The Experts below are selected from a list of 273 Experts worldwide ranked by ideXlab platform
Thierry Morel - One of the best experts on this subject based on the ideXlab platform.
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evidence of magnetic field decay in massive Main Sequence Stars
Astronomy and Astrophysics, 2016Co-Authors: L Fossati, Norbert Langer, A. De Koter, Thierry Morel, F R N Schneider, N Castro, S Simondiaz, Aline Muller, V Petit, H SanaAbstract:A significant fraction of massive Main-Sequence Stars show strong, large-scale magnetic fields. The origin of these fields, their lifetimes, and their role in shaping the characteristics and evolution of massive Stars are currently not well understood. We compile a catalogue of 389 massive Main-Sequence Stars, 61 of which are magnetic, and derive their fundamental parameters and ages. The two samples contain Stars brighter than magnitude 9 in the V-band and range in mass between 5 and 100 M⊙. We find that the fractional Main-Sequence age distribution of all considered Stars follows what is expected for a magnitude limited sample, while that of magnetic Stars shows a clear decrease towards the end of the Main Sequence. This dearth of old magnetic Stars is independent of the choice of adopted stellar evolution tracks, and appears to become more prominent when considering only the most massive Stars. We show that the decreasing trend in the distribution is significantly stronger than expected from magnetic flux conservation. We also find that binary rejuvenation and magnetic suppression of core convection are unlikely to be responsible for the observed lack of older magnetic massive Stars, and conclude that its most probable cause is the decay of the magnetic field, over a time span longer than the stellar lifetime for the lowest considered masses, and shorter for the highest masses. We then investigate the spin-down ages of the slowly rotating magnetic massive Stars and find them to exceed the stellar ages by far in many cases. The high fraction of very slowly rotating magnetic Stars thus provides an independent argument for a decay of the magnetic fields.
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amplitudes and lifetimes of solar like oscillations observed by corot red giant versus Main Sequence Stars
Astronomy and Astrophysics, 2011Co-Authors: F Baudin, Thierry Morel, C Barban, K Belkacem, S Hekker, R Samadi, O Benomar, M J Goupil, F Carrier, J BallotAbstract:Context. The advent of space-borne missions such as CoRoT or Kepler providing photometric data has brought new possibilities for asteroseismology across the H-R diagram. Solar-like oscillations are now observed in many Stars, including red giants and Main-Sequence Stars. Aims. Based on several hundred identified pulsating red giants, we aim to characterize their oscillation amplitudes and widths. These observables are compared with those of Main-Sequence Stars in order to test trends and scaling laws for these parameters for Main-Sequence Stars and red giants. Methods. An automated fitting procedure is used to analyze several hundred Fourier spectra. For each star, a modeled spectrum is fitted to the observed oscillation spectrum, and mode parameters are derived. Results. Amplitudes and widths of red-giant solar-like oscillations are estimated for several hundred modes of oscillation. Amplitudes are relatively high (several hundred ppm) and widths relatively small (very few tenths of a μHz). Conclusions. Widths measured in Main-Sequence Stars show a different variation with the effective temperature from red giants. A single scaling law is derived for mode amplitudes of red giants and Main-Sequence Stars versus their luminosity to mass ratio. However, our results suggest that two regimes may also be compatible with the observations.
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Studying the Magnetic Properties of Upper Main-Sequence Stars with FORS1
The Messenger, 2009Co-Authors: Swetlana Hubrig, Markus Schöller, Maryline Briquet, Thierry Morel, D. W. Kurtz, V. G. Elkin, Beate Stelzer, Roald Schnerr, Carol A. GradyAbstract:We summarise the results of our recent magnetic field studies in upper Main-Sequence Stars, which have exploited the spectropolarimetric capability of FORS1 at the VLT extensively.
Lee Hartmann - One of the best experts on this subject based on the ideXlab platform.
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accretion onto pre Main Sequence Stars
Annual Review of Astronomy and Astrophysics, 2016Co-Authors: Lee Hartmann, Gregory J Herczeg, Nuria CalvetAbstract:Accretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass Stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-Main-Sequence Stars of masses ≲1M⊙ (and in at least some 2–3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ∼3 Myr is modest in terms of stellar evo...
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A near-infrared survey for pre-Main Sequence Stars in Taurus
The Astronomical Journal, 1994Co-Authors: Mercedes Gómez, Scott J. Kenyon, Lee HartmannAbstract:We present a near-infrared survey of approximately 2 sq deg covering parts of L1537, L1538, and Heiles cloud 2 in the Taurus-Auriga molecular cloud. Although this study is more sensitive than previous attempts to identify pre-Main Sequence Stars in Taurus-Auriga, our survey regions contain only one new optically visible, young star. We did find several candidate embedded protoStars; additional 10 micrometer photometry is necessary to verify the pre-Main Sequence nature of these sources. Our results--combined with those of previous surveys--show that the L1537/L1538 clouds contain no pre-Main Sequence Stars. These two clouds are less dense than the active star formation sites in Taurus-Auriga, which suggests a cloud must achieve a threshold density to form Stars.
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On the Ages of Pre-Main-Sequence Stars in Taurus
The Astronomical Journal, 1992Co-Authors: Mercedes Gómez, Burton F. Jones, Lee Hartmann, Scott J. Kenyon, John Stauffer, R. Hewett, I. N. ReidAbstract:A search for pre-Main-Sequence Stars in a region of about 12.7 deg 2 near the L 1551 complex in the Taurus molecular cloud have been made. Only two new pre-Main-Sequence Stars have been identified from optical spectra of 383 proper motion candidates on the basis of Li I 6707 A absorption. These Stars have ages of ∼4 × 10 6 yr; no evidence are found for a large number of pre-Main-Sequence Stars with ages comparable to the crossing time of the cloud (∼ 10 7 yr)
J Shiode - One of the best experts on this subject based on the ideXlab platform.
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the observational signatures of convectively excited gravity modes in Main Sequence Stars
Monthly Notices of the Royal Astronomical Society, 2013Co-Authors: J Shiode, Eliot Quataert, Matteo Cantiello, Lars BildstenAbstract:We predict the flux and surface velocity perturbations produced by convectively excited gravity modes (g-modes) in Main-Sequence Stars. Core convection in massive Stars can excite g-modes to sufficient amplitudes to be detectable with high-precision photometry by Kepler and Convection, Rotation and planetary Transits (CoRoT), if the thickness of the convective overshoot region is 30percent of a pressure scale height. The g-modes manifest as excess photometric variability, with amplitudes of ∼10 μmag at frequencies 10 μHz (0.8d −1 ) near the solar metallicity zero-age Main Sequence. The flux variations are largest for Stars with M 5M� , but are potentially detectable down to M ∼ 2‐3M� . During the Main-Sequence evolution, radiative damping decreases such that ever lower frequency modes reach the stellar surface and flux perturbations reach up to ∼100 μmag at the terminal-age Main Sequence. Using the same convective excitation model, we confirm previous predictions that solar gmodesproducesurfacevelocityperturbationsof0.3mm s −1 .Thisimpliesthatstochastically excited g-modes are more easily detectable in the photometry of massive Main-Sequence Stars than in the Sun.
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the stability of massive Main Sequence Stars as a function of metallicity
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: J Shiode, Eliot Quataert, Phil ArrasAbstract:We investigate the pulsational stability of massive (M 120 M� ) Main-Sequence Stars of a range of metallicities, including primordial, Population III Stars. We include a formulation of convective damping motivated by numerical simulations of the interaction between convection and periodic shear flows. We find that convective viscosity is likely strong enough to stabilize radial pulsations whenever nuclear burning (the � -mechanism) is the dominant source of driving. This suggests that massive Main-Sequence Stars with Z 2 × 10 −3 are pulsationally stable and are unlikely to experience pulsation-driven mass loss on the Main Sequence. These conclusions are, however, sensitive to the form of the convective viscosity and highlight the need for further high-resolution simulations of the convection–oscillation interaction. For more metal-rich Stars (Z 2 × 10 −3 ), the dominant pulsational driving arises due to the κ-mechanism arising from the iron-bump in opacity and is strong enough to overcome convective damping. Our results highlight that even for oscillations with periods a few orders of magnitude shorter than the outer convective turnover time, the ‘frozen-in’ approximation for the convection–oscillation interaction is inappropriate, and convective damping should be taken into account when assessing mode stability.
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the stability of massive Main Sequence Stars as a function of metallicity
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: J Shiode, Eliot Quataert, Phil ArrasAbstract:We investigate the pulsational stability of massive (M >~ 120 Msun) Main Sequence Stars of a range of metallicities, including primordial, Population III Stars. We include a formulation of convective damping motivated by numerical simulations of the interaction between convection and periodic shear flows. We find that convective viscosity is likely strong enough to stabilize radial pulsations whenever nuclear-burning (the epsilon-mechanism) is the dominant source of driving. This suggests that massive Main Sequence Stars with Z ~ 2 x 10^-3), the dominant pulsational driving arises due to the kappa-mechanism arising from the iron-bump in opacity and is strong enough to overcome convective damping. Our results highlight that even for oscillations with periods a few orders of magnitude shorter than the outer convective turnover time, the "frozen-in" approximation for the convection-oscillation interaction is inappropriate, and convective damping should be taken into account when assessing mode stability.
Frédéric Arenou - One of the best experts on this subject based on the ideXlab platform.
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Revisiting Hipparcos data for pre-Main Sequence Stars ?
Astronomy and Astrophysics, 1999Co-Authors: Claude Bertout, N. Robichon, Frédéric ArenouAbstract:We cross-correlate the Herbig & Bell and Hippar- cos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young Stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the rele- vant associations and using Hipparcos intermediate astrometric data to derive new parallaxes of the pre-Main Sequence Stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecu- lar cloud distance, although Hipparcos distances of some faint Taurus-Auriga Stars must be viewed with caution. We then im- prove some of the solutions for the binary and multiple pre-Main Sequence Stars. In particular, we confirm three new astrometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX Oph.
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Revisiting Hipparcos data for pre-Main Sequence Stars
arXiv: Astrophysics, 1999Co-Authors: Claude Bertout, N. Robichon, Frédéric ArenouAbstract:We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young Stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the relevant associations and using Hipparcos intermediate astrometric data to derive new parallaxes of the pre-Main Sequence Stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecular cloud distance, although Hipparcos distances of some faint Taurus-Auriga Stars must be viewed with caution. We then improve some of the solutions for the binary and multiple pre-Main Sequence Stars. In particular, we confirm three new astrometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX Oph.
Carol A. Grady - One of the best experts on this subject based on the ideXlab platform.
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Studying the Magnetic Properties of Upper Main-Sequence Stars with FORS1
The Messenger, 2009Co-Authors: Swetlana Hubrig, Markus Schöller, Maryline Briquet, Thierry Morel, D. W. Kurtz, V. G. Elkin, Beate Stelzer, Roald Schnerr, Carol A. GradyAbstract:We summarise the results of our recent magnetic field studies in upper Main-Sequence Stars, which have exploited the spectropolarimetric capability of FORS1 at the VLT extensively.
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EXPORT: Optical photometry and polarimetry of Vega-type and pre-Main Sequence Stars
Astronomy and Astrophysics, 2001Co-Authors: René D. Oudmaijer, John K. Davies, Carol A. Grady, J. Palacios, Carlos Eiroa, D. De Winter, Roger Ferlet, Francisco Garzón, Andrew Collier Cameron, Hans J. DeegAbstract:This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT sample obtained at the 2.5 m Nordic Optical Telescope. The database consists of multi-epoch photo-polarimetry of 68 pre-Main-Sequence and Main-Sequence Stars. An investigation of the polarization variability indicates that 22 objects are variable at the 3 sigma level in our data. All these objects are pre-Main Sequence Stars, consisting of both T Tauri and Herbig Ae/Be objects while the Main Sequence, Vega type and post-T Tauri type objects are not variable. The polarization properties of the variable sources are mostly indicative of the UXOR-type behaviour; the objects show highest polarization when the brightness is at minimum. We add seven new objects to the class of UXOR variables (BH Cep, VX Cas, DK Tau, HK Ori, LkH alpha 234, KK Oph and RY Ori). The Main reason for their discovery is the fact that our data-set is the largest in its kind, indicating that many more young UXOR-type pre-Main Sequence Stars reMain to be discovered. The set of Vega-like systems has been investigated for the presence of intrinsic polarization. As they lack variability, this was done using indirect methods, and apart from the known case of BD+31 degrees 643, the following Stars were found to be strong candidates to exhibit polarization due to the presence of circumstellar disks: 51 Oph, BD+31 degrees 643C, HD 58647 and HD 233517.
