The first ultracompact Roche lobe-filling hot subdwarf binary

We report the discovery of the first short period binary in which a hot subdwarf star (sdOB) fills its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic Plane named the Zwicky Transient Facility and exhibits a period of Porb=39.3401(1) min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of Teff=42,400±300 K and a surface gravity of log(g)=5.77±0.05. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass MsdOB=0.337±0.015 M⊙ and a white dwarf accretor with a mass MWD=0.545±0.020 M⊙. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5−2.8 M⊙ star lost its envelope when crossing the Hertzsprung Gap. To match its current Porb, Teff, log(g), and masses, we estimate a post-common envelope period of Porb≈150 min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 M⊙ and will merge with its carbon/oxygen white dwarf companion after ≈17 Myr and presumably explode as a thermonuclear supernova or form an R CrB star

A dense 0.1-solar-mass star in a 51-minute-orbital-period eclipsing binary

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes1,2,3,4,5,6,7,8,9. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf10,11,12,13,14, and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs13,14,15,16,17,18. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs

A new class of Roche lobe–filling hot subdwarf binaries

We present the discovery of the second binary with a Roche lobe–filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of T eff = 33,700 ± 1000 K and a surface gravity of log(g) = 5.54 ± 0.11. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of M sdOB = 0.41 ± 0.04 M ⊙ and a WD mass of M WD = 0.68 ± 0.05 M ⊙. The weak eclipses imply a WD blackbody temperature of 63,000 ± 10,000 K and a radius R WD = 0.0148 ± 0.0020 R ⊙ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for ≈1 Myr at a rate of 10−9 M ⊙ yr−1, which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in ≈30 Myr. We suggest that Galactic reddening could bias discoveries toward preferentially finding Roche lobe–filling systems during the short-lived shell-burning phase. Studies using reddening-corrected samples should reveal a large population of helium core–burning hot subdwarfs with T eff ≈ 25,000 K in binaries of 60–90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational-wave emission and explode as a subluminous thermonuclear supernova or evolve into a massive single WD

Candidate tidal disruption event AT2019fdr coincident with a high-energy neutrino

High Energy Astrophysic

A tidal disruption event coincident with a high-energy neutrino

High Energy Astrophysic

The Palomar Transient Factory Sky2Night programme

Contains fulltext : 201613.pdf (publisher's version ) (Open Access

Constraining Type Ia supernova explosions and early flux excesses with the Zwicky Transient Factory

International audienceIn the new era of time-domain surveys, Type Ia supernovae are being caught sooner after explosion, which has exposed significant variation in their early light curves. Two driving factors for early-time evolution are the distribution of ^56Ni in the ejecta and the presence of flux excesses of various causes. We perform an analysis of the largest young SN Ia sample to date. We compare 115 SN Ia light curves from the Zwicky Transient Facility to the turtls model grid containing light curves of Chandrasekhar mass explosions with a range of ^56Ni masses, ^56Ni distributions, and explosion energies. We find that the majority of our observed light curves are well reproduced by Chandrasekhar mass explosion models with a preference for highly extended ^56Ni distributions. We identify six SNe Ia with an early-time flux excess in our gr-band data (four ‘blue’ and two ‘red’ flux excesses). We find an intrinsic rate of 18 ± 11 per cent of early flux excesses in SNe Ia at z < 0.07, based on three detected flux excesses out of 30 (10 per cent) observed SNe Ia with a simulated efficiency of 57 per cent. This is comparable to rates of flux excesses in the literature but also accounts for detection efficiencies. Two of these events are mostly consistent with circumstellar material interaction, while the other four have longer lifetimes in agreement with companion interaction and ^56Ni-clump models. We find a higher frequency of flux excesses in 91T/99aa-like events (44 ± 13 per cent)

SN 2019odp: A Massive Oxygen-Rich Type Ib Supernova

International audienceWe present and analyze observations of the Type Ib supernova (SN) 2019odp (a.k.a ZTF19abqwtfu) covering epochs within days of the explosion to the nebular phase at 360 d post-explosion. We discuss them in the context of recombination cooling emission for the early excess emission and consider progenitor models based on the nebular phase spectra. Our observations include photometric observations mainly in the optical and low to medium-resolution spectroscopic observations covering the complete observable time-range. We expand on existing methods to derive oxygen mass estimates from nebular phase spectroscopy. Our spectroscopic observations confirm the presence of He in the SN ejecta and we thus (re)classify it as a Type Ib supernova. From the pseudo-bolometric lightcurve we estimate a high ejecta mass $M_\text{ej} \sim 4 - 7~M_\odot$. The high ejecta mass, large nebular [O I]/[Ca II] line flux ratio ($1.2 - 1.9$) and an oxygen mass above $\gtrapprox 0.5\, M_\odot$ point towards a progenitor with pre-explosion mass higher than $18\,M_\odot$. The compact nature of the progenitor ($\lesssim 10\,R_\odot$) suggests a Wolf-Rayet (WR) star as progenitor

SN 2019odp: A Massive Oxygen-Rich Type Ib Supernova

International audienceWe present and analyze observations of the Type Ib supernova (SN) 2019odp (a.k.a ZTF19abqwtfu) covering epochs within days of the explosion to the nebular phase at 360 d post-explosion. We discuss them in the context of recombination cooling emission for the early excess emission and consider progenitor models based on the nebular phase spectra. Our observations include photometric observations mainly in the optical and low to medium-resolution spectroscopic observations covering the complete observable time-range. We expand on existing methods to derive oxygen mass estimates from nebular phase spectroscopy. Our spectroscopic observations confirm the presence of He in the SN ejecta and we thus (re)classify it as a Type Ib supernova. From the pseudo-bolometric lightcurve we estimate a high ejecta mass $M_\text{ej} \sim 4 - 7~M_\odot$. The high ejecta mass, large nebular [O I]/[Ca II] line flux ratio ($1.2 - 1.9$) and an oxygen mass above $\gtrapprox 0.5\, M_\odot$ point towards a progenitor with pre-explosion mass higher than $18\,M_\odot$. The compact nature of the progenitor ($\lesssim 10\,R_\odot$) suggests a Wolf-Rayet (WR) star as progenitor