Stargazing Two White Dwarfs in a seven minute mutual orbit

[diogenesNY]

TL;DR Summary

These two dead stars zip around each other every seven minutes

From CNN:
These two dead stars zip around each other every seven minutes
https://www.cnn.com/profiles/ashley-strickland-profile
By Ashley Strickland, CNN
Updated 2:48 PM ET, Wed July 24, 2019

(CNN)While searching the skies for brightness and blinking, the California Institute of Technology's Zwicky Transient Facility sky survey spotted an odd pair of orbiting dead stars 8,000 light-years away.

The rare discovery is the second-fastest pair ever discovered, whipping around each other at speeds reaching hundreds of kilometers per second. The two white dwarf stars complete an orbit around each other every seven minutes. It's also known as an eclipsing binary system because one of the stars repeatedly crosses in front of the other.
...
The two stars orbit so closely that they could both fit inside Saturn. The distance between them is 47,780 miles, or one-fifth the distance between the Earth and the moon.
[article continues]
https://www.cnn.com/2019/07/24/world/binary-stars-gravitational-waves-trnd-scn/index.html
From the more heavy-duty article in Nature:
Letter | Published: 24 July 2019

General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system
Kevin B. Burdge, Michael W. Coughlin, […]Thomas A. Prince
Naturevolume 571, pages528–531 (2019)
...
Here we report the discovery of an eclipsing double-white-dwarf binary system, ZTF J153932.16+502738.8, with an orbital period of 6.91 minutes. This system has an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We see rapid orbital decay, consistent with that expected from general relativity.
[article continues]

https://www.nature.com/articles/s41586-019-1403-0
or

http://dx.doi.org/10.1038/s41586-019-1403-0These url's may be a bit uncooperative... please advise if there are problems... may or may not hit a paywall

diogenesNY

 

[phinds]

Very interesting. Thanks for posting

 

[Steelwolf]

Saw that this morning and it has a lot of potentials for future study

 

[mfb]

~15 years until LISA can study the gravitational waves of these close binaries. Nice to find some sources already.

 

[jackwhirl]

Do we have an estimate of the rate of decay and time until they merge?

 

[mfb]

The paper gives 210,000 years as orbital decay timescale.
Very soon for astronomical standards, but in the far future for human timescales.

Based on Figure 2 it will be one of the easiest sources for LISA to measure, something for the first week of data-taking (as it is a continuous source you don't have to be lucky, it will be there as soon as you start measuring). Over time it should measure it with an uncertainty of less than 1%.

 

[diogenesNY]

Excerpted from the _Nature_ letter:

The orbit of ZTF J1539 + 5027 will continue to decay for about 130,000 years until it reaches a period of approximately 5 min, at which point the degenerate core of the secondary will begin to expand in response to mass loss, dramatically increasing the rate of mass transfer25. If the mass transfer is stable, which is likely given the mass ratio26 of q ≈ 1/3, the binary will evolve into an AM Canum Venaticorum system and the orbital period will increase. Alternatively, unstable mass transfer would result in a merger that could produce an R Coronae Borealis star27, or, less probably, a detonation of accreted helium on the primary could lead to a double detonation that disrupts the primary28.
--end excerpt--

Had some fun looking up AM Canum Venaticorum system and R Coronae Borealis star.

https://en.wikipedia.org/wiki/AM_Canum_Venaticorum_star
https://en.wikipedia.org/wiki/R_Coronae_Borealisand this:

http://www.mssl.ucl.ac.uk/www_astro/gal/cv_beginners.html
diogenesNY