Ultra-weak magnetic fields in Am stars: \beta UMa and \theta Leo

Ultra weak magnetic fields in Am stars: Beta UMa and theta Leo

Abstract

An extremely weak circularly-polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A (Petit et al. 2011). This signal was interpreted as a Zeeman signature related to a sub-gauss longitudinal magnetic field, constituting the first detection of a surface magnetic field in an Am star. We present here ultra-deep spectropolarimetric observations of two other bright Am stars, UMa and Leo, observed with the NARVAL spectropolarimeter. The line profiles of the two stars display circularly-polarized signatures similar in shape to the observations gathered for Sirius A. These new detections suggest that very weak magnetic fields may be present in the photospheres of a significant fraction of intermediate-mass stars, although the strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a prominent positive lobe and no detected negative lobe) are not expected in the standard theory of the Zeeman effect.

S
\TitreGlobal

SF2A 2014

tars: magnetic field, Stars: chemically peculiar

1 Introduction

Magnetic fields play an important role in the evolution of hot stars (O, B and A stars). However, the origin and even the basic properties of hot star magnetic fields are still poorly understood. About 7% of hot stars are found to be strongly magnetic with a longitudinal magnetic field in excess of 100 G (wade13). But recently, a sub-gauss longitudinal magnetic field has been discovered in the normal A star Vega (lignieres09). This detection raises the question of the ubiquity of magnetic fields in objects belonging to this mass domain. In 2011, another polarimetric signal was detected in the bright Am star Sirius A (petit11). For this object, the polarized signature in circular polarization is not of null integral over the line profile as in other massive stars, since the Stokes V line profile exhibits a positive lobe dominating over the negative one (in amplitude and area). Here, we present the results of a magnetic field search carried out for two other bright Am stars: UMa and Leo. The fundamental parameters of both targets are presented in Table 1. The Am stars are chemically peculiar stars exhibiting overabundances of iron-group elements such as zinc, strontium, zirconium and barium and deficiencies of others such as calcium and scandium. Most Am stars also feature low projected rotational velocities, as compared to normal A stars (abt09).

UMa Leo
spectral type A1V A2V
9600 K 9350 K
log g 3.83 3.65
Mass 2.7 2.5
Radius 3 4.3
vsini 23 km/s 46 km/s
boyajian12 monier05
smith93 wyatt07
royer02
Table 1: Fundamental parameters of UMa and Leo

2 Data analysis

Data were taken with the NARVAL spectropolarimeter. Narval is operated at the 2-meter Bernard Lyot Telescope (TBL), at the summit of Pic du Midi in the French Pyrénées. This fibre-fed spectropolarimeter was especially designed and optimized to detect stellar surface magnetic fields through the polarization they generate in photospheric lines and provides complete coverage of the optical spectrum from 3700 to 10500 on 40 echelle orders with a spectral resolution of 65000.

UMa was observed in March/April 2010 and March/April 2011, while observations of Leo were collected in January/March/April 2012, March/April 2013 and May 2014 (see Table 2).

date mid-HJD star (s)
17mar10 2455273.52016 UMa 16 4 107
06apr10 2455293.41173 UMa 17 4 107
10apr10 2455297.44406 UMa 19 4 107
11apr10 2455298.39660 UMa 19 4 107
25mar11 2455646.42619 UMa 25 4 107
31mar11 2455652.50367 UMa 25 4 107
02apr11 2455654.37913 UMa 03 4 107
04apr11 2455656.46150 UMa 24 4 107
22jan12 2455949.64440 Leo 05 4 180
23jan12 2455950.62844 Leo 05 4 180
24jan12 2455951.62417 Leo 05 4 180
25jan12 2455952.64032 Leo 05 4 180
14mar12 2456001.57862 Leo 05 4 180
15mar12 2456002.52449 Leo 10 4 180
24mar12 2456011.52572 Leo 05 4 180
25mar12 2456012.50225 Leo 05 4 180
27mar12 2456013.39956 Leo 10 4 180
21mar13 2456373.48791 Leo 09 4 180
23mar13 2456375.46496 Leo 09 4 180
16apr13 2456399.44433 Leo 09 4 180
17apr13 2456400.49237 Leo 09 4 180
22apr13 2456405.51179 Leo 09 4 180
23apr13 2456406.45400 Leo 09 4 180
24apr13 2456407.50188 Leo 09 4 180
14apr14 2456762.44478 Leo 05 4 180
07may14 2456785.40762 Leo 05 4 180
08may14 2456786.41135 Leo 05 4 180
09may14 2456787.41580 Leo 05 4 180
14may14 2456792.47117 Leo 05 4 180
15may14 2456793.41300 Leo 05 4 180
Table 2: Journal of observations

To test whether UMa and Leo are magnetic, we applied the well-known and commonly used Least-Squares Deconvolution (LSD) technique (donati97) on each spectrum of both stars and computed LSD pseudo line profiles from all available photospheric lines. The line lists used are created from a list of lines extracted from the VALD data base (piskunov95; kupka99) using the respective effective temperature and log g of both stars (Table 1). To further improve the signal-to-noise ratio, we then coadded all LSD profiles of each star, resulting in one single averaged LSD profile for each target. The result are shown in Figure 1.

3 Results

The line profiles of the two stars display circularly-polarized signatures similar in shape (see Figure 1) to the observations previously gathered for Sirius A. We have also separately coadded LSD profiles for each observing year (not shown here) to evaluate the temporal stability of this signal, concluding that the signatures are stable over the time-span of our observations.

Figure 1: Left: normalized Stokes I and V averaged LSD profiles for UMa. Right: same figure for Leo.

4 Conclusions

These new detections may be interpreted to suggest that sub-gauss magnetic fields are present in the photosphere of a significant fraction of intermediate-mass stars, although the strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a prominent positive lobe and essentially no negative lobe) are not expected in the standard theory of the Zeeman effect (see possible interpretations in petit11). New observations are currently being carried out to gain a better statistics of the prevalence of weak magnetic fields in intermediate-mass stars and evaluate the impact of various stellar parameters on the Vega-like magnetism.

Acknowledgements.
We acknowledge support from the ANR (Agence Nationale de la Recherche) project Imagine. This research has made use of the SIMBAD database operated at CDS, Strasbourg (France), and of NASA’s Astrophysics Data System (ADS).

References

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