531 research outputs found
Linear spectro-polarimetry: a new diagnostic tool for the classification and characterisation of asteroids
We explore the use of spectro-polarimetry as a remote sensing tool for
asteroids in addition to traditional reflectance measurements. In particular we
are interested in possible relationships between the wavelength-dependent
variation of linear polarization and the properties of the surfaces, including
albedo and composition.
We have obtained optical spectro-polarimetric measurements of a dozen
asteroids of different albedo and taxonomic classes and of two small regions at
the limb of the Moon.
We found that objects with marginally different relative reflectance spectra
(in the optical) may have totally different polarization spectra. This suggests
that spectro-polarimetry may be used to refine the classification of asteroids.
We also found that in some cases the Umov law may be violated, that is, in
contrast to what is expected from basic physical considerations, the fraction
of linear polarization and the reflectance may be positively correlated. In
agreement with a few previous studies based on multi-colour broadband
polarimetry, we found that the variation of linear polarization with wavelength
and with phase-angle is correlated with the albedo and taxonomic class of the
objects. Finally, we have serendipitously discovered that spinel-rich asteroid
(599) Luisa, located very close to the Watsonia family, is a member of the rare
class of Barbarian asteroids.
We suggest that future modelling attempts of the surface structure of
asteroids should be aimed at explaining both reflectance and polarization
spectra.Comment: Accepted for publication in MNRAS Letter
Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD2047+372
Magnetic fields have been detected in several hundred white dwarfs, with
strengths ranging from a few kG to several hundred MG. Only a few of the known
fields have a mean magnetic field modulus below about 1 MG.
We are searching for new examples of magnetic white dwarfs with very weak
fields, and trying to model the few known examples. Our search is intended to
be sensitive enough to detect fields at the few kG level.
We have been surveying bright white dwarfs for very weak fields using
spectropolarimeters at the Canada-France-Hawaii telescope, the William Herschel
telescope, the European Southern Observatory, and the Russian Special
Astrophysical Observatory. We discuss in some detail tests of the WHT
spectropolarimeter ISIS using the known magnetic strong-field Ap star HD 215441
(Babcock's star) and the long-period Ap star HD 201601 (gamma Equ).
We report the discovery of a field with a mean field modulus of about 57 kG
in the white dwarf LTT 16093 = WD2047+372. The field is clearly detected
through the Zeeman splitting of Halpha seen in two separate circularly
polarised spectra from two different spectropolarimeters. Zeeman circular
polarisation is also detected, but only barely above the 3 sigma level.
The discovery of this field is significant because it is the third weakest
field ever unambiguously discovered in a white dwarf, while still being large
enough that we should be able to model the field structure in some detail with
future observations
Monitoring and modelling of white dwarfs with extremely weak magnetic fields
Magnetic fields are detected in a few percent of white dwarfs. The number of
such magnetic white dwarfs known is now some hundreds. Fields range in strength
from a few kG to several hundred MG. Almost all the known magnetic white dwarfs
have a mean field modulus >= 1 MG. We are trying to fill a major gap in
observational knowledge at the low field limit (<= 200 kG) using circular
spectro-polarimetry.
In this paper we report the discovery and monitoring of strong, periodic
magnetic variability in two previously discovered "super-weak field" magnetic
white dwarfs, WD2047+372 and WD2359-434. WD2047+372 has a mean longitudinal
field that reverses between about -12 and +15 kG, with a period of 0.243 d,
while its mean field modulus appears nearly constant at 60 kG. The observations
can be intepreted in terms of a dipolar field tilted with respect to the
stellar rotation axis. WD2359-434 always shows a weak positive longitudinal
field with values between about 0 and +12 kG, varying only weakly with stellar
rotation, while the mean field modulus varies between about 50 and 100 kG. The
rotation period is found to be 0.112 d using the variable shape of the Halpha
line core, consistent with available photometry. The field of this star appears
to be much more complex than a dipole, and is probably not axisymmetric.
Available photometry shows that WD2359-434 is a light variable with an
amplitude of only 0.005 mag, our own photometry shows that if WD2047+372 is
photometrically variable, the amplitude is below about 0.01 mag.
These are the first models for magnetic white dwarfs with fields below about
100 kG based on magnetic measurements through the full stellar rotation. They
reveal two very different magnetic surface configurations, and that, contrary
to simple ohmic decay theory, WD2359-434 has a much more complex surface field
than the much younger WD2047+372.Comment: Accepted for publication in Astronomy & Astrophysic
Is the Eureka cluster a collisional family of Mars Trojan asteroids?
We explore the hypothesis that the Eureka family of sub-km asteroids in the
L5 region of Mars could have formed in a collision. We estimate the size
distribution index from available information on family members; model the
orbital dispersion of collisional fragments; and carry out a formal calculation
of the collisional lifetime as a function of size. We find that, as initially
conjectured by Rivkin et al (2003), the collisional lifetime of objects the
size of (5261) Eureka is at least a few Gyr, significantly longer than for
similar-sized Main Belt asteroids. In contrast, the observed degree of orbital
compactness is inconsistent with all but the least energetic family-forming
collisions. Therefore, the family asteroids may be ejecta from a cratering
event sometime in the past ~1 Gyr if the orbits are gradually dispersed by
gravitational diffusion and the Yarkovsky effect (Cuk et al, 2015). The
comparable sizes of the largest family members require either negligible target
strength or a particular impact geometry under this scenario (Durda et al,
2007; Benavidez et al, 2012). Alternatively, the family may have formed by a
series of YORP-induced fission events (Pravec.et.al, 2010). The shallow size
distribution of the family is similar to that of small MBAs (Gladman et al,
2009) interpreted as due to the dominance of this mechanism for
Eureka-family-sized asteroids (Jacobson et al, 2014). However, our population
index estimate is likely a lower limit due to the small available number of
family asteroids and observational incompleteness. Future searches for fainter
family members, further observational characterisation of the known Trojans'
physical properties as well as orbital and rotational evolution modelling will
help distinguish between different formation models.Comment: 3 Tables, 13 Figures, Accepted for publication in Icaru
A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate-mass and high-mass stars is
fundamentally a mystery. Clues toward solving this basic astrophysical problem
can likely be found at the pre-main sequence (PMS) evolutionary stage. With
this work, we perform the largest and most sensitive search for magnetic fields
in pre-main sequence Herbig Ae/Be (HAeBe) stars. Sixty-eight observations of 50
HAeBe stars have been obtained in circularly polarised light using the FORS1
spectropolarimeter at the ESO VLT. An analysis of both Balmer and metallic
lines reveals the possible presence of weak longitudinal magnetic fields in
photospheric lines of two HAeBe stars, HD 101412 and BF Ori. The intensity of
the longitudinal fields detected in HD 101412 and BF Ori suggest that they
correspond to globally-ordered magnetic fields with surface intensities of
order 1 kG. Monte Carlo simulations of the longitudinal field measurements of
the undetected stars allow us to place an upper limits of about 300 G on the
general presence of aligned magnetic dipole magnetic fields, and of about 500 G
on perpendicular dipole fields. We find that the observed bulk incidence of
magnetic HAeBe stars in our sample is 8-12%, in good agreement with that of
magnetic main sequence stars of similar masses. We also find that the rms
longitudinal field intensity of magnetically-detected HAeBe stars is similar to
that of Ap stars and consistent with magnetic flux conservation during stellar
evolution. These results are all in agreement with the hypothesis that the
magnetic fields of main sequence Ap/Bp stars are fossils, which already exist
within the stars at the pre-main sequence stage. Finally, we explore the
ability of our new magnetic data to constrain magnetospheric accretion in
Herbig Ae/Be stars.Comment: Accepted by Monthly Notices of the Royal Astronomical Society, 2007
January 11. Received 2007 January 11; in original form 2006 August 18. The
paper contains 18 pages, 11 figures and 2 table
The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?
All known Galactic Of?p stars have been shown to host strong, organized,
magnetic fields. Recently, five Of?p stars have been discovered in the
Magellanic Clouds. They posses photometric \citep{Naze} and spectroscopic
\citep{Walborn} variability compatible with the Oblique Rotator Model (ORM).
However, their magnetic fields have yet to be directly detected. We have
developed an algorithm allowing for the synthesis of photometric observables
based on the Analytic Dynamical Magnetosphere (ADM) model of \citet{Owocki}. We
apply our model to OGLE photometry in order to constrain their magnetic
geometries and surface dipole strengths. We predict that the field strengths
for some of these candidate extra-Galactic magnetic stars may be within the
detection limits of the FORS2 instrument.Comment: 5 pages, 2 figure
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