151 research outputs found
Simbol-X capability of detecting the non-thermal emission of stellar flares
We investigate the capability of detecting, with Simbol-X, non-thermal
emission during stellar flares, and distinguishing it from hot thermal
emission. We find that flare non-thermal emission is detectable when at least
~20 cts are detected with the CZT detector in the 20-80 keV band. Therefore
Simbol-X will detect the non-thermal emission from some of the X-ray brightest
nearby stars, whether the thermal vs. non-thermal relation, derived for solar
flares, holds.Comment: 2 pages, 2 postscript figures, proceedings of the workshop "Simbol-X:
the hard X-ray universe in focus", to be published in "Memorie of the Italian
Astronomical Society
Modeling X-ray emission from stellar coronae
By extrapolating from observationally derived surface magnetograms of
low-mass stars we construct models of their coronal magnetic fields and compare
the 3D field geometry with axial multipoles. AB Dor, which has a radiative
core, has a very complex field, whereas V374 Peg, which is completely
convective, has a simple dipolar field. We calculate global X-ray emission
measures assuming that the plasma trapped along the coronal loops is in
hydrostatic equilibrium and compare the differences between assuming isothermal
coronae, or by considering a loop temperature profiles. Our preliminary results
suggest that the non-isothermal model works well for the complex field of AB
Dor, but not for the simple field of V374 Peg.Comment: 4 pages, proceedings of Cool Stars 15, St Andrews, July 2008, to be
published in the Conference Proceedings Series of the American Institute of
Physic
Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot
High resolution spectroscopy, providing constraints on plasma motions and
temperatures, is a powerful means to investigate the structure of accretion
streams in CTTS. In particular, the accretion shock region, where the accreting
material is heated to temperatures of a few MK as it continues its inward bulk
motion, can be probed by X-ray spectroscopy. To attempt to detect for the first
time the motion of this X-ray-emitting post-shock material, we searched for a
Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This
test should unveil the nature of this X-ray emitting plasma component in CTTS,
and constrain the accretion stream geometry. We searched for a Doppler shift in
the X-ray emission from TW Hya with two different methods, by measuring the
position of a selected sample of emission lines, and by fitting the whole TW
Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that
the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with
respect to the stellar photosphere. This result definitively confirms that this
X-ray-emitting material originates in the post-shock region, at the base of the
accretion stream, and not in coronal structures. The comparison of the observed
velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic
velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the
footpoints of the accretion streams on TW Hya are located at low latitudes on
the stellar surface. Our results indicate that complex magnetic field
geometries, such as that of TW Hya, permit low-latitude accretion spots.
Moreover, since on TW Hya the redshift of the soft X-ray emission is very
similar to that of the narrow component of the CIV resonance doublet at 1550
Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at
0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version
after language editor corrections; 16 pages, 8 figures, 6 table
On the observability of T Tauri accretion shocks in the X-ray band
Context. High resolution X-ray observations of classical T Tauri stars
(CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13
cm^-3). This emission has been attributed to shock-heated accreting material
impacting onto the stellar surface. Aims. We investigate the observability of
the shock-heated accreting material in the X-ray band as a function of the
accretion stream properties (velocity, density, and metal abundance) in the
case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D
hydrodynamic model describing the impact of an accretion stream onto the
chromosphere, including the effects of radiative cooling, gravity and thermal
conduction. We explore the space of relevant parameters and synthesize from the
model results the X-ray emission in the [0.5-8.0] keV band and in the resonance
lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the
absorption from the chromosphere. Results. The accretion stream properties
influence the temperature and the stand-off height of the shocked slab and its
sinking in the chromosphere, determining the observability of the shocked
plasma. Our model predicts that X-ray observations preferentially detect
emission from low density and high velocity shocked accretion streams due to
the large absorption of dense post-shock plasma. In all the cases examined, the
post-shock zone exhibits quasi-periodic oscillations due to thermal
instabilities, but in the case of inhomogeneous streams and beta<<1, the shock
oscillations are hardly detectable. Conclusions. We suggest that, if accretion
streams are inhomogeneous, the selection effect introduced by the absorption on
observable plasma components may explain the discrepancy between the accretion
rate measured by optical and X-ray data as well as the different densities
measured using different He-like triplets in the X-ray band.Comment: 12 pages, 7 figures. Accepted for publication on A&
The Great Flare of 2021 November 19 on AD Leonis: Simultaneous XMM-Newton and TESS observations
We present a detailed analysis of a superflare on the active M dwarf star AD Leonis. The event presents a rare case of a stellar flare that was simultaneously observed in X-rays (with XMM-Newton) and in the optical (with the Transiting Exoplanet Survey Satellite, TESS). The radiated energy in the 0.2 - 12 keV X-ray band (1.26 +/- 0.01 x 10(33) erg) and the bolometric value (E-F,E-bol=5.57 +/- 0.03 x 10(33) erg) place this event at the lower end of the superflare class. The exceptional photon statistics deriving from the proximity of AD Leo has enabled measurements in the 1 - 8 angstrom GOES band for the peak flux (X1445 class) and integrated energy (E-F,E-GOES=4.30 +/- 0.05 x 10(32) erg), which enables a direct comparison with data on flares from our Sun. From extrapolations of empirical relations for solar flares, we estimate that a proton flux of at least 10(5)cm(-2)s(-1)sr(-1) accompanied the radiative output. With a time lag of 300 s between the peak of the TESS white-light flare and the GOES band flare peak as well as a clear Neupert effect, this event follows the standard (solar) flare scenario very closely. Time-resolved spectroscopy during the X-ray flare reveals, in addition to the time evolution of plasma temperature and emission measure, a temporary increase in electron density and elemental abundances, and a loop that extends into the corona by 13% of the stellar radius (4 x 10(9) cm). Independent estimates of the footprint area of the flare from TESS and XMM-Newton data suggest a high temperature of the optical flare (25000 K), but we consider it more likely that the optical and X-ray flare areas represent physically distinct regions in the atmosphere of AD Leo
CSI 2264: Simultaneous optical and X-ray variability in pre-Main Sequence stars. I: Time resolved X-ray spectral analysis during optical dips and accretion bursts in stars with disks
Pre-main sequence stars are variable sources. In stars with disks, this
variability is related to the morphology of the inner circumstellar region
(<0.1 AU) and that of the photosphere and corona, all impossible to be
spatially resolved with present day techniques. This has been the main
motivation for the Coordinated Synoptic Investigation of NGC 2264. In this
paper, we focus on the stars with disks. We analyze the X-ray spectral
properties extracted during optical bursts and dips in order to unveil the
nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I
observations to search for coherent optical and X-ray flux variability in stars
with disks. Then, stars are analyzed in two different samples. In stars with
variable extinction, we look for a simultaneous increase of optical extinction
and X-ray absorption during the optical dips; in stars with accretion bursts,
we search for soft X-ray emission and increasing X-ray absorption during the
bursts. Results. We find evidence for coherent optical and X-ray flux
variability among the stars with variable extinction. In 9/24 stars with
optical dips, we observe a simultaneous increase of X-ray absorption and
optical extinction. In seven dips, it is possible to calculate the NH/AV ratio
in order to infer the composition of the obscuring material. In 5/20 stars with
optical accretion bursts, we observe increasing soft X-ray emission during the
bursts that we associate to the emission of accreting gas. It is not surprising
that these properties are not observed in all the stars with dips and bursts,
since favorable geometric configurations are required. The observed variable
absorption during the dips is mainly due to dust-free material in accretion
streams. In stars with accretion bursts, we observe on average a larger soft
X-ray spectral component not observed in non accreting stars.Comment: Accepted for publication by Astronomy & Astrophysic
X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock
High spectral resolution X-ray observations of CTTSs demonstrate the presence
of plasma at T~2-3X10^6 K and n_e~10^11-10^13 cm^-3, unobserved in
non-accreting stars. Stationary models suggest that this emission is due to
shock-heated accreting material, but they do not allow to analyze the stability
of such material and its position in the stellar atmosphere. We investigate the
dynamics and the stability of shock-heated accreting material in CTTSs and the
role of the stellar chromosphere in determining the position and the thickness
of the shocked region. We perform 1-D HD simulations of the impact of the
accretion flow onto chromosphere of a CTTS, including the effects of gravity,
radiative losses from optically thin plasma, thermal conduction and a well
tested detailed model of the stellar chromosphere. Here we present the results
of a simulation based on the parameters of the CTTS MP Mus. We find that the
accretion shock generates an hot slab of material above the chromosphere with a
maximum thickness of 1.8X10^9 cm, density n_e~10^11-10^2 cm^-3, temperature
T~3X10^6 K and uniform pressure equal to the ram pressure of the accretion flow
(~450 dyn cm^-2). The base of the shocked region penetrates the chromosphere
and stays where the ram pressure is equal to the thermal pressure. The system
evolves with quasi-periodic instabilities of the material in the slab leading
to cyclic disappearance and re-formation of the slab. For an accretion rate of
~10^-10 M_sun yr^-1, the shocked region emits a time-averaged X-ray luminosity
L_X~7X10^29 erg s^-1, which is comparable to the X-ray luminosity observed in
CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the
simulation matches in detail all the main features of the O VIII and O VII
lines of the star MP Mus.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
The flaring and quiescent components of the solar corona
The solar corona is a template to understand stellar activity. The Sun is a
moderately active star, and its corona differs from active stars: active
stellar coronae have a double-peaked EM(T) with the hot peak at 8-20 MK, while
the non flaring solar corona has one peak at 1-2 MK. We study the average
contribution of flares to the solar EM(T) to investigate indirectly the
hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a
large number of unresolved solar-like flares, and to infer properties on the
flare distribution from nano- to macro-flares. We measure the disk-integrated
time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares
analyzing uninterrupted GOES/XRS light curves over time intervals of one month.
We obtain the EM_Q(T) of quiescent corona for the same time intervals from the
Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary with the solar
cycle, we evaluate them at different phases of the cycle (from Dec. 1991 to
Apr. 1998). Irrespective of the solar cycle phase, EM_F(T) appears like a peak
of the distribution significantly larger than the values of EM_Q(T) for T~5-10
MK. As a result the time-averaged EM(T) of the whole solar corona is
double-peaked, with the hot peak, due to time-averaged flares, located at
temperature similar of that of active stars, but less enhanced. The EM_F(T)
shape supports the hypothesis that the hot EM(T) peak of active coronae is due
to unresolved solar-like flares. If this is the case, quiescent and flare
components should follow different scaling laws for increasing stellar
activity. In the assumption that the heating of the corona is entirely due to
flares, from nano- to macro-flares, then either the flare distribution or the
confined plasma response to flares, or both, are bimodal.Comment: 8 pages, 7 postscript figures, accepted for publication in Astronomy
and Astrophysic
Non-stationary dynamo & magnetospheric accretion processes of the classical T Tauri star V2129 Oph
We report here the first results of a multi-wavelength campaign focussing on
magnetospheric accretion processes of the classical TTauri star (cTTS)
V2129Oph. In this paper, we present spectropolarimetric observations collected
in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT).
Circularly polarised Zeeman signatures are clearly detected, both in
photospheric absorption and accretion-powered emission lines, from time-series
of which we reconstruct new maps of the magnetic field, photospheric brightness
and accretion-powered emission at the surface of V2129Oph using our newest
tomographic imaging tool - to be compared with those derived from our old 2005
June data set, reanalyzed in the exact same way.
We find that in 2009 July, V2129Oph hosts octupolar & dipolar field
components of about 2.1 & 0.9kG respectively, both tilted by about 20deg with
respect to the rotation axis; we conclude that the large-scale magnetic
topology changed significantly since 2005 June (when the octupole and dipole
components were about 1.5 and 3 times weaker respectively), demonstrating that
the field of V2129Oph is generated by a non-stationary dynamo. We also show
that V2129Oph features a dark photospheric spot and a localised area of
accretion-powered emission, both close to the main surface magnetic region
(hosting fields of up to about 4kG in 2009 July). We finally obtain that the
surface shear of V2129Oph is about half as strong as solar.
From the fluxes of accretion-powered emission lines, we estimate that the
observed average logarithmic accretion rate (in Msun/yr) at the surface of
V2129Oph is -9.2+-0.3 at both epochs, peaking at -9.0 at magnetic maximum. It
implies in particular that the radius at which the magnetic field of V2129Oph
truncates the inner accretion disc is 0.93x and 0.50x the corotation radius in
2009 July and 2005 June respectively.Comment: MNRAS in press - 16 pages, 9 figure
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