arXiv subjects

Ignacio Araya

Publications and source records attributed to Ignacio Araya.

14 recordsLinked to original sources

arXiv2026

Quasi-Keplerian Be-Star Disks with Mimicking Viscosity

Classical Be stars are fast-rotating B-type stars with gaseous quasi-Keplerian disks formed by equatorial ejection of material. While the viscous decretion disk (VDD) model reproduces many observed properties, the role of radiative line driving in shaping these disks remains unclear. We investigated the combined influence of viscosity and radiative acceleration on the hydrodynamic structure of Be star disks by coupling the m-CAK theory of line-driven winds with a mimicking viscous prescription governed by the parameter $\gamma_{\rm vis}$. We solved the steady-state hydrodynamic equation of motion using \textsc{Hydwind} for typical B-type stellar parameters in transonic $\Omega$-slow outflows. We analyzed the velocity and density structures and derived the mass-loss rates and radial velocities at the adopted outer integration radius, $r=50\,R_\ast$. The combined action of line driving and viscosity yields regular m-CAK $\Omega$-slow solutions for equatorial outflow with a VDD-inspired rotational prescription. For quasi-Keplerian exponents ($\gamma_{\rm vis} \simeq 0.5$) and near-critical rotation ($\Omega \approx 0.96$--$0.99$), the models produce an outflowing disk with an m-CAK-type critical point at $r_{\rm c} \lesssim 20$--$30\,R_\ast$. At $50\,R_\ast$, these quasi-Keplerian solutions reach radial velocities of $76.9$--$139.2\,\mathrm{km\,s^{-1}}$. Within the present 1D parameterized framework, the m-CAK line force yields stationary solutions without imposing an outer boundary condition. Our results provide a controlled 1D test of how a VDD-inspired rotational prescription modifies the topology of stationary m-CAK $\Omega$-slow solutions. The model is an exploratory bridge toward future non-Sobolev multidimensional radiation-hydrodynamic treatments that recover quasi-Keplerian rotation and modest outflow velocities of Be disks.

astro-ph.SR↗

arXiv2026

Line-driven Radiative Winds in B-Supergiants: Bridging the Gap between Fast and Slow m-CAK Solutions

The modified Castor, Abbott, and Klein (m-CAK) theory predicts different wind regimes based on the line force parameter for changes in ionization ($\delta$) and the rotation parameter ($\Omega$). Stationary hydrodynamic studies have reported ''forbidden regions'' or gaps in this parameter space where no steady-state solution exists, suggesting physical instabilities. We investigate the stability of wind solutions within these gaps for B-supergiants to determine if they correspond to physical instabilities or numerical artifacts. We perform 1D time-dependent hydrodynamic simulations, systematically exploring the full $(\Omega, \delta)$ space for three B-supergiant models ($T_{\rm eff}=15-25$ kK), adopting a fixed density boundary condition. Our simulations reveal stable stationary solutions continuously across the entire parameter space, effectively filling the reported gaps. The transition from fast to slow regimes is smooth but structurally complex. Within the gap, the velocity profile develops a distinct ''kink'' or extended plateau in the supersonic flow, allowing the wind to reach a stable state. The mass-loss rate ($\dot{M}$) varies smoothly without artificial jumps. We find that the $\dot{M}$ gradient depends on the radiative driving strength ($k$): while $\dot{M}$ increases with $\delta$ for standard driving ($k \approx 0.32$), it decreases for the weak-driving regime ($k = 0.1$), consistent with stationary predictions. Moreover, in this regime, the final solution depends on the initial flow acceleration, confirming multiple hydrodynamic solutions. We conclude the m-CAK solution space is continuous; reported forbidden regions are artifacts of stationary methods. Time-dependent simulations effectively bridge the regimes, suggesting these transitions correspond to metastable states.

astro-ph.SR↗

arXiv2025

Effect of gravity darkening and oblate factor in rapidly rotating massive stars

Context. Rapid rotation in massive stars leads to gravity darkening and oblateness, significantly affecting their radiation-driven winds. These effects can alter wind dynamics and play a role in forming slowly equatorial outflowing winds. Aims. This work investigates the transition region where the fast solution (i.e. high terminal velocities) of radiation-driven winds in a massive rotating star, in the frame of the modified-CAK theory, switches to the Omega-slow solutions (a denser and slower wind) when the effects of gravity darkening and oblateness are considered. This Omega-slow solution appears when the rotational speed is higher and equal to 75% of the critical rotation speed. Methods. To explore the transition region for various equatorial models of B-type stars, we focus on the co-existence interval where both solutions simultaneously exist and the transition point where fast solutions switch to Omega-slow solutions. Results. Using our stationary numerical code Hydwind, we first analyse the individual effects of gravity darkening and stellar oblateness caused by high rotational speeds and then examine their combined impact on the wind solutions. Conclusions. We find that for a certain range of rotational speeds, both the fast and Omega-slow solutions can co-exist, and the co-existence range strongly depends on the initial conditions. When only gravity darkening is considered, the co-existing interval shifts towards higher rotational speeds. While in the presence of the oblateness, the co-existing interval also occurs at higher rotational speeds; however, it is less than the gravity darkening effect. We also explored how line-force parameters affect the critical point, the location of the co-existing interval, and where the solution switches.

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arXiv2023

Radiation-Driven Wind Hydrodynamics of Massive Stars: A Review

Mass loss from massive stars plays a determining role in their evolution through the upper Hertzsprung-Russell diagram. The hydrodynamic theory that describes their steady-state winds is the line-driven wind theory (m-CAK). From this theory, the mass loss rate and the velocity profile of the wind can be derived, and estimating these properly will have a profound impact on quantitative spectroscopy analyses from the spectra of these objects. Currently, the so-called beta-law, which is an approximation for the fast solution, is widely used instead of m-CAK hydrodynamics, and when the derived value is beta greater than 1.2, there is no hydrodynamic justification for these values. This review focuses on (1) a detailed topological analysis of the equation of motion (EoM), (2) solving the EoM numerically for all three different (fast and two slow) wind solutions, (3) deriving analytical approximations for the velocity profile via the LambertW function and (4) presenting a discussion of the applicability of the slow solutions.

astro-ph.SR↗

arXiv2022

Revisiting Viscous Transonic Decretion Disks of Be Stars

In the context of Be stars, we restudied the viscous transonic decretion disk model of these stars. This model is driven by a radiative force due to an ensemble of optically-thin lines and viscosity considering the Shakura Sunyaev prescription. The non-linear equation of motion presents a singularity (sonic point) and an eigenvalue, which is also the initial condition at the stellar surface. Then, to obtain this eigenvalue, we set it as a radial quantity and perform a detailed topological analysis. Thereafter, we describe a numerical method for solving either Nodal and Saddle transonic solutions. The value of the viscosity,"alpha", barely determine the location of the sonic point, but it determines the topology of the solution. We found two Nodal solutions, which are almost indistinguishable between them. Saddle solutions are founded for lower values of "alpha" than the required of the Nodal solutions. In addition, rotational velocity do not play a determine role in the velocity (and density) profile, because viscosity effects collapse all the solutions to almost a unique one in a small region above the stellar surface. A suitable combination of line force parameters and/or disk temperature, give location of the sonic point lower than 50 stellar radii, describing a truncated disk. This could explain the SED turndown observed in Be stars without needing a binary companion.

astro-ph.SR↗

arXiv2022

New self-consistent wind parameters fitting optical spectra of O-type stars observed with HERMES spectrograph

We perform spectral fitting for a set of O-type stars based on self-consistent wind solutions, which provide mass-loss rate and velocity profiles directly derived from the initial stellar parameters. The great advantage of this self-consistent spectral fitting is therefore the reduction of the number of free parameters to be tuned. Self-consistent values for the line-force parameters (k,alpha,delta) and subsequently for the mass-loss rate and terminal velocity are provided by the m-CAK prescription introduced in Paper I, updated in this work with improvements such as a temperature structure for the wind, self-consistently evaluated from the line-acceleration. Synthetic spectra are calculated using the radiative transfer code FASTWIND, replacing the classical beta-law for our new calculated velocity profiles. We found that self-consistent m-CAK solutions provide values for theoretical mass-loss rates on the order of the most recent predictions of other studies. From here, we generate synthetic spectra with self-consistent hydrodynamics to fit and obtain a new set of stellar and wind parameters for our sample of O-type stars whose spectra was taken with the high resolution echelle spectrograph HERMES (R=85000). We find a satisfactory global fit for our observations, with good accuracy for photospheric He I and He II lines and a quite acceptable fit for H lines. Although this self-consistent spectral analysis is currently constrained in the optical wavelength range only, this is an important step towards the determination of stellar and wind parameters without using a beta-law. Given these results, we expect that the values introduced here should be helpful for future studies about the stars constituting this sample, together with the prospective that the m-CAK self-consistent prescription be extended to numerous studies about massive stars in future.

astro-ph.SR↗

arXiv2022

Automatic algorithm to obtain $v \sin i$ values via Fourier Transform in BeSOS database

Be stars are found to rotate close to their critical rotation and therefore they are considered as an important laboratory of study for stellar rotation. In this context, we obtain the projected rotational velocity of a sample of classical Be Southern stars in the BeSOS database via Fourier Transform in an automated way for several absorption lines at different epochs. A Gaussian profile is fitted to eight observed photospheric HeI lines in order to select automatically the spectral signal given by areas under the curve of 95.45%, 98.75% and 99.83% from the profile to obtain $v \sin i$ via Fourier Transform technique. The values obtained are in global agreement with the literature. Analysing only one line is not enough to set the $v \sin i$ value, depending on the line the value in most cases are underestimated with respect to $\lambda$4471. When gravity darkening effects are including, apparent values increases by $\sim10$%. The resolution of the instrument PUCHEROS used for BeSOS spectra ($R \sim 17\,000$) constrain a theoretical lower bound possible at $v \sin i \sim 100$ km s$^{-1}$. The procedure has limitations using a linear limb-darkening function with $\varepsilon = 0.6$ for classical Be stars rotating close to the break-up velocity without gravity-darkening corrections, which can't be negligible. Previous works measure $v \ sin i$ values using just one spectral line and here we demonstrate that with more lines the results can varies. This could be due to the photospheric distribution of atomic transitions on classical Be stars.

astro-ph.SR↗

arXiv2020

Power spectrum of density fluctuations, halo abundances and clustering with primordial black holes

We study the effect of dark matter (DM) being encapsulated in primordial black holes (PBHs) on the power spectrum of density fluctuations $P(k)$; we also look at its effect on the abundance of haloes and their clustering. We allow the growth of Poisson fluctuations since matter and radiation equality and study both monochromatic and extended PBH mass distributions. We present updated monochromatic black hole mass constraints by demanding $<10\%$ deviations from the $\Lambda$ cold dark matter power spectrum at a scale of $k=1$hMpc$^{-1}$. Our results show that PBHs with masses $>10^4$h$^{-1}M_\odot$ are excluded from conforming all of the DM in the Universe. We also apply this condition to our extended Press-Schechter (PS) mass functions, and find that the Poisson power is scale dependent even before applying evolution. We find that characteristic masses $M^*\leq10^2 $h$^{-1}M_\odot$ are allowed, {leaving only two characteristic PBH mass windows of PS mass functions when combining with previous constraints, at $M^*\sim10^2$h$^{-1}M_\odot$ and $\sim10^{-8}$h$^{-1}M_\odot$ where all of the DM can be in PBHs. The resulting DM halo mass functions within these windows are similar} to those resulting from cold dark matter made of fundamental particles. However, as soon as the parameters produce unrealistic $P(k)$, the resulting halo mass functions and their bias as a function of halo mass deviate strongly from the behaviour measured in the real Universe.

astro-ph.CO↗

arXiv2019

Breaking the dichotomy between typical and anomalous globular clusters: the case of NGC 3201

We recently discovered that NGC 3201 has characteristics that set it outside the current twofold classification scheme for Galactic globular clusters (GCs). Most GCs are mono-metallic and show light-element abundance variations (e.g., Na-O and C-N anti-correlations); but a minority of clusters also present variations in Fe correlating with s-process element and C+N+O abundances, and they possess multiple C-N sequences. These anomalous GCs also have a broad sub-giant branch (SGB) and follow the same mass-size relation as dwarf galaxies possibly evolving into GCs. We now revealed that NGC 3201 belongs to neither group. It has multiple C-N sequences, but no broad SGB, no strong evidence of a Fe-spread, and it does not follow the mass-size relation.

astro-ph.SR↗

arXiv2018

Co-existence and switching between fast and $\Omega$-slow wind solutions in rapidly rotating massive stars

Most radiatively-driven massive star winds can be modelled with m-CAK theory resulting in so called fast solution. However, those most rapidly rotating among them, especially when the stellar rotational speed is higher than $\sim 75\%$ of the critical rotational speed, can adopt a different solution called $\Omega$-slow solution characterized by a dense and slow wind. Here, in this work we study the transition region of the solutions where the fast solution changes to the $\Omega$-slow. Using both time-steady and time-dependent numerical codes, we study this transition region for different equatorial models of B-type stars. In all the cases, at certain range of rotational speeds, we found a region where the fast and $\Omega$-slow solution can co-exist. We find that the type of solution obtained in this co-existence region depends heavily on the initial conditions of our models. We also test the stability of the solutions within the co-existence region by performing base density perturbations in the wind. We find that under certain conditions, the fast solution can switch to a $\Omega$-slow solution, or vice versa. Such solution switching may be a possible contributor of material injected into the circumstellar environment of Be stars, without requiring rotational speeds near critical values.

astro-ph.SR↗

arXiv2018

Galactic or extragalactic chemical tagging for NGC3201? Discovery of an anomalous CN-CH relation

(ABRIDGED) The origin of the globular cluster (GC) NGC3201 is under debate. Its retrograde orbit points to an extragalactic origin, but no further chemical evidence supports this idea. Light-element chemical abundances are useful to tag GCs and can be used to shed light on this discussion. We aim to derive CN and CH band strengths for red giant stars in NGC3201 and compare these with photometric indices and high-resolution spectroscopy and discuss in the context of GC chemical tagging. We found three groups in the CN-CH distribution. A main sequence (S1), a secondary less-populated sequence (S2), and a group of peculiar (pec) CN-weak and CH-weak stars, one of which was previously known. The three groups seem to have different C+N+O and/or s-process element abundances, to be confirmed by high-resolution spectroscopy. These are typical characteristics of anomalous GCs. The CN distribution of NGC 3201 is quadrimodal, which is more common in anomalous clusters. However, NGC3201 does not belong to the trend of anomalous GCs in the mass-size relation. Three scenarios are postulated here: (i) if the sequence pec-S1-S2 has increasing C+N+O and s-process element abundances, NGC3201 would be the first anomalous GC outside of the mass-size relation; (ii) if the abundances are almost constant, NGC3201 would be the first non-anomalous GC with multiple CN-CH anti-correlation groups; or (iii) it would be the first anomalous GC without variations in C+N+O and s-process element abundances. In all cases, the definition of anomalous clusters and the scenario in which they have an extragalactic origin must be revised.

astro-ph.GA↗

arXiv2014

Analytical Solutions for Radiation-Driven Winds in Massive Stars. I: The Fast Regime

Accurate massloss rate estimates are crucial keys to study wind properties of massive stars and test different evolutionary scenarios. From a theoretical point of view, this implies to solve a complex set of differential equations in which the radiation field and the hydrodynamics are strong coupled. The use of analytical expression to represent the radiation force and the solution of the equation of motion have many advantages over numerical integrations. Therefore, in this work, we present an analytical expression as solution of the equation of motion for radiation driven winds, in terms of the force multipliers parameters. This analytical expression is obtained by employing the line acceleration expression given by Villata (1992) and the methodology proposed by Mueller and Vink (2008). On the other hand, we find useful relationships to determine the parameters for the line acceleration given by Mueller and Vink (2008) in terms of the force multiplier parameters.

astro-ph.SR↗

arXiv2012

A cyclic bipolar wind in the interacting binary V393 Scorpii

V393 Scorpii is a Double Periodic Variable characterized by a relatively stable non-orbital photometric cycle of 253 days. Mennickent et al. argue for the presence of a massive optically thick disc around the more massive B-type component and describe the evolutionary stage of the system. In this paper we analyze the behavior of the main spectroscopic optical lines during the long non-orbital photometric cycle. We study the radial velocity of the donor determining their orbital elements and find a small but significant orbital eccentricity (e = 0.04). The donor spectral features are modeled and removed from the spectrum at every observing epoch using the light-curve model given by Mennickent et al. We find that the line emission is larger during eclipses and mostly comes from a bipolar wind. We find that the long cycle is explained in terms of a modulation of the wind strength; the wind has a larger line and continuum emissivity on the high state. We report the discovery of highly variable chromospheric emission in the donor, as revealed by Doppler maps of the emission lines MgII 4481 and CI 6588. We discuss notable and some novel spectroscopic features like discrete absorption components, especially visible at blue-depressed OI 7773 absorption wings during the second half-cycle, Balmer double emission with V/R-curves showing "Z-type" and "S-type" excursions around secondary and main eclipse, respectively, and H_beta emission wings extending up to +- 2000 km/s. We discuss possible causes for these phenomena and for their modulations with the long cycle.

astro-ph.SR↗

arXiv2010

Radiation driven winds with rotation: The oblate finite disc correction factor

We have incorporated the oblate distortion of the shape of the star due to the stellar rotation, which modifies the finite disk correction factor (f_D) in the m-CAK hydrodynamical model. We implement a simplified version for the f_D allowing us to solve numerically the non-linear m- CAK momentum equation.We solve this model for a classical Be star in the polar and equatorial directions. The star's oblateness modifies the polar wind, which is now much faster than the spherical one, mainly because the wind receives radiation from a larger (than the spherical) stellar surface. In the equatorial direction we obtain slow solutions, which are even slower and denser than the spherical ones. For the case when the stellar rotational velocity is about the critical velocity, the most remarkable result of our calculations is that the density contrast between the equatorial density and the polar one, is about 100. This result could explain a long-standing problem on Be stars.

astro-ph.SR↗