517 research outputs found
Photoionized features in the X-ray spectrum of Ex Hydrae
We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae ( EX Hya). In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely, O VIII lambda 18.97, Mg XII lambda 8.42, Si XIV lambda 6.18, and Fe XVII lambda 16.78. The broad and narrow components have widths of similar to 1600 km s(-1) and similar to 150 km s(-1), respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions such as O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables
Embedded Stellar Populations towards Young Massive Star Formation Regions I. G305.2+0.2
We present deep, wide-field J, H and Ks images taken with IRIS2 on the Anglo
Australian Telescope, towards the massive star formation region G305.2+0.2.
Combined with 3.6, 4.5, 5.8 and 8.0 micron data from the GLIMPSE survey on the
Spitzer Space Telescope, we investigate the properties of the embedded stellar
populations. After removing contamination from foreground stars we separate the
sources based on their IR colour. Strong extended emission in the GLIMPSE
images hampers investigation of the most embedded sources towards the known
sites of massive star formation. However, we find a sizeable population of IR
excess sources in the surrounding region free from these completeness effects.
Investigation reveals the recent star formation activity in the region is more
widespread than previously known.
Stellar density plots show the embedded cluster in the region, G305.24+0.204,
is offset from the dust emission. We discuss the effect of this cluster on the
surrounding area and argue it may have played a role in triggering sites of
star formation within the region. Finally, we investigate the distribution of
IR excess sources towards the cluster, in particular their apparent lack
towards the centre compared with its immediate environs.Comment: 16 pages, 16 figures (significantly size reduced), 2 tables, accepted
MNRA
Haemophilia patients aged 0 - 18 years in the Western Cape
Objectives. To record the number of haemophiliacs aged 0- 18 years in the Western Cape (WC), what event led to the diagnosis, the level of clotting factor, treatment, functional status of their joints and impact of the disease on the family.Design. A prospective study of patients registered with the South African National Haemophilia Registry and new patients, utilising the patients' paediatricians, hospital records, patient and guardian interviews, physicalexamination and provincial nurse haemophilia co-ordinators.Setting. Haemophilia care centres at the three WC academic hospitals, regional hospitals and homes of patients. Two elective medical students, MHH and JJH, collected the information.Subjects. All boys with confirmed haemophilia A or B in the WC.Outcome measures. Events that led to diagnosis, degree of haemophilia, use of clotting factor, functional status, and effect on family. Results. Of 78 patients (59 haemophilia A, 19 haemophilia B) identified, 49 could be studied. Forty-three per cent had severe, 29% moderate and 22% mild disease (6% unknown). Family history was present in 49%, but led to diagnosis in only 12%. The most common first symptoms were subcutaneous and mucosal bleeding. Delay in diagnosis varied from 0 to 9 months. Twenty-nine per cent of guardians were suspected of child abuse. RSA produced clotting factor was used 'on demand' in 73% of patients, for periodic prophylaxis in 20% and as continuous prophylaxis in 7%. Joints were functionally restricted in 43% of patients. The majority of guardians (59%) said the disease had a major impact on the family.Conclusions. The diagnosis of haemophilia in children with a positive family history was often delayed. Haemophilia causes significant morbidity in our patients and their families
Chemical composition of Galactic OB stars II. The fast rotator Z Oph
Z Oph, HD149757, is an O9.5 Vnn star with a very high projected rotational
velocity (vsini >= 340 km\s). It is also a classical runaway star due to its
high proper motion. We perform a quantitative analysis of its optical spectrum
in order to measure important observables of the star such as its mass,
effective temperature, luminosity and He, C, N, and O abundances. Comparing
these observed values to those predicted by the rotating evolutionary models of
the Geneva group we find that none of the two sets of models is capable of
reproducing the characteristics of the star. Nevertheless, due to its runaway
nature, the reason for this discrepancy may be that the star is not the result
of the evolution of a single object, but the product of the evolution of a
close binary system.Comment: Accepted for publication in A&
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
The origin of runaway stars
Milli-arcsecond astrometry provided by Hipparcos and by radio observations
makes it possible to retrace the orbits of some of the nearest runaway stars
and pulsars to determine their site of origin. The orbits of the runaways AE
Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each
other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these
runaways were formed in a binary-binary encounter. The path of the runaway star
zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr
ago, in the young stellar group Upper Scorpius. We propose that this neutron
star is the remnant of a supernova that occurred in a binary system which also
contained zeta Oph, and deduce that the pulsar received a kick velocity of
about 350 km/s in the explosion. These two cases provide the first specific
kinematic evidence that both mechanisms proposed for the production of runaway
stars, the dynamical ejection scenario and the binary-supernova scenario,
operate in nature.Comment: 5 pages, including 2 eps-figures and 1 table, submitted to the ApJ
Letters. The manuscript was typeset using aaste
The origin of the Guitar pulsar
Among a sample of 140 OB associations and clusters, we want to identify
probable parent associations for the Guitar pulsar (PSR B2224+65) which would
then also constrain its age. For this purpose, we are using an Euler-Cauchy
technique treating the vertical component of the galactic potential to
calculate the trajectories of the pulsar and each association into the past. To
include errors we use Monte-Carlo simulations varying the initial parameters
within their error intervals. The whole range of possible pulsar radial
velocities is taken into account during the simulations. We find that the
Guitar pulsar most probably originated from the Cygnus OB3 association ~0.8 Myr
ago inferring a current radial velocity of v_r~-30 km/s, consistent with the
inclination of its bow shock.Comment: 4 pages, 1 figure, 3 table
Neutron stars from young nearby associations the origin of RXJ1605.3+3249
Many neutron stars (NSs) and runaway stars apparently come from the same
regions on the sky. This suggests that they share the same birth places, namely
associations and clusters of young massive stars. To identify NS birth places,
we attempt to and NS-runaway pairs that could be former companions that were
disrupted in a supernova (SN). The remains of recent (<few Myr) nearby (< 150
pc) SNe should still be identifiable by observing the emission of rare
radioisotopes such as 26Al and 60Fe that can also be used as additional
indicators to confirm a possible SN event. We investigated the origin of the
isolated NS RXJ1605.3+3249 and found that it was probably born ~100 pc far from
Earth 0.45 Myr ago in the extended Corona-Australis or Octans associations, or
in Sco OB4 ~1 kpc 3.5 Myr ago. A SN in Octans is supported by the
identification of one to two possible former companions the runaway stars HIP
68228 and HIP 89394, as well as the appearance of a feature in the gamma ray
emission from 26Al decay at the predicted SN place. Both, the progenitor masses
estimated by comparison with theoretical 26Al yields as well as derived from
the life time of the progenitor star, are found to be ~11MSun.Comment: accepted for publication in PASA, special volume Astronomy with
Radioactivities; 10 pages, 4 figures, 5 table
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