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Register a new userThe first ultracompact Roche lobe-filling hot subdwarf binary
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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 $P_{rm orb}=39.3401(1)$ 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 $T_{rm eff}=42,400pm300$ K and a surface gravity of $log(g)=5.77pm0.05$. 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 $M_{rm sdOB}=0.337pm0.015$ M$_odot$ and a white dwarf accretor with a mass $M_{rm WD}=0.545pm0.020$ M$_odot$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M$_odot$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{rm orb}$, $T_{rm eff}$, $log(g)$, and masses, we estimate a post-common envelope period of $P_{rm orb}approx150$ 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 $approx0.1$ M$_odot$ and will merge with its carbon/oxygen white dwarf companion after $approx17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star.
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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 $T_{rm eff}=33,700pm1000$ K and a surface gravity of $log(g)=5.54pm0.11$. 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 $M_{rm sdOB}=0.41pm0.04$ M$_odot$ and a WD mass of $M_{rm WD}=0.68pm0.05$ M$_odot$. The weak eclipses imply a WD black-body temperature of $63,000pm10,000$ K and a radius $R_{rm WD}=0.0148pm0.0020$ M$_odot$ 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 $approx1$ Myrs at a rate of $10^{-9} M_odot {rm yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards 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 $T_{rm eff}approx25,000$ 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 sub-luminous thermonuclear supernova or evolve into a massive single WD.
One of the important issues regarding the final evolution of stars is the impact of binarity. A rich zoo of peculiar, evolved objects are born from the interaction between the loosely bound envelope of a giant, and the gravitational pull of a companion. However, binary interactions are not understood from first principles, and the theoretical models are subject to many assumptions. It is currently agreed upon that hot subdwarf stars can only be formed through binary interaction, either through common envelope ejection or stable Roche-lobe overflow (RLOF) near the tip of the red giant branch (RGB). These systems are therefore an ideal testing ground for binary interaction models. With our long term study of wide hot subdwarf (sdB) binaries we aim to improve our current understanding of stable RLOF on the RGB by comparing the results of binary population synthesis studies with the observed population. In this article we describe the current model and possible improvements, and which observables can be used to test different parts of the interaction model.
The evolution of triples has not attracted much attention in the literature, although their evolution can be dramatically different from binaries and single stars. Triples are quite common, and we find that for about 1% of the triples in the Tokovinin catalogue of multiple stellar systems in the solar neighbourhood, the tertiary star will overflow its Roche lobe at some time in its evolution, before any of the inner stars leave the main sequence. For two of these systems, Xi Tauri and HD97131 we simulate in detail this phase of mass transfer, during which stellar evolution, gravitational dynamics and hydrodynamics all play an important role. We have used the Astrophysical Multi-purpose Software Environment (AMUSE) to solve these physical processes in a self-consistent way. The resulting evolution, mass transfer and the effects on the inner as well as on the outer orbit are profound, although it is not trivial to predict the eventual consequence of the phase of mass transfer and the appearance of the resulting system.
In this study, we concentrate on the formation and evolution of hot subdwarfs binaries through the stable Roche lobe overflow (RLOF) channel of intermediate-mass binaries. We aim at setting out the properties of hot subdwarfs and their progenitors, so that we can understand the formation and evolution of hot subdwarfs comprehensively. We have obtained the ranges of the initial parameters of progenitor binaries and the properties of hot subdwarfs through the stable RLOF channel of intermediate-mass binaries, e.g. mass, envelope mass and age of hot subdwarfs. We have found that hot subdwarfs could be formed through the stable Roche lobe overflow at main sequence and Hertzsprung gap. We have also found that some subdwarf B or OB stars have anomalous high mass (around 1 solar mass) with thick envelope (0.07 solar mass to 0.16 solar mass) in our models. By comparing our theoretical results with observations on the hot subdwarfs in open clusters, we suppose a quantity of hot subdwarfs in binary systems might be found in open clusters in the future.
Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESOs Very Large Telescope Interferometer, we measure precisely the diameters of several symbiotic and related stars: HD 352, HD 190658, V1261 Ori, ER Del, FG Ser, and AG Peg. These diameters - in the range of 0.6 to 2.3 milli-arcseconds - are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer. We also provide the first spectroscopic orbit of ER Del, based on CORAVEL and HERMES/Mercator observations. The system is found to have an eccentric orbit with a period of 5.7 years. In the case of the symbiotic star FG Ser, we find that the diameter is changing by 13% over the course of 41 days, while the observations of HD 352 are indicative of an elongation. Both these stars are found to have a Roche filling factor close to 1, as is most likely the case for HD 190658 as well, while the three other stars have factors below 0.5-0.6. Our observations reveal the power of interferometry for the study of interacting binary stars - the main limitation in our conclusions being the poorly known distances of the objects.
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