Observations of Cool-Star Magnetic Fields

Reiners, A.

doi:10.12942/lrsp-2012-1

Cited by 255 publications

(245 citation statements)

References 175 publications

(230 reference statements)

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“…The line distortion is induced by a magnetic spot with one out of three different values of magnetic field strength inside the spot (B = 100 G, 600 G, and 4000 G) and one out of three different spot filling factors ( f = 1%, 3%, and 10%). While little is known about the geometric concentration of small magnetic areas on cool stars, the total magnetic energy assumed in our examples is easily justified by observations of cool star magnetic fields (Reiners 2012); our example stars have average fields of B f = 1-400 G (concentrated in one single spot) well in the range of average fields observed that can be as strong as several kG (Reiners et al 2009). …”

Section: Dependence On Wavelength and Field Strengthmentioning

confidence: 97%

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Radial velocity signatures of Zeeman broadening

Reiners

Shulyak

Anglada‐Escudé

et al. 2013

A&A

100

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Stellar activity signatures such as spots and plages can significantly limit the search for extrasolar planets. Current models of activityinduced 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 λ, 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 μm. 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 ∼1000 G can produce a similar RV amplitude as a spot temperature contrast of ∼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.

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Section: Dependence On Wavelength and Field Strengthmentioning

confidence: 97%

“…The appearance of the spectral line also depends on the geometry of the field, but this effect is often neglected assuming an "homogeneous" distribution of field lines over the stellar surface. We refer to Reiners (2012) for a deeper discussion of magnetic field observations.…”

Section: Zeeman Splitting In Stellar Spectramentioning

confidence: 99%

Radial velocity signatures of Zeeman broadening

Reiners

Shulyak

Anglada‐Escudé

et al. 2013

A&A

100

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“…The reference stars were GJ 873 (EV Lac) with |B i | f i = 3.9 kG estimated by Johns-Krull & Valenti (2000) (here f is a filling factor), and the non-magnetic GJ 1002. See Reiners (2012) for a review.…”

Section: Introductionmentioning

confidence: 99%

Exploring the magnetic field complexity in M dwarfs at the boundary to full convection

Shulyak

Reiners

Seemann

et al. 2014

A&A

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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 μm and Na i lines at 2.2 μm.Methods. A modified version of the Molecular Zeeman Library (MZL) was used to compute Landé 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.

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“…This approach has contributed significantly to our understanding of cool stars magnetic fields and stellar activity in general in recent years (e.g., Donati et al 2003Donati et al , 2006Donati et al , 2008Kochukhov et al 2004;Petit et al 2008;Hussain et al 2009;Morin et al 2010;Carroll et al 2012). However, what is good for the ZDI approach, namely rapid rotation, has an adverse effect on the Zeeman broadening approach and vice versa, such that both approaches are complementary to some degree; see also Reiners (2012) for a comprehensive overview of measuring stellar magnetic fields on cool stars.…”

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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Observations of Cool-Star Magnetic Fields

Cited by 255 publications

References 175 publications

Radial velocity signatures of Zeeman broadening

Radial velocity signatures of Zeeman broadening

Exploring the magnetic field complexity in M dwarfs at the boundary to full convection

Detecting and quantifying stellar magnetic fields

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