We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova with a fast-evolving light curve indicating an extremely low ejecta mass ( 0.33 M ) and low kinetic energy ( 1.3 × 1050 ...erg). Early-time (<4 days after explosion) photometry reveals evidence of shock cooling from an extended helium-rich envelope of ∼0.1 M located ∼1.2 × 1013 cm from the progenitor. Early-time He II line emission and subsequent spectra show signatures of interaction with helium-rich circumstellar material, which extends from 5 × 1013 cm to 2 × 1016 cm. We interpret SN2019dge as a helium-rich supernova from an ultra-stripped progenitor, which originates from a close binary system consisting of a mass-losing helium star and a low-mass main-sequence star or a compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the range of 1400-8200 Gpc − 3 yr − 1 (i.e., 2%-12% of core-collapse supernova rate). This can be compared to the observed coalescence rate of compact neutron star binaries that are not formed by dynamical capture.
Abstract
Recent discoveries of double neutron star (DNS) mergers and ultra-stripped supernovae (SNe) raise the questions of their origin and connection. We present the first 1D model of a DNS ...progenitor system that is calculated self-consistently until an ultra-stripped iron core collapse. We apply the
MESA
code starting from a post-common-envelope binary consisting of a 1.35
M
⊙
NS and a 3.20
M
⊙
zero-age main-sequence helium star and continue the modeling via Case BB Roche-lobe overflow until the infall velocity of the collapsing iron core exceeds 1000 km s
−1
. The exploding star has a total mass of ∼1.90
M
⊙
, consisting of a ∼0.29
M
⊙
He-rich envelope embedding a CO core of ∼1.61
M
⊙
and an iron-rich core of ∼1.50
M
⊙
. The resulting second-born NS has an estimated mass of ∼1.44
M
⊙
, and we discuss the fate of the post-SN system, as well as the mild recycling of the first-born NS. Depending on the initial conditions, this family of systems is anticipated to reproduce the DNS mergers detected by the LIGO network.
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 Teff = 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 MsdOB = 0.41 0.04 M and a WD mass of MWD = 0.68 0.05 M . The weak eclipses imply a WD blackbody temperature of 63,000 10,000 K and a radius RWD = 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 Teff 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.
Abstract
A large fraction of known exoplanets have short orbital periods where tidal excitation of gravity waves within the host star causes the planets’ orbits to decay. We study the effects of ...tidal resonance locking, in which the planet locks into resonance with a tidally excited stellar gravity mode. Because a star’s gravity mode frequencies typically increase as the star evolves, the planet’s orbital frequency increases in lockstep, potentially causing much faster orbital decay than predicted by other tidal theories. Due to nonlinear mode damping, resonance locking in Sun-like stars likely only operates for low-mass planets (
M
≲ 0.1
M
Jup
), but in stars with convective cores it can likely operate for all planetary masses. The orbital decay timescale with resonance locking is typically comparable to the star’s main-sequence lifetime, corresponding to a wide range in effective stellar quality factor (10
3
≲
Q
′ ≲ 10
9
), depending on the planet’s mass and orbital period. We make predictions for several individual systems and examine the orbital evolution resulting from both resonance locking and nonlinear wave dissipation. Our models demonstrate how short-period massive planets can be quickly destroyed by nonlinear mode damping, while short-period low-mass planets can survive, even though they undergo substantial inward tidal migration via resonance locking.
Abstract
Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often ...difficult. In this work, we conduct a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual Small Magellanic Cloud star and Thorne–Żytkow Object (TŻO) candidate HV 2112. Using light curves from ASAS-SN, we identify 38 stars with surface temperatures
T
< 4800 K, luminosities
(
L
/
L
⊙
) > 4.3, variability periods >400 days, and variability amplitudes Δ
V
> 2.5 mag. Eleven of these stars possess the distinctive double-peaked light-curve morphology of HV 2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large-amplitude pulsations. For the other 11 stars, we derive current masses of ∼5–10
M
⊙
, below the theoretical minimum mass of ∼15
M
⊙
for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates (MLRs), and periods of these stars are consistent with predictions for super-asymptotic giant branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of ∼(1–7) × 10
4
yr, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.
ABSTRACT
Common envelope (CE) evolution, which is crucial in creating short-period binaries and associated astrophysical events, can be constrained by reverse modelling of such binaries’ formation ...histories. Through analysis of a sample of well-constrained white dwarf (WD) binaries with low-mass primaries (seven eclipsing double WDs, two non-eclipsing double WDs, one WD-brown dwarf), we estimate the CE energy efficiency αCE needed to unbind the hydrogen envelope. We use grids of He- and CO-core WD models to determine the masses and cooling ages that match each primary WD’s radius and temperature. Assuming gravitational wave-driven orbital decay, we then calculate the associated ranges in post-CE orbital period. By mapping WD models to a grid of red giant progenitor stars, we determine the total envelope binding energies and possible orbital periods at the point CE evolution is initiated, thereby constraining αCE. Assuming He-core WDs with progenitors of 0.9–2.0 M⊙, we find αCE ∼ 0.2–0.4 is consistent with each system we model. Significantly higher values of αCE are required for higher mass progenitors and for CO-core WDs, so these scenarios are deemed unlikely. Our values are mostly consistent with previous studies of post-CE WD binaries, and they suggest a nearly constant and low envelope ejection efficiency for CE events that produce He-core WDs.
We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just 8.8 minutes, the second known eclipsing binary with an orbital period of less than 10 ...minutes. The system likely consists of two low-mass white dwarfs and will merge in approximately 400,000 yr to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its 6.91 minute counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational-wave sources detectable by the space-based gravitational-wave detector the Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of , LISA should detect the source within its first few months of operation and achieve a signal-to-noise ratio of 63 7 after 4 yr. We find component masses of and , radii of and , and effective temperatures of and . We determine all of these properties and the distance to this system using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint (r > 21 mAB) gravitational-wave sources that the Vera Rubin Observatory and LISA should identify.
ABSTRACT
Rotation and mass-loss are crucially interlinked properties of massive stars, strongly affecting their evolution and ultimate fate. Massive stars rotating near their break-up limit shed mass ...centrifugally, creating Be stars with circumstellar discs and possibly driving outbursts. Using the mesa stellar evolution code, we examine the effects of efficient angular momentum transport on the main-sequence and post-main-sequence rotational evolution of massive stars. In rapid rotators, angular momentum transported from the contracting core to the expanding envelope can spin-up the surface layers past the break-up rate, particularly for stars near (or beyond) the end of the main-sequence and in low-metallicity environments. We also demonstrate that centrifugal instabilities could arise in rapidly rotating massive stars, potentially triggering the S Doradus outbursts observed in luminous blue variable stars. Prior mass accretion from a binary companion increases both the likelihood and the intensity of centrifugal mass-loss. We discuss implications for massive stellar evolution, Be stars, and luminous blue variables.
ABSTRACT
Recent asteroseismic measurements have revealed a small population of stars in close binaries, containing primaries with extremely slow rotation rates. Such stars defy the standard ...expectation of tidal synchronization in such systems, but they can potentially be explained if they are trapped in a spin-orbit equilibrium known as Cassini state 2 (CS2). This state is maintained by orbital precession due to an outer tertiary star, and it typically results in a very sub-synchronous rotation rate and high degree of spin-orbit misalignment. We examine how CS2 is affected by magnetic braking and different types of tidal dissipation. Magnetic braking results in a slower equilibrium rotation rate, while tidal dissipation via gravity waves can result in a slightly higher rotation rate than predicted by equilibrium tidal theory, and dissipation via inertial waves can result in much slower rotation rates. For seven binary systems with slowly rotating primaries, we predict the location of the outer tertiary predicted by the CS2 theory. In five of these systems, a tertiary companion has already been detected, although it is closer than expected in three of these, potentially indicating tidal dissipation via inertial waves. We also identify a few new candidate systems among a population of eclipsing binaries with rotation measurements via spot modulation.
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