arXiv subjects

S. Hubrig

Publications and source records attributed to S. Hubrig.

At least 73 records · Page 4Linked to original sources

arXiv2017

Characterizing the magnetic field and spectral variability of the rigidly rotating magnetosphere star HD345439

A team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, recently announced the discovery of two rare rigidly rotating magnetosphere stars, HD345439 and HD23478. Near-infrared spectra of these objects revealed emission-line behavior identical to that previously discovered in the helium-strong star sigma Ori E, which has a strong magnetic field and rotates fast. A single spectropolarimetric observation of HD345439 with FORS2 at the VLT in 2014 over 88min indicated that HD345439 may host a strong, rapidly varying magnetic field. In this work, we present the results of our spectropolarimetric monitoring of this star with FORS2, which revealed the presence of a strong longitudinal magnetic field dominated by a dipolar component. The analysis of spectral variability indicates an opposite behaviour of He and Si lines, which is usually attributed to differences in the distribution of surface He and Si abundance spots.

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arXiv2017

B fields in OB stars (BOB): The magnetic triple stellar system HD 164492C in the Trifid nebula

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 timespan 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 days, 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 timescales of a few days, with a most probable period in the range of 1.4-1.6 days. Star Cb with Teff~25000 K is the apparently 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 24000 K presents a normal chemical pattern, while the least massive star Ca2 is a mid-B type star (Teff~15000 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.

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arXiv2017

Fast spectral variations of OBA stars

Our search for magnetic fields in selected OBA stars using FOcal Reducer low dispersion Spectrographs FORS 1 and FORS 2 in spectropolarimetric mode revealed the presence of strong line profile variations (LPV) in spectra of magnetic OBA stars. More specifically, we detected moderate line profile variations of selected lines on a time scale of minutes in FORS 2 spectra of the A0 supergiant HD 92207 (Hubrig et al. 2014). Recently, we investigated the variability of line profiles of selected OBA stars with the multi-mode focal reducer SCORPIO at the 6-meter BTA telescope. We discovered regular variations of H and He lines in the spectra of the O-type star HD 93521 (O9.5III) with periods ~4-5 and ~32-36 minutes. The possible origin of short time-scale spectral variations is discussed.

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arXiv2017

Statistics of magnetic field measurements in OB stars

We review the measurements of magnetic fields of OB stars and compile a catalog of magnetic OB stars. Based on available data we confirm that magnetic field values are distributed according to a log--normal law with a mean log(B)=2.53 and a standard deviation $σ=0.54$. We also investigate the formation of the magnetic field of OBA stars before the Main Sequence (MS).

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arXiv2016

B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD-62 2124

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 23650$\pm$250 K, a surface gravity of 3.95$\pm$0.10 dex and a surface helium fraction of 0.35$\pm$0.02 by number. The metal composition is in agreement with the cosmic abundance standard, except for Mg, Si and S, which are slightly non-solar. The strong and broad ($\sim$300 km/s) disc-like emission displayed by the H$α$ 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.

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arXiv2016

B fields in OB stars (BOB): Concluding the FORS2 observing campaign

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. In the framework of this programme, we carried out low-resolution spectropolarimetric observations of a large sample of massive stars using FORS2 installed at the ESO VLT 8-m telescope. 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 3sigma errors. From the 32 observations of 28 OB stars, we were able to monitor the magnetic fields in CPD-57 3509 and HD164492C, confirm the magnetic field in HD54879, 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 FORS2 within the BOB programme. For the pre-selected objects with a v sin i below 60 km/s, we obtain a magnetic field detection rate of 5+-5%. We also discuss X-ray properties and multiplicity of the objects in our FORS2 sample with respect to the magnetic field detections.

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arXiv2016

The challenge of measuring magnetic fields in strongly pulsating stars: the case of HD 96446

Among the early B-type stars, He-rich Bp stars exhibit the strongest large-scale organized magnetic fields with a predominant dipole contribution. The presence of $β$ Cep-like pulsations in the typical magnetic early Bp-type star HD 96446 was announced a few years ago, but the analysis of the magnetic field geometry was hampered by the absence of a reliable rotation period and a sophisticated procedure for accounting for the impact of pulsations on the magnetic field measurements. Using new spectropolarimetric observations and a recently determined rotation period based on an extensive spectroscopic time series, we investigate the magnetic field model parameters of this star under more detailed considerations of the pulsation behaviour of the line profiles.

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arXiv2016

Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars. I. The case of HD101412

Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53+-1.68 d. The discovery of this period, about half of the magnetic rotation period P_m = 42.076 d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars.

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arXiv2016

The first spectropolarimetric monitoring of the peculiar O4Ief supergiant zeta Puppis

The origin of the magnetic field in massive O-type stars is still under debate. To model the physical processes responsible for the generation of O star magnetic fields, it is important to understand whether correlations between the presence of a magnetic field and stellar evolutionary state, rotation velocity, kinematical status, and surface composition can be identified. The O4Ief supergiant zeta Pup is a fast rotator and a runaway star, which may be a product of a past binary interaction, possibly having had an encounter with the cluster Trumper 10 some 2Myr ago. The currently available observational material suggests that certain observed phenomena in this star may be related to the presence of a magnetic field. We acquired spectropolarimetric observations of zeta Pup with FORS2 mounted on the 8-m Antu telescope of the VLT to investigate if a magnetic field is indeed present in this star. We show that many spectral lines are highly variable and probably vary with the recently detected period of 1.78d. No magnetic field is detected in zeta Pup, as no magnetic field measurement has a significance level higher than 2.4sigma. Still, we studied the probability of a single sinusoidal explaining the variation of the longitudinal magnetic field measurements.

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arXiv2016

Searching for a magnetic field in Wolf-Rayet stars using FORS2 spectropolarimetry

To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the ESO FORS2 instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR6, WR7, WR18, and WR23 in our Galaxy. The second run in service mode was focused on monitoring the star WR6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR6 was measured at a significance level of 3.3sigma ( = 258+-78G). Among the other targets, the highest value for the longitudinal magnetic field, = 327+-141G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR6 revealed a sinusoidal nature of the variations with the known rotation period of 3.77d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR6. BAT99 7, WR7, and WR,23 do not show variability of the linear polarization over two nights.

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arXiv2016

Magnetic Fields in Massive Stars: New Insights

Substantial progress has been achieved over the last decade in studies of stellar magnetism due to the improvement of magnetic field measurement methods. We review recent results on the magnetic field characteristics of early B- and O-type stars obtained by various teams using different measurement techniques.

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arXiv2016

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp-lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well-studied targets among the Herbig Ae stars. High-resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low-resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron-peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines.

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arXiv2016

B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD-57° 3509

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 NGC3293, and characterise the star's atmospheric and fundamental parameters. 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. 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 23750+-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.

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arXiv2015

HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights

Several arguments have been presented that favour a scenario in which the low detection rate of magnetic fields in Herbig Ae stars can be explained by the weakness of these fields and rather large measurement uncertainties. Spectropolarimetric studies involving sharp-lined Herbig Ae stars appear to be promising for the detection of such weak magnetic fields. These studies offer a clear spectrum interpretation with respect to the effects of blending, local velocity fields, and chemical abundances, and allow us to identify a proper sample of spectral lines appropriate for magnetic field determination. High-resolution spectropolarimetric observations of the three sharp-lined Herbig Ae stars HD101412, HD104237, and HD190073 have been obtained in recent years with the HARPS. We used these archival observations to investigate the behaviour of their longitudinal magnetic fields. To carry out the magnetic field measurements, we used the multi-line singular value decomposition method. We discovered that different line lists for HD101412 yield differences in both the shape of the Stokes V signatures and their field strengths. They could be interpreted in the context of the impact of the circumstellar matter and elemental abundance inhomogeneities on the measurements of the magnetic field. Due to the small size of the Zeeman features on the first three epochs and the lack of near-IR observations, circumstellar and photospheric contributions cannot be estimated unambiguously. In the Stokes V spectrum of HD104237, we detect that the secondary component, a T Tauri star, possesses a rather strong magnetic field =129G, while no significant field is present in the primary component. Our measurements of HD190073 confirm the presence of a variable magnetic field and indicate that the circumstellar environment may have a significant impact on the observed polarization features.

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arXiv2015

B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars

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.

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arXiv2015

B fields in OB stars (BOB): Detection of a strong magnetic field in the O9.7 V star HD54879

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 FORS2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD54879, 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 33000$\pm1000$ K and 4.0$\pm0.1$ 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 HD54879 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 H$α$ shows a variable emission. The H$α$ 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 HD54879 the most strongly magnetic, non-variable single O-star detected to date.

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arXiv2015

B fields in OB stars (BOB): FORS2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD23478 and HD345439

Massive B-type stars with strong magnetic fields and fast rotation are very rare and provide a mystery for theories of both star formation and magnetic field evolution. Only two such stars, called sigma Ori E analogs, were previously known. 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 presence of magnetic fields in these newly discovered sigma Ori E analogs was not investigated in the past. In the framework of our ESO Large Programme, and one normal ESO programme, we carried out low-resolution FORS2 spectropolarimetric observations of HD23478 and HD345439. From the measurements using hydrogen lines, we discover a rather strong longitudinal magnetic field of the order of up to 1.5kG in HD23478, and up to 1.3kG using the entire spectrum. The analysis of HD345439 using four subsequent spectropolarimetric subexposures does not reveal the presence of a magnetic field at a significance level of 3sigma. On the other hand, the inspection of individual subexposures indicates that HD345439 may host a strong magnetic field, rapidly varying over 88 minutes. A hint at the fast rotation of HD345439 is also given by the behaviour of several metallic and He I lines in the low-resolution FORS2 spectra, showing profile variations already on such a short time scale.

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arXiv2015

The Puzzling Spectrum of HD 94509

The spectral features of HD 94509 are highly unusual, adding an extreme to the zoo of Be and shell stars. The shell dominates the spectrum, showing lines typical for spectral types mid-A to early-F, while the presence of a late/mid B-type central star is indicated by photospheric hydrogen line wings and helium lines. Numerous metallic absorption lines have broad wings but taper to narrow cores. They cannot be fit by Voigt profiles. We aim to describe and illustrate unusual spectral features of this star, and make rough calculations to estimate physical conditions and abundances in the shell. Furthermore, the central star is characterized. We assume mean conditions for the shell. An electron density estimate is made from the Inglis-Teller formula. Excitation temperatures and column densities for Fe I and Fe II are derived from curves of growth. The neutral H column density is estimated from high Paschen members. The column densities are compared with calculations made with the photoionization code Cloudy. Atmospheric parameters of the central star are constrained employing non-LTE spectrum synthesis. Overall chemical abundances are close to solar. Column densities of the dominant ions of several elements, as well as excitation temperatures and the mean electron density are well accounted for by a simple model. Several features, including the degree of ionization, are less well described. HD 94509 is a Be star with a stable shell, close to the terminal-age main sequence. The dynamical state of the shell and the unusually shaped, but symmetric line profiles, require a separate study.

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