Shulyak, Denis V.

[["dc.bibliographiccitation.firstpage","2548"],["dc.bibliographiccitation.issue","4"],["dc.bibliographiccitation.journal","Monthly Notices of the Royal Astronomical Society"],["dc.bibliographiccitation.lastpage","2557"],["dc.bibliographiccitation.volume","418"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Seifahrt, Andreas"],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.date.accessioned","2018-11-07T08:49:03Z"],["dc.date.available","2018-11-07T08:49:03Z"],["dc.date.issued","2011"],["dc.description.abstract","Close M dwarf binaries and higher multiples allow the investigation of rotational evolution and mean magnetic flux unbiased from scatter in inclination angle and age since the orientation of the spin axis of the components is most likely parallel and the individual systems are coeval. Systems composed of an early-type (M0.0-M4.0) and a late-type (M4.0-M8.0) component offer the possibility to study differences in rotation and magnetism between partially and fully convective stars. We have selected 10 of the closest dM systems to determine the rotation velocities and the mean magnetic field strengths based on spectroscopic analysis of FeH lines of Wing-Ford transitions at 1 mu m observed with Very Large Telescope/CRIRES. We also studied the quality of our spectroscopic model regarding atmospheric parameters including metallicity. A modified version of the Molecular Zeeman Library (MZL) was used to compute Landeg-factors for FeH lines. Magnetic spectral synthesis was performed with the synmast code. We confirmed previously reported findings that less massive M dwarfs are braked less effectively than objects of earlier types. Strong surface magnetic fields were detected in primaries of four systems (GJ 852, GJ 234, LP 717-36 and GJ 3322), and in the secondary of the triple system GJ 852. We also confirm strong 2-kG magnetic field in the primary of the triple system GJ 2005. No fields could be accurately determined in rapidly rotating stars with nu sin i > 10 km s(-1). For slowly and moderately rotating stars, we find the surface magnetic field strength to increase with the rotational velocity nu sin i which is consistent with other results from studying field stars."],["dc.identifier.doi","10.1111/j.1365-2966.2011.19644.x"],["dc.identifier.isi","000298088100036"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/21363"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Wiley-blackwell"],["dc.relation.issn","0035-8711"],["dc.title","Rotation, magnetism and metallicity of M dwarf systems"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A151"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","537"],["dc.contributor.author","Nesvacil, N."],["dc.contributor.author","Lueftinger, T."],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Obbrugger, M."],["dc.contributor.author","Weiss, W."],["dc.contributor.author","Drake, N. A."],["dc.contributor.author","Hubrig, S."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.contributor.author","Polosukhina, N."],["dc.date.accessioned","2018-11-07T09:15:34Z"],["dc.date.available","2018-11-07T09:15:34Z"],["dc.date.issued","2012"],["dc.description.abstract","Context. In atmospheres of magnetic main-sequence stars, the diffusion of chemical elements leads to a number of observed anomalies, such as abundance spots across the stellar surface. Aims. The aim of this study was to derive a detailed picture of the surface abundance distribution of the magnetic chemically peculiar star HD 3980. Methods. Based on high-resolution, phase-resolved spectroscopic observations of the magnetic A-type star HD 3980, the inhomogeneous surface distribution of 13 chemical elements (Li, O, Si, Ca, Cr, Mn, Fe, La, Ce, Pr, Nd, Eu, and Gd) has been reconstructed. The INVERS12 code was used to invert the rotational variability in line profiles to elemental surface distributions. Results. Assuming a centered, dominantly dipolar magnetic field configuration, we find that Li, O, Mg, Pr, and Nd are mainly concentrated in the area of the magnetic poles and depleted in the regions around the magnetic equator. The high abundance spots of Si, La, Ce, Eu, and Gd are located between the magnetic poles and the magnetic equator. Except for La, which is clearly depleted in the area of the magnetic poles, no obvious correlation with the magnetic field has been found for these elements otherwise. Ca, Cr, and Fe appear enhanced along the rotational equator and the area around the magnetic poles. The intersection between the magnetic and the rotational equator constitutes an exception, especially for Ca and Cr, which are depleted in that region. Conclusions. No obvious correlation between the theoretically predicted abundance patterns and those determined in this study could be found. This can be attributed to a lack of up-to-date theoretical models, especially for rare earth elements."],["dc.identifier.doi","10.1051/0004-6361/201117097"],["dc.identifier.fs","596778"],["dc.identifier.isi","000300416800151"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9586"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/27720"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Multi-element Doppler imaging of the CP2 star HD 3980"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A35"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","563"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Seemann, U."],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.date.accessioned","2018-11-07T09:43:00Z"],["dc.date.available","2018-11-07T09:43:00Z"],["dc.date.issued","2014"],["dc.description.abstract","Context. Magnetic fields play a pivotal role in the formation and evolution of low-mass stars, but the dynamo mechanisms generating these fields are poorly understood. Measuring cool star magnetism is a complicated task because of the complexity of cool star spectra and the subtle signatures of magnetic fields. Aims. Based on detailed spectral synthesis, we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 that populate the mass regime around the boundary between partially and fully convective stars. Very high-resolution (R = 100 000), high signal-to-noise (up to 400), near-infrared Stokes I spectra were obtained with CRIRES at ESO's Very Large Telescope covering regions of the FeH Wing-Ford transitions at 1 mu m and Na I lines at 2.2 mu m. Methods. A modified version of the Molecular Zeeman Library (MZL) was used to compute Lande g-factors for FeH lines. We determined the distribution of magnetic fields by magnetic spectral synthesis performed with the SYNMAST code. We tested two different magnetic geometries to probe the influence of field orientation effects. Results. Our analysis confirms that FeH lines are excellent indicators of surface magnetic fields in low-mass stars of type M, particularly in comparison to profiles of Na I lines that are heavily affected by water lines and that suffer problems with continuum normalization. The field distributions in all four stars are characterized by three distinct groups of field components, and the data are consistent neither with a smooth distribution of different field strengths nor with one average field strength covering the full star. We find evidence of a subtle difference in the field distribution of GJ 285 compared to the other three targets. GJ 285 also has the highest average field of 3.5 kG and the strongest maximum field component of 7-7.5 kG. The maximum local field strengths in our sample seem to be correlated with rotation rate. While the average field strength is saturated, the maximum local field strengths in our sample show no evidence of saturation. Conclusions. We find no difference between the field distributions of partially and fully convective stars. The one star with evidence of field distribution different from the other three is the most active star (i.e. with X-ray luminosity and mean surface magnetic field) rotating relatively fast. A possible explanation is that rotation determines the distribution of surface magnetic fields, and that local field strengths grow with rotation even in stars in which the average field is already saturated."],["dc.identifier.doi","10.1051/0004-6361/201322136"],["dc.identifier.fs","609676"],["dc.identifier.isi","000333798000035"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10910"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/34081"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Exploring the magnetic field complexity in M dwarfs at the boundary to full convection"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A103"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","552"],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Anglada-Escude, Guillem"],["dc.contributor.author","Jeffers, Sandra V."],["dc.contributor.author","Morin, Julien"],["dc.contributor.author","Zechmeister, Mathias"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.date.accessioned","2018-11-07T09:26:23Z"],["dc.date.available","2018-11-07T09:26:23Z"],["dc.date.issued","2013"],["dc.description.abstract","Stellar activity signatures such as spots and plages can significantly limit the search for extrasolar planets. Current models of activity-induced radial velocity (RV) signals focus on the impact of temperature contrast in spots according to which they predict the signal to diminish toward longer wavelengths. The Zeeman effect on RV measurements counteracts this: the relative importance of the Zeeman effect on RV measurements should grow with wavelength because the Zeeman displacement itself grows with lambda, and because a magnetic and cool spot contributes more to the total flux at longer wavelengths. In this paper, we model the impact of active regions on stellar RV measurements including both temperature contrast in spots and line broadening by the Zeeman effect. We calculate stellar line profiles using polarized radiative transfer models including atomic and molecular Zeeman splitting over large wavelength regions from 0.5 to 2.3 mu m. Our results show that the amplitude of the RV signal caused by the Zeeman effect alone can be comparable to that caused by temperature contrast; a spot magnetic field of similar to 1000 G can produce a similar RV amplitude as a spot temperature contrast of similar to 1000 K. Furthermore, the RV signal caused by cool and magnetic spots increases with wavelength, in contrast to the expectation from temperature contrast alone. We also calculate the RV signal caused by variations in average magnetic field strength from one observation to the next, for example due to a magnetic cycle, but find it unlikely that this can significantly influence the search for extrasolar planets. As an example, we derive the RV amplitude of the active M dwarf AD Leo as a function of wavelength using data from the HARPS spectrograph. Across this limited wavelength range, the RV signal does not diminish at longer wavelengths but shows evidence for the opposite behavior, consistent with a strong influence of the Zeeman effect. We conclude that the RV signal of active stars does not vanish at longer wavelength but sensitively depends on the combination of spot temperature and magnetic field; in active low-mass stars, it is even likely to grow with wavelength."],["dc.identifier.doi","10.1051/0004-6361/201220437"],["dc.identifier.fs","602592"],["dc.identifier.isi","000317912000102"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10133"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/30285"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/279347/EU//WAVELENGTH STANDARDS"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Radial velocity signatures of Zeeman broadening"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Astronomy Reports"],["dc.bibliographiccitation.lastpage","42"],["dc.bibliographiccitation.volume","60"],["dc.contributor.author","Boyarchuk, A. A."],["dc.contributor.author","Shustov, B. M."],["dc.contributor.author","Savanov, I. S."],["dc.contributor.author","Sachkov, M. E."],["dc.contributor.author","Bisikalo, D. V."],["dc.contributor.author","Mashonkina, L. I."],["dc.contributor.author","Wiebe, D. Z."],["dc.contributor.author","Shematovich, V. I."],["dc.contributor.author","Shchekinov, Yu. A."],["dc.contributor.author","Chugai, N. N."],["dc.contributor.author","Ivanov, P. B."],["dc.contributor.author","Voshchinnikov, N. V."],["dc.contributor.author","Lamzin, S. A."],["dc.contributor.author","Ayres, T."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Jeffrey, S."],["dc.contributor.author","Zwintz, S. K."],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Hubert, B."],["dc.contributor.author","Lammer, H."],["dc.contributor.author","Zhilkin, A. G."],["dc.contributor.author","Kaigorodov, P. V."],["dc.contributor.author","Sichevskii, S. G."],["dc.contributor.author","Ustamuich, S."],["dc.contributor.author","Kanev, E. N."],["dc.contributor.author","Ryabchikova, Tanya"],["dc.contributor.author","Gomez de Castro, Ana I."],["dc.contributor.author","Piskunov, Nikolai E."],["dc.contributor.author","Gerard, J.-C."],["dc.contributor.author","Fossati, Luca"],["dc.contributor.author","Werner, Katharina"],["dc.contributor.author","Kil'pio, E. Yu."],["dc.date.accessioned","2020-12-10T18:37:11Z"],["dc.date.available","2020-12-10T18:37:11Z"],["dc.date.issued","2016"],["dc.description.abstract","The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the interstellar medium and protoplanetary disks, the physics of accretion and outflows in astrophysical objects, from Active Galactic Nuclei to close binary stars, stellar activity (for both low-mass and high-mass stars), and processes occurring in the atmospheres of both planets in the solar system and exoplanets. Technological progress in UV astronomy achieved in recent years is also considered. The well advanced, international, Russian-led Spektr-UV (World Space Observatory-Ultraviolet) project is described in more detail. This project is directed at creating a major space observatory operational in the ultraviolet (115-310 nm). This observatory will provide an effective, and possibly the only, powerful means of observing in this spectral range over the next ten years, and will be an powerful tool for resolving many topical scientific problems."],["dc.identifier.doi","10.1134/S1063772916010017"],["dc.identifier.eissn","1562-6881"],["dc.identifier.isi","000368685900001"],["dc.identifier.issn","1063-7729"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/76867"],["dc.language.iso","en"],["dc.notes.intern","DOI Import GROB-354"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Maik Nauka/interperiodica/springer"],["dc.relation.issn","1562-6881"],["dc.relation.issn","1063-7729"],["dc.title","Scientific problems addressed by the Spektr-UV space project (world space Observatory-Ultraviolet)"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A37"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","523"],["dc.contributor.author","Shulyak, D."],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Wende, Sebastian"],["dc.contributor.author","Kochukhov, Oleg"],["dc.contributor.author","Piskunov, Nikolai E."],["dc.contributor.author","Seifahrt, Andreas"],["dc.date.accessioned","2018-11-07T08:37:20Z"],["dc.date.available","2018-11-07T08:37:20Z"],["dc.date.issued","2010"],["dc.description.abstract","Aims. We present first quantitative results of the surface magnetic field measurements in selected M-dwarfs based on detailed spectra synthesis conducted simultaneously in atomic and molecular lines of the FeH Wing-Ford F-4 Delta - X-4 Delta transitions. Methods. A modified version of the Molecular Zeeman Library (MZL) was used to compute Lande g-factors for FeH lines in different Hund's cases. Magnetic spectra synthesis was performed with the SYNMAST code. Results. We show that the implementation of different Hund's case for FeH states depending on their quantum numbers allows us to achieve a good fit to the majority of lines in a sunspot spectrum in an automatic regime. Strong magnetic fields are confirmed via the modelling of atomic and FeH lines for three M-dwarfs YZ CMi, EV Lac, and AD Leo, but their mean intensities are found to be systematically lower than previously reported. A much weaker field (1.7-2 kG against 2.7 kG) is required to fit FeH lines in the spectra of GJ 1224. Conclusions. Our method allows us to measure average magnetic fields in very low-mass stars from polarized radiative transfer. The obtained results indicate that the fields reported in earlier works were probably overestimated by about 15-30%. Higher quality observations are needed for more definite results."],["dc.identifier.doi","10.1051/0004-6361/201015229"],["dc.identifier.fs","582306"],["dc.identifier.isi","000285346600041"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9681"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/18505"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Modelling the molecular Zeeman-effect in M-dwarfs: methods and first results"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]