1,380 research outputs found
Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?
We have analyzed a number of intense X-ray flares observed in the Chandra
Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula
Cluster (ONC). Analysis of the flare decay allows to determine the size, peak
density and magnetic field of the flaring structure. A total of 32 events (the
most powerful 1% of COUP flares), have sufficient statistics for the analysis.
A broad range of decay times (from 10 to 400 ks) are present in the sample.
Peak flare temperatures are often very high, with half of the flares in the
sample showing temperatures in excess of 100 MK. Significant sustained heating
is present in the majority of the flares. The magnetic structures which are
found, are in a number of cases very long, with semi-lengths up to 10^12 cm,
implying the presence of magnetic fields of hundreds of G extending to
comparable distance from the stellar photosphere. These very large sizes for
the flaring structures ($ >> R_*) are not found in more evolved stars, where,
almost invariably, the same type of analysis results in structures with L <=
R_*. As the majority of young stars in the ONC are surrounded by disks, we
speculate that the large magnetic structures which confine the flaring plasma
are actually the same type of structures which channel the plasma in the
magnetospheric accretion paradigm, connecting the star's photosphere with the
accretion disk.Comment: Accepted to ApJS, COUP special issu
Rotational modulation of X-ray emission in Orion Nebula young stars
We investigate the spatial distribution of X-ray emitting plasma in a sample
of young Orion Nebula Cluster stars by modulation of their X-ray light-curves
due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project
(COUP), is made possible by the exceptional length of the observation: 10 days
of ACIS integration during a time span of 13 days, yielding a total of 1616
detected sources in the 17x17 arcmin field of view. We here focus on a
subsample of 233 X-ray-bright stars with known rotational periods. We search
for X-ray modulation using the Lomb Normalized Periodogram method.
X-ray modulation related to the rotation period is detected in at least 23
stars with periods between 2 and 12 days and relative amplitudes ranging from
20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar
rotation period while in seven cases it is about half that value, possibly due
to the presence of X-ray emitting structures at opposite stellar longitudes.
These results constitute the largest sample of low mass stars in which X-ray
rotational modulation has been observed. The detection of rotational modulation
indicates that the X-ray emitting regions are distributed inhomogeneneously in
longitude and do not extend to distances significantly larger than the stellar
radius. Modulation is observed in stars with saturated activity levels
(L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the
stellar surface with X-ray emitting regions.Comment: 41 pages, 15 figures, ApJS in press. Figure 15 (34 panels) is an
on-line only figure and is not included. A pdf file which includes figure 15
as well as full resolution versions of figure 10 and 11 is available at:
http://www.astropa.unipa.it/~ettoref/COUP_RotMod.pd
The photospheric abundances of active binaries I Detailed analysis of HD 113816 (IS Vir) and HD 119285 (V851 Cen)
The high-resolution optical spectra of the two X-ray active binaries RS CVn
stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed and their Na,
Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined. The analysis of IS Vir
and V851 Cen is performed with three different LTE methods. In the first one,
abundances are derived using measured equivalent widths and Kurucz LTE model
atmospheres as input for the MOOG software package. The input atmospheric
parameters and abundances are iteratively modified until (i) the Fe I
abundances exhibit no trend with excitation potential or equivalent width, (ii)
Fe I and Fe II average abundances are the same and (iii) Fe and Alpha elements
average abundances are consistent with the input values. The second method
follows a similar approach, but relies on the B-V and V-I colour indices to
determine the temperature. The third method relies on fitting the 6162 A Ca I
line wing profiles to derive the surface gravity. The reliability of these
methods is investigated in the context of single line RS CVn stars. It is shown
that the V-I photometric index gives, on a broader sample of stars,
significantly cooler estimates of the effective temperature than the B-V index.
All other approaches give results in good agreement with each other. The
analysis of IS Vir and V851 Cen results in both cases in their primaries being
giant stars of near-solar metallicity. Their parameters as derived with the
first method are respectively Teff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and
Teff = 4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen the
derived iron content is significantly higher than a previous determination in
the literature. Both stars exhibit relative overabundances of several elements
(e.g. Ca) with respect to the solar mix.Comment: 12 Pages, 2 Figures, Accepted for publication in A&
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