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Issue
A&A
Volume 528, April 2011
Article Number A151
Number of page(s) 7
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361/201016345
Published online 18 March 2011

© ESO, 2011

1. Introduction

Magnetic fields play an important role in astrophysical phenomena of the universe on various scales. For galaxies, dynamo models associated with various MHD instabilities occurring in the interstellar medium (ISM) are used to explain the formation of the galactic structure (e.g., Gomez & Cox 2004; Bonanno & Urpin 2008). Magnetic fields play a role in the evolution of interstellar molecular clouds and the star formation process, where the cloud collapse is probably taking place along the magnetic field lines (e.g. Alves et al. 2008). They are also present at all stages of stellar evolution, from young T Tauri stars and Ap/Bp stars to the end products: white dwarfs, neutron stars, and magnetars. On the other hand, the role of magnetic fields in massive O-type stars and Wolf-Rayet (WR) stars remains unknown. No definitive magnetic field has ever been detected in WR stars and presently only less than a dozen O stars have published magnetic fields. In addition, the theories about the origin of magnetic fields in O-type stars remain poorly developed, mostly because the distribution of magnetic field strengths in massive stars from the ZAMS to more evolved stages has not yet been studied.

In our study, we focus on magnetic fields of massive stars observed in different environments: in open clusters at different ages and in the field. The results of our recent kinematic analysis of known magnetic O-type stars using the best available astrometric, spectroscopic, and photometric data indicates that a magnetic field is more frequently detected in candidate runaway stars than in stars belonging to clusters or associations (Hubrig et al. 2011a). As the sample of stars with magnetic field detections is still very small, a study of a larger sample is urgently needed to confirm the detected trend by performing dedicated magnetic field surveys of O stars in clusters/associations and the field. We were granted four nights in 2010 May with FORS 2 at the VLT to survey magnetic fields in massive stars, but because of poor weather conditions, only half of the granted time could be used for observations. Notwithstanding, the results obtained allow us to constrain preliminarily the conditions conducive to the presence of magnetic fields and derive the first trends for their occurrence rate and field strength distribution. This information is critical for answering the principal question of the possible origin of magnetic fields in massive stars.

In the following, we present 41 new measurements of magnetic fields in 36 massive stars using FORS 2 at the VLT in spectropolarimetric mode. Our observations and the obtained results are described in Sect. 2, and their discussion is presented in Sect. 3.

2. Observations and results

Spectropolarimetric observations were carried out in 2010 May 20–23 in visitor mode at the European Southern Observatory with FORS 2 mounted on the 8-m Antu telescope of the VLT. This multi-mode instrument is equipped with polarization analyzing optics, comprising super-achromatic half-wave and quarter-wave phase retarder plates, and a Wollaston prism with a beam divergence of 22″ in standard resolution mode1. Polarimetric spectra were obtained with the GRISM 600B and the narrowest slit width of 0\hbox{$\farcs$}4 to achieve a spectral resolving power of R ~ 2000. The use of the mosaic detector made of blue optimized E2V chips and a pixel size of 15 μm allowed us to cover a large spectral range, from 3250 to 6215 Å, which includes all hydrogen Balmer lines from Hβ to the Balmer jump. The spectral types and the visual magnitudes of the studied stars are listed in Table 1.

A detailed description of the assessment of the longitudinal magnetic-field measurements using FORS 2 was presented in our previous papers (e.g., Hubrig et al. 2004a,b, and references therein). The mean longitudinal magnetic field, ⟨Bz⟩, was derived using VI=−geffeλ24πmec21IdIdλ⟨Bz⟩,\begin{equation} \frac{V}{I} = -\frac{g_{\rm eff} e \lambda^2}{4\pi{}m_{\rm e}c^2}\ \frac{1}{I}\ \frac{{\rm d}I}{{\rm d}\lambda} \left<B_{z}\right>, \label{eqn:one} \end{equation}(1)where V is the Stokes parameter that measures the circular polarisation, I is the intensity in the unpolarised spectrum, geff is the effective Landé factor, e is the electron charge, λ is the wavelength, me the electron mass, c the speed of light, dI / dλ is the derivative of Stokes I, and ⟨Bz⟩ is the mean longitudinal magnetic field.

Table 1

List of O-type stars observed with FORS 2.

Longitudinal magnetic fields were measured in two ways: using only the absorption hydrogen Balmer lines or the entire spectrum including all available absorption lines. The lines that show evidence of emission were not used to determine the magnetic field strength. The feasibility of longitudinal magnetic field measurements in massive stars using low-resolution spectropolarimetric observations has been demonstrated by previous studies of O and B-type stars (e.g., Hubrig et al. 2006; 2008; 2009a; 2011b). To verify that the instrument was functioning properly, we observed the magnetic Ap star HD 187474, which has a well-studied longitudinal magnetic field, during the night of May 23 at rotation phase 0.66. HD 187474 has a rotation period of 6.4 yr and a longitudinal magnetic field ranging roughly from −2 kG to 2 kG. The measured value of the magnetic field, ⟨Bz⟩all =  −1249 ± 47 G, fits very well to the observations at the same phase presented by Landstreet & Mathys (2000).

Table 2

Longitudinal magnetic fields measured with FORS 2 in the studied sample.

Although we were granted four nights for our survey, owing to unfavorable weather conditions (snow and high humidity) only four stars could be observed during the first night, 14 stars during the second night, none during the third night, and 23 during the last night. Most of the targets were observed only once. The exceptions were the stars HD 328856, HD 153426, and HD 168112, which we were able to observe twice. HD 148937 was observed three times to assess the magnetic field variability over the rotation cycle. Apart from this star, which has a rotation period of seven days (Nazé et al. 2008), no exact rotation periods are known for the other stars in our sample.

The results of our magnetic field measurements are presented in Table 2. In the first two columns, we provide the star names and the modified Julian dates at the middle of the exposures. In Cols. 3 and 4, we present the longitudinal magnetic field ⟨Bz⟩all using the whole spectrum, and the longitudinal magnetic field ⟨Bz⟩hyd using only the hydrogen lines. All quoted errors are 1σ uncertainties. In Col. 5, we identify new detections by ND and in the case of HD 148937 the confirmed detection is marked by CD.

Ten stars of our sample, CPD−28 2561, CPD−47 2963, HD 93843, HD 130298, HD 148937, HD 328856, HD 153426, HD 153919, HD 154643, and HD 156154 show evidence of a magnetic field.

Most importantly, the strongest magnetic fields are detected in both Of?p stars CPD−28 2561 and HD 148937. Walborn (1973) introduced the Of?p category for massive O stars displaying recurrent spectral variations in certain spectral lines, sharp emission or P Cygni profiles in He I and the Balmer lines, and strong C III emission lines around 4650 Å. Only five Galactic Of?p stars are presently known: HD 108, NGC 1624-2, CPD−28 2561, HD 148937, and HD 191612 (Walborn et al. 2010). Our observations of CPD−28 2561 enabled us to detect a magnetic field at the 3.1σ level using the whole spectrum and at the 3.2σ level using Balmer lines. The study of the radial velocity variation by Levato et al. (1988) found variability of a few emission lines with a probable period of 17 days. Walborn et al. (2010) report that CPD−28 2561 undergoes extreme spectral transformations very similar to those of HD 191612, on a timescale of weeks, inferred from the variable emission intensity of the C III λλ 4647-4650-4652 triplet. The detection of a mean longitudinal magnetic field  ⟨ Bz ⟩  =  −254  ±  81 G in the Of?p star HD 148937 using FORS 1 at the VLT was previously reported by Hubrig et al. (2008). An extensive multiwavelength study of HD 148937 was carried out by Nazé et al. (2008), who detected the small-scale variations of He ii 4686 and the Balmer lines with a period of seven days and an overabundance of nitrogen by a factor of four compared to the Sun. The periodicity of spectral variations in hydrogen and helium lines was re-confirmed using additional higher resolution spectroscopic material indicating the similarity to the other Of?p stars HD 108 and HD 191612 (Naze et al. 2010). Our spectropolarimetric observations of this star indicate that the magnetic field is variable, but owing to the low number of measurements it is not possible to verify the period deduced from spectroscopic observations. The magnetic field of this star was observed at 4.8σ, 2.9σ, and 3.4σ levels on three different nights using all absorption lines.

The remaining three Of?p stars are located in the northern hemisphere and cannot be reached with FORS 2 at the VLT. To study magnetic fields in HD 108 and HD 191612 we used polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope (Hubrig et al. 2010). As a result, we detected a longitudinal magnetic field  ⟨ Bz ⟩  =  −168  ±  35 G in the Of?p star HD 108, which agrees with the longitudinal magnetic field measurement of the order of −150 G reported by Martins et al. (2010). For the star HD 191612 with a rotation period of 537.6 d (Howarth et al. 2007), we measured a longitudinal magnetic field  ⟨ Bz ⟩  = 450  ±  153 G at rotation phase 0.43 (Hubrig et al. 2010). The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field  ⟨ Bz ⟩  =  −220  ±  38 G at rotation phase 0.24 (Donati et al. 2006), indicating a change of polarity over  ~ 100 days. No attempt has yet been made to measure the magnetic field of NGC 1624-2. Clearly these magnetic field measurements in Of?p stars imply that there is a tight relation between the observed properties of the Of?p star group and the presence of a magnetic field.

For the star CPD−47 2963, we achieved a 3.1σ detection using all absorption lines. According to Walborn et al. (2010), this star belongs to the Ofc category, which consists of normal spectra with C III λλ 4647-4650-4652 emission lines of comparable intensity to those of the Of defining lines N III λλ 4634-4640-4642. The authors indicate that the Ofc phenomenon occurs primarily in certain associations and young clusters. However, the available kinematic and photometric data do not indicate cluster or association membership for CPD−47 2963. The origin of the magnetic field in this star probably differs from those of other magnetic O-type stars, because non-thermal radio emission, which is frequently observed in binary systems with colliding winds, was detected by Benaglia et al. (2001). On the other hand, the membership of CPD−47 2963 in a binary or multiple system has not yet been investigated. The authors suggest that the non-thermal radiation from this star possibly originates from strong shocks in the wind itself and/or in the wind colliding region if the star has a massive early-type companion. Both optical and radio observations reveal the presence of a second source separated by 5″.

According to Walborn et al. (2010), the star HD 93843, with a 3.7σ detection achieved using all absorption lines, belongs to the Ofc category. Prinja et al. (1998) monitored the stellar wind of this star using IUE time series. They identified systematic changes in the absorption troughs of the Si iv and N v resonance lines with a repeatability of wind structures with a period of 7.1 days. The authors suggest that a magnetic field might help explain the cyclical wind perturbation. On the other hand, three other stars of the Ofc category included in our survey, HD 93204, HDE 303308, and HD 93403, do not have a detectable magnetic field at the 3σ level.

The star HD 130298 with a longitudinal magnetic field observed at the 3.1σ level using the Balmer lines, is known as an object with a bow shock. Noriega-Crespo et al. (1997) used ISSA/IRAS archival spectra to identify stars surrounded by extended infrared emission at 60 μm, which is a signature of wind bow shocks. The bow shocks are usually associated with runaway early-type stars with typical wind velocities of 500–3000 km s-1 and mass loss rates  ~ 10-5–10-6 M⊙ yr-1 (see e.g. Puls et al. 1996).

Table 3

Probable members in open clusters and associations.

The two stars HD 328856 and HD 153426, which both have magnetic field detections, were observed on two different nights, namely the first and the fourth night of our observing run. For HD 328856, we obtained on these nights 3.3σ and 3.1σ level detections, respectively, using all absorption lines. On the basis of the photometric membership probability, this star is a member of the compact open cluster Hogg 22 in the Ara region at an age of 5 Myr and a distance of about 1300 pc (see Sect. 3). On the other hand, its proper motions indicate that HD 328856 is not fully co-moving with the other cluster members, deviating from the cluster mean proper motion by  ~ 2σ (for more details of the membership probabilities, see Kharchenko et al. 2004). The observations of the star HD 153426 detected a mean longitudinal magnetic field at the 3.9σ level using Balmer lines on the fourth night. The non-detection of the magnetic field for HD 153426 on the first observing night can probably be explained by the strong dependence of the longitudinal magnetic field on the rotational aspect. HD 153426 is a double-lined spectroscopic binary with unknown orbit parameters and was considered by de Wit et al. (2005) as a star in a newly detected cluster. Using the highest quality kinematic data on young open clusters presently available, Schilbach & Röser (2008) suggested that HD 153426 was ejected from the cluster Hogg 22. Their back-tracing procedure indicates that the encounter time for HD 153426, i.e. the time when the star was ejected, is about 8.1 Myr, while the age of the cluster Hogg 22 is only 5 Myr.

The star HD 153919 was observed only once, revealing the presence of a mean longitudinal magnetic field at the 3.9σ level, using all absorption lines. The study of Ankay et al. (2001) suggested that this star is a runaway X-ray binary, ejected from the OB association Sco OB1 about 2 Myr ago by the supernova of 4U1700-37’s progenitor. They considered this star to be a companion of 4U1700-37, most likely a neutron star powered by wind accretion (e.g., Jones et al. 1973). Since 4U1700-37 is a candidate low-mass black hole (Brown et al. 1996), this system can be similar to the optical component (the O9.7 Iab supergiant) in the system Cyg X-1, for which a variable weak magnetic field was detected using a FORS 1 spectropolarimetric time series over the orbital period of 5.6 days (Karitskaya et al. 2010). Schilbach & Röser (2008) identified the origin of this field star in the cluster NGC 6231 (the open cluster inside Sco OB1) at an age of about 6.5 Myr and their back-tracing procedure indicates that the star was ejected from the cluster 1.1 Myr ago.

The longitudinal magnetic field for the star HD 154643 was observed at the 3.2σ level using all absorption lines. De Wit et al. (2005) characterise this star as a candidate runaway star associated with the young cluster Bochum 13. However, Schilbach & Röser (2008) identified the origin of this field star in the cluster ASCC 88, which has an an age of about 14.8 Myr, and proposed that HD 154643 was ejected 1.4 Myr ago.

The star HD 156154, for which we achieved a 3.1σ detection using all absorption lines, seems to be the only star with a high cluster membership probability among the O-type stars with detected magnetic fields we have considered here (see Sect. 3). According to kinematic and photometric criteria, this star belongs to the open cluster Bochum 13 at an age of 12 Myr and a distance of about 1 kpc.

3. Discussion

There has been much research of massive stars in recent years in order to properly model the effects of their rotation, stellar winds, and surface chemical composition. However, possible paths for the formation of magnetic O-type stars have not been yet analysed by modern evolution theories of single and binary stars. The number of massive stars with detected magnetic fields remains small, and the available data are insufficient to prove statistically whether magnetic fields in massive stars are ubiquitous or appear in specific stars with certain stellar parameters and a special environment. On the other hand, the observations of magnetic fields in massive stars accumulated over the past few years can be used to preliminarily constrain the conditions that enable the appearance of magnetic fields and provide the first trends about their occurrence and field strength distribution.

Since no longitudinal magnetic fields stronger than 300 G have been detected in our study (apart from the rather strong field in the Of?p star CPD−28 2561), we have confirmed our previous conclusion (Hubrig et al. 2008) that large-scale, dipole-like, magnetic fields with polar field strengths higher than 1 kG are not widespread among O type stars. Our study presents the results of a magnetic field survey of 36 massive stars. Among them, 19 stars have been found to be related to open clusters and associations of different ages. The data on the cluster membership of these probable cluster O-type stars are presented in Table 3. To increase the significance of our statistic assessment, we present in the same table the data for an additional six probable cluster O-type stars, which have been studied in recent years by Hubrig et al. (2008; 2009b; 2011a), marked for convenience by an asterisk. As database for the compilation of Table 3, we used the All-sky Compiled Catalogue of 2.5 million stars (ASCC-2.5, 3rd version) of Kharchenko & Roeser (2009). We note that for the calculation of kinematic membership probability only proper motions were used. According to Dias et al. (2002; catalogue version 3.1 (24/11/2010)), one of the previously studied O-type stars, the star HD 152408 (Hubrig et al. 2008), is projected onto the cluster Collinder 316, which represents a large group of bright stars superposed on Trumpler 24 at the age log t = 6.92. We have not included this star in Table 3, as no membership criteria were discussed in this work. According to Mason et al. (1998) and Pourbaix et al. (2004)2, among the stars presented in Table 3, six stars, HD 47839, HD 135240, HD 152233, HD 153919, HD 154368, and HD 167263, are members of spectroscopic binary systems, with orbital periods between 3.4 and 9247 days.

thumbnail Fig. 1

The positions of the stars studied for cluster membership in the colour-magnitude diagram. Different symbols indicate stars with different membership probabilities: squares represent stars with cluster membership probability higher than 60%, circles stars with a probability between 14% and 60%, and triangles membership probability between 1% and 14%. Non-members and runaway stars are marked by dots and crosses, respectively. One runaway star, HD 171589, does not appear in this figure, as its colour and magnitude do not fit the presented parameter space (see Table 4). The three stars with magnetic fields, HD 155806, HD 156154, and HD 164794, with high cluster membership probabilities are denoted by filled squares. Isochrones for log t = 6.6, 6.8, 7.0, and 7.2 are presented by solid lines and the zero-age main sequence is shown as a dashed line.

thumbnail Fig. 2

The age distribution of studied probable cluster members. The three stars with magnetic fields, HD 155806, HD 156154, and HD 164794, are denoted by horizontal lines.

From the inspection of kinematic and photometric membership probabilities, and following the membership criteria described by Kharchenko et al. (2004), five stars in Table 3, CPD−58 2620, HD 93129B, HDE 303308, HD 105056, HD 152233, have a rather low cluster membership probability. Two other stars, HD 120521 and HD 153919, are not kinematic members of the oldest clusters Platais 10 and NGC 6281, respectively, and should be regarded as field stars projected against the clusters by chance. Among the remaining stars, only in three stars, HD 155806, HD 156154, and HD 164794, have weak magnetic fields been detected (more details about the kinematic study of HD 155806 and HD 164794 can be found in Hubrig et al. 2011a). In Fig. 1, we display the positions of the stars listed in Table 3 in the colour–magnitude diagram. The absolute magnitudes MV and intrinsic colours (B − V)0 were calculated using cluster distances and E(B − V) values presented by Kharchenko et al. (2005a,b). The theoretical isochrones were calculated by the Padova group (Girardi et al. 2002) and the values for the zero-age main sequence were retrieved from Schmidt-Kaler (1982). The absolute magnitudes and colours are presented in Table 4. The uncertainty in the distance modulus is assumed to be 0.2. The positions of the magnetic stars with high cluster-membership probabilities, HD 156154, HD 155806, and HD 164794, do not represent any specific distribution that would indicate that their magnetic fields appear at a certain evolutionary age.

Table 4

Absolute magnitudes and intrinsic colours of stars studied for the cluster membership.

In Fig. 2, we present the age distribution of the most probable cluster members. While the age of HD 155806 and HD 164794 is similar to the bulk of the studied cluster O-type stars, the star HD 156154 is somewhat older at an age of  ~ 12 Myr. HD 155806 is classified as an Oe star, possibly representing the higher mass analogues of classical Be stars (e.g. Walborn 1973). Only six members have been proposed to belong to this group of stars (e.g. Negueruela et al. 2004). The star HD 164794 is a spectroscopic double-lined system with an orbital period of 2.4 yr, that is known to emit non-thermal radio-emission, probably associated with colliding winds (Nazé et al. 2010). No specific information can be found in the literature about the luminous supergiant HD 156154.

The available observations seem to indicate that a magnetic field is more frequently detected in field stars than in stars belonging to clusters or associations. It is generally accepted that the majority of massive stars form in star clusters and associations, and studies of kinematical properties of the massive star field population indicate that a major part of these stars can be traced back to their parent open clusters or associations (e.g. Schilbach & Roeser 2008). Pflamm-Altenburg & Kroupa (2010) discussed in their work whether massive stars can form in isolation in the galactic field. According to de Wit et al. (2005), only a few per cent of all O-type stars can be considered as having formed outside a cluster environment. Pflamm-Altenburg & Kroupa considered the two-step-ejection process, which represents the combination of the dynamical and the supernova ejection scenario with the result that massive field stars produced via this ejection process in the vast majority of cases cannot be traced back to their parent star clusters. These stars might be mistakenly considered as massive stars formed in isolation. While this cannot be proven, the observed numbers of field O stars is consistent with this idea.

For the newly detected magnetic O-type stars, HD 153426, HD 153919, and HD 154643, Schilbach & Roeser (2008) suggested that the three stars were ejected from the clusters Hogg 22, NGC 6231, and ASCC 88, respectively. On the other hand, none of the four magnetic Of?p stars is known to belong to a cluster or an association. The study of the evolutionary state of HD 108 and HD 191612 indicates that both stars have evolved significantly (Martins et al. 2010). Our kinematic study of the Of?p star HD 148937 has shown that it possesses a space velocity of 32 km s-1 with respect to the Galactic open cluster system, with the velocity component U =  −26 being directed in the opposite direction from the Galactic center and the velocity component W =  −13 directed from the Galactic plane (Hubrig et al. 2011a). These rather large velocities indicate that this star can be considered as a candidate runaway star.

It is striking that most previously detected magnetic O-type stars are candidate runaway stars (Hubrig et al. 2011a,c). In the sample of O-stars with magnetic fields detected in this work, four other stars, HD 130298, HD 153426, HD 153919, and HD 154643, are also mentioned in the literature as candidate runaway stars. In the past, two mechanisms have been discussed to explain the existence of runaway stars: in one scenario, close multi-body interactions in a dense cluster environment cause one or more stars to be scattered out of the region (e.g. Leonard & Duncan 1990). For this mechanism, runaways are ejected in dynamical three- or four-body interactions. An alternative mechanism involves a supernova explosion within a close binary, ejecting the secondary according to the conservation of momentum (Zwicky 1957; Blaauw 1961). However, none of these scenarios consider the possibility that a massive star can acquire a magnetic field during the ejection process. These findings clearly provide a strong motivation to carry out a kinematic study of all stars previously surveyed for magnetic fields to search for a correlation between the kinematic status and the presence of a magnetic field.

On the basis of these still very limited magnetic surveys of massive stars, we are unable to conclude whether O-type stars are magnetic in certain evolutionary states and a specific environment. Open star clusters and associations are very useful laboratories for testing star formation and stellar evolution. The ages of our subsample of three stars with magnetic fields do not contradict the idea that it is drawn from the general distribution of cluster ages. We have to keep in mind though that we have a very small number statistics. It appears that our observations are consistent with the assumption that the presence of a magnetic field can be expected in stars of different classification categories. Although a few hot magnetic stars might be peculiar on the basis of their spectral morphology, prior to their field detection (e.g. Of?p; Walborn 2006), the presence of a magnetic field can also be expected in stars of other classifications. Future magnetic field measurements are urgently needed to constrain the conditions controlling the presence of magnetic fields in hot stars, and the implications of these fields for their physical parameters and evolution.

1

The spectropolarimetric capabilities of FORS 1 were moved to FORS 2 in 2009.

Acknowledgments

A.E.P. acknowledges support of the RFBR grant 10-02-91338.

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All Tables

Table 1

List of O-type stars observed with FORS 2.

In the text
Table 2

Longitudinal magnetic fields measured with FORS 2 in the studied sample.

In the text
Table 3

Probable members in open clusters and associations.

In the text
Table 4

Absolute magnitudes and intrinsic colours of stars studied for the cluster membership.

In the text

All Figures

thumbnail Fig. 1

The positions of the stars studied for cluster membership in the colour-magnitude diagram. Different symbols indicate stars with different membership probabilities: squares represent stars with cluster membership probability higher than 60%, circles stars with a probability between 14% and 60%, and triangles membership probability between 1% and 14%. Non-members and runaway stars are marked by dots and crosses, respectively. One runaway star, HD 171589, does not appear in this figure, as its colour and magnitude do not fit the presented parameter space (see Table 4). The three stars with magnetic fields, HD 155806, HD 156154, and HD 164794, with high cluster membership probabilities are denoted by filled squares. Isochrones for log t = 6.6, 6.8, 7.0, and 7.2 are presented by solid lines and the zero-age main sequence is shown as a dashed line.
In the text
thumbnail Fig. 2

The age distribution of studied probable cluster members. The three stars with magnetic fields, HD 155806, HD 156154, and HD 164794, are denoted by horizontal lines.
In the text

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