Common Envelope Transients - Progenitors, Precursors, and Properties of their Outbursts

Project Information

CET-3PO

Grant agreement ID: 101042610

DOI

10.3030/101042610

EC signature date 9 February 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 489 225,00

EU contribution

€ 1 489 225,00

Coordinated by

UNIVERSITAT DE BARCELONA
Spain

Periodic Reporting for period 1 - CET-3PO (Common Envelope Transients - Progenitors, Precursors, and Properties of their Outbursts)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

The majority of massive stars (more than 8 times our Sun) are born in binary and multiple-star systems. When these stars evolve during their lifetimes, they can interact with their companions. When large amount of mass is quickly transferred from one system to another, an instability in this system can occur: one of the components can spiral into the gaseous envelope of its companion, forming a Common Envelope. The evolution of the binaries during this phase is necessary to form many exciting systems in astrophysics, including sources of gravitational waves.

Despite its importance, there are several unanswered questions in this field: What systems enter CEE? What happens during CEE? How CEE remnants evolve? This problem is difficult to tackle from theoretical and computational perspectives, as it involves different scales (e.g. a red supergiant donor with a neutron star accretor) and timescales (millions of years of evolution vs. dynamical interactions lasting just days). The problem can be broken into smaller pieces, and studied separately. However, to put back all these parts together and test these results, observations are needed.

Recently, a new type of astrophysical transients called luminous red novae (LRNe) has emerged as direct observational evidence of the dynamical ejection of the CE in binaries, followed by a stellar merger. Their progenitor systems and slow brightening precursor emission can be identified in archival and multi-wavelength time-domain data. Their outbursts can be observed to obtain new information about the energetic outbursts. Finally, their late-time evolution allows us to study the dust formed quickly after the merger has happened. These multiple phases have the potential to provide a complete observational evolution of binary systems entering CEE.

The aim of this project is to study the different stages of CEE in massive binaries using observations of extragalactic LRNe. The observational sample will contain transients within 15 Mpc from massive binary progenitors with HST archival data. The project will develop novel transient selection strategies to identify a fraction of these LRNe even years before their main outburst, and study the extensive mass loss leading to coalescence. The study will provide observational evidence of the physical processes that occur before, during, and after the ejection of the CE in massive binary systems, the characteristics of their progenitors, and their rate in our Local Universe.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main activities performed as part of the project have focused on three main areas:
-1. Building the framework to quickly identify and study LRNe and their precursors in time-domain surveys data. Here my research group has:
a) Published a catalogue of yellow supergiant stars detected in archival data from the Hubble Space Telescope. According to our hypothesis, these sources are likely progenitors of future LRNe outbursts.
b) Developed filtering algorithms that prioritize under-luminous transients detected by ongoing public time-domain surveys (both inside and outside the Milky Way) and transients related to our yellow supergiants catalogue.
c) Set a daily scanning team to select the best candidates and obtained regular time on ground-based and space-based telescopes to classify and follow-up these transients.
d) Built the necessary pipelines to quickly reduce and analyze the data.
e) Carried a pilot study to find potential LRNe precursors in the Milky Way and identify possible contaminants that are in the same color-magnitude parameter space.

-2. Providing new observational evidence on the progenitors and outburst properties of LRNe with state-of-the-art observational datasets. The work performed implied:
a) Detailed modelling of the progenitor systems of two LRNe where we tested different mass-transfer instability criteria.
b) Detailed analysis of medium-resolution spectroscopy of LRNe outbursts. The unprecedented resolution of our data has allowed us to discover previously missed low velocity components in the LRNe ejecta, likely associated with different mass loss episodes.

-3. Obtaining new observational data on the remnant evolution of LRNe. My research team and collaborators obtained JWST time to study the dust properties of four well-studied LRNe. The observations and data analysis are ongoing.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The modelling of LRNe progenitors developed by the CET-3PO group goes beyond the state of the art in two main aspects. First, it uses detailed binary evolution models to track the primary star as it evolves through mass transfer. Second, it uses several instability criteria to ensure that the progenitor was already in an unstable mass transfer phase when the system merged. Our results show that the progenitor primary stars that lead LRNe are 15-35% more massive than estimated before from their archival data. The reason is that when the binary nature of the system is considered, the mass transfer towards the companion causes a substantial drop in the luminosity of the star, making it to appear as having a lower mass. In addition, the structure of the envelope also substantially changes. Our modelling shows the importance of setting realistic initial conditions for simulating the dynamical ejection of the common envelope.
The spectroscopic analysis of LRNe outbursts with new higher resolution data shows that the ejecta has different velocity components, usually a low- and a high-velocity components that evolve differently as the outburst progresses. This is the first study of this kind for an extragalactic high-mass system.
Finally, our pilot search for LRNe precursors in the Milky Way has shown that the parameter space occupied by yellow supergiants (LRNe likely progenitors) is shared with other variable systems, making it more challenging to identify them. Although the most common “contaminats” are self-extinguished Be stars, we have also identified young stellar objects, Herbing Be stars, and pulsating stars.