Report

An unusual white dwarf star may be a surviving remnant of a subluminous Type Ia supernova

See allHide authors and affiliations

Science  18 Aug 2017:
Vol. 357, Issue 6352, pp. 680-683
DOI: 10.1126/science.aam8378

Unusual star may be supernova debris

Type Ia supernovae occur when a white dwarf star is completely destroyed in a thermonuclear explosion. Recently, another class of supernova has been found, dubbed type Iax; these look like type Ia but are much fainter and may be the result of only partial destruction of a white dwarf. In support of this notion, Vennes et al. found a white dwarf star in our Galaxy that is low-mass, is moving quickly, and has an unusual composition. These properties suggest that it could be the predicted leftover remains from a type Iax supernova.

Science, this issue p. 680

Abstract

Subluminous Type Ia supernovae, such as the Type Iax–class prototype SN 2002cx, are described by a variety of models such as the failed detonation and partial deflagration of an accreting carbon-oxygen white dwarf star or the explosion of an accreting, hybrid carbon-oxygen-neon core. These models predict that bound remnants survive such events with, according to some simulations, a high kick velocity. We report the discovery of a high proper motion, low-mass white dwarf (LP 40-365) that travels at a velocity greater than the Galactic escape velocity and whose peculiar atmosphere is dominated by intermediate-mass elements. Strong evidence indicates that this partially burnt remnant was ejected following a subluminous Type Ia supernova event. This supports the viability of single-degenerate supernova progenitors.

Type Ia supernova (SN Ia) explosions are powered by the detonation of a Chandrasekhar-mass white dwarf with a degenerate carbon-oxygen core (1). Models show that the explosion may be triggered by the high internal pressure caused either by matter accreted from a close donor star [the single-degenerate (SD) scenario] or by the merger with another white dwarf (the double-degenerate scenario) (2). Although Type Ia supernovae are used to calibrate the cosmological distance scale (1) and constrain cosmological models (3, 4), our knowledge of these objects is incomplete, and their progenitors have remained elusive (5, 6). The possibility of detecting surviving remnants from subluminous SN Ia events may help illuminate the SN Ia progenitor problem in general. Models (79) proposed to explain observed properties of subluminous SN Ia such as the SN Iax–class prototype SN 2002cx (10, 11) involve failed detonation and partial deflagration of a massive white dwarf (79) or the explosion of a hybrid carbon-oxygen-neon (CONe) core (12, 13), with both scenarios expected, under the right circumstances, to leave a bound remnant. Direct evidence for such remnants is missing (5).

We have observed the high proper motion star LP 40-365 (14). An identification spectrum was obtained on 2015 February 21 using the Richtey-Chretien spectrograph attached to the Mayall 4-m telescope at Kitt Peak National Observatory (KPNO) (Fig. 1). The main characteristics are a blue continuum indicating a temperature of ≈10,000 K (circa B9 star); the complete absence of neutral hydrogen or helium absorption lines, unlike in normal B stars; and the appearance of strong magnesium (Mg I and Mg II) and sodium (Na I) line series and weaker lines of oxygen (O I). Table 1 lists astrometric (15) and photometric (16) data for this object. We followed up this unusual spectrum using low- to high-dispersion spectra obtained between June 2015 and June 2016 with the William Herschel 4.2-m telescope on La Palma, the Hiltner 2.4-m telescope on Kitt Peak, and, finally, with the Gemini-North 8-m telescope on Mauna Kea (17).

Fig. 1 Normalized optical spectrum of LP 40-365 as a function of wavelength.

The spectrum was obtained with the Richey-Chretien Spectrograph at the 4-m telescope (KPNO). Dominant spectral lines of sodium (Na I) and magnesium (Mg I and Mg II) are labeled, along with weaker lines of oxygen (O I), aluminum (Al I) and silicon (Si II). A broad feature near 5230 Å is tentatively identified with a resonance in the Mg I 3p2 photoionization cross section.

Table 1 Stellar data and parameters.

The celestial coordinates are provided along the right ascension (RA ≡ α) and declination (Dec ≡ δ) and the apparent motion of the star, i.e., the proper motion μ, is decomposed into μα cos δ along the right ascension and μδ along the declination.

View this table:

We performed a spectral line analysis using an iterative procedure that adjusts a parametrized spectral synthesis to the observed line spectrum using χ2 minimization techniques. These calculations were performed by using a multiparameter fitting procedure that constrains simultaneously the effective temperature (Teff) and surface gravity (log g) of the star and each individual element abundance in the atmosphere (17). We analyzed the high-resolution spectra obtained with the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) fed by optical fibers attached to the Gemini-North telescope using the Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES) (CFHT, Canada-France-Hawaii Telescope).

The model atmospheres and synthetic spectra supporting our analysis were calculated in full non–local thermodynamic equilibrium (non-LTE) by using the computer codes TLUSTY version 204 and SYNSPEC version 49 (17, 18). The chemical composition includes elements with atomic numbers from Z = 1 (H) to 30 (Zn) with all relevant ionized atoms. The atmosphere is in radiative equilibrium; convective energy transport was found to be inefficient. Detailed line profiles were calculated by using line-broadening parameters dominated by electronic collisions (Stark effect). Table 1 lists best-fitting stellar parameters (with 1σ statistical error bars) and Fig. 2 shows the corresponding abundances. The high Teff and the log g, which is intermediate between normal white dwarfs and the main sequence stars, indicate that this object is most likely a low-mass degenerate star (19). The line profiles demonstrate that the star is rotating with a projected rotation velocity v sin i = 30.5 km s–1, where i is the apparent inclination of the rotation axis and v is the equatorial rotation velocity, suggesting that the parent body was spun up during binary interaction. The abundance analysis shows that the main atmospheric constituents are oxygen and neon, with substantial traces of intermediate-mass elements such as aluminum and silicon.

Fig. 2 Elemental abundances for LP 40-365.

Photospheric abundances, expressed as the number fraction versus the atomic number, were measured in the high-dispersion spectrum obtained with GRACES at the Gemini-North telescope on Mauna Kea. The atmosphere is dominated by oxygen and neon, followed by sodium and magnesium. Iron is more abundant than nickel and other elements in the iron group by at least a factor of 10. Upper limits are shown with arrows.

We measured a large Doppler wavelength shift in the spectral line analysis. From a sample of 21 velocity measurements taken at different epochs and after correcting for Earth’s motion, we measured an average radial velocity vr = 497.6 ± 1.1 km s–1, without significant variations (Embedded Image). Therefore, the star is apparently single and moving at a velocity characteristic of hypervelocity stars (20). Those objects are former members of binary systems that were ejected during three-body encounters with the Galactic center (GC) or ejected following the demise of a massive white dwarf companion in a SD SN Ia (21).

In addition to its large radial velocity, LP 40-365 shows a large apparent motion across the celestial sphere of μ = 158 milli–arc sec per year (mas year–1). With knowledge of the distance d, the proper motion vector may be converted into the tangential velocity vector vT, which, combined with the radial velocity vr, provides a complete three-dimensional description of LP 40-365’s motion. We estimated the distance toward LP 40-365 with a photometric method (17), using as inputs the apparent luminosity of the star and an estimate of its absolute lumimosity. The absolute luminosity is calculated by using the surface temperature measurements described earlier and an estimate of the stellar radius, which is model dependent. However, that distance estimate will eventually be superseded by Gaia parallax measurements (22).

The radius of a low-mass, degenerate or partly degenerate body is sensitive to finite-temperature effects (23, 24) that would inflate the radius of a young, extremely low-mass white dwarf such as LP 40-365. Models for carbon, oxygen, silicon, or iron cores are available (25), but unfortunately, the predicted surface gravity of available models (log g > 6.0, where g is expressed in cm s–2) largely exceeds the measured gravity of LP 40-365, indicating that its mass should be much lower than 0.3 solar mass (MEmbedded Image). Lower-mass models with helium interiors are available (23) and indicate that a body with a mass of ≈0.14 MEmbedded Image and ≈8% of the solar radius (REmbedded Image) reproduces the log g and Teff of LP 40-365, assuming a cooling age between 5 and 50 million years (17). Although we have concluded that the interior of LP 40-365 is most likely composed of carbon, oxygen, and neon, or heavier elements, helium models characterized by identical mean electronic weight (μe = 2) represent a reasonable proxy. The central temperature of the adopted model, Tc ≈ 30 × 106 K, is lower than that of a typical inert core of normal white dwarfs (23). Adopting a radius of 0.078 REmbedded Image, we estimated an absolute magnitude in the Johnson V band MV = 8.14 magnitude (mag). Thus, the apparent MV magnitude listed in Table 1 implies a distance of Embedded Image pc. The tangential velocity at a distance of 298 pc is vT = 224 km s–1, for a total space velocity relative to the Sun of 546 km s–1.

To retrace the past history of this object, we converted the apparent velocity components (radial and tangential) into the Galactic velocity vector (26) (U, V, W) = (–346, 360, 217) km s–1. This instantaneous velocity vector may be projected back in time by adopting an appropriate Galactic potential model (27). We followed the Galactic orbit of LP 40-365 from the present time (t = 0) back to t = –100 million years. The projected trajectories displayed in Fig. 3 indicate that, for an assumed starting point set at distances between 100 and 1000 pc, the object did not encounter the GC and, therefore, is not the product of a three-body dynamical interaction with the GC (28). None of the resultant trajectories, which allowed for uncertainties in the distance, are bound Galactic orbits either. The total velocity in the Galactic rest frame varies between 675 km s–1, assuming d = 100 pc, and 1016 km s–1, assuming d = 1000 pc, with a velocity of 709 km s–1 at the distance (298 pc) set by the photometric method. All exceed the Galactic escape velocity at 8.5 kpc from the GC (20). The object must have originated along one of those projected trajectories, and the trajectory that took LP 40-365 to the present-day distance of 298 pc entered the plane <5 million years ago. The simulated cooling time scale for a 0.15 MEmbedded Image compact object with an Teff of 104 K is only ≈5 to 50 million years (23).

Fig. 3 Calculated Galactic motion of LP 40-365.

The orbits are drawn (A) in the Galactic plane (z versus x) and (B) perpendicular to the plane (y versus x) with the GC (solid circle) at the origin. The Sun (Embedded Image) is located 8.5 kpc along the x axis. The current (t = 0) position of LP 40-365 is estimated assuming a distance to the Sun of, from uppermost to lowermost curve, 1000, 800, 600, 400, 300, 200, and 100 pc. The past trajectory resulting from an assumed distance of 300 pc is marked with open circles at, from rightmost to leftmost curve, –3, –10, and –30 million years. Schematic views of the Galactic arms are shown in gray.

Combining the peculiar surface composition of this compact object, the results of the trajectory analysis, and the evolutionary age estimate, it appears likely that LP 40-365 is the surviving remnant of a subluminous SN Ia event that took place below the Galactic plane, a few kiloparsecs away and earlier than 50 million years ago. The stellar properties and the kinematics of LP 40-365 are comparable to some simulated events (7), suggesting that this object is indeed a fragment that survived the failed detonation of an SN Iax. The mass estimate is somewhat less than accounted for in these simulations (>0.3 MEmbedded Image). However, other models (9) successfully achieve remnants with masses as low as 0.09 MEmbedded Image but without delivering a large kick velocity. Variations in the adopted ignition geometry, such as centered versus off-center ignition, may affect the kinematical outcome for the surviving remnant. Simulations involving hybrid CONe cores (13) successfully generated low-mass remnants, but these simulations did not explore postexplosion kinematics.

Intermediate-mass elements detected in the atmosphere of LP 40-365 are expected to contaminate bound remnants after a typical SN Iax event (9), but we found only minute traces of iron-group elements, which normally dominate the supernova ejecta. The paucity of iron-group elements and the prevalence of lighter elements indicate that gravitational settling and chemical separation may have occurred with light elements dominating over heavier ones. It is not possible to estimate the fraction of iron material produced in the explosion that would manage to diffuse to the star’s center. Diffusion time scales at the star’s surface may be comparable to or longer than the age of the object (29). Conversely, the absence of carbon and prevalence of oxygen and neon at the surface of LP 40-365 would preferably match the configuration of a hybrid CONe core (13). None of the simulations take element diffusion explicitly into account; therefore, a detailed comparison of predicted and observed surface compositions would not constitute a definitive test for any models.

It has been suggested that dynamical instability in a low-mass x-ray binary orbiting a distant main sequence star could result in the high-velocity ejection of the donor star (30, 31). Apart from an unspecified surface composition, the predicted high-velocity star could resemble LP 40-365. However, only ~10–8 such events are expected per year in the Milky Way (31), compared to a rate of ~10–3 for SN Iax events (32); therefore, this scenario is less probable.

The actual donor star that must also have been ejected (21) along with LP 40-365 should be detectable as well. For example, the high-velocity, helium-rich subdwarf star US 708 (33) is a representative of the class of donor stars that emerged from a SD SN Ia event, and a similar object would have been ejected along with LP 40-365 after the proposed underluminous SN Ia event. The possible detection of a bound remnant in the aftermath of the SN Iax event SN 2008ha has been reported, although it may be a chance alignment (32). The properties of that object are unknown. The tentative progenitor of SN 2012Z has been described as novalike (34), suggesting the likely presence of an accreting white dwarf in a SD progenitor system akin to that of LP 40-365. No bound remnant has been identified. The atmospheric properties of LP 40-365 share some similarities with those of another extreme white dwarf (35) but exhibit clear distinctions as well: Both WD 1238+674 and LP 40-365 are oxygen rich, but WD 1238+674 is more massive (0.6 MEmbedded Image versus 0.14 MEmbedded Image) and its kinematical properties do not appear as extreme. The discovery of the oxygen-neon white dwarf WD 1238+674 lends support to the hybrid CONe formation model (36) and, indirectly, to the subluminous SN Ia explosion models involving hybrid CONe white dwarfs (13).

Supplementary Materials

www.sciencemag.org/content/357/6352/680/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S8

Tables S1 and S2

References (3758)

References and Notes

  1. Materials and methods are available as supplementary materials.
  2. A description of the instrument is currently available at www.noao.edu/kpno/manuals/rcspec/rcsp.html.
  3. A description of the instrument is currently available at www.ing.iac.es/astronomy/instruments/isis/.
  4. A description of the instrument is currently available at http://mdm.kpno.noao.edu/Manuals/ModSpec/modspec_man.html.
  5. A description of the instrument is currently available at www.gemini.edu/sciops/instruments/graces/.
  6. Image Reduction and Analysis Facility (IRAF) is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
  7. See http://nova.astro.umd.edu/.
  8. See http://kurucz.harvard.edu/atoms.html.
  9. See www.nist.gov/pml/atomic-spectra-database.
Acknowledgments: A.K. and S.V. acknowledge support from the Grant Agency of the Czech Republic (15-15943S). A.K. was a visiting astronomer at KPNO, National Optical Astronomy Observatory. This work was also supported by the project RVO:67985815 in the Czech Republic. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2013-2016) under grant agreement number 312430 [Optical Infrared Coordination Network for Astronomy (OPTICON)]. J.R.T. acknowledges support from NSF grant AST-1008217. This research made use of services at Astroserver.org under the reference number NWKZKA and is based on observations obtained at KPNO, National Optical Astronomy Observatory, and at the Gemini Observatory, program GN-2016A-FT-24, using ESPaDOnS, located at the CFHT. This publication makes use of data products from the Wide-field Infrared Survey Explorer, the Two Micron All-Sky Survey, and the American Association of Variable Sky Observers (AAVSO) Photometric All-Sky Survey (APASS). Complete acknowledgments are included in the supplementary material. Observational data are available in publicly available archives, as are our calculated model atmospheres. Full details and URLs are given in the supplementary material.
View Abstract

Navigate This Article