Stellar mergers are expected to take place in numerous circumstences in the evolution of stellar systems. In particular, they are considered as a plausible origin of stellar eruptions of the V838 Mon type. V1309 Sco is the most recent eruption of this type in our Galaxy. The object was discovered in September 2008. Our aim is to investigate the nature of V1309 Sco. V1309 Sco has been photometrically observed in course of the OGLE project since August 2001. We analyse these observations in different ways. In particular, periodogram analyses were done to investigate the nature of the observed short term variability of the progenitor. We find out that the progenitor of V1309 Sco was a contact binary with an orbital period of ~1.4 day. This period was decreasing with time. Similarly the light curve of the binary was also evolving, indicating that the system evolved toward its merger. The violent phase of the merger, marked by the systematic brightenning of the object, started in March 2008, i.e. half a year before the outburst discovery. We also investigate the observations of V1309 Sco during the outburst and the decline and show that they can be fully accounted for within the merger hypothesis. For the first time in the literature we show, from direct observations, that contact binaries indeed end up by merging into a single object, as it was suggested in numerous theoretical studies of these systems. Our study also shows that stellar mergers indeed result in eruptions of the V838 Mon type.
It was recently demonstrated that the eruption of V1309 Sco was a result of a merger of the components of a cool contact binary. We computed a set of evolutionary models of the detached binaries with different initial parameters to compare it with pre-burst observations of V1309 Sco. The models are based on our recently developed evolutionary model of the formation of cool contact binaries. The best agreement with observations was obtained for binaries with initial masses of 1.8-2.0 solar masses and initial periods of 2.5-3.1 d. The evolution of these binaries consists of three phases: at firs…
V1309 Sco was proposed to be a stellar merger and a common envelope transient based on the pre-outburst light curve of a contact eclipsing binary with a rapidly decaying orbital period. Using published data, I show that the period decay timescale P/Pdot of V1309 Sco decreased from ~1000 to ~170 years in less than about 6 years, which implies a very high value of second period derivative. I argue that V1309 Sco experienced an onset of dynamical mass loss through the outer Lagrange point, which eventually obscured the binary. The photosphere of the resulting continuous optically-thick outflow ex…
One very important object for understanding the nature of red novae is V1309 Sco. Its pre-outburst observations showed that, before its red-nova eruption in 2008, it was a contact binary quickly evolving to the merger of the components. It thus provided us with a direct evidence that the red novae result from stellar mergers. We analyse the spectral energy distribution (SED) of the object and its evolution with time. From various optical and infrared surveys and observing programmes carried out with OGLE, HST, VVV, Gemini South, WISE, Spitzer, and Herschel we constructed observed SED in 2010 a…
We report high-precision observations of the linear polarization of the F1$\,$III star $θ$ Scorpii. The polarization has a wavelength dependence of the form expected for a rapid rotator, but with an amplitude several times larger than seen in otherwise similar main-sequence stars. This confirms the expectation that lower-gravity stars should have stronger rotational-polarization signatures as a consequence of the density dependence of the ratio of scattering to absorption opacities. By modelling the polarization, together with additional observational constraints (incorporating a revised analy…
We present optical and infrared spectroscopy of V1309 Sco, an object that erupted in 2008 in a stellar-merger event. During the outburst, V1309 Sco displayed characteristics typical of red transients, a class of objects similar to V838 Mon. Our observations were obtained in 2009 and 2012, i.e. months and years after the eruption of V1309 Sco, and illustrate severe changes in the remnant, mainly in its circumstellar surroundings. In addition to atomic gas observed in earlier epochs, we identified molecular bands of TiO, VO, H$_2$O, ScO, AlO, and CrO. The infrared bands of CrO we analyse are the…
Stellar mergers are estimated to be common events in the Galaxy. The best studied stellar merger case to date is V1309 Sco (= Nova Scorpii 2008) which was originally misclassified as a Nova event. Later identified as the merger of the components of a cool overcontact binary system with 1.52 Msun and 0.16 Msun, V1309 Sco showed an initial period of P = 1.4 days before the merger. Post-outburst evolution demonstrated that V1309 Sco was unlike the typical Classical Novae and Symbiotic Recurrent Novae with significant dust production around it, and indicated that the system may become a post-AGB (…
Nova Sco 2008 (=V1309 Sco) is an example of a V838 Mon type eruption rather than a typical classical nova. This enigmatic object was recently shown to have resulted from the merger of two stars in a contact binary. It is the first stellar merger that was identified to be undergoing a common envelope transient. To understand the properties of its binary progenitor, the pre-outburst light curves were analyzed by using the W-D method. The photometric solution of the 2002 light curve shows that it is a deep contact binary (f = 89.5(+-40.5)%) with a mass ratio of 0.094. The asymmetry of the light c…
ZZ PsA is a neglected bright southern contact binary system with maximum V magnitude of 9.26. We present the first multi-band photometric analysis and find the system to be in deep contact (>95%) with an extremely low mass ratio of 0.078. The primary has a mass of 1.213 M? in keeping with its reported spectral class of F6. In order to determine if ZZ PsA is a merger candidate we outline the current status regarding the instability mass ratio and develop new relationship linking the mass of the primary to the instability mass ratio of the system and the degree of contact. We find that ZZ PsA al…
Inspired by the discovery of the red nova V1309 Sco (Nova Scorpii 2008) and the fact that its progenitor was a binary system with a rapidly decreasing orbital period, we have searched for period changes in OGLE binary stars. We have selected a sample of 22 462 short-period (P_orb < 4d) eclipsing binary stars observed toward the Galactic bulge by the OGLE-III survey in years 2001-2009. This dataset was extended with photometry from OGLE-II (1997-2000) and the first six years of OGLE-IV (2010-2015). For some stars, the data were supplemented with OGLE-I photometry (1992-1995). After close inspec…
VFTS102 has a projected rotational velocity ($>$500\,km s$^{-1}$) and would appear to be the fastest rotating O-type star. We show that its high rotational velocity could be understood within the framework of the binary merger. In the binary merger channel, the progenitor binary of VFTS102 would evolve into contact while two components are still on the main sequence, and then merge into a rapidly rotating single star. Employing Eggleton's stellar evolution code, we performed binary stellar evolution calculations and mapped out the initial parameters of the progenitor of VFTS102 in the orbital …
We study the initial conditions of a common envelope (CE) event resulting in a stellar merger. A merger's dynamics could be understood through its light curve, but no synthetic light curve has yet been created for the full evolution. Using the Smoothed Particle Hydrodynamics (SPH) code StarSmasher, we have created three-dimensional (3D) models of a 1.52 $M_\odot$ star that is a plausible donor in the V1309 Sco progenitor. The integrated total energy profiles of our 3D models match their initial one-dimensional (1D) models to within a 0.1 per cent difference in the top 0.1 $M_\odot$ of their en…
Detailed Investigation of a W UMa Contact Binary with an Ultralow Mass Ratio and a Third-Body as a Potential Merger Candidate
Binary Systems of South and North (BSN) Project（南北二元星项目）；Observatoire de Paris, CNRS, PSL（巴黎天文台，法国国家科学研究中心，巴黎文理研究大学）；Çanakkale Onsekiz Mart University（恰纳卡莱十三三月大学）；Instituto de Astronomía, UNAM（墨西哥国立自治大学天文学研究所）
It was recently demonstrated that contact binaries occur in globular clusters (GCs) only immediately below turn-off point and in the region of blue straggler stars (BSs). In addition, observations indicate that at least a significant fraction of BSs in these clusters was formed by the binary mass-transfer mechanism. The aim of our present investigation is to obtain and analyze a set of evolutionary models of cool, close detached binaries with a low metal abundance, which are characteristic of GC. We computed the evolution of 975 models of initially detached, cool close binaries with different …
The only well-studied red nova progenitor (V1309 Sco) was a contact binary with a 1.4-day period. The prospects for searching for similar systems, as well as stellar merger candidates in general, are explored in this work. The photospheric temperatures of 128 variables with periods P = 1.1 - 1.8 d classified as W UMa-type binaries are calculated using their colors listed in the SDSS catalog. A selection of 15 contact binaries with similar temperatures and periods as the V1309 Sco progenitor is thus compiled. The Kepler Eclipsing Binary Catalog is used to analyse systems with eclipse timing var…
Binary evolution at the extremes of mass-transfer efficiency: Contact, mergers, and population signatures
Heidelberger Institut für Theoretische Studien（海得尔堡理论研究所）；School of Mathematics, Statistics and Physics, Newcastle University（纽卡斯尔大学数学、统计与物理学院）；Universität Heidelberg, Department of Physics and Astronomy（海德堡大学物理与天文学系）；Institute for Mathematics, Astrophysics and Particle Physics, Radboud University（拉德堡德大学数学、天体物理与粒子物理研究所）；Zentrum für Astronomie, Astronomisches Rechen-Institut (ZAH/ARI), Heidelberg University（海德堡大学天文中心，天文计算研究所）
Binary and multiple stellar systems are numerous in our solar neighborhood with 80 per cent of the solar-type stars being members of systems with high order multiplicity. The Contact Binaries Towards Merging (CoBiToM) Project is a programme that focuses on contact binaries and multiple stellar systems, as a key for understanding stellar nature. The goal is to investigate stellar coalescence and merging processes, as the final state of stellar evolution of low-mass contact binary systems. Obtaining observational data of approximately 100 eclipsing binaries and multiple systems and more than 400…
KIC 9832227 is a contact binary whose 11 hr orbital period is rapidly changing. Based on the apparent exponential decay of its period, the two stars were predicted to merge in early 2022 resulting in a rare red nova outburst. Fortunately KIC 832227 was observed in 2003 as part of the NASA Ames pre-Kepler Vulcan Project to search for transiting exoplanets. We find that the Vulcan timing measurement does not agree with the previous exponential decay model. This led us to re-evaluate the other early epoch non-Kepler data sets, the Northern Sky Variability Survey (NSVS) and Wide Angle Search for P…
Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform understanding of binary evolutionary pathways and end products. We use light curves and new optical spectroscopy to conduct a pilot study of ten (near-)contact systems in the long-period ($P$>0.5 d) tail of close binaries in the Kepler field. We use PHOEBE light curve models to compute Bayesian probabilities on five principal system parameters. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the…
Multi band photometry and light curve analysis for two newly recognized contact binary systems, TYC 6995-813-1 and NSVS 13602901 are presented. Both were found to be of extreme low mass ratios 0.11 and 0.17, respectively. The secondary components of both systems show evidence of considerable evolution with elevated densities as well as both luminosity and radii well above their main sequence counterparts. Even in the absence of significant spot activity at least one of the systems, TYC 6995-813-1, shows features of magnetic and chromospheric activity. TYC 6995-813-1 is also determined to be a …
The evolution of contact binary star systems in mass asymmetry (transfer) coordinate is considered. The orbital period changes is explained by an evolution in mass asymmetry towards the symmetry (symmetrization of binary system). It is predicted that a decreasing and an increasing orbital periods are related, respectively, with the non-overlapping and overlapping stage of the binary star during its symmetrization. A huge amount of energy $ΔU\approx 10^{41}$ J is converted from the potential energy into internal energy of the stars during the symmetrization. As shown, the merger of stars in the…
Context: It has been predicted that the object KIC 9832227 - a contact binary star - will undergo a merger in 2022.2+/-0.7. We describe the near infrared spectrum of this object as an impetus to obtain pre-merger data. Aims: We aim to characterise (i)~the nature of the individual components of the binary and (ii)~the likely circumbinary environment, so that the merger - if and when it occurs - can be interpreted in an informed manner. Methods: We use infrared spectroscopy in the wavelength range 0.7micron - 2.5microns, to which we fit model atmospheres to represent the individual stars. We use…
Evidence suggests that the direct progenitor stars of some core-collapse supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some `superluminous supernovae' (SLSNe). We examine models in which massive stars gain mass soon after the end of core hydrogen burning. These are mainly intended to represent mergers following a brief contact phase during early Case B mass transfer, but may also represent stars which gain mass in the Hertzsprung Gap or extremely late during the main-sequence phase for other reasons. The post-accretion stars spend their core helium-burning phase as b…
The origin of dust grains in the interstellar medium is still open problem. \cite{Nicholls2013} found the presence of a significant amount of dust around V1309 Sco which maybe originate from the merger of a contact binary. We investigate the origin of dust around V1309 Sco, and suggest that these dust grains are efficiently produced in the binary-merger ejecta. By means of \emph{AGBDUST} code, we estimate that $\sim 5.2\times10^{-4} M_\odot$ of dust grains are produced, and their radii are $\sim 10^{-5}$ cm. These dust grains mainly are composed of silicate and iron grains. Because the mass of…
Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the smallest and least massive stars. These systems are faint and rare, and it is believed that they have reached a contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer and mass loss through stellar wind processes. This study is conducted in the frame of Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries, with the aim to investigate the possibility of them being red nov…
We first discuss a large set of evolutionary calculations of close binaries with a B-type primary at birth and with a period such that the Roche lobe overflow starts during the core hydrogen burning phase of the primary (intermediate mass and massive case A binaries). The evolution of both components is followed simultaneously allowing us to check for the occurrence of contact binaries. We consider various models to treat a contact system and the influence of these models on the predicted Algol-type system population is investigated. We critically discuss the available observations of Algol-ty…
Currently there is great interest in the outcomes and astrophysical implications of mergers of double degenerate binaries. In a commonly adopted approximation, the components of such binaries are represented by polytropes with an index n=3/2. We present detailed comparisons of stellar mass-transfer and merger simulations of polytropic binaries that have been carried out using two very different numerical algorithms --- a finite-volume "grid" code and a smoothed-particle hydrodynamics (SPH) code. We find that there is agreement in both the ultimate outcomes of the evolutions and the intermediat…
Photometric and Spectroscopic study of Ten Low Mass Ratio Contact Binary Systems: Orbital Stability, O'Connell Effect and Infra-red Calcium Line Filling
In this study, all unpublished time series photometric data of BM UMa ($q \sim$ 2.0, P = 0.2712\,d) from available archives were re-investigated together with new data taken from the TNT-2.4m of the Thai National Observatory (TNO). Based on period analysis, there is a short-term variation superimposed on the long-term period decrease. The trend of period change can be fitted with a downward parabolic curve indicating a period decrease at a rate of $\mathrm{d}P/\mathrm{d}t = -3.36(\pm 0.02)\times10^{-8}$ d $\textrm{yr}^{-1}$. This long-term period decrease can be explained by mass transfer from…
We present photometric analysis of three bright red nova progenitor contact binary systems: ASAS J082151-0612.6, TYC 7281-269-1 and TYC 7275-1968-1. The primary components in all three systems are solar-type low mass stars with radii somewhat larger than their Zero Age Main Sequence counterparts. The secondaries, as in most contact binary systems, have radii and luminosities well above their main sequence counterparts. All three have extreme low mass ratios ranging from 0.075 to 0.097 and two have high degrees of contact, in excess of 75%. All three have mass ratios and separations below the t…
The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerf…
Multi-band photometric observations of eleven totally eclipsing contact binaries were carried out. Applying the Wilson-Devinney program, photometric solutions were obtained. There are two W-subtype systems, which are CRTS J133031.1+161202 and CRTS J154254.0+324652, and the rest systems are A-subtype systems. CRTS J154254.0+324652 has the highest fill-out factor with 94.3$\%$, and the lowest object is CRTS J155009.2+493639 with only 18.9$\%$. The mass ratios of the eleven systems are all less than 0.1, which means that they are extremely low mass ratio binary systems. We performed period variat…
V4332 Sgr is a red transient (red nova) whose eruption was observed in 1994. The remnant of the eruption shows a unique optical spectrum: strong emission lines of atoms and molecules superimposed on an M-type stellar spectrum. The stellar-like remnant is presumably embedded in a disc-like dusty envelope seen almost face-on. The observed optical spectrum is assumed to result from interactions of the central-star radiation with dust and gas in the disc and outflows initiated in 1994. We reduced and measured a high-resolution (R = ~40 000) spectrum of V4332 Sgr obtained with VLT/UVES in April/May…
TIC 470710327, a massive compact hierarchical triple-star system, was recently identified by NASA's Transiting Exoplanet Survey Satellite (TESS). TIC 470710327 is comprised of a compact (1.10 d) circular eclipsing binary, with total mass $\approx 10.9-13.2\ \rm{M_{\odot}}$, and a more massive ($\approx 14-17\ \rm{M_{\odot}}$) eccentric non-eclipsing tertiary in a $52.04$ d orbit. Here we present a progenitor scenario for TIC 470710327 in which '2+2' quadruple dynamics result in Zeipel-Lidov-Kozai (ZLK) resonances that lead to a contact phase of the more massive binary. In this scenario, the tw…
CONTEXT: V1309 Sco is the only certain noncompact stellar merger, due to its indisputable preoutburst light curve matching that of a contact binary of almost equal mass stars. Therefore, anything that can be deduced from the existing observations serves as benchmark constraints for models. AIMS: We present some observational evidences to guide future hydrodynamical simulations and common envelope studies. METHODS: Using archive spectra taken at high and mid spectral resolution during the V1309 Sco outburst and late decline, together with the inferential methods we developed to study nova eject…
BAT99 126 is a multiple system in the Large Magellanic Cloud containing a Wolf-Rayet (WR) star, which has a reported spectroscopic (orbital) period of 25.5 days and a photometric (orbital) period of 1.55 days, and hence is potentially one of the shortest WR binaries known to date. Such short-period binary systems containing a WR star in low-metallicity environments are prime candidate progenitors of black-hole (BH) mergers. By thoroughly analysing the spectroscopic and photometric data, we aim to establish the true multiplicity of BAT99 126, characterise the orbit(s) of the system, measure the…
Photometric observations for the totally eclipsing binary system TYC 4002-2628-1, were obtained between November 2020 and November 2021. To determine the stellar atmospheric parameters, a spectral image was taken with the 2.16 m telescope at National Astronomical Observatory of China (NAOC). TYC 4002-2628-1 is a low-amplitude (about 0.15 mag for $V$ band), short-period (0.3670495 d), contact eclipsing binary with a total secondary eclipse. Intrinsic light curve variations and the reversal of the O'Connell effect are detected in the light curves, which may be due to spot activity. Based on the …
A new sample of massive B-type contact binary candidates from the OGLE survey of the Magellanic Clouds
Instituto de Astrofísica de Canarias（加那利群岛天体物理研究所）；Universidad de La Laguna（拉古纳大学）；Columbia University（哥伦比亚大学）；Lund University（隆德大学）；Nicolaus Copernicus University（尼古拉·哥白尼大学）；University of Arizona（亚利桑那大学）；Argelander Institut für Astronomie（阿尔兰德天文学研究所）；Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）
Kerzendorf et al. (2012) recently reported the startling discovery of a metal-poor ([Fe/H]=-1 +/- 0.4) A-type star near the center of the Tycho supernova remnant. We propose two possible explanations. In the first, Tycho B is a blue straggler, formed from the merger of a close K- or G-type binary system, which was previously in a quadruple system with the binary that produced SN 1572. Both binaries were likely brought to tidal contact by Kozai-Lidov oscillations acting in concert with tidal friction. Analogous progenitor systems may include CzeV343, VW LMi, and KIC 4247791. In the second, Tych…
Nicolaus Copernicus Astronomical Center（尼古拉·哥白尼天文学中心）；Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB)（巴塞罗那大学宇宙科学研究所）；Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB)（巴塞罗那大学量子物理与天体物理学系）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究研究所）
