The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Abstract

Aims. In a follow-up investigation we present Zeeman-Doppler maps of the weak-lined T Tauri star (WTTS) V410 Tau. As a rapid rotating star and a typical WTTS the stellar surface of V410 Tau is accessible to surface imaging techniques and allows us to detect and reconstruct the major magnetic surface features on this pre-main sequence star. Methods. The polarized signals we are measuring are on the order of 10 −4 to 10 −3 and are hidden well below the noise level of a single observation. A new line profile reconstruction technique based on a singular value decomposition (SVD) allows us to extract the weak polarized line profiles (Stokes V) as well as the intensity profiles (Stokes I). One of the key features of the line profile reconstruction is that the SVD line profiles are amenable to radiative transfer modeling within our Zeeman-Doppler Imaging code iMap. The code also utilizes a new iterative regularization scheme which is independent of any additional surface constraints. To provide more stability a vital part of our inversion strategy is to invert both Stokes I and Stokes V profiles to simultaneously reconstruct the temperature and magnetic field surface distribution of V410 Tau. A new image-shear analysis is also implemented to allow the search for image and line profile distortions induced by a differential rotation of the star. Results. The magnetic field structure we obtain for V410 Tau shows a good spatial correlation with the surface temperature and is dominated by a strong field within the cool polar spot. The Zeeman-Doppler maps exhibit a large-scale organization of both polarities around the polar cap in the form of a twisted bipolar structure. The magnetic field reaches a value of almost 2 kG within the polar region but smaller fields are also present down to lower latitudes. The pronounced non-axisymmetric field structure and the nondetection of a differential rotation for V410 Tau supports the idea of an underlying α 2 -type dynamo, which is predicted for WTTS.

Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517

“…We use our Doppler-imaging code iMap (Carroll et al 2007(Carroll et al , 2009) in its latest version described in Carroll et al (2012). Reconstructing a stellar surface from a series of spectral line profiles is an inverse problem that amounts to solving the integral equation relating the surface temperature distribution to the observed line profiles while controlling the effects of noise in the data through a regularizing functional (see, e.g., the review by Rice 2002).…”

“…Spectral lines at the edges of the échelle orders are either completely removed, or averaged if repeated in two adjacent orders. Although the averaging of many line profiles is computationally more demanding because the same averaging must be done for the synthetic line profiles during the inversion, this process can lead to significantly lower least-squares errors (Carroll et al 2012). Notes.…”

“…By using only a limited number of eigenprofiles, we can significantly de-noise the line profiles and reduce high frequency noise as well as systematic noise. The number of significant eigenprofiles used for the reconstruction is determined by a randomized Monte Carlo procedure as described by Carroll et al (2012). Figure 8 shows the results from the line-profile inversion of the Li i 670.8-nm line.…”

HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights

“…The software package used to study the magnetic fields in this paper, the SVD method for Stokes profile reconstruction, was introduced by Carroll et al (2012). The basic idea of SVD is similar to the principal component analysis (PCA) approach, where the similarity of the individual Stokes V profiles allows one to describe the most coherent and systematic features present in all spectral line profiles as a projection onto a small number of eigenprofiles.…”

B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD −62° 2124

“…The B z values, derived from the HARPSpol spectra, were estimated adopting two techniques: the least-squares deconvolution technique (LSD; Donati et al 1997;Kochukhov et al 2010) and the single value decomposition technique (SVD; Carroll et al 2012). We derived the B z values considering 90 metal lines and ignoring He lines because of their large width and the inhomogeneous surface distribution that biases the magnetic field measurements (see Table 1).…”