Publications of Constantin Cazorla
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See detailAbundance determination in massive stars: challenges for mixing processes
Cazorla, Constantin ULiege

Doctoral thesis (2017)

Massive stars, the most luminous stars, are the true “cosmic engines” of our Universe. They eject large quantity of material throughout their life, which strongly influences their evolutionary path as ... [more ▼]

Massive stars, the most luminous stars, are the true “cosmic engines” of our Universe. They eject large quantity of material throughout their life, which strongly influences their evolutionary path as well as their environment. An important feature of massive stars is their high rotational velocities that are either acquired at birth or due to the influence of a companion. Rotation is believed to transport nitrogen-rich and carbon/oxygen-poor material generated in the stellar core through the CNO cycle, to the surface. A way to test the e ciency of rotational mixing is to study the chemical composition at the surface of stars, in particular the fastest rotators. The incentive for this study was the discovery, in the context of the VLT- FLAMES Survey of Massive Stars, of fast rotators exhibiting an unenriched nitrogen composition at their surface, contrary to predictions from single-star evolutionary models including rotation. However, their multiplicity may a ect this conclusion, since both rotation and abundances can change as a result of binary interactions. In this work, we combined, for the first time, a detailed surface abundance analysis with a radial-velocity study to quantify the importance of binary e ects. This work was conducted for a sample of 40 bright, OB fast rotators in our Galaxy. Statistical tests and period-search techniques revealed that ≥ 40% of our targets whose multiplicity status can be probed, are binary or binary candidates. We derived the projected rotational velocity of our targets and model atmosphere codes were then used to derive stellar parameters and surface abundances of all sample stars. This abundance study revealed a correlation between the helium and nitrogen abundances of our targets, which is predicted by the rotational mixing theory. Finally, we compared our results to predictions of single-star evolutionary models. We found that 10 – 20% of our 40 targets exhibit no enhancement of the [N/O] abundance ratio, in line with results of the VLT-FLAMES Survey of Massive Stars. The properties of only half of our sample are explained by such models, and surprisingly we also uncovered a quite common large abundance of helium at the surface of our targets. Modifying the di usion coe cient in single-star models and models of non-rotating mergers did not reproduce simultaneously both the observed helium abundances and the [N/O] abundance ratios. Binary models considering a mass-transfer episode can, however, reproduce the [N/O] values of the majority of our targets and even the helium abundances of some of the most helium-enriched targets, but they cannot explain stars displaying little helium enrichment but high [N/O] values. In conclusion, we found that not every feature of massive stars can be explained by models, suggesting that they lack a physical ingredient and thus require further improvements. The second part of this thesis aimed at improving our knowledge of the X-ray emission of early B-type stars. We studied 11 such stars at high resolution thanks to two X-ray facilities, XMM-Newton and Chandra, doubling the number of B-stars analysed at high resolution. In many aspects, our study confirmed previous ones: early B-stars display rather narrow and unshifted lines arising from a warm (of typically 0.2 – 0.6 keV) plasma located at a few stellar radii over the stellar surface. We also found that abundances derived in the X-ray domain are in fair agreement with photospheric ones derived in the optical domain. Furthermore, most early B-stars are moderately bright X-ray emitters – though we also unexpectedly found that this X-ray emission varies, on short and/or long timescales, for half of our sample. A few stars display peculiar features: the presence of a very hot (1.6 – 4.4 keV) component and strong variations. These features suggest that the recorded X-ray emission may not be entirely linked to the B-stars, but could be contaminated by emission from a companion or an interaction with it. Indeed, in one case, HD 79351, a flare was detected, of a luminosity compatible with those from PMS stars, and which could be associated to its companion. Finally, the data used also led to the discovery of the second case of X-ray pulsations associated to beta Cephei activity. [less ▲]

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See detailChemical abundances of fast-rotating massive stars. II. Interpretation and comparison with evolutionary models
Cazorla, Constantin ULiege; Nazé, Yaël ULiege; Morel, Thierry ULiege et al

in Astronomy and Astrophysics (2017), 604(A123),

Aims: Past observations of fast-rotating massive stars exhibiting normal nitrogen abundances at their surface have raised questions about the rotational mixing paradigm. We revisit this question thanks to ... [more ▼]

Aims: Past observations of fast-rotating massive stars exhibiting normal nitrogen abundances at their surface have raised questions about the rotational mixing paradigm. We revisit this question thanks to a spectroscopic analysis of a sample of bright fast-rotating OB stars, with the goal of quantifying the efficiency of rotational mixing at high rotation rates. Methods: Our sample consists of 40 fast rotators on the main sequence, with spectral types comprised between B0.5 and O4. We compare the abundances of some key element indicators of mixing (He, CNO) with the predictions of evolutionary models for single objects and for stars in interacting binary systems. Results: The properties of half of the sample stars can be reproduced by single evolutionary models, even in the case of probable or confirmed binaries that can therefore be true single stars in a pre-interaction configuration. The main problem for the rest of the sample is a mismatch for the [N/O] abundance ratio (we confirm the existence of fast rotators with a lack of nitrogen enrichment) and/or a high helium abundance that cannot be accounted for by models. Modifying the diffusion coefficient implemented in single- star models does not solve the problem as it cannot simultaneously reproduce the helium abundances and [N/O] abundance ratios of our targets. Since part of them actually are binaries, we also compared their chemical properties with predictions for post-mass transfer systems. We found that these models can explain the abundances measured for a majority of our targets, including some of the most helium-enriched, but fail to reproduce them in other cases. Our study thus reveals that some physical ingredients are still missing in current models. [less ▲]

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See detailChemical abundances of fast-rotating massive stars. I. Description of the methods and individual results
Cazorla, Constantin ULiege; Morel, Thierry ULiege; Nazé, Yaël ULiege et al

in Astronomy and Astrophysics (2017), 603(A56),

Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast- rotating objects with apparently normal surface nitrogen abundances ... [more ▼]

Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast- rotating objects with apparently normal surface nitrogen abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing. Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity. Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way. [less ▲]

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See detailπ Aquarii is another γ Cassiopeiae object
Nazé, Yaël ULiege; Rauw, Grégor ULiege; Cazorla, Constantin ULiege

in Astronomy and Astrophysics (2017), 602

The γ Cas category is a subgroup of Be stars displaying a strong, hard, and variable thermal X-ray emission. An XMM-Newton observation of π Aqr reveals spectral and temporal characteristics that clearly ... [more ▼]

The γ Cas category is a subgroup of Be stars displaying a strong, hard, and variable thermal X-ray emission. An XMM-Newton observation of π Aqr reveals spectral and temporal characteristics that clearly make this Be star another member of the γ Cas category. Furthermore, π Aqr is a binary but, contrary to γ Cas, the nature of the companion to the Be star is known; it is a non-degenerate (stellar) object and its small separation from the Be star does not leave much room for a putative compact object close to the Be disk. This renders the accretion scenario difficult to apply in this system, and, hence, this discovery favors a disk-related origin for the γ Cas phenomenon. Based on observations collected with the ESA science mission XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). [less ▲]

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See detailChemical abundances of fast-rotating OB stars
Cazorla, Constantin ULiege; Morel, Thierry ULiege; Nazé, Yaël ULiege et al

in Proceedings of the International Astronomical Union - New windows on massive stars: asteroseismology, interferometry, and spectropolarimetry (2015)

Fast rotation in massive stars is predicted to induce mixing in their interior, but a population of fast-rotating stars with normal nitrogen abundances at their surface has recently been revealed (Hunter ... [more ▼]

Fast rotation in massive stars is predicted to induce mixing in their interior, but a population of fast-rotating stars with normal nitrogen abundances at their surface has recently been revealed (Hunter et al. 2009; Brott et al. 2011, but see Maeder et al. 2014). However, as the binary fraction of these stars is unknown, no definitive statements about the ability of single-star evolutionary models including rotation to reproduce these observations can be made. Our work combines for the first time a detailed surface abundance analysis with a radial-velocity monitoring for a sample of bright, fast-rotating Galactic OB stars to put strong constraints on stellar evolutionary and interior models. [less ▲]

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See detailThe X-ray bright massive stars in Cyg OB2
Nazé, Yaël ULiege; Cazorla, Constantin ULiege; Rauw, Grégor ULiege

Poster (2014, July 01)

As a complement to the results from the Cyg OB2 Chandra Legacy program, we present in this contribution the detailed analysis of the four X-ray bright massive stars dominating the cluster. Cyg OB2 #5, #8A ... [more ▼]

As a complement to the results from the Cyg OB2 Chandra Legacy program, we present in this contribution the detailed analysis of the four X-ray bright massive stars dominating the cluster. Cyg OB2 #5, #8A, and #9 are binary or multiple massive stars in the Cyg OB2 association displaying several peculiarities, such as bright X-ray emission and non-thermal radio emission. Our X-ray monitoring of these stars reveals the details of their behavior at high energies, which can be directly linked to wind-wind collisions (WWCs). In addition, the X-ray emission of Cyg OB2 #12, an evolved massive star, shows a long-term decrease, which could hint at the presence of a companion (with associated colliding winds) or indicate the return to quiescence of the system following a recent eruption. [less ▲]

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See detailChemical abundances of fast-rotating OB stars
Cazorla, Constantin ULiege; Morel, Thierry ULiege; Nazé, Yaël ULiege et al

Poster (2014, June 23)

Fast rotation in massive stars is predicted to induce mixing in their interior, but recent observations have challenged this concept by revealing a population of fast-rotating stars with normal nitrogen ... [more ▼]

Fast rotation in massive stars is predicted to induce mixing in their interior, but recent observations have challenged this concept by revealing a population of fast-rotating stars with normal nitrogen abundances at their surface (Hunter et al. 2009, A&A, 496, 841; Brott et al. 2011, A&A, 530, A116, but see Maeder et al. 2014, A&A, 565, A39). However, as the binary fraction of these stars is unknown, the importance of mass-transfer processes cannot be quantified. As a result, no definitive statements about the ability of single-star evolutionary models including rotation to reproduce these observations can be made. Our work combines for the first time a detailed surface abundance analysis with a radial-velocity monitoring for a sample of bright, fast-rotating Galactic OB stars to put strong constraints on stellar evolutionary and interior models. [less ▲]

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See detailAbundances of fast-rotating massive stars
Cazorla, Constantin ULiege

Diverse speeche and writing (2014)

PhD student day presentation

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See detailWind collisions in three massive stars of Cygnus OB2
Cazorla, Constantin ULiege; Nazé, Yaël ULiege; Rauw, Grégor ULiege

in Astronomy and Astrophysics (2014), 561

Aims. We wish to study the origin of the X-ray emission of three massive stars in the CygOB2 association: CygOB2#5, Cyg OB2 #8A, and Cyg OB2 #12. Methods. To this aim, dedicated X-ray observations from ... [more ▼]

Aims. We wish to study the origin of the X-ray emission of three massive stars in the CygOB2 association: CygOB2#5, Cyg OB2 #8A, and Cyg OB2 #12. Methods. To this aim, dedicated X-ray observations from XMM-Newton and Swift are used, as well as archival ROSAT and Suzaku data. Results. Our results on Cyg OB2 #8A improve the phase coverage of the orbit and confirm previous studies: the signature of a wind- wind collision is conspicuous. In addition, signatures of a wind-wind collision are also detected in Cyg OB2 #5, but the X-ray emission appears to be associated with the collision between the inner binary and the tertiary component orbiting it with a 6.7 yr period, without a putative collision inside the binary. The X-ray properties strongly constrain the orbital parameters, notably allowing us to discard some proposed orbital solutions. To improve the knowledge of the orbit, we revisit the light curves and radial velocity of the inner binary, looking for reflex motion induced by the third star. Finally, the X-ray emission of Cyg OB2 #12 is also analyzed. It shows a marked decrease in recent years, compatible with either a wind-wind collision in a wide binary or the aftermath of a recent eruption. [less ▲]

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See detailX-ray emission of interacting wind binaries in Cyg OB2
Nazé, Yaël ULiege; Cazorla, Constantin ULiege; Rauw, Grégor ULiege

Poster (2013, September)

Cyg OB2 #5, #8A, and #9 are binary or multiple massive stars in the Cyg OB2 association displaying several peculiarities, such as bright X-ray emission and non-thermal radio emission. Our X-ray monitoring ... [more ▼]

Cyg OB2 #5, #8A, and #9 are binary or multiple massive stars in the Cyg OB2 association displaying several peculiarities, such as bright X-ray emission and non-thermal radio emission. Our X-ray monitoring of these stars reveals the details of their behaviours at high energies, which can be directly linked to wind-wind collisions (WWCs). In addition, the X-ray emission of Cyg OB2 #12, an evolved massive star, shows a long-term decrease, which could hint at the presence of a companion (with associated colliding winds) or indicate the return to quiescence of the system following a recent eruption. [less ▲]

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See detailDévoiler les secrets des étoiles au coeur du Cygne grâce aux rayons X
Cazorla, Constantin ULiege

Master's dissertation (2013)

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See detailX-ray emission of interacting wind binaries in Cyg OB2
Nazé, Yaël ULiege; Cazorla, Constantin ULiege; Rauw, Grégor ULiege

in EAS Publications Series (2013, February 01)

Cyg OB2 #5, #8A, and #9 are binary or multiple massive stars in the Cyg OB2 association displaying several peculiarities, such as bright X-ray emission and non-thermal radio emission. Our X-ray monitoring ... [more ▼]

Cyg OB2 #5, #8A, and #9 are binary or multiple massive stars in the Cyg OB2 association displaying several peculiarities, such as bright X-ray emission and non-thermal radio emission. Our X-ray monitoring of these stars reveals the details of their behaviours at high energies, which can be directly linked to wind-wind collisions (WWCs). In addition, the X-ray emission of Cyg OB2 #12, an evolved massive star, shows a long-term decrease, which could hint at the presence of a companion (with associated colliding winds) or indicate the return to quiescence of the star following a recent eruption. [less ▲]

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