Research Article

A hot and fast ultra-stripped supernova that likely formed a compact neutron star binary

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Science  12 Oct 2018:
Vol. 362, Issue 6411, pp. 201-206
DOI: 10.1126/science.aas8693

Explosive origin of a binary neutron star

Some types of core-collapse supernovae are known to produce a neutron star (NS). A binary NS merger was recently detected from its gravitational wave emission, but it is unclear how such a tight binary system can be formed. De et al. discovered a core-collapse supernova with unusual properties, including the removal of the outer layers of the star before the explosion. They interpret this as the second supernova in an interacting binary system that already contains one NS. Because the explosion probably produced a second NS (rather than a black hole) in a tight orbit, it could be an example of how binary NS systems form.

Science, this issue p. 201


Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 1050 ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

Core-collapse supernovae (SNe) are the violent deaths of massive stars when they run out of nuclear fuel in their cores and collapse, forming a neutron star (NS) or black hole (BH) (1). For massive stars that have lost some or all of their outer hydrogen (H) and helium (He) envelope, the resulting collapse produces a stripped-envelope supernova (SN) (2). The amount of material stripped from the star is a sensitive function of the initial mass of the star and its environment; if the star was born in a binary system, it also depends on the orbital properties of the system and the nature of the companion (2, 3).

Because most massive stars are born in close binary systems (4), stripping via binary interactions likely plays a large role in producing the observed diversity of stripped-envelope SNe (5, 6). For the most compact companions in close orbits, the stripping of massive stars may be extensive enough to completely remove their outer layers, leaving behind a naked metal core close to the minimum mass required for the core to collapse (the Chandrasekhar mass). If massive enough, the highly stripped core eventually collapses to produce a faint and fast-evolving SN explosion that ejects a small amount of material (7, 8). Although it has been difficult to securely identify these explosions, such “ultra-stripped” SNe have been suggested to lead to the formation of a variety of compact NS binary systems [i.e., a NS in orbit around another NS, white dwarf (WD), or BH] (7, 9).

Discovery and follow-up of iPTF 14gqr

iPTF 14gqr (SN 2014ft) was discovered by the intermediate Palomar Transient Factory (iPTF) (10, 11) on 14.18 October 2014 UTC (universal time coordinated) at a g-band optical magnitude of ≈20.2 mag. The source was not detected in the previous observation on 13.32 October 2014 UTC (0.86 days before discovery), with a limiting magnitude of g ≥ 21.5 mag. The transient was found in the outskirts (at a projected offset of ≈29 kpc from the center) of a tidally interacting spiral galaxy (IV Zw 155) at redshift z = 0.063 and luminosity distance D = 284.5 megaparsecs (Fig. 1). We obtained rapid ultraviolet (UV), optical, and near-infrared follow-up observations of the source, including a sequence of four spectra within 24 hours of the first detection (12).

Fig. 1 Discovery field and host galaxy of iPTF 14gqr.

(A) Optical image of the field from the Sloan Digital Sky Survey (SDSS) (51); R and G filter images have been used for red and cyan colors, respectively. (B) Composite RGB image (R, G, and B filter images have been used for red, green, and blue colors, respectively) of the iPTF 14gqr field from images taken near the second peak (19 October 2014) with the Palomar 60-inch telescope (P60), showing a blue transient inside the white dashed circle at the discovery location. (C) Late-time composite R+G image (R and G filter images have been used for red and cyan colors, respectively) of the host galaxy taken with the Low Resolution Imaging Spectrograph on the Keck I telescope.

We also obtained multiepoch x-ray and radio observations and found that the source remained undetected at these wavelengths (12). These upper limits rule out luminous nonthermal emission, such as that typically seen in relativistic and gamma-ray burst–associated SNe, but are not stringent enough to constrain the environment of the progenitor (figs. S11 and S12).

Our photometric follow-up indicated that the source rapidly faded within a day of detection, followed by rebrightening to a second peak on a longer time scale (rising over ≈7 days) (12) (Fig. 2). The early decline was detected in all optical and UV photometric bands and was characterized by a blackbody spectrum that cooled rapidly from a temperature T > 32,000 K near first detection to T ~ 10,000 K at 1 day after discovery (Figs. 3 and 4). Our early spectra also exhibit blackbody continua with temperatures consistent with those inferred from the photometry, superimposed with intermediate-width emission lines of He ii, C iii, and C iv. Such high ionization lines, which are typically associated with elevated pre-explosion mass-loss episodes in massive stars, have not been seen in early spectra of previously observed hydrogen-poor SNe. Although similar features are present in the early spectra of some hydrogen-rich core-collapse SNe (1315) (fig. S6), the relatively large widths of the lines (full width at half maximum ~ 2000 to 4000 km s−1) and the rapid evolution of the 4686-Å emission feature (Fig. 3) are not.

Fig. 2 Multicolor photometric observations of iPTF 14gqr.

(A) Multicolor light curves of iPTF 14gqr from our photometric follow-up observations (magnitudes are corrected for galactic extinction and offset vertically as indicated in the legend). Inverted triangles denote 5σ upper limits, whereas other symbols denote detections. Hollow inverted triangles are upper limits from P48/P60 imaging, and the filled inverted triangles are upper limits from Swift observations (filled green triangles are V band limits from Swift). Epochs when spectra were obtained are marked in both panels by vertical black dashed lines. (B) Zoomed-in view of the early evolution of the light curve. The black solid line shows the assumed explosion epoch. The colored solid lines show the best-fitting shock-cooling model for extended progenitors (25). Only photometric data before the cyan dot-dashed vertical line were used in the fitting (12).

Fig. 3 Spectroscopic evolution of iPTF 14gqr.

(A) Observed spectra before (gray) and after (black) binning. The epochs of the spectra, along with the scaling and vertical shifts used, are indicated next to each spectrum. (B) Zoomed-in view of the early spectra, indicated by the black dashed box in (A), showing rapid evolution of the λ4686 feature within 24 hours of discovery. The x axis indicates the velocity shift from the He ii λ4686 line. The orange and cyan lines mark the locations of the λ4686 line and the C iii λ4650 line, respectively. For the +13.9-hour and +25.2-hour spectra, additional magenta lines show the profiles of the C iv λ5801 and the C iii λ5696 features, respectively, at the same epochs. (C) Scaled optical/UV SEDs of the photometry and spectra obtained within the first light curve peak (see Fig. 2) in magenta, along with photometry near the second peak in orange. The circles indicate observed photometric fluxes, whereas the triangle is a 5σ upper limit. The dashed black lines indicate the best-fitting blackbody SEDs, including all optical/UV data points for the first peak and only the optical data points for the second peak (12).

Fig. 4 Bolometric light curve and Arnett modeling of iPTF 14gqr.

(A) Bolometric light curve of iPTF 14gqr. The filled black points indicate blackbody (BB) luminosities obtained from fitting multicolor photometry, whereas the magenta points correspond to pseudo-bolometric luminosities (12). The empty black circles indicate g-band luminosities obtained by multiplying the g-band flux Fλ with the wavelength λ of the filter. The inverted triangles denote estimated predetection 5σ upper limits on the respective luminosities (12). The inset shows the bolometric light curves zoomed into the region of the first peak. (B) Radius and temperature evolution of the fitted blackbody functions. (C) Best-fitting Arnett model of the pseudo-bolometric light curve of the main (second) peak of iPTF 14gqr. The 56Ni mass MNi and diffusion time scale τM corresponding to the model are indicated in the legend (12).

Spectra obtained near the second peak are dominated by emission from the expanding photosphere and exhibit relatively blue continua, with broad absorption features reminiscent of normal stripped-envelope SNe of type Ic that do not exhibit absorption lines of H or He in the spectra (16) (fig. S7). We find associated absorption velocities of ~10,000 km s−1 (12). The photometric properties of the second peak are broadly consistent with a number of previously observed fast type Ic events (figs. S3 and S4), but the rapidly declining first peak and the fast rise time to the second peak are unlike previously observed events. The source quickly faded after the second peak, declining at a rate of 0.21 mag day−1 in the g band (12). Our final spectrum taken at ≈34 days after explosion shows that the source exhibited an early transition to the nebular phase on a time scale faster than that for previously observed core-collapse SNe. The nebular phase spectrum exhibits prominent [Ca ii] emission similar to several other type Ic SNe (fig. S8).

Multicolor photometry at multiple epochs allow us to trace the evolution of the optical/UV spectral energy distribution (SED), which we use to construct bolometric light curves that contain flux integrated over all wavelengths (Figs. 3 and 4) (12). We fit the pseudo-bolometric light curve of iPTF 14gqr with a simple Arnett model (17) to estimate the explosion parameters. Allowing the explosion time to vary as a free parameter, we estimate an ejecta mass Mej ≈ 0.15 to 0.30 solar masses Embedded Image, an explosion kinetic energy EK ≈ (1.0 to 1.9) × 1050 ergs, and a synthesized Ni mass Embedded Image (12) (Fig. 4 and fig. S9). The inferred ejecta mass is lower than known core-collapse type Ic SNe (1820), which have ejecta masses in the higher range of ~0.7 to 15 Embedded Image, with a mean of 2 to 3 Embedded Image over a sample of ≈20 SNe. However, the parameters of iPTF 14gqr are similar to those inferred for SN 2005ek (21) and 2010X (22), rapidly evolving type I SNe whose physical origins remain a matter of debate.

The rapid decline of the first peak observed in iPTF 14gqr is reminiscent of shock-cooling emission from the outer layers of a progenitor after the core-collapse SN shock breaks out (23, 24) (fig. S5). We consider alternative explanations (12) and find them to be inconsistent with the data. In particular, the observed double-peaked light curve in the redder optical bands requires the presence of an extended low-mass envelope around the progenitor (24, 25). To constrain the properties of such an envelope, we use models (25) to construct multicolor light curves for a range of masses and radii of the envelope (Me and Re, respectively). We find a best-fitting model of Embedded Image and Re ~ 3 × 1013 cm [~450 solar radii Embedded Image] (12) (Fig. 2 and fig. S10). Even though the model considered here is simplified (e.g., it ignores the density structure of the envelope), we expect the estimated parameters to be accurate within an order of magnitude (26), leading us to conclude that the progenitor was surrounded by an extended envelope with a mass of Embedded Image at a radius of Embedded Image.

We use the early spectra to constrain the composition of the outer envelope. The emission lines observed in the early spectra of iPTF 14gqr can be understood as arising from recombination in the outer regions of the extended circumstellar material (CSM), which was ionized by the high-energy radiation produced in the shock breakout (13, 15) (fig. S6). We estimate the location and mass of the emitting He ii from the luminosity of the early 4686-Å line, assuming a CSM density profile that varies with radius r as ∝ r−2 (15). We find the emitting region to be located at r ~ 6 × 1014τ−2 cm and to contain a helium mass Embedded Image, where τ is the optical depth of the region (12). The absence of prominent Lorentzian scattering profiles in the lines suggests that the optical depth is small, and assuming τ ≈ 1, we find r ~ 6 × 1014 cm (8 × 103 Embedded Image) and Embedded Image. Because our calculations are based on fitting a simple two-component Gaussian profile to the 4686-Å emission line (to estimate the unknown contamination of C iii at 4650 Å), these estimates are uncertain by a factor of a few (2 to 4).

Using the C iv 5801-Å lines and similar methods as above, we estimate a CSM carbon mass of ~4 × 10−3 Embedded Image, whereas the hydrogen mass is constrained to be Embedded Image. Additional constraints based on light travel time arguments also suggest that the envelope was located at r ≤ 6 × 1015 cm from the progenitor (12). The flash-ionized emission lines exhibit complex asymmetric profiles (Fig. 3) that we attribute to light travel time effects, given the large size of the envelope and the high inferred wind velocities (12, 27).

An ultra-stripped progenitor

The low ejecta mass and explosion energy, as well as the presence of an extended He-rich envelope, indicate an unusual progenitor channel for iPTF 14gqr. The detection of the early shock-cooling emission indicates a core-collapse origin of the explosion, whereas the bright radioactivity powered emission suggests that this explosion is associated with the class of iron core-collapse explosions. The low ejecta mass, together with the small remaining amount of He in the progenitor, rules out models of single star evolution as well as a nondegenerate massive star companion for the progenitor of iPTF 14gqr (12), leaving only the most compact companions (such as a NS, WD, or BH) as possible explanations of the highly stripped (or ultra-stripped) progenitor.

Ultra-stripped explosions have been modeled in the case of He star–NS binaries, in which stripping of the He star by a NS in a close orbit leads to the subsequent collapse of an ultra-stripped He star (7, 8, 28). Hence, we compare theoretical bolometric light curves for ultra-stripped explosions (28) to those of iPTF 14gqr in Fig. 5 for a model with Embedded Image, Embedded Image, and EK = 2 × 1050 ergs. To account for the early declining emission, we also add a component corresponding to shock cooling of an extended envelope for Embedded Image and Re = 6 × 1013 cm. The two-component light curve matches the light curve data. We also compare the spectroscopic properties of iPTF 14gqr to those of ultra-stripped SN models in Fig. 5. The models (28) assumed fully mixed ejecta that led to the production of strong line blanketing features below 4000 Å, unlike this source. Thus, we recalculated the models for ejecta with no mixing (as with the light curve calculations) and were able to match to the spectra of iPTF 14gqr near the second peak (Fig. 5 and fig. S13).

Fig. 5 Comparison of iPTF 14gqr to theoretical models of ultra-stripped SNe.

(A) Bolometric light curve of iPTF 14gqr shown with a composite light curve consisting of ultra-stripped type Ic SN models (28) and early shock-cooling emission (25). The blue dashed line corresponds to the 56Ni powered peak in the ultra-stripped SN models for Embedded Image, Embedded Image, and EK = 2 × 1050 ergs; the magenta line corresponds to the early shock-cooling emission; and the orange line represents the total luminosity from the sum of the two components. Blackbody (BB) luminosities represent the early emission, whereas pseudo-bolometric (pB) luminosities are used for the second peak (12). (B) Comparison of the peak photospheric spectra of iPTF 14gqr [the epoch is indicated by the cyan dashed line in (A)] to that of the model in (A). The overall continuum shape, as well as absorption features of O i, Ca ii, Fe ii, and Mg ii, are reproduced (12).

Our observations indicate the presence of an extended He-rich envelope around the progenitor at the time of collapse, thus providing insight into the terminal evolution of the progenitors of ultra-stripped SNe and, more broadly, the lowest-mass progenitors of core-collapse SNe. By using the line widths in our early spectra, we estimate that the emitting envelope was expanding with a velocity of ~1000 to 2000 km s−1 at the time of collapse, consistent with the escape velocity from a compact He star (12). When considered with the inferred size of the envelope (at least Embedded Image), the velocities suggest that the envelope was ejected ~8 to 20 days before the explosion.

The temporal coincidence of the ejection with the final SN suggests that the envelope was likely associated with an intense pre-SN mass-loss episode of the progenitor (12). Despite the close stripping, ultra-stripped progenitors are expected to retain a small amount of He Embedded Image in their outer layers. The prominent He and C lines in the early spectra are consistent with eruptive mass loss when considering the expected surface compositions of ultra-stripped progenitors (8). The time scale of the ejection is similar to that expected for silicon flashes (~2 weeks before explosion) in the terminal evolution of low-mass metal cores (29) that have been suggested to lead to elevated mass-loss episodes before the explosion. Such mass-loss episodes are relevant to ultra-stripped progenitors as well (2830).

iPTF 14gqr exhibits a projected offset of ~15 kpc from the nearest spiral arms of its star-forming host galaxy (12), which is puzzling when compared to the expected locations of ultra-stripped SNe (8). Although we do not find evidence of an underlying stellar association or of galaxy emission features in late-time imaging and spectroscopy, the limits are not sensitive enough to rule out the presence of a dwarf galaxy or a star-forming H ii region (characterized by its H α emission) at or near the transient location (12). Nonetheless, the tidally interacting environment of the host galaxy suggests that outlying star formation in collisional debris is likely in this system (12, 31), which could harbor young stellar systems (with ages of ~5 to 100 million years) in the faint tidal tails (fig. S14). Hence, the discovery of a core-collapse SN in these outskirts is consistent with our interpretation.

Although a number of previously observed fast type Ic SNe [e.g., SN 2005ek (21) and SN 2010X (22)] were suggested to be members of the ultra-stripped SN class, it has been difficult to confirm a core-collapse origin for these explosions because these events were discovered only near maximum of the radioactively powered peak. Specifically, without early photometry and spectroscopy that can reveal the presence of a shock-cooling component, these fast transients are also consistent with variants of models involving thermonuclear detonations on WDs (3234). The early discovery and prompt follow-up of iPTF 14gqr establish the presence of a shock-cooling emission component that requires an extended progenitor consistent with a core-collapse explosion. In the probable scenario that iPTF 14gqr formed a NS in the explosion [we find a BH remnant to be unlikely given the observed properties of the SN (12)], the low ejecta mass in the system suggests that the SN results in the formation of a bound and compact NS binary system (12).

Implications for formation of compact NS binaries

Our interpretation of iPTF 14gqr as an ultra-stripped SN has implications in the wider context of stellar evolution. Compact NS binary systems evolve from binary massive stars that undergo several phases of mass transfer over their lifetime (Fig. 6). The initial phases of such evolution, in which two massive stars evolve into interacting binaries consisting of a compact object in orbit around a massive star (x-ray binaries), have been observed in several systems in the local Universe (35, 36). However, the subsequent phases that lead to the formation of compact NS binary systems have not been observed. This is due to the low occurrence rates of such systems, the short lifetimes (~106 years) of the final stages, and observational selection effects disfavoring their detection (8, 37, 38).

Fig. 6 Stellar evolutionary sequence leading from a binary system of massive stars (starting from the top left) to a NS-NS system.

NS-BH systems are expected to arise from binaries in which the first formed compact object is a BH. NS-WD systems follow a similar evolutionary sequence starting from the HMXB (high-mass x-ray binary) stage (where the NS is replaced by the WD) but require additional mass transfer in the earlier stages (52). The material composition of the stars is indicated by their colors: red, H-rich material; cyan/blue, He-rich material; gray, CO-rich material; green, degenerate matter (in NS). The specific phase of the evolution is indicated by the text under each diagram, with black text indicating previously observed phases, red text denoting phases that have not been previously observed, and bold red text indicating phases observed in this work. [Adapted from (9)]

Binary evolution models suggest that the subsequent evolution proceeds via a common envelope phase, during which the loss of angular momentum via dynamical friction leads to the formation of a close He star–compact object binary (9, 39, 40). An additional phase of close gravitational stripping by the compact companion then leads to the formation of an ultrastripped SN progenitor (9), with properties that can be inferred from our observations of iPTF 14gqr. The measured orbital properties of known double NS systems suggest that the second NSs were created in weak and low–ejecta mass explosions that impart a small natal kick to the newborn NS (41, 42).

The presence of the extended He-rich envelope in iPTF 14gqr, along with the lack of He in the low mass of ejecta, suggests that the progenitor was highly stripped by a compact companion, such that only a thin He layer was retained on its surface. This He layer was then ejected in an intense pre-SN mass-loss episode, as shown by the high velocity of the envelope. Taken together, these observations provide evidence of the terminal evolution of a post–common envelope He star–compact object binary leading to the formation of a compact NS binary system (Fig. 6).

Although wide binaries containing a NS and another compact object may be formed in noninteracting systems of binary massive stars, ultra-stripped SNe have been suggested to precede the formation of almost all compact NS binary systems (8). Thus, these explosions likely represent the only channel to forming NS-NS and NS-BH systems that are sufficiently compact to merge within the age of the Universe and produce observable merger signals for joint gravitational wave (43) and electromagnetic (4446) observations (8, 47, 48). Given that only a fraction of the systems produced by these explosions will merge within that time, the rates of ultra-stripped explosions must be higher than the rates of their mergers.

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S15

Tables S1 to S7

References (53181)

Data S1

References and Notes

  1. See supplementary materials.
  2. Compact systems may also form by dynamical capture in dense stellar environments (49).
Acknowledgments: We thank the anonymous referees for a careful reading of the manuscript, which helped improve the quality of the paper. We thank C. Steidel, N. Stone, D. Stern, P. Hopkins, S. de Mink, Y. Suwa, A. Heger, and T. M. Tauris for valuable discussions. M.M.K. thanks J. Fuller, E. S. Phinney, L. Bildsten, and E. Quataert for stimulating discussions at the Skyhouse during a PTF-TN meeting. We also thank T. Staley and G. Anderson for help with scheduling of the AMI observations. Additional facility acknowledgments are provided in the supplementary materials. Funding: The Intermediate Palomar Transient Factory project is a scientific collaboration among the California Institute of Technology; Los Alamos National Laboratory; the University of Wisconsin, Milwaukee; the Oskar Klein Center; the Weizmann Institute of Science; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe. This work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE Grant 1545949. GROWTH is a collaborative project among California Institute of Technology (USA); University of Maryland, College Park (USA); University of Wisconsin, Milwaukee (USA); Texas Tech University (USA); San Diego State University (USA); Los Alamos National Laboratory (USA); Tokyo Institute of Technology (Japan); National Central University (Taiwan); Indian Institute of Astrophysics (India); Indian Institute of Technology Bombay (India); Weizmann Institute of Science (Israel); The Oskar Klein Centre at Stockholm University (Sweden); Humboldt University (Germany); and Liverpool John Moores University (UK). A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. A.G.-Y. is supported by the EU via ERC grant 725161, the Quantum Universe I-Core program, the ISF, the BSF Transformative program, and a Kimmel award. E.O.O. is grateful for support by grants from the Willner Family Leadership Institute Ilan Gluzman (Secaucus, NJ), Israel Science Foundation, Minerva, BSF, BSF-transformative, and the I-Core program by the Israeli Committee for Planning and Budgeting and the Israel Science Foundation (ISF). F.T. and J.S. gratefully acknowledge support from the Knut and Alice Wallenberg Foundation. The Oskar Klein Centre is funded by the Swedish Research Council. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-05CH11231. M.S. acknowledges support from EU/FP7 ERC grant 615929. P.E.N. acknowledges support from the DOE through DE-FOA-0001088, Analytical Modeling for Extreme-Scale Computing Environments. T.J.M. is supported by Grants-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (16H07413 and 17H02864). Numerical computations were, in part, carried out on the PC cluster at the Center for Computational Astrophysics, National Astronomical Observatory of Japan. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Author contributions: K.D. and M.M.K. initiated the study, conducted analysis, and wrote the manuscript. I.M. initiated the follow-up of the young transient. D.A.P., G.E.D., and Y.C. conducted Keck and Palomar observations and contributed to data reduction and manuscript preparation. S.B.C. conducted Keck and Swift observations and contributed to data reduction and manuscript preparation. M.S. conducted the WHT observations and data reduction. F.T. and J.S. conducted NOT observations and data analysis and contributed to manuscript preparation. J.B. conducted the LCO observations and data reduction. T.P. conducted Gemini observations and data analysis. C.R. and R.P.F. conducted the AMI observations and data reduction. A.H. conducted the VLA observations and data reduction. S.R.K. is iPTF PI and contributed to manuscript preparation. T.J.M. and P.A.M. prepared the ultra-stripped SN models presented in the paper. E.O.O., C.F., A.G.-Y., R.L., P.E.N., and A.L.P. contributed to manuscript preparation. G.B.D., R.R.L., and F.M. contributed to the machine learning codes used to search for young transients. Competing interests: The authors declare no competing interests. Data and materials availability: All photometric data used in this paper are provided in the supplementary materials (tables S1 and S2), and all observed spectra are available via the WISeREP repository at (under source name iPTF 14gqr). The software used for the ultra-stripped SN modeling is presented in (50), and the synthetic model spectra are available in data S1.
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