| 1. |
- de Boer, J., et al.
(författare)
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Multiple rings in the transition disk and companion candidates around RX J1615.3-3255 High contrast imaging with VLT/SPHERE
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
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Tidskriftsartikel (refereegranskat)abstract
- Context. The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found. Aims. We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at the surface of the transition disk around RXJ1615.3-3255 (RX J1615). Methods. We observed RXJ1615 with VLT/SPHERE. From these observations, we obtained polarimetric imaging with ZIMPOL (R'-band) and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous spatially resolved spectra with the IFS (YJ). Results. We image the disk for the first time in scattered light and detect two arcs, two rings, a gap and an inner disk with marginal evidence for an inner cavity. The shapes of the arcs suggest that they are probably segments of full rings. Ellipse fitting for the two rings and inner disk yield a disk inclination i = 47 +/- 2 degrees and find semi-major axes of 1.50 +/- 0.01 '' (278 au), 1.06 +/- 0.01 '' (196 au) and 0.30 +/- 0.01 '' (56 au), respectively. We determine the scattering surface height above the midplane, based on the projected ring center offsets. Nine point sources are detected between 2.1 '' and 8.0 '' separation and considered as companion candidates. With NACO data we recover four of the nine point sources, which we determine to be not co-moving, and therefore unbound to the system. Conclusions. We present the first detection of the transition disk of RXJ1615 in scattered light. The height of the rings indicate limited flaring of the disk surface, which enables partial self-shadowing in the disk. The outermost arc either traces the bottom of the disk or it is another ring with semi-major axis greater than or similar to 2.35 '' (435 au). We explore both scenarios, extrapolating the complete shape of the feature, which will allow us to distinguish between the two in future observations. The most attractive scenario, where the arc traces the bottom of the outer ring, requires the disk to be truncated at r approximate to 360 au. If the closest companion candidate is indeed orbiting the disk at 540 au, then it would be the most likely cause for such truncation. This companion candidate, as well as the remaining four, all require follow up observations to determine if they are bound to the system.
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| 2. |
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| 3. |
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| 4. |
- Kochukhov, Oleg, et al.
(författare)
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Are there tangled magnetic fields on HgMn stars?
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 554, s. A61-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects. Aims. We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections of weak longitudinal magnetic fields in HgMn stars based on the moment technique. Methods. Our spectrum synthesis code calculated the Zeeman broadening of metal lines in HARPS spectra, assuming an unstructured, turbulent magnetic field. A simple line formation model with a homogeneous radial field distribution was applied to assess compatibility between previous longitudinal field measurements and the observed mean circular polarization signatures. Results. Our analysis of the Zeeman broadening of magnetically sensitive spectral lines reveals no evidence of tangled magnetic fields in any of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the mean magnetic field modulus - much smaller than the field strengths implied by studies based on differential magnetic line intensification and quadratic field diagnostics. The new HARPSpol longitudinal field measurements for the extreme HgMn star HD65949 and the normal late-B star 21 Peg are consistent with zero at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD11753 shows that the recent moment technique measurements retrieved from the same data are incompatible with the lack of circular polarization signatures in the spectrum of this star. Conclusions. We conclude that there is no evidence for substantial tangled magnetic fields on the surfaces of studied HgMn stars. We cannot independently confirm the presence of very strong quadratic or marginal longitudinal fields for these stars, so results from the moment technique are likely to be spurious.
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| 5. |
- Kochukhov, Oleg, et al.
(författare)
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First detection of linear polarization in the line profiles of active cool stars
- 2011
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Ingår i: Astrophysical Journal Letters. - 2041-8205. ; 732:2, s. L19-
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Tidskriftsartikel (refereegranskat)abstract
- The application of high-resolution spectropolarimetry has led to major progress in understanding the magnetism and activity of late-type stars. During the last decade, magnetic fields have been discovered and mapped for many types of active cool stars using spectropolarimetric data. However, these observations and modeling attempts are fundamentally incomplete since they are based on the interpretation of the circular polarization alone. Taking advantage of the newly built HARPS polarimeter, we have obtained the first systematic observations of several cool active stars in all four Stokes parameters. Here we report the detection of magnetically induced linear polarization for the primary component of the very active RS CVn binary HR 1099 and for the moderately active K dwarf epsilon Eri. For both stars the amplitude of linear polarization signatures is measured to be similar to 10(-4) of the unpolarized continuum, which is approximately a factor of 10 lower than for circular polarization. This is the first detection of the linear polarization in line profiles of cool active stars. Our observations of the inactive solar-like star alpha Cen A show neither circular nor linear polarization above the level of similar to 10(-5), indicating the absence of a net longitudinal magnetic field stronger than 0.2 G.
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| 6. |
- Kochukhov, Oleg, et al.
(författare)
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Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Aims. Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of similar to 30 kG, one of the strongest known for any non-degenerate star. Methods. We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field. Results. Based on high-precision measurements of the mean magnetic field modulus, we refined the rotational period of HD 75049 to P-rot = 4.048267 +/- 0.000036 d. We also derived basic stellar parameters, T-eff = 10 250 +/- 250 K and log g = 4.3 +/- 0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Of the chemical elements, Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole. Conclusions. The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediate-mass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths.
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| 7. |
- Kochukhov, Oleg, et al.
(författare)
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No magnetic field in the spotted HgMn star mu Leporis
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 534, s. L13-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Chemically peculiar stars of the mercury-manganese (HgMn) type represent a new class of spotted late-B stars, in which evolving surface chemical inhomogeneities are apparently unrelated to the presence of strong magnetic fields but are produced by some hitherto unknown astrophysical mechanism.Aims. The goal of this study is to perform a detailed line profile variability analysis and carry out a sensitive magnetic field search for one of the brightest HgMn stars -mu Lep.Methods. We acquired a set of very high-quality intensity and polarization spectra of mu Lep with the HARPSpol polarimeter. These data were analyzed with the multiline technique of least-squares deconvolution in order to extract information on the magnetic field and line profile variability.Results. Our spectra show very weak but definite variability in the lines of Sc, all Fe-peak elements represented in the spectrum of mu Lep, as well as Y, Sr, and Hg. Variability might also be present in the lines of Si and Mg. Anomalous profile shapes of Ti II and YII lines suggest a dominant axisymmetric distribution of these elements. At the same time, we found no evidence of the magnetic field in mu Lep, with the 3 sigma upper limit of only 3 G for the mean longitudinal magnetic field. This is the most stringent upper limit on the possible magnetic field derived for a spotted HgMn star.Conclusions. The very weak variability detected for many elements in the spectrum mu Lep suggests that low-contrast chemical inhomogeneities may be common in HgMn stars and that they have not been recognized until now due to the limited precision of previous spectroscopic observations and a lack of time-series data. The null result of the magnetic field search reinforces the conclusion that formation of chemical spots in HgMn stars is not magnetically driven.
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| 8. |
- Makaganiuk, Vitalii, et al.
(författare)
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Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 529, s. A160-
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Tidskriftsartikel (refereegranskat)abstract
- Context. According to our current understanding, a subclass of the upper main-sequence chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were recently discovered on their surfaces. At the same time, no global magnetic fields stronger than 1-100 G are detected by systematic studies. Aims. The goals of our study are to search for a magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar surfaces of both components.Methods. Our analysis is based on high-quality spectropolarimetric time-series observations obtained during 10 consecutive nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search we employed a least-squares deconvolution (LSD). We used spectral disentangling to measure radial velocities and study the line profile variability. Chemical spot geometry was reconstructed using multi-line Doppler imaging.Results. We report a non-detection of magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field. Circular polarization profiles also do not indicate any signatures of complex surface magnetic fields. For a simple dipolar field configuration we estimated an upper limit of the polar field strength to be 60-70 G. For the HgMn component we found variability in spectral lines of Ti, Ba, Y, and Sr with the rotational period equal to the orbital one. The surface maps of these elements reconstructed with the Doppler imaging technique show a relative underabundance on the hemisphere facing the secondary component. The contrast of chemical inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.
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| 9. |
- Makaganiuk, Vitalii, et al.
(författare)
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The search for magnetic fields in mercury-manganese stars
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 525, s. A97-
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Tidskriftsartikel (refereegranskat)abstract
- Context. A subclass of the upper main-sequence chemically peculiar stars, mercury-manganese (HgMn) stars were traditionally considered to be non-magnetic, showing no evidence of variability in their spectral line profiles. However, discoveries of chemical inhomogeneities on their surfaces imply that this assumption should be investigated. In particular, spectroscopic time-series of AR Aur, a And, and five other HgMn stars indicate the presence of chemical spots. At the same time, no signatures of global magnetic fields have been detected. Aims. We attempt to understand the physical mechanism that causes the formation of chemical spots in HgMn stars and gain insight into the potential magnetic field properties at their surfaces; we performed a highly sensitive search for magnetic fields for a large set of HgMn stars. Methods. With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6 m-telescope, we obtained high-quality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines to ensure significantly greater sensitivity to the presence of magnetic fields, including very weak fields. Results. For the 47 individual objects studied, including six components of SB2 systems, we do not detect any magnetic fields at greater than the 3 sigma level. The lack of detection in the circular polarization profiles indicates that if strong fields are present on these stars, they must have complex surface topologies. For simple global fields, our detection limits imply upper limits to the fields present of 2-10 Gauss in the best cases. Conclusions. We conclude that HgMn stars lack large-scale magnetic fields, which is typical of spotted magnetic Ap stars, of sufficient strength to form and sustain the chemical spots observed on HgMn stars. Our study confirms that in addition to magnetically altered atomic diffusion, there exists another differentiation mechanism operating in the atmospheres of late-B main sequence stars that can produce compositional inhomogeneities on their surfaces.
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| 10. |
- Piskunov, Nikolai, et al.
(författare)
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HARPSpol - The New Polarimetric Mode for HARPS
- 2011
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Ingår i: The Messenger. ; 143, s. 7-10
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The HARPS spectrograph can now perform a full polarisation analysis of spectra. It has been equipped with a polarimetric unit, HARPSpol, which was jointly designed and produced by Uppsala, Utrecht and Rice Universities and by the STScI. Here we present the new instrument, demonstrate its polarisation capabilities and show the first scientific results.
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| 11. |
- Rusomarov, Naum, et al.
(författare)
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Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 : I. Spectropolarimetric observations in all four Stokes parameters
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 558, s. A8-
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Tidskriftsartikel (refereegranskat)abstract
- Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter. Aims. The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. Methods. HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300600 and resolving power exceeding 10(5). The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements. Results. We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, < Bz >, with an accuracy of 510 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous < Bz > measurements with our data allowed us to determine an improved rotational period of the star, P-rot = 12.45812 +/- 0.00019 d. We also measured the longitudinal magnetic field from the cores of H alpha and H beta lines. The analysis of < Bz > measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our < Bz > and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the H alpha core and related it to a non-uniform surface distribution of rare earth elements.
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| 12. |
- Snik, F., et al.
(författare)
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The HARPS Polarimeter
- 2011
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Ingår i: Solar Polarization 6.
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Konferensbidrag (refereegranskat)abstract
- We recently commissioned the polarimetric upgrade of the HARPS spectrograph at ESO’s 3.6-m telescope at La Silla, Chile. The HARPS polarimeter is capable of full Stokes spectropolarimetry with large sensitivity and accuracy, taking advantage of the large spectral resolution and stability of HARPS. In this paper we present the instrument design and its polarimetric performance. The first HARPSpol observations show that it can attain a polarimetric sensitivity of \tilde10$^-5$ (after addition of many lines) and that no significant instrumental polarization effects are present.
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| 13. |
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