1,225 research outputs found
A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. II. A Global View of the Wind Variability
This study is the second part of a survey searching for large-scale
spectroscopic variability in apparently single Wolf-Rayet (WR) stars. In a
previous paper (Paper I), we described and characterized the spectroscopic
variability level of 25 WR stars observable from the northern hemisphere and
found 3 new candidates presenting large-scale wind variability, potentially
originating from large-scale structures named Co-rotating Interaction Regions
(CIRs). In this second paper, we discuss an additional 39 stars observable from
the southern hemisphere. For each star in our sample, we obtained 4-5
high-resolution spectra with a signal-to-noise ratio of ~100 and determined its
variability level using the approach described in Paper I. In total, 10 new
stars are found to show large-scale spectral variability of which 7 present
CIR-type changes (WR 8, WR 44, WR 55, WR 58, WR 61, WR 63, WR 100). Of the
remaining stars, 20 were found to show small-amplitude changes and 9 were found
to show no spectral variability as far as can be concluded from the data in
hand. Also, we discuss the spectroscopic variability level of all single
galactic WR stars that are brighter than v~12.5, and some WR stars with 12.5 <
v <= 13.5; i.e. all the stars presented in our two papers and 4 more stars for
which spectra have already been published in the literature. We find that 23/68
stars (33.8 %) present large-scale variability, but only 12/54 stars (~22.1 %)
are potentially of CIR-type. Also, we find 31/68 stars (45.6 %) that only show
small-scale variability, most likely due to clumping in the wind. Finally, no
spectral variability is detected based on the data in hand for 14/68 (20.6 %)
stars. Interestingly, the variability with the highest amplitude also have the
widest mean velocity dispersion.Comment: 14 pages, 24 figures, 2 tables, Accepted in Ap
Polarimetric modeling of corotating interaction regions (CIRs) threading massive-star winds
Massive star winds are complex radiation-hydrodynamic (sometimes
magnetohydrodynamic) outflows that are propelled by their enormously strong
luminosities. The winds are often found to be structured and variable, but can
also display periodic or quasi-periodic behavior in a variety of wind
diagnostics. The regular variations observed in putatively single stars,
especially in UV wind lines, have often been attributed to corotating
interaction regions (CIRs) like those seen in the solar wind. We present light
curves for variable polarization from winds with CIR structures. We develop a
model for a time-independent CIR based on a kinematical description. Assuming
optically thin electron scattering, we explore the range of polarimetric light
curves that result as the curvature, latitude, and number of CIRs are varied.
We find that a diverse array of variable polarizations result from an
exploration of cases. The net polarization from an unresolved source is
weighted more toward the inner radii of the wind. Given that most massive stars
have relatively fast winds compared to their rotation speeds, CIRs tend to be
conical at inner radii, transitioning to a spiral shape at a few to several
stellar radii in the wind. Winds with a single CIR structure lead to easily
identifiable polarization signatures. By contrast allowing for multiple CIRs,
all emerging from a range of azimuth and latitude positions at the star, can
yield complex polarimetric behavior. Although our model is based on some
simplifying assumptions, it produces qualitative behavior that we expect to be
robust, and this has allowed us to explore a wide range of CIR configurations
that will prove useful for interpreting polarimetric data.Comment: accepted to A&
An Investigation of the Large-scale Variability of the Apparently Single Wolf-Rayet Star WR 1
In recent years, much studies have focused on determining the origin of the
large-scale line-profile and/or photometric patterns of variability displayed
by some apparently single Wolf-Rayet stars, with the existence of an unseen
(collapsed?) companion or of spatially extended wind structures as potential
candidates. We present observations of WR 1 which highlight the unusual
character of the variations in this object. Our narrowband photometric
observations reveal a gradual increase of the stellar continuum flux amounting
to Delta v = 0.09 mag followed by a decline on about the same timescale (3-4
days). Only marginal evidence for variability is found during the 11 following
nights.
Strong, daily line-profile variations are also observed but they cannot be
easily linked to the photometric variations.
Similarly to the continuum flux variations, coherent time-dependent changes
are observed in 1996 in the centroid, equivalent width, and skewness of He II
4686. Despite the generally coherent nature of the variations, we do not find
evidence in our data for the periods claimed in previous studies. While the
issue of a cyclical pattern of variability in WR 1 is still controversial, it
is clear that this object might constitute in the future a cornerstone for our
understanding of the mechanisms leading to the formation of largely anisotropic
outflows in Wolf-Rayet stars.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
A near-IR [Fe II] line imaging survey of supernova remnants in M33
We report on the first near-IR [Fe II] line imaging survey of extragalactic
SNRs.
Observations of a sample of 42 objects drawn from an optically-selected
catalogue of SNRs in M33 provide evidence for a wide range in the [Fe II] 1.644
microns luminosities. This can be understood as being primarily due to
variations in the chemical abundances and density of the local ISM, although
shock conditions may also play a significant role. We briefly discuss how these
results may be used to better calibrate the supernova rate of star-forming
galaxies.Comment: To be published in the proceedings of the conference: ``The
Interstellar Medium in M31 and M33'' (E. M. Berkhuijsen and R. Beck eds.). 4
pages, 1 figur
WR 35a: A new double-lined spectroscopic binary
Aims. We present the first orbital solution for the Wolf-Rayet star, WR 35a, that was discovered by a spectroscopic monitoring of faint WN-type stars. Methods. Spectral features of two different components were identified, and thus a method of disentangling the individual spectra of both components was applied. Radial velocities were determined for each component in the binary system. Results. The orbital solution and component properties of the system were derived. We determined that WR 35a is composed of a WN6 star with a O8.5 V companion orbiting at a 41.90-day period.Facultad de Ciencias Astronómicas y Geofísica
WR 35a: A new double-lined spectroscopic binary
Aims. We present the first orbital solution for the Wolf-Rayet star, WR 35a, that was discovered by a spectroscopic monitoring of faint WN-type stars. Methods. Spectral features of two different components were identified, and thus a method of disentangling the individual spectra of both components was applied. Radial velocities were determined for each component in the binary system. Results. The orbital solution and component properties of the system were derived. We determined that WR 35a is composed of a WN6 star with a O8.5 V companion orbiting at a 41.90-day period.Facultad de Ciencias Astronómicas y Geofísica
A 10-hour period revealed in optical spectra of the highly variable WN8 Wolf-Rayet star WR 123
Aims. What is the origin of the large-amplitude variability in Wolf-Rayet WN8
stars in general and WR123 in particular? A dedicated spectroscopic campaign
targets the ten-hour period previously found in the high-precision photometric
data obtained by the MOST satellite. Methods. In June-August 2003 we obtained a
series of high signal-to-noise, mid-resolution spectra from several sites in
the {\lambda}{\lambda} 4000 - 6940 A^{\circ} domain. We also followed the star
with occasional broadband (Johnson V) photometry. The acquired spectroscopy
allowed a detailed study of spectral variability on timescales from \sim 5
minutes to months. Results. We find that all observed spectral lines of a given
chemical element tend to show similar variations and that there is a good
correlation between the lines of different elements, without any significant
time delays, save the strong absorption components of the Hei lines, which tend
to vary differently from the emission parts. We find a single sustained
periodicity, P \sim 9.8 h, which is likely related to the relatively stable
pulsations found in MOST photometry obtained one year later. In addition,
seemingly stochastic, large-amplitude variations are also seen in all spectral
lines on timescales of several hours to several days.Comment: 6 pages, 4 figures, 2 tables, data available on-line, accepted in A&A
Research Note
Biases in Thorpe-scale estimates of turbulence dissipation. Part I : Assessments from large-scale overturns in oceanographic data
Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2497–2521, doi:10.1175/JPO-D-14-0128.1.Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LT ~ LO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT ~ LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.B.D.M. and S.K.V. gratefully acknowledge the support of the Office of Naval Research under Grants N00014-12-1-0279, N00014-12-1-0282, and N00014-12-1-0938 (Program Manager: Dr. Terri Paluszkiewicz). S.K.V. also acknowledges support of the National Science Foundation under Grant OCE-1151838. L.S.L. acknowledges support for BBTRE by the National Science Foundation by Contract OCE94-15589 and NATRE and IWISE by the Office of Naval Research by Contracts N00014-92-1323 and N00014-10-10315. J.N.M. was supported through Grant 1256620 from the National Science Foundation and the Office of Naval Research (IWISE Project).2016-04-0
The interstellar medium local to HD 10125
Based on an H I line and 408- and 1420-MHz radio continuum survey carried out at the Dominion Radio Astrophysical Observatory (DRAO), the environment of the O star HD 10125 has been studied. In addition, complementary radio continuum images, as well as infrared data of the same region have been analysed. An arc-like structure is found in all the radio continuum data. From the 21 -cm line data, an H I minimum is found in the velocity range ‒27 to ‒32kms-1. Although HD 10125 is not at the centre of the H I cavity, its eccentric position is consistent with the observed stellar proper motion. The H I cavity and the continuum arc-like structure show an excellent morphological correlation. The radio continuum emission has a spectral index (Sv ∼ vα) α = 0.0 ± 0.1, which establishes the thermal nature of the arc-like feature. The dust temperature obtained from the infrared data is higher in the area where the continuum emission is present. A distance of 3 kpc is derived for the star, the H I cavity and the radio continuum structure. We conclude that all the features we have found are physically related to each other. The O star has enough energetic photons to both ionize the surrounding gas and heat up the dust and, through its powerful wind, also sweep up the H I and H II gas.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí
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