88 research outputs found
Quantum statistics of interacting dimer spin systems
The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wurtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important
Scaling laws of solar and stellar flares
In this study we compile for the first time comprehensive data sets of solar
and stellar flare parameters, including flare peak temperatures T_p, flare peak
volume emission measures EM_p, and flare durations t_f from both solar and
stellar data, as well as flare length scales L from solar data. Key results are
that both the solar and stellar data are consistent with a common scaling law
of EM_p ~ T_p^4.7, but the stellar flares exhibit ~250 times higher emission
measures (at the same flare peak temperature). For solar flares we observe also
systematic trends for the flare length scale L(T_p) ~ T_p^0.9 and the flare
duration t_F(T_p) ~ T_p^0.9 as a function of the flare peak temperature. Using
the theoretical RTV scaling law and the fractal volume scaling observed for
solar flares, i.e., V(L) ~ L^2.4, we predict a scaling law of EM_p ~ T_p^4.3,
which is consistent with observations, and a scaling law for electron densities
in flare loops, n_p ~ T_p^2/L ~ T_p^1.1. The RTV-predicted electron densities
were also found to be consistent with densities inferred from total emission
measures, n_p=(EM_p/q_V*V)^1/2, using volume filling factors of q_V=0.03-0.08
constrained by fractal dimensions measured in solar flares. Our results affect
also the determination of radiative and conductive cooling times, thermal
energies, and frequency distributions of solar and stellar flare energies.Comment: 9 Figs., (paper in press, The Astrophsycial Journal
Search for low instability strip variables in the young open cluster NGC 2516
In this paper we revise and complete the photometric survey of the
instability strip of the southern open cluster NGC 2516 published by Antonello
and Mantegazza (1986). No variable stars with amplitudes larger than
were found. However by means of an accurate analysis based on a new statistical
method two groups of small amplitude variables have been disentangled: one with
periods (probably Scuti stars) and one with periods
. The position in the HR diagram and the apparent time-scale may
suggest that the stars of the second group belong to a recently discovered new
class of variables, named Dor variables. They certainly deserve
further study. We also present a comparison between the results of the
photometric survey and the available pointed ROSAT observations of this
cluster.Comment: 7 pages, 2 ps figures. Accepted for P.A.S.
The Corona of the Young Solar Analog EK Draconis
First coronal microwave and new soft X-ray observations of the very active, near-Zero-Age Main-Sequence (ZAMS) dGOe star EK Dra = HD 129333 show that this analog of the young Sun is more luminous in both emissions than most single M-dwarf flare stars. Variations in the 8.4 GHz flux include modulation with the optically determined rotation period of 2.7 days. This result points to a non-uniform filling of the corona with energetic electrons due to an incomplete coverage of the surface with active regions and a source volume that is not concentric with the star. The radio luminosity varying between log L(sub R) = 13.6 and 14.6 (L(sub R) in erg/s/Hz) shows evidence for unpolarized gyrosynchrotron flares, while strongly polarized flares were absent during the observations. This star is the first young, truly solar-like main sequence G star discovered in microwaves. Having just arrived on the main sequence, it conclusively proves that young, solar-like G stars can maintain very high levels of radio emission after their T Tau phase. The X-ray observations were obtained from the ROSAT All-Sky Survey (RASS). The average X-ray luminosity amounts to log L(sub x) = 29.9 (L(sub x) in erg/s). A Raymond-Smith type plasma model fit yields two plasma components at temperatures of 1.9 and 10 MK, with volume emission measures of 1.2 and 2.5 x 10 (exp 52)/cu cm, respectively. The X-ray light curve is significantly variable, with the photon count rate from the cooler plasma being strongly modulated by the rotation period; the emission from the hotter plasma is only weakly variable. Modeling of the source distribution in the stellar corona yields electron densities of the order of 4 x 10(exp 10)/cu cm or higher for the cool plasma component. It indicates that a considerable portion of EK Dra's high X-ray luminosity is due to high-density plasma rather than large emission volume. Parameters for an X-ray flare indicate an electron density of 1.75 x 10(exp 11)/cu cm and a source height of (1-2) x 10(exp 10) cm, compatible with a few times the scale height of the cooler plasma component
Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares
(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet
emission from coronae of magnetically active stars is entirely (or to a large
part) due to the superposition of flares, using an analytic approach to
determine the amplitude distribution of flares in light curves. The
flare-heating hypothesis is motivated by time series that show continuous
variability suggesting the presence of a large number of superimposed flares
with similar rise and decay time scales. We rigorously relate the amplitude
distribution of stellar flares to the observed histograms of binned counts and
photon waiting times, under the assumption that the flares occur at random and
have similar shapes. Applying these results to EUVE/DS observations of the
flaring star AD Leo, we find that the flare amplitude distribution can be
represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our
analytical results agree with existing Monte Carlo results of Kashyap et al.
(2002) and Guedel et al. (2003). The method is applicable to a wide range of
further stochastically bursting astrophysical sources such as cataclysmic
variables, Gamma Ray Burst substructures, X-ray binaries, and spatially
resolved observations of solar flares.Comment: accepted for publication in Ap
Solar Wind and its Evolution
By using our previous results of magnetohydrodynamical simulations for the
solar wind from open flux tubes, I discuss how the solar wind in the past is
different from the current solar wind. The simulations are performed in fixed
one-dimensional super-radially open magnetic flux tubes by inputing various
types of fluctuations from the photosphere, which automatically determines
solar wind properties in a forward manner. The three important parameters which
determine physical properties of the solar wind are surface fluctuation,
magnetic field strengths, and the configuration of magnetic flux tubes.
Adjusting these parameters to the sun at earlier times in a qualitative sense,
I infer that the quasi-steady-state component of the solar wind in the past was
denser and slightly slower if the effect of the magneto-centrifugal force is
not significant. I also discuss effects of magneto-centrifugal force and roles
of coronal mass ejections.Comment: 6 pages, 1 figure, Earth, Planets, & Space in press (based on 5th
Alfven Conference) correction of discussion on a related pape
Coronal density diagnostics with Si X: CHANDRA/LETGS observations of Procyon, Cen A&B, Capella and Eri
Electron density diagnostics based on a line intensity ratio of Si X are
applied to the X-ray spectra of Procyon, Cen A&B, Capella and
Eri measured with the Low Energy Transmission Grating Spectrometer
(LETGS) combined with High-resolution Camera (HRC) on board the {\it Chandra
X-ray Observatory}. The ratio of the intensities of the Si X lines at
50.524 \AA and 50.691 \AA is adopted. A certain of the temperature effect in
appears near the low-density limit region, which is due to the
contamination of Si X line at 50.703 \AA . Using the emission measure
distribution (EMD) model derived by Audard et al. (2001) for Capella and
emissivities calculated with APEC model by Smith et al. (2001), we successfully
estimate contributions of Fe XVI lines at 50.367 \AA and 50.576 \AA (73% and
62%, respectively). A comparison between observed ratios and theoretical
predictions constrains the electron densities (in logarithmic) for Procyon to
be 8.61 cm, while for Cen A&B, Capella and
Eri to be 8.81 cm, 8.60
cm, 9.30 cm and 9.11 cm,
respectively. The comparison of our results with those constrained by the
triplet of He-like carbon shows a good agreement. For normal stars, our results
display a narrow uncertainty, while for active stars, a relatively larger
uncertainty, due to the contamination from Fe XVI lines, is found. Another
possible reason may be that the determination of the continuum level, since the
emission lines of Si X become weak for the active stars. For Eri, an
electron density in the C V forming region was estimated firstly through Si X
emissions.Comment: 24 pages. The Astronomical Journal Accepte
The Unusual X-ray Spectrum of FU Orionis
FU Orionis stars (FUors) are young low-mass eruptive stars that are thought
to be accreting at high rates. They could give rise to soft X-ray emission from
accretion shocks, but their X-ray properties are largely unknown. We report the
first X-ray detection of the prototype star FU Orionis with XMM-Newton. Its
X-ray spectrum is unusual compared to those of classical T Tauri stars (cTTS).
The cool and hot components typically detected in cTTS are present but are seen
through different absorption column densities. A heavily-absorbed hot component
at kT > 5 keV accounts for most of the observed flux and a strong Fe K emission
line is present. Such high temperatures are characteristic of magnetic
processes rather than shock emission. We discuss possible origins of the
unusual spectrum in the context of a complex physical environment that likely
includes disk accretion, a strong wind, magnetic activity, and close binarity.Comment: ApJ, vol. 643, in press. 23 pages, 4 figures, 1 tabl
Some Like it Hot: The X-Ray Emission of The Giant Star YY Mensae
(Abridged abstract) We present an analysis of the X-ray emission of the
rapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton.
Although no obvious flare was detected, the X-ray luminosity changed by a
factor of two between the XMM-Newton and Chandra observations taken 4 months
apart. The coronal abundances and the emission measure distribution have been
derived from three different methods using optically thin collisional
ionization equilibrium models. The abundances show an inverse first ionization
potential (FIP) effect. We further find a high N abundance which we interpret
as a signature of material processed in the CNO cycle. The corona is dominated
by a very high temperature (20-40 MK) plasma, which places YY Men among the
magnetically active stars with the hottest coronae. Lower temperature plasma
also coexists, albeit with much lower emission measure. Line broadening is
reported, which we interpret as Doppler thermal broadening, although rotational
broadening due to X-ray emitting material high above the surface could be
present as well. We use two different formalisms to discuss the shape of the
emission measure distribution. The first one infers the properties of coronal
loops, whereas the second formalism uses flares as a statistical ensemble. We
find that most of the loops in the corona of YY Men have their maximum
temperature equal to or slightly larger than about 30 MK. We also find that
small flares could contribute significantly to the coronal heating in YY Men.
Although there is no evidence of flare variability in the X-ray light curves,
we argue that YY Men's distance and X-ray brightness does not allow us to
detect flares with peak luminosities Lx <= 10^{31} erg/s with current
detectors.Comment: Accepted paper to appear in Astrophysical Journal, issue Nov 10, 2004
(v615). This a revised version. Small typos are corrected. Figure 7 and its
caption and some related text in Sct 7.2 are changed, without incidence for
the conclusion
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