LP 40-365-Type Stars

• LP 40-365-type stars are partly burnt, runaway stellar remnants produced in subluminous thermonuclear supernovae with unique high velocities and inflated radii.
• They are characterized by atmospheres dominated by nuclear ash, typically enriched in oxygen, neon, and magnesium, indicating incomplete burning.
• Observations from Gaia and spectroscopy reveal extreme kinematics and distinct structural properties, supporting binary-ejection and partial deflagration models.

Searching arXiv for recent and foundational papers on LP 40-365-type stars and related survivor channels. LP 40-365-type stars are a class of high-velocity, chemically peculiar, low-mass stellar remnants associated with thermonuclear supernovae. The prototype, LP 40-365 (GD 492), was identified as an unusual compact object with an atmosphere lacking detectable hydrogen and helium and with kinematics exceeding the local Galactic escape speed, leading to its interpretation as a surviving remnant of a subluminous Type Ia supernova (Vennes et al., 2017). Subsequent work generalized this into a class of partly burnt runaway remnants, usually linked to partial deflagrations in near-Chandrasekhar-mass white dwarfs that leave bound survivors and eject them from compact binaries (Raddi et al., 2019, Hermes et al., 2021). The class is observationally defined by extreme kinematics, low surface gravity, inflated radii relative to canonical white dwarfs, and photospheres dominated by nuclear ash; however, the detailed atmospheric composition of the prototype has been modeled both as helium-dominated with a large neon fraction and as O/Ne-dominated, so the taxonomy includes an important spectroscopic debate (Raddi et al., 2018, Raddi et al., 2019).

1. Definition and taxonomic position

LP 40-365-type stars are described as partly burnt, runaway stellar remnants that survived peculiar thermonuclear supernovae (Raddi et al., 2019). In the interpretation emphasized by the discovery paper for LP 40-365, they are partially burnt, bound white dwarf remnants produced in subluminous Type Ia supernovae, especially events similar to the Type Iax subclass, in which failed detonation or partial deflagration does not fully disrupt the accreting white dwarf (Vennes et al., 2017). Later work sharpened this picture by identifying LP 40-365-type stars as the former accretors rather than the donor stars: they are interpreted as partially deflagrated white dwarf remnants ejected in Type Iax supernovae, with neon- and oxygen-dominated atmospheres and low remnant masses (Bhat et al., 27 Feb 2026).

This definition is primarily relational. LP 40-365-type stars are distinct from donor-ejected hypervelocity stars such as US 708, which are non-degenerate former companions launched when the primary explodes and whose atmospheres remain hot-subdwarf-like rather than ash-dominated (Raddi et al., 2018). They are also distinct from the D$^6$ hypervelocity survivors discussed in double-degenerate double-detonation scenarios, where the runaway is the surviving donor white dwarf rather than the partially disrupted accretor (Shen, 6 Feb 2025). Normal white dwarfs are likewise excluded because their atmospheres are typically H- or He-dominated with trace external pollution, not pervasive enrichment by partial O- and Si-burning products (Raddi et al., 2018).

A recurrent source of ambiguity is atmospheric classification. One detailed spectroscopic analysis of LP 40-365 found that a helium-dominated atmosphere, with $\sim 33$ percent neon and $2$ percent oxygen by mass, could reproduce most observed properties and emphasized strong hydrogen deficiency and a super-Solar Mn excess (Raddi et al., 2018). In contrast, later work on the broader class characterized the confirmed LP 40-365 stars as having O/Ne-dominated atmospheres with Mg as the third most abundant element and with H and He undetected at stringent limits (Raddi et al., 2019). This suggests that “LP 40-365-type” is most secure as a dynamical and evolutionary category, while the exact photospheric stratification of individual members remains model-dependent.

2. Discovery history and current census

LP 40-365 itself was first established as a high proper motion, low-mass white dwarf traveling faster than the Galactic escape velocity and showing an atmosphere dominated by intermediate-mass elements, which was interpreted as direct evidence that bound remnants can survive underluminous Type Ia/Iax events (Vennes et al., 2017). Gaia DR2 then fixed its parallax at $\pi = 1.58 \pm 0.03$ mas and showed that it is a subluminous compact star with $R = 0.18 \pm 0.01\,R_\odot$, a rest-frame velocity of $852 \pm 10\ \mathrm{km\ s^{-1}}$, and a Galactic disc crossing $5.0 \pm 0.3$ Myr ago (Raddi et al., 2018).

The class expanded significantly in 2019. Raddi et al. reported three stars that, together with the prototype, formed a distinct class of chemically peculiar runaway stars surviving thermonuclear explosions: LP 40-365, J1603$-$6613, and J1825$-$3757, with J0905+2510 identified as a likely additional member (Raddi et al., 2019). In that sample, the confirmed objects had masses and radii ranging between $0.20$–$\sim 33$0 and $\sim 33$1–$\sim 33$2, respectively, and showed closely similar atmospheric abundance patterns (Raddi et al., 2019).

By 2026, the census had grown further. A new object, Gaia DR3 5446737753669901568, J102230.26$\sim 33$3341420.5 (J1022$\sim 33$43414), was identified as an LP 40-365-type runaway star, and the discovery paper stated that seven such objects were known before that work, raising the sample to eight (Bhat et al., 27 Feb 2026). J1022$\sim 33$53414 is notable because it is substantially hotter than previously studied members, with $\sim 33$6 and $\sim 33$7, while retaining the same qualitative atmospheric character: neon- and oxygen-dominated, strongly H/He deficient, and dynamically extreme (Bhat et al., 27 Feb 2026).

The census itself matters because the class is rare and short-lived. Raddi et al. estimated that approximately 20 LP 40-365-type stars brighter than 19 mag should be detectable within 2 kpc by the end of the Gaia mission, while a more detailed detectability discussion in the same work yielded $\sim 33$8 potentially detectable objects within 2 kpc under restrictive visibility assumptions (Raddi et al., 2019). The 2025 evolutionary study did not recompute the event rate for LP 40-365 stars, but referenced an estimate of $\sim 33$9 LP 40-365-like survivors with $2$0 currently in the Milky Way when Galactic-potential integration is included (Shen, 6 Feb 2025).

3. Atmospheric composition and spectroscopic phenomenology

The defining observational signature of the class is a photosphere dominated by nuclear ash rather than by primordial H or He. In the 2017 discovery analysis, LP 40-365 was described as having an atmosphere dominated by oxygen and neon, followed by sodium and magnesium, with aluminum and silicon also present and with hydrogen and helium not detected (Vennes et al., 2017). The 2019 class study strengthened this pattern by reporting O/Ne-dominated atmospheres with Mg as the third-most abundant element in all confirmed members, together comprising almost the entire atmospheric mass, with homogeneous trace metals from partial O- and Si-burning (Raddi et al., 2019). Their average atmospheric mass fractions were given as Ne $2$1–$2$2, O $2$3–$2$4, and Mg $2$5–$2$6 by mass, with all other elements contributing $2$7–$2$8 in total (Raddi et al., 2019).

The prototype, however, remains spectroscopically contentious. The 2018 reanalysis concluded that a convective helium atmosphere with mixing-length ML2/$2$9 and with Ne $\pi = 1.58 \pm 0.03$0 by number, corresponding to $\pi = 1.58 \pm 0.03$1 by mass, reproduced the overall spectrum and SED more convincingly than prior O/Ne-dominated models (Raddi et al., 2018). In that model, hydrogen was strongly deficient with $\pi = 1.58 \pm 0.03$2, carbon was undetected with $\pi = 1.58 \pm 0.03$3, and the atmosphere displayed 13 species including He, Ne, O, Mg, Na, Al, Si, Ca, Fe, Ni, S, Cr, Ti, and Mn (Raddi et al., 2018). A major result was the measured [Mn/Fe] $\pi = 1.58 \pm 0.03$4 dex, or approximately seven times Solar, which was argued to diagnose high-density thermonuclear burning in a near-Chandrasekhar-mass progenitor (Raddi et al., 2018).

Spectral diagnostics are correspondingly unusual. Across the class, key features include strong Mg I and Mg II lines, the Mg I jump near $\pi = 1.58 \pm 0.03$5 Å, Ne I lines such as $\pi = 1.58 \pm 0.03$6 Å in cooler members, O I optical multiplets, Na I, Al II/III, Si II/III, Ca II H+K, and multiple iron-peak transitions including Mn II, Cr II, Fe I/II, and Ni I/II (Raddi et al., 2018, Raddi et al., 2019). The hotter J1022$\pi = 1.58 \pm 0.03$73414 extends this phenomenology into a different temperature regime: its FORS2 spectra clearly show O I, Mg II, and blue-visible Ne I lines, with weaker C II, O II, Mg I, Al II/III, Si II, and Ca II, while the continuum opacity at $\pi = 1.58 \pm 0.03$8 Å is dominated by O I, Ne I, and Mg II bound-free absorption (Bhat et al., 27 Feb 2026).

These abundance patterns are central to physical interpretation. The 2019 sample paper argued that enhanced O, Ne, and Mg plus moderately super-solar iron-peak ratios relative to Fe indicate incomplete O- and Si-burning in a thermonuclear explosion that did not fully disrupt the accretor (Raddi et al., 2019). The 2026 discovery paper made the tension with extant deflagration models explicit: CO white dwarf deflagration survivors do not reproduce the high observed $\pi = 1.58 \pm 0.03$9Ne abundance, ONe white dwarf deflagration models generally predict O $R = 0.18 \pm 0.01\,R_\odot$0 Ne by mass fraction, and current 3D deflagration simulations also struggle to match the observed kick velocities (Bhat et al., 27 Feb 2026). A plausible implication is that the observed atmospheres encode both explosion products and substantial post-explosion processing by convection, diffusion, levitation, and stratification.

4. Structure, inflation, rotation, and thermal evolution

LP 40-365-type stars are structurally unlike canonical white dwarfs. They occupy a regime of low mass, low surface gravity, and large radius, indicating partial degeneracy and post-explosion inflation. For the three confirmed members in the 2019 sample, the reported parameters were: LP 40-365 with $R = 0.18 \pm 0.01\,R_\odot$1, $R = 0.18 \pm 0.01\,R_\odot$2, and $R = 0.18 \pm 0.01\,R_\odot$3; J1603$R = 0.18 \pm 0.01\,R_\odot$46613 with $R = 0.18 \pm 0.01\,R_\odot$5, $R = 0.18 \pm 0.01\,R_\odot$6, and $R = 0.18 \pm 0.01\,R_\odot$7; and J1825$R = 0.18 \pm 0.01\,R_\odot$83757 with $R = 0.18 \pm 0.01\,R_\odot$9, $852 \pm 10\ \mathrm{km\ s^{-1}}$0, and $852 \pm 10\ \mathrm{km\ s^{-1}}$1 (Raddi et al., 2019). Their compactness is far below canonical white dwarf values, and the paper noted that these remnants are inflated by roughly an order of magnitude in radius relative to typical white dwarfs (Raddi et al., 2019).

The basic structural relations used throughout the literature are

$852 \pm 10\ \mathrm{km\ s^{-1}}$2

and

$852 \pm 10\ \mathrm{km\ s^{-1}}$3

For LP 40-365, the Gaia-era reanalysis used the parallax and SED scaling to derive $852 \pm 10\ \mathrm{km\ s^{-1}}$4, $852 \pm 10\ \mathrm{km\ s^{-1}}$5, and $852 \pm 10\ \mathrm{km\ s^{-1}}$6 from $852 \pm 10\ \mathrm{km\ s^{-1}}$7 and $852 \pm 10\ \mathrm{km\ s^{-1}}$8 (Raddi et al., 2018). The discovery paper had instead obtained $852 \pm 10\ \mathrm{km\ s^{-1}}$9, $5.0 \pm 0.3$0, $5.0 \pm 0.3$1, and $5.0 \pm 0.3$2 from pre-Gaia modeling, illustrating the early dependence on model-derived distances (Vennes et al., 2017).

Rotation provides an additional diagnostic of origin. Time-series photometry revealed coherent 8.914-hr variability in LP 40-365, with semi-amplitudes of $5.0 \pm 0.3$3 in the ultraviolet, $5.0 \pm 0.3$4 in the optical, and a lower-significance infrared modulation consistent with the same period and phase (Hermes et al., 2021). The authors interpreted this as rotational modulation by surface inhomogeneities, with an ephemeris

$5.0 \pm 0.3$5

and inferred an equatorial speed of $5.0 \pm 0.3$6 for $5.0 \pm 0.3$7, consistent with the spectroscopic limit $5.0 \pm 0.3$8 (Hermes et al., 2021). Under approximate angular-momentum conservation, this period favors a bound-remnant interpretation over a donor-star interpretation, because tidally locked donor remnants in comparable channels are expected to rotate much faster (Hermes et al., 2021). The absence of detectable Zeeman splitting further implies $5.0 \pm 0.3$9 kG (Hermes et al., 2021).

Thermal evolution is generally described as Kelvin-Helmholtz contraction of a hot, inflated remnant. The 2019 sample paper estimated $-$0 Myr for LP 40-365, $-$1 Myr for J1603$-$26613, and $-$3 Myr for J1825$-$43757 using $-$5 (Raddi et al., 2019). A 2025 MESA study evolved hot, fully convective O/Ne stars with masses $-$6, $-$7, $-$8, $-$9, and $-$0 and found that they contract, cool, and migrate through $-$1–$-$2 at luminosities comparable to observed LP 40-365 objects before developing radiative cores and ultimately entering white-dwarf-like cooling tracks (Shen, 6 Feb 2025). The HR-diagram match was strong, but most objects with inferred midplane travel times of about $-$3–$-$4 Myr lay on model tracks at ages $-$5 Myr, with J1825$-$63757 being the main exception (Shen, 6 Feb 2025).

5. Kinematics and dynamical ejection

Extreme motion through the Galaxy is one of the defining observables of the class. The prototype LP 40-365 has a barycentric radial velocity of about $-$7 and a Galactic rest-frame speed of $-$8, making it unbound and implying an ejection from the Galactic disc at least $-$9 if rotational assist is included (Raddi et al., 2018). In the 2019 sample, LP 40-365 had $0.20$0, J1603$0.20$16613 had $0.20$2, and J1825$0.20$33757 had $0.20$4; the first two were unbound, while J1825$0.20$53757 remained bound on an eccentric, retrograde halo-like orbit (Raddi et al., 2019).

The standard conversion between astrometry and tangential motion is

$0.20$6

with $0.20$7 in arcsec yr$0.20$8 and $0.20$9 in pc (Vennes et al., 2017). Orbit calculations in the class literature use Galactic potentials such as Allen & Santillán or MWPotential2014 and incorporate full Gaia covariance when available (Vennes et al., 2017, Raddi et al., 2019, Raddi et al., 2018). For LP 40-365, backward integration showed that its trajectory does not intersect the Galactic Center and crossed the Galactic plane about $\sim 33$00 Myr ago, supporting a binary-supernova origin rather than a supermassive-black-hole slingshot (Vennes et al., 2017, Raddi et al., 2018).

J1022$\sim 33$013414 extends the kinematic parameter space in a prior-dependent but clearly extreme way. Its Gaia DR3 astrometry gives $\sim 33$02 mas and proper motion components $\sim 33$03 mas yr$\sim 33$04 and $\sim 33$05 mas yr$\sim 33$06 (Bhat et al., 27 Feb 2026). With a flat $\sim 33$07 prior, the paper derived a median distance of $\sim 33$08 kpc, a Galactocentric rest-frame speed $\sim 33$09, and an ejection velocity $\sim 33$10; only $\sim 33$11 of Monte Carlo trajectories remained bound (Bhat et al., 27 Feb 2026). The inferred flight time since disk crossing is $\sim 33$12 Myr (Bhat et al., 27 Feb 2026).

These velocities constrain progenitor compactness. In the binary-ejection interpretation used for the 2019 sample, minimum ejection speeds of $\sim 33$13–$\sim 33$14 arise naturally from compact binaries with near-$\sim 33$15 accretors and He-burning donors, using the scaling

$\sim 33$16

for separations of a few $\sim 33$17 cm and orbital periods of order 1 hr (Raddi et al., 2019). For LP 40-365-type stars specifically, however, many authors attribute the final runaway speed not merely to binary disruption but to an additional kick from asymmetric ejecta. The 2026 discovery paper summarized this using

$\sim 33$18

with $\sim 33$19–$\sim 33$20, and noted that current 3D deflagration models typically peak at $\sim 33$21 for ONe white dwarf remnants, well below the observed velocities of many LP 40-365 stars (Bhat et al., 27 Feb 2026).

6. Progenitor channels, competing interpretations, and unresolved issues

The dominant interpretation across the foundational LP 40-365 literature is that these stars are bound remnants of underluminous thermonuclear explosions, especially Type Iax-like partial deflagrations in near-Chandrasekhar-mass white dwarfs (Vennes et al., 2017, Hermes et al., 2021). In this view, a CO, hybrid CONe, or ONe white dwarf undergoes incomplete burning, ejects part of its mass asymmetrically, and leaves behind a low-mass, inflated remnant with a photosphere enriched in products of partial O- and Si-burning and, in some analyses, signatures of high-density burning through enhanced Mn (Raddi et al., 2018, Raddi et al., 2019, Bhat et al., 27 Feb 2026). The 2025 evolutionary study made this more specific for one channel by suggesting that LP 40-365 stars are kicked remnants of near-Chandrasekhar-mass O/Ne white dwarfs that were partially disrupted by oxygen deflagrations (Shen, 6 Feb 2025).

Competing or complementary interpretations remain active. The same 2025 study distinguished LP 40-365 remnants from D$\sim 33$22 survivors: LP 40-365 stars were modeled as hot, low-mass O/Ne stars contracting on Kelvin-Helmholtz timescales, whereas D$\sim 33$23 survivors are donor white dwarfs from double-detonation events and have different compositions and thermal histories (Shen, 6 Feb 2025). A separate 2025 paper on the hypervelocity star D6-2 then reopened the comparison from the other side. Using MESA and MSE, it found that compact hot subdwarf + white dwarf binaries with nearly exhausted He-star donors can produce donor ejection velocities spanning $\sim 33$24–$\sim 33$25, overlapping the velocities reported for LP 40-365-like stars, and that ejecta stripping of thin residual He envelopes could yield C/O-dominated surfaces with possible intermediate-mass-element enrichment (Rajamuthukumar et al., 15 Nov 2025). That work explicitly stated that the consistency refers primarily to kinematics and that detailed atmospheric abundances require ejecta stripping and pollution modeling, with full 3D impact simulations still needed (Rajamuthukumar et al., 15 Nov 2025).

The central unresolved issues are therefore not merely classificatory but physical. First, abundance yields remain problematic: no single published yield set fully reproduces the observed photospheric composition of LP 40-365, and the high neon content of the class remains difficult for both CO and ONe deflagration models (Raddi et al., 2018, Bhat et al., 27 Feb 2026). Second, kick speeds and remnant masses are not well matched by current simulations: many models leave remnants more massive than the commonly inferred $\sim 33$26–$\sim 33$27 and predict kicks too small to explain the observed $\sim 33$28–$\sim 33$29 velocities (Raddi et al., 2019, Bhat et al., 27 Feb 2026). Third, post-explosion atmospheric processing is incompletely modeled. Several studies stress that diffusion, convective dredge-up, radiative levitation, and rotation can alter the surface abundances over Myr timescales, so present-day spectra need not equal bulk remnant composition (Raddi et al., 2018, Raddi et al., 2019).

The observational program implied by these uncertainties is already clear in the literature. High-S/N spectroscopy, especially targeting Mg edges, Ne/He diagnostics, Mn, and weak iron-group features, remains essential for abundance constraints (Raddi et al., 2018). UV and time-domain data are particularly valuable because LP 40-365 shows stronger rotational modulation in the ultraviolet than in the optical, and because hotter members such as J1022$\sim 33$303414 expose blue Ne I diagnostics inaccessible in cooler stars (Hermes et al., 2021, Bhat et al., 27 Feb 2026). Improved remnant-evolution calculations, multidimensional deflagration and ejecta-impact simulations, and Galactic-potential orbit modeling are repeatedly identified as necessary to reconcile composition, age, mass, and velocity within a single formation framework (Shen, 6 Feb 2025, Rajamuthukumar et al., 15 Nov 2025, Bhat et al., 27 Feb 2026).