V1309 Scorpii: merger of a contact binary

Abstract: Stellar mergers are expected to take place in numerous circumstences in the evolution of stellar systems. In particular, they are considered as a plausible origin of stellar eruptions of the V838 Mon type. V1309 Sco is the most recent eruption of this type in our Galaxy. The object was discovered in September 2008. Our aim is to investigate the nature of V1309 Sco. V1309 Sco has been photometrically observed in course of the OGLE project since August 2001. We analyse these observations in different ways. In particular, periodogram analyses were done to investigate the nature of the observed short term variability of the progenitor. We find out that the progenitor of V1309 Sco was a contact binary with an orbital period of ~1.4 day. This period was decreasing with time. Similarly the light curve of the binary was also evolving, indicating that the system evolved toward its merger. The violent phase of the merger, marked by the systematic brightenning of the object, started in March 2008, i.e. half a year before the outburst discovery. We also investigate the observations of V1309 Sco during the outburst and the decline and show that they can be fully accounted for within the merger hypothesis. For the first time in the literature we show, from direct observations, that contact binaries indeed end up by merging into a single object, as it was suggested in numerous theoretical studies of these systems. Our study also shows that stellar mergers indeed result in eruptions of the V838 Mon type.

• The paper provides direct observational evidence of a contact binary merger driven by a decaying orbital period.
• Analysis of OGLE light curves revealed a six-month exponential brightening, marking the transition to merger.
• Results validate models predicting V838 Mon-type eruptions and illustrate the energetic and spectral changes post-merger.

Overview of "V1309 Scorpii: Merger of a Contact Binary"

The study focuses on V1309 Scorpii (V1309 Sco), which is an exemplary case of a stellar merger. The research presented in this paper utilized extensive photometric data from the OGLE project to provide the first direct observational evidence that contact binaries can evolve into single objects through a merger. In particular, V1309 Sco confirms theoretical predictions about the end stages of binary evolution and establishes the occurrence of V838 Mon-type eruptions as stellar mergers.

Key Findings

• Progenitor Characterization: The progenitor of V1309 Sco was identified as a contact binary with an orbital period of approximately 1.4 days, which was decreasing over time. This decrease in the orbital period was concluded to result from a merger event.
• Evolution towards Merger: Detailed analysis of the light curves indicated that the system underwent significant changes leading to its merger. Variations in the light curve prior to the eruption were consistent with those expected from a contact binary nearing a merger phase.
• Outburst Description: The analysis encompassed the photometric data of V1309 Sco's outburst and subsequent decline. The rise to maximum brightness followed an exponential formula and was characterized by a smooth brightening over approximately six months. This behavior corroborates hypotheses regarding stellar eruptions resulting from mergers.
• Post-Merger Phase: During the decline, the remaining object evolved towards reduced luminosity, still in cooler M-type spectral classifications. The study provides a comprehensive assessment of the physical parameters post-event, showing notable differences when compared to pre-merger parameters.

Implications

• Stellar Merger Conformation: The event of V1309 Sco serves as direct observational support for theoretical models that predict the merging of contact binaries due to orbital instabilities like Darwin instability or evolution through deep contact.
• Understanding V838 Mon-type Eruptions: The study suggests that eruptions similar to the one observed in V838 Mon can indeed result from binary mergers, providing a framework for future observations and potential identification of other such events in the universe.
• Merger Energetics: An analysis of the energy involved in the merger highlights that a significant proportion of primary orbital energy is radiated away, supporting the hypothesis that these catastrophic transformations are driven by orbital dynamics.

Future Directions

The study of V1309 Sco opens new avenues for observing other potential stellar mergers and understanding their long-term evolutionary consequences. Future research may focus on:

• More extensive surveys of binary systems to identify candidates for merger events.
• Detailed simulations of contact binary evolution to predict observable characteristics pre and post merger.
• Investigation into the remnant structures formed following the merger, particularly regarding their rotation rates and potential formation of circumstellar discs.

In conclusion, the paper provides compelling evidence and insights into the phenomenon of contact binary mergers, advancing our understanding of stellar evolution and the onset of rare eruption events akin to V838 Mon types. The data and interpretations set a solid foundation for further astrophysical exploration of binary interactions and stellar lifecycle endpoints.