1,090 research outputs found
Cool Stars and Space Weather
Stellar flares, winds and coronal mass ejections form the space weather. They
are signatures of the magnetic activity of cool stars and, since activity
varies with age, mass and rotation, the space weather that extra-solar planets
experience can be very different from the one encountered by the solar system
planets. How do stellar activity and magnetism influence the space weather of
exoplanets orbiting main-sequence stars? How do the environments surrounding
exoplanets differ from those around the planets in our own solar system? How
can the detailed knowledge acquired by the solar system community be applied in
exoplanetary systems? How does space weather affect habitability? These were
questions that were addressed in the splinter session "Cool stars and Space
Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In
this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar
Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur
X-ray emission from T Tauri stars
We have modelled the X-ray emission of T Tauri stars assuming that they have
isothermal, magnetically-confined coronae. These coronae extend outwards until
either the pressure of the hot coronal gas overcomes the magnetic field, or, if
the corona interacts with a disk before this happens, by the action of the disk
itself. This work is motivated by the results of the Chandra Orion Ultradeep
Project (COUP) that show an increase in the X-ray emission measure with
increasing stellar mass. We find that this variation (and its large scatter)
result naturally from the variation in the sizes of the stellar coronae. The
reduction in the magnitude of the X-ray emission due to the presence of a disk
stripping the outer parts of the stellar corona is most pronounced for the
lower mass stars. The higher mass stars with their greater surface gravities
have coronae than typically do not extend out as far as the inner edge of the
disk and so are less affected by it. For these stars, accretion takes place
along open field lines that connect to the disk. By extrapolating surface
magnetograms of young main sequence stars we have examined the effect on the
X-ray emission of a realistic degree of field complexity. We find densities
consistent with estimates from modelling of individual flares. A simple dipole
field in contrast gives densities typically an order of magnitude less. We
suggest that T Tauri stars have coronal fields that are slightly more extended
than their main sequence counterparts, but not as extended as a purely dipolar
fields.Comment: 12 pages, 13 figures, to appear in Monthly Notices of the Royal
Astronomical Societ
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
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
Multi-wavelength observing of a forming solar-like star
V2129 Oph is a 1.35 solar mass classical T Tauri star, known to possess a
strong and complex magnetic field. By extrapolating from an observationally
derived magnetic surface map, obtained through Zeeman-Doppler imaging, models
of V2129 Oph's corona have been constructed, and used to make predictions
regarding the global X-ray emission measure, the amount of modulation of X-ray
emission, and the density of accretion shocks. In late June 2009 we will under
take an ambitious multi-wavelength, multi-observing site, and near
contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray,
spectropolarimetric and photometric monitoring. This will allow the validity of
the 3D field topologies derived via field extrapolation to be determined.Comment: 4 pages, proceedings of the 3rd MSSL workshop on High Resolution
X-ray Spectroscopy: towards IX
Variable X-ray emission from the accretion shock in the classical T Tauri star V2129 Ophiuchi
Context. The soft X-ray emission from high density plasma observed in several CTTS is usually associated with the accretion process. However, it is still unclear whether this high density âcoolâ plasma is heated in the accretion shock, or if it is coronal plasma fed or modified by the accretion process.
Aims. We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph. In this paper, we analyze Chandra grating spectrometer data and attempt to correlate the observed X-ray emitting plasma components with the characteristics of the accretion process and the stellar magnetic field constrained by simultaneous optical observations.
Methods. We analyze a 200 ks Chandra/HETGS observation, subdivided into two 100 ks segments, of the CTTS V2129 Oph. For the two observing segments corresponding to two different phases within one stellar rotation, we measure the density of the cool plasma component and the emission measure distribution.
Results. The X-ray emitting plasma covers a wide range of temperatures: from 2 up to 34 MK. The cool plasma component of V2129 Oph (T â 3â4 MK) varies between the two segments of the Chandra observation: during the first observing segment high density plasma (log N_c = 12.1_(-1.1)^(+0.6)) with high EM at ~3â4 MK is present, whereas, during the second segment, this plasma component has lower EM and lower density (logâN_e 3 R_â).
Conclusions. Our observation provides additional confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock. The variability of this cool plasma component on V2129 Oph may be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation. In particular, during the first time interval a direct view of the shock region is possible, while, during the second, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing the accretion-driven X-rays
The coronal structure of AB Dor determined from contemporaneous Doppler imaging and X-ray spectroscopy
We obtain contemporaneous observations of the surface and corona of AB Dor
using ground-based circularly polarised spectra from the Anglo-Australian
Telescope and X-ray data from the Chandra satellite. The ground-based data are
used to construct surface magnetic field maps, which are extrapolated to
produce detailed models of the quiescent corona. The X-ray data serve as a new
test for the validity of these coronal models.
The high coronal density and complex multi-polar magnetic field indicate a
compact X-ray corona, which is concentrated close to the surface, with a
height, H~0.3-0.4R*. There is also significant correlation between the surface
and coronal active region locations. At this epoch AB Dor appears to possess
one very large active longitude region; displaying enhanced activity in the
form of large dark spots, strong magnetic fields and chromospheric emission.
Finally, the level of rotational modulation and shape of the X-ray lightcurve
depend on the distribution of magnetic field in the obscured hemisphere. The
models that best reproduce the rotational modulation observed in the
contemporaneous Chandra X-ray lightcurve and spectra require the magnetic field
in the obscured hemisphere to be of the same polarity as that in the observed
hemisphere. The Sun shows different behaviour, with the leading polarity
reversed in the opposite hemisphere. The X-ray observations provide a unique
constraint on the magnetic structure in the obscured hemisphere.Comment: 17 pages, 14 figures, accepted by MNRAS. This version has cropped
figures. For a preprint with the original figures please go to
http://star-www.st-and.ac.uk/~gajh/papers0
Antimicrobial susceptibility of bacteria isolated from newborns with suspected or confirmed necrotising enterocolitis
Aim. This audit aimed to identify which bacteria were associated with necrotising enterocolitis (NEC) and determine their antibiotic sensitivities. Methods. A retrospective audit of all infants with a diagnosis of suspected NEC or confirmed NEC and a positive culture (blood/faeces/operative specimen or vascular access device) between January 2000 and September 2007 was performed. Results. Ninety nine infants had a diagnosis of suspected (45) or confirmed NEC (54). Seventeen patients had suspected (5) or confirmed (12) NEC and a positive culture result. 12 babies had positive blood cultures associated with their NEC. Only 4 of the 12 cases of NEC with a positive blood culture received adequate first line cover for their subsequently identified infecting organism. Conclusions. Due to the limitations of this study we are unable to make general recommendations on the first line antibiotic choice for babies with suspected or confirmed NEC. Our current regime of Ampicillin, Gentamicin and Metronidazole failed to adequately treat 8 of the 12 organisms subsequently isolated in blood cultures. Only the combination of Vancomycin and Meropenem would have adequately treated all the bacteria identified. The concern with this approach is the possible emergence of multi drug resistant bacteria
Computational analysis of anti-HIV-1 antibody neutralization panel data to identify potential functional epitope residues
Advances in single-cell antibody cloning methods have led to the identification of a variety of broadly neutralizing antiâHIV-1 antibodies. We developed a computational tool (Antibody Database) to help identify critical residues on the HIV-1 envelope protein whose natural variation affects antibody activity. Our simplifying assumption was that, for a given antibody, a significant portion of the dispersion of neutralization activity across a panel of HIV-1 strains is due to the amino acid identity or glycosylation state at a small number of specific sites, each acting independently. A model of an antibodyâs neutralization IC_(50) was developed in which each site contributes a term to the logarithm of the modeled IC_(50). The analysis program attempts to determine the set of rules that minimizes the sum of the residuals between observed and modeled IC_(50) values. The predictive quality of the identified rules may be assessed in part by whether there is support for rules within individual viral clades. As a test case, we analyzed antibody 8ANC195, an anti-glycoprotein gp120 antibody of unknown specificity. The model for this antibody indicated that several glycosylation sites were critical for neutralization. We evaluated this prediction by measuring neutralization potencies of 8ANC195 against HIV-1 in vitro and in an antibody therapy experiment in humanized mice. These experiments confirmed that 8ANC195 represents a distinct class of glycan-dependent antiâHIV-1 antibody and validated the utility of computational analysis of neutralization panel data
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