285 research outputs found
Polarization due to rotational distortion in the bright star Regulus
This is the full published article (retrieved from the 6 months post-publication posting on arXiv) including the Methods and Supplementary Information sections: 33 pages, 10 figures, 8 tablesPolarization in stars was first predicted by Chandrasekhar [1] who calculated a substantial linear polarization at the stellar limb for a pure electron-scattering atmosphere. This polarization will average to zero when integrated over a spherical star but could be detected if the symmetry is broken, for example by the eclipse of a binary companion. Nearly 50 years ago, Harrington and Collins [2] modeled another way of breaking the symmetry and producing net polarization - the distortion of a rapidly rotating hot star. Here we report the first detection of this effect. Observations of the linear polarization of Regulus, with two different high-precision polarimeters, range from +42 parts-per-million (ppm) at a wavelength of 741 nm to -22 ppm at 395 nm. The reversal from red to blue is a distinctive feature of rotation-induced polarization. Using a new set of models for the polarization of rapidly rotating stars we find that Regulus is rotating at 96.5(+0.6/-0.8)% of its critical angular velocity for breakup, and has an inclination greater than 76.5 degrees. The rotation axis of the star is at a position angle of 79.5+/-0.7 degrees. The conclusions are independent of, but in good agreement with, the results of previously published interferometric observations of Regulus [3]. The accurate measurement of rotation in early-type stars is important for understanding their stellar environments [4], and course of their evolution [5].Peer reviewedFinal Accepted Versio
The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel
We report new radial velocity observations of GP Vel/HD77581, the optical
companion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more than
two complete orbits of the system, we detect evidence for tidally induced
non-radial oscillations on the surface of GP Vel, apparent as peaks in the
power spectrum of the residuals to the radial velocity curve fit. By removing
the effect of these oscillations (to first order) and binning the radial
velocities, we have determined the semi-amplitude of the radial velocity curve
of GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measured
semi-amplitude of the pulsar's orbit, the mass ratio of the system is
0.081+/-0.005. We are able to set upper and lower limits on the masses of the
component stars as follows. Assuming GP Vel fills its Roche lobe then the
inclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain the
masses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star and
M_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle is
i=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe is
beta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun and
M_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits is
also possible, corresponding to other combinations of i and beta. In addition,
we note that if the zero phase of the radial velocity curve is allowed as a
free parameter, rather than constrained by the X-ray ephemeris, a significantly
improved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shift
of 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of the
radial velocity curve may indicate the presence of an additional radial
velocity component at the orbital period.Comment: Accepted for publication in Astronomy & Astrophysic
Pre-main sequence variable stars in young open cluster NGC 1893
We present results of multi-epoch (fourteen nights during 2007-2010) -band
photometry of the cluster NGC 1893 region to identify photometric variable
stars in the cluster. The study identified a total of 53 stars showing
photometric variability. The members associated with the region are identified
on the basis of spectral energy distribution, two colour diagram and
colour-magnitude diagram. The ages and masses of the majority of
pre-main-sequence sources are found to be 5 Myr and in the range 0.5
4, respectively. These pre-main-sequence
sources hence could be T Tauri stars. We also determined the physical
parameters like disk mass and accretion rate from the spectral energy
distribution of these T Tauri stars. The periods of majority of the T Tauri
stars range from 0.1 to 20 day. The brightness of Classical T Tauri stars is
found to vary with larger amplitude in comparison to Weak line T Tauri stars.
It is found that the amplitude decreases with increase in mass, which could be
due to the dispersal of disks of massive stars.Comment: 15pages, 16 figures and 5 tables, Accepted for publication in MNRA
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Optical and submillimetre observations of Bok globules -- tracing the magnetic field from low to high density
We present optical and submillimetre polarimetry data of the Bok globule CB3
and optical polarimetry data of the Bok globule CB246. We use each set of
polarimetry data to infer the B-field orientation in each of the clouds. The
optical data can only be used in the low density, low extinction edge regions
of clouds. The submillimetre data can only be used in the high column-density,
central regions of the clouds.
It has previously been found that near-infrared polarisation mapping of
background stars does not accurately trace the magnetic field in dense cloud
regions. This may be due to a lack of aligned grains in dense regions. We test
this by comparing the field orientations measured by our two independent
polarimetry methods. We find that the field orientation deduced from the
optical data matches up well with the orientation estimated from the
submillimetre data. We therefore claim that both methods are accurately tracing
the same magnetic field in CB3. Hence, in this case, there must be significant
numbers of aligned dust grains in the high density region, and they do indeed
trace the magnetic field in the submillimetre.
We find an offset of 4014 degrees between the magnetic field orientation
and the short axis of the globule. This is consistent with the mean value of
313 degrees found in our previous work on prestellar cores, even though
CB3 is a protostellar core. Taken together, the six prestellar cores that we
have now studied in this way show a mean offset between magnetic field
orientation and core short axis of 3 degrees, in apparent
contradiction with some models of magnetically dominated star formation.Comment: 8 pages, 3 figures, accepted for publication in MNRA
One Hundred Years of Observations of the Be Star HDE 245770 (the X-ray Binary A0535+26/V725 Tau): The End of an Active Phase
UBV observations of the X-ray binary system A0535+26/V725 Tau at the Crimean
Station of the Sternberg Astronomical Institute in 1980-1998 are presented.
Based on our and published data, we analyze the photometric history of the star
from 1898.Comment: Translated from Pis'ma Astronomicheskii Zhurnal, Vol. 26, No. 1,
2000, pp. 13-2
Chandrasekhar-Kendall functions in astrophysical dynamos
Some of the contributions of Chandrasekhar to the field of
magnetohydrodynamics are highlighted. Particular emphasis is placed on the
Chandrasekhar-Kendall functions that allow a decomposition of a vector field
into right- and left-handed contributions. Magnetic energy spectra of both
contributions are shown for a new set of helically forced simulations at
resolutions higher than what has been available so far. For a forcing function
with positive helicity, these simulations show a forward cascade of the
right-handed contributions to the magnetic field and nonlocal inverse transfer
for the left-handed contributions. The speed of inverse transfer is shown to
decrease with increasing value of the magnetic Reynolds number.Comment: 10 pages, 5 figures, proceedings of the Chandrasekhar Centenary
Conference, to be published in PRAMANA - Journal of Physic
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
3EG J2016+3657: Confirming an EGRET Blazar Behind the Galactic Plane
We recently identified the blazar-like radio source G74.87+1.22 (B2013+370)
as the counterpart of the high-energy gamma-ray source 3EG J2016+3657 in the
Galactic plane. However, since most blazar identifications of EGRET sources are
only probabilistic in quality even at high Galactic latitude, and since there
also exists a population of unidentified Galactic EGRET sources, we sought to
obtain additional evidence to support our assertion that 3EG J2016+3657 is a
blazar. These new observations consist of a complete set of classifications for
the 14 brightest ROSAT X-ray sources in the error circle, of which B2013+370
remains the most likely source of the gamma-rays. We also obtained further
optical photometry of B2013+370 itself which shows that it is variable,
providing additional evidence of its blazar nature. Interestingly, this field
contains, in addition to the blazar, the plerionic supernova remnant CTB 87,
which is too distant to be the EGRET source, and three newly discovered
cataclysmic variables, all five of these X-ray sources falling within 16' of
each other. This illustrates the daunting problem of obtaining complete
identifications of EGRET sources in the Galactic plane.Comment: 16 pages, 6 figures, accepted for publication in The Astrophysical
Journa
X-ray Source Populations in the Region of the Open Clusters NGC 6633 and IC 4756
Using deep exposures (~10^5 s) with the ROSAT HRI, we have performed
flux-limited surveys for X-ray sources in the vicinity of the Hyades-age open
stellar clusters NGC 6633 and IC 4756, detecting 31 and 13 sources
respectively. Our primary aim is to search for so-far unrecognised cluster
members. We propose identifications or classifications (cluster member, field
star, extragalactic field object) for the X-ray sources, based on published
membership lists, and on X-ray:optical flux ratios and optical
colour--magnitude diagrams. Results of simulating the expected X-ray-emitting
source populations are compared with the ROSAT measurements and with the
expected capabilities of the XMM mission. The simulations provide a novel
method of comparing the activity levels of NGC 6633 and IC 4756 with that of
the Hyades. The measurements and simulations confirm that cluster members are
the major class of X-ray emitter in these fields at flux levels f_x >
10^{-14}/erg/cm^2/s (0.1--2.4 keV), contributing ~40 per cent of the total
X-ray sources. We find 6 possible new members in NGC 6633, and 4 candidates in
IC 4756; all require further observation to establish membership probability.Comment: 12 pages, 4 figure
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