arXiv subjects

Véronique Petit

Publications and source records attributed to Véronique Petit.

16 recordsLinked to original sources

arXiv2025

New Frontiers in the Study of Magnetic Massive Stars with the Habitable Worlds Observatory

High-mass stars are notable for several reasons: they are characterized by strong winds, which inject momentum and enriched material into their surroundings, and die spectacularly as supernovae, leaving behind compact remnants and heavy elements (such as those that make life on Earth possible). Despite their relative rarity, they play a disproportionate role in the evolution of the galaxies that host them, and likely also played a significant role in the early days of the Universe. A subset ($\sim$10\%) of these stars was also found to host magnetic fields on their surface. These fields impact their evolution, and may lead to exotic physics (e.g., heavy stellar-mass black holes, pair-instability supernovae, magnetars, etc.). However, the detection and measurement of magnetic fields is limited, due to current instrumentation, to nearby massive stars in the Milky Way. To truly understand how magnetism arises in massive stars, and what role it might have played in earlier stages of our Universe, we require next-generation hardware, such as the proposed near-infrared-to-ultraviolet spectropolarimeter Pollux, on the Habitable Worlds Observatory (HWO). In this contribution, we detail how Pollux @ HWO will enable new frontiers in the study of magnetic massive stars, delivering results that will profoundly impact the fields of stellar formation, stellar evolution, compact objects, and stellar feedback.

astro-ph.IM↗

arXiv2025

Discoveries of fine structures and secondary pulses in coherent radio emission from a magnetic massive star

In this paper, we report auroral radio emission from a magnetic B star HD 142990 using the MeerKAT radio telescope at $900-1670$ MHz. The star is known to produce such emission (observed as periodic radio pulses) via electron cyclotron maser emission (ECME). However, past studies on ECME from this star were confined to observations at specific rotational phase ranges where one expects to see such pulses. We, for the first time, observed the star for its one complete rotation cycle and discovered that the star also produces 'off-pulse' emission, which we term as secondary enhancements. Two such enhancements were observed, one of which is left circularly polarized (LCP) and the other is right circularly polarized (RCP), the latter is confirmed to be persistent. Using simulation, we infer that such pulses are likely related to the large misalignment between the stellar rotation and magnetic dipole axes ($>80^\circ$), leading to the formation of highly complex magnetospheric plasma distribution. In addition, by extracting dynamic spectra for the primary pulses, we discovered prominent fine structures in one of the LCP pulses, with timescales as small as the instrumental time resolution (8 seconds). This is the first time that such structures are seen from a magnetic hot star, and has the potential to reveal detailed information about how the emission is driven, and the nature of the elementary sources of radiation. To pinpoint the origin of these fine structures and their significance, higher time and spectral resolution observations should be conducted in the future.

astro-ph.SR↗

arXiv2024

Using ZDI maps to determine magnetic forces and torques at the photospheres of Early-type stars

We use the magnetic field components measured by Zeeman Doppler imaging (ZDI) to calculate the stellar surface force and torque due to magnetic stresses for the fast rotators $σ$ Ori E, 36 Lyn and CU Vir, and the slow rotator $τ$ Sco. If we assume the stars have spherical photospheres, the estimated torques give spin down time scales no larger than $7 \times 10^5$ yr. For $σ$ Ori E, the predicted spin down time scale, $\simeq 6000$ yr, is much less than the observationally measured time scale of $\simeq 10^6$ yr. However, for CU Vir, we find that the spin down time scale from its ZDI map is $7 \times 10^5$ yr in good agreement with its average rate of spin down from 1960 to 2010. With the exception of $τ$ Sco, the net force due to magnetic stresses at the stellar surface are large compared to the surface-integrated pressure. We discuss possible reasons for the large values of the forces (and torques), and suggest that the likely explanation is that rotation and the magnetic stresses create significant departures from spherical symmetry.

astro-ph.SR↗

arXiv2022

Testing a scaling relation between coherent radio emission and physical parameters of hot magnetic stars

Coherent radio emission via electron cyclotron maser emission (ECME) from hot magnetic stars was discovered more than two decades ago, but the physical conditions that make the generation of ECME favourable remain uncertain. Only recently was an empirical relation, connecting ECME luminosity with the stellar magnetic field and temperature, proposed to explain what makes a hot magnetic star capable of producing ECME. This relation was, however, obtained with just fourteen stars. Therefore, it is important to examine whether this relation is robust. With the aim of testing the robustness, we conducted radio observations of five hot magnetic stars. This led to the discovery of three more stars producing ECME. We find that the proposed scaling relation remains valid after the addition of the newly discovered stars. However we discovered that the magnetic field and effective temperature correlate for $T_\mathrm{eff}\lesssim 16$ kK (likely an artifact of the small sample size), rendering the proposed connection between ECME luminosity and $T_\mathrm{eff}$ unreliable. By examining the empirical relation in light of the scaling law for incoherent radio emission, we arrive at the conclusion that both types of emission are powered by the same magnetospheric phenomenon. Like the incoherent emission, coherent radio emission is indifferent to $T_\mathrm{eff}$ for late-B and A-type stars, but $T_\mathrm{eff}$ appears to become important for early-B type stars, possibly due to higher absorption, or, higher plasma density at the emission sites suppressing the production of the emission.

astro-ph.SR↗

arXiv2022

Helium-like X-ray line complexes show that the hottest plasma on the O supergiant zeta Puppis is in its wind

We present an analysis of Chandra grating spectra of key helium-like line complexes to put constraints on the location with respect to the photosphere of the hottest ($T \gtrsim{6 \times 10^6}$ K) plasma in the wind of the O supergiant zeta Pup and to explore changes in the 18 years between two sets of observations of this star. We fit two models -- one empirical and one wind-shock-based -- to the S XV, Si XIII, and Mg XI line complexes and show that an origin in the wind flow, above $r \approx 1.5$ R$_{\ast}$, is strongly favored over an origin less than 0.3 R$_{\ast}$ above the photosphere ($r \lesssim 1.3$ R$_{\ast}$), especially in the more recent, very long-exposure data set. There is a modest increase in the line and continuum fluxes, line widths, wind absorption signatures, and of the hot plasma's distance from the photosphere in the 18 years since the first Chandra grating observation of zeta Pup. Both modes of modeling include the effects of dielectronic recombination satellite emission line blending on the helium-like complexes -- the first time this has been accounted for in the analysis of He-like line ratios in O stars.

astro-ph.SR↗

arXiv2021

Chandra grating spectroscopy of embedded wind shock X-ray emission from O stars shows low plasma temperatures and significant wind absorption

We present a uniform analysis of six examples of embedded wind shock (EWS) O star X-ray sources observed at high resolution with the Chandra grating spectrometers. By modeling both the hot plasma emission and the continuum absorption of the soft X-rays by the cool, partially ionized bulk of the wind we derive the temperature distribution of the shock-heated plasma and the wind mass-loss rate of each star. We find a similar temperature distribution for each star's hot wind plasma, consistent with a power-law differential emission measure, $\frac{d\log EM}{d\log T}$, with a slope a little steeper than -2, up to temperatures of only about $10^7$ K. The wind mass-loss rates, which are derived from the broadband X-ray absorption signatures in the spectra, are consistent with those found from other diagnostics. The most notable conclusion of this study is that wind absorption is a very important effect, especially at longer wavelengths. More than 90 per cent of the X-rays between 18 and 25 Angstrom units produced by shocks in the wind of $ζ$ Puppis are absorbed, for example. It appears that the empirical trend of X-ray hardness with spectral subtype among O stars is primarily an absorption effect.

astro-ph.HE↗

arXiv2020

Chandra spectral measurements of the O supergiant $ζ$ Puppis indicate a surprising increase in the wind mass-loss rate over 18 years

New long Chandra grating observations of the O supergiant $ζ$ Pup show not only a brightening of the x-ray emission line flux of 13 per cent in the 18 years since Chandra's first observing cycle, but also clear evidence - at more than four sigma significance - of increased wind absorption signatures in its Doppler-broadened x-ray emission line profiles. We demonstrate this with non-parametric analysis of the profiles as well as Gaussian fitting and then use the line-profile model fitting to derive a mass-loss rate of $2.47 \pm 0.09 \times 10^{-6}$ Msun/yr, which is a 40 per cent increase over the value obtained from the cycle 1 data. The increase in the individual emission line fluxes is greater for short-wavelength lines than long-wavelength lines, as would be expected if a uniform increase in line emission is accompanied by an increase in the wavelength-dependent absorption by the cold wind in which the shock-heated plasma is embedded.

astro-ph.SR↗

arXiv2020

The evolution of magnetic stars in a single-age population

Observational and theoretical work has now established that the fossil fields of magnetic massive stars are surviving remnants from an earlier event, or an earlier evolutionary phase. However, many important questions remain regarding the effects of these fields on the late stages of stellar evolution, as well as their impact on the core-collapse mechanism and the formation of exotic compact objects such as magnetars and gravitational wave progenitors. There is currently a critical need to incorporate the impact of fossil fields in models of the structure and evolution of magnetic stars, and to determine the evolutionary history of magnetic massive stars. We present a preliminary population study of a cluster of co-evolving stars based on MESA evolutionary tracks that account for the effect of magnetic mass-loss quenching.

astro-ph.SR↗

arXiv2019

MOBSTER: Establishing a Picture of Magnetic Massive Stars as a Population

Magnetic massive and intermediate-mass stars constitute a separate population whose properties are still not fully understood. Increasing the sample of known objects of this type would help answer fundamental questions regarding the origins and characteristics of their magnetic fields. The MOBSTER Collaboration seeks to identify candidate magnetic A, B and O stars and explore the incidence and origins of photometric rotational modulation using high-precision photometry from the Transiting Exoplanet Survey Satellite (\textit{TESS}) mission. In this contribution, we present an overview of our methods and planned targeted spectropolarimetric follow-up surveys.

astro-ph.SR↗

arXiv2019

Magnetic inhibition of convection in O star envelopes

It has been suggested that the absence of macroturbulence in the atmosphere of NGC 1624 - 2 is due its strong magnetic field (the strongest known for a massive O star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. Here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. We find upper mass limits of ~55 Msun and ~30 Msun for magnetic suppression to be possible in Zero Age Main Sequence and Terminal Age Main Sequence stars, respectively. For evolved stars, magnetic suppression of convection can significantly alter the stars' evolution. For NGC 1624 - 2, we find a polar dipole strength of 16.5 +/- 5.9 kG is required to suppress convection, in good agreement with the value ~20 kG measured by spectropolarimetry.

astro-ph.SR↗

arXiv2017

Rotationally modulated photometric variations in B supergiants?

In this contribution, we present BRITE observations of the early-B supergiants $ε$ Ori and $κ$ Ori. We perform a preliminary analysis of the data acquired over the first two Orion observing runs. We evaluate whether they are compatible with co-rotating bright spots and discuss the challenges of such an approach.

astro-ph.SR↗

arXiv2017

The impact and evolution of magnetic confinement in hot stars

Magnetic confinement of the winds of hot, massive stars has far-reaching consequences on timescales ranging from hours to Myr. Understanding the long-term effects of this interplay has already led to the identification of two new evolutionary pathways to form `heavy' stellar mass black holes and pair-instability supernova even at galactic metallicity. We are performing 1D stellar evolution model calculations that, for the first time, account for the surface effects and the time evolution of fossil magnetic fields. These models will be thoroughly confronted with observations and will potentially lead to a significant revision of the derived parameters of observed magnetic massive stars.

astro-ph.SR↗

arXiv2017

Whole Earth Telescope discovery of a strongly distorted quadrupole pulsation in the largest amplitude rapidly oscillating Ap star

We present a new analysis of the rapidly oscillating Ap (roAp) star, 2MASS J$19400781-4420093$ (J1940; $V=13.1$). The star was discovered using SuperWASP broadband photometry to have a frequency of 176.39 d$^{-1}$ (2041.55 $μ$Hz; $P = 8.2$ min; Holdsworth et al. 2014a) and is shown here to have a peak-to-peak amplitude of 34 mmag. J1940 has been observed during three seasons at the South African Astronomical Observatory, and has been the target of a Whole Earth Telescope campaign. The observations reveal that J1940 pulsates in a distorted quadrupole mode with unusual pulsational phase variations. A higher signal-to-noise ratio spectrum has been obtained since J1940's first announcement, which allows us to classify the star as A7 Vp Eu(Cr). The observing campaigns presented here reveal no pulsations other than the initially detected frequency. We model the pulsation in J1940 and conclude that the pulsation is distorted by a magnetic field of strength $1.5$ kG. A difference in the times of rotational maximum light and pulsation maximum suggests a significant offset between the spots and pulsation axis, as can be seen in roAp stars.

astro-ph.SR↗

arXiv2017

Possible pair-instability supernovae at solar metallicity from magnetic stellar progenitors

Near-solar metallicity (and low-redshift) Pair-Instability Supernova (PISN) candidates challenge stellar evolution models. Indeed, at such a metallicity, even an initially very massive star generally loses so much mass by stellar winds that it will avoid the electron-positron pair-creation instability. We use recent results showing that a magnetic field at the surface of a massive star can significantly reduce its effective mass-loss rate to compute magnetic models of very massive stars (VMSs) at solar metallicity and explore the possibility that such stars end as PISNe. We implement the quenching of the mass loss produced by a surface dipolar magnetic field into the Geneva stellar evolution code and compute new stellar models with an initial mass of $200\,M_\odot$ at solar metallicity, with and without rotation. It considerably reduces the total amount of mass lost by the star during its life. For the non-rotating model, the total (CO-core) mass of the models is $72.8\,M_\odot$ ($70.1\,M_\odot$) at the onset of the electron-positron pair-creation instability. For the rotating model, we obtain $65.6\,M_\odot$ ($62.4\,M\odot$). In both cases, a significant fraction of the internal mass lies in the region where pair instability occurs in the $\log(T)-\log(ρ)$ plane. The interaction of the reduced mass loss with the magnetic field efficiently brakes the surface of the rotating model, producing a strong shear and hence a very efficient mixing that makes the star evolve nearly homogeneously. The core characteristics of our models indicate that solar metallicity models of magnetic VMSs may evolve to PISNe (and pulsation PISNe).

astro-ph.SR↗

arXiv2015

Confirming HD 23478 as a new magnetic B star hosting an H$α$-bright centrifugal magnetosphere

In this paper we report 23 magnetic field measurements of the B3IV star HD 23478: 12 obtained from high resolution Stokes $V$ spectra using the ESPaDOnS (CFHT) and Narval (TBL) spectropolarimeters, and 11 from medium resolution Stokes $V$ spectra obtained with the DimaPol spectropolarimeter (DAO). HD 23478 was one of two rapidly rotating stars identified as potential "centrifugal magnetosphere" hosts based on IR observations from the Apache Point Observatory Galactic Evolution Experiment survey. We derive basic physical properties of this star including its mass ($M=6.1^{+0.8}_{-0.7}\,M_\odot$), effective temperature ($T_{\rm eff}=20\pm2\,$kK), radius ($R=2.7^{+1.6}_{-0.9}\,R_\odot$), and age ($τ_{\rm age}=3^{+37}_{-1}\,$Myr). We repeatedly detect weakly-variable Zeeman signatures in metal, He and H lines in all our observations corresponding to a longitudinal magnetic field of $\langle B_z\rangle\approx-2.0\,$kG. The rotational period is inferred from Hipparcos photometry ($P_{\rm rot}=1.0498(4)\,$d). Under the assumption of the Oblique Rotator Model, our obsevations yield a surface dipole magnetic field of strength $B_d\geq9.5\,$kG that is approximately aligned with the stellar rotation axis. We confirm the presence of strong and broad H$α$ emission and gauge the volume of this star's centrifugal magnetosphere to be consistent with those of other H$α$ emitting centrifugal magnetosphere stars based on the large inferred Alfvén to Kepler radius ratio.

astro-ph.SR↗

arXiv2014

Plasma Leakage from the Centrifugal Magnetospheres of Magnetic B-Type Stars

Magnetic B-type stars are often host to Centrifugal Magnetospheres (CMs). Here we describe the results of a population study encompassing the full sample of known magnetic early B-type stars, focusing on those with detectable CMs. We present revised rotational and magnetic parameters for some stars, clarifying their positions on the rotation-confinement diagram, and? find that plasma densities within their CMs are much lower than those predicted by centrifugal breakout.

astro-ph.SR↗