-sequence star. secondary may close binary involving 2 lower-mass main sequence stars, or more complex system.

the low mass model, popularised citizen sky project, proposes primary evolved asymptotic giant branch star of 2–4 m☉. relies on distance , luminosity estimates lower observations. star unusually large , bright giant star given mass, possibly result of high mass loss. match observed eclipse , orbital data, secondary normal b main sequence star of 6 m☉ embedded in thick disc seen edge on.

the orbit determined, inclined @ on 87 degrees earth. primary , secondary around 35 au apart (in high mass model), further planet neptune sun. in low mass model, separation 18 au.

visible component

low inclination view of system (illustration nico camargo)

the visible component, epsilon aurigae a, semiregular pulsating post-asymptotic giant branch star belonging spectral class f0. f-type star has around 143 358 times diameter of sun, , 37,875 times luminous. (reliable sources vary considerably in estimates of both quantities.) if star in position of sun, envelop mercury , possibly venus. f-type stars epsilon aurigae tend glow white , display strong ionized calcium absorption lines , weak hydrogen absorption lines; being class above sun (which g-type star), f-type stars typically hotter sunlike stars. other f-type stars include procyon s primary star, brightest star in constellation canis minor.

eclipsing component

the eclipsing component emits comparatively insignificant amount of light, , not visible naked eye. heated region, however, has been discovered in center of object. thought dusty disc surrounding class b main sequence star. modelling spectral energy distribution ε aurigae whole produces best fit b5v star @ centre of disc. such star have mass around 5.9 m☉. observed orbit, assuming normal f-type supergiant primary star, requires secondary mass on 13 m☉. low mass model accepts 5.9 m☉ secondary , requires low-mass primary. high-mass model accepts normal mass supergiant primary , argues pair of b-type stars, or unusual single higher-mass star.

a variable star chart ε aurigae. numbered stars comparison stars numbers giving comparison star brightness in magnitudes

the disc around secondary star 3.8 au wide, 0.475 au thick, , blocks 70% of light passing through it, allowing light primary star seen during eclipses. radiates 550 k black body.