Common Envelope Evolution

The evolution of interacting binary stars

By Dirk Goës

Many of the stars visible in the night sky are binary star systems with two stars quietly orbiting each other around a common centre of mass.  Many of these will evolve to become interacting binary stars where mass, mainly in the form of hydrogen gas, is being transferred from one star to another with often explosive results.  Examples include cataclysmic variables, X-ray binaries, Type Ia Supernovae and the merger of two stars into one.  These types of star systems are very important in astrophysics for understanding the physics of stars and the history of our galaxy and universe.  For example observations of Type Ia Supernovae were famously used to determine that the universe is expanding at an accelerating rate. 

But how do these stable binary star systems evolve to become unstable interacting binary systems? 

They evolve through a process known as common envelope evolution (CEE).  It occurs when one star in a binary system expands to such a degree that it engulfs its companion.  Both stars are now encased in a common envelope of gas.  The friction within this common envelope causes the orbits of both stars to spiral into each other creating a binary system with a much tighter orbit. 

The common envelope may become be lost resulting in a compact binary system or the envelope may be retained resulting in a merger of the two stars.  Researchers are working to better understand this process through a combination of theory, simulations and observations as explained below. 

Figure 1: A false colour radio light image of binary star system HD101584.  The binary is in the central green dot and the green ring of gas is possibly an ejected common envelope.  Jets of gas emanating from the binary create the blue gas (moving towards the viewer) and the red gas (moving away from the viewer)  (Credit: ALMA (ESO/NAOJ/NRAO), Olofsson et al. Acknowledgement: Robert Cumming). 

Common envelope evolution – how it works

A common envelope event can occur when a stable binary system evolves and becomes unstable.  There are many different pathways to a common envelope event, but the example described here starts with a stable binary system where both stars are in their main-sequence hydrogen fusion phase, like our Sun, and are orbiting each other and interacting only through gravity. 

Most often the two stars will be of different mass and the higher mass star will burn through its nuclear fuel faster and therefore evolve faster than its companion.  If the higher mass star is about the mass of our Sun to about eight times the mass of the Sun, it will evolve into a red giant star.  This occurs once it has exhausted its supply of hydrogen fuel in its core, and nuclear fusion in the core ceases. 

As the red giant expands it may expand to such a degree that the outer layers of gas are no longer gravitationally attached to the star.  In this case it is said to have overflowed its Roche-lobe, which is the gravitational domain of a star.  When this occurs, the red giant may engulf its binary companion and form a common envelope of gas around both stars. 

The friction in this envelope causes drag and the two stars spiral inward into a tighter orbit.  This process also causes orbital energy to be transferred from the stars to the surrounding envelope.  It is theorised that the amount of orbital energy transferred to the envelope influences whether the envelope of gas is ejected from the binary system or retained.  In addition, other factors such as tidal forces, magnetic braking, and gravitational waves are thought to influence how the envelope behaves. 

The common envelope will be ejected and lost from the binary system if the envelope becomes detached and stops orbiting with the binary system.  The outer layers of the red giant star will be lost together with the common envelope and only its core will remain, forming a white dwarf star.  In this case the result is a tight binary system with a white dwarf and main-sequence star closely orbiting each other. 

If the envelope is not lost it is expected that the two stars will continue to spiral into each other and merge into one.  Most of the material making up the common envelope will be retained in the merged system.  The process is depicted in figure 2. 

Figure 2: A conceptual diagram of the main stages of a simple common envelope scenario.  Not to scale (Diagram: Dirk Goës).    

If the resulting system is a tight binary (as shown in step 5 in figure 2), it may further evolve to become an interacting binary system.  This can occur when the second also star evolves into a red giant and overflows its Roche-lobe.  At this point the white dwarf will start to pull material off the red giant and mass transfer commences.  Depending on the nature of the white dwarf, especially its mass, this can result in a cataclysmic variable (a white dwarf that experiences a periodic thermonuclear explosion on its surface) or a Type Ia Supernovae (the complete destruction of the white dwarf). 

Figure 3: A conceptual diagram of mass transfer in an interacting binary system that results in a Type Ia Supernova explosion.  The explosion occurs when the mass of the white dwarf exceeds the Chandrasekhar Limit.  Not to scale (Diagram: Dirk Goës). 

The above description is a simple example of a much more complex process with many possible evolutionary pathways and outcomes.  For example, the evolution of a binary system may experience more than one common envelope phase.  Or if the initial binary system consists of very high mass stars, then neutron star or black hole binaries may result. 

Theory, simulations and observations

That common envelope evolution occurs in binary star systems was first proposed in the 1970’s.  Since then, astrophysicists understanding of the mechanism has improved through a combination of theory, computer simulations and observations.  However, it is a tough astrophysical problem with many unknowns.

Computer simulations have played an important role in advancing astrophysicists understanding of the process.  Professor Natalia Ivanova (University of Alberta, Canada), a leading theorist in this area and the co-author of an important book on the subject writes via email correspondence that “The reasons for envelope loss or retention are not fully understood in all cases” however “We are quite good at modelling low-mass donors, where common envelope events are modelled cleanly and behave well in dynamical codes [computer simulations]. The field is steadily progressing, with several [research] groups now beginning to model more massive stars (10 solar masses or more)”. 

CEE events are difficult to observe and record through telescopes because they are short lived and when and where they will happen are not easily predicted.  They are considered transient events which means they involve detecting a change in part of the sky such as a star suddenly brightening.  However, two types of observations have been made.  One is the observation of luminous red novae (LRNe) which are theorised to be a common envelope event in progress.  The other is of the remnant of a common envelope that has been ejected from a binary system. 

Luminous Red Novae

Like a supernova, but not as bright, a luminous red nova (LRN) is a sudden brightening around a star system that is very red in colour.  Several luminous red novae have been observed including a system named V838 Monocerotis, which was spectacularly captured by the Hubble Space Telescope as shown in figure 4. 

Figure 4: The luminous red nova V838 Monocerotis, believed to result from a common envelope event around the binary system contained in the central yellow pulse of light (Credit: NASA, the Hubble Heritage Team (AURA/STScI) and ESA). 

In 2008 a LRN around the star system V1309 Scorpii was observed and a subsequent 2011 research study found that it was the result of a binary star system merging into a single star.  This determination was possible because V1309 Scorpii had been observed and tracked since 2001 by the Optical Gravitational Lensing Experiment (OGLE) survey.  This allowed the research team, led by Dr Romuald Tylenda of the Nicolaus Copernicus Astronomical Center in Poland, to observe and measure how this binary star system evolved and merged. 

A 2014 research study at the University of Alberta Canada, developed a set of computer simulations that recreate the V1309 Scorpii merger.  One of the simulations show that as the red giant star expands, the white dwarf star begins to draw material from the giant.  Subsequently both stars become encased in a common envelope and the observed LRN outburst occurs and the stars merge.  This all occurs over a very short timescale measured in days as shown in figure 5. 

Figure 5: A simulation recreating the common envelope event, LRN and merger of binary star system V1309 Scorpii.  The final frame (bottom right) shows the two stars in the common envelope shortly before they merge (Credit: Nandez, J. L. A., et al. 2014, ApJ, 786, 39).

Common envelope remnants

In a 2022 research study Chinese and Australian astronomers reported the discovery of a possible remnant of a common envelope ejection around a binary system called J1920-2001 that is estimated to have occurred approximately 10,000 years ago.  The system consists of a white dwarf and a hot-subdwarf star in a tight binary orbit with mass being transferred from the hot-subdwarf to the white dwarf.  Hot-subdwarfs are blue in colour and research has found them to be closely associated with common envelope evolution.  The system was first identified in a survey of blue stellar objects with the Australian National University’s SkyMapper telescope. 

Spectral analysis of J1920-2001 revealed that it closely matched the spectrum of a hot-subdwarf except for the presence of ionised calcium absorption lines (Calcium II).  Calcium II lines would normally only appear for much cooler objects.  Radial velocity measurements revealed that the material containing the Calcium II is surrounding but not orbiting with the binary system.  This led to the conclusion that this detached material was the leftover ejected common envelope. 

Figure 6: Spectrum of J1920-2001 from the Wide Field Spectrograph (WiFeS) on the Australian National University 2.3-m telescope. The top panel shows the spectrum of J1920-2001 in grey with the red line representing the best fit for a hot-subdwarf star of spectral class O (sdO).  The outliers are the strong Callium II lines (CaII K&H).  (Credit: Li, J., et al. 2022, MNRAS, 515, 3370). 

The team that discovered J1920-2001, led by Dr Jiangdan Li of the Chinese Academy of Sciences, were inspired by this discovery to undertake a targeted study of similar systems.  The team interrogated the spectra collected by the Chinese Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) looking for hot-subdwarf systems with prominent Calcium II features.  The resulting 2025 research paper reported the discovery of a further 145 binary systems with similar characteristics and evidence of ejected common envelopes. 

The Vera C Rubin Observatory

Most observations of possible common envelope events have been detected after they have occurred.  Researchers hope that in the future they will be able to catch these events while they are in progress.  A telescope facility that may be successful in achieving this is the new Vera C. Rubin Observatory located in Chile.  Starting in 2025 it will conduct a ten-year survey of the southern sky called the Legacy Survey of Space and Time (LSST).  With an 8.4 metre mirror and the largest digital camera ever built it will image the entire southern sky every three nights. 

One of the science goals of the Rubin Observatory is “Exploring objects that change position or brightness over time” or “Exploring Transients”.  A common envelope event, like a supernova, is an object that changes brightness.  By constantly re-imaging the sky the observatory will detect millions of changes in the night sky that will be sent out automatically as alerts to astronomers around the world.  Astronomers will filter the alerts for the types of objects they are interested in, such as supernova or possible common envelope events, and then use other telescopes and observation facilities to study the details of specific objects. 

Figure 7: The Vera C. Rubin Observatory on top of Cerro Pachón (2,672 m), in the Coquimbo region of Chile.  It is named after Vera Rubin (pictured), the pioneering astronomer who showed that dark matter exists.  It is funded by the US National Science Foundation, United States Department of Energy and private funds (Credit: Olivier Bonin/SLAC National Accelerator Laboratory) (Credit: Cornell University). 

Exciting future

Facilities such as the Rubin Observatory rely not only on the precision engineering of telescope mirrors and cameras but also on raw computing power to store, process and distribute data to researchers.  This ability combined with the research teams who are constantly improving algorithms to run better simulations means our understanding of the evolution of binary star systems and the role of common envelope evolution is likely to advance rapidly. 

Australian researchers across multiple institutions have been granted access to the data being collected by the Rubin Observatory. 

Thank you to Professor Natalia Ivanova (University of Alberta, Canada) and Dr Jiangdan Li (Chinese Academy of Sciences) for answering my questions via email.