Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

Abstract

We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0 • in slower rotating stars of mass M > 3 M too, analog to the behaviour of angles β in slowly rotating stars of M < 3 M . The obliquity angle distribution as inferred from the distribution of r-values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle β tends to reach values close to either 90 • or 0 • for M < 3 M . The evolution of the obliquity angle β seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of β with the elapsed time on the main sequence for stars with M > 3 M , no similar trend is found for stars with M < 3 M . However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories.

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks

“…1 we show the significance distributions of our measurements. These distributions are different from the distribution shown by Wade et al (2007) (Hubrig et al 2004b(Hubrig et al , 2007b, the magnetic field in the present study was diagnosed only at a level of 1.8σ and 1.6σ, respectively. Wade et al (2005) failed to detect a magnetic field in HD 139614 with quoted uncertainties of 25 G on two consecutive nights with the high resolution spectropolarimeter ESPaDOnS.…”

“…on the rotation phase. For the Herbig Ae star HD 163296, we found no indication for the presence of a photospheric magnetic field, in agreement with our previous studies (Hubrig et al 2006b(Hubrig et al , 2007b.…”

“…Certainly, the advantage of using high resolution spectropolarimeters such as ESPaDOnS at the Canada-France-Hawaii Bernacca & Perinotto (1970), c Grady et al (1996), d van den Ancker et al (1998), e Acke & Waelkens (2004), f Hamidouche et al (2008), g this work, h Dunkin et al (1997), i Hubrig et al (2007b), j Alencar et al (2003), k Royer et al (2007), l Mora et al (2001), m Slettebak (1982), n de la Reza & Pinzón (2004), o Beskrovnaya et al (2004), p Alecian et al (2008), q Landstreet et al (2008) Pontoppidan et al (2007b), aa Siebenmorgen et al (2000), bb Kessler-Silacci et al (2005), cc Okamoto et al (2004), dd Schütz et al (2005b), ee Dommanget & Nys (1994), f f Baines et al (2006), gg Corporon & Lagrange (1999), hh Stelzer et al (2006), ii Chen et al (2006a), j j Augereau et al (2001), kk Carmona et al (2007), ll Stecklum et al (1995), mm Reipurth & Zinnecker (1993), nn Roberge et al (2002), oo Grady et al (2007), pp Stauffer et al (1995), qq SIMBAD, rr Eisner et al (2004), ss Grady et al (2004), tt Pirzkal et al (1997), uu Schnerr et al (2007), vv Blondel et al (2006), ww van den Ancker et al (1997), xx Wade et al (2007), yy …”

New X-ray observations of IQ Aurigae andα2 Canum Venaticorum

“…The field A0p star IQ Aur (HD 34452, HR 1732) belongs to the class of α 2 CVn spectrum variable stars and is thought to be a single star with V = 5.4 and small E B−V 0.01. Babel & Montmerle (1997), who report the ROSAT X-ray detection at log L X = 29.6 erg s −1 , assume a 4 kG dipolar field for IQ Aur and determine an inclination of i ≈ 32 • and an obliquity of β ≈ 50 • , whereas Hubrig et al (2007) find i = 87 • and of β = 7 • . Bohlender et al (1993) state: 'a large range of values for i is possible: if i is large β is fairly small and if i is as small as allowed by our assumptions, β may be more than 80 • .…”

“…The stellar parameter differ somewhat as well, e.g. BM97 assume log L/L = 2.9 ± 0.2, M = 4.8 M , R = 5.1 ± 1.1 R and V rot = 105 km s −1 whereas Hubrig et al (2007) give log L/L = 2.3 ± 0.1, M = 3.2 ± 0.2 M , R = 2.2 ± 0.4 R and V rot = 46 km s −1 . Because the adopted distance is the same, the stellar data and viewing geometry of IQ Aur appear to be only moderately constrained and time-variability might play an additional role.…”