arXiv subjects

A. C. Rubio

Publications and source records attributed to A. C. Rubio.

7 recordsLinked to original sources

arXiv2026

The birth of Be star disks III. SPH models of localised mass ejections

Classical Be stars exhibit mass ejection events that feed their viscous decretion disks. Recent TESS space photometry and simultaneous spectroscopy revealed that these flickers are localised, short-lived, and associated with near-Keplerian rotating material close to the stellar surface. We aim to constrain the geometrical and dynamical conditions required for a localised surface ejection to generate a Keplerian decretion disk and to predict the corresponding photometric, spectroscopic, and polarimetric observables. Material, and the injection radius. The SPH outputs were post-processed with the radiative transfer code HDUST to obtain synthetic observables. We scaled the density of the models to match the reference flicker for the Be star f\,Car. A mildly super-Keplerian rotation of the injection volume, a high viscosity, and a mass-loss rate of the order $10^{-6}\,\rm M_\odot \, yr^{-1} \, str^{-1}$ are required for the ejected material to remain in orbit and form a small disk. The synthetic observables reproduce the behaviour of the reference flicker. The simulations confirm that during mass ejection the disk is asymmetric and dynamically evolving, and circularises within a few days after the end of the flicker. Models with too wide mass ejection angle or too high angular velocity fail to reproduce the observed light curve and line profile behaviour. The models are consistent with mass ejection happening very close to the stellar equator. Localised, short-duration, mildly super-Keplerian ejections combined with high viscosity and high mass-loss rates can account for the short-timescale variability of the circumstellar environment of Be stars. Be disks can be formed from such outbursts and realistic 3D injection geometries are essential to connect surface dynamics to disk build-up within the framework of the viscous decretion disk model.

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arXiv2025

Hidden massive eclipsing binaries in red supergiant systems: The hierarchical triple system KQ Puppis and other candidates

The majority of massive stars are part of binary systems that may interact during their evolution. However, not many RSGs are known binaries, and only a few have constrained orbital parameters. We search the available TESS photometry for eclipsing companions of RSGs. We focus on the best candidate, VV Cephei type binary KQ Pup, which is made up of a RSG, KQ Pup A, and a B-type companion, KQ Pup B (orbital period of 26 yr). We use photometry, spectroscopy, and newly taken interferometric data with VLTI-GRAVITY. Using TESS, we discovered eclipses with a period of $17.2596 \: \rm d$, associated with KQ Pup B, making it a Ba+Bb binary. The detection of the hydrogen Br$\gamma$ line with VLTI-GRAVITY enabled us to track the orbital motion of the Ba+Bb pair relative to A and determine the astrometric orbit of A+B. The dynamical masses agree with independent estimates from asteroseismology and evolutionary models. The results give a mass of $ \sim 10 \: \rm M_{\odot} $ for the RSG KQ Pup A and $ \sim 14 \: \rm M_{\odot} $ for the sum of the hot components Ba+Bb. We determined an orbital parallax of $\pi = 1.24^{+0.05}_{-0.04}\, \rm mas $, which is the first such parallax measurement for a RSG. KQ Pup represents a unique demonstration of mass transfer mechanism in wide eccentric RSG systems. The variability of Balmer emission lines and the detection of Br$\gamma$ are a strong signature of accretion to Ba+Bb near periastron. With the RSG filling its Roche lobe only by $\sim 70\%$ at periastron, the mass transfer is instead driven by accretion from its extended atmosphere via the Wind Roche Lobe Overflow. The accretion disk dissipates by apastron. Overall, we discovered that several previously assumed RSG binaries host eclipsing inner systems, corresponding to $\sim 10 \%$ of all known Galactic RSG binaries. This suggests that many of the other RSG binaries may also be hierarchical triples.

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arXiv2025

Calibration of Binary Population Synthesis Models Using White Dwarf Binaries from APOGEE, GALEX and Gaia

The effectiveness and stability of mass transfer in binaries system are crucial in determining its final product. Rapid binary population synthesis (BPS) codes simplify the complex physics of mass transfer by adopting parameterized prescriptions for the stability of mass transfer, accretion efficiency in stable mass transfer, and the efficiency of common-envelope ejection. We calibrate these uncertain parameters by comparing BPS models with observational data. White dwarf and main sequence binaries are an ideal population to study binary interaction, as they can be formed through stable or unstable mass transfer, or without interaction, which affect the orbital period and masses of the present-day population. The APOGEE-GALEX-Gaia catalog provides a homogeneous sample of over 500 systems with well measured radial velocities that can be used as a comparison baseline for BPS simulations of such binaries. We compare the distribution of observed maximum radial velocity variation ($ΔRV_{\rm max}$) and estimated masses to BPS models simulated with COSMIC, varying the mass transfer and common-envelope ejection efficiency, and the criteria for mass transfer stability at key evolutionary stages. The $ΔRV_{\rm max}$ comparison shows clear preference for a higher fraction of stable mass transfer during the first ascent giant branch, and for highly effective envelope ejection. For the systems with WD masses, there is a slight preference for non-conservative mass transfer. In COSMIC and similar codes, the envelope ejection efficiency and the envelope binding energy are degenerate parameters. Our result of high ejection efficiency may indicate that either additional sources of energy are required to eject the envelope, or that its binding energy is lower than traditionally assumed. Future comparisons to BPS simulations can be drawn for other datasets as they become available.

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arXiv2025

The birth of Be star disks I. From localized ejection to circularization

Classical Be stars are well known to eject mass, but the details governing the initial distribution and evolution of this matter into a disk are poorly constrained by observations. By combining high-cadence spectroscopy with contemporaneous space photometry from TESS, we have sampled about 30 mass ejection events in 13 Be stars. Our goal is to constrain the geometrical and kinematic properties of the ejecta, facilitating the investigation into the initial conditions and evolution, and understanding its interactions with preexisting material. The photometric variability is analyzed together with measurements of the rapidly changing emission features to identify the onset of outburst events and obtain information about the geometry of the ejecta and its evolution. All Be stars observed with sufficiently high cadence exhibit rapid oscillations of line asymmetry with a single frequency in the days following the start of the event. The emission asymmetry cycles break down after roughly 5 - 10 cycles, with the emission line profile converging toward approximate symmetry. In photometry, several frequencies typically emerge at relatively high amplitude at some point during the mass ejection process. In all observed cases, freshly ejected material was initially within a narrow azimuthal range, indicating it was launched from a localized region on the star. The material orbits the star with a frequency consistent with the near-surface Keplerian orbital frequency. This material circularizes into a disk configuration after several orbital timescales. This is true whether or not there was a preexisting disk. We find no evidence for precursor phases prior to the ejection of mass in our sample. The several photometric frequencies that emerge during outburst are at least partially stellar in origin. (Abstract abridged)

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arXiv2025

High-spatial-resolution simulations of Be star disks in binary systems: I. Structure and kinematics of coplanar disks

Binarity in massive stars has proven to be an important aspect in the their evolution. For Be stars, it might be the cause of their spin up, and thus part of the mechanism behind the formation of their viscous decretion disks. Detecting companions in systems with Be stars is challenging, making it difficult to obtain observational constraints on their binary fraction. We explore the effects of a binary companion in a system with a Be star, from disk formation to quasi steady-state using smoothed particle hydrodynamics (SPH) simulations of coplanar, circular binary systems. High spatial resolution is achieved by adopting particle splitting in the SPH code, as well as a more realistic description of the secondary star and the disk viscosity. The tidal forces considerably affect the Be disk, forming distinct regions in the system, with observational consequences that can be used to infer the presence of a otherwise undetectable companion. With the upgraded code, we can probe a region approximately 4 times larger than previously possible. We describe the configuration and kinematics of each part of the system, and provide a summary of their expected observational signals. Material that enters the Roche lobe of the companion is partially captured by it, forming a rotationally supported, disk-like structure. Material not accreted escapes and forms a circumbinary disk around the system. This is the first work to describe the region beyond the truncation region of the Be disk and its observational consequences with detail. We argue that observational features of previously unclear origin, such as the intermittent shell features and emission features of HR 2142 and HD 55606, originate in areas beyond the truncation region. This new understanding of the behavior of disks in Be binaries will allow not just for better interpretation of existing data, but also for the planning of future observations.

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arXiv2023

Bayesian sampling with BeAtlas, a grid of synthetic Be star spectra I. Recovering the fundamental parameters of αEri and βCMi

Classical Be stars are fast rotating, near main sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of 61.600 NLTE radiative transfer models for Be stars, calculated with the HDUST code. The grid was coupled with a Monte Carlo Markov chain code to sample the posterior distribution. We test our method on two well-studied Be stars, $α$ Eri and $β$ CMi, using photometric, polarimetric and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of $α$ Eri, the disc parameters of $β$ CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in $β$ CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: the fundamental parameters of any Be star can now be estimated in a matter of hours or days.

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arXiv2020

The Large Scale Behaviour in the Disk of $δ$ Scorpii from 2000-2018

We model the circumstellar disk of $δ$ Sco using the 3-dimensional Monte Carlo radiative transfer code HDUST in order to quantify the large scale changes in the disk through the years 2000 to 2018, and to see if these changes can be attributed to the secondary star affecting the disk throughout its orbit. We determine our best-fitting models through matching simulated observations to actual H$\rm α$ spectroscopy and V-band photometric observations. Our modelling results confirm previous findings that the disk of $δ$ Sco was forming early in the century. We also find a period of disk dissipation when the companion is at apastron, as well as a significant growth of the disk between 2009 and 2011, prior to the periastron of 2011. Due to the steady-state nature of the disk after 2011, it is difficult to say whether the variations seen are due to the effect of the close passage of the binary companion.

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