1,037 research outputs found
Search for an infrared counterpart of IGR J16358-4756
We report here on near infrared observations of the field around IGR
J16358-4756. The source belongs to the new class of highly absorbed X-ray
binaries discovered by IBIS/INTEGRAL. Our primary goal was to identify the
infrared counterpart of the source, previously suggested to be a LMXB and then
further reclassified as a HMXB. We have made use of Chandra observations of the
source in order to better constrain the number of possible counterparts. Using
the differential photometry technique, in observations spanning a timescale of
1 month, we found no long term variability in our observations. This is
compatible, and we suggest here, that the source is a HMXB.Comment: 5 pages, 2 figures, all styles included; to be published in: "The
Transient Milky Way: a perspective for MIRAX", eds. F. D'Amico, J. Braga, and
R. E. Rothschild, AIP Conference Proceeding
Correlating the interstellar magnetic field with protostellar jets and its sources
This article combines new CCD polarimetric data with previous information
about protostellar objects in a search for correlations involving the
interstellar magnetic field. Specifically, we carried out an optical
polarimetric study of a sample of 28 fields of 10 X 10 arcmin^2 located in the
neighborhood of protostellar jets and randomly spread over the Galaxy. The
polarimetry of a large number of field stars is used to estimate both the
average and dispersion of the interstellar magnetic field (ISMF) direction in
each region. The results of the applied statistical tests are as follows.
Concerning the alignment between the jet direction and the interstellar
magnetic field, the whole sample does not show alignment. There is, however, a
statistically significant alignment for objects of Classes 0 and I. Regarding
the interstellar magnetic field dispersion, our sample presents values slightly
larger for regions containing T Tauri objects than for those harboring younger
protostars. Moreover the ISMF dispersion in regions containing high-mass
objects tends to be larger than in those including only low-mass protostars. In
our sample, the mean interstellar polarization as a function of the average
interstellar extinction in a region reaches a maximum value around 3% for A(V)
= 5, after which it decreases. Our data also show a clear correlation of the
mean value of the interstellar polarization with the dispersion of the
interstellar magnetic field: the larger the dispersion, the smaller the
polarization. Based on a comparison of our and previous results, we suggest
that the dispersion in regions forming stars is larger than in quiescent
regions.Comment: ApJ accepte
Evidence for reduced magnetic braking in polars from binary population models
We present the first population synthesis of synchronous magnetic cataclysmic variables, called polars, taking into account the effect of the white dwarf (WD) magnetic field on angular momentum loss. We implemented the reduced magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary Stellar Evolution (BSE) code recently calibrated for cataclysmic variable (CV) evolution. We then compared separately our predictions for polars and non-magnetic CVs with a large and homogeneous sample of observed CVs from the Sloan Digital Sky Survey. We found that the predicted orbital period distributions and space densities agree with the observations if period bouncers are excluded. For polars, we also find agreement between predicted and observed mass transfer rates, while the mass transfer rates of non-magnetic CVs with periods ≳3 h drastically disagree with those derived from observations. Our results provide strong evidence that the reduced MB model for the evolution of highly magnetized accreting WDs can explain the observed properties of polars. The remaining main issues in our understanding of CV evolution are the origin of the large number of highly magnetic WDs, the large scatter of the observed mass transfer rates for non-magnetic systems with periods ≳3 h, and the absence of period bouncers in observed samples
Assessing the sensitivity of leeches as indicators of water quality
The objective of this work was assessing the sensitivity of leeches to several water quality attributes in lowlands streams.Weused occupancymodelling that account explicitly for detectability, to estimate the influence of four variables (dissolved oxygen, 5-days biochemicals oxygen demand, conductivity, and dissolved inorganic nitrogen) affecting nine species. We described the sensitivity as a change in the occupancy along the range of water quality attributes.We found at least one species of Helobdella in 81% of sites and Helobdella, as genus, was detected along the entire gradient of each attribute. However, differences in the sensitivitywere observed between species. For example, if we analyse the sensitivity of the genus Helobdella to dissolved oxygen, we can say that it is very tolerant.
However, if we analyse the response to dissolved oxygen of each one of the species of Helobdella,wewill realize that H. michaelseni, and H. simplex showed a high occupancy at high levels of dissolved oxygen; while H. hyalina and H. triserialis lineata showed high occupancy at low levels. Describe the sensitivity of the species in terms of occupancy, offers a new methodology to understand how the species behave along a stressor gradient.Instituto de Limnología "Dr. Raúl A. Ringuelet"Facultad de Ciencias Naturales y Muse
Blobs in Wolf-Rayet Winds: Random Photometric and Polarimetric Variability
Some isolated Wolf-Rayet stars present random variability in their optical
flux and polarization. We make the assumption that such variability is caused
by the presence of regions of enhanced density, i.e. blobs, in their envelopes.
In order to find the physical characteristics of such regions we have modeled
the stellar emission using a Monte Carlo code to treat the radiative transfer
in an inhomogeneous electron scattering envelope. We are able to treat multiple
scattering in the regions of enhanced density as well as in the envelope
itself. The finite sizes of the source and structures in the wind are also
taken into account. Most of the results presented here are based on a parameter
study of models with a single blob. The effects due to multiple blobs in the
envelope are considered to a more limited extent. Our simulations indicate that
the density enhancements must have a large geometric cross section in order to
produce the observed photopolarimetric variability. The sizes must be of the
order of one stellar radius and the blobs must be located near the base of the
envelope. These sizes are the same inferred from the widths of the sub-peaks in
optical emission lines of Wolf-Rayet stars. Other early-type stars show random
polarimetric fluctuations with characteristics similar to those observed in
Wolf-Rayet stars, which may also be interpreted in terms of a clumpy wind.
Although the origin of such structures is still unclear, the same mechanism may
be working in different types of hot stars envelopes to produce such
inhomogeneities.Comment: Accepted to ApJ. 17 pages + 6 figure
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