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Multi epoch spectroscopic variability of the B supergiant HD75149

This pilot study analyzes 25 years of multi-epoch optical spectroscopy of the B supergiant HD75149 to reveal that its short-term variability is driven by episodic mass-loss rate increases of up to a factor of 1.8 within four days, rather than by a close binary companion.

Original authors: J. Chamoun-Contreras, C. Arcos, N. Machuca, C. E. Perez-Ramirez, L. S. Cidale, M. Curé, I. Araya, D. Turis-Gallo, M. Hadjara

Published 2026-05-29

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Original authors: J. Chamoun-Contreras, C. Arcos, N. Machuca, C. E. Perez-Ramirez, L. S. Cidale, M. Curé, I. Araya, D. Turis-Gallo, M. Hadjara

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a massive star, HD 75149, as a giant, fiery lighthouse in the constellation Vela. This isn't a quiet lighthouse, though. It's a "B-type supergiant," a stellar giant that is constantly blowing a powerful, invisible wind made of its own hot gas into space. This wind is so strong that it shapes the star's future and feeds the surrounding galaxy with new material.

The astronomers in this study acted like detectives, watching this star over a long period (from 2004 to 2025) to figure out why its "light signature" keeps changing. They took 25 snapshots of the star's light using different telescopes, looking closely at how specific colors of light (spectral lines) behaved.

Here is what they found, explained simply:

1. The Mystery of the Shifting Colors

When they looked at the star's light, they saw two very different behaviors:

The "Heavy" Elements (Silicon, Carbon, Nitrogen): These lines were relatively calm. They wiggled a little bit, like a pendulum swinging gently. This suggested the star's surface was pulsating (breathing in and out), but not violently.
The "Hydrogen" Wind (H-alpha): This was the wild card. The hydrogen lines, which tell us about the star's wind, were chaotic. Sometimes they looked like a deep dip (absorption), sometimes a bright spike (emission), and sometimes a mix of both (a P-Cygni profile). These changes happened incredibly fast—sometimes within just a few days.

The Analogy: Imagine the star is a person blowing on a campfire. The "heavy elements" are like the person's steady breathing rhythm. But the "hydrogen wind" is the fire itself. One moment the fire is a small, steady flame; the next, a sudden gust of wind makes it roar and change shape completely. The star's wind is doing the same thing, flaring up and dying down rapidly.

2. Was it a Secret Twin?

A common reason stars change their light is if they have a hidden partner (a binary star) orbiting them. As the partner moves, it pulls the star, causing it to wobble back and forth, which changes the light's color (like a siren passing by).

The researchers checked for this "wobble" in the heavy elements. They found no evidence of a close, fast-orbiting partner. The wobbles they saw were too small and irregular to be caused by a hidden twin. However, they couldn't rule out a very distant partner that we just can't see yet, but the main cause of the changes is definitely the star itself.

3. The "Slow Wind" Discovery

The team used a special computer model (called ISOSCELES) to simulate the star's wind. They were testing a specific theory called the "δ-slow" solution.

The Old Idea: Scientists used to think these stars blew wind at a steady, high speed, like a jet engine.
The New Finding: This star behaves more like a slow, thick fog rolling out. The model that worked best was the "slow wind" one. It showed that the star's wind speed and the amount of material it loses (mass-loss rate) can change drastically in just a few days.

The Analogy: Think of the star's wind not as a steady stream from a garden hose, but as a faucet that someone is frantically turning on and off. In just four days, the star increased the amount of gas it was losing by 1.8 times, yet the speed of the gas didn't change much. It was like turning up the volume on a speaker without changing the pitch.

4. The Connection: Breathing and Blowing

The study concludes that the star's "breathing" (pulsations) is likely causing the "blowing" (wind changes).

The star's surface pulses (expands and contracts) like a beating heart.
These pulses shake the base of the wind, causing the wind to get denser or thinner rapidly.
This explains why the hydrogen wind changes so fast, while the heavy elements on the surface just wiggle gently.

Summary

HD 75149 is a massive, unstable star that is constantly reshaping its own atmosphere. It isn't being shaken by a secret partner; it is shaking itself. Its wind is a "slow, thick" flow that can suddenly surge in intensity due to the star's internal pulsations. This study is important because it shows us that massive stars are much more dynamic and changeable on short timescales than we previously thought, acting like living, breathing entities rather than static balls of fire.

Technical Summary: Multi-epoch spectroscopic variability of the B supergiant HD 75149

Problem Statement
Massive stars, particularly B-type supergiants (BSGs), continuously enrich the interstellar medium through powerful stellar winds. These stars exhibit complex spectroscopic and photometric variability, including changes in line broadening, flux intensity, radial velocities (RVs), and equivalent widths (EWs). While the physical origins of this variability are debated—ranging from binarity to intrinsic pulsations and stochastic wind instabilities—there is a need to investigate short-term variability within a consistent hydrodynamic and radiative-transfer framework. Specifically, the authors aim to characterize the multi-epoch optical spectroscopic variability of the B3 Ia supergiant HD 75149 (OP Vel) to determine whether its variability is driven by a close binary companion or intrinsic stellar processes, such as pulsation-wind coupling.

Methodology
The study analyzes a dataset of 25 nightly averaged optical spectra obtained between 2004 and 2025 using three different instrument configurations:

REOSC-DC (CASLEO): 19 spectra (2006–2017) with a resolving power of ~13,000 and S/N ≈ 300.
NRES/LCO: 2 spectra (2025) with high resolution (~53,000) and S/N ≈ 100.
FEROS (La Silla): 4 spectra (2004–2006) with R=48,000 and S/N ≈ 200–230.

The authors measured radial velocities (RVs) and equivalent widths (EWs) for 17 spectral lines, including Hydrogen (H), Helium (He I), Silicon (Si III), Nitrogen (N II), Magnesium (Mg II), and Carbon (C II). To assess variability, they calculated the moments of the line profiles (M0 for EW, M1 for centroid/RV, and M3 for skewness).

To model the wind variability, the authors utilized the ISOSCELES grid, which incorporates the Hydwind code to solve hydrodynamic equations and the FASTWIND code for non-LTE radiative transfer. Crucially, the analysis focused on the $\delta$ -slow hydrodynamic solution (Curé et al. 2011), a regime characterized by slower terminal velocities and denser winds compared to standard fast wind models. The H $\alpha$ line profiles were fitted to derive mass-loss rates ( $\dot{M}$ ) and terminal velocities ( $v_\infty$ ) for specific time intervals, fixing stellar parameters ( $\log g = 2.5$ , $T_{\text{eff}} = 16,500$ K) based on literature values.

Key Results

Hydrogen vs. Metal Line Variability: The H $\alpha$ line exhibits pronounced short-term variability, with P-Cygni profiles changing from absorption to emission and vice versa over intervals of a few days. In contrast, metal lines (Si III, N II, Mg II, C II) show significantly smaller RV amplitudes (typically < 10 km s $^{-1}$ ), consistent with pulsating oscillations rather than orbital motion.
Mass-Loss Rate Fluctuations: The derived mass-loss rates vary between $1.06 \times 10^{-7}$ and $2.81 \times 10^{-7} M_\odot \text{yr}^{-1}$ . The most significant variation observed was an increase in $\dot{M}$ by a factor of 1.8 within a four-day period (March 2015). Conversely, the terminal velocity ( $v_\infty$ ) remained relatively stable, with variations generally not exceeding 10 km s $^{-1}$ over similar intervals, though a larger variation (~26 km s $^{-1}$ ) was noted in 2013.
Binary Hypothesis: The lack of coherent, orbit-like RV shifts in the metallic lines argues against the presence of a close binary companion as the primary driver of the variability. While a wide or low-inclination companion cannot be entirely ruled out, the data do not support a short-period binary scenario.
Wind Modeling: The $\delta$ -slow solution provided a better fit to the observed H $\alpha$ profiles than standard fast wind models, successfully reproducing the lower terminal velocities and denser wind signatures typical of this BSG.

Significance and Claims
The paper concludes that the pronounced variability in hydrogen lines, contrasted with the stability of metal lines, suggests that the observed changes are intrinsic to the star, driven by episodes of mass loss associated with a slow wind occurring on timescales comparable to photometric variations. The authors propose a pulsation–wind coupling scenario, where photospheric oscillations (likely g- and s-modes) perturb the base of the wind, modulating its density structure and mass-loss rate.

The study highlights the diagnostic power of combining high-cadence spectroscopy with hydrodynamic wind models (specifically the $\delta$ -slow regime) to understand BSG variability. It positions HD 75149 in a transitional evolutionary regime between stable BSGs and objects with enhanced mass loss, where non-radial pulsations, internal gravity waves, and wind instabilities interact. The authors emphasize that while their results are robust regarding the detection of variability, formal uncertainties on the mass-loss rates remain unconstrained due to the lack of a Bayesian error analysis in the current fitting procedure, a limitation they acknowledge for future work.

1. The Mystery of the Shifting Colors

2. Was it a Secret Twin?

3. The "Slow Wind" Discovery

4. The Connection: Breathing and Blowing

Summary

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