Institute of Astronomy

Abstracts (Posters)

(updated 28 July 2016 at 1135hrs)

Carlo Abate

Making a Long Period Short. Observed and Modelled Orbital Periods in CEMP-s Stars

CEMP-s stars are metal-poor stars of the Galactic halo with enhanced abundances of carbon and s-elements. Most CEMP-s stars are observed in binary systems, suggesting that the enrichment in carbon and s-elements is due to mass transfer from an AGB primary. However, binary population synthesis models do not reproduce the observed period distribution of CEMP-s stars because most syntetic CEMP-s stars are formed at longer periods than observed. In this work we study binary mass transfer and we investigate how different assumptions on the accretion efficiency, angular-momentum loss and stability of Roche-lobe overflow modify the cumulative period distributions of synthetic population of CEMP-s stars.

Jeff Andrews

Beyond Population Synthesis: Markov-Chain Monte Carlo Models of High Mass X-ray Binaries

Recent studies of high mass binaries have traditionally employed population synthesis codes to link evolutionary models with observed populations. However for certain types of binaries, particularly those that are rare or short-lived, population synthesis is a poor technique; only a small fraction of simulated systems appear similar to those observed. For more complex problems, the computational expense can be prohibitive. Correlating high mass X-ray binaries (HMXB) with regions of recent star-formation in nearby galaxies is one such problem. I will demonstrate an alternative to population synthesis based on a Markov-Chain Monte Carlo method which uses the spatially resolved star formation history as a prior on the HMXBs’ birth location and age. We apply our model to HMXBs in the Magellanic Clouds, quantitatively constraining their evolutionary histories and mapping their birth locations. With adaptations, this method can be applied to other populations of compact object binaries.

Alexey Bobrick

Mass Transfer in Eccentric Compact Binaries

We perform hydrodynamic simulations of mass transfer in eccentric white dwarf neutron star binaries. By measuring non-conservative mass transfer parameters we link the results of our simulations to a long-term evolution model. We find that the binaries containing a low mass helium white dwarf evolve into ultracompact X-ray binaries. The systems with higher mass white dwarfs instead experience unstable mass transfer leading to a tidal disruption of the white dwarf, which is possibly observed as Ca-rich gap transients. We find that the critical mass dividing stable and unstable systems is significantly lower than predicted by standard jet-only model of mass loss. The difference is explained by the importance of mechanical disc winds at highly super-Eddington mass transfer rates, which lead to a much more efficient loss of angular momentum from the binary than a jet. We find that the eccentricity does not affect the loss of angular momentum and may only affect the long-term evolution if it changes quickly enough. Our results are broadly consistent with the observations of binary radio pulsars, Ca-rich gap transients, possibly produced by the unstable systems, and ultracompact X-ray binaries.

Conor Byrne

The Formation of Helium-Rich Hot Subdwarfs via Common Envelope Evolution

Subdwarf B stars are unusual core helium burning stars whose origins can only explained through binary interactions. Their surface chemistries are quite intriguing. Whilst most are helium-poor a few are helium-rich. The merger of two helium white dwarfs provides one route to form helium-rich subdwarfs, but it does not explain the presence of helium-rich subdwarfs which have a binary companion. We present results of simulations computed with the MESA stellar evolution code to investigate if common envelope ejection by a red giant could produce such helium-rich subdwarfs. It is seen that flash-driven convection in post-common-envelope objects can produce subdwarfs with helium rich atmospheres if a sufficient amount of the red giant's envelope is removed, but the transition from hydrogen rich to helium rich atmospheres appears to take place at effective temperatures higher than that of an observed helium-rich subdwarf binary.

Nareemas Chehlaeh

Eclipsing Binary Stars in the Open Cluster NGC 2126

We present the first results of photometric time series observations of the open cluster NGC2126 acquired during the years 2004 and 2013-2015 with the aim to detect new variable stars, to study binary systems as well as the oscillation spectra of pulsating stars. We present the B and V light curves of the binary systems and a detailed analysis of the oscillating eclipsing binary star V6.

Xuefei Chen

EL CVn-type Binaries and Mass Transfer

EL CVn-type binaries contain an A-type dwarf star and a very low mass (~0.2Msun) helium white dwarf precursor. The WASP and Kepler photometric surveys have found a number of such binaries. Recent observations discovered that some pre-He WDs have multi-periodic pulsations, opening up the possibility of using asteroseismology to study the interior structure of these stars. In this talk, I will show that long-term stable mass transfer between low-mass stars in close binary systems can excellently reproduce the observational properties of such objects, such as the evolutionary track, the WD mass-period relation, the companion mass etc. In particular, the mass ratio-period (q, P) plane gives some clues on how conservative of the mass transfer is when the donor is on MS or during HG, i.e. near the bifurcation period. Our study predicts a very strong dependence of the lifetime of EL CVn-type stars on the mass of the pre-He WDs. This leads to an intrinsic peak mass around the minimum mass of pre-He WDs (~0.16Msun) allowed from stellar evolution. We predict that many more EL CVn-type binaries remain to be discovered and that these binaries will be predominantly low-mass systems in old stellar populations.

Xiao Cui

Progenitors of Type Ia Supernovae: the Element Abundance for an Accretion WD

Type Ia supernovae (SNe Ia) play an important role in the field of astrophysics, and the mass increase process during the accretion is very important for the single-degenerate(SD) model. In this work, we employed the stellar evolution code MESA to simulate the evolution process of solar-composition material accreting onto a 1.0 M⊙ CO WD at a high accretion rate of 4.3× 10−7 M⊙ yr−1, There will be a He shell burning around the WD and a H shell burning around the He shell. The results indicate that the ratio of carbon to oxygen resulting from the helium shell flash in an accreting WD is higher and more complicated than previously thought. Besides, such a high ratio of C/O may have some influences on SNe Ia.

Barlomiej Debski (2 posters)

1) Observations of the Peculiar Dwarf Nova CSS160603:162117+441254

Here we present the preliminary results of the ongoing observational campaign of the close binary system CSS160603:162117+441254. The object, initially thought to be a contact binary, went through an outburst phase in the beginning of June 2016, thus revealing its true nature of a dwarf nova.  In quiescence, the object's lightcurve resembled those of contact binaries, both in the lightcurve shape (classified as EW-type) and in its colour. This rises a question on to how many objects currently classified as contact binaries solely by lightcurve shape and colour are in fact contact binaries and not cataclysmic variables in their quiescence.

2) The Lightcurve Morphology Analysis for Contact Binary Systems

Lightcurve Morphology Analysis is a quick method for studying the general properties of the surface phenomena in close binary stars observed in a long-time base photometry.  Its application to the Kepler data allowed to find the indicators for starspot migration, the direction of the starspot migration, constraints on the starspot co-⁠latitude and size parameters and many other.  One of the most promising prospects for this method is the determination of which component of the contact binary is the more massive one.  In this work I present some applications of the Analysis to the systems with the EW-⁠type lightcurves from the Kepler Eclipsing Binary Catalogue.

Katarzyna Drozd

Symbiotic Stars in the Local Group of Galaxies

Since not a single supernova type Ia progenitor has been observed, symbiotic stars (SySt) with their complex structure are putative candidates for a group of stars that one day may go ballistic.  Since it became possible to study individual objects with sensitive enough spectroscopy, one can collect a sample of the brightest SySt in the Local Group of Galaxies. For over 1 100 objects observed in M31 we found 165 counterparts in POINT-AGAPE catalog (PAC).  We decided to revisit PAC original data and and retrieve missing light curves. Here we present a sample of first results - light curves of SySt, SySt candidates, and other emission line objects in M31.

Sárka Dvoráková​, P. Koubsky, V. Votruba

Spectroscopycal Study of Tripple Star V421 Cep

R. F. Sanford (1926) revealed the nature of spectroscopic triple star system V421 Cephei and got orbital elements of both orbit. Our task was to disentangle the spectra of individual components of triple star system with help of program named KOREL (P. Hadrava, 2004) and with using orbital parameters specified by Sanford. So far we have managed to find a shape of spectral lines for two visible components of triple star in the He I, P 12, P 14 and H gama spectral lines. We have tried to confirm our results by using another program for disentangling spectra written by Ilijic (2003). Finally, we have proceeded to validate the orbital parameters which were independently determined from radial velocity curve. Work is still in progress.

Alba Aller Egea

Search for Binary Central Stars of Planetary Nebulae: Exploiting Archival Data

With the new binary central stars identified in the last years, binary interactions have become the main proposed mechanism to explain the complex morphologies seen in PNe. There are around 50 close binary central stars detected so far. However, this number remains very small as compared to the more than 3000 known PNe in the Milky Way. In this context, we are involved in a project to study how binary interaction processes influence the shape of PNe.

Archival data offer a great opportunity to search for new candidates of binary CSs since surveys like, e.g., Catalina, ASAS and OMC provide light curves of a large number of objects in a long timespan. We are exploiting these surveys to search for new candidates that will be subsequently followed-up by means of multi-epoch photometric and spectroscopic observations. We are also using other techniques, like the search for infrared excess from archival photometry in the spectral energy distributions, which could indicate the presence of late-type stars in the nuclei of PNe.

Ahmed Essam

Frequency and Photometric Analysis of the New Short Period Pulsating Star GSC 02444-00241

BVRI photometric observations by using the 1.88 m telescope of Kottamia Astronomical Observatory (Egypt) revealed that the star GSC 02444-00241 is high amplitude and short period δ Scuti star whose variability is recently discovered by Essam (2013). CCD observations of this newly discovered variable star are analyzed for a preliminary investigation of its pulsation properties. Photometric analysis conducted by the authors yields a period of 0d.0425 and peak-to-peak amplitude of 0.014 magnitudes. The frequency analysis exhibits that this pulsating star has multi periods with a period ratio f0 / f1=0.775. The type of variability is discussed, and the pulsation frequencies and Fourier spectra are presented. By using the empirical relations for the δ Scuti stars we determined the global parameters of GSC 02444-00241: Teff = 6888±60 K, L/Lsun = 3.503±0.261, R/Rsun = 1.317±0.043, M/Msun = (1.410±0.05)^8, log g = 4.348±0.088, the pulsation constant, Q = 0.033±0.002 day and the distance d=980.884±20.555 pc. From the study of the evolution status of the system GSC 02444-00241 we found that the age (τ) equal to (6.609±4.469)*10^8 years.

Nancy Remage Evans

Binary Framework: Cepheids

Cepheids are a class of intermediate mass stars (6 Msun) with sharp spectral lines, and a well determined distance and evolutionary state.  We have surveyed approximately 70 stars in radial velocities and HST imaging to determine binary parameters, in particular the distribution of separations and mass ratios. Of the stars with possible resolved companions, we have observed a subsample with XMM-Newton to use X-ray activity to differentiate between young physical companions and chance alignments with old field stars.  Results include an outer limit of 4000 AU beyond which companions are not found, and the distribution of mass ratios at different separations.

Cezary Galan

Chemical Abundances and their Peculiarities as Tracers of Symbiotic Binary Evolution

Large collection of near-infrared spectra has been used to measure photospheric chemical abundances in symbiotic giants and to perform the first statistical analyses. We have obtained the number of interesting results concerning metallicities, evolutionary status, and the Galactic populations membership. I will also discuss the s-process elements pollution, which we have detected in roughly half of our sample with medium-resolution optical spectra.

Shaung Gao

Does Binarity Change with Metallicity?

We estimate the fraction of F,G,K stars with close binary companions by analysing multi-epoch stellar spectra from SDSS and LAMOST for radial velocity (RV) variations. We employ a Bayesian method to infer the maximum likelihood of the fraction of binary stars with orbital periods of 1000 days or shorter, assuming a simple model distribution for a binary population with circular orbits. The overall inferred fraction of stars with such a close binary companion is 43.0% \pm 2.0% for a sample of F, G, K stars from SDSS SEGUE, and 30% \pm 8.0% in a similar sample from LAMOST. The apparent close binary fraction decreases with the stellar effective temperature. We divide the SEGUE and LEGUE data into three subsamples with different metallicity ([Fe/H] < -1.1; -1.1 < [Fe/H] < -0.6; -0.6 < [Fe/H]), for which the inferred close binary fractions are 56% \pm 5.0%, 56.0% \pm 3%, and 30% \pm 5.7%. The metal-rich stars from our sample are therefore substantially less likely to possess a close binary companion than otherwise similar stars drawn from metal-poor populations. The different ages and formation environments of the Milky Way's thin disk, thick disk and halo may contribute to explaining these observations. Alternatively metallicity may have a significant effect on the formation and/or evolution of binary stars.

Zhanwen Han

Accreting White Dwarfs: their Population, the Related SNe Ia, the Soft X-ray and Extreme UV Emission and Nova Production

Accreting, nuclear-burning white dwarfs (WDs) are believed to be candidate progenitors of Type Ia supernovae (SNe Ia), and to account for supersoft X-ray sources, novae, etc. depending on their accretion rates.  In this talk, we will show how a WD accretes matter, burns hydrogen and helium, grows in mass and radiates in soft X-ray and extreme UV. We carried out a binary population synthsis study on the production of SNe Ia via single degenerate scenario, the soft X-ray and extreme UV emission of elliptical galaxies, and nova production.  We then compared the results of our model with that of observations.

Andrei Igoshev

On the Velocities that BH- and NS-XRBs Receive at Formation: Bayesian Approach

In the light of the recent first ever LIGO detection of two merging two merging black holes due to their gravitational waves emission, the study of the initial conditions at the formation of black holes gains additional value. The birth of a black hole in a supernova explosion continues to be a mysterious process in many aspects, in particular the initial kick velocity has challenged many theoretical models and it potentially can give us more insight into the formation mechanism and has a significant influence on the estimated rate of gravitational wave detections. Recent publications challenged the high black hole kick hypothesis. Thus it is essential to carefully study any available evidence for and against this hypothesis. The gravitational Galactic potential together with some minimal assumptions about the possible birth-place for a binary allow us to estimate the plausibility of high kicks. One of the most striking indication for a high kick velocity is the position of the BH X-ray binary H1705-250, see Repetto & Nelemans (2015). We wrote the Bayesian posterior probability for the initial peculiar velocity of this binary and applied the Markov chain Monte Carlo algorithm to draw samples from it. We have found that it is plausible, and with high probability, that the absolute value for the kick velocity exceeds 100-200 km/s.

Amanda Karakas

R Coronae Borealis Stars as Viable Factories of Pre-solar Grains

We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB stars to be approximately 25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon-oxygen and helium-rich white dwarfs. Our new RCB birthrate (1.8e-3 per year) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180-540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, we calculate the total dust production from RCB stars and compare this rate to dust production from novae and  born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.

Jakub Klencki

Dynamical Formation of Black Hole Low-Mass X-Ray Binaries in the Field – an Alternative to Common Envelope

Very wide binaries (> 500 AU) are subject to numerous encounters with flying-by stars in the Galactic field and can be perturbated into highly eccentric orbits (e ∼ 0.99). For such systems tidal interactions at close pericenter passages can lead to orbit circularization and possibly mass transfer, consequently producing X-Ray binaries without the need for common envelope. We test this scenario for the case of Black Hole Low-Mass X-Ray Binaries (BH LMXBs) by performing a population synthesis from primordial binaries with numerical treatment of random stellar encounters. We test various models for the threshold pericenter distance under which tidal forces cause circularization. We find that fly-by interactions can produce a current population of ∼ 60–220 BH LMXBs in the Galactic field and the results are sensitive to the assumption on tidal circularization efficiency. We show that the most likely donors are low-mass stars (< 1 sollar mass, at the onset of mass transfer) as observed in the population of known sources (∼ 20). However, the low number of systems formed along this route is in tension with most recent observational estimate of a number of dormant BH LMXBs in Galaxy 10^4 –10^8 (Tetarenko et al. 2016). If indeed the numbers are so high, alternative formation channels of BHs with very low-mass donors need to be identified.

Angela Kochoska

Automated Discovery and Classification of Binary Star Candidates in Spectroscopic and Photometric Sky Surveys

Sky surveys have become a fundamental tool of astronomy and astrophysics following the technological advancements of the previous century. They have revolutionized the field by shifting the research focus from isolated observations of single objects to the continuous gathering of massive amounts of data of various types and for an extensive range of scientific goals. However, the immense amount of collected data from past and ongoing sky surveys has rendered the detection and classification of potentially interesting objects far from trivial.   We propose a method that combines t-Distributed Stochastic Neighbor Embedding (t-SNE) and Density-Based Spatial Clustering of Applications with Noise (DBSCAN) to identify potential binary star candidates in spectroscopic data from the Gaia-ESO and GALAH survey. We apply the method to the broadening functions of the spectra as it simplifies the distinction between normal and peculiar objects and efficiently isolates potential binary candidates. The underlying structure of the low-dimensional map of the objects obtained with this approach can be further used to classify the objects by proposed morphological types. We also show the performance of the method on Kepler and Gaia-simulated Kepler data, which has proved to be a powerful tool in identification, classification and visualization of binary star light curves.

Michalis Kourniotis

Massive Binaries in the Danks Clusters

We aim to increase the number of hot, massive stars with well-constrained parameters resolved from the study of eclipsing, double-lined systems. We employ near-infrared spectroscopy with ISAAC/VLT to explore the kinematics and thereby the fundamental parameters, of obscured binaries in the galactic, young, massive Danks clusters. Our analysis of a system containing two late O-type dwarfs yields masses of 22 and 25 Mo, and a direct distance with a precision of 2%. Preliminary results from the study of a system that comprises of extreme O-type/Wolf Rayet stars, indicate high masses ~67 Mo for each component. The latter system may provide valuable constraints on modeling the pre-supernova state of stars characterized by high mass-loss. It can therefore contribute towards a better understanding of the physics of poorly-studied extreme massive objects.

Thomas Kupfer

Ultracompact AMCVn Type Binaries and their Progenitors

AMCVn systems are a small group of mass transferring ultracompact binaries with orbital periods between 5.4 and 65 min. They consist of a white dwarf (WD) primary, and a WD or semi-degenerate helium star secondary and will act as verification sources for upcoming space based gravitational wave detectors like eLISA/NGO. A way to distinguish between these scenarios is to obtain the chemical composition of the donor, and in particular the C/O, N/O and N/C ratios, due to different levels of CNO and He burning in the progenitor of the donor.  In this talk I will give an overview of the observed abundances in AMCVn type systems and show that none of them show evidence of strong carbon enhancement indicating that the preferred channel is the WD donor channel rather than the helium star channel. I will link this result to possible progenitor systems such as double white dwarfs and helium stars with white dwarf companions.

Eleni Lalounta

The First Multi-colour Photometric Study of the Short-period Contact Eclipsing Binary V2790 Ori 

Eleni Lalounta1,  Eleftheria-Panagiota Christopoulou1, Athanasios Papageorgiou2  and  Alessandro Siviero3
1 Department of Physics, University of Patras, Greece 2 Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile 3 Department of Physics and Astronomy University of Padua, Italy

We present the first precision BVRcIc light curves of the short period contact binary, V2790 Ori, preliminary synthetic light curve solutions with PHOEBE and a period study. The photometric observations taken at Mythodia Observatory, University of Patras, Greece on 2015 are going to be combined with newly obtained spectroscopic data with 1.82 m telescope at Asiago Observatory, Italy. 

Emily Leiner

An Evolved Blue Straggler in M67 from K2 Mission Asteroseismology

Yellow stragglers (YSs) are stars that fall above the subgiant branch in optical color-magnitude diagrams, between the blue stragglers and the red giants. YSs may represent a population of evolved blue stragglers, but none have the mass and radius measurements needed to determine their evolutionary states and formation histories. Here we report the first asteroseismic mass and radius measurements of such a star, the yellow straggler S1237 in the open cluster M67. We apply asteroseismic scaling relations to a frequency analysis of Kepler K2 light curves and find a mass of 2.9 ± 0.2 Msun and a radius of 9.2 ± 0.2 Rsun. This is more than twice the mass of the main sequence turnoff in M67, suggesting S1237 is indeed an evolved blue straggler with a formation history involving at least three stars. S1237 is the primary in a spectroscopic binary. Through SED analysis we find that the secondary is likely an upper main-sequence star near the turnoff, but a slightly hotter blue straggler companion is also possible. We also compare the asteroseismic mass of the primary to the mass from CMD fitting and consider possible formation mechanisms for this star. We suggest that S1237 may have formed from dynamical encounters resulting in stellar collisions or a binary merger.

Lifang Li

The Influences of the Mixing Length Parameters and Metallicities on the Structure and Evolution of W UMa Type Binaries

In this work, we have discussed the convection mixing parameter, and the metallicities on the structure and evolution of low-mass W UMa type binary systems based on Eggleton's code, and found that the convection mixing length parameter and metallicities have a significant influence on the mass-radius and mass-effective temperature relationships of ZAMS stars. The smaller the convection mixing length parameter, $\alpha$, is taken to be, the lower the effective temperature of each component is and the larger the radius is, so that the convection mixing length has a significant influence on the location of the theoretical models in the period-color diagram of observed W UMa systems. Our models can cover the whole region of the period-color diagram of the observed systems if the convection mixing length parameter $\alpha$ is taken to be in a region of $0.90\la\alpha\la1.884$ so that our models with different values of mixing length parameter can explain the scatter of the observed systems in period-color diagram. Meanwhile, the metallicity are found to be a significant influence on the structure and evolution of low-mass W UMa systems. The models with the different metallicities are located in the different region in the period-color diagram, so that the scatter of the observed W UMa systems in the period-color diagram might be caused by the observed W UMa systems with different convective mixing length parameters or different metallicities.

Zhongmu Li

Powerful CMD: a Tool for Studying Binary Fraction of Star Clusters

A tool for studying the binary fraction of star clusters from the colour-magnitude diagrams is presented. Binary fractions are determined via the effects of binaries on the width of main-seqence,the fraction of blue stragglers and the shape of red clump. It is possible that the binary fraction of many star clusters can be determined via this tool. This will help us to understand more clearly about star clusters and to put forward the modeling of stellar populations of star clusters.  

Zhengwei Liu

Constraining the Progenitors of Type Iax Supernovae

Type Iax supernovae (SNe Iax) are proposed as one new sub-class of SNe Ia since they present observational properties that are sufficiently distinct from the bulk of SNe Ia. SNe Iax are the most common of all types of peculiar SNe by both number and rate, with an estimated rate of occurrence of about 5%–30% of the total SN Ia rate. However, the progenitor systems of SNe Iax are still unknown. We compare current observations for SNe Iax with theoretical predictions from binary evolution and population synthesis calculations for a variety of potential progenitor scenarios to put constraints on the progenitors of SNe Iax.

Rajeev Manick

Binarity amongst RV Tauri Stars with a Disc

Context. RV Tauri stars have long thought to be post-AGB stars as a result of their observational properties. During the last few decades, it became more overt that binarity is a prevalent phenomenon amongst RV Tauri stars displaying a broad dust excess already starting at H or K band. The binary orbital parameters in numerous cases, gives strong indication that these stars could have had a different fate on the AGB due to strong interactions with the unseen companion. This study fronts the possible impacts of binarity upon late stages of stellar evolution.
Aims. In this paper we determine the orbital parameters of 6 binary RV Tauri stars with a disc. The orbits are contaminated by the high amplitude pulsations and we aim to disentangle the pulsations from the orbits to constrain accurate orbital parameters. We also locate them on the HR diagram to constrain their evolutionary stage.
Methods. We used high resolution spectroscopic time series obtained from the HERMES and CORALIE spectrographs mounted on the Mercator and Swiss Leonard Telescopes, respectively. An updated ASAS/AAVSO photometric time series is analysed to complement the pulsations found in the spectroscopy and to clean the radial velocities from the pulsations. The pulsation-cleaned orbits are fit with a Keplerian model from which the orbital parameters are derived. We also present a novel PLC relationship using type II cepheids in the LMC, which is used to obtain accurate distances to the galactic targets and hence their luminosities.
Results. The main result is that all the 6 stars included in this study are binaries with orbital periods ranging between 660 and 1800 days with eccentricities between 0.2 and 0.6. The mass functions have values between 0.08 to 0.5 which points to an unevolved low mass companion. In the photometric time series we detect a long-term variation on the scale of the orbital period for IRAS 17038-4815 and IRAS 09144-4933, proving their RVb nature. The stellar luminosities we derive indicates that all except DY Ori and EP Lyr, are good post-AGB candidates.
Conclusions. The orbital parameters strongly suggests that the evolution of these stars was interrupted by a strong phase of binary interaction on the AGB or even prior to the AGB. These systems are expected to have already circularized, given their size on the AGB compared to their orbital separations. The high observed eccentricities and long orbital periods among these stars remains unexplained.

Elvijs Matrozis

Radiative Levitation in Carbon-enhanced Metal-poor Stars

It is commonly assumed that the surface abundances of carbon-enhanced metal-poor stars with s-process enrichment (CEMP-s stars) reflect the nucleosynthesis occurring in low-metallicity asymptotic giant branch stars, an important contributor to the chemical evolution of the Universe. By modelling the post-mass-transfer evolution of CEMP-s stars in detail, we show that this assumption should not hold because of gravitational settling and radiative levitation modifying the composition of the radiative regions of CEMP-s stars. However, observations indicate that additional physical processes, like mass loss or turbulence, must largely counteract gravitational settling and radiative levitation in these stars.

Maxwell Pollack

Modeling Blue Straggler Formation through Case C Mass Transfer with MESA

Case C mass transfer (Roche Lobe overflow from an AGB donor to a MS accretor) is known to be a common formation mechanism for blue stragglers (BSs), however much is still not well understood about this mass transfer process. Using the stellar evolution code MESA (Modules for Experiments in Stellar Astrophysics) and its newly implemented binary module, simulated BSs have been produced through accretion onto a main sequence progenitor star by Roche lobe overflow (RLOF) from an AGB companion. Depending upon the evolutionary stage of the donor at RLOF, the mass transfer rate is either steady and continuous, indicating an early-AGB donor, or follows an intermittent, start-stop pattern of mass loss, indicating a TP-AGB donor star. While these models make great improvements upon previous simulations with other codes, uncertainties in the treatment of angular momentum transfer and Helium flash mass loss still present strong barriers to their use in estimating progenitor system binary properties (masses, periods, etc.), and will be the subject of future work.

Theodor Pribulla

Physical Parameters and 0.1% Parallax of the Detached Eclipsing Binary V923 Scorpii

Context. V923 Sco is a bright (V = 5.91), nearby (π = 15.46±0.40 mas) southern eclipsing binary. With both components being slow rotators the projected masses of the components are known with about 0.2% precision. The system seems ideal for very precise mass, radius and luminosity determination.
Aims. We aim at very precise determination of absolute stellar parameters for both components of the eclipsing binary and model-independent determination of the distance.
Methods. We obtained high-precision photometry of both eclipses of V923 Sco with the MOST satellite as well as spatially resolved the orbit with the VLTI at 8 epochs. Combining projected size of the spectroscopic orbit (in km) and visual orbit (in mas) we derive distance to the system.
Results. The photometric observations show that both eclipses occur in V923 Sco. We could determine very precise absolute parameters of the components in spite of the parameter correlations. We determined the distance to V923 Sco with better than 0.1 percent precision combining spectroscopic and interferometric observations and therefore provide a stringent test of Gaia parallaxes.
Conclusions. We show that combining spectroscopic and interferometric observations can lead to parallaxes with about 10μas precision.

Françoise Raucq

Observational Signatures of Past Mass-exchange Episodes in Massive Binaries : The Cases of LSS 3074 and HD 17505

Mass and angular momentum exchanges through Roche Lobe Overflow interactions within close massive binaries are known to play an important role in the subsequent evolution of the components of such systems, and produce several observational signatures, such as asynchronous rotation and altered chemical compositions, that remain once the stars detach again. We have started to investigate these effects in a sample of massive O-star binaries that are thought to have previously experienced a Case A Roche Lobe Overflow episode. Using phase-resolved spectroscopy, we perform the disentangling of the optical spectra of the two stars. The reconstructed primary and secondary spectra are then analysed with the CMFGEN model atmosphere code to determine stellar parameters such as the effective temperatures, surface gravities and rotational velocities, and to constrain the chemical composition of the components. In this contribution, we present the results of our analyses of LSS 3074 (O5.5I + O6.5-7I, P = 2.1852 days), together with the analyses of its photometric lightcurve and orbital solution. We also present the first results of our analyses of the triple system HD17505 ([O7.5V + O7.5V, P = 8.57 days] + O6.5III).

Martha Irene Saladino Rosas

AMUSEing Winds in Binary Stars

Most stars are found in binary systems. During their evolution, especially during the AGB phase, stars lose material due to stellar winds; a fraction of this material can be gravitationally accreted by the companion, producing changes in the system. We investigate mass transfer in low-mass binaries to see how the mass accreted by the companion depends on the orbital parameters of the system and how it affects the evolution of the orbit. Using the AMUSE framework, we perform SPH simulations of the outflow from the primary star and by coupling it with a gravity code, we study the interaction of the matter with the secondary. We have studied the flow structure for a few simple cases: a constant wind velocity profile with isothermal and adiabatic equations of state, as well as more realistic simulations in which cooling of the gas has been included. Our preliminary results show a clear spiral structure in the outflow and evidence for an accretion disk forming around the secondary star.

Veronika Schaffenroth

The EREBOS Project - Studying the Influence of Substellar or Very Low Mass Stellar Companions on Late Stellar Evolution

Planets and brown dwarfs in close orbits will interact with their host stars, as soon as they evolve to become red giants. However, the outcome of those interactions is still unclear. Recently, several brown dwarfs have been discovered orbiting hot subdwarf stars in very short orbital periods of 0.065 - 0.096 d. More than 5% of those stars might have close substellar companions. This shows that such companions can significantly affect late stellar evolu tion and that sdB binaries are ideal objects to study this influence. Thirty-six new eclipsing sdB binary systems with cool low-mass companions with periods from 0.05 to 0.5 d were discovered based on their lightcurves by the OGLE project. We want to use this unique and homogeneously selected sample to derive the mass distribution of the companions, constrain the fraction of sub- stellar companions and determine the minimum mass needed to strip of the red-giant envelope. We are especially interested in testing models that predict hot Jupiter planets as possible companions. Therefore, we started the EREBOS (Eclipsing Reflection Effect Binaries from the OGLE Survey), which aims at analyzing all newly discovered HW Vir systems based on a spectroscopic and photometric follow-up of all targets. For this we were granted with an ESO Large Program for ESO/VLT-FORS. Here we will introduce this new project and give the current status together with first results.

Tuğça Şener

Infrared Spectra Analysis of HW Vir

We present a preliminary analysis of infrared spectroscopy of HW Vir, a post-common-envelope binary system, comprising of a hot subdwarf B and a cool M dwarf. At the end of April 2016, we obtained the data from McDonald Observatory with the Immersion Grating Infrared Spectrograph (IGRINS) with high resolution of R ≃ 45,000 between 1.5-2.5μm. Until now, there has been various attempts to solve the system in optical wavelengths, yet this study aims to improve the solution set with the contribution of IR data. From the IGRINS Spectral Library program, which provides high quality stellar spectra covering a wide range of effective temperature and surface gravity as well as the metallicity, we aim to use the spectra of B and M stars; to constrain the characteristics of cold companion more explicitly.

Yong Shao

Orbital Evolution of Low Mass X-ray Binaries

The general picture that drives the evolution of low mass X-ray binaries (LMXBs) has been described in the past decades, while the detailed process is still unclear. We use the orbital period derivative measured at the current epoch to diagnose the physical mechanisms during the LMXB evolution. The orbital behaviors of LMXBs are complex, the observed period derivatives can not be explained by the orbital angular momentum losses due to gravitational radiation, magnetic braking and mass loss.  We propose that X-ray irradiation from the accretor and magnetic activity on the stellar surface may perturb the structure of the donor stars. As a result of tidal interactions, the orbit-spin coupling in LMXBs can produce the observed rapid changing rate of orbital periods.  Compared to the stable mass transfer in the standard LMXB evolution, the fluctuant mass transfer may be the true situation. 

Sagiv Shiber

3D Simulations of the Grazing Envelope Evolution

We present three-dimensional simulations of the grazing envelope evolution (GEE), with the goal of exploring the basic flow properties and the role that jets play in removing the envelope. In the simulated run, a secondary main-sequence star grazes the envelope of the primary asymptotic giant branch star as it spirals-in. We inject two opposite jets perpendicular to the equatorial plane from the location of the secondary star, and follow the evolution of the envelope and of the ejected mass for several orbital periods.

Amaury Triaud

To follow

Gregor Traven

Exploring Peculiar Morphologies in Large Spectroscopic Surveys using t-SNE Reduction of Spectral Information

Peculiar spectra and objects to which they belong are relatively abundant among targets of general all-sky surveys such as RAVE, Gaia-ESO, and GALAH. Detection of such objects is important because the automatic evaluation of their stellar and chemical properties might turn out to be very challenging and therefore introduces a complication in achieving scientific goals such as galactic archaeology. On the other hand, the nature of peculiar objects can be quite intriguing and might include various types of stars such as cataclysmic variables, symbiotic stars, stars with massive outflows or inflows, stars exhibiting chromospheric emission, different binary systems, and others. We employ the novel dimensionality reduction technique t-SNE (t-Distributed Stochastic Neighbor Embedding) to alleviate the discovery and overview of distinct morphological types that occur among the thousands of spectra in large surveys. Using all available spectral information, t-SNE analysis exposes recognizable morphological groups, detecting also diverse influences or errors made in the spectral reduction procedures. A variety of the technique called parametric t-SNE enables us to assign distinct morphological flags related to the input catalogue of spectra with known peculiar morphologies. For a convenient overview of the results of our method, we present a very user-friendly utility called t-SNE Explorer, which was already tested in several surveys and can be readily adopted as a discovery and examination tool for particular types of spectra.

Kenny Van

Stability of Mass Transfer from Massive Giants

The mass transfer in binaries with massive donors and compact companions, when the donors rapidly evolve after their main sequence, is one of the dominant formation channels of merging double stellar-mass black hole binaries. This mass transfer was previously postulated to be unstable and was expected to lead to a common envelope event. The common envelope event then would end with either double black hole formation, or with the merger of the two sts. We re-visit the stability of this mass transfer, and find that for a large range of the binary orbital separations this mass transfer is stable. This newly found stability allows us to reconcile the theoretical rate for double black hole binary mergers predicted by population synthesis studies, and the empirical rate obtained by LIGO. Furthermore, the stability of the mass transfer leads to the formation of ultra-luminous X-ray sources. The theoretically predicted formation rates of ultra-luminous X-ray sources powered by a stellar-mass BH, as well as the range of produced X-ray luminosity, can explain the observed bright ultraluminous X-ray sources.

Joris Vos

The Unexpected Eccentricity of Long Period sdB Binaries

Hot subdwarf binaries are evolved core He burning stars with very thin hydrogen envelopes (<0.1 Msol). The only way to form these stars is through binary evolution, which makes then interesting objects to study binary evolution methods.

Hot subdwarf-B stars in long period binaries are found to be on eccentric orbits, even though current binary evolution theory predicts these systems to circularize before the onset of Roche-lobe overflow.

We aimed to find binary evolution mechanisms that can explain these eccentric long period orbits, and reproduce the currently observed period-eccentricity diagram. Three different processes are considered; tidally enhanced wind mass loss, phase dependent RLOF on eccentric orbits and the interaction between a circumbinary disk and the binary. To test these processes they were implemented in the binary module of the stellar evolution code MESA.

We find that a combination of phase dependent mass loss and circumbinary disk interaction can explain the eccentricities of the observed systems, but the models are unable to reproduce the observed trend of higher eccentricities at longer orbital periods. Further observations in combination with theoretical modeling will hopefully solve this discrepancy.

Maja Vuckovic

HW Vir: a Direct Detection of the Secondary Star

HW Virginis is an eclipsing binary, consisting of a hot subdwarf-B star primary and an M-dwarf secondary on a 2.8 h orbit. The hot sdB star of almost 30 000K strongly irradiates the facing hemisphere of the M-dwarf, which is otherwise invisible. The close binary may also host a planetary system with 2 Jovian planets. HW Vir has been studied extensively in order to determine the stellar and binary parameters. However, because the secondary star has never been directly detected in spectroscopic data, it has not been possible to determine accurate stellar masses.  Here we present the first direct detection of the secondary star in phase - resolved spectroscopic VLT + X-shooter data. These data contain both absorption and emission features from the low-mass secondary star, originating in the inflated, irradiated hemisphere.

Bo Wang

The Hybrid CONeWD + He Star Binaries as the Progenitors of Type la Supernovae

The hybrid CONe white dwarfs (WDs) have been suggested to be possible progenitors of type Ia supernovae (SNe Ia). In this work, we systematically studied the hybrid CONe WD + He star channel for the progenitors of SNe Ia, in which a hybrid CONe WD increases its mass to the Chandrasekhar mass limit by accreting He-rich material from a non-degenerate He star. According to a series of detailed binary population synthesis simulations, we obtained the SN Ia birthrates and delay times for this channel. The SN Ia birthrates for this channel roughly accounts for 1%-18% of all SNe Ia. The estimated delay times are ~28Myr-178Myr, which are the youngest SNe Ia predicted by any progenitor model so far. We suggest that SNe Ia from this channel may provide an alternative explanation of type Iax SNe. We also presented some properties of the donors at the point when the WDs reach the Chandrasekhar mass. These properties may be a good starting point for investigating the surviving companions of SNe Ia, and for constraining the progenitor channel studied in this work.

You Wu

The Formation of sdB+Neutron Star Binaries

Utilizing binary evolution theory, we predicted that sdB+NS/BH systems exist in our Galaxy, even though we haven't found any direct evidence to support this so far. Here, we present the results of a numerical simulation of the formation of sdB binaries that have a neutron star companion. These systems are believed to arise from MS+NS systems which experience either common envelope(CE) ejection or stable mass transfer. Our models initially consist of a NS(1.4M⊙) and a main sequence star, where the initial mass of the main sequence star ranges from 2 to 5M⊙. By means of simulations, we find that (1) for each study sample, the sdB+NS systems will have the maximum RV semi-amplitude (K) only if the main sequence star fills its Roche lobe at the end of the main sequence phase, or the beginning of the Hertzsprung gap phase;(2) the ultimate mass of the neutron star is inversely proportional to the mass of the sdB star;(3) the age of the sdB+NS systems becomes younger with increasing initial mass of the main sequence star.

Xiaojie Xu

The Importance of Non-magnetic Cataclysmic Variables in our Galaxy

Cataclysmic Variables (CVs) are binaries where a white dwarf (WD) accretes matter from a main sequence or a sub-giant star. CVs are important X-ray emitters below 10^34 ergs/s in the solar neighborhood. CVs with strong magnetized WDs (mCVs) were proposed as the main contributors of the galactic ridge X-ray emission (GRXE) after Revnivtsev et al. (2009) based on the analogy of their luminosity functions and hard X-ray spectra. However, recent works suggested that non-magnetic CVs, although less luminous, are more populous than we thought. We carried out a systematic study on Suzaku observed CVs, including mCVs and non-magnetic ones, and compare their Fe line widths and ratios with those of GRXE. We found the values of line ratios and widths of non-magnetic CVs, instead of mCVs, are consistent with those of GRXE. We conclude that non-magnetic Cvs are more important contributors of the Galactic X-ray background. A more detailed investigation of of their evolution and population is required. 

Miloslav Zejda

Contact Binaries at the Period Cut-off

The period distribution shows a very sharp decline below 0.27 days (Terrell et al. 2012, Drake et al. 2014) and a short-period limit at about 0.22 d. Contact binaries below the short-period limit are very rare (e.g., Rucinski 1992; Becker et al. 2011).  We have selected some short-period binaries with an orbital period near and below the limit. The first BVRI photometry and new light curve solutions based on it are presented.  We discussed found parameters and geometrical structures.

Fenghui Zhang

EPS Models with Binaries – YUNNAN Models

Recently, observations found that the binary fraction is a steep, monotonic function of stellar mass (Duchêne and & Kraus 2013 ARA&A 51 269). For massive O stars, ~70% of them are in binary systems and would merge, be stripped of their envelope, spun-up either by accretion or coalescence (Sana et al. 2012 Sci. 337 444). Massive stars play an important role in the galaxy evolution. So, we must consider the binaries in the evolutionary population synthesis (EPS) models, which is a basic tool to derive the galaxy properties.  We have built such a set of EPS models, in which various binary systems are considered (Zhang et al. 2004, 2005a,b, 2006, 2009, YUNNAN-II models). To construct a stellar population (SP) comprising binaries, the following input distributions are needed: (i) the initial mass function of the primaries; (ii) the mass-ratio distribution of the binaries; (iii) the distribution of orbital separations; and (iv) the eccentricity distribution. In our constructed SPs, approximately 50% of stellar systems are binary systems with orbital periods less than 100 yr (the fraction is a typical value for the Galaxy).  We found that the inclusion of binaries would (i) increase the UV flux by 102 times at log(t/yr) > 9, (ii) increase the far UV flux by 102-5 times at 7<log(t/yr)<8, (iii) increase the (M/L)K by 0.2 dex, and (iv) make the colours bluer, and so on.   The above results are closely related to the UV-upturn problem of elliptical galaxies, HII regions, high-redshift Lyman-break galaxies (Han et al. 2007, Zhang et al. 2015, Smit et al. 2014), and so on. Moreover, the results are related to many galaxy parameters (such as, the total stellar mass M*, metallicity Z*, gaseous metallicity Zgas, star-formation rate SFR, age t, and so on). Applying the above results to the determinations of galaxy parameters, we found the inclusion of binary interactions make M*, Z* or t larger, SFR and Zgas smaller (Zhang et al. 2012, 2015). Because that the first three parameters are accumulated ones, the later two are the current ones, it reveals that galaxies maybe have experienced violet activities at early epoch of galaxy evolution and are relatively quiet recently.

Page last updated: 28 July 2016 at 11:36