First detection of a weak magnetic field on the giant Arcturus: remnants of a solar dynamo?

Sennhauser, C.; Berdyugina, S. V.

doi:10.1051/0004-6361/201015445

Cited by 21 publications

(21 citation statements)

References 37 publications

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“…An extense study of RV in this star spanning 20 years by Hatzes et al (2015) reports the presence of a giant planet with a period of ∼ 630 days and modulation of active regions with a period of ∼ 520 days. Finally, a weak magnetic field was also detected in Arcturus (α Boo) by Sennhauser & Berdyugina (2011) with a possible period of 208 days (only 3 measurements were made). The detection was confirmed by Aurière et al (2015) but no period is provided.…”

Section: Discussionmentioning

confidence: 97%

Planets around evolved intermediate-mass stars

Mena

Santos

Silva

et al. 2018

A&A

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Aims. The aim of this work is to search for planets around intermediate-mass stars in open clusters by using data from an extensive survey with more than 15 years of observations. Methods. We obtain high-precision radial velocities (RV) with the HARPS spectrograph for a sample of 142 giant stars in 17 open clusters. We fit Keplerian orbits when a significant periodic signal is detected. We also study the variation of stellar activity indicators and line-profile variations to discard stellar-induced signals.Results. We present the discovery of a periodic RV signal compatible with the presence of a planet candidate in the 1.15 Gyr open cluster IC4651 orbiting the 2.06 M star No. 9122. If confirmed, the planet candidate would have a minimum mass of 7.2M J and a period of 747 days. However, we also find that the FWHM of the CCF varies with a period close to the RV, casting doubts on the planetary nature of the signal. We also provide refined parameters for the previously discovered planet around NGC2423 No. 3 but show evidence that the BIS of the CCF is correlated with the RV during some of the observing periods. This fact advises us that this might not be a real planet and that the RV variations could be caused by stellar activity and/or pulsations. Finally, we show that the previously reported signal by a brown dwarf around NGC4349 No. 127 is presumably produced by stellar activity modulation. Conclusions. The long-term monitoring of several red giants in open clusters has allowed us to find periodic RV variations in several stars. However, we also show that the follow-up of this kind of stars should last more than one orbital period to detect long-term signals of stellar origin. This work warns that although it is possible to detect planets around red giants, large-amplitude, long-period RV modulations do exist in such stars that can mimic the presence of an orbiting planetary body. Therefore, we need to better understand how such RV modulations behave as stars evolve along the Red Giant Branch and perform a detailed study of all the possible stellar-induced signals (e.g. spots, pulsations, granulation) to comprehend the origin of RV variations.

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Section: Discussionmentioning

confidence: 97%

Planets around evolved intermediate-mass stars

Mena

Santos

Silva

et al. 2018

A&A

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“…2007; Airapetian et al 2010), given the detection of a magnetic field with a surface-averaged strength of ∼0.5 G (Sennhauser & Berdyugina 2011). However, a self-consistent model that simultaneously computes the thermal structure and dynamics for the weakly ionized atmosphere of red giant stars is lacking so far.…”

Section: Introductionmentioning

confidence: 99%

Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 μm CO lines

Ohnaka

Marín

2018

A&A

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Aims. The outer atmosphere of K giants shows thermally inhomogeneous structures consisting of the hot chromospheric gas and the cool molecular gas. We present spectro-interferometric observations of the multicomponent outer atmosphere of the well-studied K1.5 giant Arcturus (α Boo) in the CO first overtone lines near 2.3 µm. Methods. We observed Arcturus with the AMBER instrument at the Very Large Telescope Interferometer (VLTI) at 2.28-2.31 µm with a spectral resolution of 12 000 and at projected baselines of 7.3, 14.6, and 21.8 m.Results. The high spectral resolution of the VLTI/AMBER instrument allowed us to spatially resolve Arcturus in the individual CO lines. Comparison of the observed interferometric data with the MARCS photospheric model shows that the star appears to be significantly larger than predicted by the model. It indicates the presence of an extended component that is not accounted for by the current photospheric models for this well-studied star. We found out that the observed AMBER data can be explained by a model with two additional CO layers above the photosphere. The inner CO layer is located just above the photosphere, at 1.04 ± 0.02 R ⋆ , with a temperature of 1600 ± 400 K and a CO column density of 10 20±0.3 cm −2 . On the other hand, the outer CO layer is found to be as extended as to 2.6 ± 0.2 R ⋆ with a temperature of 1800 ± 100 K and a CO column density of 10 19±0.15 cm −2 . Conclusions. The properties of the inner CO layer are in broad agreement with those previously inferred from the spatially unresolved spectroscopic analyses. However, our AMBER observations have revealed that the quasi-static cool molecular component extends out to 2-3 R ⋆ , within which region the chromospheric wind steeply accelerates.

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“…This is the typical situation for stars with long rotation periods and/or in situations where the amount A&A 563, A56 (2014) of telescope time is limited. Such observations are responsible for a number of exciting magnetic field detection in recent times (e.g., Aurière et al 2007Aurière et al , 2009Lignières et al 2009;Aurière et al 2010;Grunhut et al 2010;Konstantinova-Antova et al 2010;Sennhauser & Berdyugina 2011;Petit et al 2011;Fossati et al 2013). Because many of the observed polarimetric line profiles are buried in noise, the investigations rely on a prior line profile reconstruction technique, such as the least-square-deconvolution (LSD; Donati et al 1997), principalcomponent-analysis (PCA;, or singularvalue-decomposition (SVD; Carroll et al 2012).…”

Section: Introductionmentioning

confidence: 99%

Detecting and quantifying stellar magnetic fields

Carroll¹,

Strassmeier²

2014

A&A

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Context. In recent years, we have seen a rapidly growing number of stellar magnetic field detections for various types of stars. Many of these magnetic fields are estimated from spectropolarimetric observations (Stokes V) by using the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of this method rapidly deteriorates with increasing noise and thus calls for a more robust procedure that combines signal detection and field estimation. Aims. We introduce an estimation method that provides not only the effective or mean longitudinal magnetic field from an observed Stokes V profile but also uses the net absolute polarization of the profile to obtain an estimate of the apparent (i.e., velocity resolved) absolute longitudinal magnetic field. Methods. By combining the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able to decompose observed Stokes profiles with an overcomplete dictionary of wavelet-basis functions to reliably reconstruct the observed Stokes profiles in the presence of noise. The elementary wave functions of the sparse reconstruction process were utilized to estimate the effective longitudinal magnetic field and the apparent absolute longitudinal magnetic field. A multiresolution analysis complements the OMP algorithm to provide a robust detection and estimation method.Results. An extensive Monte-Carlo simulation confirms the reliability and accuracy of the magnetic OMP approach where a mean error of under 2% is found. Its full potential is obtained for heavily noise-corrupted Stokes profiles with signal-to-noise variance ratios down to unity. In this case a conventional COG method yields a mean error for the effective longitudinal magnetic field of up to 50%, whereas the OMP method gives a maximum error of 18%. It is, moreover, shown that even in the case of very small residual noise on a level between 10 −3 and 10 −5 , a regime reached by current multiline reconstruction techniques, the conventional COG method incorrectly interprets a large portion of the residual noise as a magnetic field, with values of up to 100 G. The magnetic OMP method, on the other hand, remains largely unaffected by the noise, regardless of the noise level the maximum error is no greater than 0.7 G.

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First detection of a weak magnetic field on the giant Arcturus: remnants of a solar dynamo?

Cited by 21 publications

References 37 publications

Planets around evolved intermediate-mass stars

Planets around evolved intermediate-mass stars

Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 μm CO lines

Detecting and quantifying stellar magnetic fields

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