P. Petit

The dependence of differential rotation on temperature and rotation

We use Doppler imaging techniques to determine the dependence of starspot rotation rates on latitude in an homogeneous sample of young, rapidly-rotating solar analogues. A solar-like differential rotation law is used, where the rot ation depends on sin 2 (θ), where θ is the stellar latitude. By including this term in the image rec onstruction process, using starspots as tracers, we are able to determine the magnitude of the shear over more than one rotation cycle. We also consider results from matched filter starspot tracking techniques, where individual starspot rotation rates are determined. In addition we have re-analysed published results and present a new measurement for the K3 dwarf, Speedy Mic. A total of 10 stars of spectral type G2 - M2 are considered. We fi nd a trend towards decreasing surface differential rotation with decreasing effective temperature. The implied approach to solid body rotation with increasing relative co nvection zone depth implies that the dynamo mechanism operating in low-mass stars may be substantially different from that in the Sun.

The magnetic field and confined wind of the O star theta(1) Orionis C

Aims. In this paper we confirm the presence of a globally-ordered, k G-strength magnetic field in the photosphere of the young O st ar θ 1 Orionis C, and examine the properties of its optical line profile variations. Methods. A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the rotational cycle ofθ 1 Orionis C. Using the Least-Squares Deconvolution (LSD) multiline technique, we have succeeded in detecting variable Stokes V Zeeman signatures associated with the LSD mean line profile. These signatures have been modeled to determine the magnetic field geometry. We have furthermore examined the profile variatio ns of lines formed in both the wind and photosphere using dynamic spectra. Results. Based on spectrum synthesis fitting of the LSD profiles, we det ermine that the polar strength of the magnetic dipole component is 1150 < Bd < 1800 G and that the magnetic obliquity is 27 ◦ < β < 68 ◦ , assuming i = 45± 20 ◦ . The best-fit values for i = 45 ◦ are Bd = 1300± 150 (1σ) G andβ = 50 ◦ ± 6 ◦ (1σ). Our data confirm the previous detection of a magnetic field i n this star, and furthermore demonstrate the sinusoidal variability of the longitudina l field and accurately determine the phases and intensities o f the magnetic extrema. The analysis of “photospheric” and “wind” line profile varia tions supports previous reports of the optical spectroscop ic characteristics, and provides evidence for infall of material within the magnetic equatorial plane.

Large‐scale magnetic field of the G8 dwarf ξ Bootis A

We investigate the magnetic geometry of the active G8 dwarf ξ Bootis A (ξ Boo A), from spectropolarimetric observations obtained in 2003 with the MuSiCoS echelle spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We repeatedly detect a photospheric magnetic field, with periodic variations consistent with rotational modulation. Circularly polarized (Stokes V) line profiles present a systematic asymmetry, showing up as an excess in amplitude and area of the blue lobe of the profiles. Direct modelling of Stokes V profiles suggests that the global magnetic field is composed of two main components, with an inclined dipole and a large-scale toroidal field. We derive a dipole intensity of about 40 G, with an inclination of 35° of the dipole with respect to the rotation axis. The toroidal field strength is of the order of 120 G. A noticeable evolution of the field geometry is observed over the 40 nights of our observation window and results in an increase in field strength and dipole inclination. n nThis study is the first step of a long-term monitoring of ξ Boo A and other active solar-type stars, with the aim of investigating secular fluctuations of stellar magnetic geometries induced by activity cycles.

Rotational periods of four roAp stars

Abstract. Forty-five new measurements of the mean longitudinal magnetic fields and mean equivalent widths of 4 roAp stars have obtained using theMuSiCoS spectropolarimeter at Pic duMidi observatory. These new high-precision data have been combined with archival measurements in order to constrain the rotational periods of HD 12098, HD 24712 = HR 1217, HD 122970 and HD 176232 = 10 Aql. We report a revised rotational period for HD 24712 (Prot = 12.45877 ± 0.00016 d, crucial for interpretation nof upcoming MOST observations of this star), new rotational periods for HD 12098 and HD 122970 (Prot = 5.460 ± n0.001 d and Prot = 3.877 ± 0.001 d, respectively) and evidence for an extremely long period for HD 176232.

The magnetic Bp star 36 Lyncis I. Magnetic and photospheric properties

Aims.This paper reports the photospheric, magnetic and circumstellar gas characteristics of the magnetic B8p star 36 Lyncis (HD 79158). Methods: .Using archival data and new polarised and unpolarised high-resolution spectra, we redetermine the basic physical properties, the rotational period and the geometry of the magnetic field, and the photospheric abundances of various elements. Results: .Based on magnetic and spectroscopic measurements, we infer an improved rotational period of 3.83475± 0.00002 d. We determine a current epoch of the longitudinal magnetic field positive extremum (HJD 2 452 246.033), and provide constraints on the geometry of the dipole magnetic field (i? 56°, 3210~G? B_d? 3930 G, ? unconstrained). We redetermine the effective temperature and surface gravity using the optical and UV energy distributions, optical photometry and Balmer line profiles (T_eff=13 300± 300 K, log g=3.7-4.2), and based on the Hipparcos parallax we redetermine the luminosity, mass, radius and true rotational speed (L=2.54± 0.16~L_?, M=4.0± 0.2~M_?, R=3.4± 0.7~R_?, v_eq=45-61.5 km s-1). We measure photospheric abundances for 21 elements using optical and UV spectra, and constrain the presence of vertical stratification of these elements. We perform preliminary Doppler Imaging of the surface distribution of Fe, finding that Fe is distributed in a patchy belt near the rotational equator. Most remarkably, we confirm strong variations of the H? line core which we interpret as due to occultations of the star by magnetically-confined circumstellar gas.

A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star for the Gravity Probe B Mission

We present the first detection of the secondary of the spectroscopic binary system IM Pegasi (HR 8703), the guide star for the NASA-Stanford relativity gyroscope mission Gravity Probe B. In support of this mission, high-resolution echelle spectra of IM Peg have been obtained on an almost nightly basis. Applying the technique of least-squares deconvolution, we achieve very high signal-to-noise ratio line profiles and detect the orbit of the secondary of the system. Combining almost 700 new radial velocity measurements of both the primary and secondary of the system with previous measurements, we derive improved orbital parameters of the IM Peg system. Using these estimates along with the previously determined range of orbital inclination angles for the system, we find that the primary of IM Peg is a giant of mass 1.8+/-0.2 Msolar, while the secondary is a dwarf of mass 1.0+/-0.1 Msolar.

A survey of the weakest-field magnetic Ap stars: discovery of a threshold magnetic field strength?

We are conducting a magnetic survey of a sample of about 30 spectroscopically identified Ap stars, with weak or previously undetected magnetic fields. For 28 studied stars, we have obtained 25 detections of Stokes V Zeeman signatures. Our results suggest that all Ap stars are magnetic. Further there may exist a minimum field strength for which Ap-type characteristics are produced.

Prominence mapping of the RS CVn system HR 1099

We investigate temporal fluctuations in the H

A spectropolarimetric survey of the coolest magnetic Ap stars

alpha

First Direct Detection of Magnetic Fields in Starspots and Stellar Chromospheres

emission profiles of the RS CVn system HR 1099 from a monitoring using the MuSiCoS spectropolarimeter (Observatoire du Pic du Midi, France) in 2001, between December 01 and December 18. Part of the observed emission fluctuations is consistent with rotational modulation, which we interpret as the spectral signature of a dense and complex prominence system trapped in the magnetosphere of HR 1099 and forced to co-rotate with the binary system. The distribution of emitting material is mapped by means of Doppler tomography. We discuss the evolution of prominences over the observing window. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html