Measuring Magnetic fields in Ultracool stars & Brown dwarfs Dong-hyun Lee

How can we measure B-field in star? Using Zeeman Effect –The splitting of a spectral line into several components in the presence of a B-field –Stellar B-fields are usually measured through Zeeman broadening in atomic lines that have large Lande g-values –The measurement is usually carried out by comparing the profiles of magnetically sensitive & insensitive absorption lines b/w observations & model spectra –Alternate method that relies on the change in line equivalent width has been developed by Basri (1992, ApJ, 390, 622) –Both methods require the use of a polarized radiative transfer code & knowledge of the Zeeman shift for each Zeeman component in the B-field –But atomic lines vanish in the low-excitation atmospheres & among the ubiquitous molecular lines appear in the spectra of cool stars

How can we measure B-field in star? Using the Wing-Ford bands of FeH(M & L spectral type) –FeH bands show a systematic growth as the star gets cooler –Model cool & rapidly rotating spectra from warmer, slowly rotating spectra utilizing an interpolation scheme based on curve-of-growth analysis –FeH features can distinguish b/w negligible, moderate, high magnetic fluxes on low-mass dwarfs, with a accuracy of about 1kG –B-fields are responsible for the generation of stellar activity –Stellar flares are observed in some of the ultracool obj.s although they seem to be different than in the solar case –Look for Zeeman broadening in molecular lines in ultracool obj.s –Strong magnetic sensitivity of the W-F band of FeH just before 1 microm is cleary demonstrated –Investigate the possibility of detecting B-fields in FeH lines of ultracool dwarfs through comparison b/w the spectrum of a star with unknow B-field can be calibrated in atomic lines

Wing-Ford band of FeH in Ultracool stars –Obtain spectra that cover the wavelength region from H alpha to 1 micro m –Strong FeH absorption around 9900 Angstrom in spectra of M dwarfs –A high-resolution spectroscopic sequence of the Wing-Ford band in the spectral types M2 – L0 in –Amplified absorption spectrum A(lambda) : the normalized residual intensity at lambda Alpha : the optical depth scale factor C : a const. that controls the maximum absorption depth due to saturation

Magnetic Sensitivity in the FeH band –B-field measurment utilize the space quantization of the atomic angular moment J in a B-field –Sensitivity of atomic absorption lines to a B-field is approx. prop.to the Lande factor g –Magnetic splitting in atomic lines can be calculated very precisely –Molecular Zeeman effect is more complex : J vector has more quantization states due to nuclear rotation (cf. in atomic case) –Magnetic sensitivity in FeH lines would be high in transitions with very large values of J (J <= 15) –Intermediate coupling of J makes it difficult to make precise calculations for Lande factor g for molecule, and despite the efforts to understand the FeH spectrum, its coupling const.s have not yet measured and are still unknown. –FeH has an excellent potential for measuring B-fields in cool dwarfs (by observational evidence) –B-fields have been measured in early- & mid-type M dwarfs using well-understood atomic lines. In these stars, FeH band is already prominent. –Compare a spectrum of an inactive star that presumably has no measurable B-field & one of an active star with a B-field measured from atomic lines

Magnetic Sensitivity in the FeH band –High-resolution spectra of the inactive star GJ1227(lower line) & the active star GI873(upper line) –For comparison the sunspot spectrum is overplotted with an offset –Magnetically insensitive lines are dark gray, sensitive lines are light gray –Positions of atomic lines are marked as hatched regions

Detectability of B-fields on ultracool obj.s Magnetic measurement by line ratios –One can determine line broadening due to rotation by comparison of magnetically insensitive lines to the same lines in a reference star with known rotational velocity –Zeeman signal is analyzed by comparing the shape of magnetically sensitive lines b/w the target spectrum & the reference spectrum. –One method of obtaining the magnetic signal is to employ line depth ratios b/w magnetically sensitive & insensitive lines –2 lines should be chosen in close proximity to each other, so that differential errors in continuum placement are less of a concern –Rotation & resolution have the same effect on the line widths, the limiting resolution is the one that corresponds to the maximum rotation velocity –We identified 4 ratios of neighboring absorption features that are particularly useful to measure the magnetic flux : 2 for slow rotators & 2 for rapid rotators –For the rapid rotators, the positions are not centered on a physical absorption line, but rather on a feature that is a blend of several lines at that rotation vel. –Use the template spectra of active & inactive stars shown in –A linear interpolation of the observed reference spectra

Detectability of B-fields on ultracool obj.s –The left panels show the case of no rotation ; right panels the spectra are spun up –The top panels show the case of nonsaturated lines (alpha = 2, ~M6) ; bottom panels the FeH band is heavily saturated (alpha =16, ~L4) –The B-field increases from top to bottom in the spectra –The ratio b/w the depths of the 2 absorption features is plotted as func. Of Bf=p(3.9kG) in the small plot below each panel

Magnetic measurement by chi^2 fitting –Fit(solid gray line) of a linear interpolation b/w the spectra of GJ1227 & GJ873 to the spectrum of GI729 –Parameter chi^2 is calculated from the regions b/w the vertical dashed lines –Best fit is achieved for Bf=2.0kG ~50%GJ1227 & ~50% GI873