Stellar differential rotation from direct starspot tracking

On the Sun, the rotation periods of individual sunspots not only trace the latitude dependence of the surface rotation rate, but also provide clues as to the amount of subsurface fluid shear. In this paper we present the first measurements of stellar differential rotation made by tracking the rotation of individual starspots with sizes comparable to the largest sunspots. To achieve this we re-analyse four sequences of densely-sampled, high signal-to-noise echelle spectra of AB Doradus spanning several stellar rotations in 1996 December. Using spectral subtraction, least-squares deconvolution and matched-filter analysis, we demonstrate that it is possible to measure directly the velocity amplitudes and rotation periods of large numbers of individual starspots at low to intermediate latitude. We derive values for the equatorial rotation rate and the magnitude of the surface differential rotation, both of which are in excellent agreement with those obtained by Donati & Collier Cameron (1997) from cross-correlation of Doppler images derived a year earlier in 1995 December, and with a re-analysis of the 1996 data by the method of Donati et al (2000). The differences between the rotation rates of individual spots and the fitted differential rotation law are substantially greater than the observational errors. The smaller spots show a greater scatter about the mean relation than the larger ones, which suggests that buffeting by turbulent supergranular flows could be responsible.Comment: 9 pages, 7 figures, accepted for publication in MNRA

Massive O- and B-type stars velocities

The main observational sample discussed in this paper comprises high-resolution, single snapshot spectra of 431 O- and B-type Galactic stars. The IACOB database includes spectra from two different instruments: the FIES and HERMES spectrographs attached to the 2.56m Nordic Optical Telescope and the 1.2m Mercator telescope, respectively. Both instruments provide a complete wavelength coverage between 3800 and 7000Å (9000Å for the case of HERMES spectra), and the associated resolving power (R) of the spectra is 25000, 46000 (FIES) and 85000 (HERMES). By default, all the spectra in the IACOB database are reduced using the corresponding available pipelines (FIEStool and HermesDRS, respectively) and they are normalized by means of our own procedures implemented in IDL

The IACOB project . III. New observational clues to understand macroturbulent broadening in massive O- and B-type stars

The term macroturbulent broadening is commonly used to refer to a certain type of non-rotational broadening affecting the spectral line profiles of O- and B-type stars. It has been proposed to be a spectroscopic signature of the presence of stellar oscillations; however, we still lack a definitive confirmation of this hypothesis. Aims: We aim to provide new empirical clues about macroturbulent spectral line broadening in O- and B-type stars to evaluate its physical origin. Methods: We used high-resolution spectra of 430 stars with spectral types in the range O4 - B9 (all luminosity classes) compiled in the framework of the IACOB project. We characterized the line broadening of adequate diagnostic metal lines using a combined Fourier transform and goodness-of-fit technique. We performed a quantitative spectroscopic analysis of the whole sample using automatic tools coupled with a huge grid of fastwind models to determine their effective temperatures and gravities. We also incorporated quantitative information about line asymmetries into our observational description of the characteristics of the line profiles, and performed a comparison of the shape and type of line-profile variability found in a small sample of O stars and B supergiants with still undefined pulsational properties and B main-sequence stars with variable line profiles owing to a well-identified type of stellar oscillations or to the presence of spots in the stellar surface. Results: We present a homogeneous and statistically significant overview of the (single snapshot) line-broadening properties of stars in the whole O and B star domain. We find empirical evidence of the existence of various types of non-rotational broadening agents acting in the realm of massive stars. Even though all these additional sources of line-broadening could be quoted and quantified as a macroturbulent broadening from a practical point of view, their physical origin can be different. Contrarily to the early- to late-B dwarfs and giants, which present a mixture of cases in terms of line-profile shape and variability, the whole O-type and B supergiant domain (or, roughly speaking, stars with MZAMS ≳ 15 M⊙) is fully dominated by stars with a remarkable non-rotational broadening component and very similar profiles (including type of variability). We provide some examples illustrating how this observational dataset can be used to evaluate scenarios aimed at explaining the existence of sources of non-rotational broadening in massive stars.17 pages, 11 figures, 3 tables. Accepted for publication in A&Astatus: publishe

Massive O- and B-type stars velocities

Item does not contain fulltextThe main observational sample discussed in this paper comprises high-resolution, single snapshot spectra of 431 O- and B-type Galactic stars. The IACOB database includes spectra from two different instruments: the FIES and HERMES spectrographs attached to the 2.56m Nordic Optical Telescope and the 1.2m Mercator telescope, respectively. Both instruments provide a complete wavelength coverage between 3800 and 7000Å (9000Å for the case of HERMES spectra), and the associated resolving power (R) of the spectra is 25000, 46000 (FIES) and 85000 (HERMES). By default, all the spectra in the IACOB database are reduced using the corresponding available pipelines (FIEStool and HermesDRS, respectively) and they are normalized by means of our own procedures implemented in IDL