More luminous red novae that require jets

I study two intermediate luminosity optical transients (ILOTs) classified as luminous red novae (LRNe) and argue that their modeling with a common envelope evolution (CEE) without jets encounters challenges. LRNe are ILOTs powered by violent binary interaction. Although popular in the literature is to assume a CEE is the cause of LRNe, I here repeat an old claim that many LRNe are powered by grazing envelope evolution (GEE) events; the GEE might end in a CEE or a detached binary system. I find that the LRN AT 2021biy might have continued to experience mass ejection episodes after its eruption and, therefore, might not suffered a full CEE during the outburst. This adds to an earlier finding that a jet-less model does not account for some of its properties. I find that a suggested jet-less CEE model for the LRN AT 2019zhd does not reproduce its photosphere radius evolution. These results that challenge jet-less models of two LRNe strengthen a previous claim that jets play major roles in powering ILOTs and shaping their ejecta and that in many LRNe, the more compact companion launches the jets during a GEE.

One subclass of ILOTs is luminous red novae (LRNe).There is an observational definition of LRNe, e.g., Cai et al. (2022a), as those ILOTs that have a slow and long luminosity rise before the outburst, followed by two luminosity peaks; the second peak might be a plateau.LRN early-time optical spectra are similar to those of type IIn supernovae with a blue continuum with superposed narrow hydrogen emission lines.A theoretical common definition of LRNe is of ILOTs, fainter than typical supernovae, that are powered by full merger, including common envelope evolution (CEE; e.g., Ivanova 2017), whether the companion survives or not the CEE.I note that there is no consensus on the exact definitions of these terms and the subclasses of ILOTs, e.g., Kashi & Soker (2016) versus Pastorello & Fraser (2019), Pastorello et al. (2019), andCai et al. 2022b).In this study, I claim that there might be no one-to-one correspondence between the observational classification of LRNe and events experiencing a full CEE merger.Instead, many LRNe might experience the grazing envelope evolution (GEE) during their outburst.The GEE might end with a CEE or with a detached binary system.
One of the motivations for considering the jet-powering of ILOTs is the bipolar morphologies of spatially resolved ILOTs, some with point symmetry.Such is the bipolar ejecta of the nineteenth-century Great eruption of Eta Carinae, called the Homunculus (e.g., Davidson, & Humphreys 1997).The Great Eruption was a luminous blue variable major eruption, a subclass of ILOTs (see Section 3).Observations show bipolar morphologies in the ILOTs V4332 Sgr (Kamiński et al. 2018), Nova 1670 (Shara et al. 1985;Kamiński et al. 2020Kamiński et al. , 2021a)), and V838 Mon (Chesneau et al. 2014;Kamiński et al. 2021;Mobeen et al. 2021Mobeen et al. , 2023)).The point-symmetric morphological features of some ILOTs, e.g., the Homunculus of Eta Carinae (Steffen et al. 2014) and Nova 1670 (CK Vulpeculae; e.g., Kamiński 2024), are key elements in claiming jet powering of ILOTs: while equatorial mass ejection during binary interaction can, in principle, explain bipolar structure, this process by itself cannot explain point symmetry.Jets are most likely to shape point-symmetric structures.
Processes that release gravitational energy in binary interaction and can power ILOTs include mass transfer via an accretion disk where the mass-accreting (more compact) companion launches jets (e.g., Kashi et al. 2010;Soker 2016aSoker , 2020a;;Soker, & Kashi 2016;Kashi 2018), and/or the system ejecta mass in and near the equatorial plane (e.g., Pejcha et al. 2017;Hubová & Pejcha 2019) or the merger of two stars.The latter includes the onset of a CEE, whether the companion survives or not.Extreme cases are the spiraling-in of a neutron star or a black hole inside the envelope of a red supergiant.These events are expected to be as bright as luminous core-collapse supernovae (CCSNe) and are termed common envelope jets supernovae (CEJSNe; e.g., Soker, & Gilkis 2018;Gilkis et al. 2019;Grichener & Soker 2019;Yalinewich, & Matzner 2019;Schreier et al. 2021).CCSNe, which are most likely also powered by jets (e.g., Soker 2024 for a recent review), and CEJSNe, are not grouped under ILOTs despite being gravitationally-power through jets because most of the gravitational energy in these systems is carried by neutrinos rather than jets or the ejecta in general.
Other energy sources to power the radiation of an ILOT in a CEE are the hot ejected gas (e.g., Chen & Ivanova 2024), which the spiraling-in process heats, and recombination energy (e.g., Ivanova et al. 2013;Howitt et al. 2020;Matsumoto & Metzger 2022).In Soker (2023b), I compared these two energy sources with jet powering and concluded that only jets could power rapidly rising lightcurves of bright ILOTs.In general, the collision of wide jets with a slower expanding ejecta efficiently channels the kinetic energy of the jets to radiation (Soker 2020a); more efficient than equatorial ejecta collision with a shell (e.g., Pejcha et al. 2016a,b) and a shell collision with a slow equatorial outflow (e.g., Metzger, & Pejcha 2017;Andrews, & Smith 2018;Kurfürst & Krtička 2019) proposed by these studies as an energy source of radiation.In Section 2, I examine the jet-less model that Chen & Ivanova (2024) proposed that is based on the ejected hot gas and recombination energy.I find that it does not reproduce all the observed properties of LRN AT 2019zhd.
Earlier studies of jet-powering focused on bumps of short-period increased luminosity, e.g., Soker & Kaplan (2021) studied the ILOT SNhunt120 (observations by Stritzinger et al. 2020a) and the LRN AT 2014ej (observations by Stritzinger et al. 2020a).In section 3 I consider bumps in the lightcurve of AT 2021biy of decreased luminosity while the radius increases.I argue that these bumps suggest a continues binary interaction that might imply that no CEE took place in this outburst.I also find that the ejecta is likely to be bipolar.I summarize this short study in Section 4.

CHALLENGING A JET-LESS MODEL OF AT 2019ZHD
This section addresses the dispute between models of LRNe that attribute significant role to jets (see Section 1), and those that are not, i.e., jet-less models.
Chen & Ivanova (2024) construct a jet-less CEE LRN model, where the emission results from recombination and cooling of hot gas ejected by the CEE.They fit their parameters to (almost) exactly fit the lightcurve of the LRN AT 2019zhd as reported by Pastorello et al. (2021).Pastorello et al. (2021) calculated the blackbody temperature and radius of the photosphere of AT 2019zhd; I present two panels from their paper in Figure 1.Chen & Ivanova (2024) do not present the blackbody temperature nor the radius of their model.They do present the optical depth as a function of radius and time.I take their value of τ (r, t) = 1 to approximately give the radius of the photosphere and added the line R(τ = 1, t) on the right panel of Figure 1 by a light-blue line marked CI2024.
From the two lines for AT 2019zhd on the right panel of Figure 1, I conclude that the model of Chen & Ivanova (2024) does not reproduce the photospheric radius that Pastorello et al. ( 2021) calculated from their observations of AT 2019zhd, not even qualitatively.Chen & Ivanova (2024) writes also that the time evolution of the gas at T g = 5000 K resembles the shape of the lightcurve.However, as the left panel of Figure 1 shows, the blackbody temperature that Pastorello et al. (2021) calculate for AT 2019zhd varies from about 7000 K near maximum light to 2400 K at t = 34d.
The jet-less model of Chen & Ivanova (2024) and the calculations of Pastorello et al. (2021) do not agree with each other.Either one of them is wrong, or both miss the real behavior because they assume spherically symmetric ejecta and do not acknowledge the primary role that jets might play.I tend to accept the calculations of Pastorello et al. (2021) as being approximately correct, but expect non-spherical effects to play significant roles (see Section 1 for observational indications of bipolar ILOTs).In any case, the jet-less model that Chen & Ivanova (2024) constructed seems not to explain the behavior of the LRN AT 2019zhd.The jets are probably needed.

THE DIPS IN THE LIGHT-CURVE OF AT 2021BIY
Many studies attribute the bipolar morphology of many planetary nebulae to jets, whether there are direct signatures of jets, e.g., MyCn 18 (e.g., Bryce et al. 1997;O'Connor et al. 2000), or indirect, e.g., NGC 6302 (e.g., Balick et al. 2023) and NGC 7027 (e.g., Moraga Baez et al. 2023).The same holds for many bipolar symbiotic nebulae, e.g., R Aquarii, which was shaped by precessing jets (e.g., Santamaría et al. 2024), and preplanetary nebulae (e.g., Sahai 2020), e.g., M 1-92 (e.g., Alcolea et al. 2022).Based on bipolar planetary nebulae, some past studies attributed the bipolar structure of the ejecta of the nineteenth-century Great Eruption of Eta Carinae (about 1837-1856; Humphreys, Davidson, & Smith 1999), i.e., the Homunculus, to powering by jets (e.g., Soker 2001;Kashi & Soker 2010).The Great Eruption of Eta Carinae was a luminous blue variable major eruption (outburst) that belongs to a subclass of ILOTs.I argued before (Soker 2023b) that this claim of shaping by jets applies to the Great Eruption binary scenario models that involve no CEE (e.g., Soker 2001Soker , 2007) ) and to the Great Eruption triple star scenarios where two of the three stars entered a CEE (e.g., Livio & Pringle 1998;Portegies Zwart & van den Heuvel 2016;Hirai et al. 2021).The difference is that in the non-CEE scenario, the binary system interaction might repeat itself with several peaks in the same event or repeat in consecutive events.The non-repeating nature is one of the difficulties with the triple-star scenarios of the Great Eruption (Portegies Zwart & van den Heuvel 2016;Hirai et al. 2021).Specifically, the problems with the triple-star merger scenarios are as follows.(i) There was a second outburst in 1890-1895 (the Lesser Eruption; Humphreys, Davidson, & Smith 1999) that, according to the triple-star scenario, requires another merger.Namely, a system of four stars.(ii) The Homunculus and the present binary system share the same equatorial plane (e.g., Madura et al. 2012).This is difficult to explain in a non-coplanar triple-stellar system; merger scenarios require an unstable triple system likely not to be coplanar.(iii) The scenario proposed by Hirai et al. ( 2021) predicts the presence of a dense gas in the equatorial plane; this is not observed in the Homunculus.For these, I consider it unnecessary and problematic to apply a triple-star or a quadruple-star model to the two nineteenth-century eruptions of Eta Carinae.In the binary model for the Great Eruption of Eta Carinae, the secondary star grazed the primary stellar envelope during periastron passages.The secondary star accreted mass via an accretion disk that launched the jets that powered the event and shaped the Homunculus (e.g., Kashi & Soker 2010).
With the dispute between a one-time CEE event (e.g., Ivanova 2017)  Figure 2 from Cai et al. (2022a) presents the luminosity, blackbody temperature, and photosphere radius of four ILOTs.I refer to AT 2021biy.I examine three radius rises as marked on the lower panel.In the insets, I give the start and end times of the three time periods and the radii at these times.I take the times and radii from table A.2 of Cai et al. (2022a).From the start and end points, I calculate the average velocity of the increasing radius.The radii of these points are highly uncertain, and therefore, so is the velocity I calculate.Assuming that this velocity is the velocity of a mass, I calculated the time the central stellar system ejected the mass.I mark these times with the green-double-lined arrow on the time axis.
A rapid rise in radius can result from a mass that the stellar system ejected late and catches up with the photosphere.What is different in the present case for the two late radius rises is that decreases in luminosity rather than increases accompany these increases in radius.A very moderate rise in luminosity accompanies the first radius rise.In the case of shell collision, the expectation is an increase in luminosity because kinetic energy is channeled to thermal energy that is partially radiated away.The two late radius rises end with a temperature where most of the gas is recombined.Therefore, it seems that the shell that catches up with the photosphere is mainly neutral, and the collision with the rarefied photospheric gas does not heat it much.
The flow structure is likely to be non-spherical.For example, the main photosphere might be from an equatorial mass ejection before the rapid radius rise.The late ejecta might be then a wide bipolar outflow, say of a half opening angle of α j ∼ 45 • −60 • .If we observe this LRN along the polar direction, the cold polar ejecta obscures the equatorial photosphere for several weeks; this explains a cool expanding photosphere.In that case, there might be more mass-ejection episodes of lower mass, those that do not obscure the equatorial gas.The possible parameter space of these late bipolar mass-ejection episodes is large.It deserves a study by itself.
The mass in the latest ejecta that caused the third rapid radius rise can be estimated as follows.The blackbody temperature of the gas in the last rise at maximum radius r 3e ≃ 10 15 cm, at t ≃ 410 − 430 days, is T BB,3e ≃ 2000 K.At that low temperature, molecular opacity dominates, and the opacity is κ ≃ 0.01 cm 2 g −1 (e.g., Ferguson et al. 2005Ferguson et al. , 2007)), for a density of ≃ 10 −11 g cm −1 .The required mass to have an optical depth of 1 at that radius is (1) The total mass is smaller if the mass is in a wide polar outflow rather than a complete shell over a solid angle of Ω = 4π4.For a half opening angle of 60 • (45 • ; measured from the polar axis), the ejected mass is If the same approach is applied to the second rise, the required ejected mass is too high.The temperature at the largest radius in the second rise, r 2e ≃ 6.4 × 10 14 cm, is T BB,2e ≃ 3000 K where the opacity is only κ ≃ 3 × 10 −4 cm 2 g −1 (e.g., Ferguson et al. 2005Ferguson et al. , 2007)).This implies, for a spherical shell by equation (1), an ejected shell mass of M 2 ≃ 8.6M ⊙ .This is too large for the inferred progenitor system.A more likely explanation is that the ejecta is wide polar ejecta (to two pools).This reduces the required ejected mass from two effects.The first effect is that Ω < 4π.The second is that the polar ejecta does not cover the entire photosphere of the equatorial ejecta.In that case, it is possible that the infrared temperature of T BB,2e ≃ 3000 K is a combination of two photospheres: One of the equatorial ejecta at ≈ 4000 K, and one is of the polar ejecta at ≈ 2000 K.At both these temperatures, the opacity is higher than at ≈ 3000 K, which implies a lower required mass.This implies that the ejected polar mass that obscures some of the equatorial photosphere is M 2 ≈ 0.1M ⊙ .
In the case of AT 2021biy, my suggestion is that the fast polar outflow recombines before it covers a large area, and, therefore, when it expands enough to start obscuring the equatorial photosphere, it is already cold at ≈ 2000 K.The polar and equatorial outflows collide on their overlapping solid angles, and the polar outflow further heats the equatorial outflow and prolongs the plateau.As said, I expect that there have been earlier jet-launching episodes that did not obscure the equatorial photosphere but heated it up.
Further detailed research is needed on the structure of two photospheres at different temperatures in a bipolar outflow: a denser and slower thick equatorial outflow and a faster eruptive bipolar outflow.My conclusion from this short discussion is that the lightcurve of LRN AT2021biy, that, as noticed by Cai et al. (2022a), the jet-less model of Matsumoto & Metzger (2022) cannot explain, was powered by late jet-launching episodes.This implies, in turn, that the interaction of the main outburst was more likely to have been a GEE than a CEE.Whether the binary system eventually entered a CEE or survived as a detached binary system is for future observations to determine.In the latter case, this system can experience another ILOT event, much as the Lesser Eruption of Eta Carinae at the end of the nineteenth century.

SUMMARY
This study is motivated by two recent ILOTs classified as LRNe, AT 2019zhd and AT 2019zhd, which jet-less models based on recombination energy have difficulty explaining.In Section 2, I found that the jet-less model of Chen & Ivanova (2024) does not reproduce the radius evolution of LRN AT 2019zhd as Pastorello et al. (2021) calculated from their observations.As noticed by Cai et al. (2022a), the jet-less model of Matsumoto & Metzger (2022) cannot reproduce both the luminosity and duration of LRN AT 2021biy.In section 3, I further argued that the late lightcurve of AT 2021biy requires late mass-ejection episodes.As earlier studies suggested (see section 1), a late activity that powers more energy to an ILOT and which also accounts for bumps in the lightcurve might be the launching of jets by the more compact companion in a binary system.I suggest this late binary activity to the LRN AT 2019zhd.This implies that the binary system did not enter a CEE at the main outburst.
For both these LRNe, I take the view that the binary interaction during the main outburst involved GEE rather than CEE.In Soker (2016a), the last sentence of the Abstract reads: "Some future ILOTs of giant stars might be better explained by the GEE than by merger and CEE without jets."the progenitor of AT 2021biy was a yellow giant (R * ≃ 300R ⊙ ).I attribute the LRN AT 2021biy to a GEE event.It might have ended in a final CEE, or the two stars might have stayed detached.I repeat my earlier claim that many ILOTs are powered by a GEE event; it might end with a CEE or a detached binary system.There are no observational indications for the progenitor of AT 2019zhd, but I expect jets to have powered this ILOT as well.
This study adds to the accumulating shreds of evidence, observationally and theoretically, that jets play major roles in powering most ILOTs, including all subclasses of LRNe and luminous blue variable major eruptions, whether the system enters a CEE during the outburst or experiencing a GEE.Note that in some LRNe, the binary interaction might be with a sub-stellar companion.
Fig. 1.-The evolution of the black-body temperature (left panel) and photospheric radius (right) panel that Pastorello et al. (2021) calculated from their observations under the assumption of spherically symmetric ejecta.I added the light-blue line on the right panel, which is the radius of optical depth τ R = 1 from the jet-less model that Chen & Ivanova (2024) built for LRN AT 2019zhd.Their model fails to reproduce the radius.
and repeating jet-launching episodes, likely in a GEE (e.g., Soker 2020b), I examine the properties of the ILOT AT2021biy that Cai et al. (2022a) studied in detail.AT2021biy presents the longest plateau among LRNe, with a duration of 210 days.Cai et al. (2022a) find the progenitor to be a yellow supergiant of mass ≈ 20M ⊙ , a luminosity and radius at explosion of L * ≃ 10 5 L ⊙ and R * ≃ 300R ⊙ .Cai et al. (2022a) tried to fit the light-curve with the jet-less recombination model of Matsumoto & Metzger (2022): From the plateau duration of t pl = 210 days and the expansion velocity of vE = 430 km s −1 that they deduced from the Hα line they estimated the ejected mass to be M ej ≃ 9.9M ⊙ .With these parameters, they found the jet-less model of Matsumoto & Metzger (2022) to give a plateau luminosity of ≈ 3 × 10 39 erg s −1 , which is an order of magnitude below the observed plateau luminosity L pl ≃ 5 × 10 40 erg s −1 .Therefore, Cai et al. (2022a) claimed an extra energy source powered the plateau.I attribute the extra energy to jets, or more accurately, most of the energy to jets.
Fig. 2.-A figure adapted from Cai et al. (2022a) of the bolometric lightcurves, blackbody temperature evolution, and blackbody (photosphere) radius evolution of four ILOTs.I added the estimated starting and ending times and radii (three insets) of three rapid radius increase periods of AT2021biy, as indicated by the three arrows in the lower panel.The insets also indicate the expansion velocity of each rise, namely v ej = (re − rs)/te − ts) for each of the three rapid rises.The green arrows indicate the estimated ejection time of the material calculated by t ej = ts − rs/v ej .I expect that there were other jet-launching episodes that did not obscure the equatorial photosphere.