103 research outputs found
No evidence of a strong magnetic field in the Blazhko star RR Lyrae
Astronomy and Astrophysics, v. 413, p. 1087-1093, 2004. http://dx.doi.org/10.1051/0004-6361%3a20031600International audienc
Emission Lines in the Spectrum of the 3He Star 3 Cen A
Emission in the 4d - 4f transitions of MnII (multiplet 13, 6122-6132 Ang), in
the 4f - 6g transitions of PII, and in 6149.5 Ang of HgII has been detected in
the spectrum of the helium weak star 3 Centauri A (B5 III-IVp). Weaker emission
from the same MnII multiplet is also seen in the hot, mild HgMn star 46 Aquila
(B9 III).It is suggested that the emission is of photospheric origin and may be
evidence for the stratification of manganese, phosphorus and mercury in the
photosphere of 3 CenA, and of manganese in 46Aql.Comment: 16 pages, 3 figure
Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a
The detailed spiral structure in the outer Galactic disk is still poorly
known, and for several Galactic directions we rely on model extrapolations. One
of these regions is the fourth Galactic quadrant, in the sector comprised
between Vela and Carina (270 <l< 300) where no spiral arms have been detected
so far in the optical beyond 270. By means of deep UBVI photometry, we search
for spiral features in known low absorption windows.U photometry, although
demanding, constitutes a powerful tool to detect and characterize distant
aggregates, and allows to derive firmer distance estimates. We studied a
direction close to the tangent (l=290) to the Carina arm, in an attempt to
detect optical spiral tracers beyond the Carina branch, where radio
observations and models predictions indicate the presence of the extension of
the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line
of sight, we detect three distinct groups of young stars. Two of them, at 2.5
and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this
particular direction).The latter is here detected for the first time. The third
group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond
the Carina arm, and constitutes the first optical detection of this arm in the
fourth Galactic quadrant. The position of this feature is compatible with HI
observations and model predictions. We furthermore present evidence that this
extremely distant group, formerly thought to be a star cluster (Shorlin 1), is
in fact a diffuse young population. In addition, our data-set does not support
the possible presence of the Monoceros Ring toward this direction. This study
highlights how multicolor optical studies can be effective to probe the spiral
structure in the outer Galactic disk.Comment: 9 pages, 13 eps figure, in press in A&A, abstract rephrased and a few
figures degraded in resolution to fit i
Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur
AR Aur is the only eclipsing binary known to contain a HgMn star, making it
an ideal case for a detailed study of the HgMn phenomenon. HgMn stars are a
poorly understood class of chemically peculiar stars, which have traditionally
been thought not to possess significant magnetic fields. However, the recent
discovery of line profile variability in some HgMn stars, apparently
attributable to surface abundance patches, has brought this belief into
question. In this paper we investigate the chemical abundances, line profile
variability, and magnetic field of the primary and secondary of the AR Aur
system, using a series of high resolution spectropolarimetric observations. We
find the primary is indeed a HgMn star, and present the most precise abundances
yet determined for this star. We find the secondary is a weak Am star, and is
possibly still on the pre-main sequence. Line profile variability was observed
in a range of lines in the primary, and is attributed to inhomogeneous surface
distributions of some elements. No magnetic field was detected in any
observation of either stars, with an upper limit on the longitudinal magnetic
field in both stars of 100 G. Modeling of the phase-resolve longitudinal field
measurements leads to a 3 sigma upper limit on any dipole surface magnetic
field of about 400 G.Comment: Accepted for publication in MNRAS, 11 pages, 9 figure
Photometric distances to young stars in the inner Galactic disk. I. The L = 314 direction
The spiral structure of the Milky Way is nowadays receiving renewed attention
thanks to the combined efforts of observational campaigns in different
wavelength regimes, from the optical to the radio. We start in the paper the
exploration of a number of key sectors in the inner Milky Way, where the spiral
structure is still poorly known. We search for density enhancements of young
stars that might plausibly be associated with spiral structure. To this aim we
collect sufficiently wide-field UBVI photometry. The intensive usage of U-band
photometry ensures robust determination of reddening and hence distance for
stars of spectral type earlier than A0, which are well-known spiral arm
tracers. The fields we use are large enough to include in most cases
well-studied Galactic clusters, which we use as bench-marks to assess the
quality and standardisation of the data, and to validate our method. We focus
in this paper on the line of sight to the Galactic longitude l= 314. First
of all, we made exhaustive tests to show that our data-set is in the standard
system, and calibrated our method using the two open clusters NGC 5617 and
Pismis 19 which happen to be in the field, and for which we found estimates of
the basic parameters in full agreement with the literature. We then applied the
method to the general field stars and detected signatures of three different
groups of stars, evenly distributed across the field of view, at
1.5, 2.5, and 5.1 kpc,
respectively. These distances are compatible with the location of the nowadays
commonly accepted description of the Carina-Sagittarius and Scutum-Crux arms,
at heliocentric distance of 2 an 5 kpc,Comment: 23 pages, 13 eps figure, in press in Astronomy and Astrophysic
First evidence of a magnetic field on Vega. Towards a new class of magnetic A-type stars
We report the detection of a magnetic field on Vega through
spectropolarimetric observations. We acquired 257 Stokes V, high
signal-to-noise and high-resolution echelle spectra during four consecutive
nights with the NARVAL spectropolarimeter at the 2-m Telescope Bernard Lyot of
Observatoire du Pic du Midi (France). A circularly polarized signal in line
profiles is unambiguously detected after combining the contribution of about
1200 spectral lines for each spectrum and summing the signal over the 257
spectra. Due to the low amplitude of the polarized signal, various tests have
been performed to discard the possibility of a spurious polarized signal. They
all point towards a stellar origin of the polarized signal. Interpreting this
polarization as a Zeeman signature leads to a value of G for the
disk-averaged line-of-sight component of the surface magnetic field. This is
the first strong evidence of a magnetic field in an A-type star which is not an
Ap chemically peculiar star. Moreover, this longitudinal magnetic field is
smaller by about two orders of magnitude than the longitudinal magnetic field
(taken at its maximum phase) of the most weakly magnetic Ap stars. Magnetic
fields similar to the Vega magnetic field could be present but still undetected
in many other A-type stars.Comment: 4 pages, accepted for publication as a Letter in Astronomy &
Astrophysic
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
Stable propagation of an ordered array of cracks during directional drying
We study the appearance and evolution of an array of parallel cracks in a
thin slab of material that is directionally dried, and show that the cracks
penetrate the material uniformly if the drying front is sufficiently sharp. We
also show that cracks have a tendency to become evenly spaced during the
penetration. The typical distance between cracks is mainly governed by the
typical distance of the pattern at the surface, and it is not modified during
the penetration. Our results agree with recent experimental work, and can be
extended to three dimensions to describe the properties of columnar polygonal
patterns observed in some geological formations.Comment: 8 pages, 4 figures, to appear in PR
The search for magnetic fields in mercury-manganese stars
We performed a highly sensitive search for magnetic fields on a large set of
HgMn stars. With the aid of a new polarimeter attached to the HARPS
spectrometer at the ESO 3.6m-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 resulting in significantly greater sensitivity to the presence
of magnetic fields, including very weak fields. For the 47 individual objects
studied, including 6 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. We conclude that HgMn stars lack large-scale magnetic fields,
typical for spotted magnetic Ap stars, sufficient 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 which can
compositional inhomogeneities on their surfaces.Comment: 12 pages, 8 figures, 2 table
Spiral cracks in drying precipitates
We investigate the formation of spiral crack patterns during the desiccation
of thin layers of precipitates in contact with a substrate. This
symmetry-breaking fracturing mode is found to arise naturally not from torsion
forces, but from a propagating stress front induced by the fold-up of the
fragments. We model their formation mechanism using a coarse-grain model for
fragmentation and successfully reproduce the spiral cracks. Fittings of
experimental and simulation data show that the spirals are logarithmic,
corresponding to constant deviation from a circular crack path. Theoretical
aspects of the logarithmic spirals are discussed. In particular we show that
this occurs generally when the crack speed is proportional to the propagating
speed of stress front.Comment: 4 pages, 5 figures, RevTe
- …