Helium Star Donor Research Articles

Long-term Evolution of Postexplosion Helium-star Companions of Type Iax Supernovae

Supernovae of Type Iax (SNe Iax) are an accepted faint subclass of hydrogen-free supernovae. Their origin, the nature of the progenitor systems, however, is an open question. Recent studies suggest that the weak deflagration explosion of a near-Chandrasekhar-mass white dwarf in a binary system with a helium-star donor could be the origin of SNe Iax. In this scenario, the helium-star donor is expected to survive the explosion. We use the one-dimensional stellar evolution codes MESA and Kepler to follow the postimpact evolution of the surviving helium companion stars. The stellar models are based on our previous hydrodynamical simulations of ejecta–donor interaction, and we explore the observational characteristics of these surviving helium companions. We find that the luminosities of the surviving helium companions increase significantly after the impact: they could vary from 2500 L ⊙ to 16,000 L ⊙ for a Kelvin–Helmholtz timescale of about 104 yr. After the star reaches thermal equilibrium, it evolves as an O-type hot subdwarf (sdO) star and continues its evolution along the evolutionary track of a normal sdO star with the same mass. Our results will help to identify the surviving helium companions of SNe Iax in future observations and to place new constraints on their progenitor models.

Pre-explosion Properties of Helium Star Donors to Thermonuclear Supernovae

Abstract Helium star–carbon-oxygen white dwarf (CO WD) binaries are potential single-degenerate progenitor systems of thermonuclear supernovae. Revisiting a set of binary evolution calculations using the stellar evolution code MESA, we refine our previous predictions about which systems can lead to a thermonuclear supernova and then characterize the properties of the helium star donor at the time of explosion. We convert these model properties to near-UV/optical magnitudes assuming a blackbody spectrum and support this approach using a matched stellar atmosphere model. These models will be valuable to compare with pre-explosion imaging for future supernovae, though we emphasize the observational difficulty of detecting extremely blue companions. The pre-explosion source detected in association with SN 2012Z has been interpreted as a helium star binary containing an initially ultra-massive WD in a multiday orbit. However, extending our binary models to initial CO WD masses of up to 1.2 M ⊙, we find that these systems undergo off-center carbon ignitions and thus are not expected to produce thermonuclear supernovae. This tension suggests that, if SN 2012Z is associated with a helium star–WD binary, then the pre-explosion optical light from the system must be significantly modified by the binary environment and/or the WD does not have a carbon-rich interior composition.

The Interaction of Type Iax Supernova Ejecta with a Helium Companion Star

Abstract Type Iax supernovae (SNe Iax) are an important subluminous class of SNe Ia. However, their progenitors and explosion mechanisms are not yet understood. It has been suggested that SNe Iax may be produced from weak deflagration explosions of Chandrasekhar-mass white dwarfs (WDs) in binary systems with a helium (He) star donor. In such progenitor systems, the stripped He companion material caused by the ejecta-companion interaction is expected to present some He features in their late-time spectra. However, the detection of He lines in late-time spectra of SNe Iax has not yet been successful, which gives an upper limit on the amount of stripped He mass of . In this work, we study the interaction between SN Iax ejecta and a He star companion by performing three-dimensional hydrodynamical simulations with a weak pure deflagration explosion model. We find that about of He material can be stripped off from the companion star by SN explosion, which is very close to (or lower than) the observational upper limit on the total stripped He mass in SNe Iax. We, therefore, conclude that nondetection of He lines in late-time spectra of SNe Iax could be reasonably explained if they indeed resulted from weak pure deflagration explosions of Chandrasekhar-mass WDs in progenitor systems with a He star donor.

Circumstellar properties of Type Ia supernovae from the helium star donor channel

ABSTRACT We investigate the predicted circumstellar properties of Type Ia supernova progenitor systems with non-degenerate helium star donors. It has been suggested that systems consisting of a carbon + oxygen white dwarf and a helium star can lead to Type Ia supernova explosions. Binary evolution calculations for the helium star donor channel predict that such a progenitor system is in either a stable helium-shell burning phase or a weak helium-shell flash phase at the time of the Type Ia supernova explosion. By using the binary evolution models from our previous study, we show that a large fraction of the progenitor systems with a helium star donor have a low enough density to explain the current non-detection of radio emission from Type Ia supernovae. Most of the progenitor systems in the weak helium-shell flash phase at the time of the Type Ia supernova explosion, which may dominate the prompt (short delay time) Type Ia supernova population, have both a low circumstellar density and a faint helium star donor, which could account for the non-detection of radio emission and a pre-explosion companion star in SN 2011fe and SN 2014J. We also find some progenitor systems that are consistent with the properties of the companion star candidate identified at the explosion location of the Type Iax SN 2012Z.

Anatomy of the hyper-runaway star LP 40–365 with Gaia

Abstract LP 40–365 (aka GD 492) is a nearby low-luminosity hyper-runaway star with an extremely unusual atmospheric composition, which has been proposed as the remnant of a white dwarf that survived a peculiar Type Iax supernova (SN Iax) in a single-degenerate scenario. Adopting the Gaia Data Release 2 (DR2) parallax, ϖ = 1.58 ± 0.03 mas, we estimate a radius of 0.18 ± 0.01 R⊙, confirming LP 40–365 as a subluminous star that is ≃ 15 times larger than a typical white dwarf and is compatible with the SN Iax remnant scenario. We present an updated kinematic analysis, making use of the Gaia parallax and proper motion, and confirm that LP 40–365 is leaving the Milky Way at about 1.5 times the escape velocity of the Solar neighbourhood with a rest-frame velocity of 852 ± 10 km s−1. Integrating the past trajectories of LP 40–365, we confirm that it crossed the Galactic disc 5.3 ± 0.5 Myr ago in the direction of Carina, likely coming from beneath the plane. Finally, we estimate that LP 40–365 was ejected from its progenitor binary with a velocity of at least 600 km s−1, which is compatible with theoretical predictions for close binaries containing a white dwarf and a helium-star donor.

Early ultraviolet signatures from the interaction of Type Ia supernova ejecta with a stellar companion

The progenitors of SNe Ia are not yet fully understood. The two leading progenitor scenarios are the single-degenerate (SD) scenario and the double-degenerate scenario. In the SD scenario, the collision of the SN Ia ejecta with its companion star is expected to produce detectable ultraviolet (UV) emission in the first few days after the SN explosion within certain viewing angles. A strong UV flash has recently been detected in an SN 2002es-like peculiar SN Ia iPTF14atg by Cao et al., which is interpreted as evidence of an early-time UV signature due to SN ejecta interacting with its companion star, supporting the SD scenario. In this paper, we present the expected luminosity distributions of early-time UV emission arising from SN Ia ejecta-companion interaction by performing binary population synthesis calculations for different progenitor systems in the SD scenario. Our theoretical predictions will be helpful for future early-time observations of SNe Ia to constrain their possible progenitors. Assuming the observed strong UV pulse of iPTF14atg was indeed produced by the SN ejecta-companion interaction, our population synthesis model suggests that the progenitor system of iPTF14atg is most likely a red-giant donor binary system, and it is unlikely to have been a main-sequence or helium-star donor system.

Ultra-stripped supernovae: progenitors and fate

The explosion of ultra-stripped stars in close binaries can lead to ejecta masses < 0.1 M_sun and may explain some of the recent discoveries of weak and fast optical transients. In Tauris et al. (2013), it was demonstrated that helium star companions to neutron stars (NSs) may experience mass transfer and evolve into naked ~1.5 M_sun metal cores, barely above the Chandrasekhar mass limit. Here we present a systematic investigation of the progenitor evolution leading to ultra-stripped supernovae (SNe). In particular, we examine the binary parameter space leading to electron-capture (EC SNe) and iron core-collapse SNe (Fe CCSNe), respectively, and determine the amount of helium ejected with applications to their observational classification as Type Ib or Type Ic. We mainly evolve systems where the SN progenitors are helium star donors of initial mass M_He = 2.5 - 3.5 M_sun in tight binaries with orbital periods of P_orb = 0.06 - 2.0 days, and hosting an accreting NS, but we also discuss the evolution of wider systems and of both more massive and lighter - as well as single - helium stars. In some cases we are able to follow the evolution until the onset of silicon burning, just a few days prior to the SN explosion. We find that ultra-stripped SNe are possible for both EC SNe and Fe CCSNe, and that the amount of helium ejected is correlated with P_orb - the tightest systems even having donors being stripped down to envelopes of less than 0.01 M_sun. We estimate the rise time of ultra-stripped SNe to be in the range 12 hr - 8 days, and light curve decay times between 1 and 50 days. Ultra-stripped SNe may produce NSs in the mass range 1.10 - 1.80 M_sun and are highly relevant for LIGO/VIRGO since most (possibly all) merging double NS systems have evolved through this phase. Finally, we discuss the low-velocity kicks which might be imparted on these resulting NSs at birth. [Abridged]

GALACTIC ULTRACOMPACT X-RAY BINARIES: DISK STABILITY AND EVOLUTION

We study the mass transfer rates and disk stability conditions of ultracompact X-ray binaries (UCXBs) using empirical time-averaged X-ray luminosities from Paper I (Cartwright et al. 2013) and compiled information from the literature. The majority of UCXBs are consistent with evolutionary tracks for white dwarf donors. Three UCXBs with orbital periods longer than 40 minutes have mass transfer rates above 10^{-10} Msun/year, inconsistent with white dwarf donor tracks. We show that if helium star donors can retain their initial high entropy, they can explain the observed mass transfer rates of these UCXBs. Several UCXBs show persistent luminosities apparently below the disk instability limit for irradiated He accretion disks. We point out that a predominantly C and/or O disk (as observed in the optical spectra of several) lowers the disk instability limit, explaining this disagreement. The orbital period and low time-averaged mass transfer rate of 2S 0918-549 provide evidence that the donor star is a low-entropy C/O white dwarf, consistent with optical spectra. We combine existing information to constrain the masses of the donors in 4U 1916-053 (0.064+-0.010 Msun) and 4U 1626-67 (<0.036 Msun for a 1.4 Msun neutron star). We show that 4U 1626-67 is indeed persistent, and not undergoing a transient outburst, leaving He star models as the best explanation for the donor.

Population synthesis of ultra-compact X-ray binaries

Ultra-compact X-ray binaries (UCXBs) are very interesting and important objects. By taking the population synthesis approach to the evolution of binaries, we carry out a detailed study of UCXBs. We estimate that there are ∼ 5000–10000 UCXBs in the Galaxy, and their birthrates are ∼ 2.6–7.5 × 10−4 yr−1. Most UCXBs are transient X-ray sources, but their X-ray luminosities are much lower than those of persistent sources. Therefore, the majority of observed UCXBs should be persistent sources. About 40%–70% of neutron stars (NSs) in UCXBs form via an accretion-induced collapse from an accreting ONe white dwarf (WD), 1%–10% of NSs in UCXBs form via core-collapse supernovae and others form via the evolution-induced collapse of a naked helium star. About 50%–80% of UCXBs have naked helium star donors, 5%–10% of UCXBs have HeWD donors, 15%–40% of UCXBs have COWD donors and UCXBs with ONeWD donors are negligible. Our investigation indicates that the uncertainty mainly comes from evolution of the common-envelope which develops in these systems.

Donors of Persistent Neutron-Star Low-Mass X-Ray Binaries

ABSTRACTProperties of X-ray luminosities in low-mass X-ray binaries (LMXBs) mainly depend on donors. We have carried out a detailed study of donors in persistent neutron-star LMXBs (PLMXBs) by means of a population synthesis code. PLMXBs with different donors have different formation channels. Our numerical simulations show that more than 90% of PLMXBs have main-sequence (MS) donors, and PLMXBs with red giant (RG) donors via stellar wind (wind) are negligible. In our model, most of the neutron stars (NSs) in PLMXBs with hydrogen-rich donors form via core-collapse supernovae, while more than 90% of the NSs in PLMXBs with naked helium star (He) donors or white dwarf (WD) donors form via an evolution-induced collapse via helium star (1.4 ≤ MHe/M⊙ ≤ 2.5) or an accretion-induced collapse for an accreting ONeMg WD. PLMXBs with different donors have different properties. In PLMXBs with MS donors, the orbital periods are between and 100 hr, and the mass transfer is driven by donor evolution or magnetic braking. Our population synthesis code shows that their X-ray luminosities are mainly around . Similarly, in PLMXBs with RG donors via Roche lobe overflow (Roche), the mass transfer is driven by donor evolution, but orbital periods are between and 1000 hr. Their X-ray luminosities are . The two known LMXBs (Cyg X-2 and GX 13 + 1) can belong to PLMXBs with RG (Roche) donors. PLMXBs with RG (wind) donors have the longest orbital periods and low X-ray luminosities (). Their contributions to X-ray luminosities can be negligible. In PLMXBs with He donors, the orbital periods are shorter than 80 minutes, and the mass transfer is mainly driven by magnetic braking. Results of our numerical simulations predict that PLMXBs with X-ray luminosities of mainly come from binaries with He donors. In PLMXBs with WD donors, the orbital periods are shorter than 1 hr, and the mass transfer is mainly driven by gravitational radiation. According to results of our population synthesis code, their X-ray luminosities are between ∼6 × 1035 and , and most of the LMXBs with WD donors are transient.

The helium star donor channel for the progenitors of type Ia supernovae with different metallicities

Context: The nature of type Ia supernovae (SNe Ia) is still unclear. Metallicities may have an important effect on their properties. Aims: In this paper, we study the He star donor channel towards SNe Ia comprehensively and systematically at various metallicities. Methods: Employing Eggleton's stellar evolution code with the optically thick wind assumption, we calculated about 10 000 WD + He star systems and obtained SN Ia production regions of the He star donor channel with metallicities Z=0.03, 0.02, 0.004 and 0.0001. According to a detailed binary population synthesis approach, we also obtained SN Ia birthrates at various metallicities. Results: Our study shows that both the initial mass of the He donor star and the initial orbital period for SNe Ia increase with metallicity, while the minimum initial mass of the carbon--oxygen white dwarfs producing SNe Ia decreases with metallicity. For a constant star-formation galaxy, SN Ia birthrates increase with metallicity. If a single starburst is assumed, SNe Ia occur systemically earlier and the peak value of the birthrate is larger for a high Z, and the He star donor channel with different metallicities can produce the young SNe Ia with delay times $\sim$45-220Myr.

The chemical composition of donors in AM CVn stars and ultracompact X-ray binaries: observational tests of their formation

We study the formation of ultra-compact binaries (AM CVn stars and ultra-compact X-ray binaries) with emphasis on the surface chemical abundances of the donors in these systems. Hydrogen is not convincingly detected in the spectra of these systems. Three different proposed formation scenarios involve different donor stars, white dwarfs, helium stars or evolved main-sequence stars. Using detailed evolutionary calculations we show that the abundances of helium WD donors and evolved main-sequence stars are close to equilibrium CNO-processed material, and the detailed abundances correlate with the core temperature and thus mass of the MS progenitors. Evolved MS donors typically have traces of H left. For hybrid or CO white dwarf donors, the carbon and oxygen abundances depend on the temperature of the helium burning and thus on the helium core mass of the progenitors. For helium star donors in addition to their mass, the abundances depend strongly on the amount of helium burnt before mass transfer starts and can range from unprocessed and thus almost equal to CNO-processed matter, to strongly processed and thus C/O rich and N-deficient. We briefly discuss the relative frequency of these cases for helium star donors, based on population synthesis results. Finally we give diagnostics for applying our results to observed systems and find that the most important test is the N/C ratio, which can indicate the formation scenario as well as, in some cases, the mass of the progenitor of the donor. In addition, if observed, the N/O, O/He and O/C ratios can distinguish between helium star and WD donors. Applied to the known systems we find evidence for WD donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid WD or very evolved helium star donors in the UCXBs 4U 1626-67 and 4U 0614+09. [Abridged]

The helium star donor channel for the progenitors of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. There are several progenitor models for SNe Ia proposed in the past years. In this paper, we have carried out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggleton's stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu (2004) for the mass accumulation efficiency of the He-shell flashes onto the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period--secondary mass ($\log P^{\rm i}-M^{\rm i}_2$) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (i.e. $\sim 1.2\times10^{-3} {\rm yr}^{-1}$ in the Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times ($\la 10^{8}$ yr), which is consistent with recent observational implications of young populations of SN Ia progenitors.

CXOU J121538.2+361921 in the galaxy NGC 4214: a double neutron star in the making?

Abstract CXOU J121538.2+361921 is the brightest X-ray source in the galaxy NGC 4214, with an X-ray luminosity of up to 0.7 × 1039 erg s−1. The observed periodicity of 3.62 h, derived from eclipse timing, is interpreted as the orbital period of the system. It has been suggested that the system is a low-mass helium star with a lower-mass compact companion. If this idea is correct, then CXOU J121538.2+361921 will evolve into a double neutron star, a binary consisting of a radio pulsar and another neutron star. In this study we investigate this possibility further. We find that the X-ray luminosity is consistent with super-Eddington accretion in a helium star-neutron star binary. The binary is in a state of mass transfer which is initiated when the helium-star donor is at the helium shell burning stage. A donor star with a current mass in the range of around 2.2–3.6 M⊙ is required to explain the observed orbital period. Helium stars in this mass range are massive enough to collapse in a supernova explosion, making CXOU J121538.2+361921 the immediate progenitor of a double neutron star.

Population synthesis for double white dwarfs

We study two models for AM CVn stars: white dwarfs accreting (i) from a helium white dwarf companion and (ii) from a helium-star donor. We show that in the first model possibly no accretion disk forms at the onset of mass transfer. The stability and the rate of mass transfer then depend on the tidal coupling between the accretor and the orbital motion. In the second model the formation of AM CVn stars may be prevented by detonation of the CO white dwarf accretor and the disruption of the system. With the most favourable conditions for the formation of AM CVn stars we find a current Galactic birth rate of 6.8 10^{-3} yr^{-1}. Unfavourable conditions give 1.1 10^{-3} yr^{-1}. The expected total number of the systems in the Galaxy is 9.4 10^{7} and 1.6 10^{7}, respectively. We model very simple selection effects to get some idea about the currently expected observable population and discuss the (quite good) agreement with the observed systems.