19 research outputs found
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
The Herschel Digit Survey Of Weak-Line T Tauri Stars: Implications For Disk Evolution And Dissipation
As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key Program, we present Herschel photometry (at 70, 160, 250, 350, and 500 mu m) of 31 weak-line T Tauri star (WTTS) candidates in order to investigate the evolutionary status of their circumstellar disks. Of the stars in our sample, 13 had circumstellar disks previously known from infrared observations at shorter wavelengths, while 18 of them had no previous evidence for a disk. We detect a total of 15 disks as all previously known disks are detected at one or more Herschel wavelengths and two additional disks are identified for the first time. The spectral energy distributions (SEDs) of our targets seem to trace the dissipation of the primordial disk and the transition to the debris disk regime. Of the 15 disks, 7 appear to be optically thick primordial disks, including 2 objects with SEDs indistinguishable from those of typical Classical T Tauri stars, 4 objects that have significant deficit of excess emission at all IR wavelengths, and 1 "pre-transitional" object with a known gap in the disk. Despite their previous WTTS classification, we find that the seven targets in our sample with optically thick disks show evidence for accretion. The remaining eight disks have weaker IR excesses similar to those of optically thin debris disks. Six of them are warm and show significant 24 mu m Spitzer excesses, while the last two are newly identified cold debris-like disks with photospheric 24 mu m fluxes, but significant excess emission at longer wavelengths. The Herschel photometry also places strong constraints on the non-detections, where systems with F-70/F-70,(*) greater than or similar to 5-15 and L-disk/L-* greater than or similar to 10(-3) to 10(-4) can be ruled out. We present preliminary models for both the optically thick and optically thin disks and discuss our results in the context of the evolution and dissipation of circumstellar disks.NASA through JPL/CaltechNASA through the Sagan Fellowship ProgramEuropean Commission PERG06-GA-2009-256513Agence Nationale pour la Recherche (ANR) of France ANR-2010-JCJC-0504-01CFHT 11AH96Astronom
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
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
Detection of the Neupert Effect in the Corona of an RS CVn Binary System by XMM-Newton and the VLA
The RS CVn-type binary Geminorum was observed during a large,
long-duration flare simultaneously with {\it XMM-Newton} and the VLA. The light
curves show a characteristic time dependence that is compatible with the
Neupert effect observed in solar flares: The time derivative of the X-ray light
curve resembles the radio light curve. This observation can be interpreted in
terms of a standard flare scenario in which accelerated coronal electrons reach
the chromosphere where they heat the cool plasma and induce chromospheric
evaporation. Such a scenario can only hold if the amount of energy in the fast
electrons is sufficient to explain the X-ray radiative losses. We present a
plausibility analysis that supports the chromospheric evaporation model.Comment: Accepted for publication in Ap
Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1-1700 A)
We report on the results of the Sun in Time multi-wavelength program (X-rays
to the UV) of solar analogs with ages covering ~0.1-7 Gyr. The chief science
goals are to study the solar magnetic dynamo and to determine the radiative and
magnetic properties of the Sun during its evolution across the main sequence.
The present paper focuses on the latter goal, which has the ultimate purpose of
providing the spectral irradiance evolution of solar-type stars to be used in
the study and modeling of planetary atmospheres. The results from the Sun in
Time program suggest that the coronal X-ray-EUV emissions of the young
main-sequence Sun were ~100-1000 times stronger than those of the present Sun.
Similarly, the transition region and chromospheric FUV-UV emissions of the
young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than
at present. When considering the integrated high-energy emission from 1 to 1200
A, the resulting relationship indicates that the solar high-energy flux was
about 2.5 times the present value 2.5 Gyr ago and about 6 times the present
value about 3.5 Gyr ago (when life supposedly arose on Earth). The strong
radiation emissions inferred should have had major influences on the thermal
structure, photochemistry, and photoionization of planetary atmospheres and
also played an important role in the development of primitive life in the Solar
System. Some examples of the application of the Sun in Time results on
exoplanets and on early Solar System planets are discussed.Comment: 20 pages, 8 figures, accepted for publication in Ap
An XMM-Newton Study of the Coronae of Coronae Borealis
(Abridged) We present results of XMM-Newton observations of the RS CVn binary
Coronae Borealis. The RGS and EPIC MOS2 spectra were simultaneously
fitted with collisional ionization equilibrium plasma models to determine
coronal abundances of various elements. Contrary to the solar first ionization
potential (FIP) effect in which elements with a low FIP are overabundant in the
corona compared to the solar photosphere, and contrary to the ``inverse'' FIP
effect observed in several active RS CVn binaries, coronal abundance ratios in
CrB show a complex pattern as supported by similar findings in the
Chandra HETGS analysis of CrB with a different methodology (Osten et
al. 2003). Low-FIP elements ( eV) have their abundance ratios relative to
Fe consistent with the solar photospheric ratios, whereas high-FIP elements
have their abundance ratios increase with increasing FIP. We find that the
coronal Fe abundance is consistent with the stellar photospheric value,
indicating that there is no metal depletion in CrB. However, we
obtain a higher Fe absolute abundance than in Osten et al. (2003). Except for
Ar and S, our absolute abundances are about 1.5 times larger than those
reported by Osten et al. (2003). However, a comparison of their model with our
XMM-Newton data (and vice versa) shows that both models work adequately in
general. We find, therefore, no preference for one methodology over the other
to derive coronal abundances. Despite the systematic discrepancy in absolute
abundances, our abundance ratios are very close to those obtained by Osten et
al. (2003). Finally, we confirm the measurement of a low density in \ion{O}{7}
( cm), but could not confirm the higher densities
measured in spectral lines formed at higher temperatures.Comment: To appear in Astrophysical Journal (ApJ 10 September 2005, v630 2
issue
XMM-Newton Detection of Hard X-ray Emission in the Nitrogen-Type Wolf-Rayet Star WR110
We have used the excellent sensitivity of XMM-Newton to obtain the first
high-quality X-ray spectrum of a Wolf-Rayet (WR) star which is not known to be
a member of a binary system. Our target, the nitrogen-type star WR 110 (= HD
165688) was also observed and detected with the VLA at four different
frequencies. The radio data are in excellent agreement with that expected for
free-free wind emission. and the ionized mass-loss rate is derived. The X-ray
emission measure distribution shows a dominant contribution from cool plasma at
kT = 0.5 keV (6 MK) which is only weakly absorbed. We argue that this
cool emission originates at hundreds of radii if the wind is spherical and
homogeneous and derive shock velocities and the X-ray filling factor using
radiative shock models. A surprising result is the unambiguous detection of a
hard X-ray component clearly seen in the hard-band images and the spectra. This
hard component accounts for about half of the observed flux and can be
acceptably fitted by a hot optically thin thermal plasma or a power-law model.
If the emission is thermal, then a temperature kT 3 keV is
derived. Such high temperatures are not predicted by current instability-driven
wind shock models. We examine several alternatives and show that the hard
emission could be accounted for by the WR wind shocking onto a close stellar
companion which has so far escaped detection. However, until persuasive
evidence for binarity is found we are left with the intriguing possibility that
the hard X-ray emission is produced entirely by the Wolf-Rayet star.Comment: 2 tables, 7 figure