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Contact binary
Contact binary
Contact binary
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Artist's rendering of the massive contact binary star VFTS 352, located in the Large Magellanic Cloud

In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overcontact binary.[1][2] The term "contact binary" was introduced by astronomer Gerard Kuiper in 1941.[3] Almost all known contact binary systems are eclipsing binaries;[4] eclipsing contact binaries are known as W Ursae Majoris variables, after their archetype star, W Ursae Majoris.[5]

In a contact binary, both stars have filled their Roche lobes, allowing the more massive primary component to transfer both mass and luminosity to the secondary member. As a result, the components in a contact binary often have similar effective temperatures and luminosities, regardless of their respective masses. The rate of energy transfer between the components is dependent on their mass ratio and luminosity ratio. In cases where the stars are in geometric contact but the thermal contact is poor, there can exist wide differences between their respective temperatures.[6]

Contact binaries are not to be confused with common envelopes. Whereas the configuration of two touching stars in a contact binary has a typical lifetime of millions to billions of years, the common envelope is a dynamically unstable phase in binary evolution that either expels the stellar envelope or merges the binary in a timescale of months to years.[7]

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Contact binary

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A contact binary is a system in which the two component stars are so close that they fill their respective lobes and share a common convective envelope, allowing for direct interaction between their outer atmospheres. These systems represent an extreme stage of binary evolution, where the stars' proximity leads to and energy exchange, often resulting in observable photometric and spectroscopic variability. Contact binaries are classified as overcontact systems when both stars exceed their Roche lobes, forming a single shared envelope that envelops the pair; this configuration distinguishes them from binaries, where only one star overflows its lobe. The prototype for this class is the W Ursae Majoris (W UMa) system, a short-period eclipsing variable where the components are late-type main-sequence stars of spectral classes F, G, or K, with orbital periods typically ranging from 0.2 to 1 day. Due to their geometry, contact binaries produce continuous light curves with two shallow minima from mutual eclipses, and they often emit X-rays from coronal activity enhanced by magnetic interactions in the shared envelope. These systems are common among close binaries, comprising a significant fraction of eclipsing variables detected in surveys, and they play a key role in understanding , including pathways to common-envelope phases and potential mergers. Observations suggest that contact binaries form through loss primarily via magnetic braking, leading to orbital shrinkage until contact is achieved. Their study provides insights into mass-ratio distributions, with many showing nearly equal masses, and luminosity transfer from the primary to the secondary, maintaining .

Definition and Characteristics

Definition

A contact binary is a binary star system in which both component stars fill their s and share a common gaseous envelope, resulting in continuous mass exchange and causing the system to appear as a single, distorted object from a distance. The represents the gravitational boundary around each star in a , consisting of an surface where material is bound primarily to that star; it is shaped like a teardrop and terminates at the inner L1, the between the stars where their gravitational influences are equal. Filling the occurs when a star expands such that its reaches the L1 point, enabling the transfer of material through this point to the companion star. This configuration distinguishes contact binaries from other binary types: detached binaries maintain separation greater than their combined Roche lobes, preventing direct interaction or , while binaries involve only one star filling its lobe, leading to unidirectional overflow from the donor to the acceptor. In contrast, contact binaries feature both stars overfilling their lobes, with the shared envelope facilitating bidirectional energy and . Contact binaries were first recognized in the early through observations of eclipsing variables, exemplified by the discovery of the short-period light variations in the prototype system W Ursae Majoris by Gustav Müller and Paul Kempf in 1903. The term "contact binary" was formally introduced by in 1941 to describe such closely interacting systems.

Physical Characteristics

While the following describes typical low-mass, late-type systems, contact binaries also include higher-mass, early-type examples with hotter primaries and more disparate properties. Contact binaries feature a shared convective that envelops both stellar components, causing their photospheres to come into direct contact at the inner (L1), and imparting a distinctive peanut-shaped or egg-like morphology to the system as a whole. This common facilitates continuous energy and mass exchange between the two stars, maintaining across their surfaces. Contact binaries typically have mass ratios ranging from about 0.1 to 1.0, with many systems showing values below 0.5; total system masses for late-type examples are between 0.5 and 2.5 solar masses (M_\sun). Their orbital periods are short, spanning 0.2 to 1 day, while each component has a radius of 1 to 3 solar radii (R_\sun), reflecting their evolved, bloated structures due to the envelope sharing. In typical late-type contact binaries, the components possess similar effective temperatures between 4000 and 7000 K, corresponding to F- to K-type spectral classifications, resulting in nearly identical surface brightness. Hot spots often appear at the interface where the envelopes meet, marking regions of enhanced heating from dynamical interactions. Dynamically, these systems achieve tidal locking, with rotational periods synchronized to the orbital period, minimizing shear in the shared envelope. Energy dissipation within the convective common envelope drives rapid evolutionary changes, including adjustments in mass ratio and fill-out factor, on timescales comparable to the thermal adjustment of the secondary star. The condition for contact is defined by both stars filling their Roche lobes, approximated by the volume-equivalent radius formula: RLa≈0.49q2/30.6q2/3+ln⁡(1+q1/3)\frac{R_L}{a} \approx \frac{0.49 q^{2/3}}{0.6 q^{2/3} + \ln(1 + q^{1/3})} where RLR_L is the Roche lobe radius, aa is the semi-major axis, and qq is the mass ratio (secondary to primary mass). This relation ensures the photospheres overflow the L1 point, enabling the envelope connection.

Classification

W-type Contact Binaries

W-type contact binaries represent the primary subtype of overcontact systems within the W Ursae Majoris (W UMa) class of eclipsing variables, characterized by two main-sequence components of late spectral types ranging from F7 to K or M, both filling their Roche lobes and enveloped in a shared gaseous structure. This classification, introduced by Binnendijk in 1970, distinguishes them from hotter A-type systems based on light curve morphology, where the deeper primary minimum arises from the eclipse of the hotter, smaller secondary by the cooler, larger primary. These binaries typically exhibit orbital periods between 0.2 and 0.5 days, with a median value of approximately 0.35 days, reflecting their compact configurations driven by angular momentum loss mechanisms. The components of W-type systems possess shallow convective envelopes, which facilitate stable, conservative through the common envelope without significant loss, maintaining between the . Their total luminosities are comparable to those of single main-sequence of equivalent type and , owing to the efficient redistribution in the shared envelope. Fill-out factors, measuring the degree of overcontact as the fraction of the common envelope volume occupied by the , often exceed 20% in these systems, indicating deep contact and enhanced envelope sharing. W-type contact binaries are the most prevalent form of overcontact systems, accounting for nearly half of all cataloged W UMa binaries in comprehensive samples, with a relative frequency of about 0.2% among main-sequence FGK stars in the solar neighborhood. Their stability stems from comparable component masses, typically yielding mass ratios between 0.3 and 0.7, which prevent the onset of Darwin instability—a tidal disruption mechanism that would otherwise lead to rapid merger—thus enabling these systems to persist for billions of years without immediate coalescence. Notable examples include W Ursae Majoris itself, a prototypical system with a period of 0.333 days and spectral types around G0 for both components.

A-type Contact Binaries

A-type contact binaries represent a subtype of overcontact systems in which both stellar components fill their lobes and share a common envelope, distinguished by light curves where the deeper primary minimum arises from the of the hotter, more massive primary by the cooler secondary. This classification, originally proposed by Binnendijk based on photometric observations, contrasts with W-type systems where the secondary is hotter. These binaries typically feature components of early spectral types (A to F), with effective temperatures reaching up to approximately 10,000 for the primary, and luminosities higher than those of their W-type counterparts due to their more massive constituents (total masses often exceeding 1.5 solar masses). Orbital periods for A-type systems generally range from 0.5 to 1 day, longer than the shorter periods (<0.5 day) typical of W-type binaries, reflecting their larger separations and lower densities. Key structural traits of A-type contact binaries include deeper radiative envelopes compared to the convective envelopes dominant in W-type systems, which facilitates more rapid rates between components owing to reduced in radiative regions. This envelope structure leads to a higher degree of overcontact, with fill-out factors often exceeding 20-30%, though variability in fill-out can occur due to dynamical instabilities. The systems exhibit smaller ratios (typically q < 0.5) and more pronounced differences between components, with the primary maintaining higher surface temperatures despite shared envelope heating. These characteristics contribute to asymmetric light variations and potential hot-spot activity from streams. Evolutionarily, A-type contact binaries are generally post-main-sequence systems, where at least one component—often the initially less massive secondary—has evolved into a or giant phase, driving the onset of overcontact through overflow. This evolutionary stage implies prior detached or phases, with the secondary's expansion reversing the and promoting instability. Such systems are prone to asymmetric mass loss via stellar winds or ejection events, which can alter orbital parameters and lead to rapid evolution toward merger. In dense environments like star clusters, their merger products may manifest as blue stragglers, appearing younger and more luminous than cluster main-sequence stars. A-type contact binaries are less frequent than W-type systems, comprising roughly 20-30% of known overcontact binaries, as their formation requires specific evolutionary paths involving higher initial masses and loss mechanisms like magnetic braking. They are preferentially detected in older stellar populations, such as galactic fields or globular clusters, where evolved progenitors are abundant. Detection relies on photometric surveys revealing their distinct symmetries, though variable fill-out factors can occasionally mimic detached phases, complicating identification without spectroscopic confirmation of radial velocities and envelope sharing.

Formation

Initial Formation Mechanisms

Contact binaries originate from primordial binary systems formed during the early stages of star formation, where most emerge in multiple configurations through the gravitational fragmentation of dense cores. These cores, embedded within turbulent , collapse under gravity, often splitting into binary or multiple protostellar systems due to and thermal support variations. Turbulent fragmentation promotes the formation of such multiples, with simulations indicating that approximately 50% of form in binary or higher-order systems. For contact binaries, the initial orbital separations must be exceptionally small, typically less than 10 solar radii (about 0.05 ), to allow later envelope contact without invoking post-main-sequence evolution. Close binaries with such tight orbits arise primarily from disk instability in protostellar disks, where gravitational fragmentation produces companion clumps that migrate inward via accretion torques in circumbinary disks. Hydrodynamical simulations demonstrate that turbulent conditions in these disks lead to fragmentation at initial separations of tens of , followed by rapid inward migration driven by transfer, resulting in final periods short enough (P < 10 days) for ~10-20% of systems to become precursors to contact binaries. Dynamical capture during the pre-main-sequence phase in dense environments can also contribute, though it is less dominant for the closest pairs. Environmental factors significantly influence these primordial processes, with dense star-forming clusters favoring closer initial binaries through enhanced three-body interactions and disk instabilities. In regions like the Cluster, high stellar densities (~10^3-10^4 stars pc^{-3}) promote the survival and tightening of primordial binaries via and migration. Magnetic fields, with strengths typically 10–100 μG in protostellar cores, play a key role by aligning stellar spins with orbits, suppressing excessive disk expansion, and facilitating fragmentation scales that yield tighter binaries, reducing separations by up to a factor of two compared to non-magnetized cases. Observational evidence from young star-forming regions supports these mechanisms, as surveys of Class 0/I protostars in the reveal a high fraction (~40-60%) of binaries with separations below 100 AU, including tight orbits indicative of disk fragmentation and early migration. and radio observations confirm bimodal separation distributions peaking at ~75 AU (close pairs) and ~3000 AU (wider), consistent with simulations of primordial formation without significant dynamical hardening post-formation. These young systems in Orion, aged <1 Myr, serve as direct precursors to evolved contact binaries, highlighting the efficiency of initial close-pair formation in clustered environments.

Binary Evolution Leading to Contact

Contact binaries typically form through the evolution of initially detached binary systems consisting of low-mass stars, where the components gradually approach each other until their envelopes make contact. As the primary star evolves off the , its radius expands due to hydrogen shell burning, eventually filling its and initiating to the secondary. Concurrently, the orbit shrinks due to loss, primarily through magnetic braking in convective envelopes of cool stars, with a minor contribution from emission in tighter systems. This dual process—stellar expansion and —bridges the detached phase to overcontact, with the secondary often expanding in response to mass accretion, leading both stars to overfill their s and share a common envelope. The timescales for this evolution vary with initial and stellar masses but generally span 1 to 10 billion years for low-mass systems (total mass < 2 M⊙). For binaries resembling solar-type stars (masses ~0.8–1.2 M⊙) with initial periods of 2–3 days, contact is reached in approximately 5 Gyr, aligning with the age of the Galactic disk and allowing such systems to form continuously over . Magnetic braking dominates the orbital shrinkage in these non-compact systems, acting over gigayear scales, while gravitational waves become relevant only for periods below ~0.2 days but play a negligible role prior to contact. Mass ratio plays a crucial role in the pathway to contact. In systems with nearly equal masses (q ≈ 1), evolution proceeds symmetrically, with both stars expanding comparably and reaching overcontact simultaneously without a prolonged semi-detached phase. For unequal masses (q << 1), the more massive primary fills its Roche lobe first, leading to a temporary semi-detached configuration before the secondary's response drives full contact; this asymmetry can delay overcontact but accelerates overall evolution due to enhanced angular momentum loss. Orbital decay due to magnetic braking can be approximated by relating the rate of period change to loss, often modeled in terms of an effective loss rate carrying specific angular momentum from the . A simplified form for the fractional period change is given by P˙P=−6M1+M2Menv×(JorbJtotal)M˙,\frac{\dot{P}}{P} = -6 \frac{M_1 + M_2}{M_{\rm env}} \times \left( \frac{J_{\rm orb}}{J_{\rm total}} \right) \dot{M}, where M1M_1 and M2M_2 are the component , MenvM_{\rm env} is the envelope , JorbJ_{\rm orb} and JtotalJ_{\rm total} are the orbital and total , and M˙\dot{M} represents the effective loss rate induced by magnetic braking. This formulation highlights how braking preferentially removes orbital , shrinking the separation until contact.

Evolutionary Processes

Mass Transfer and Envelope Sharing

In contact binaries, mass transfer primarily occurs via a narrow stream of material originating from the inner (L1), where the more massive primary component overflows its . This stream, deflected by the , circulates within the common envelope, transporting and high-entropy material around the less massive secondary component before partially returning to the primary. As a result, the circulation process effectively equalizes the surface temperatures and chemical compositions between the two stars, maintaining thermal and compositional homogeneity across the shared envelope. Typical rates in these systems range from approximately 10−710^{-7} to 10−510^{-5} solar masses per year, driven by evolutionary expansion of the primary and loss mechanisms such as magnetic braking. The shared envelope in contact binaries features vigorous convective mixing in the outer layers, which homogenizes the composition and thermal structure, preventing significant gradients between the components. This mixing is facilitated by large-scale circulations induced by baroclinic instabilities from unequal internal heating at the boundaries. At the points of stellar contact—often manifesting as a "peanut-shaped" —frictional heating and impact of the circulating stream generate localized hot spots, typically on the facing hemispheres of both stars, with temperature excesses of several thousand . These hot spots contribute to the observed asymmetries and enhanced emission in and wavelengths. Stability in the contact phase is governed by the relative timescales of thermal relaxation and orbital motion, with the thermal relaxation timescale being shorter than the orbital period, enabling the system to achieve a quasi-equilibrium configuration through ongoing adjustments in the envelope structure. This balance inhibits dynamical instabilities, particularly in near-equal-mass systems where the mass ratio prevents excessive donor expansion. In such cases, runaway mass transfer is avoided, as the convective envelope's response to perturbations dissipates energy efficiently without leading to rapid detachment or merger. Seminal analyses, including thermal relaxation oscillation models, underscore how these dynamics sustain the overcontact state over evolutionary timescales. Observationally, the and envelope sharing manifest in synchronized rotation of both components with the , due to efficient tidal torques within the common envelope. Orbits are highly circular, with eccentricities typically less than 0.01, reflecting rapid tidal circularization and the absence of significant perturbations during the stable contact phase. These signatures are evident in photometric light curves showing continuous variability and in curves indicating co-rotation without detectable eccentricity.

End States and Mergers

Contact binaries reach their end states through a variety of merger scenarios triggered by excessive mass loss, such as via stellar winds or overflow through the outer L2, or by dynamical instabilities like the Darwin instability, which causes and inspiral when the orbital becomes comparable to the stars' spin . These processes lead to the eventual coalescence of the components, with the timescale potentially accelerated in the final phases by emission, particularly for systems with short orbital periods. The -driven inspiral timescale for such systems can be approximated as τGW≈1012(P1 day)8/3(MM⊙)−5/3 years,\tau_{\rm GW} \approx 10^{12} \left( \frac{P}{1\,{\rm day}} \right)^{8/3} \left( \frac{M}{{\rm M}_\odot} \right)^{-5/3} \, {\rm years}, where PP is the orbital period and MM is the total mass; this provides a prediction for the end-state merger timing, though for typical contact binary parameters (P∼0.3P \sim 0.3–1 day, M∼1M \sim 1–20 M⊙_\odot), τGW\tau_{\rm GW} often exceeds the Hubble time except in the closest configurations. The primary outcomes of these mergers are single stars that appear as rapid rotators, often manifesting as blue stragglers due to the combined mass and rejuvenated appearance on the Hertzsprung-Russell diagram. Approximately 10–30% of single white dwarfs in the field may originate as merger products, suggesting a notable contribution to the population of apparently single stars from binary interactions, with higher fractions (up to ~30%) estimated for massive main-sequence stars. Recent evolutionary models of contact binaries, particularly those using detailed simulations like MESA, indicate that a substantial fraction—over 50% in nuclear-timescale integrations for massive systems—ultimately merge within the Hubble time, driven by non-conservative and tidal effects; for primaries in the range 4.8–20.8 M⊙, at least 40% enter contact phases, with 12–19% leading to mergers. The remnants exhibit enhanced surface due to deep mixing during the merger, dredging up - and nitrogen-rich material from the cores to the surface, which alters their chemical profiles compared to single-star . These models predict period-mass ratio distributions consistent with observations, with mergers producing rapidly rotating that may display or other anomalies from the violent interaction.

Observation and Detection

Photometric Observations

Contact binaries are detected primarily through their photometric variability, which arises from their eclipsing nature. Due to the close proximity and shared of the components, these systems exhibit continuous s throughout the orbital cycle, resulting in curves characterized by two minima of nearly equal depth and lacking flat bottoms, as the total eclipse persists without interruption from the contact configuration. This distinct morphology distinguishes contact binaries from detached or systems, where minima depths often differ and flat-bottomed eclipses may occur during partial phases. Key features of contact binary light curves include the O'Connell effect, an asymmetry between the two maxima often attributed to starspots on the cooler component, which causes one maximum to be brighter than the other. Additionally, changes, typically on the order of ΔP/P ≈ 10^{-6} yr^{-1}, reflect ongoing between the components, leading to secular variations observable in long-term monitoring. Large-scale photometric surveys have significantly expanded the catalog of known contact binaries, with over 70,000 systems identified in surveys such as ASAS-SN as of 2020, and thousands more from missions like TESS and ZTF by 2025. Notable contributions come from the All Sky Automated Survey (ASAS), the Optical Gravitational Lensing Experiment (OGLE), and the Kepler mission, which have detected thousands of these variables through their periodic brightness modulations, alongside more recent efforts from ASAS-SN, TESS, and ZTF. Photometric colors from these surveys, such as (V - I), provide estimates of effective temperatures, typically ranging from 4000 K to 7000 K for the components, aiding in classification as W-type (cooler) or A-type (hotter) systems. To derive physical parameters, light curves are modeled using the Wilson-Devinney code, which accounts for geometric effects like overflow and illumination. This approach fits key quantities such as the fill-out factor (the degree of common envelope filling, often 10–80%) and (nearly edge-on at i ≈ 90° for eclipsing systems), enabling determination of mass ratios and component radii.

Spectroscopic and Other Methods

Spectroscopic observations of contact binaries primarily rely on double-lined spectra to derive curves, which reveal the orbital speeds of both components, typically on the order of K1K_1 and K2∼100K_2 \sim 100 km/s for systems with periods around 0.3–1 day. These measurements enable the determination of minimum masses through the relations for circular orbits, M1sin⁡3i=P(K1+K2)2K22πGM_1 \sin^3 i = \frac{P (K_1 + K_2)^2 K_2}{2\pi G} and M2sin⁡3i=P(K1+K2)2K12πGM_2 \sin^3 i = \frac{P (K_1 + K_2)^2 K_1}{2\pi G}, where PP is the , K1K_1 and K2K_2 are the semi-amplitudes of the primary and secondary, ii is the inclination, and GG is the . This approach provides dynamical masses when combined with inclinations from photometry, offering insights into the and total mass, which are crucial for understanding the shared envelope dynamics. Spectral analysis of contact binaries often reveals composite spectra with similar spectral types for both components, reflecting their nearly equal temperatures due to the common envelope. Rotational broadening of spectral lines, arising from the rapid spin-orbit synchronization in these close systems, complicates line profile fitting but indicates equatorial velocities up to several hundred km/s. Evidence of chemical anomalies, such as enhanced nitrogen abundances from convective mixing or mass exchange across the envelope, has been detected in some systems, suggesting internal processing that alters surface compositions. Beyond , with arrays like CHARA resolves the angular separation of components or the extended envelope in nearby contact binaries, providing direct geometric constraints on sizes and filling factors at sub-milliarcsecond scales. observations detect emission from hot spots where stream impacts heat the surface, as seen in flares from accretion-like processes, revealing temperatures exceeding 10^6 and luminosities up to 10^{30} erg/s. Astrometric data from refine distances to contact binaries, enabling absolute parameter calibration with precisions better than 10% out to several kpc, which is essential for evolutionary modeling. A key challenge in spectroscopic studies of contact binaries is the blending of lines from the two rapidly rotating stars, necessitating spectral disentangling algorithms that iteratively separate component spectra using Doppler shifts across multiple epochs. These methods, such as those implemented in tools like Spectangular, achieve separation accuracies of 1–5% in line strengths, allowing reliable abundance and velocity determinations despite the overlap.

Notable Examples

W Ursae Majoris

W Ursae Majoris serves as the prototypical example of a W-type contact binary, characterized by two main-sequence stars in an overcontact configuration with nearly equal surface temperatures and short s. Discovered as a in 1903 by William J. Hussey, who identified its variability and determined an of approximately 0.334 days along with a mean visual magnitude of 7.8, the system has since become the namesake for this class of binaries. The binary consists of two low-mass main-sequence components, with masses of approximately 1.14 and 0.99 solar masses for the primary and secondary, respectively, effective temperatures around 4500 , a fill-out factor of about 70%, and a combined of approximately 1.5 solar luminosities. These parameters place it firmly within the W-type subclass, where the stars share a common convective envelope while maintaining distinct cores. A distinctive feature of W Ursae Majoris is its pronounced O'Connell effect, an asymmetry in the light curve maxima often attributed to starspots or circumstellar material, alongside a secular increase in the that signals active from the less massive to the more massive component. Extensively studied since its identification, the system exemplifies the stable overcontact phase typical of W-type binaries, providing key insights into their formation and evolution. Recent measurements place it at a distance of about 120 pc, facilitating precise absolute parameter determinations.

VFTS 352

VFTS 352 is a contact binary system discovered in 2015 through the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Tarantula Nebula within the Large Magellanic Cloud. This system represents one of the most massive known contact binaries, with an orbital period of approximately 1.12 days. The binary consists of two O-type stars with masses of about 28.6 and 28.9 solar masses, both classified as O4.5 V(n)((fc))z and O5.5 V(n)((fc))z, respectively, and effective temperatures exceeding 40,000 K. It exhibits overcontact configuration, where both components significantly overfill their Roche lobes and share a common envelope with a high fill-out factor of around 1.29. The stars are rapidly rotating at velocities of approximately 325 km/s, consistent with tidal locking in this tight orbit. As a young, massive system in a low-metallicity environment like the , shows signs of enhanced internal mixing, potentially indicating chemically homogeneous evolution or an impending merger. Recent studies (as of 2021) continue to explore its mixing processes and role as a progenitor for binaries. This configuration challenges existing models of stability in massive binaries and suggests it could serve as a progenitor for systems involving mergers or long-duration gamma-ray bursts.

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