The rapid rotation and complex magnetic field geometry of Vega
TL;DR: In this article, the authors used a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20 000.
Abstract: Context. The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields. Aims. We aim to confirm the field detection reported by Lignieres et al. (2009, A&A, 500, L41) and provide additional observational constraints about the field characteristics, by modelling the large-scale magnetic geometry of the star and by investigating a possible seasonal variability of the reconstructed field topology. Methods. We analyse a total of 799 high-resolution circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. Using about 1100 spectral lines, we employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20 000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. Results. We confirm the detection of circularly polarized signatures in the mean line profiles. The signal shows up in four independent data sets acquired with both NARVAL and ESPaDOnS. The amplitude of the polarized signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 ± 0.008 d. The reconstruction of the magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. Conclusions. The repeated observational evidence that Vega possesses a weak photospheric magnetic field strongly suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars now investigatable with the current generation of stellar spectropolarimeters.
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TL;DR: In this paper, the authors present Spitzer 10-35 micrometers spectroscopic data centered at both stars and identify warm, unresolved excess emission in the close vicinity of Vega for the first time.
Abstract: Vega and Fomalhaut are similar in terms of mass, ages, and global debris disk properties; therefore, they are often referred to as debris disk twins. We present Spitzer 10-35 micrometers spectroscopic data centered at both stars and identify warm, unresolved excess emission in the close vicinity of Vega for the first time. The properties of the warm excess in Vega are further characterized with ancillary photometry in the mid-infrared and resolved images in the far-infrared and submillimeter wavelengths. The Vega warm excess shares many similar properties with the one found around Fomalhaut. The emission shortward of approximately 30 micrometers from both warm components is well described as a blackbody emission of approximately 170 K. Interestingly, two other systems, Eri and HR 8799, also show such an unresolved warm dust using the same approach. These warm components may be analogous to the solar system s zodiacal dust cloud, but of far greater mass (fractional luminosity of approximately 10(exp-5) to 10(exp-6) compared to 10(exp-8) to 10(exp-7). The dust temperature and tentative detections in the submillimeter suggest that the warm excess arises from dust associated with a planetesimal ring located near the water-frost line and presumably created by processes occurring at similar locations in other debris systems as well. We also review the properties of the 2 micrometers hot excess around Vega and Fomalhaut, showing that the dust responsible for the hot excess is not spatially associated with the dust we detected in the warm belt.We suggest it may arise from hot nano grains trapped in the magnetic field of the star. Finally, the separation between the warm and cold belt is rather large with an orbital ratio greater than or approximately 10 in all four systems. In light of the current upper limits on the masses of planetary objects and the large gap, we discuss the possible implications for their underlying planetary architecture and suggest that multiple, low-mass planets likely reside between the two belts in Vega and Fomalhaut.
174 citations
Cites background from "The rapid rotation and complex magn..."
...Vega has a magnetic field strength of∼1.2 G, with about a quarter in a dipolar component and the rest in higher order terms (Petit et al. 2010)....
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TL;DR: In this paper, the authors investigated the occurrence of subsurface magnetism in OB stars and found that magnetic fields of sufficient amplitude to affect the wind could emerge at the surface via magnetic buoyancy.
Abstract: Context. Hot luminous stars show a variety of phenomena in their photospheres and winds which still lack clear physical explanation. Among these phenomena are photospheric turbulence, line profile variability (LPV), non-thermal emission, non-radial pulsations, discrete absorption components (DACs) and wind clumping. It has been argued that a convection zone close to the stellar surface could be responsible for some of these phenomena. This convective zone is caused by a peak in the opacity associated with iron-group elements and is referred to as the “iron convection zone” (FeCZ). Aims. Assuming dynamo action producing magnetic fields at equipartition in the FeCZ, we investigate the occurrence of subsurface magnetism in OB stars. Then we study the surface emergence of these magnetic fields and discuss possible observational signatures of magnetic spots. Methods. Simple estimates are made using the subsurface properties of massive stars, as calculated in 1D stellar evolution models. Results. We find that magnetic fields of sufficient amplitude to affect the wind could emerge at the surface via magnetic buoyancy. While at this stage it is difficult to predict the geometry of these features, we show that magnetic spots of size comparable to the local pressure scale height can manifest themselves as hot, bright spots. Conclusions. Localized magnetic fields could be widespread in those early type stars that have subsurface convection. This type of surface magnetism could be responsible for photometric variability and play a role in X-ray emission and wind clumping.
147 citations
TL;DR: In this paper, the authors presented the first two-dimensional models of an isolated rapidly rotating star that include the derivation of the differential rotation and meridional circulation in a self-consistent way.
Abstract: This work aims at presenting the first two-dimensional models of an isolated rapidly rotating star that include the derivation of the differential rotation and meridional circulation in a self-consistent way.We use spectral methods in multidomains, together with a Newton algorithm to determine the steady state solutions including differential rotation and meridional circulation for an isolated non-magnetic, rapidly rotating early-type star. In particular we devise an asymptotic method for small Ekman numbers (small viscosities) that removes the Ekman boundary layer and lifts the degeneracy of the inviscid baroclinic solutions.For the first time, realistic two-dimensional models of fast-rotating stars are computed with the actual baroclinic flows that predict the differential rotation and the meridional circulation for intermediate-mass and massive stars. These models nicely compare with available data of some nearby fast-rotating early-type stars like Ras Alhague ($\alpha$ Oph), Regulus ($\alpha$ Leo), and Vega ($\alpha$ Lyr). It is shown that baroclinicity drives a differential rotation with a slow pole, a fast equator, a fast core, and a slow envelope. The differential rotation is found to increase with mass, with evolution (here measured by the hydrogen mass fraction in the core), and with metallicity. The core-envelope interface is found to be a place of strong shear where mixing will be efficient.Two-dimensional models offer a new view of fast-rotating stars, especially of their differential rotation, which turns out to be strong at the core-envelope interface. They also offer more accurate models for interpreting the interferometric and spectroscopic data of early-type stars.
130 citations
Cites background from "The rapid rotation and complex magn..."
...Indeed, the new detailed observations of stars that report either on magneticfi lds (Petit et al. 2009, 2010) or on abundance patterns in clusters (Hunter et al. 2008; Brott et al. 2011) both crucially dependo the fluid flows that pervade the interior of stars....
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...This may not be so bad for some A stars like Vega, whose surface magnetic fields are rather weak (Petit et al. 2010)....
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TL;DR: In this paper, the authors used the six-beam Michigan Infrared Combiner on the Center for High Angular Resolution Astronomy Array (CIRCA) to study the photometric evolution of the Lyrae star.
Abstract: Optical and infrared interferometers definitively established that the photometric standard Vega (={alpha} Lyrae) is a rapidly rotating star viewed nearly pole-on. Recent independent spectroscopic analyses could not reconcile the inferred inclination angle with the observed line profiles, preferring a larger inclination. In order to resolve this controversy, we observed Vega using the six-beam Michigan Infrared Combiner on the Center for High Angular Resolution Astronomy Array. With our greater angular resolution and dense (u, v)-coverage, we find that Vega is rotating less rapidly and with a smaller gravity darkening coefficient than previous interferometric results. Our models are compatible with low photospheric macroturbulence and are also consistent with the possible rotational period of {approx}0.71 days recently reported based on magnetic field observations. Our updated evolutionary analysis explicitly incorporates rapid rotation, finding Vega to have a mass of 2.15{sup +0.10}{sub -0.15} M{sub Sun} and an age 700{sup -75}{sub +150} Myr, substantially older than previous estimates with errors dominated by lingering metallicity uncertainties (Z = 0.006{sup +0.003}{sub -0.002}).
121 citations
Cites background or methods from "The rapid rotation and complex magn..."
...Petit et al. (2010) carried out Zeeman Doppler imaging, finding a detectable weak polar field concentration (∼0.6 Gauss)....
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...We can reduce our errors by including constraints on the period (0.71±0.03 days; Petit et al. 2010; Alina et al. 2012) and v sin i (22±2 km s−1, Takeda et al. 2008ab) as statistical priors during the Markov Chain calculation....
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...Although the periodic signal is indeed weak, there is growing confidence after many years of observations that a persistent signal at 0.71±0.03 days (Petit et al. 2010; Alina et al. 2012) represents the rotational period of Vega....
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TL;DR: In this paper, the authors presented the first two-dimensional model of an isolated rapidly rotating star that includes the derivation of the differential rotation and meridional circulation in a self-conistent way.
Abstract: Aims. This work aims at presenting the first two-dimensional model s of an isolated rapidly rotating star that include the deriv ation of the differential rotation and meridional circulation in a self-con sistent way. Methods. We use spectral methods in multidomains, together with a Newton algorithm to determine the steady state solutions including differential rotation and meridional circulation for an isolat ed non-magnetic, rapidly rotating early-type star. In part icular we devise an asymptotic method for small Ekman numbers (small viscosities) that removes the Ekman boundary layer and lifts the degeneracy of the inviscid baroclinic solutions. Results. For the first time, realistic two-dimensional models of fast -rotating stars are computed with the actual baroclinic flow s that predict the differential rotation and the meridional circulation for inter mediate-mass and massive stars. These models nicely compare with available data of some nearby fast-rotating early-typ e stars like Ras Alhague (α Oph), Regulus (α Leo), and Vega (α Lyr). It is shown that baroclinicity drives a differential rotation with a slow pole, a fast equator, a fast cor e, and a slow envelope. The differential rotation is found to increase with mass, with evolution (her e measured by the hydrogen mass fraction in the core), and with metallicity. The core-envelope interface is found to be a place of strong shear where mixing will be effi cient. Conclusions. Two-dimensional models offer a new view of fast-rotating stars, especially of their differential rotation, which turns out to be strong at the core-envelope interface. They also offer more accurate models for interpreting the interferometr ic and spectroscopic data of early-type stars.
121 citations
References
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31 Jan 1986TL;DR: Numerical Recipes: The Art of Scientific Computing as discussed by the authors is a complete text and reference book on scientific computing with over 100 new routines (now well over 300 in all), plus upgraded versions of many of the original routines, with many new topics presented at the same accessible level.
Abstract: From the Publisher:
This is the revised and greatly expanded Second Edition of the hugely popular Numerical Recipes: The Art of Scientific Computing. The product of a unique collaboration among four leading scientists in academic research and industry, Numerical Recipes is a complete text and reference book on scientific computing. In a self-contained manner it proceeds from mathematical and theoretical considerations to actual practical computer routines. With over 100 new routines (now well over 300 in all), plus upgraded versions of many of the original routines, this book is more than ever the most practical, comprehensive handbook of scientific computing available today. The book retains the informal, easy-to-read style that made the first edition so popular, with many new topics presented at the same accessible level. In addition, some sections of more advanced material have been introduced, set off in small type from the main body of the text. Numerical Recipes is an ideal textbook for scientists and engineers and an indispensable reference for anyone who works in scientific computing. Highlights of the new material include a new chapter on integral equations and inverse methods; multigrid methods for solving partial differential equations; improved random number routines; wavelet transforms; the statistical bootstrap method; a new chapter on "less-numerical" algorithms including compression coding and arbitrary precision arithmetic; band diagonal linear systems; linear algebra on sparse matrices; Cholesky and QR decomposition; calculation of numerical derivatives; Pade approximants, and rational Chebyshev approximation; new special functions; Monte Carlo integration in high-dimensional spaces; globally convergent methods for sets of nonlinear equations; an expanded chapter on fast Fourier methods; spectral analysis on unevenly sampled data; Savitzky-Golay smoothing filters; and two-dimensional Kolmogorov-Smirnoff tests. All this is in addition to material on such basic top
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01 Dec 19921,818 citations
"The rapid rotation and complex magn..." refers background in this paper
...From the common minimum, we derive a rotation period of 0.725 ± 0.008 d in July 2008, against 0.729 ± 0.008 d in September 2009 (error bars are calculated according to Press et al. 1992)....
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TL;DR: In this article, an optimal spectrum extraction procedure is described, and examples of its performance with CCD data are presented, which delivers the maximum possible signal-to-noise ratio while preserving spectrophotometric accuracy.
Abstract: An optimal spectrum extraction procedure is described, and examples of its performance with CCD data are presented. The algorithm delivers the maximum possible signal-to-noise ratio while preserving spectrophotometric accuracy. The effects of moderate geometric distortion and of cosmic-ray hits on the spectrum are automatically accounted for. In tests with background-noise limited CCD spectra, optimal extraction offers a 70-percent gain in effective exposure time in comparison with conventional extraction procedures.
1,779 citations
"The rapid rotation and complex magn..." refers methods in this paper
...The data reduction is performed by Libre-Esprit, a dedicated, fully automated software described by Donati et al. (1997) and implementing the optimal spectral extraction principle of Horne (1986) and Marsh (1989)....
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TL;DR: In this paper, the authors describe the design and construction of a formatted fiber field unit, SparsePak, and characterize its optical and astrometric performance for spectroscopy of low surface brightness extended sources in the visible and near-infrared.
Abstract: We describe the design and construction of a formatted fiber field unit, SparsePak, and characterize its optical and astrometric performance. This array is optimized for spectroscopy of low surface brightness extended sources in the visible and near‐infrared. SparsePak contains 82, 4 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc}
ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2}
ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\farcs$\end{document} 7 fibers subtending an area of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pif...
1,634 citations