Dusty, Self-Obscured Transients from Stellar Mergers and Common Envelope Phases

MacLeod, Morgan; De, Kishalay; Loeb, Abraham

doi:10.48550/arxiv.2205.07929

“…We based the triple type II obscuring ILOT scenario on the type II ILOT scenario in binaries where an equatorial outflow obscures the central star after the eruption that the binary interaction induces (Kashi & Soker 2017b; Soker 2021; see also MacLeod et al 2022). The dusty equatorial outflow scatters and reradiates energy to all directions, and therefore reduces the emission toward an equatorial observer.…”

Section: The Scenariomentioning

confidence: 99%

A Rapidly Fading Star as a Type II Obscuring Intermediate Luminosity Optical Transient (ILOT) in a Triple-star System

Bear

¹

,

Soker

²

,

Kashi

³

2022

ApJ

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We propose a triple-star scenario where the merger of two pre-main-sequence low-mass stars, ≲0.5 M ⊙, ejects a dusty equatorial outflow that obscures and temporarily causes the disappearance of a massive star, ≳8 M ⊙. The merger of the low-mass inner binary powers a faint outburst, i.e., a faint intermediate luminosity optical transient (ILOT), but its main effect that can last for decades is to (almost) disappear the luminous massive star of the triple system. The typical orbital period of the triple system is about 1 yr. The merger process proceeds as the more-massive star of the two low-mass pre-main-sequence stars starts to transfer mass to the least-massive star in the triple system and as a result of that expands. This type II obscuring ILOT scenario in a triple-star system might account for the fading, rebrightening, and then refading of the massive post-main-sequence star M101-OC1. It might recover in about 20–100 yr. Our study strengthens the claim that there are alternative scenarios to account for the (almost) disappearing of massive stars, removing the need for failed supernovae. In these scenarios the disappearing is temporary, lasting from months to decades, and therefore at a later time the massive star explodes as a core collapse supernova, even if it forms a black hole.

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“…We base these constraints on models of circumbinary mass loss in systems trending toward merger. These models have successfully explained the dynamics of orbital decay and the observations of increasing obscuration in V1309 Sco 28,30,32,162 . As the binary system approaches merger, mass loss near the outer, L 2 Lagrange point progressively drains angular momentum of the binary, causing it to tighten and further enhancing the mass transfer rate.…”

Section: Distance Constraints and Bolometric Light Curvementioning

confidence: 82%

An infrared transient from a star engulfing a planet

De

¹

,

MacLeod

²

,

Karambelkar

³

et al. 2023

Nature

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It is well known that planets with short orbital periods (≲ 10 days) are common around stars like the Sun 1-3 . Stars expand as they evolve, and thus we expect their close planetary companions to be engulfed [4][5][6] . However, this phase has never been directly observed. Here, we present the discovery of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae 7, 8 -a class of eruptions now confirmed 9 to arise from mergers of binary stars. Its exceptionally low optical luminosity (≈ 10 35 erg s −1 ) and radiated energy (≈ 6.5 × 10 41 erg) point to the engulfment of a planet (of 1 − 10 Jupiter masses) by its Sun-like host star. We estimate the Galactic rate of such Sub-luminous Red Novae (SLRNe) to be ∼ 0.1−few yr −1 . Future Galactic plane surveys are well-poised to routinely identify them, revealing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.Using data from the Zwicky Transient Facility (ZTF) time domain survey 10 , we searched 2 for slowly evolving outbursts near the Galactic plane (Methods). We identified a transient optical source named ZTF 20aazusyv (hereafter ZTF SLRN-2020) that exhibited a fast rise from quiescence to peak outburst flux in ≈ 10 days, subsequently fading by ≈ 10× over six months (Figure 1).The long optical outburst duration together with its faint peak flux distinguishes it from common Galactic plane transients resulting from white dwarfs with close binary companions (the 'dwarf' and 'classical' novae 11 ). The transient also exhibits a mid-IR brightening starting ≈ 7 months prior to the optical outburst, together with bright mid-infrared (IR) emission (> 50× brighter than the optical r-band at ≈ 4 months after optical peak) that lasted for ≳ 15 months. No X-ray emission was detected in follow-up observations during the outburst using the Swift telescope 12 , ruling out an unstable disk accretion episode around a neutron star or black hole 13 (Methods).The bright mid-IR emission during the outburst is suggestive of emission from a warm dust shell surrounding the stellar photosphere. We model the optical to mid-IR spectral energy distribution (SED; Methods) at ≈ 120 days after the optical peak. The analysis reveal a relatively hot inner photosphere (≈ 9000 K) surrounded by a warm dust shell with temperature ≈ 1000 K, located behind a dust visual extinction column of A V ≈ 3.6 mag (Figure 2). Using the 90% confidence interval on the foreground extinction together with three-dimensional Galactic dust distribution maps (Methods), we infer the source to be located at a distance of 2 to 7 kpc. A joint analysis of the overlap between the different dust maps suggests a likely distance of ≈ 4 kpc (Methods). Performing the same analysis at ≈ 320 days after peak, we find the SED to have predominantly shifted into the IR bands caused by an increase in the dust optical depth, that can be att...

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“…At the end of the first bump (MJD 58759), fitting a single blackbody to the g and r detections suggests a source with = . This photosphere is most likely formed by mass outflowing from the outer Lagrangian (L2) point (see, e.g., Pejcha et al 2017or MacLeod et al 2022). On the second bump, the transient is only detected in r, with the most constraining g-band upper limit giving g −r > 0.8 mag, suggesting that the optically thick photosphere has cooled.…”

Section: Lrn-goldmentioning

confidence: 99%

Volumetric Rates of Luminous Red Novae and Intermediate-luminosity Red Transients with the Zwicky Transient Facility

Karambelkar

¹

,

Kasliwal

²

,

Blagorodnova

³

et al. 2023

ApJ

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Luminous red novae (LRNe) are transients characterized by low luminosities and expansion velocities, and they are associated with mergers or common-envelope ejections in stellar binaries. Intermediate-luminosity red transients (ILRTs) are an observationally similar class with unknown origins, but they are generally believed to be either electron-capture supernovae in super-asymptotic giant branch stars or outbursts in dusty luminous blue variables (LBVs). In this paper, we present a systematic sample of eight LRNe and eight ILRTs detected as part of the Census of the Local Universe (CLU) experiment on the Zwicky Transient Facility (ZTF). The CLU experiment spectroscopically classifies ZTF transients associated with nearby (<150 Mpc) galaxies, achieving 80% completeness for m r < 20 mag. Using the ZTF-CLU sample, we derive the first systematic LRNe volumetric rate of 7.8 − 3.7 + 6.5 × 10 − 5 Mpc−3 yr−1 in the luminosity range −16 ≤ M r ≤ −11 mag. We find that, in this luminosity range, the LRN rate scales as dN / dL ∝ L − 2.5 ± 0.3 —significantly steeper than the previously derived scaling of L −1.4±0.3 for lower-luminosity LRNe (M V ≥ −10 mag). The steeper power law for LRNe at high luminosities is consistent with the massive merger rates predicted by binary population synthesis models. We find that the rates of the brightest LRNe (M r ≤ −13 mag) are consistent with a significant fraction of them being progenitors of double compact objects that merge within a Hubble time. For ILRTs, we derive a volumetric rate of 2.6 − 1.4 + 1.8 × 10 − 6 Mpc−3 yr−1 for M r ≤ −13.5 mag, which scales as dN / dL ∝ L − 2.5 ± 0.5 . This rate is ∼1%–5% of the local core-collapse supernova rate and is consistent with theoretical ECSN rate estimates.

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Dusty, Self-Obscured Transients from Stellar Mergers and Common Envelope Phases

Cited by 4 publications

References 84 publications

A Rapidly Fading Star as a Type II Obscuring Intermediate Luminosity Optical Transient (ILOT) in a Triple-star System

A Rapidly Fading Star as a Type II Obscuring Intermediate Luminosity Optical Transient (ILOT) in a Triple-star System

An infrared transient from a star engulfing a planet

Volumetric Rates of Luminous Red Novae and Intermediate-luminosity Red Transients with the Zwicky Transient Facility

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