641 research outputs found
Multi-wavelength observations of Proxima Centauri
We report simultaneous observations of the nearby flare star Proxima Centauri
with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and
X-ray observations cover the star's quiescent state, as well as its flaring
activity and allow us to probe the stellar atmospheric conditions from the
photosphere into the chromosphere, and then the corona during its different
activity stages. Using the X-ray data, we investigate variations in coronal
densities and abundances and infer loop properties for an intermediate-sized
flare. The optical data are used to investigate the magnetic field and its
possible variability, to construct an emission line list for the chromosphere,
and use certain emission lines to construct physical models of Proxima
Centauri's chromosphere.
We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA
during the more active states of Proxima Centauri. For the intermediate flare,
we find two secondary flare events that may originate in neighbouring loops,
and discuss the line asymmetries observed during this flare in H i, He i, and
Ca ii lines. The high time-resolution in the H alpha line highlights strong
temporal variations in the observed line asymmetries, which re-appear during a
secondary flare event. We also present theoretical modelling with the stellar
atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&
X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri
Using the XMM-Newton X-ray observatory to monitor the nearest star to the
Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically
active star ever observed. Correlated X-ray and optical variability provide
strong support for coronal energy and mass supply by a nearly continuous
sequence of rapid explosive energy releases. Variable emission line fluxes were
observed in the He-like triplets of OVII and NeIX during a giant flare. They
give direct X-ray evidence for density variations, implying densities between
2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the
volume of the line-emitting plasma. We discuss the data in the context of the
chromospheric evaporation scenario.Comment: 10 pages, 2 figures, accepted by The Astrophysical Journal, Letters;
improved calculations of radiative loss of cool plasma (toward end of paper
S-particles at their naturalness limits
We draw attention on a particular configuration of supersymmetric particle
masses, motivated by naturalness and flavour considerations. All its relevant
phenomenological properties for the LHC are described in terms of a few
physical parameters, irrespective of the underlying theoretical model. This
allows a simple characterization of its main features, useful to define a
strategy for its discovery.Comment: 13 pages, 8 figures, added reference
L-band (3.5 micron) IR-excess in massive star formation, II. RCW 57/NGC 3576
We present a JHKL survey of the massive star forming region RCW 57 (NGC 3576)
based on L-band data at 3.5 micron taken with SPIREX (South Pole Infrared
Explorer), and 2MASS JHK data at 1.25-2.2 micron. This is the second of two
papers, the first one concerning a similar JHKL survey of 30 Doradus.
Colour-colour and colour-magnitude diagrams are used to detect sources with
infrared excess. This excess emission is interpreted as coming from
circumstellar disks, and hence gives the cluster disk fraction (CDF). Based on
the CDF and the age of RCW 57, it is possible to draw conclusions on the
formation and early evolution of massive stars. The infrared excess is detected
by comparing the locations of sources in JHKL colour-colour and L vs. (K-L)
colour-magnitude diagrams to the reddening band due to interstellar extinction.
A total of 251 sources were detected. More than 50% of the 209 sources included
in the diagrams have an infrared excess. Comparison with other JHKL surveys,
including the results on 30 Doradus from the first paper, support a very high
initial disk fraction (>80%) even for massive stars, although there is an
indication of a possible faster evolution of circumstellar disks around high
mass stars. 33 sources only found in the L-band indicate the presence of
heavily embedded, massive Class I protostars. We also report the detection of
diffuse PAHs emission throughout the RCW 57 region.Comment: 15 pages, 13 figure
Binaries in star clusters and the origin of the field stellar population
Many, possibly most, stars form in binary and higher-order multiple systems.
Therefore, the properties and frequency of binary systems provide strong clues
to the star-formation process, and constraints on star-formation models.
However, the majority of stars also form in star clusters in which the birth
binary properties and frequency can be altered rapidly by dynamical processing.
Thus, we almost never see the birth population, which makes it very difficult
to know if star formation (as traced by binaries, at least) is universal, or if
it depends on environment. In addition, the field population consists of a
mixture of systems from different clusters which have all been processed in
different ways.Comment: 16 pages, no figures. To appear as invited review article in a
special issue of the Phil. Trans. Royal Soc. A: Ch. 8 "Star clusters as
tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer
reviewed. LaTeX, requires rspublic.cls style fil
Determining the Sign of the Z-Penguin Amplitude
We point out that the precision measurements of the pseudo observables R_b^0,
A_b, and A_FB^0,b performed at LEP and SLC suggest that in models with
minimal-flavor-violation the sign of the Z-penguin amplitude is identical to
the one present in the standard model. We determine the allowed range for the
non-standard contribution to the Inami-Lim function C and show by analyzing
possible scenarios with positive and negative interference of standard model
and new physics contributions, that the derived bound holds in each given case.
Finally, we derive lower and upper limits for the branching ratios of K^+ ->
pi^+ nu nubar, K_L -> pi^0 nu nubar, K_L -> mu^+ mu^-, B -> X_d,s nu nubar, and
B_d,s -> mu^+ mu^- within constrained minimal-flavor-violation making use of
the wealth of available data collected at the Z-pole.Comment: 20 pages, 5 pdf figures, 5 tables, uses pdflatex; further typos
corrected, matches PRD versio
Recent developments in radiative B decays
We report on recent theoretical progress in radiative B decays. We focus on a
calculation of logarithmically enhanced QED corrections to the branching ratio
and forward-backward asymmetry in the inclusive rare decay anti-B --> X(s) l+
l-, and present the results of a detailed phenomenological analysis. We also
report on the calculation of NNLO QCD corrections to the inclusive decay anti-B
--> X(s) gamma. As far as exclusive modes are concerned we consider
transversity amplitudes and the impact of right-handed currents in the
exclusive anti-B --> K^* l+ l- decay. Finally, we state results for exclusive B
--> V gamma decays, notably the time-dependent CP-asymmetry in the exclusive B
--> K^* gamma decay and its potential to serve as a so-called ``null test'' of
the Standard Model, and the extraction of CKM and unitarity triangle parameters
from B --> (rho,omega) gamma and B --> K^* gamma decays.Comment: 5 pages, 2 figures. Accepted for publication in the proceedings of
International Europhysics Conference on High Energy Physics (EPS-HEP2007),
Manchester, England, 19-25 Jul 200
The effect of flares on total solar irradiance
Flares are powerful energy releases occurring in stellar atmospheres. Solar
flares, the most intense energy bursts in the solar system, are however hardly
noticeable in the total solar luminosity. Consequently, the total amount of
energy they radiate 1) remains largely unknown and 2) has been overlooked as a
potential contributor to variations in the Total Solar Irradiance (TSI), i.e.
the total solar flux received at Earth. Here, we report on the detection of the
flare signal in the TSI even for moderate flares. We find that the total energy
radiated by flares exceeds the soft X-ray emission by two orders of magnitude,
with an important contribution in the visible domain. These results have
implications for the physics of flares and the variability of our star.Comment: accepted in Nature Physic
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