The study of evolutionary changes in intermediate mass magnetic CP stars across the HR diagram

Evgeny Semenko

Special Astrophysical Observatory of the Russian Academy of Sciences

“Putting A Stars Into Context: Evolution, Environment, and Related Stars”Moscow, SAI MSU

Introduction

Significant magnetic field occurs in a wide range of A and B stars from Ae/Be Herbig objects to the right side of HR diagram.

Young Bp stars generally have stronger magnetic fields with complex structure: HD 215441, HD 37776.

There are few Ap stars with strongest magnetic field exceeding 20 kG: HD 154708, HD 178892, BD +00°4535 and some others.

Evolved Ap stars near TAMS are weakly magnetic with practically invisible REEs.

Introduction

Group of evolved stars with accurate measurements of longitudinal magnetic field and other physical parameters consists of at least 6 A stars.

The number of young stars near ZAMS with detailed measurements is quit low. Individual results – individual masses and other parameters.

In order to make detailed study of the evolutional changes of magnetic field and chemical anomalies in Ap/Bp star we need to fill a gap between ZAMS and more evolved objects within one evolutional track.

Introduction

All evolved stars are located within the range of 2.5-3.0 solar masses.

Target selection

We carried out an observational program aimed the searches of new magnetic stars and their further detailed study.

Object preselection is based on the next criteria:

1) spectral class from B7 to A0;

2) IR excess along with the absence of any other Ae/Be stars signatures;

3) relatively slow rotation;

4) abundance anomalies.

Instruments

Russian 6-m telescope equipped with:

1. Main stellar spectrograph (http://www.sao.ru/hq/lizm/mss/en/index.html): R=0.12 Å/pix, Vlim=11.5m, 500Å sp. range – circular spectropolarimetry.

2. Nasmyth echelle spectrometer (http://www.sao.ru/hq/ssl/NES.html): R=0.026Å/pix, Vlim=10m, 4000-7000Å.

1-m Telescope Zeiss-1000 of the SAO RAS. Low resolution spectroscopy.

An observational sample

About dozen of late B stars were selected as observational candidates.

Polarized spectra obtained for less than half.

Echelle spectra were collected for two objects: HD 63347 and HD 50341.

Most objects from current sample are winter objects.

The study of magnetic field

Two early Bp stars: HD 63347 and HD 50341.

Age about 20 million yrs.

SrCrEu-type spectral anomalies.

Rotational velocities are about 40-50 km/s.

The study of magnetic field

Longitudinal magnetic field of HD 63347 (left) and HD 50341 (right) phased with rotational period. Both periods are smaller than 3 days. Surface field not exceed 3 kG.

Physical parameters of two stars

Effective temperature: 11 300 K (HD 63347) and 11 800 K (HD 50341). Derived from photometry and profiles of hydrogen lines.

Surface gravity: 4.2 and 4.1 dex, resp.

Luminocities: 1.64 (HD 63347), 1.60 (HD 50341).

vsin i = 42 km/s (HD 63347), 51 km/s (HD 50341).

Chemical composition of HD 63347

Both stars demonstrates strong spectral variability.

Chemical composition of HD 63347

Conclusions

There are stars with weak magnetic fields and prominent chemical peculiarities among young objects.

Overabundance of Fe and Cr in some young stars intrinsically the same as in evolved magnetic stars of equal mass.

While the longitudinal magnetic field of young stars is quite a small, their structure could be more complex than a simple dipole.

Thanks for your attention!