Aurelie Fumel

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Herbig Ae/Be stars are intermediate-mass pre-main sequence (PMS) stars showing signs of intense activity and strong stellar winds, whose origin is not yet understood in the frame of current theoretical models of stellar evolution for young stars. The evolutionary tracks of the earlier Herbig Ae stars cross a recently discovered PMS instability strip. Many of these stars exhibit pulsations of delta Scuti type. HD 104237 is a well-known pulsating Herbig Ae star. In this article, we reinvestigated an extensive high-resolution quasi-continuous spectroscopic data set in order to search for very faint indications of non-radial pulsations in the line profile. To do this, we worked on dynamical spectra of equivalent photospheric (LSD) profiles of HD 104237. A 2D Fourier analysis (F2D) was performed of the entire profile and the temporal variation of the central depth of the line was studied with the time-series analysis tools Period04 and SigSpec. We present a mode identification corresponding to the detected dominant frequency. We perform a new accurate determination of the fundamental stellar parameters in view of a forthcoming asteroseismic modeling. Following the previous studies on this star, our analysis of the dynamical spectrum of recentered LSD profiles corresponding to the 22nd -25th of April 1999 nights spectra has confirmed the presence of multiple oscillation modes of low-degree l in HD 104237 and led to the first direct detection of a non-radial pulsation mode in this star: the dominant mode F1 was identified by the Fourier 2D method having a degree l value comprised between 1 and 2, the symmetry of the pattern variation indicating an azimuthal order of +1 or -1. The detailed study of the fundamental stellar parameters has provided a Teff, log g and iron abundance of 8550 +/- 150K, 3.9 +/- 0.3 and -4.38 +/- 0.19 (i.e. [Fe/H]=+0.16 +/- 0.19), respectively.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not known to host magnetic fields. We aim to confirm the field detection and provide additional observational constraints about the field characteristics, by modelling the magnetic geometry of the star and by investigating the seasonal variability of the reconstructed field. We analyse a total of 799 circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. We employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20,000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. We confirm the detection of circularly polarized signatures in the mean line profiles. The amplitude of the signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 \pm 0.008 d. The reconstructed magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. The repeated observation of a weak photospheric magnetic field on Vega suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars.