[Show abstract][Hide abstract] ABSTRACT: Despite of the importance of magnetic fields for the full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied over the rotation cycle until now. One reason for the paucity of such observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field became in the last years one of the most studied targets among the Herbig Ae/Be stars. A few months ago we obtained multi-epoch polarimetric spectra of this star with FORS2 to search for a rotation period and to constrain the geometry of the magnetic field. We measured longitudinal magnetic fields on 13 different epochs distributed over 62 days. These new measurements together with our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. The search of the rotation period resulted in P=42.076+-0.01d. According to near-infrared imaging studies the star is observed nearly edge-on. The star exhibits a single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. These observations are usually considered as evidence for a dominant dipolar contribution to the magnetic field topology. Comment: 6 pages, 7 figures, 1 table, accepted for publication as a Letter in A&A
Astronomy and Astrophysics 11/2010; 525(13). DOI:10.1051/0004-6361/201015806 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Our recent studies of late B-type stars with HgMn peculiarity revealed for
the first time the presence of fast dynamical evolution of chemical spots on
their surfaces. These observations suggest a hitherto unknown physical process
operating in the stars with radiative outer envelopes. Furthermore, we have
also discovered existence of magnetic fields on these stars that have up to now
been thought to be non-magnetic. Here we will discuss the dynamical spot
evolution on HD 11753 and our new results on magnetic fields on AR Aur.
Proceedings of the International Astronomical Union 10/2010; 6(S273). DOI:10.1017/S1743921311015109
[Show abstract][Hide abstract] ABSTRACT: Observations at various wavelengths of late B-type stars exhibiting strong overabundances of the chemical elements Hg and Mn in their atmospheres indicate that these stars are frequently found in binary and multiple systems. We intend to study the multiplicity of this type of chemically peculiar stars, looking for visual companions in the range of angular separation between 0.05" and 8". We carried out a survey of 56 stars using diffraction-limited near-infrared imaging with NAOS-CONICA at the VLT. Thirty-three companion candidates in 24 binaries, three triples, and one quadruple system were detected. Nine companion candidates were found for the first time in this study. Five objects are likely chance projections. The detected companion candidates have K magnitudes between 5.95m and 18.07m and angular separations ranging from <0.05" to 7.8", corresponding to linear projected separations of 13.5-1700 AU. Our study clearly confirms that HgMn stars are frequently members of binary and multiple systems. Taking into account companions found by other techniques, the multiplicity fraction in our sample may be as high as 91%. The membership in binary and multiple systems seems to be a key point to understanding the abundance patterns in these stars. Comment: 12 pages, 5 figures, 3 tables, accepted for publication in A&A
Astronomy and Astrophysics 10/2010; 522(7). DOI:10.1051/0004-6361/201014246 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The system AR Aur is a young late B-type double-lined eclipsing binary with a primary star of HgMn peculiarity. We applied
the Doppler imaging method to reconstruct the distribution of Fe and Y over the surface of the primary using spectroscopic
time series obtained in 2005 and from 2008 October to 2009 February. The results show a remarkable evolution of the element
distribution and overabundances. Measurements of the magnetic field with the moment technique using several elements reveal
the presence of a longitudinal magnetic field of the order of a few hundred gauss in both stellar components and a quadratic
field of the order of 8 kG on the surface of the primary star.
Monthly Notices of the Royal Astronomical Society Letters 08/2010; 408(1):L61 - L65. DOI:10.1111/j.1745-3933.2010.00928.x · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Context: Recent attention has been directed to abundance variations among very young stars. Aims: To perform a detailed abundance study of the Herbig Ae star HD 101412, taking advantage of its unusually sharp spectral lines. Methods: High-resolution spectra are measured for accurate wavelengths and equivalent widths. Balmer-line fits and ionization equlibria give a relation between Teff, and log(g). Abundance anomalies and uncertain reddening preclude the use of spectral type or photometry to fix Teff. Excitation temperatures are used to break the degeneracy between Teff and log(g). Results: Strong lines are subject to an anomalous saturation that cannot be removed by assuming a low microturbulence. By restricting the analysis to weak (<= 20 m[A]) lines, we find consistent results for neutral and ionized species, based on a model with Teff = 8300K, and log(g)=3.8. The photosphere is depleted in the most refractory elements, while volatiles are normal or, in the case of nitrogen, overabundant with respect to the sun. The anomalies are unlike those of Ap or Am stars. Conclusions: We suggest the anomalous saturation of strong lines arises from heating of the upper atmospheric layers by infalling material from a disk. The overall abundance pattern may be related to those found for the Lambda Boo stars, though the depletions of the refractory elements are milder, more like those of Vega. However, the intermediate volatile zinc is depleted, precluding a straightforward interpretation of the abundance pattern in terms of gas-grain separation. Comment: Accepted for publication in Astronomy and Astrophysics; 7 pages, 7 figs., 2 tables
Astronomy and Astrophysics 08/2010; 523(0004-6361). DOI:10.1051/0004-6361/201015361 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Large abundance anomalies have previously been detected in horizontal-branch B-type stars. We present the first high-resolution study of isotopic anomalies and chemical abundances in six horizontal-branch B-type stars in the globular clusters NGC 6397 and NGC 6752, carried out with UVES on the VLT and compare them to those observed in chemically peculiar main-sequence stars.
Proceedings of the International Astronomical Union 08/2010; 266:421-421. DOI:10.1017/S1743921309991578
[Show abstract][Hide abstract] ABSTRACT: ESO Very Large Telescope observations revealed on a high confidence level the presence of a variable magnetic field in the optical component (O-supergiant) and in the outer parts of the accreting structure around BH. This result creates observational basis for models of magnetic disc accretion on BH in X-ray binaries, quasars and other AGN. Scaling measured magnetic field to BH vicinities permits to explain Cyg X-1 X-ray flickering.
[Show abstract][Hide abstract] ABSTRACT: We report the results of our search for magnetic fields in a representative sample of classical Be stars carried out during 2006-2008 using low-resolution spectropolarimetry with FORS1 at the VLT. Among the 28 classical Be stars studied, detections of a magnetic field were achieved in seven stars (i.e. ~25%). The detected magnetic fields are rather weak, not stronger than ~150G. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min.
Proceedings of the International Astronomical Union 06/2010; 6:222 - 223. DOI:10.1017/S1743921311010441
[Show abstract][Hide abstract] ABSTRACT: We present the results of magnetic field measurements for a sample of 23 young Herbig Ae/Be (HAEBEs) stars and 12 stars with debris disks. The spectropolarimetric data were obtained during four observing runs in 2003-2008 at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8 m Kueyen telescope. Among the 23 HAEBEs studied, stellar magnetic fields of about 100-150G have been detected in 11 stars (i.e. ~50%). The presence of circumstellar polarization signatures formed in the stellar wind supports the assumption that the magnetic centrifuge is one of the main mechanisms of the wind acceleration. No field detection at a significance level of 3σ was achieved in stars with debris disks.
Proceedings of the International Astronomical Union 06/2010; 6(S272):224 - 226. DOI:10.1017/S1743921311010453
[Show abstract][Hide abstract] ABSTRACT: Aims. We gathered about 100 high-resolution spectra of three typical HgMn (mercury-manganese) stars, HD 11753, HD 53244, and HD 221507, to search for slowly pulsating B-like pulsations and surface inhomogeneous distribution of various chemical elements. Methods. Classical frequency analysis methods were used to detect line profile variability and to determine the variation period. Doppler imaging reconstruction was performed to obtain abundance maps of chemical elements on the stellar surface. Results. For HD 11753, which is the star with the most pronounced variability, distinct spectral line profile changes were detected for Ti, Sr, Y, Zr, and Hg, whereas for HD 53244 and HD 221507 the most variable line profiles belong to the elements Hg and Y, respectively. We derived rotation periods for all three stars from the variations of radial velocities and equivalent widths of spectral lines belonging to inhomogeneously distributed elements: P_rot (HD 11753)=9.54 d, P_rot (HD 53244)=6.16 d, and P_rot (HD 221507)=1.93 d. For HD 11753 the Doppler imaging technique was applied to derive the distribution of the most variable elements Ti, Sr, and Y using two datasets separated by ~65 days. Results of Doppler imaging reconstruction revealed noticeable changes in the surface distributions of TiII, SrII, and YII between the datasets, indicating the hitherto not well understood physical processes in stars with radiative envelopes that cause a rather fast dynamical chemical spot evolution. Comment: Accepted for publication in Astronomy and Astrophysics, 6 pages, 9 figures, 2 tables.
Astronomy and Astrophysics 03/2010; 511(1). DOI:10.1051/0004-6361/200913775 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot=13.86d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field. Comment: 7 pages, 6 figures, 1 table, to appear in Astronomische Nachrichten
[Show abstract][Hide abstract] ABSTRACT: HD 65949 is a late B star with exceptionally strong Hg II at 3984[A], but it is not a typical HgMn star. The Re II spectrum is of extraordinary strength. Abundances, or upper limits are derived here for 58 elements based on a model with Teff = 13100K, and log(g) = 4.0. Even-Z elements through nickel show minor deviations from solar abundances. Anomalies among the odd-Z elements through copper are mostly small. Beyond the iron peak, a huge scatter is found. The abundance pattern of the heaviest elements resembles the N=126 r-process peak of solar material, though not in detail. We find a significant correlation of the abundance excesses with second ionization potentials for elements with Z > 30. This indicates the relevance of photospheric or near-photospheric processes. We explore a model with mass accretion of exotic material followed by the more commonly accepted differentiation by diffusion. That model leads to a number of predictions which challenge future work. Likely primary and secondary masses are near 3.3 and 1.6 M(solar), with a separation of ca. 0.25 AU. New atomic structure calculations are presented in two appendices. Comment: Accepted by MNRAS: 16 pages, 5 figures
Monthly Notices of the Royal Astronomical Society 02/2010; 405(2). DOI:10.1111/j.1365-2966.2010.16529.x · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present high-resolution, high signal-to-noise UVES spectra of AO Vel, a quadruple system containing an eclipsing BpSi star. From these observations we reconstruct the spectra of the individual components and perform an abundance analysis of all four stellar members. We found that all components are chemically peculiar with different abundances patters. In particular, the two less massive stars show typical characteristics of HgMn stars. The two most massive stars in the system show variable line profiles indicating the presence of chemical spots. Given the youth of the system and the notable chemical peculiarities of their components, this system could give important insights in the origin of chemical anomalies. Comment: 8 pages, 7 figures, 1 appendix. To be published in the Monthly Notices of the Royal Astronomical Society
Monthly Notices of the Royal Astronomical Society 01/2010; 402(4). DOI:10.1111/j.1365-2966.2009.16061.x · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Context. Z CMa is a binary system which consists of two young stars: a Herbig AeBe component Z CMa NW embedded in a dust cocoon and a less massive component Z CMa SE, which is classified as a FU Orionis type star. Associated to the binary system is a giant parsec-size jet. Past spectropolarimetric observations showed that the position angle of the linear optical polarization is perpendicular to the jet axis, indicating that the visual light escapes the cocoon via cavities aligned with the jet axis and is then scattered back into the line of sight of the observer. Recently the system showed the largest outburst reported during the almost 90 years of available observations. Aims: We present new spectrophotometric and spectropolarimetric data obtained in 2008 during the recent outburst phase. Methods: The data obtained in the visual spectral range at medium spectral resolution were used to study the geometry of the system from the linear polarization spectra as well as its magnetic field from the circular polarization spectra. Results: During the recent outburst we detected that the Z CMa system is polarized by 2.6% in the continuum and emission line spectrum, with a position angle still perpendicular to the jet. From the high level of polarization we concluded that the outburst is associated with the dust-embedded Herbig AeBe NW component. The deep absorption components of the Balmer lines in the velocity frame which extend from zero velocity and reach a wind velocity of 700 km s-1, together with the absence of a red-shifted broad emission at similar velocities, indicate a bi-polar wind. We did not detect a significant mean longitudinal magnetic field during the outburst, but in the data obtained in 2004 we detected the possible presence of a rather strong magnetic field of the order of 1 kG. However, we critically review the applied method of magnetic field measurements in the presence of a strong stellar wind. The main result of our studies is that the bolometric luminosity of Z CMa remained surprisingly constant during the recent outburst. We conclude that either the geometry of the cavity through which the light escapes from the cocoon has opened a new path or that the screen of dust, which reflects the light toward the observer became more efficient, causing the observed increase of the visual brightness by about 2fm5. Based on observations made with ESO Telescopes at the Paranal Observatories under programme IDs: 074.C-0442, 081.C-0410, and 282.C-5041.Appendix A, Tables 1 and 2, and Fig. 6 are only available at http://www.aanda.org
Astronomy and Astrophysics 01/2010; 509(1). DOI:10.1051/0004-6361/200913704 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: HD 65949 is a late B star in NGC 2516. Two X-ray sources are within 1". The spectrum arguably falls outside of the usual CP star classifications. It may be most closely related to the HgMn stars, having strong Hg II lambda3984, and Pt II. But while ? is exceptionally strong, Mn II is only mildly enhanced. The rarely observed Re II spectrum is extraordinarily well developed. Other uncommon identifications are Kr II, Os II, and Th III. Preliminary quantitative work by Cowley, Hubrig, and Wahlgren (JPhCS, 130, 012005, 2008) is now extended to abundances or upper limits of 58 elements. Calculations of atomic structures by the Mons group, assessed through comparisons with experiment, have provided a large number of oscillator strengths making it possible to derive reliable abundances from rare 5d ions, as well as the 4d spectrum of Ru II. Nb II (4d) is confirmed, and its abundance determined using recent work of Nilsson and Ivarsson (A&A, 492, 609, 2008). Many Re II (5d) lines show broad hyperfine structure, easily resolved on ESO HARPS spectra. We provide new orbital elements for this known SB1. The period is 21.28 days and is slowly increasing, indicating the presence of a third body. Using the spectroscopic Teff=13100 and log(g) = 4.0, as well as the orbital data, we estimate primary and secondary masses near 3.3 and 1.6 M&sun;, with a separation of some 0.25 AU. Observations: ESO (UVES Progs. 076.D-0172, 081.D-0498), HARPS, and from Complejo Astronomico El Leoncito. Support from Belgian FRS-FNRS is gratefully acknowledged.
[Show abstract][Hide abstract] ABSTRACT: The system ARAur is a young late B-type double-lined eclipsing binary with a primary star of HgMn peculiarity. We applied the Doppler imaging method to reconstruct the distribution of Fe and Y over the surface of the primary using spectroscopic time series obtained in 2005 and from 2008 October to 2009 February. The results show a remarkable evolution of the element distribution and overabundances. Measurements of the magnetic field with the moment technique using several elements reveal the presence of a longitudinal magnetic field of the order of a few hundred gauss in both stellar components and a quadratic field of the order of 8kG on the surface of the primary star. Based on observations obtained at the 2.56-m Nordic Optical Telescope on La Palma, the Karl-Schwarzschild-Observatorium in Tautenburg and the STELLA robotic telescope on Tenerife. E-mail: email@example.com
Monthly Notices of the Royal Astronomical Society 01/2010; 408. · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Z CMa is a binary system which consists of two young stars: A Herbig AeBe component "Z CMa NW" embedded in a dust cocoon and a less massive component "Z CMa SE", which is classified as a FU Orionis type star. Recently, the system showed the largest outburst reported during the almost 90 years of available observations. During the recent outburst we detect that the Z CMa system is polarized by 2.6% in the continuum and emission line spectrum, with a position angle still perpendicular to the jet. From the high level of polarization we conclude that the outburst is associated with the dust embedded Herbig AeBe NW component. The main result of our studies is that the bolometric luminosity of Z CMa remained surprisingly constant during the recent "outburst". We conclude that either the geometry of the cavity through which the light escapes from the cocoon has opened a new path, or that the screen of dust, which reflects the light toward the observer became more efficient causing the observed increase of the visual brightness by about 2.5 magnitudes. Comment: letter to A&A, accepted 17/12/2009
[Show abstract][Hide abstract] ABSTRACT: The low resolution spectropolarimetric observations obtained with FORS1 at the VLT have often been used for investigating magnetic fields in hot stars. Here we describe the first investigation of the magnetic field over the stellar rotation in a cool late-type star, FK Com, based on FORS1 spectropolarimetry. We measure the mean longitudinal magnetic field from nine circularly polarised spectra, and study its behaviour over the stellar rotation. The magnetic field measurements are compared to a simultaneous stellar surface temperature map obtained with Doppler imaging techniques. These observations reveal two cool spots on the surface, and indicate that the main cool region coincides with the maximum value of the mean longitudinal magnetic field. Additionally, only 0.25 in phase apart from the main spot, the secondary spot is located at a similar phase to the field minimum. The observations can be interpreted as two spots having different magnetic field polarities, implying that the starspot configuration on FK Com is similar to that observed in the Sun.