S. Hubrig

Leibniz Institute for Astrophysics Potsdam, Potsdam, Brandenburg, Germany

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Publications (264)492.59 Total impact

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    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; · 5.08 Impact Factor
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    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
    02/2010;
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    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; · 5.52 Impact Factor
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    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; · 5.52 Impact Factor
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    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 01/2010; 266:421-421.
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    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.
    01/2010;
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    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: shubrig@aip.de
    Monthly Notices of the Royal Astronomical Society 01/2010; 408. · 5.52 Impact Factor
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    ABSTRACT: Context. There is considerable interest in the helium variable a Cen as a bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce iii and other possible lanthanides in the spectrum the of hottest chemically peculiar (CP) star in which these elements have been found. A Kr ii line appears within a broad absorption which we suggest may be due to a high-level transition in C ii. Methods: Wavelengths and equivalent widths are measured on high-resolution UVES spectra, analyzed, and their phase-variations investigated. Results: New, robust identifications of Ce iii and Kr ii are demonstrated. Nd iii is likely present. A broad absorption near lambda4619 is present at all phases of a Cen, and in some other early B stars. Conclusions: The presence of lanthanides in a Cen strengthens the view that this star is a significant link between the cooler CP stars and the hotter helium-peculiar stars. Broad absorptions in a Cen are not well explained. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO programmes 65.L-0316(A), 073.D-0504(A), and 076.B-0055(A)).
    Astronomy and Astrophysics 01/2010; 523. · 5.08 Impact Factor
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    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. · 5.08 Impact Factor
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    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
    12/2009;
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    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.
    The Messenger. 12/2009; 138:15-18.
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    ABSTRACT: The atmospheres of chemically peculiar stars can be highly structured in both the horizontal and vertical dimensions. While most prevalent in the magnetic stars, these structures can also exist in non-magnetic stars. In addition to providing an important window to understanding the physical processes at play in these complex atmospheres, they can also be exploited to study stellar pulsations. This article reviews contributions to the session “A 3D look into the atmosphere” of the Joint Discussion “Progress in understanding the physics of Ap and related stars”. It is divided into 3 sections: “Magnetic field and surface structures”, “Pulsations in the atmospheres of roAp stars/inversions”, and “Spectral synthesis/atmospheric models”.
    Proceedings of the International Astronomical Union 10/2009; 5:151 - 160.
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    ABSTRACT: The analysis of the high-resolution UVES spectra of the CP stars HR 6000 and 46 Aql revealed the presence of an impressive number of unidentified lines, in particular in the 5000 - 5400 A region. Because numerous 4d-4f transitions of FeII lie in this spectral range, and because both stars are iron overabundant, we investigated whether the unidentified lines can be due to FeII. ATLAS12 model atmospheres with parameters [13450K, 4.3] and [12560K, 3.8] were computed for the individual abundances of HR 6000 and 46 Aql, respectively, in order to use the stars as spectroscopic sources to identify FeII lines and to determine FeII gf-values. After having identified several unknown lines in the stellar spectra as due to (3H)4d - (3H)4f transitions of FeII, we derived stellar log gf's for them by comparing observed and computed profiles. The energies of the upper levels were assigned on the basis of both laboratory iron spectra and predicted energy levels. We fixed 21 new levels of FeII with energies between 122910.9 cm^-1 and 123441.1 cm^-1. They allowed us to add 1700 new lines to the Fe II line list in the range 810 - 15011 A. Comment: accepted by Astronomy and Astrophysics -- The abstract is shortened here. Note: This study is the result of a collaboration with S. Johansson, who unfortunately left us before this paper started to be written
    09/2009;
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    ABSTRACT: X-ray binary Cygnus X-1 is microquasar containing historically first candidate to black hole. Paradigm of magnetic disc accretion dominates in theoretical models describing processes taking place in objects containing black holes such as microquasars and active galactic nuclei. Nevertheless up to now there were no reliable measurements of magnetic fields in these systems. The first prediction of the Cyg X-1 magnetic field was done by V.F.Shvartsman. He wrote that Cyg X-1 X-ray emission millisecond flickering evidences the presence of a black hole and points to the magnetic field role in accretion onto a black hole. From that times there were many of attempts to search for the Cyg X-1 magnetic field but all of these efforts indicated upper limits only. Our VLT FORS1 2007 and 2008 observations revealed a presence of a magnetic field in the system. For the first time we obtained on the level of 6 standard deviations a magnetic field of the order of 130 G on the surface of the Cyg X-1 optical component (O-supergiant) and observational estimation on a 4 sigma level of a magnetic field on a outer part of accretion structure (about 600 G) in accordance with theoretical prediction. Scaling this field value to black hole vicinity we showed that the field is strong enough to explain the X-ray millisecond flickering and can explain it. Our result presents the first direct determination of magnetic field in accreting disk around a black hole. Comment: 6 pages, 3 figures
    08/2009;
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    ABSTRACT: The extraordinary magnetic Ap star HD75049 has been studied with data obtained with the ESO VLT and 2.2-m telescopes. Direct measurements reveal that the magnetic field modulus at maximum reaches 30kG. The star shows photometric, spectral and magnetic variability with a rotation period of 4.049d. Variations of the mean longitudinal magnetic field can be described to first order by a centred dipole model with an inclination i = 25 degrees, an obliquity beta = 60 degrees, and a polar field B_p = 42kG. The combination of the longitudinal and surface magnetic field measurements imply a radius of R = 1.7 R sun, suggesting the star is close to the zero-age main sequence. HD75049 displays moderate overabundances of Si, Ti, Cr, Fe and large overabundances of rare earth elements. This star has the second strongest magnetic field of any main sequence star after Babcock's star, HD215441, which it rivals.
    Monthly Notices of the Royal Astronomical Society 08/2009; · 5.52 Impact Factor
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    Swetlana Hubrig
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    ABSTRACT: In comparison with stellar magnetic field studies in the eighties and nineties, where most effort was focused on A, B, and active solar-type stars, magnetic fields are currently directly measured in massive early B and O stars with radiative envelopes, as well as in the lower mass tail consisting of fully convective late-M dwarfs. Knowledge of the magnetic field topology of stars of different mass and at different evolutionary stages is important to understand the underlying magnetic field generation mechanisms. I review the present status of magnetic field studies along with the results of theoretical modeling.
    Proceedings of the International Astronomical Union 07/2009; 5:171 - 180.
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    ABSTRACT: We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. 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. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 07/2009; 330(7):708 - 716. · 1.40 Impact Factor
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    ABSTRACT: Context. Recently, evidence for the presence of weak magnetic fields in Herbig Ae/Be stars has been found in several studies. Aims. We seek to expand the sample of intermediate-mass pre-main sequence stars with circular polarization data to measure their magnetic fields, and to determine whether magnetic field properties in these stars are correlated with mass-accretion rate, disk inclination, companions, silicates, PAHs, or show a correlation with age and X-ray emission as expected for the decay of a remnant dynamo. Methods. Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debris disk stars have been obtained at the European Southern Observatory with FORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM 600B in the wavelength range 3250–6215 Å we were able to cover all hydrogen Balmer lines from H$\beta$ to the Balmer jump. In all observations a slit width of 0$\farcs$4 was used to obtain a spectral resolving power of $R$ $\approx$ 2000. Results. Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous magnetic field detections. The largest longitudinal magnetic field, $\left<B_{\rm z}\right>$ = -454$\pm$42 G, was detected in the Herbig Ae/Be star HD 101412 using hydrogen lines. No field detection at a significance level of 3$\sigma$ was achieved in stars with debris disks. Our study does not indicate any correlation of the strength of the longitudinal magnetic field with disk orientation, disk geometry, or the presence of a companion. We also do not see any simple dependence on the mass-accretion rate. However, it is likely that the range of observed field values qualitatively supports the expectations from magnetospheric accretion models giving support for dipole-like field geometries. Both the magnetic field strength and the X-ray emission show hints of a decline with age in the range of ~2–14 Myr probed by our sample, supporting a dynamo mechanism that decays with age. However, our study of rotation does not show any obvious trend of the strength of the longitudinal magnetic field with rotation period. Furthermore, the stars seem to obey the universal power-law relation between magnetic flux and X-ray luminosity established for the Sun and main-sequence active dwarf stars.
    Astronomy and Astrophysics 07/2009; · 5.08 Impact Factor
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    ABSTRACT: We obtained 13 spectropolarimetric observations of the strongly magnetic rapidly oscillating Ap star HD 154708 over 3 months with the multimode instrument FORS 1, installed at the 8-m Kueyen telescope of the Very Large Telescope. These observations have been used for the determination of the rotation period of P= 5.3666 ± 0.0007 d. Using stellar fundamental parameters and the longitudinal magnetic field phase curve, we briefly discuss the magnetic field geometry. The star is observed nearly pole-on and the magnetic field geometry can be described by a centred dipole with a surface polar magnetic field strength Bd between 26.1 and 28.8 kG and an inclination of the magnetic axis to the rotation axis in the range to .
    Monthly Notices of the Royal Astronomical Society 06/2009; 396(2):1018 - 1022. · 5.52 Impact Factor
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    ABSTRACT: Accretion shocks have been recognized as important X-ray emission mechanism for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events that are caused by violent accretion, have been given little attention. We have observed the FUor object Z CMa during optical outburst and quiescence with Chandra. No significant changes in X-ray brightness and spectral shape are found, suggesting that the X-ray emission is of coronal nature. Due to the binary nature of Z CMa the origin of the X-ray source is ambiguous. However, the moderate hydrogen column density derived from our data makes it unlikely that the embedded primary star is the X-ray source. The secondary star, which is the FUor object, is thus responsible for both the X-ray emission and the presently ongoing accretion outburst, which seem however to be unrelated phenomena. The secondary is also known to drive a large outflow and jet, that we detect here for the first time in X-rays. The distance of the X-ray emitting outflow source to the central star is higher than in jets of low-mass stars.
    04/2009;

Publication Stats

1k Citations
492.59 Total Impact Points

Institutions

  • 2011–2014
    • Leibniz Institute for Astrophysics Potsdam
      Potsdam, Brandenburg, Germany
  • 2003–2011
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
    • University of Michigan-Dearborn
      • Department of Natural Sciences
      Dearborn, MI, United States
  • 1996–2009
    • University of Santiago, Chile
      CiudadSantiago, Santiago, Chile
  • 2004–2007
    • European Southern Observatory
      Arching, Bavaria, Germany
    • Case Western Reserve University
      • Department of Astronomy
      Cleveland, OH, United States
  • 2006
    • The International Society for Optics and Photonics
      International Falls, Minnesota, United States
  • 1999–2000
    • Lund University
      • Department of Physics
      Lund, Skane, Sweden
    • Universität Potsdam
      Potsdam, Brandenburg, Germany