arXiv subjects

Gloria Koenigsberger

Publications and source records attributed to Gloria Koenigsberger.

At least 19 recordsLinked to original sources

arXiv2026

Revised orbital parameters of the gamma2 Velorum system

Context. gamma2 Velorum is the closest and visually brightest Wolf-Rayet binary system. Its eccentric orbit modulates the X-rays observed from the wind-wind interaction, and its large separation allows for spatially resolving both components. Aims. We aim to strengthen the constraints on gamma2 Velorum's properties and, in particular, solve the discrepancy between the eccentricity determined from the emission lines and that from the absorption lines. Methods. We obtained VLT/GRAVITY observations and combined them with earlier spatially-resolved data at different orbital phases. Results. Strong constraints on all orbital parameters were determined and, in particular, we find that e=0.322, close to what was derived from the emission lines. The X-ray light curve declines as s to power -3 after periastron, where s is the separation of the two stars, but its modulation is likely affected by absorption and occultation of the X-ray emitting region at other orbital phases. We find that previous discrepancies in the reddening value can be traced to a brighter K-band magnitude than that predicted by the WR wind models. We conclude E(B-V)=0.02+-0.02 mag. Our now more precise mass and radius values combined with previously determined effective temperatures provide very strong constraints on evolutionary models. The closest match for the O-star is provided by an initial mass M=28.7 Mo rotationally mixed model and a M=32 Mo model for the WR star, with negligible accretion onto the O-star during the WR progenitor's Roche Lobe overflow phase. However, the temperature of the O star is higher and the mass of the WR star is found to be smaller than predicted by the evolutionary tracks for the current epoch, consistent with the well-known "mass-discrepancy problem" in massive stars.

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arXiv2026

Elemental Abundances in the Binary Star V505 Per

We present a detailed chemical abundance analysis of the eclipsing binary system V505 Per. High resolution spectra were analyzed using the MOOG spectrum analysis code, and we determined abundances not only for iron and lithium but also for Si, Na, Ca, Mn, and Ni, elements that have not previously been analyzed in detail for this system. Abundances were computed across 15 temperature points using model atmospheres, with stellar parameters refined by minimizing abundance trends with excitation potential. We determined effective temperatures of T_eff = 6650 +/- 50 K for the primary and T_eff = 6550 +/- 50 K for the secondary, with iron abundances of [Fe/H] = -0.10 +/- 0.06 and [Fe/H] = -0.19 +/- 0.07, respectively. Most [X/Fe] ratios are consistent with solar values, though manganese is deficient. Our analysis of the effective temperatures shows that both stars lie on the hot edge of the lithium dip, consistent with Koenigsberger et al. (2025), which may help resolve the inconsistency noted of the stars lithium abundance within the dip by Baugh et al. (2013).

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arXiv2024

Transitory tidal heating and its impact on cluster isochrones

The kinetic energy in tidal flows, when converted into heat, can affect the internal structure of a star and shift its location on a color-magnitude diagram from that of standard models. In this paper we explore the impact of injecting heat into stars with masses near the main sequence turnoff mass (1.26 $M_\odot$) of the open cluster M67. The heating rate is obtained from the tidal shear energy dissipation rate which is calculated from first principles by simultaneously solving the equations that describe orbital motion and the response of a star's layers to the gravitational, Coriolis, centrifugal, gas pressure and viscous forces. The stellar structure models are computed with MESA. We focus on the effects of injecting heat in pulses lasting 0.01 Gyr, a timeframe consistent with the synchonization timescale in binary systems. We find that the location of the tidally perturbed stars in the M67 color-magnitude diagram is shifted to significantly higher luminosities and effective temperatures than predicted by the standard model isochrone and include locations corresponding to some of the Blue Straggler Stars. Because tidal heating takes energy from the orbit causing it to shrink, Blue Straggler Stars could be merger or mass-transfer progenitors as well as products of these processes.

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arXiv2024

Eccentric binaries: Periastron events and tidal heating

Periastron brightening events, also known as the heartbeat phenomenon, are a clear manifestation of interaction effects in binary systems. We explore the role of tidal shear energy dissipation in stars undergoing periastron brightening events by performing a computation from first principles that uses a quasi-hydrodynamic Lagrangian scheme to simultaneously solve the orbital motion and the equations of motion of a 3D grid of volume elements covering the inner, rigidly rotating region of a tidally perturbed star. The equations of motion include the gravitational acceleration of both stars, the centrifugal, Coriolis, gas pressure accelerations, and viscous coupling between volume elements. The method is illustrated for a grid of model binary systems with a 10 M$_\odot$ primary that is perturbed by a 6.97 M$_\odot$ companion in eccentric orbits (e=0 $-$ 0.7). The model is then applied to the heartbeat star MACHO 80.7443.1718. We find an increase by factors 10$^{-6}$ $-$10$^{-3}$ in tidal shear energy dissipation at periastron, consistent with the majority of observed heartbeat stars. The magnitude of the periastron effect correlates with the degree of departure from synchronicity: stars rotating much faster or much slower than the synchronous rate at periastron present the strongest effect. We confirm that for eccentricities $\leq$0.3, pseudo-synchronization occurs for 0.8$< ω/Ω_{ave} <$1, where $Ω_{ave}$ is the average orbital angular velocity. However, we find that the rotation rate with minimum energy for e=0.5 and 0.7 occurs for $ω/Ω_{ave}>$1 . The tidal shear energy dissipation model reproduces from first principles the 23% maximum brightness enhancement at periastron of MACHO 80.7443.1718. The extraordinarily large hearbeat amplitude is likely due to a rotation rate that differs considerably from the synchronous rate at periastron.

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arXiv2023

Structure and evolution of a tidally heated star

The shearing motion of tidal flows that are excited in non-equilibrium binary stars transform kinetic energy into heat via a process referred to as tidal heating. In this paper we aim to explore the way tidal heating affects the stellar structure. We used the TIDES code, which solves the equations of motion of the three-dimensional (3D) grid of volume elements that conform multiple layers of a rotating binary star to obtain an instantaneous value for the angular velocity, $ω''$, as a function of position in the presence of gravitational, centrifugal, Coriolis, gas pressure, and viscous forces. The released energy, $\dot{E,}$ was computed using a prescription for turbulent viscosity that depends on the instantaneous velocity gradients. The $\dot{E}$ values for each radius were injected into a MESA stellar structure calculation. The method is illustrated for a 1.0+0.8 M$_\odot$ binary system, with an orbital period of $P$=1.44d and departures from synchronous rotation of 5% and 10%. We find that heated models have a larger radius and surface luminosity, a smaller surface convection zone, and lower nuclear reaction rates than the equivalent standard stellar models, and their evolutionary tracks extend to higher temperatures. The magnitude of these effects depends on the amount of injected energy, which, for a fixed set of stellar, rotation and orbital parameters, depends on the perturbed star's density structure and turbulent viscosity. Tidal heating offers a possible alternative for describing phenomena such as bloated or overluminous binary components, age discrepancies, and aspherical mass ejection, as well as the extended main sequence turnoff in clusters. However, establishing its actual role requires 3D stellar structure models commensurate with the nonspherically symmetric properties of tidal perturbations.

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arXiv2021

Induced differential rotation and mixing in asynchronous binary stars

Rotation contributes to internal mixing processes and observed variability in massive stars. A significant number of binary stars are not in strict synchronous rotation, including all eccentric systems. This leads to a tidally induced and time-variable differential rotation structure. We present a method for exploring the rotation structure of asynchronously rotating binaries. We solve the equations of motion of a 3D grid of volume elements located above the rigidly rotating core in the presence of gravitational, centrifugal, Coriolis, gas pressure and viscous forces to obtain the angular velocity as a function of the three spatial coordinates and time. We find that the induced rotation structure and its temporal variability depend on the degree of departure from synchronicity. In eccentric systems, the structure changes over the orbital cycle with maximum amplitudes occurring potentially at orbital phases other than periastron passage. We discuss the possible role of the time-dependent tidal flows in enhancing the mixing efficiency and speculate that, in this context, slowly rotating asynchronous binaries could have more efficient mixing than the analogous more rapidly rotating but tidally locked systems. We find that some observed nitrogen abundances depend on the orbital inclination, which, if real, would imply an inhomogeneous chemical distribution over the stellar surface or that tidally induced spectral line variability, which is strongest near the equator, affects the abundance determinations. Our models predict that, neglecting other angular momentum transfer mechanisms, a pronounced initial differential rotation structure converges toward average uniform rotation on the viscous timescale. We suggest that by taking into account the processes that are triggered by asynchronous rotation, a broader perspective of binary star structure, evolution and variability may be gleaned.

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arXiv2020

Internal circulation in tidally locked massive binary stars -- Consequences for double black hole formation

Steady-state circulation currents are predicted in tidally deformed binary stars, which are believed to be progenitors of double black-hole merger events. This work aims to quantitatively characterise the steady-state circulation currents in components of a tidally locked binary system and to explore the effects of such currents on numerical models. Previous results describing the circulation in a single rotating star and a binary star are used to deduce a new prescription for the internal circulation in tidally locked binaries. We explore the effect of this prescription numerically for binary systems with primary masses between 25 and 100 solar masses. When comparing circulation velocities in the radial direction for the single rotating star and binary star, it is found that the average circulation velocity in the binary star may be described as an enhancement to the circulation velocity in a single rotating star. This velocity enhancement is a simple function depending on the masses of the binary components and amounts to a factor of approximately two when the components have equal masses. It is found that the ehancement causes the formation of double helium stars through efficient mixing to occur for systems with higher initial orbital periods, lower primary masses and lower mass ratios, compared to the standard circulation scenario. Taking into account appropriate distributions for primary mass, initial period and mass ratio, models with enhanced mixing predict 2.4 times more double helium stars being produced in the parameter space than models without. We conclude that the effects of companion-induced circulation have strong implications for the formation of close binary black holes. Not only do the predicted detection rates increase but double black-hole systems with mass ratios as low as 0.8 may be formed when companion-induced circulation is taken into account.

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arXiv2020

The nature of the companion in the Wolf-Rayet system EZ Canis Majoris

EZ Canis Majoris is a classical Wolf-Rayet star whose binary nature has been debated for decades. It was recently modeled as an eccentric binary with a periodic brightening at periastron of the emission originating in a shock heated zone near the companion. The focus of this paper is to further test the binary model and to constrain the nature of the unseen close companion by searching for emission arising in the shock-heated region. We analyze over 400 high resolution the International Ultraviolet Explorer spectra obtained between 1983 and 1995 and XMM-Newton observations obtained in 2010. The light curve and radial velocity (RV) variations were fit with the eccentric binary model and the orbital elements were constrained. We find RV variations in the primary emission lines with a semi-amplitude K$_1\sim$30 km/s in 1992 and 1995, and a second set of emissions with an anti-phase RV curve with K$_2\sim$150 km/s. The simultaneous model fit to the RVs and the light curve yields the orbital elements for each epoch. Adopting a Wolf-Rayet mass M$_1\sim$20 M$_\odot$ leads to M$_2\sim$3-5 M$_\odot$, which implies that the companion could be a late B-type star. The eccentric (e=0.1) binary model also explains the hard X-ray light curve obtained by XMM-Newton and the fit to these data indicates that the duration of maximum is shorter than the typical exposure times. The anti-phase RV variations of two emission components and the simultaneous fit to the RVs and the light curve are concrete evidence in favor of the binary nature of EZ Canis Majoris. The assumption that the emission from the shock-heated region closely traces the orbit of the companion is less certain, although it is feasible because the companion is significantly heated by the WR radiation field and impacted by the WR wind.

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arXiv2020

Asymmetrical nebula of the M33 variable GR290 (WR/LBV)

Context: GR290 (M33 V0532=Romano's star) is a luminous M33 object undergoing photometric variability typical for luminous blue variable (LBV) stars. It lies inside Wolf-Rayet region in the Hertzsprung-Russell diagram and possesses a WN8 type spectrum at the light minima. Analysis of Gran Telescopio Canarias (GTC) spectra obtained in 2016 led to the conclusion that it is surrounded by an unresolved HII region formed mostly of ejected material from the central star, and disclosed the presence of a second, more extended asymmetrical emission region. Aims: The aim of this paper is to further explore the structure of the nearby environment of GR290. Methods: Long-slit spectra of GR290 were obtained with three slit orientations in the visual and red spectral regions. The emission-line distribution for each slit was analyzed. Results: We confirm the presence of an asymmetric HII region that extends ~50 pc to the south; ~30pc to the north and southeast; ~20 pc to the east and northwest and ~10pc to the west. We also present the first spectrum to be acquired of a star belonging to the neighboring OB88 association, J013501.87+304157.3, which we classify as a B-type supergiant with a possible binary companion.

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arXiv2019

The History Goes On: Century Long Study of Romano's Star

GR 290 (M 33 V0532 = Romano's Star) is a unique variable star in the M33 galaxy, which simultaneously displays variability typical for luminous blue variable (LBV) stars and physical parameters typical for nitrogen-rich Wolf-Rayet (WR) stars (WN). As of now, GR 290 is the first object which is confidently classified as a post-LBV star. In this paper, we outline the main results achieved from extensive photometric and spectroscopic observations of the star: the structure and chemical composition of its wind and its evolution over time, the systematic increase of the bolometric luminosity during the light maxima, the circumstellar environment. These results show that the current state of Romano's Star constitutes a fundamental link in the evolutionary path of very massive stars.

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arXiv2019

Long uninterrupted photometric observations of the Wolf-Rayet star EZ CMa by the Toronto {\em{BRITE}} satellite reveal a very fast apsidal motion

Context. The variability of the Wolf-Rayet star EZ CMa has been documented for close to half a century, and a clear periodicity of $\sim$3.7 days is established. However, all attempts to prove that it is a binary have failed because the photometric, spectroscopic, and polarimetric variations are not coherent over more than a few orbital cycles. Aims. In this letter we show that the lack of coherence in the variability can be explained with a very rapid apsidal motion in a binary orbit.} Methods. We measured the times of minima in a recently published exceptionally long photometric light curve obtained by the Toronto {\emph{BRITE}} satellite. The apsidal motion and the system eccentricity are determined from the length of the time intervals between these minima, which alternate in their duration, following a pattern that is clearly associated with apsidal motion. These minima are superposed on brightness enhancements of the emission from a shock zone, which occur at about the times of periastron phases. Results. We determine the orbital periodicity, $P_{a}=3.63\, $d, and the period of the apsidal motion, $U\simeq 100\, $d, which together yield an average sidereal period of $P_{s}=3.77\,$d. The eccentricity is found to be close to 0.1. The rate of periapsis retreat changes significantly over the period of observation and is determined to be $-16^\circ\,\mathrm{P}^{-1}_a$ at the beginning of the observing period and $-10^\circ\,\mathrm{P}^{-1}_a$ at the end. Conclusions. We demonstrate that by introducing a fast apsidal motion, the basic photometric variability is very well explained. The binary nature of EZ CMa is now established. This might imply that other apparently single Wolf-Rayet stars that emit hard X-rays, similar to EZ CMa, are also binaries.

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arXiv2018

Wind and nebula of the M33 variable GR290 (WR/LBV)

Context: GR290 (M33/V532=Romano's Star) is a suspected post-LBV star located in M33 galaxy that shows a rare Wolf-Rayet spectrum during its minimum light phase. In spite of many studies, its atmospheric structure, its circumstellar environment and its place in the general context of massive stars evolution is poorly known. Aims: Detailed study of its wind and mass loss, and study of the circumstellar environment associated to the star. Methods: Long-slit spectra of GR290 were obtained during its present minimum luminosity phase with the GTC together with contemporaneous BVRI photometry. The data were compared with non-LTE model atmosphere synthetic spectra computed with CMFGEN and with CLOUDY models for ionized interstellar medium regions. Results: The current $m_V=18.8$ mag, is the faintest at which this source has ever been observed. The non-LTE models indicate effective temperature $T_{eff}$=27-30 kK at radius $R_{2/3}$=27-21 Rsun and mass loss rate $\dot{M}=1.5\times10^{-5}$ Msun yr$^{-1}$. The terminal wind speed $V_\infty$=620 ${\rm km~s^{-1}}$ is faster than ever before recorded while the current luminosity $L_*=(3.1-3.7)\times 10^5$ Lsun is the lowest ever deduced. It is overabundant in He and N and underabundant in C and O. It is surrounded by an unresolved compact HII region with dimensions $\leq$4 pc, from where H-Balmer, HeI lines and [OIII] and [NII] are detected. In addition, we find emission from a more extended interstellar medium (ISM) region which appears to be asymmetric, with a larger extent to the East (16-40 pc) than to the West. Conclusions: In the present long lasting visual minimum, GR290 is in a lower bolometric luminosity state with higher mass loss rate. The nearby nebular emission seems to suggest that the star has undergone significant mass loss over the past $10^4-10^5$ years and is nearing the end stages of its evolution.

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arXiv2018

BRITE-Constellation reveals evidence for pulsations in the enigmatic binary $η$ Carinae

$η$ Car is a massive, eccentric binary with a rich observational history. We obtained the first high-cadence, high-precision light curves with the BRITE-Constellation nanosatellites over 6 months in 2016 and 6 months in 2017. The light curve is contaminated by several sources including the Homunculus nebula and neighboring stars, including the eclipsing binary CPD$-$59$^\circ$2628. However, we found two coherent oscillations in the light curve. These may represent pulsations that are not yet understood but we postulate that they are related to tidally excited oscillations of $η$ Car's primary star, and would be similar to those detected in lower-mass eccentric binaries. In particular, one frequency was previously detected by van Genderen et al. and Sterken et al. through the time period of 1974 to 1995 through timing measurements of photometric maxima. Thus, this frequency seems to have been detected for nearly four decades, indicating that it has been stable in frequency over this time span. These pulsations could help provide the first direct constraints on the fundamental parameters of the primary star if confirmed and refined with future observations.

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arXiv2017

A changing wind collision

We report on the first detection of a global change in the X-ray emitting properties of a wind-wind collision, thanks to XMM-Newton observations of the massive SMC system HD5980. While its lightcurve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the lightcurve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found, without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind-wind collision, considering the varying wind properties of the eruptive component in HD5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions.

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arXiv2016

Alpha Virginis: line-profile variations and orbital elements

Abridged: Alpha Virginis is a binary system whose proximity and brightness allow detailed investigations of the internal structure and evolution of stars undergoing time-variable tidal interactions. Previous studies have led to the conclusion that the internal structure of Spica's primary star may be more centrally condensed than predicted by theoretical models of single stars, raising the possibility that the interactions could lead to effects that are currently neglected in structure and evolution calculations. The key parameters in confirming this result are the values of the orbital eccentricity $e$, the apsidal period $U$, and the primary star's radius, R_1. We analyze the impact that line profile variability has on the derivation of its orbital elements and R_1. We use high SNR observations obtained in 2000, 2008, and 2013 to derive the orbital elements from fits to the radial velocity curves. We produce synthetic line profiles using an ab initio tidal interaction model. Results: The variations in the line profiles can be understood in terms of the tidal flows, whose large-scale structure is relatively fixed in the rotating binary system reference frame. Fits to the radial velocity curves yield $e$=0.108$\pm$0.014. However, the analogous RV curves from theoretical line profiles indicate that the distortion in the lines causes the fitted value of $e$ to depend on the argument of periastron; i.e., on the epoch of observation. As a result, the actual value of $e$ may be as high as 0.125. We find that $U$=117.9$\pm$1.8, which is in agreement with previous determinations. Using the value $R_1=6.8 R_\odot$ derived by Palate et al. (2013) the value of the observational internal structure constant $k_{2,obs}$ is consistent with theory. We confirm the presence of variability in the line profiles of the secondary star.

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arXiv2015

Planet heating prevents inward migration of planetary cores

Planetary systems are born in the disks of gas, dust and rocky fragments that surround newly formed stars. Solid content assembles into ever-larger rocky fragments that eventually become planetary embryos. These then continue their growth by accreting leftover material in the disc. Concurrently, tidal effects in the disc cause a radial drift in the embryo orbits, a process known as migration. Fast inward migration is predicted by theory for embryos smaller than three to five Earth masses. With only inward migration, these embryos can only rarely become giant planets located at Earth's distance from the Sun and beyond, in contrast with observations. Here we report that asymmetries in the temperature rise associated with accreting infalling material produce a force (which gives rise to an effect that we call "heating torque") that counteracts inward migration. This provides a channel for the formation of giant planets and also explains the strong planet-metallicity correlation found between the incidence of giant planets and the heavy-element abundance of the host stars.

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arXiv2014

The HD5980 multiple system: Masses and evolutionary status

New spectroscopic observations of the LBV/WR multiple system HD5980 in the Small Magellanic Cloud are used to address the question of the masses and evolutionary status of the two very luminous stars in the 19.3d eclipsing binary system. Two distinct components of the N V 4944 A line are detected in emission and their radial velocity variations are used to derive masses of 61 and 66 Mo, under the assumption that binary interaction effects on this atomic transition are negligible. We propose that this binary system is the product of quasi-chemically homogeneous evolution with little or no mass transfer. Thus, both of these binary stars may be candidates for gamma-ray burst progenitors or even pair instability supernovae. Analysis of the photospheric absorption lines belonging to the third-light object in the system confirm that it consists of an O-type star in a 96.56d eccentric orbit (e=0.82) around an unseen companion. The 5:1 period ratio and high eccentricities of the two binaries suggest that they may constitute a hierarchical quadruple system.

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arXiv2013

Tidal Flows in asynchronous binaries: The beta-factor

We discuss the potential role that tidal flows in asynchronous binary stars may play in transporting chemically enriched material from deep layers towards the surface and the corresponding observational consequences of these processes. We suggest that the time-dependent velocity field induced by asynchronous rotation may contribute significantly to the mixing, thus providing a channel for the formation of chemically enriched slowly rotating massive stars.

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