arXiv subjects

M. Schöller

Publications and source records attributed to M. Schöller.

At least 55 records · Page 3Linked to original sources

arXiv2018

A magnetic study of the $δ$ Scuti variable HD 21190 and the close solar-type background star CPD -83$^{\circ}$ 64B

HD 21190 is a known $δ$ Scuti star showing Ap star characteristics and a variability period of 3.6 h discovered by the Hipparcos mission. Using Gaia DR1 data for an astrometric analysis, it was recently suggested that HD 21190 forms a physical binary system with the companion CPD -83$^{\circ}$ 64B. An atmospheric chemical analysis based on HARPS observations revealed the presence of overabundances of heavy and rare-earth elements, which are typically observed in chemically peculiar stars with large-scale organized magnetic fields. Previous observations of HD 21190 indicated a magnetic field strength of a few hundred Gauss. The presence of a magnetic field in CPD -83$^{\circ}$ 64B remained unexplored. In this work, we reanalyse this system using Gaia DR2 data and present our search for the magnetic field in both stars based on multi-epoch HARPSpol high-resolution and FORS 2 low-resolution spectropolarimetric observations. The Gaia DR2 results clearly indicate that the two star are not physically associated. A magnetic field detection at a significance level of more than 6$σ$ ($\left< B_{\rm z}\right>_{\rm all}=230\pm38$ G) was achieved for the $δ$ Scuti variable HD 21190 in FORS 2 observations using the entire spectrum for the measurements. The magnetic field appears to be stronger in CPD -83$^{\circ}$ 64B. The highest value for the longitudinal magnetic field in CPD -83$^{\circ}$ 64B, $\left< B_{\rm z}\right>_{\rm all}=509\pm104$ G, is measured at a significance level of 4.9$σ$. Furthermore, the high-resolution HARPS\-pol observations of this component indicate the presence of pulsational variability on a time scale of tens of minutes.

astro-ph.SR↗

arXiv2018

Detection of magnetic fields in He-rich early B-type stars using HARPSpol

We focus on early-B type stars with helium overabundance, for which the presence of a magnetic field has not previously been reported. The measurements were carried out using high-spectral-resolution spectropolarimetric observations obtained with the High Accuracy Radial velocity Planet Searcher (HARPS) in polarimetric mode, installed at the ESO La Silla 3.6m telescope. For five He-rich stars, the longitudinal magnetic field was detected for the first time. For one target, HD58260, the presence of a longitudinal magnetic field of the order of 1.8 kG has already been reported in the literature, but the magnetic field has remained constant over tens of years. Our measurement carried out using the polarimetric spectra obtained in 2015 March indicates a slight decrease of the longitudinal magnetic field strength compared to measurements reported in previous works. A search for periodic modulation in available photometric data allowed us to confidently establish a period of 2.64119$\pm$0.00420 d in archival ASAS3 data for CPD -27$^{\circ}$ 1791. No period could be determined for the other five stars. The obtained results support the scenario that all He-rich stars are detectably magnetic and form an extension of the Ap star phenomenon to higher temperatures.

astro-ph.SR↗

arXiv2018

HD 149277: a rare short-period SB2 system with a subsynchronously rotating magnetic He-rich primary

HD 149277 is a rare SB2 system with a slowly rotating magnetic He-rich primary with $P_{\rm rot}=25.4$ d. The CFHT/ESPaDOnS archive spectra revealed $P_{\rm orb}=11.5192 \pm 0.0005$ d indicating strong subsynchronous rotation of the primary component. Such a strong subsynchronous rotation was not detected in any other SB2 system with a magnetic chemically peculiar component. Our inspection of the spectra revealed the presence of resolved Zeeman split spectral lines allowing us to determine the variability of the mean magnetic field modulus over the rotation period. The maximum of the magnetic field modulus concides roughly with the positive extremum of the longitudinal field, whereas the minimum of the modulus with the negative extremum of the longitudinal field. No evidence for a longitudinal magnetic field was seen in the circularly polarized spectra of the secondary component. Using archival data from the ASAS3 survey, we find in the frequency spectrum only one significant peak, corresponding to the period $P_{\rm phot}=25.390 \pm 0.014$ d. This value is in good agreements with the previous determination of the rotation period, $P_{\rm rot}=25.380\pm0.007$ d, which was based on longitudinal magnetic field measurements.

astro-ph.SR↗

arXiv2018

GRAVITY chromatic imaging of Eta Car's core

Eta Car is one of the most intriguing luminous blue variables in the Galaxy. Observations and models at different wavelengths suggest a central binary with a 5.54 yr period residing in its core. 2D and 3D radiative transfer and hydrodynamic simulations predict a primary with a dense and slow stellar wind that interacts with the faster and lower density wind of the secondary. The wind-wind collision scenario suggests that the secondary's wind penetrates the primary's wind creating a low-density cavity in it, with dense walls where the two winds interact. We aim to trace the inner ~5-50 au structure of Eta Car's wind-wind interaction, as seen through BrG and, for the first time, through the He I 2s-2p line. We have used spectro-interferometric observations with GRAVITY at the VLTI. Our modeling of the continuum allows us to estimate its FWHM angular size close to 2 mas and an elongation ratio of 1.06 +/- 0.05 over a PA = 130 +/- 20 deg. Our CMFGEN modeling helped us to confirm that the role of the secondary should be taken into account to properly reproduce the observed BrG and He I lines. Chromatic images across BrG reveal a southeast arc-like feature, possibly associated to the hot post-shocked winds flowing along the cavity wall. The images of He I 2s-2p served to constrain the 20 mas structure of the line-emitting region. The observed morphology of He I suggests that the secondary is responsible for the ionized material that produces the line profile. Both the BrG and the He I 2s-2p maps are consistent with previous hydrodynamical models of the colliding wind scenario. Future dedicated simulations together with an extensive interferometric campaign are necessary to refine our constraints on the wind and stellar parameters of the binary, which finally will help us predict the evolutionary path of Eta Car.

astro-ph.SR↗

arXiv2018

Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.

astro-ph.GA↗

arXiv2018

Weak Magnetic Fields in Two Herbig Ae Systems: The SB2 AK Sco and the Presumed Binary HD 95881

We report the detection of weak mean longitudinal magnetic fields in the Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD95881 using observations with the High Accuracy Radial velocity Planet Searcher polarimeter (HARPSpol) attached to the European Southern Observatory's (ESO's) 3.6m telescope. Employing a multi-line singular value decomposition (SVD) method, we detect a mean longitudinal magnetic field =-83+-31G in the secondary component of AK Sco on one occasion. For HD95881, we measure =-93+-25G and =105+-29G at two different observing epochs. For all the detections the false alarm probability is smaller than 10^-5. For AK Sco system, we discover that accretion diagnostic Na I doublet lines and photospheric lines show intensity variations over the observing nights. The double-lined spectral appearance of HD95881 is presented here for the first time.

astro-ph.SR↗

arXiv2018

Magnetic and pulsational variability of Przybylski's star (HD101065)

Since its discovery more than half a century ago Przybylski's star (HD101065) continues to excite the astronomical community by the unusual nature of its spectrum, exhibiting exotic element abundances. This star was also the first magnetic chemically peculiar A-type star for which the presence of rapid oscillations was established. Our analysis of newly acquired and historic longitudinal magnetic field measurements indicates that Przybylski's star is also unusual with respect to its extremely slow rotation. Adopting a dipolar structure for the magnetic field and using a sine wave fit to all reported longitudinal magnetic field values over the last 43yr, we find a probable rotation period P_rot ~ 188yr, which however has to be considered tentative as it does not represent a unique solution and has to be verified by future observations. Additionally, based on our own spectropolarimetric material obtained with HARPSpol, we discuss the impact of the anomalous structure of its atmosphere, in particular of the non-uniform horizontal and vertical distributions of chemical elements on the magnetic field measurements and the pulsational variability. Anomalies related to the vertical abundance stratification of Pr and Nd are for the first time used to establish the presence of a radial magnetic field gradient.

astro-ph.SR↗

arXiv2018

The magnetic variability of the $β$~Cep star $ξ^{1}$\,CMa

$ξ^{1}$ CMa is a known magnetic star showing rotationally modulated magnetic variability with a period of 2.17937 d. However, recent work based on high-resolution spectropolarimetry suggests that the rotation period is longer than 30 years. We compare our new spectropolarimetric measurements with FORS 2 at the VLT acquired on three consecutive nights in 2017 to previous FORS 1/2 measurements of the longitudinal magnetic field strength. The new longitudinal magnetic field values are in the range from 115 to 240 G and do not support the presence of a long period.

astro-ph.SR↗

arXiv2017

Detection of a centrifugal magnetosphere in one of the most massive stars in the $ρ$ Oph star-forming cloud

Recent XMM-Newton observations of the B2 type star rho Oph A indicated a periodicity of 1.205d, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS2 spectropolarimetric observations in different rotation phases revealed the presence of a negative ( _all=-419+-101G) and positive ( _all=538+-69G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as B_d = 1.9+-0.2kG. Our calculations of the Kepler and Alfven radii imply the presence of a centrifugally supported, magnetically confined plasma around rho Oph A. The study of the spectral variability indicates a behaviour similar to that observed in typical magnetic early-type Bp stars.

astro-ph.SR↗

arXiv2017

A search for the presence of magnetic fields in the two Supergiant Fast X-ray Transients IGR J08408-4503 and IGR J11215-5952

A significant fraction of high-mass X-ray binaries are supergiant fast X-ray transients (SFXTs). The prime model for the physics governing their X-ray behaviour suggests that the winds of donor OB supergiants are magnetized. To investigate if magnetic fields are indeed present in the optical counterparts of such systems, we acquired low-resolution spectropolarimetric observations of the two optically brightest SFXTs, IGR J08408-4503 and IGR J11215-5952 with the ESO FORS2 instrument during two different observing runs. No field detection at a significance level of 3sigma was achieved for IGR J08408-4503. For IGR J11215-5952, we obtain 3.2sigma and 3.8sigma detections ( _hydr = -978+-308G and _hydr = 416+-110G) on two different nights in 2016. These results indicate that the model involving the interaction of a magnetized stellar wind with the neutron star magnetosphere can indeed be considered to characterize the behaviour of SFXTs. We detected long-term spectral variability in IGR J11215-5952, while for IGR J08408-4503 we find an indication of the presence of short-term variability on a time scale of minutes.

astro-ph.SR↗

arXiv2017

The wind and the magnetospheric accretion onto the T Tauri star S Coronae Australis at sub-au resolution

To investigate the inner regions of protoplanetary disks, we performed near-infrared interferometric observations of the classical TTauri binary system S CrA. We present the first VLTI-GRAVITY high spectral resolution ($R\sim$4000) observations of a classical TTauri binary, S CrA (composed of S CrA N and S CrA S and separated by $\sim$1.4"), combining the four 8-m telescopes in dual-field mode. Our observations in the near-infrared K-band continuum reveal a disk around each binary component, with similar half-flux radii of about 0.1 au at d$\sim$130 pc, inclinations ($i=$28$\pm$3$^o$\ and $i=$22$\pm$6$^o$), and position angles (PA=0$^o\pm$6$^o$ and PA=-2$^o\pm$12$^o$), suggesting that they formed from the fragmentation of a common disk. The S CrA N spectrum shows bright HeI and Br$γ$ line emission exhibiting inverse P-Cygni profiles, typically associated with infalling gas. The continuum-compensated Br$γ$ line visibilities of S CrA N show the presence of a compact Br$γ$ emitting region the radius of which is about $\sim$0.06 au, which is twice as big as the truncation radius. This component is mostly tracing a wind. Moreover, a slight radius change between the blue- and red-shifted Br$γ$ line components is marginally detected. The presence of an inverse P-Cygni profile in the HeI and Br$γ$ lines, along with the tentative detection of a slightly larger size of the blue-shifted Br$γ$ line component, hint at the simultaneous presence of a wind and magnetospheric accretion in S CrA N.

astro-ph.SR↗

arXiv2017

Rotationally modulated variability and pulsations of the He-rich star CPD -62 2124 with an extraordinarily strong magnetic field

A longitudinal magnetic field with a strength of 5.2kG was recently detected in CPD -62 2124, which has a fractional main-sequence lifetime of about 60%. Strongly magnetic early-B type chemically peculiar stars in an advanced evolutionary state are of special interest to understand the evolution of the angular momentum and spin-down timescales in the presence of a global magnetic field. We exploited 17 FORS2 low-resolution spectropolarimetric observations and 844 ASAS3 photometric measurements for the determination of the rotation period, pulsations, and the magnetic field geometry of the star. We calculated periodograms and applied phenomenological models of photometric, spectral and spectropolarimetric variability. We found that all quantities studied, specifically equivalent widths, the mean longitudinal magnetic field , and the flux in the V filter, vary with the same period P = 2.628d, which was identified as the rotation period. The observed variations can be fully explained by a rigidly rotating main-sequence star with an uneven distribution of chemical elements, photometric spots, and a stable, nearly dipolar magnetic field with a polar field strength of about 21kG, frozen into the body of the star. The magnetic field of CPD -62 2124 is tilted to the rotation axis by beta=28+-7deg, while the inclination of the rotation axis towards the line of sight is only i=20+-5deg. In the acquired FORS2 spectra, we detect short-term line profile variations indicating the presence of beta Cephei type pulsations. As of today, no other pulsating star of this type is known to possess such a strong magnetic field.

astro-ph.SR↗

arXiv2017

Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509

The magnetic field of CPD -57 3509 was recently detected in the framework of the BOB (B fields in OB stars) collaboration. We acquired low-resolution spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity, to study the magnetic field geometry, and to determine the surface distribution of silicon and helium. We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations. A period of 6.36d was detected in the measurements of the mean longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed the presence of He I spots located around the magnetic poles, with a strong concentration at the positive pole and a weaker one around the negative pole. In contrast, high concentration Si III spots are located close to the magnetic equator. Further, our analysis of the spectral variability of CPD -57 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of beta Cep-like pulsations. A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data.

astro-ph.SR↗

arXiv2017

An astrometric and spectroscopic study of the $δ$ Scuti variable HD21190 and its wide companion CPD -83$^{\circ}$ 64B

Although pulsations of $δ$ Scuti type are not expected among Ap stars from a theoretical point of view, previous observations of the known $δ$ Scuti star HD21190 indicated a spectral classification F2 III SrEuSi:, making it the most evolved Ap star known. Our atmospheric chemical analysis based on recent HARPS observations confirms the presence of chemical peculiarities in HD21190. This star is also the only target known to host a magnetic field along with its $δ$ Scuti pulsation properties. Using an astrometric analysis, we show that HD21190 forms a physical binary system with the companion CPD -83$^{\circ}$ 64B. The presented astrometric and spectroscopic study of the binary components is important to understand the complex interplay between stellar pulsations, magnetic fields, and chemical composition.

astro-ph.SR↗

arXiv2017

Accretion-ejection morphology of the microquasar SS433 resolved at sub-au scale

We present the first optical observation at sub-milliarcsecond (mas) scale of the microquasar SS 433 obtained with the GRAVITY instrument on the VLT interferometer. The 3.5 hour exposure reveals a rich K-band spectrum dominated by hydrogen Br$γ $ and \ion{He}{i} lines, as well as (red-shifted) emission lines coming from the jets. The K-band continuum emitting region is dominated by a marginally resolved point source ($<$ 1 mas) embedded inside a diffuse background accounting for 10\% of the total flux. The jet line positions agree well with the ones expected from the jet kinematic model, an interpretation also supported by the consistent sign (i.e. negative/positive for the receding/approaching jet component) of the phase shifts observed in the lines. The significant visibility drop across the jet lines, together with the small and nearly identical phases for all baselines, point toward a jet that is offset by less than 0.5 mas from the continuum source and resolved in the direction of propagation, with a typical size of 2 mas. The jet position angle of $\sim$80$^{\circ}$ is consistent with the expected one at the observation date. Jet emission so close to the central binary system would suggest that line locking, if relevant to explain the amplitude and stability of the 0.26c jet velocity, operates on elements heavier than hydrogen. The Br$γ $ profile is broad and double peaked. It is better resolved than the continuum and the change of the phase signal sign across the line on all baselines suggests an East-West oriented geometry alike the jet direction and supporting a (polar) disk wind origin.

astro-ph.HE↗

arXiv2017

Sub-milliarcsecond Optical Interferometry of the HMXB BP Cru with VLTI/GRAVITY

We observe the HMXB BP Cru using interferometry in the near-infrared K band with VLTI/GRAVITY. Continuum visibilities are at most partially resolved, consistent with the predicted size of the hypergiant. Differential visibility amplitude ($Δ|V| \sim 5\%$) and phase ($Δϕ\sim 2 °$) signatures are observed across the HeI $2.059 μ$m and Br$γ$ lines, the latter seen strongly in emission, unusual for the donor star's spectral type. For a baseline $B \sim 100$m, the differential phase RMS $\sim 0.2 °$ corresponds to an astrometric precision of $\sim 2 μ$as. A model-independent analysis in the marginally resolved limit of interferometry reveals asymmetric and extended emission with a strong wavelength dependence. We propose geometric models based on an extended and distorted wind and/or a high density gas stream, which has long been predicted to be present in this system. The observations show that optical interferometry is now able to resolve HMXBs at the spatial scale at which accretion takes place, and therefore probe the effects of the gravitational and radiation fields of the compact object on its environment.

astro-ph.SR↗

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.

astro-ph.SR↗

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.

astro-ph.SR↗