Massive stars are key agents in the Universe, driving the evolution of star forming galaxies through their photons, winds and violent deaths at all redshifts. The community devotes large efforts to characterize those processes which have been identified to affect the evolution of massive stars: rotation, binarity and stellar winds. Recently, important effects of magnetic fields in massive stars are suggested by observations and by models, and spectacular objects such as gamma-ray bursts and magnetars can not be understood without their consideration. But what is their frequency and field strength distribution, their origin, and what are the evolutionary consequences of magnetic fields in massive stars?

We aim at directly addressing these questions through the study of a systematically selected sample of massive OB stars of different ages, considering their spin and their hard X-ray emission. We wish to measure the polarization induced in spectral lines by the Zeeman effect. Recent observations showed that FORS 2 and HARPS are ideal instruments for this. Through (funded) parallel theory projects, we aim to clarify the role of magnetic fields in the advanced stages of massive stars, including their fate as supernova or gamma-ray burst.





People Involved:

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Name Affiliation
T. Morel AGO Department, University of Liège Principal Investigator
N. Castro University of Michigan, Ann Arbor Co-investigator
N. Langer Argelander Institute for Astronomy (AIfA), Bonn Co-investigator
R. Barbá Departamento de Física, Universidad de La Serena, La Serena, Chile Co-investigator
G. Mathys European Southern Observatory (ESO), Garching Co-investigator
M. Schöller European Southern Observatory (ESO), Garching Co-investigator
L. Fossati Space Research Institute, Graz Co-investigator
N. Przybilla Insbruck University, Insbruck Co-investigator
A. Piskunov Institute of Astronomy (INASAN), Moscow Co-investigator
A. Herrero Instituto de Astrofísica de Canarias (IAC), Tenerife Co-investigator
J. Maíz Apellániz Instituto de Astrofísica de Andalucía (IAA), Granada Co-investigator
S. Simon-Díaz Instituto de Astrofísica de Canarias (IAC), Tenerife Co-investigator
J. F. González Instituto de Ciencias Astronómicas, San Juan Co-investigator
F. Schneider Department of Physics, Oxford Co-investigator
R. Arlt Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
M. Briquet Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
T. A. Carroll Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
S. Hubrig Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
I. Ilyin Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
S. Järvinen Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
A. Liermann Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
R. Scholz Leibniz Institute for Astrophysics (AIP), Potsdam Co-investigator
N. Kharchenko Main Astronomical Observatory, Kiev Co-investigator
H. Spruit MPA, Garching Co-investigator
A. Reisenegger Pontificia Universidad Católica de Chile (PUC), Santiago Co-investigator
W.-R. Hamann Potsdam Universität, Potsdam Co-investigator
L. Oskinova Potsdam Universität, Potsdam Co-investigator
P. L. Dufton Queen's University, Belfast Co-investigator
A. Kholtygin Saint-Petersburg State University, St. Petersburg Co-investigator
S.-C. Yoon Seoul National University, Seoul Co-investigator
A. Irrgang Sternwarte, Universität Erlangen, Bamberg Co-investigator
M.-F. Nieva Insbruck University, Insbruck Co-investigator
A. de Koter University of Amsterdam (UvA), Amsterdam Co-investigator
H. Sana KU Leuven Co-investigator

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  • Talks and Posters

    talk_Morel-AG-Bochum.pdf

    talk_IAUS307_Geneva_Morel.pdf

    talk_SAO_Kholtygin.pdf

    talk_Amsterdam_Schoeller.pdf

    talk_EWASS2015_Morel.pdf

    talk_Przybilla-AG-Kiel.pdf

  • Accepted Papers

    Hubrig et al. (2014), A&A, 564, L10, "B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid Nebula, one of the youngest star forming regions"

    Morel et al., 2015, IAUS307, 342, "The B Fields in OB Stars (BOB) Survey"

    Fossati et al. (2015), A&A, 574, A20, "B fields in OB stars (BOB): On the detection of weak magnetic fields in the two early B-type stars beta CMa and ε CMa. Possible lack of a "magnetic desert" in massive stars"

    Hubrig et al. (2015), A&A Letters, 578, L3, "B fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439"

    Castro et al. (2015), A&A, 581, 81, "B fields in OB stars (BOB): Detection of a strong magnetic field in the O9.7 V star HD 54879"

    Fossati et al. (2015), A&A, 582, A45, "B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars"

    Przybilla et al. (2016), A&A, 587, A7, "B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD−57◦ 3509"

    Schöller et al. (2017), A&A, 599, A66, "B fields in OB stars (BOB): Concluding the FORS 2 observing campaign"

    Castro et al. (2017), A&A, 597, L6, "B fields in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD -62° 2124"

    González et al. (2017), MNRAS, 467, 437, "B fields in OB stars (BOB): The magnetic triple stellar system HD 164492C in the Trifid nebula"

    ABSTRACT.
    Aims:

    Recent magnetic field surveys in O- and B-type stars revealed that about 10% of the core-hydrogen-burning massive stars host large-scale magnetic fields. The physical origin of these fields is highly debated. To identify and model the physical processes responsible for the generation of magnetic fields in massive stars, it is important to establish whether magnetic massive stars are found in very young star-forming regions or whether they are formed in close interacting binary systems.


    Methods:

    In the framework of our ESO Large Program, we carried out low-resolution spectropolarimetric observations with FORS 2 in 2013 April of the three most massive central stars in the Trifid nebula, HD 164492A, HD 164492C, and HD 164492D. These observations indicated a strong longitudinal magnetic field of about 500–600 G in the poorly studied component HD 164492C. To confirm this detection, we used HARPS in spectropolarimetric mode on two consecutive nights in 2013 June.


    Results:

    Our HARPS observations confirmed the longitudinal magnetic field in HD 164492C. Furthermore, the HARPS observations revealed that HD 164492C cannot be considered as a single star as it possesses one or two companions. The spectral appearance indicates that the primary is most likely of spectral type B1–B1.5 V. Since in both observing nights most spectral lines appear blended, it is currently unclear which components are magnetic. Long-term monitoring using high-resolution spectropolarimetry is necessary to separate the contribution of each component to the magnetic signal. Given the location of the system HD 164492C in one of the youngest star formation regions, this system can be considered as a Rosetta Stone for our understanding of the origin of magnetic fields in massive stars.

    ABSTRACT.

    The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects

    ABSTRACT.

    Only a small fraction of massive stars seem to host a measurable structured magnetic field, whose origin is still unknown and whose implications for stellar evolution still need to be assessed. Within the context of the "B fields in OB stars (BOB)" collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars beta CMa (HD 44743; B1 II/III) and ε CMa (HD 52089; B1.5II) in December 2013 and April 2014. For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field, approximately constant with time. We determined the physical parameters of both stars and characterise their X-ray spectrum. For the beta Cep star beta CMa, our mode identification analysis led to determining a rotation period of 13.6 ± 1.2 days and of an inclination angle of the rotation axis of 57.6±1.7°, with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of about 22° and a dipolar magnetic field strength (Bd) of about 100 G (60<Bd<230 G within the 1sigma level), below what is typically found for other magnetic massive stars. This conclusion is strengthened further by considerations of the star's X-ray spectrum. For ε CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Our results imply that both stars are expected to have a dynamical magnetosphere, so the magnetic field is not able to support a circumstellar disk. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. A histogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field "desert" observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational effort is probably still needed to properly access the magnetic field incidence.

    ABSTRACT.

    Aims. Massive B-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. Only two such stars, called σOri E analogues, were known until recently. A team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD23478 and HD345439. The magnetic fields in these newly discovered σOri E analogues have not been investigated so far. Methods. In the framework of our ESO Large Programme and one normal ESO programme, we carried out low-resolution FORS 2 spectropolarimetric observations of HD23478 and HD345439.

    Results. In the measurements of hydrogen lines, we discover a rather strong longitudinal magnetic field of up to 1.5 kG in HD23478 and up to 1.3 kG using the entire spectrum. The analysis of HD345439 using four subsequent spectropolarimetric subexposures does not reveal a magnetic field at a significance level of 3σ. On the other hand, individual subexposures indicate that HD345439 may host a strong magnetic field that rapidly varies over 88min. The fast rotation of HD345439 is also indicated by the behaviour of several metallic and He i lines in the low-resolution FORS 2 spectra that show profile variations already on this short time-scale.

    ABSTRACT.

    The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS 2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD 54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code fastwind results in an effective temperature and a surface gravity of 33 000 ± 1000 K and 4.0 ± 0.1 dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible within the errors. We investigate line-profile variability in HD 54879 by complementing our spectra with spectroscopic data from other recent OB-star surveys. The photospheric lines remain constant in shape between 2009 and 2014, although Hα shows a variable emission. The Hα emission is too strong for a standard O9.7 V and is probably linked to the magnetic field and the presence of circumstellar material. Its normal chemical composition and the absence of photospheric line profile variations make HD 54879 the most strongly magnetic, non-variable single O-star detected to date.

    ABSTRACT.

    Within the context of the collaboration "B fields in OB stars (BOB)", we used the FORS2 low-resolution spectropolarimeter to search for a magnetic field in 50 massive stars, including two reference magnetic massive stars. Because of the many controversies of magnetic field detections obtained with the FORS instruments, we derived the magnetic field values with two completely independent reduction and analysis pipelines. We compare and discuss the results obtained from the two pipelines. We obtained a general good agreement, indicating that most of the discrepancies on magnetic field detections reported in the literature are caused by the interpretation of the significance of the results (i.e., 3-4 sigma detections considered as genuine, or not), instead of by significant differences in the derived magnetic field values. By combining our results with past FORS1 measurements of HD46328, we improve the estimate of the stellar rotation period, obtaining P = 2.17950+/-0.00009 days. For HD125823, our FORS2 measurements do not fit the available magnetic field model, based on magnetic field values obtained 30 years ago. We repeatedly detect a magnetic field for the O9.7V star HD54879, the HD164492C massive binary, and the He-rich star CPD -57 3509. We obtain a magnetic field detection rate of 6+/-4%, while by considering only the apparently slow rotators we derive a detection rate of 8+/-5%, both comparable with what was previously reported by other similar surveys. We are left with the intriguing result that, although the large majority of magnetic massive stars is rotating slowly, our detection rate is not a strong function of the stellar rotational velocity.

    ABSTRACT.

    Aims. We report the detection of a magnetic field in the helium-strong star CPD −57° 3509 (B2 IV), a member of the Galactic open cluster NGC 3293, and characterise the star’s atmospheric and fundamental parameters. Methods. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. Results. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD −57° 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in Hα is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23 750 ± 250 K and 4.05 ± 0.10, respectively, and a surface helium fraction of 0.28 ± 0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD −57° 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its cluster membership.

    ABSTRACT.

    Aims: The B fields in OB stars (BOB) Collaboration is based on an ESO Large Programme to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. Methods: In the framework of this program, we carried out low-resolution spectropolarimetric observations of a large sample of massive stars using FORS 2 installed at the ESO VLT 8 m telescope. Results: We determined the magnetic field values with two completely independent reduction and analysis pipelines. Our in-depth study of the magnetic field measurements shows that differences between our two pipelines are usually well within 3σ errors. From the 32 observations of 28 OB stars, we were able to monitor the magnetic fields in CPD -57° 3509 and HD 164492C, confirm the magnetic field in HD 54879, and detect a magnetic field in CPD -62° 2124. We obtain a magnetic field detection rate of 6 ± 3% for the full sample of 69 OB stars observed with FORS 2 within the BOB program. For the preselected objects with a v sin i below 60 km s-1, we obtain a magnetic field detection rate of 5 ± 5%. We also discuss X-ray properties and multiplicity of the objects in our FORS 2 sample with respect to the magnetic field detections.

    ABSTRACT.

    The origin and evolution of magnetism in OB stars is far from being well understood. With approximately 70 magnetic OB stars known, any new object with unusual characteristics may turn out to be a key piece of the puzzle. We report the detection of an exceptionally strong magnetic field in the He-strong B2IV star CPD -62° 2124. Spectropolarimetric FORS2 and HARPSpol observations were analysed by two independent teams and procedures, concluding on a strong longitudinal magnetic field of approximately 5.2 kG. The quantitative characterisation of the stellar atmosphere yields an effective temperature of 23 650 ± 250 K, a surface gravity of 3.95 ± 0.10 dex and a surface helium fraction of 0.35 ± 0.02 by number. The metal composition is in agreement with the cosmic abundance standard, except for Mg, Si and S, which are slightly non-solar. The strong and broad ( 300 km s-1) disc-like emission displayed by the Hα line suggests a centrifugal magnetosphere supported by the strong magnetic field. Our results imply that CPD -62° 2124 is an early B-type star hosting one of the strongest magnetic fields discovered to date, and one of the most evolved He-strong stars known, with a fractional main-sequence lifetime of approximately 0.6.

    ABSTRACT.

    HD 164492C is a spectroscopic triple stellar system that has been recently detected to possess a strong magnetic field. We have obtained high-resolution spectroscopic and spectropolarimetric observations over a time span of two years and derived physical, chemical and magnetic properties for this object. The system is formed by an eccentric close spectroscopic binary (Ca1–Ca2) with a period of 12.5 d, and a massive tertiary Cb. We calculated the orbital parameters of the close pair, reconstructed the spectra of the three components and determined atmospheric parameters and chemical abundances by spectral synthesis. From spectropolarimetric observations, multi-epoch measurements of the longitudinal magnetic fields were obtained. The magnetic field is strongly variable on time-scales of a few days, with a most probable period in the range of 1.4–1.6 d. Star Cb with Teff ∼ 25 000 K is apparently the fastest rotator and the most massive star of this triple system and has anomalous chemical abundances with a marked overabundance of helium, 0.35±0.04 by number. We identified this star as being responsible for the observed magnetic field, although the presence of magnetic fields in the components of the Ca pair cannot be excluded. Star Ca1 with a temperature of about 24 000 K presents a normal chemical pattern, while the least massive star Ca2 is a mid-B-type star (Teff ∼ 15 000 K) with an overabundance of silicon. The obtained stellar parameters of the system components suggest a distance of 1.5 kpc and an age of 10–15 Myr.