System 176

System Report

System 176 is astrophysically very interesting.  It consists of a close binary of white dwarfs surrounded by gas and dust ejected from the stars. this material has formed a disk close to the binary where new planets are beginning to form from it, clearing out their orbits to form ring-like gaps in the disk. The more massive white dwarf is the smaller, as we would expect from degenerate matter that is not supported by thermal pressure. Indeed, the more massive white dwarf has a mass of 1.43 solar masses which makes it one of the most massive white dwarfs known. It must be very close to its stable limit (the Chandrasekhar mass).

System 176 binary white dwarf

When the stars were still on the main sequence, the more massive of the two would have entered the red giant phase first, overflowing its Roche lobe and causing material to stream away from the older star onto its younger companion. This would have reduced the distance between the two stars in order to conserve angular momentum (since the flowing material would have been moving away from the system's center of mass). This reduction in separation would have further reduced the Roche lobe, accelerating the overflow and increasing the rate of mass transfer. This runaway positive feedback would have led to rapid transfer that would have been complete after just a few years. This would have been too rapid for the smaller companion to assimilate the material and the Roche lobes of both stars would have overflowed, resulting in a common envelope phase in which the two stellar cores orbited within a common atmospheric envelope. The core of the older star would at this point be a white dwarf essentially, orbiting within its now red giant companion.

The drag of the envelope on the stellar cores would have drained the orbital energy and the two stars would have approached one another even more closely. The system likely became a propeller system, propelling the envelope outwards. The younger star would then have been reduced to a red dwarf, orbiting close to its white dwarf companion. our models suggest that the red dwarf, overflowing its Roche lobe, transferred matter to its white dwarf companion. This time, in order to conserve angular momentum (since the material was now flowing towards the system's center of mass) the separation between the two stars would have increased until mass transfer ceased. The red dwarf would transition into a hot degenerate white dwarf due to the loss of material. The loss of mass would have led to its radius increasing and we propose that the lighter white dwarf in this system (with a mass of 0.6 solar masses) with the larger radius would be this red dwarf remnant and that the mass of the heavier white dwarf can be explained in part by the accretion of mass from its companion.

This raises several questions. Did the mode of accretion enable the more massive white dwarf to approach its Chandrasekhar limit without detonating in a supernova explosion? Models place the Chandrasekhar limit at around 0.44 solar masses, but this depends on the model and perhaps on the mode of formation.

The further loss of angular momentum from the binary, in the form of strong gravitational waves due to the intense gravitational fields and closeness of the two stars to one-another, is expected to result eventually in a white dwarf merger. The more massive white dwarf is so close to its limit of stability that it would likely detonate at this point in a type Ia supernova - a type of supernova which lacks hydrogen lines in its spectrum since the outer hydrogen-rich layers of both stars have already been ejected. This could destroy or expel planets beginning to form in the dust disk. Alternatively, however, the merger might transition into a more compact neutron star, preserving the surrounding planets. The fate of this system could be as little as a century away and probably no further than a few thousand years.

This rare system provides an excellent opportunity to refine our mathematical models of white dwarf physics, from predictions of the maximum mass, to equations of state, to atmospheric phenomena.

System 176 binary white dwarf

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System 176 planetary nebula

Above: a remote view of system 176 taken at a distance of 2365 ly, showing the system as it appeared 2365 ly ago. The nebula of hot gas has been expelled, likely by the propeller action of the binary at its center. The nebula rapidly faded but some of the material remains as a central protoplanetary disk.

Recommendation

Release a probe to take measurements to refine current models of white dwarf physics and planet formation; leave the probe behind to carry out long-term observations over the coming decades before setting a course for a new target system.