Eclipsing Binary Variables
Eclipsing binary variables are binary stars which orbit their common center of gravity, but the orbit happens to have a special orientation, inclined 90 degrees to Earth, so that during their orbit, each star temporarily blocks from our view part or all of the light from the other star. Most stars are so far away that most such binary systems still appear to us on the sky as a single star. When the stars eclipse each other this way, it causes the brightness of the system to vary and the light curve to dim. The relative brightness, size, and type of the stars in an eclipsing binary, as well as the distance between them, determines the appearance of the light curve. If the stars differ in brightness, the brighter one is called the primary, and the dimmer one the secondary. The light curve may dip twice during the orbit, once when the secondary passes in front of the primary and once when the primary passes in front of the secondary. There are three main types of eclipsing binaries that are distinguished on the basis of their light curves: Algol type, Beta-Lyrae type, and W Ursae Majoris-type.
Algol-type Eclipsing Binary
EA, or Algol-type eclipsing binary systems are made up of two stars that are widely separated in their mutual orbit and do not interact, so that the light curve remains flat (shows little variation) outside of the eclipses. Since the stars do not interact, the binary is referred to as detached. If the two stars have very different temperatures or sizes, then the light curve will show two different-shaped eclipses. For instance, when a cooler, dimmer (secondary) star eclipses a hotter, brighter (primary) star, there will be a larger decrease in brightness, so a deeper eclipse in the light curve. This is called the primary minimum. When the hotter primary star passes in front of the secondary star, there may be a smaller decrease in total brightness, referred to as the secondary minimum.
Below is a sonified video of some observations of an EA type eclipsing binary. The 107 observations take place over only about 2 and a half hours. The video shows the primary minimum of this system, or when the secondary star eclipses the primary. The observations are impressively high in resolution, with each beat corresponding to a minute in real-time. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
The next video is the phased light curve for the observed EA type eclipsing binary. The binary system was determined to have a period of about 12 hours based on 594 total observations taken over nearly 600 days. In the video, you should hear two full phases, each consisting of the primary minimum followed by the secondary minimum with near constant brightness in between. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Below is the spectrum for the observed EA type eclipsing binary. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 angstroms. Lower pitch represents weaker flux. The spectrum will gradually increase in flux (pitch) as wavelength increases before leveling off around halfway through.
This EA type eclipsing binary, J161320.56+301231.0 was targeted for SDSS-IV spectroscopy as a variable in the TDSS project (Roulston in prep.). The light curve is an optical g-band from the ZTF.
Beta Lyrae-type Eclipsing Binary
Beta Lyrae type (EB) eclipsing binary systems have closer, short period orbits. The two components of the binary are referred to as semi-detached - so close that the stars distort each other gravitationally, from spherical to ellipsoidal shapes, but not so close that they exchange mass. Because of the ellipsoidal shape of the stars, there is no flat, well-defined phase of constant brightness between the eclipses.
Below is a sonified video of observations taken of an EB type eclipsing binary. The 42 observations that can be heard took place over a span of 20 nights, so that each beat corresponds to 3 hours in real time. Because of the long times between observations compared to the binary period, there are no trends to be heard in the video, but listen for the quick fluctuations in brightness. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Shown next is the phased light curve for the EB type eclipsing binary. The phased light curves of EB type binaries are much smoother than EA type binaries. In other words, the eclipses, or dips in brightness, occur so gradually that it can be hard to pinpoint the start and end of an eclipse. Also, there is not a well-defined baseline brightness when the stars aren’t eclipsing like there is for detached EA-type eclipsing systems. Listen and hear two full phases, each consisting of the secondary minimum followed by the primary minimum with smooth transitions in between the two eclipses. One phase corresponds to a period of about 6 hours and 40 minutes. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Below is the spectrum for the observed EB type eclipsing binary. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 angstroms. Lower pitch represents weaker flux. The spectrum will gradually increase in flux (pitch) as wavelength increases before leveling off around halfway through. Note that the eclipses have different depths, which means that the two stars in the binary must have different brightness.
This EB, J171738.45+405904.6 was targeted for SDSS-IV spectroscopy as a variable in the TDSS project (Roulston in prep.). The light curve is an optical r-band from the ZTF.
W Ursae Majoris-type Eclipsing Binary
EW, or W Ursae Majoris eclipsing binary stars are also known as low mass contact binaries. They are close enough that their outer atmospheres touch. Effectively, the stars in an EW contact binary share a common envelope of material and transfer mass between them. Thus, the two stars usually have very similar masses.
Below is a sonified video of some observations of an EW type eclipsing binary. The 25 second long video contains 126 observations and takes place over about 8 hours total. Each beat therefore corresponds to a minute in real time, which is phenomenal resolution. The video shows one of the minimums of this system, or when one star eclipses the other. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
The next video shows the phased light curve of the observed EW type eclipsing binary. Similar to the EB type binary, the EW binary has a smooth, ellipsoidal phased light curve because the stars are gravitationally distorted by one another and the visible area of the stars is constantly changing. Because both stars in an EW type binary are in contact, they have nearly equal masses and surface temperatures, so that the depths of the brightness minima are almost the same. This system has a period of about 6 and a half hours. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Below is the spectrum for the observed EW type eclipsing binary. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 angstroms. Lower pitch represents weaker flux. The spectrum will gradually increase in flux (pitch) as wavelength increases before leveling off around halfway through. EW eclipsing binaries usually contain low mass stars, which are cool, and therefore red. Cool stars also have many spectral absorption features from atoms and molecules that form in the outer layers of the stellar atmosphere.
This EW, J162659.25+354021.2 was targeted for SDSS-IV spectroscopy as a variable in the TDSS project (Roulston in prep.). The light curve is an optical r-band from the ZTF.
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