85 research outputs found
Imaging Polarimetric Observations of a New Circumstellar Disk System
Few circumstellar disks have been directly observed. Here we use sensitive
differential polarimetric techniques to overcome atmospheric speckle noise in
order to image the circumstellar material around HD 169142. The detected
envelope or disk is considerably smaller than expectations based on the
measured strength of the far-IR excess from this system
A Search for Intrinsic Polarization in O Stars with Variable Winds
New observations of 9 of the brightest northern O stars have been made with
the Breger polarimeter on the 0.9~m telescope at McDonald Observatory and the
AnyPol polarimeter on the 0.4~m telescope at Limber Observatory, using the
Johnson-Cousins UBVRI broadband filter system. Comparison with earlier
measurements shows no clearly defined long-term polarization variability. For
all 9 stars the wavelength dependence of the degree of polarization in the
optical range can be fit by a normal interstellar polarization law. The
polarization position angles are practically constant with wavelength and are
consistent with those of neighboring stars. Thus the simplest conclusion is
that the polarization of all the program stars is primarily interstellar.
The O stars chosen for this study are generally known from ultraviolet and
optical spectroscopy to have substantial mass loss rates and variable winds, as
well as occasional circumstellar emission. Their lack of intrinsic polarization
in comparison with the similar Be stars may be explained by the dominance of
radiation as a wind driving force due to higher luminosity, which results in
lower density and less rotational flattening in the electron scattering inner
envelopes where the polarization is produced. However, time series of
polarization measurements taken simultaneously with H-alpha and UV spectroscopy
during several coordinated multiwavelength campaigns suggest two cases of
possible small-amplitude, periodic short-term polarization variability, and
therefore intrinsic polarization, which may be correlated with the more widely
recognized spectroscopic variations.Comment: LaTeX2e, 22 pages including 11 tables; 12 separate gif figures; uses
aastex.cls preprint package; accepted by The Astronomical Journa
Optical polarimetry and photometry of comet 17P/Holmes
Comet 17P/Holmes was observed for linear polarisation using the optical
polarimeter mounted on the 1.2m telescope atop Gurushikhar peak near Mt. Abu
during the period November-December 2007. Observations were conducted through
the IHW narrow band (continuum) filters. During the observing run the phase
angle was near at which the comet showed negative polarisation. On
the basis of the observed polarisation data we find comet 17P/Holmes to be a
typical comet with usual dust characteristics. We note that radial rate of
change of brightness in coma in red band is higher than that in blue band; it
has decreased by a factor of 3.6 and 2.5 respectively in red and blue bands
during the November - December run, indicating relative increase in the
abundance of smaller dust particles out ward. Radial brightness variation seen
near the nucleus on November 6 is indicative of the presence of a blob or
shocked region beyond 10" from the nucleus which has gradually smoothened by
December 13. The brightness distribution is found steeper during November 5-7
as compared to on December 13.Comment: 11pages, 7 figures, accepted for publication in MNRA
Multi-wavelength polarimetric study towards the open cluster NGC 1893
We present multi-wavelength linear polarimetric observations for 44 stars of
the NGC 1893 young open cluster region along with V-band polarimetric
observations of stars of other four open clusters located between l ~160 to
~175 degree. We found evidence for the presence of two dust layers located at a
distance of ~170 pc and ~360 pc. The dust layers produce a polarization Pv
~2.2%. It is evident from the clusters studied in the present work that, in the
Galactic longitude range l ~160 to 175 degree and within the Galactic plane
(|b| < 2 degree), the polarization angles remain almost constant, with a mean
~163 degree and a dispersion of 6 degree. The small dispersion in polarization
angle could be due to the presence of uniform dust layer beyond 1 kpc. Present
observations reveal that in case of NGC 1893, the foreground two dust layers,
in addition to the intracluster medium, seems to be responsible for the
polarization effects. It is also found that towards the direction of NGC 1893,
the dust layer that exists between 2-3 kpc has a negligible contribution
towards the total observed polarization. The weighted mean for percentage of
polarization (Pmax) and the wavelength at maximum polarization ({\lambda}max)
are found to be 2.59 \pm 0.02% and 0.55 \pm 0.01 \mum respectively. The
estimated mean value of {\lambda}max indicates that the average size of the
dust grains within the cluster is similar to that in the general interstellar
medium. The spatial variation of the polarization is found to decrease towards
the outer region of the cluster. In the present work, we support the notion, as
already has been shown in previous studies, that polarimetry, in combination
with (U-B)/(B-V) colour-colour diagram, is a useful tool for identifying
non-members in a cluster.Comment: 36 pages, 12 figures, 10 tables, accepted for the publication in
MNRA
Error Analysis for Dual-Beam Optical Linear Polarimetry
In this paper we present an error analysis for polarimetric data obtained
with dual-beam instruments. After recalling the basic concepts, we introduce
the analytical expressions for the uncertainties of polarization degree and
angle. These are then compared with the results of Monte-Carlo simulations,
which are also used to briefly discuss the statistical bias. Then we approach
the problem of background subtraction and the errors introduced by a
non-perfect Wollaston prism, flat-fielding and retarder plate defects. We
finally investigate the effects of instrumental polarization and we propose a
simple test to detect and characterize it. The application of this method to
real VLT-FORS1 data has shown the presence of a spurious polarization, which is
of the order of 1.5% at the edges of the field of view. The cause of this
effect has been identified with the presence of rather curved lenses in the
collimator, combined with the non complete removal of reflections by the
coatings. This problem is probably common to all focal-reducer instruments
equipped with a polarimetric mode. An additional spurious and asymmetric
polarization field, whose cause is still unclear, is visible in the B band.Comment: 13 pages, 11 figures. Accepted for publication in PAS
Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective
We examine patterns of variation of 32 primarily main sequence stars,
extending our previous 7-12 year time series to 13-20 years by combining b, y
data from Lowell Observatory with similar data from Fairborn Observatory.
Parallel chromospheric Ca II H and K emission data from the Mount Wilson
Observatory span the entire interval. The extended data strengthen the
relationship between chromospheric and photometric variation derived
previously. Twenty-seven stars are deemed variable. On a year-to-year timescale
young active stars become fainter when their Ca II emission increases while
older less active stars such as the Sun become brighter when their Ca II
emission increases. The Sun's total irradiance variation, scaled to the b and y
filter photometry, still appears to be somewhat smaller than stars in our
limited sample with similar mean chromospheric activity, but we now regard this
discrepancy as probably due mainly to our limited stellar sampl
Probing Dust in the Atmosphere of Brown Dwarfs Through Polarization
Theoretical analysis and observational evidences indicate that a brown dwarf
with effective temperature greater than 1400 K would have dust cloud in its
atmosphere. In this letter, we show that dust scattering should yield polarized
continuum radiation from the relatively warm brown dwarfs and the polarized
flux profile could be a potential diagnosis tool for the optical and the
physical properties of dust grains. The degree of polarization due to multiple
scattering will be more in the optical region if the particle size is small
while significant polarization should be detected in the infra-red region if
the particle size is large. It is pointed out that the departure from
sphericity in the shape of the object due to rapid rotation and due to tidal
effect by the companion in a binary system ensures the disc integrated
polarization to be non-zero.Comment: 9 pages (Latex AAS v4.0), 2 postscript figures, Accepted by The
Astrophysical Journal Letter
Polarimetric variations of binary stars. IV. Pre-main-sequence spectroscopic binaries located in Taurus, Auriga, and Orion
We present polarimetric observations of 14 pre-main-sequence (PMS) binaries
located in the Taurus, Auriga, and Orion star forming regions. The majority of
the average observed polarizations are below 0.5%, and none are above 0.9%.
After removal of estimates of the interstellar polarization, about half the
binaries have an intrinsic polarization above 0.5%, even though most of them do
not present other evidences for the presence of circumstellar dust. Various
tests reveal that 77% of the PMS binaries have or possibly have a variable
polarization. LkCa3, Par1540, and Par2494 present detectable periodic and
phase-locked variations. The periodic polarimetric variations are noisier and
of a lesser amplitude (~0.1%) than for other types of binaries, such as hot
stars. This could be due to stochastic events that produce deviations in the
average polarization, a non-favorable geometry (circumbinary envelope), or the
nature of the scatterers (dust grains are less efficient polarizers than
electrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dust
in its environment to produce detectable levels of polarization and variations.
A fourth interesting case is W134, which displays rapid changes in polarization
that could be due to eclipses. We compare the observations with some of our
numerical simulations, and also show that an analysis of the periodic
polarimetric variations with the Brown, McLean, & Emslie (BME) formalism to
find the orbital inclination is for the moment premature: non-periodic events
introduce stochastic noise that partially masks the periodic low-amplitude
variations and prevents the BME formalism from finding a reasonable estimate of
the orbital inclination.Comment: 70 pages, 20 figures, to be published in the Astronomical Journa
Interstellar polarization and grain alignment: the role of iron and silicon
We compiled the polarimetric data for a sample of lines of sight with known
abundances of Mg, Si, and Fe. We correlated the degree of interstellar
polarization and polarization efficiency (the ratio of to the colour
excess or extinction ) with dust phase abundances. We detect an
anticorrelation between and the dust phase abundance of iron in non
silicate - containing grains ]_\rm d, a correlation
between and the abundance of Si, and no correlation between or
and dust phase abundances. These findings can be explained if mainly
the silicate grains aligned by the radiative mechanism are responsible for the
observed interstellar linear polarization.Comment: Accepted for publication in Astronomy and Astrophysic
EXPORT: optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT
sample obtained at the 2.5m Nordic Optical Telescope. The database consists of
multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars.
An investigation of the polarization variability indicates that 22 objects are
variable at the 3sigma level in our data. All these objects are pre-main
sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the
main sequence, Vega type and post-T Tauri type objects are not variable. The
polarization properties of the variable sources are mostly indicative of the
UXOR-type behaviour; the objects show highest polarization when the brightness
is at minimum. We add seven new objects to the class of UXOR variables (BH Cep,
VX Cas, DK Tau, HK Ori, LkHa 234, KK Oph and RY Ori). The main reason for their
discovery is the fact that our data-set is the largest in its kind, indicating
that many more young UXOR-type pre-main sequence stars remain to be discovered.
The set of Vega-like systems has been investigated for the presence of
intrinsic polarization. As they lack variability, this was done using indirect
methods, and apart from the known case of BD +31.643, the following stars were
found to be strong candidates to exhibit polarization due to the presence of
circumstellar disks: 51 Oph, BD +31.643C, HD 58647 and HD 233517.Comment: A&A accepte
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