495 research outputs found
Searching for a link between the presence of chemical spots on the surface of HgMn stars and their weak magnetic fields
We present the results of mapping the HgMn star AR Aur using the Doppler
Imaging technique for several elements and discuss the obtained distributions
in the framework of a magnetic field topology.Comment: 2 pages, 1 figure, to appear in Proceedings of IAU Symposium 259
"Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife,
Spain, November 3-7, 200
A refined analysis of the remarkable Bp star HR 6000
UVES spectra of the very young (~10^7 years) peculiar B-type star HR 6000
were analyzed in the near-UV and visual spectral regions (3050-9460 A) with the
aim to extend to other spectral ranges the study made previously in the UV
using IUE spectra. Stellar parameters Teff=12850K, logg=4.10, and xi=0km/s, as
determined from H_beta, H_gamma, H_delta Balmer profiles and from the Fe I, Fe
II ionization equilibrium, were used to compute an individual abundances
ATLAS12 model. We identified spectral peculiarities and obtained final stellar
abundances by comparing observed and computed equivalent widths and line
profiles. The adopted model fails to reproduce the (b-y) and c color indices.
The spectral analysis has revealed: the presence of emission lines for Mn II,
Cr II, and Fe II; isotopic anomalies for Hg, Ca; the presence of interstellar
lines of Na I at lambda lambda 3302.3, 3302.9, 5890, 5896 A, and of K I at
7665, 7699 A; the presence of a huge quantity of unidentified lines, which we
presume to be mostly due to Fe II transitions owing to the large Fe
overabundance amounting to [+0.7]. The main chemical peculiarities are an
extreme overabundance of Xe, followed by those of Hg, P, Y, Mn, Fe, Be, and Ti.
The most underabundant element is Si, followed by C, N, Al, S, Mg, V, Sr, Co,
Cl, Sc, and Ni. The silicon underabundance [-2.9] is the lowest value for Si
ever observed in any HgMn star. The observed lines of He I can not be
reproduced by a single value of the He abundance, but they require values
ranging from [-0.8] to [-1.6]. Furthermore, when the observed and computed
wings of He I lines are fitted, the observed line cores are much weaker than
the computed ones. From the present analysis we infer the presence of vertical
abundance stratification for He, Mn, and possibly also P.Comment: 14 pages, 8 figures, 6 tables, accepted for publication in A&
The narrow, inner CO ring around the magnetic Herbig Ae star, HD 101412
We describe and model emission lines in the first overtone band of CO in the
magnetic Herbig Ae star HD 101412. High-resolution CRIRES spectra reveal
unusually sharp features which suggest the emission is formed in a thin disk
centered at 1 AU with a width 0.32 AU or less. A wider disk will not fit the
observations. Previous observations have reached similar conclusions, but the
crispness of the new material brings the emitting region into sharp focus.Comment: Accepted as Astronomy and Astrophysics Letter; 4 pages, 5 figure
Characterising the magnetic fields of the Herbig Ae/Be stars HD97048, HD150193, HD176386, and MWC480
Our knowledge of the presence and the role of magnetic fields in
intermediate-mass pre-main-sequence stars remains very poor.
We present the magnetic properties of four Herbig Ae/Be stars that have not
been previously studied in detail.
Our results for the three Herbig Ae/Be stars HD97048, HD150193, and HD176386
are based on multi-epoch low-resolution spectra obtained in spectropolarimetric
mode with FORS2 mounted on the VLT. New high-resolution polarimetric spectra
were obtained for MWC480 with the SOFIN spectrograph installed at the Nordic
Optical Telescope. We discuss 41 FORS2 low-resolution observations of the
Herbig Ae/Be stars HD97048, HD150193, and HD176386 and determine their
rotational periods. Using stellar fundamental parameters and the longitudinal
magnetic field phase curves, we place constraints on the magnetic field
geometry. Three high-resolution circularly polarised SOFIN spectra obtained for
MWC480 were measured using the moment technique where wavelength shifts between
right- and left-hand side circularly polarised spectra are interpreted in terms
of a longitudinal magnetic field .
Our search for periodicities resulted in P=0.693d for HD97048, P=1.317d for
HD150193, and P=0.899d for HD176386. The magnetic field geometry can likely be
described by a centred dipole with a polar magnetic field strength B_d of
several hundred Gauss. The longitudinal magnetic-field measurements of MWC480
reveal the presence of a strong kG field, which was undetected in our previous
low-resolution polarimetric observations with FORS1. A weak magnetic field was
detected in the circumstellar components of the Ca II H&K lines and the Na I D
lines, indicating a complex interaction between the stellar magnetic field and
the circumstellar environment.Comment: 8 pages, 9 figures, 5 tables, accepted for publication in A&
Dynamical evolution of titanium, strontium, and yttrium spots on the surface of the HgMn star HD 11753
Aims. We gathered about 100 high-resolution spectra of three typical HgMn
(mercury-manganese) stars, HD 11753, HD 53244, and HD 221507, to search for
slowly pulsating B-like pulsations and surface inhomogeneous distribution of
various chemical elements. Methods. Classical frequency analysis methods were
used to detect line profile variability and to determine the variation period.
Doppler imaging reconstruction was performed to obtain abundance maps of
chemical elements on the stellar surface. Results. For HD 11753, which is the
star with the most pronounced variability, distinct spectral line profile
changes were detected for Ti, Sr, Y, Zr, and Hg, whereas for HD 53244 and HD
221507 the most variable line profiles belong to the elements Hg and Y,
respectively. We derived rotation periods for all three stars from the
variations of radial velocities and equivalent widths of spectral lines
belonging to inhomogeneously distributed elements: P_rot (HD 11753)=9.54 d,
P_rot (HD 53244)=6.16 d, and P_rot (HD 221507)=1.93 d. For HD 11753 the Doppler
imaging technique was applied to derive the distribution of the most variable
elements Ti, Sr, and Y using two datasets separated by ~65 days. Results of
Doppler imaging reconstruction revealed noticeable changes in the surface
distributions of TiII, SrII, and YII between the datasets, indicating the
hitherto not well understood physical processes in stars with radiative
envelopes that cause a rather fast dynamical chemical spot evolution.Comment: Accepted for publication in Astronomy and Astrophysics, 6 pages, 9
figures, 2 tables
Magnetic fields in O-type stars measured with FORS1 at the VLT
The presence of magnetic fields in O-type stars has been suspected for a long
time. The discovery of such fields would explain a wide range of well
documented enigmatic phenomena in massive stars, in particular cyclical wind
variability, Halpha emission variations, chemical peculiarity, narrow X-ray
emission lines and non-thermal radio/X-ray emission. Here we present the
results of our studies of magnetic fields in O-type stars, carried out over the
last years.Comment: 2 pages, 1 figure, to appear in Proceedings of IAU Symposium 259
"Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife,
Spain, November 3-7, 200
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