Magnetic flux determination in late-type dwarfs (original) (raw)

Magnetic field measurements on moderately active cool dwarfs

Astronomy and Astrophysics

We present a careful analysis of 13 high-quality optical spectra of low to moderately active late-type dwarfs (G1-K5) aimed at determining their magnetic parameters. Among our sample only one star, Eri (spatially averaged field strength ≈ 165 ± 30 G), exhibits the unambiguous signature of a magnetic field, a few are candidates and the remaining show no sign of a magnetic field in the observed spectra. Our analysis is based on an inversion of the spectra using detailed numerical solutions of the Unno-Rachkovsky equations, for multiple spectral lines at different positions on the stellar disk, and including magneto-optical effects. It gives results for Eri which are in good agreement with the detailed analysis of infrared spectra by . However, the low value of the spatially averaged field strength of these recent analyses imply that most values of the magnetic flux determined previously for moderately active stars are probably too large, often by considerable amounts. We find that the magnetic flux can be reliably determined if considerable care is taken in the analysis, but the magnetic field strength and filling factor cannot be determined separately for moderately active stars with optical spectra of spectral resolution ≤ 10 5 and S/N ≤ 250.

Methods and Results for Detecting Magnetic Fields on Late-Type Stars

Solar and Stellar Magnetic Fields: Origins and Coronal Effects, 1983

We discuss our program to detect and measure magnetic flux on the surfaces of late-type stars. We adopt a novel technique to deconvolve magnetically insensitive lines from similar, magnetically sensitive lines to infer the degree of Zeeman splitting in the latter lines. These measurements yield values for the magnetic field strength and filling factor (flux). To illustrate our approach we present multiple observations of the RS CVn star X And. At the epoch of observation 26 April 1981 we find a field strength of 1290 ± 50 gauss covering 48 ± 2 percent of this star's surface. Observations at other epochs clearly demonstrate magnetic flux variability on X And. Standard polarization methods for measuring stellar magnetic fields are inappropriate for solar-type field topologies where the field polarities are tangled, and where polarization effects cancel.

Magnetic fields in M-dwarfs: quantitative results from detailed spectral synthesis in FeH lines

Arxiv preprint arXiv: …, 2010

Strong surface magnetic fields are ubiquitously found in M-dwarfs with mean intensities on the order of few thousand Gauss -three orders of magnitude higher than the mean surface magnetic field of the Sun. These fields and their interaction with photospheric convection are the main source of stellar activity, which is of big interest to study links between parent stars and their planets. Moreover, the understanding of stellar magnetism, as well as the role of different dynamo-actions in particular, is impossible without explaining magnetic fields in M-dwarfs. Measuring magnetic field intensities and geometries in such cool objects, however, is strongly limited to our ability to simulate the Zeeman effect in molecular lines. In this work, we present quantitative results of modelling and analysis of the magnetic fields in selected M-dwarfs in FeH Wing-Ford lines and strong atomic lines. Some particular FeH lines are found to be the excellent probes of the magnetic field.

Magnetic Field Topology in Low-Mass Stars: Spectropolarimetric Observations of M Dwarfs

The Astrophysical Journal, 2009

The magnetic field topology plays an important role in the understanding of stellar magnetic activity. While it is widely accepted that the dynamo action present in low-mass partially convective stars (e.g., the Sun) results in predominantly toroidal magnetic flux, the field topology in fully convective stars (masses below ∼ 0.35 M ⊙ ) is still under debate. We report here our mapping of the magnetic field topology of the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based on time series data collected from 20 hours of observations spread over 3 successive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging technique applied to time series of rotationally modulated circularly polarized profiles reveals an axisymmetric large-scale poloidal magnetic field on the M4 dwarf. We then apply a synthetic spectrum fitting technique for measuring the average magnetic flux on the star. The flux measured in G 164-31 is |Bf | = 3.2 ± 0.4 kG, which is significantly greater than the average value of 0.68 kG determined from the imaging technique. The difference indicates that a significant fraction of the stellar magnetic energy is stored in small-scale structures at the surface of G 164-31. Our Hα emission light curve shows evidence for rotational modulation suggesting the presence of localized structure in the chromosphere of this M dwarf. The radius of the M4 dwarf derived from the rotational period and the projected equatorial velocity is at least 30% larger than that predicted from theoretical models. We argue that this discrepancy is likely primarily due to the young nature of G 164-31 rather than primarily due to magnetic field effects, indicating that age is an important factor which should be considered in the interpretation of this observational result. We also report here our polarimetric observations of five other M dwarfs with spectral types from M0 to M4.5, three of them showing strong Zeeman signatures.

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

Astronomy and Astrophysics, 2009

Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims. This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere and of a solar-like chromosphere and corona. Methods. We carried out a spectroscopic monitoring campaign in which we observed ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. The objects were monitored over several rotation periods spectroscopically. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. From the spectra, we determined the temperature of the objects in each spectrum, and measured the equivalent width in a number of diagnostically important lines. Temperature variations would indicate the presence of warm and cold regions, variations in the equivalent widths of photospheric lines are sensitive to the structure of cloud layers, and H α is a diagnostic for chromospheric structures. Results. Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the T eff-variations are ≤50 K, and the rmsvariations in the equivalent widths of H α small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. We find that Hα-line is in emission but the equivalent width is only −4.4 ± 0.3 Å. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions. We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

Recent advances in the observation and analysis of stellar magnetic fields

Lecture Notes in Physics, 1991

A b s t r a c t : There has been considerable progress recently in the study of magnetic fields on late-type stars. Advances in the theory include investigation of systematic effects, new and improved methods of analysis for unpolarized and circularly polarized lines, new models of stellar dynamos and of broadband linear polarization, and the first studies of the thermodynamic nature of stellar magnetic regions and their vertical structure. Observationally, there have been new measurements, particularly of young and active stars (including the first detection of a field on a pre-main sequence object), a new monitoring campaign, the first indications of the relative temperatures in stellar plages, and the first measurements of fields in a single stellar active region. I discuss the results in the framework of stellar activity and surface structure.

Infrared Measurements of Stellar Magnetic Fields

Infrared Solar Physics, 1994

A b s t r a c t . I review the advantages, techniques, and results of measurement of magnetic fields on cool stars in the infrared ( I R ) . These measurements have generated several important results, including the following: the first data on the magnetic parameters of d M e and R S C V n variables; evidence for field strength confinement b y photospheric gas pressure; support for the correlation between magnetic flux and rotation, with possible saturation at high rotation rates; indications of horizontal a n d / o r vertical magnetic field structure; and evidence of spatial variations in Β over a stellar surface. I discuss these results in detail, and suggest future directions for I R magnetic field research. K e y w o r d s : infrared: stars -line: formation -stars: activity -stars: magnetic fields 437 D. M. Rabin et al. (eds.), Infrared Solar Physics, 437-447.