Chandra Shekhar Saraf (NCAC, Warsaw)
The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H₀, the σ₈-S₈ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 σ tension between the Planck CMB estimate of the Hubble constant H₀ and the SH0ES collaboration measurements. After showing the H₀ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωₘ, and the amplitude or rate of the growth of structure (σ₈ , fσ₈). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H₀-S₈ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions.
Abdalla, E. et al., Journal of High Energy Astrophysics (2022)
Prasad R (IISER Bhopal)
The existence of quark stars is an open problem in astrophysics, and their formation is possible in several astrophysical scenarios via the quark-hadron phase transition. We addressed the spin-down induced phase transition scenario, wherein magnetic braking drives neutron stars from their birth (Keplerian rotation) to later stages of life (slow spin). The central density is found to rise during the slowing down stages, and on reaching a critical phase transition density, the neutron star transits to a hybrid star branch, and a quark core is seeded. The further slowing down results in the growth of the quark core. We computed the mass and size of the quark core during different stages of evolutionary history. The phase transition onset leads to an anomalous change in the magnetic braking index. Also, it can excite the star's f-mode oscillations, leading to burst-type gravitational wave signals in the range of present detectors. The other emissions could be neutrino bursts and GRBs. Detection of these signals and their sky localization may help in finding the quark/hybrid stars formed via phase transition events. This talk is based on: https://doi.org/10.1093/mnras/stac2324
Christian Eze (NCAC, Warsaw)
Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell . Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.
Gergely Hajdu (NCAC, Warsaw)
We use images collected with the near-infrared camera (NIRCam) on board the James Webb Space Telescope and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main sequence (MS) of 47 Tucanae. The F115W vs. F115W-F322W2 CMD from NIRCam shows that, below the knee, the MS stars span a wide color range, where the majority of M-dwarfs exhibit blue colors, and a tail of stars are distributed toward the red. A similar pattern is observed from the F160W vs. F110W-F160W CMD from HST, and multiple populations of M-dwarfs are also visible in the optical F606W vs. F606W-F814W CMD. The NIRCam CMD shows a poorly-populated sequence of faint MS stars that we tentatively associate with a population of very low-mass stars. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colors and magnitudes of different stellar populations in the NIRCam filters (at metallicities [Fe/H]=-1.5 and [Fe/H]=-0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs' chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colors of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, challenging the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.
Fatima Kayanikhoo (NCAC, Warsaw)
In at least 400 European caves such as Lascaux, Chauvet and Altamira, Upper Palaeolithic Homo sapiens groups drew, painted and engraved non-figurative signs from at least ∼42,000 BP and figurative images (notably animals) from at least 37,000 BP. Since their discovery ∼150 years ago, the purpose or meaning of European Upper Palaeolithic non-figurative signs has eluded researchers. Despite this, specialists assume that they were notational in some way. Using a database of images spanning the European Upper Palaeolithic, we suggest how three of the most frequently occurring signs—the line <|>, the dot <•>, and the
20000-year Ice Age drawings mystery
An Upper Palaeolithic Proto-writing System and Phenological Calendar
https://doi.org/10.1017/S0959774322000415
Lami Suleiman (NCAC, Warsaw)
The third generation of Gravitational Wave detectors includes the space based triangular detector LISA and the two ground based detectors Cosmic Explorer and Einstein telescope. If Cosmic Explorer is a brute force improvement on the LIGO/Virgo perpendicular arms configuration, Einstein telescope is exploring much more “exotic” features for signal improvement. For this Journal Club, I will first discuss the underground and triangle shape configurations in discussion for Einstein Telescope. Then, I will present two papers that propose the detection of Gravitational Waves in LHC like particles accelerators.
Articles for the discussion:
Amedeo Romagnolo (NCAC, Warsaw)
Of the roughly 3000 neutron stars known, only a handful have sub-stellar companions. The most famous of these are the low-mass planets around the millisecond pulsar B1257+12. New evidence indicates that observational biases could still hide a wide variety of planetary systems around most neutron stars. We consider the environment and physical processes relevant to neutron star planets, in particular the effect of X-ray irradiation and the relativistic pulsar wind on the planetary atmosphere. We discuss the survival time of planet atmospheres and the planetary surface conditions around different classes of neutron stars, and define a neutron star habitable zone based on the presence of liquid water and retention of an atmosphere. Depending on as-yet poorly constrained aspects of the pulsar wind, both Super-Earths around B1257+12 could lie within its habitable zone.
Patruno, A., & Kama, M. (2017) Astronomy & Astrophysics, Volume 608
Angelos Karakonstantakis (NCAC, Warsaw)
Krzysztof Nalewajko (NCAC, Warsaw)
We present unprecedented high-fidelity radio images of the M87 jet. We analyzed Jansky Very Large Array broadband full-polarization radio data from 4 to 18 GHz. The observations were taken with the most extended configuration (A configuration), which allows the study of the emission of the jet up to kiloparsec scales with a linear resolution of ~10 pc. The high sensitivity and resolution of our data allow us to resolve the jet width. We confirm a double-helix morphology of the jet material between ~300 pc and ~1 kpc. We found a gradient of the polarization degree with a minimum at the projected axis and maxima at the jet edges and a gradient in the Faraday depth with opposite signs at the jet edges. We also found that the behavior of the polarization properties along the wide range of frequencies is consistent with internal Faraday depolarization. All of these characteristics strongly support the presence of a helical magnetic field in the M87 jet up to 1 kpc from the central black hole, although the jet is most likely particle-dominated at these large scales. Therefore, we propose a plausible scenario in which the helical configuration of the magnetic field has been maintained to large scales thanks to the presence of Kelvin-Helmholtz instabilities.
Aikaterini-Niovi Triantafyllaki (Tartu Observatory, Estonia)
Large concentrations of mass are now understood to be the products of a Hubble time’s worth of merging and accretion. This history is preserved in the outer regions of galaxies' halos, where the dynamical scales are longer. This makes it possible to preserve fossil records of these events in the form of longlasting substructures imprinted in the physical properties of their stellar populations. In practice, this information is often hidden at surface brightness values below the sky. Planetary Nebulae (PNe) can solve this observational challenge: owing to their strong [OIII] emission line— they are easily detected— PNe offer a unique tool to investigate low surface brightness regions and gather detailed observational proof of the structures' evolution. In order to search for hierarchical processes, I analysed data from the Virgo Planetary Nebula Survey (VPNS) with the aim to study physical properties of its PN population and how they relate to the cluster properties as well as tracing variations in metallicity as a consequence of the presence of accretion events. As a result, the different values of the PN α-parameter are consistent with a gradient from more metal rich stars in the galaxies' centres towards more metal poor populations of stars at large radii until the galaxy stellar population mixes with the IC component, consistent with a late built-up of the galaxies' halos. By comparing the α-parameter values of galaxies subject to environmental effects with those in close regions of intracluster (IC) regions, this work has shown that the Virgo intracluster light (ICL) is built up over time as a consequence of the tidal forces acting on both late- and small early-type galaxies. This, in turn, causes the ICL to be characterised by different metallicity values, especially in the north-west region where the IC component is highly unrelaxed. With this work I also provided evidence for an unknown accretion event in the halo of the central galaxy, M87, that has caused an important modification of the metallicity of its outer stellar populations.
Aikaterini-Niovi Triantafyllaki & Alessia Longobardi (in preparation)
Gergely Hajdu (NCAC, Warsaw)
The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z=0.964. Keck LRIS spectra show that the [OIII]/Hβ ratio varies from ~1 to ~10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [OIII] knot with a luminosity of 1.9x1041 ergs/s. The stellar continuum colors vary along the feature, and are well-fit by a simple model that has a monotonically increasing age with distance from the tip. The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation. The best-fit time since ejection is ~39 Myr and the implied velocity is v~1600 km/s. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected in [OIII] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.
Unnikrishnan Sureshkumar (University of the Witwatersrand, Johannesburg, South Africa)
Galaxies live in dark matter haloes and hence the galaxy properties are majorly defined by the properties of the haloes. Thus the environmental dependence of dark matter halo properties prompts a correlation between galaxy properties and the environment. In this talk, I will discuss the results from our works (arXiv:2102.04177 and arXiv:2201.10480) that explored how luminosities in optical to mid-infrared bands, stellar mass, and star formation rate are correlated with the environment. We use a set of stellar mass-selected and 3.4 μm luminosity-selected galaxies from the Galaxy and Mass Assembly (GAMA) survey. We utilize the galaxy two-point correlation functions and marked correlation functions to investigate the environmental correlations. I will also discuss the impact of various selection effects on the galaxy clustering measurements. Additionally, I will show the results of our ongoing work with data from simulations.
Dragana Ilic (University of Belgrade)
Victor Oknyansky (University of Haifa)
Tomasz Krajewski (NCAC, Warsaw)
Terminal velocity reached by bubble walls in cosmological first order phase transitions is an important parameter determining both primordial gravitational-wave spectrum and production of baryon asymmetry in models of electroweak baryogenesis. We developed a numerical code to study the real-time evolution of expanding bubbles and investigate how their walls reach stationary states. Our results agree with profiles obtained within the so-called bag model with very good accuracy, however, not all such solutions are stable and realised in dynamical systems. Depending on the exact shape of the potential there is always a range of wall velocities where no steady state solutions exist. This behaviour in deflagrations was explained by hydrodynamical obstruction where solutions that would heat the plasma outside the wall above the critical temperature and cause local symmetry restoration are forbidden. For even more affected hybrid solutions causes are less straight forward, however, we provide a simple numerical fit allowing one to verify if a solution with a given velocity is allowed simply by computing the ratio of the nucleation temperature to the critical one for the potential in question.
Biswaraj Palit (NCAC, Warsaw)
How black holes consume and eject matter has been the subject of intense studies for more than 60 years. The luminosity of these systems are often compared to the Eddington limit, the border at which the spherical accretion is inhibited by the radiation pressure of photons it produces. The discovery of ultraluminous X-ray sources (ULXs) showed that accretion can proceed even when the apparent luminosity exceeds the Eddington limit (Kaaret et al. 2017). High apparent luminosity might be produced by the beaming of the incident radiation by a thick collimated outflow or by a truly super-Eddington accretion flow. However, possibilities to study these outflows in detail are limited, as ULXs are typically found in distant galaxies. Using the Imaging X-ray Polarimetry Explorer (IXPE, Weisskopf et al. 2022), we made the first measurement of X-ray polarization in Galactic X-ray binary Cyg X-3. The detection of high, ≈25\%, nearly energy-independent linear polarization, orthogonal to the direction of the radio ejections, unambiguously indicates the primary source is obscured and the observer on Earth only sees reflected and scattered light. Modelling shows there is an optically thick envelope with a narrow funnel around the primary X-ray source in the system. We derive an upper limit on the opening angle of the funnel that implies a lower limit on the beamed luminosity exceeding the Eddington value. We show that Cyg X-3 is viewed as a ULX to an extragalactic observer located along the axis of the funnel. Our findings reveal this unique persistent source as an ideal laboratory for the study of the inner workings of ULX central engines.
Sudhagar Suyamprakasam (NCAC, Warsaw)
Most all-sky searches for continuous gravitational waves assume the source to be isolated. In this paper, we allow for an unknown companion object in a long-period orbit and opportunistically use previous results from an all-sky search for isolated sources to constrain the continuous gravitational-wave amplitude over a large and unexplored range of binary orbital parameters without explicitly performing a dedicated search for binary systems. The resulting limits are significantly more constraining than any existing upper limits for unknown binary systems, albeit the latter apply to different orbital parameter ranges that are computationally much costlier to explore.
Sudhagar Suyamprakasam (NCAC, Warsaw)
Despite achieving sensitivities capable of detecting the extremely small amplitude of gravitational waves (GWs), LIGO and Virgo detector data contain frequent bursts of non-Gaussian transient noise, commonly known as 'glitches'. Glitches come in various time-frequency morphologies, and they are particularly challenging when they mimic the form of real GWs. Given the higher expected event rate in the next observing run (O4), LIGO-Virgo GW event candidate validation will require increased levels of automation. Gravity Spy, a machine learning tool that successfully classified common types of LIGO and Virgo glitches in previous observing runs, has the potential to be restructured as a signal-vs-glitch classifier to accurately distinguish between glitches and GW signals. A signal-vs-glitch classifier used for automation must be robust and compatible with a broad array of background noise, new sources of glitches, and the likely occurrence of overlapping glitches and GWs. We present GSpyNetTree, the Gravity Spy Convolutional Neural Network Decision Tree: a multi-CNN classifier using CNNs in a decision tree sorted via total GW candidate mass tested under these realistic O4-era scenarios.
Helena Ciechowska (Institute of Geophysics, PAN, Warsaw)
Man-made changes to the environment can trigger seismic events. The Castanhao region (Ceara, Brazil) is an example of such activity. The dam built over the Jaguaribe River was followed by the creation of the Castanhao reservoir, which soon before the opening of the dam started to impact the occurrence of earthquake swarms in the area. In my work, I analyze the continuous data recorded on a seismic network of 6 seismological stations, between January 2010 and December 2010. In order to process such a vast dataset, the PyMPA template matching software is used along with STA/LTA algorithm.
Adhikary Jyotismita (NCBJ, Warsaw)
Due to notoriously small value of the neutrino magnetic moment, the phenomena of neutrino spin flavour precession (SFP) requires very high magnetic field. This makes only a handful of systems suitable to study this phenomena. By the observation of SFP, the Dirac and Majorana nature of neutrinos is expected to be distinguished. My talk will be based on the paper: "Neutrino spin flavour precession in magnetised white dwarf" DOI 10.1088/1361-6471/acd46b. In this work, we pointed out the potential of white dwarf (WD) system in studying the spin-flavour oscillation of neutrinos. The findings include: spin flavour transition probability of Dirac neutrinos is much higher in comparison to the Majorana neutrino which converts the active neutrino flavours to sterile in a significant amount and along with that, the neutrino magnetic moment plays a very sensitive role in the process.
Felipe Espinoza Arancibia (NCAC, Warsaw)
We present an analysis of the theoretical and observed light-curve parameters of the fundamental mode (FU) classical Cepheids in the Magellanic Clouds in V- and I- photometric bands. The state-of-the-art 1D non-linear radial stellar pulsation (RSP) code in MESA (MESA-RSP) has been utilized to generate the theoretical light curves using four sets of convection parameters. Theoretical light curves with two chemical compositions: Z = 0.008 and Z = 0.004 appropriate for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, covered a wide range of periods (3
Parikshit Biswas (NCAC, Warsaw)
Stellar mass black holes in X-ray binaries (XRBs) are known to display different states characterized by different spectral and timing properties, understood in the framework of a hot corona coexisting with a thin accretion disk whose inner edge is truncated. There are several open questions related to the nature and properties of the corona, the thin disk, and dynamics behind the hard state. This motivated us to perform two-dimensional hydrodynamical simulations of accretion flows onto a 10 solar masses black hole. We consider a two-temperature plasma, incorporate radiative cooling with bremmstrahlung, synchrotron and comptonization losses and approximate the Schwarzschild spacetime via a pseudo-Newtonian potential. We varied the mass accretion rate in the range 0.02 ≤ Ṁ/Ṁ_Edd ≤ 0.35. Our simulations show the natural emergence of a colder truncated thin disk embedded in a hot corona, as required to explain the hard state of XRBs. We found that as Ṁ increases, the corona contracts and the inner edge of the thin disk gets closer to the event horizon. At a critical accretion rate 0.02 ≤ Ṁ_crit \Ṁ_Edd ≤ 0.06, the thin disk disappears entirely. We discuss how our simulations compare with XRB observations in the hard state.
Dr. Miljenko Cemeljic (CAMK, Warsaw)
With the era of Software Defined Radio, amateur radio-astronomy obtained a new impulse. It is possible to build a small radio telescope at low cost (less than 100 eur) and use it to observe the neutral hydrogen line (21cm) in our Galaxy and also radio emissions from the Sun and Jupiter. There are even amateur observations of pulsars with such devices. Such projects, involving astronomy, radio technics and data analysis are a welcome feature for STEM teaching at various levels. I will explain how to build such a telescope and observe the Galaxy rotation curve.
Amin Mosallanezhad (CAMK, Warsaw)
Black hole astrophysics is a rapidly growing field of study that has captured the attention of many researchers in recent years. The extreme physical conditions and super-gravity of black holes make them a fascinating subject of research. Black holes play a fundamental role in many active phenomena in the universe, such as active galactic nuclei, stellar-mass black holes, ultra-luminous X-ray sources, gamma-ray bursts, and stellar and galactic jets. Moreover, black hole astrophysics is closely related to other crucial topics in astrophysics. Observational evidence has established a strong correlation between the mass of the black hole and the luminosity, stellar velocity dispersion, or stellar mass in the galaxy spheroid. This correlation suggests that central supermassive black holes (SMBHs) play a critical role in the formation and evolution of galaxies. Active galactic nuclei (AGN) emit intense radiation, jets, and wind, which can spread from the interior of the galaxy to the spatial scale of the dark matter halo outside the galaxy. These outputs can interact with the interstellar medium, affecting the temperature, density, and spatial distribution of these gases, and ultimately impacting the formation of stars in the galaxy and even leading to the extinction of the galaxy. At the extra-galactic scale, the nuclear output of these active galaxies can interact with the gas in the ring galaxy medium and the galaxy's halo, thereby affecting the gas supply of the galaxy's outer gas to the galaxy, which in turn will also affect the star formation and galaxy evolution.
Angelos Karakonstantakis (NCAC, Warsaw)
Motivated by the increasing number of detections of merging black holes by LIGO-VIRGO-KAGRA, black hole (BH) binary mergers in the discs of active galactic nuclei (AGNs) is investigated as a possible merger channel. In this pathway, BH encounters in the gas disc form mutually bound BH binary systems through interaction with the gas in the disc and subsequently inspiral through gravitational torques induced by the local gas. To determine the feasibility of this merger pathway, we present the first three-dimensional global hydrodynamic simulations of the formation and evolution of a stellar-mass BH binaries AGN discs with three different AGN disc masses and five different initial radial separations. These 15 simulations show binary capture of prograde and retrograde binaries can be successful in a range of disc densities including cases well below that of a standard radiatively efficient alpha disc, identifying that the majority of these captured binaries are then subsequently hardened by the surrounding gas. The eccentricity evolution depends strongly on the orbital rotation where prograde binaries are governed by gravitational torques form their circumbinary mini disc, with eccentricities being damped, while for retrograde binaries the eccentricities are excited to >~ 0.9 by accretion torques. In two cases, retrograde binaries ultimately undergo a close periapsis passage which results in a merger via gravitational waves after only a few thousand binary orbits. Thus, the merger time-scale can be far shorter than the AGN disc lifetime. These simulations support an efficient AGN disc merger pathway for BHs.
Parikshit Partha Biswas (NCAC, Warsaw)
The mass (𝘔ₜₒᵥ) and radius (𝘙ₜₒᵥ) of the maximum-mass nonrotating neutron star (NS) play a crucial role in constraining the elusive equation of state (EOS) of cold dense matter and in predicting the fate of remnants from binary neutron star (BNS) mergers. In this study, we introduce a novel method to deduce these parameters by examining the mergers of second-generation (2G) BHs with NSs. These 2G BHs are assumed to originate from superamassive neutron stars (SMNSs) formed in BNS mergers. Since the properties of the remnant BHs arising from the collapse of SMNSs follow a universal relation governed by 𝘔ₜₒᵥ and 𝘙ₜₒᵥ, we anticipate that by analyzing a series (∼100 detections) of mass and spin measurements of the 2G BHs using the third-generation ground-based gravitational wave detectors, 𝘔ₜₒᵥ and 𝘙ₜₒᵥ can be determined with a precision of ∼0.01M⊙ and ∼0.6 km, respectively.
Special session (NCAC, Warsaw)
Special session focusing on three recent works published by CAMK employees and students.
Gonzalo Ignacio Rojas García (NCAC, Warsaw)
Exoplanets smaller than Neptune are common around red dwarf stars (M dwarfs), with those that transit their host star constituting the bulk of known temperate worlds amenable for atmospheric characterization. We analyze the masses and radii of all known small transiting planets around M dwarfs, identifying three populations: rocky, water-rich, and gas-rich. Our results are inconsistent with the previously known bimodal radius distribution arising from atmospheric loss of a hydrogen/helium envelope. Instead, we propose that a density gap separates rocky from water-rich exoplanets. Formation models that include orbital migration can explain the observations: Rocky planets form within the snow line, whereas water-rich worlds form outside it and later migrate inward.
Luque, R., & Pallé, E. (2022) https://doi.org/10.1126/science.abl7164
Special session (NCAC, Warsaw)
1. Andrzej Zdziarski
"What are the spins of stellar-mass black holes?"
based on the papers:
"Black hole spin measurements in LMC X-1 and Cyg X-1 are highly
model-dependent"
by Zdiarski et al.:
https://arxiv.org/abs/2308.06167
and
"Common origin of black holes in high mass X-ray binaries and in
gravitation-wave sources"
by Belczynski et a.:
https://arxiv.org/abs/2111.09401
2. Michał Bejger
"Astrophysics with continuous gravitational waves"
by Haskell & Bejger:
https://ui.adsabs.harvard.edu/abs/2023NatAs...7.1160H/abstract
Monica Sanjinez Ortiz (NCAC, Warsaw)
Phase diagrams of fully ionized binary ionic mixtures are considered within the framework of the linear mixing formalism taking into account recent advances in understanding quantum one-component plasma thermodynamics. We have followed a transformation of azeotropic phase diagrams into peritectic and eutectic types with increase of the charge ratio. For solid 12C/16O and 16O/20Ne mixtures, we have found extensive miscibility gaps. Their appearance seems to be a robust feature of the theory. The gaps evolve naturally into two-solid regions of eutectic phase diagrams at higher Z2/Z1. They do not depend on thermodynamic fit extensions beyond their applicability limits. The gaps are sensitive to binary mixture composition and physics, being strongly different for C/O and O/Ne mixtures and for the three variants of corrections to linear-mixing solid-state energies available in the literature. When matter cools to its miscibility gap temperature, the exsolution process takes place. It results in a separation of heavier and lighter solid solutions. This may represent a significant reservoir of gravitational energy and should be included in future white dwarf (WD) cooling simulations. Ion quantum effects mostly resulted in moderate modifications; however, for certain Z2/Z1, these effects can produce qualitative restructuring of the phase diagram. This may be important for the model with 22Ne distillation in cooling C/O/Ne WD proposed as a solution for the ultramassive WD cooling anomaly.
Felipe Espinoza Arancibia (NCAC, Warsaw)
The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparison of the theoretical IS edges with those obtained empirically, using the most recent Cepheids catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. In this study, we investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC), considering any effect that increases its width, to obtain intrinsic edges that can be compared with theoretical models. We use data of classical fundamental-mode (F) and first-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. Our final sample includes 2058 F and 1387 1O Cepheids. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. We obtain the blue and red edges of the IS in V- and I-photometric bands, in addition to logTeff and logL. The results obtained show a break located at the Cepheids' period of about 3 days, which was not reported before. We compare our empirical borders with theoretical ones published in the literature obtaining a good agreement for specific parameter sets. The break in the IS borders is most likely explained by the depopulation of second and third crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Results from the comparison of our empirical borders with theoretical ones prove that our empirical IS is a useful tool for constraining theoretical models.
Espinoza Arancibia et al. 2023:
https://ui.adsabs.harvard.edu/abs/2023arXiv231115849E/abstract
