278 research outputs found
Detection of nonthermal emission from the bow shock of a massive runaway star
The environs of massive, early-type stars have been inspected in recent years
in the search for sites where particles can be accelerated up to relativistic
energies. Wind regions of massive binaries that collide have already been
established as sources of high-energy emission; however, there is a different
scenario for massive stars where strong shocks can also be produced: the
bow-shaped region of matter piled up by the action of the stellar strong wind
of a runaway star interacting with the interstellar medium. We study the
bow-shock region produced by a very massive runaway star, BD+43 3654, to look
for nonthermal radio emission as evidence of a relativistic particle
population. We observed the field of BD+43 3654 at two frequencies, 1.42 and
4.86 GHz, with the Very Large Array (VLA), and obtained a spectral index map of
the radio emission. We have detected, for the first time, nonthermal radio
emission from the bow shock of a massive runaway star. After analyzing the
radiative mechanisms that can be at work, we conclude that the region under
study could produce enough relativistic particles whose radiation might be
detectable by forthcoming gamma-ray instruments, like CTA North.Comment: Accepted in Astronomy and Astrophysics Letter
X-ray Light Curves and Accretion Disk Structure of EX Hydrae
We present X-ray light curves for the cataclysmic variable EX Hydrae obtained
with the Chandra High Energy Transmission Grating Spectrometer and the Extreme
Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the
shape and amplitude of the binary light curve and discuss a new feature: the
phase of the minimum in the binary light curve, associated with absorption by
the bulge on the accretion disk, increases with wavelength. We discuss several
scenarios that could account for this trend and conclude that, most likely, the
ionization state of the bulge gas is not constant, but rather decreases with
binary phase. We also conclude that photoionization of the bulge by radiation
originating from the white dwarf is not the main source of ionization, but that
it is heated by shocks originating from the interaction between the inflowing
material from the companion and the accretion disk. The findings in this paper
provide a strong test for accretion disk models in close binary systems.Comment: 19 pages, 4 figures, accepted for publication in the Ap
Inferring coronal structure from X-ray lightcurves and Doppler shifts: a Chandra study of AB Doradus
The Chandra X-ray observatory monitored the single cool star, AB Doradus,
continuously for a period lasting 88 ksec (1.98 Prot) in 2002 December with the
LETG/HRC-S. The X-ray lightcurve shows rotational modulation, with three peaks
that repeat in two consecutive rotation cycles. These peaks may indicate the
presence of compact emitting regions in the quiescent corona. Centroid shifts
as a function of phase in the strongest line profile, O VIII 18.97 A, indicate
Doppler rotational velocities with a semi-amplitude of 30 +/- 10 km/s. By
taking these diagnostics into account along with constraints on the rotational
broadening of line profiles (provided by archival Chandra HETG Fe XVII and FUSE
Fe XVIII profile) we can construct a simple model of the X-ray corona that
requires two components. One of these components is responsible for 80% of the
X-ray emission, and arises from the pole and/or a homogeneously distributed
corona. The second component consists of two or three compact active regions
that cause modulation in the lightcurve and contribute to the O VIII centroid
shifts. These compact regions account for 16% of the emission and are located
near the stellar surface with heights of less than 0.3R*. At least one of the
compact active regions is located in the partially obscured hemisphere of the
inclined star, while one of the other active regions may be located at 40
degrees. High quality X-ray data such as these can test the models of the
coronal magnetic field configuration as inferred from magnetic Zeeman Doppler
imaging.Comment: 28 pages, 11 figures, accepted by Ap
OB Stars & Stellar Bowshocks in Cygnus-X: A Novel Laboratory Estimating Stellar Mass Loss Rates
We use mid-IR images from the Spitzer Cygnus~X Legacy Survey to search for
stellar bowshocks, a signature of early type "runaway" stars with high space
velocities. We identify ten arc-shaped nebulae containing centrally located
stars as candidate bowshocks. New spectroscopic observations of five stars show
that all are late O to early B dwarfs. Our morphologically selected sample of
bowshock candidates encompasses diverse physical phenomena. Three of the stars
appear to be pre-main-sequence objects on the basis of rising SEDs in the
mid-IR, and their nebulae may be photon-dominated regions (PDRs). Four objects
have ambiguous classification. These may be partial dust shells or bubbles. We
conclude that three of the objects are probable bowshocks, based on their
morphological similarity to analytic prescriptions. Their nebular morphologies
reveal no systematic pattern of orientations that might indicate either a
population of stars ejected from or large-scale hydrodynamic outflows from Cyg
OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by
radial velocity or bowshock techniques is ~0.5%, much smaller than the 8%
estimated among field OB stars. We also obtained a heliocentric radial velocity
for the previously known bowshock star, BD+43\degr3654, of -66.2+/-9.4 km/s,
solidifying its runaway status and implying a space velocity of 77+/-10 km/s.
We use the principles of momentum-driven bowshocks to arrive at a novel method
for estimating stellar mass loss rates. Derived mass loss rates range between
10^-7 and few x10^-6 solar masses/yr for the three O5V -- ~B2V stars identified
as generating bowshocks. These values are at the upper range of, but broadly
consistent with, estimates from other methods. (Abridged)Comment: 49 pages, 19 figures; Accepted for publication in ApJ;
full-resolution color figure version available at
http://physics.uwyo.edu/~chip/Papers/CygXBowshocks; comments invite
Block copolymer micelles as switchable templates for nanofabrication
Block copolymer inverse micelles from polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP) deposited as monolayer films onto surfaces show responsive behavior and are reversibly switchable between two states of different topography and surface chemistry. The as-coated films are in the form of arrays of nanoscale bumps, which can be transformed into arrays of nanoscale holes by switching through exposure to methanol. The use of these micellar films to act as switchable etch masks for the structuring of the underlying material to form either pillars or holes depending on the switching state is demonstrated
Open Clusters IC 4665 and Cr 359 and a Probable Birthplace of the Pulsar PSR B1929+10
Based on the epicyclic approximation, we have simulated the motion of the
young open star clusters IC 4665 and Collinder 359. The separation between the
cluster centers is shown to have been minimal 7 Myr ago, 36 pc. We have
established a close evolutionary connection between IC 4665 and the
Scorpius-Centaurus association -- the separation between the centers of these
structures was pc 15 Myr ago. In addition, the center of IC 4665
at this time was near two well-known regions of coronal gas: the Local Bubble
and the North Polar Spur. The star HIP 86768 is shown to be one of the
candidates for a binary (in the past) with the pulsar PSR B1929+10. At the
model radial velocity of the pulsar km s, a close
encounter of this pair occurs in the vicinity of IC 4665 at a time of -1.1 Myr.
At the same time, using currently available data for the pulsar B1929+10 at its
model radial velocity km s, we show that the hypothesis
of Hoogerwerf et al. (2001) about the breakup of the Oph--B1929+10
binary in the vicinity of Upper Scorpius (US) about 0.9 Myr ago is more
plausible.Comment: 19 pages, 8 figure
Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project
(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determine
the proper motions of ~113500 stars using a 23-year baseline. The proper
motions will use the Hipparcos data, with epoch 1991.25, as first epoch and the
first intermediate-release Gaia astrometry, with epoch ~2014.5, as second
epoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr,
depending on stellar magnitude. Depending on the characteristics of an object,
in particular its distance and velocity, its radial velocity can have a
significant impact on the determination of its proper motion. The impact of
this perspective acceleration is largest for fast-moving, nearby stars. Our
goal is to determine, for each star in the Hipparcos catalogue, the
radial-velocity standard error that is required to guarantee a negligible
contribution of perspective acceleration to the HTPM proper-motion precision.
We employ two evaluation criteria, both based on Monte-Carlo simulations, with
which we determine which stars need to be spectroscopically (re-)measured. Both
criteria take the Hipparcos measurement errors into account. For each star in
the Hipparcos catalogue, we determine the confidence level with which the
available radial velocity and its standard error, taken from the XHIP
compilation catalogue, are acceptable. We find that for 97 stars, the radial
velocities available in the literature are insufficiently precise for a 68.27%
confidence level. We also identify 109 stars for which radial velocities are
currently unknown yet need to be acquired to meet the 68.27% confidence level.
To satisfy the radial-velocity requirements coming from our study will be a
daunting task consuming a significant amount of spectroscopic telescope time.
Fortunately, the follow-up spectroscopy is not time-critical since the HTPM
proper motions can be corrected a posteriori once (improved) radial velocities
become available.Comment: Accepted in A&
High-mass X-ray binaries and OB-runaway stars
High-mass X-ray binaries (HMXBs) represent an important phase in the
evolution of massive binary systems. HMXBs provide unique diagnostics to test
massive-star evolution, to probe the physics of radiation-driven winds, to
study the process of mass accretion, and to measure fundamental parameters of
compact objects. As a consequence of the supernova explosion that produced the
neutron star (or black hole) in these systems, HMXBs have high space velocities
and thus are runaways. Alternatively, OB-runaway stars can be ejected from a
cluster through dynamical interactions. Observations obtained with the
Hipparcos satellite indicate that both scenarios are at work. Only for a
minority of the OB runaways (and HMXBs) a wind bow shock has been detected.
This might be explained by the varying local conditions of the interstellar
medium.Comment: 15 pages, latex (sty file included) with 5 embedded figures (one in
jpg format), to appear in Proc. "Influence of binaries on stellar population
studies", Eds. Vanbeveren, Van Rensberge
The proper motion of the isolated neutron star RX J1605.3+3249
We obtained deep optical imaging of the thermally emitting X-ray bright and
radio-quiet isolated neutron star RX J1605.3+3249 with the Subaru telescope in
1999 and 2003. Together with archival HST images acquired in 2001 these data
reveal a proper motion of mu = 144.5 +/- 13.2 mas/yr. This implies a relatively
high spatial velocity and indicates that the star is unlikely to be re-heated
by accretion of matter from the interstellar medium. Assuming that RX
J1605.3+3249 is a young (10^5-10^6 yr) cooling neutron star, its apparent
trajectory is consistent with a birth in the nearby Sco OB2 OB association at a
location close to that derived for RX J1856.5-3754 and perhaps also to that of
RX J0720.4-3125. This suggests that the X-ray bright part of ROSAT-discovered
isolated neutron stars is dominated by the production of the Sco OB2 complex
which is the closest OB association and a part of the Gould belt. The B and R
magnitudes of the faint optical counterpart did not vary from 1999 to 2003 at B
= 27.22 +/- 0.10. Its B-R colour index of +0.32 +/- 0.17 is significantly
redder than that of other isolated neutron stars and the optical flux lies a
factor 11.5 above the extrapolation of the X-ray blackbody-like spectrum. The
red optical colour reveals the presence of an additional emitting component in
the optical regime over the main neutron star thermal emission. We also
discovered a small elongated Halpha nebula approximately centered on the
neutron star and aligned with the direction of motion. The width of the nebula
is unresolved and smaller than ~ 0.4" for a length of about 1". The shape of
the Balmer emitting nebula is very different from those seen close to other
neutron stars and should be confirmed by follow-up observations. We shortly
discuss the possible mechanisms which could give rise to such a geometry.Comment: 9 pages, 6 figures, resolution of some figures reduced to meet
astro-ph file size restriction, accepted for publication in A&
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