uu.seUppsala University Publications
Change search
ReferencesLink to record
Permanent link

Direct link
The search for magnetic fields in mercury-manganese stars
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Show others and affiliations
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 525, A97- p.Article in journal (Refereed) Published
Abstract [en]

Context. A subclass of the upper main-sequence chemically peculiar stars, mercury-manganese (HgMn) stars were traditionally considered to be non-magnetic, showing no evidence of variability in their spectral line profiles. However, discoveries of chemical inhomogeneities on their surfaces imply that this assumption should be investigated. In particular, spectroscopic time-series of AR Aur, a And, and five other HgMn stars indicate the presence of chemical spots. At the same time, no signatures of global magnetic fields have been detected. Aims. We attempt to understand the physical mechanism that causes the formation of chemical spots in HgMn stars and gain insight into the potential magnetic field properties at their surfaces; we performed a highly sensitive search for magnetic fields for a large set of HgMn stars. Methods. With the aid of a new polarimeter attached to the HARPS spectrometer at the ESO 3.6 m-telescope, we obtained high-quality circular polarization spectra of 41 single and double HgMn stars. Using a multi-line analysis technique on each star, we co-added information from hundreds of spectral lines to ensure significantly greater sensitivity to the presence of magnetic fields, including very weak fields. Results. For the 47 individual objects studied, including six components of SB2 systems, we do not detect any magnetic fields at greater than the 3 sigma level. The lack of detection in the circular polarization profiles indicates that if strong fields are present on these stars, they must have complex surface topologies. For simple global fields, our detection limits imply upper limits to the fields present of 2-10 Gauss in the best cases. Conclusions. We conclude that HgMn stars lack large-scale magnetic fields, which is typical of spotted magnetic Ap stars, of sufficient strength to form and sustain the chemical spots observed on HgMn stars. Our study confirms that in addition to magnetically altered atomic diffusion, there exists another differentiation mechanism operating in the atmospheres of late-B main sequence stars that can produce compositional inhomogeneities on their surfaces.

Place, publisher, year, edition, pages
2011. Vol. 525, A97- p.
Keyword [en]
stars: chemically peculiar, stars: magnetic field, polarization
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-143686DOI: 10.1051/0004-6361/201015666ISI: 000285193700145OAI: oai:DiVA.org:uu-143686DiVA: diva2:390924
Available from: 2011-01-24 Created: 2011-01-24 Last updated: 2012-10-18Bibliographically approved
In thesis
1. Magnetic Fields and Chemical Spots in HgMn Stars
Open this publication in new window or tab >>Magnetic Fields and Chemical Spots in HgMn Stars
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mercury-manganese (HgMn) stars belong to the class of chemically peculiar (CP) stars. It was recently discovered that some HgMn stars have spots of chemical elements on their surfaces. According to conventional picture of CP stars, magnetic field facilitates the formation and long term stability of chemical spots by controlling stratification of elements in stellar atmosphere. However, previous attempts to find magnetic field in HgMn stars set an upper limit on its strength at the level of about 20-100 Gauss. Observational evidence suggested that even weaker magnetic fields can be responsible for the formation of chemical spots. The main goal of our work was to verify this possibility.

The search for weak magnetic fields requires the use of least-squares deconvolution (LSD) technique.  This method combines information from many spectral lines providing a mean line profile with increased signal-to-noise ratio. Up to now there was no extensive comparison of the LSD profile with real spectral lines. We showed that the LSD profile of the intensity spectrum does not behave like a real spectral line as a function of chemical composition. However, for circular polarization, LSD profile resembles the profile of a spectral line with mean atomic parameters.

We performed a comprehensive search for magnetic field in 47 HgMn stars and their companions, based on high-quality spectropolarimetric data obtained with the HARPSpol polarimeter at the ESO 3.6-m telescope. With the help of LSD technique, an upper limit on the mean longitudinal magnetic field was brought down to 2-10 G for most stars. We concluded that magnetic field is not responsible for the spot formation in HgMn stars.

We obtained full rotational phase coverage for the HgMn stars φ Phe and 66 Eri. This enabled us to investigate line profile variability, reconstruct surface maps of chemical elements, and perform a search for magnetic field with very high sensitivity. For φ Phe we derived surface maps of Y, Sr, Ti, Cr, and obtained an upper limit of 4 G on the field strength. We also found marginal indication of vertical stratification of Y and Ti. No magnetic field was detected in both components of 66 Eri, with an upper limit of 10-24 G. We discovered chemical spots of Y, Sr, Ba, and Ti, in the primary star. We demonstrated a relation between the binary orbit and the morphology of these spots.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 46 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 873
Chemically peculiar stars; magnetic fields; spectropolarimetry; Doppler imaging
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
urn:nbn:se:uu:diva-160308 (URN)978-91-554-8199-5 (ISBN)
Public defence
2011-12-08, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Available from: 2011-11-09 Created: 2011-10-20 Last updated: 2012-10-16

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Makaganiuk, VitaliiKochukhov, OlegPiskunov, NikolaiStempels, H. C.
By organisation
Observational AstronomyDepartment of Physics and Astronomy
In the same journal
Astronomy and Astrophysics
Physical Sciences

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 523 hits
ReferencesLink to record
Permanent link

Direct link