Rise of the Phoenix Giants: A Rich History of Dusty Post-merger Stellar Remnants

The first confirmed stellar merger event was serendipitously discovered in the database of the OGLE bulge photometric monitoring project. V1309 Sco, initially being composed of a post-main sequence (giant star) contact binary system, experienced orbital decay and eventually underwent a merger and intense outburst in 2008 (Stȩpień 2011; Tylenda et al. 2011). Nicholls et al. (2013) and McCollum et al. (2014) investigated the pre- and post-outburst infrared characteristics of this object finding that it produced copious amounts of dust during the merger event and possibly presented mass loss prior to the merger. While the central star has faded post-outburst (and is very faint in the optical), the dust emission level has stayed in its elevated state indicating that the dust is detached from the central post-merger stellar product (e.g., Tylenda & Kamiński 2016). In parallel to the realization of the nature of V1309 Sco, several other similar dust-enshrouded outbursting systems in the Milky Way were attributed to merger events (the red novae: V838 Mon, V4332 Sgr, CK Vul, and OGLE-2002-BLG-360; e.g., Soker & Tylenda 2003; Kamiński et al. 2010, 2015; Tylenda et al. 2013; Loebman et al. 2015).

While known red novae central stars remain enshrouded in dust and challenging to study, the existence of optically visible post-main sequence stars with infrared excess emission indicative of dusty circumstellar material has been known for some time (e.g., Zuckerman et al. 1995; Plets et al. 1997) and are the topic of continued study (e.g., Bharat Kumar et al. 2015; Rebull et al. 2015). Some of the initially discovered dusty first-ascent giant stars were also found to host other unusual characteristics: enhanced lithium content of their atmosphere, sometimes above cosmic values, and rapid rotation (e.g., de La Reza et al. 1996; Fekel et al. 1996). Merger events (including between a star and close-separation giant planet) continue to be favorably regarded as the origin for some of these unusual systems (e.g., Siess & Livio 1999; Denissenkov & Weiss 2000; Drake et al. 2002; Reddy et al. 2002; Sandquist et al. 2002; Jura 2003; Carlberg et al. 2009, 2010, 2012, 2013; Rebull et al. 2015; Aguilera-Gómez et al. 2016; Reddy & Lambert 2016; Punzi et al. 2018; Martell et al. 2020; Soares-Furtado et al. 2020).

Cross-correlation of optical stellar catalogs and infrared all-sky survey catalogs has produced a sample of exceptionally dusty, sometimes actively accreting, first-ascent giant stars (e.g., Zuckerman et al. 2008; Melis 2009; Melis et al. 2009). These giant stars—who are nearing the end of their lives—are experiencing a rebirth into characteristics typically associated with young, planet-forming, stars. As such we have come to call these systems "Phoenix Giants."

Among this sample are objects with characteristics in common with that of the "blue ring nebula" system. BP Psc hosts an extended nebula of gas and dust, dramatic light year-long bipolar jets, and a disk of material that the host giant star actively accretes (Zuckerman et al. 2008). TYC 4144 329 2 also hosts an accretion disk and has host star properties similar to the central star of the blue ring nebula (Melis et al. 2009). Following the legacy of the late Prof. Michael Jura in explaining the existence of unusual dusty giant star systems (Jura 2003), a binary merger model was explored for BP Psc and TYC 4144 329 2 and found to be reasonable.

The blue ring nebula system builds on a rich history of similar such stars documented over the past few decades. The advancement in merger modeling presented by Hoadley et al. (2020) provides increasing support for the existence of optically visible merger remnants in the Milky Way. What is unknown at this time is how common such merger events are and what binary orbital configurations give rise to mergers. Sufficient characterization of merger pathways would help constrain what sort of binary properties are incapable of giving rise to the type of systems that will eventually produce supernovae or detectable gravitational-wave systems. Understanding their mass loss history and dust production capability will help to inform galaxy evolution studies, especially those relevant to forthcoming dust-sensitive instruments like those on the James Webb Space Telescope.