1,302 research outputs found
On a mechanism for enhancing magnetic activity in tidally interacting binaries
We suggest a mechanism for enhancing magnetic activity in tidally interacting
binaries. We suppose that the deviation of the primary star from spherical
symmetry due to the tidal influence of the companion leads to stellar pulsation
in its fundamental mode. It is shown that stellar radial pulsation amplifies
torsional Alfv{\'e}n waves in a dipole-like magnetic field, buried in the
interior, according to the recently proposed swing wave-wave interaction
(Zaqarashvili 2001). Then amplified Alfv{\'e}n waves lead to the onset of
large-scale torsional oscillations, and magnetic flux tubes arising towards the
surface owing to magnetic buoyancy diffuse into the atmosphere producing
enhanced chromospheric and coronal emission.Comment: Accepted in Ap
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
FUSE ultraviolet spectra of 8 giant and supergiant stars reveal that high
temperature (3 X 10^5 K) atmospheres are common in luminous cool stars and
extend across the color-magnitude diagram from Alpha Car (F0 II) to the cool
giant Alpha Tau (K5 III). Emission present in these spectra includes
chromospheric H-Lyman Beta, Fe II, C I, and transition region lines of C III, O
VI, Si III, Si IV. Emission lines of Fe XVIII and Fe XIX signaling temperatures
of ~10^7 K and coronal material are found in the most active stars, Beta Cet
and 31 Com. A short-term flux variation, perhaps a flare, was detected in Beta
Cet during our observation. Stellar surface fluxes of the emission of C III and
O VI are correlated and decrease rapidly towards the cooler stars, reminiscent
of the decay of magnetically-heated atmospheres. Profiles of the C III (977A)
lines suggest that mass outflow is underway at T~80,000 K, and the winds are
warm. Indications of outflow at higher temperatures (3 X 10^5K) are revealed by
O VI asymmetries and the line widths themselves. High temperature species are
absent in the M-supergiant Alpha Ori. Narrow fluorescent lines of Fe II appear
in the spectra of many giants and supergiants, apparently pumped by H Lyman
Alpha, and formed in extended atmospheres. Instrumental characteristics that
affect cool star spectra are discussed.Comment: Accept for publication in The Astrophysical Journal; 22 pages of
text, 23 figures and 8 table
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
HST Observations of Chromospheres in Metal Deficient Field Giants
HST high resolution spectra of metal-deficient field giants more than double
the stars in previous studies, span about 3 magnitudes on the red giant branch,
and sample an abundance range [Fe/H]= -1 to -3. These stars, in spite of their
age and low metallicity, possess chromospheric fluxes of Mg II (2800 Angstrom)
that are within a factor of 4 of Population I stars, and give signs of a
dependence on the metal abundance at the lowest metallicities. The Mg II k-line
widths depend on luminosity and correlate with metallicity. Line profile
asymmetries reveal outflows that occur at lower luminosities (M_V = -0.8) than
detected in Ca K and H-alpha lines in metal-poor giants, suggesting mass
outflow occurs over a larger span of the red giant branch than previously
thought, and confirming that the Mg II lines are good wind diagnostics. These
results do not support a magnetically dominated chromosphere, but appear more
consistent with some sort of hydrodynamic, or acoustic heating of the outer
atmospheres.Comment: 36 pages, 12 figures, 7 tables, and accepted for publication in The
Astronomical Journa
Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J1655--40)
We have obtained intermediate resolution optical spectra of the black-hole
candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical
active state (R~15.05). We measure the radial velocity curve of the secondary
star and obtain a semi-amplitude of 279+/-10 km/s; a value which is 30 per cent
larger than the value obtained when the source is in quiescence. Our large
value for K_2 is consistent with 60 +9,-7 per cent of the secondary star's
surface being heated; compared to 35 per cent, which is what one would expect
if only the inner face of the secondary star were irradiated. Effects such as
irradiation-induced flows on the secondary star may be important in explaining
the observed large value for K_2.Comment: 5 pages, 2 figures, accepted by MNRA
Magnetic Flux Cancellation in Ellerman Bombs
Ellerman Bombs (EBs) are often found to be co-spatial with bipolar photospheric magnetic fields. We use Hα
imaging spectroscopy along with Fe I 6302.5 Ă
spectropolarimetry from the Swedish 1 m Solar Telescope (SST),
combined with data from the Solar Dynamic Observatory, to study EBs and the evolution of the local magnetic
fields at EB locations. EBs are found via an EB detection and tracking algorithm. Using NICOLE inversions of the
spectropolarimetric data, we find that, on average, (3.43 ± 0.49) à 1024 erg of stored magnetic energy disappears
from the bipolar region during EB burning. The inversions also show flux cancellation rates of 1014â1015 Mx sâ1
and temperature enhancements of 200 K at the detection footpoints. We investigate the near-simultaneous flaring
of EBs due to co-temporal flux emergence from a sunspot, which shows a decrease in transverse velocity when
interacting with an existing, stationary area of opposite polarity magnetic flux, resulting in the formation of the
EBs. We also show that these EBs can be fueled further by additional, faster moving, negative magnetic flux
regions
GJ 900: A new hierarchical system with low-mass components
Speckle interferometric observations made with the 6 m telescope of the
Special Astrophysical Observatory of the Russian Academy of Sciences in 2000
revealed the triple nature of the nearby ( mas)
low-mass young ( Myr) star GJ 900. The configuration of the triple
system allowed it to be dynamically unstable. Differential photometry performed
from 2000 through 2004 yielded - and -band absolute magnitudes and
spectral types for the components to be =6.660.08,
=9.150.11, =10.080.26, =4.840.08,
=6.760.20, =7.390.31, K5--K7,
M3--M4, M5--M6. The ``mass--luminosity''
relation is used to estimate the individual masses of the components:
,
,
. From the observations of the
components relative motion in the period 2000--2006, we conclude that GJ 900 is
a hierarchical triple star with the possible orbital periods
P80 yrs and P20 yrs. An analysis of the 2MASS
images of the region around GJ 900 leads us to suggest that the system can
include other very-low-mass components.Comment: 7 pages, 5 figure
- âŠ