arXiv subjects

R. Prinja

Publications and source records attributed to R. Prinja.

9 recordsLinked to original sources

arXiv2026

Testing the theory of colliding winds: The periastron passage of 9 Sagittarii. II. Radio monitoring

Colliding winds in massive-star binaries generate non-thermal radio emission and an increased X-ray emission. We study the radio emission of the non-thermal emitter 9 Sgr, which is a long-period and highly eccentric binary. We conducted a monitoring campaign of 9 Sgr around its 2013 - 2014 periastron passage, obtaining radio, X-ray and optical spectroscopy data. In this paper, we analyse the radio data. The radio data at 3.6, 6 and 20 cm show a maximum near periastron. Other non-thermal radio emitters among O-type binaries have a minimum flux near periastron, because the synchrotron emission is absorbed by the free-free absorption in the two stellar winds. This is not the case for 9 Sgr, because even at periastron the orbital separation is sufficient to maintain the wind-wind collision and its associated synchrotron emission outside the effective free-free radius of both stars. Our standard modelling with a local magnetic field that decreases inversely proportional to the distance from the stars (1/r) fails to explain the observed fluxes. Instead, we need to adopt a local magnetic field that decreases faster than 1/r. While our model provides an acceptably good fit to the data, there is clearly still room for improvement. More advanced modelling should include magnetohydrodynamics and more detailed physics of the injection, acceleration and cooling of the relativistic electrons. Radio observations at orbital phases away from periastron can put further constraints on the models.

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arXiv2026

First spectroscopic identification of the main sequence in Westerlund 1

Being the most massive known young stellar cluster in the Milky Way, Westerlund 1 (Wd1) constitutes an ideal benchmark for understanding the evolution of massive stars. However, the cluster age remains highly controversial (~4-10 Myr), hindering the use of Wd1 as a reference for massive star evolution. One of the main issues is high foreground extinction, which has so far prevented the detection of the main sequence. Using infrared spectroscopy we seek to detect the cluster's main sequence for the first time, to characterise the Hertzsprung-Russell diagram, and to use the cluster's turn-off to obtain a robust age estimate. We obtained multi-epoch, near-infrared VLT/KMOS spectroscopic observations of Wd1 to map its population of massive stars. The spectra of ~110 members were analysed with CMFGEN models to derive stellar parameters, populate the cluster Hertzsprung-Russell diagram, and compare it with isochrones from evolutionary models. Our observations returned 47 new spectroscopically identified cluster members, with spectral types O9-B1 III-V. The cluster turn-off indicates an age of 5.5+/-1.0 Myr at a distance of 4.23+0.23-0.21 kpc, displaying a moderate degree of coevality. We demonstrate that our estimate of the age of Wd1 is robust against reasonable changes in the distance and extinction law, and the adopted rotational velocity and metallicity of the stellar isochrones. We further find that ~65% of the OB stars with multi-epoch coverage exhibit radial-velocity variability. Infrared observations of the unevolved stellar population support a single episode of star formation with an age of ~5.5 Myr, reinforcing its potential as a benchmark for massive star evolution and providing a reference sample for future binary population studies.

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arXiv2023

X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars

The winds of massive stars have an impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with HST, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of terminal wind speed with a number of fundamental stellar parameters, namely effective temperature, metallicity, and surface escape velocity. We empirically determined the terminal wind speed for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the Civ 1548-1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models. We find strong trends of terminal wind speed with effective temperature and surface escape speed when the wind is strong enough to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03 when we compared our results to previous Galactic studies. Our results suggest that effective temperature rather than surface escape speed should be used as a straightforward empirical prediction of terminal wind speed and that the observed metallicity dependence is steeper than suggested by earlier works.

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arXiv2022

Ultraviolet Spectropolarimetric Diagnostics of Hot Star Magnetospheres

Several space missions and instruments for UV spectropolarimetry are in preparation, such as the proposed NASA MIDEX Polstar project, the proposed ESA M mission Arago, and the Pollux instrument on the future LUVOIR-like NASA flagship mission. In the frame of Polstar, we have studied the capabilities these observatories would offer to gain information on the magnetic and plasma properties of the magnetospheres of hot stars, helping us test the fundamental hypothesis that magnetospheres should act to rapidly drain angular momentum, thereby spinning the star down, whilst simultaneously reducing the net mass-loss rate. Both effects are expected to lead to dramatic differences in the evolution of magnetic vs. non-magnetic stars.

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arXiv2022

The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV 476: A comprehensive analysis reveals surprisingly low stellar masses

Massive stars at low metallicity are among the main feedback agents in the early Universe and in present-day star forming galaxies. When in binaries, these stars are potential progenitors of gravitational-wave events. Knowledge of stellar masses is a prerequisite to understanding evolution and feedback of low-metallicity massive stars. Using abundant spectroscopic and photometric measurements of an outstandingly bright eclipsing binary, we compare its dynamic, spectroscopic, and evolutionary mass estimates and develop a binary evolution scenario. We comprehensively studied the eclipsing binary system, AzV 476, in the Small Magellanic Cloud. The light curve and radial velocities were analyzed to obtain the orbital parameters. The photometric and spectroscopic data in the UV and optical were analyzed using the Potsdam Wolf-Rayet model atmospheres. The obtained results are interpreted using binary-evolution tracks. AzV 476 consists of an O4IV-III((f))p primary and an O9.5:Vn secondary. Both components have similar current masses (~20 M$_{\odot}$) obtained from both the orbital and spectroscopic analysis. The wind mass-loss rate of log($\dot{M}$/(M$_{\odot}$/yr))=-6.2 of the primary is a factor of ten higher than a recent empirical prescription for single O stars in the SMC. Only close-binary evolution with mass transfer can reproduce the current stellar and orbital parameters. The binary evolutionary model reveals that the primary has lost about half of its initial mass and is already core helium burning. Our comprehensive analysis of AzV 476 yields a consistent set of parameters and suggests previous case B mass transfer. The derived stellar masses agree within their uncertainties. The moderate masses of AzV 476 underline the scarcity of bright massive stars in the SMC. The core helium burning nature of the primary indicates that stripped stars might be hidden among OB-type populations.

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arXiv2015

The MiMeS Survey of Magnetism in Massive Stars: Introduction and overview

The MiMeS project is a large-scale, high resolution, sensitive spectropolarimetric investigation of the magnetic properties of O and early B type stars. Initiated in 2008 and completed in 2013, the project was supported by 3 Large Program allocations, as well as various programs initiated by independent PIs and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS at the Canada-France-Hawaii Telescope, Narval at the T\'elescope Bernard Lyot, and HARPSpol at the European Southern Observatory La Silla 3.6m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.

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arXiv2014

The variable stellar wind of Rigel probed at high spatial and spectral resolution

We present a spatially resolved, high-spectral resolution (R=12000) K-band temporal monitoring of Rigel using AMBER at the VLTI. Rigel was observed in the Bracket Gamma line and its nearby continuum in 2006-2007, and 2009-2010. These unprecedented observations were complemented by contemporaneous optical high-resolution spectroscopy. We analyse the near-IR spectra and visibilities with the 1D non-LTE radiative-transfer code CMFGEN. The differential and closure phase signal exhibit asymmetries that are interpreted as perturbations of the wind. A systematic visibility decrease is observed across the Bracket Gamma. During the 2006-2007 period the Bracket Gamma and likely the continuum forming regions were larger than in the 2009-2010 epoch. Using CMFGEN, we infer a mass-loss rate change of about 20% between the two epochs. We further find time variations in the differential visibilities and phases. The 2006-2007 period is characterized by noticeable variations of the differential visibilities in Doppler position and width and by weak variations in differential and closure phase. The 2009-2010 period is much more quiet with virtually no detectable variations in the dispersed visibilities but a strong S-shape signal is observed in differential phase coinciding with a strong ejection event discernible in the optical spectra. The differential phase signal that is sometimes detected is reminiscent of the signal computed from hydrodynamical models of corotating interaction regions. For some epochs the temporal evolution of the signal suggests the rotation of the circumstellar structures.

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arXiv2010

High spatial resolution monitoring of the activity of BA supergiant winds

There are currently two optical interferometry recombiners that can provide spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands, and VEGA/CHARA, recently commissioned, operating in the visible. These instruments are well suited to study the wind activity of the brightest AB supergiants in our vicinity, in lines such as H$\alpha$ or BrGamma. We present here the first observations of this kind, performed on Rigel (B8Ia) and Deneb (A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and 2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in 2008-2009 by VEGA. The extension of the Halpha and BrGamma line forming regions were accurately measured and compared with CMFGEN models of both stars. Moreover, clear signs of activity were observed in the differential visibility and phases. These pioneer observations are still limited, but show the path for a better understanding of the spatial structure and temporal evolution of localized ejections using optical interferometry.

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arXiv2008

Wind structure of late B supergiants I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)

We provide a quantitative analysis of time-variable phenomena in the photospheric, near-star, and outflow regions of the late-B supergiant (SG) HD 199478. The analysis is based primarily on optical spectroscopic datasets secured between 1999 and 2000 from the Bulgarian NAO, Tartu, and Ritter Observatories. The temporal behaviour of HD 199478 is characterised by three key empirical properties: (i) systematic central velocity shifts in the photospheric absorption lines, including C II and He I, over a characteristic time-scale of abou 20 days; (ii) extremely strong, variable H alpha emission with no clear modulation signal, and (iii) the occurrence in 2000 of a (rare) high-velocity absorption (HVA) event in H alpha, which evolved over about 60 days, showing the clear signature of mass infall and outflows. In these properties HD 199478 resembles few other late-B SGs with peculiar emission and HVAs in H alpha (HD 91619, HD 34085, HD 96919). Non-LTE line synthesis modelling is conducted using FASTWIND for these late-B SGs to constrain and compare their fundamental parameters within the context of extreme behaviour in the H alpha lines. Our analysis indicate that at the cooler temperature edge of B SGs, there are objects whose wind properties, as traced by H alpha, are inconsistent with the predictions of the smooth, spherically symmetric wind approximation. This discordance is still not fully understood and may highlight the role of a non-spherical, disk-like, geometry, which may result from magnetically-driven equatorial compression of the gas. Ordered dipole magnetic fields may also lead to confined plasma held above the stellar surface, which ultimately gives rise to transient HVA events.

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