134 research outputs found
Meridional flow and differential rotation by gravity darkening in fast rotating solar-type stars
An explanation is presented for the rather strong total surface differential
rotation of the observed very young solar-type stars like AB Dor and PZ Tel.
Due to its rapid rotation a nonuniform energy flux leaves the stellar core so
that the outer convection zone is nonuniformly heated from below. Due to this
`gravity darkening' of the equator a meridional flow is created flowing
equatorwards at the surface and thus accelerating the equatorial rotation. The
effect linearly grows with the normalized pole-equator difference, \epsilon, of
the heat-flux at the bottom of the convection zone. A rotation rate of about 9
h leads to \epsilon=0.1 for a solar-type star. In this case the resulting
equator-pole differences of the angular velocity at the stellar surface,
\delta\Omega, varies from unobservable 0.005/day to the (desired) value of 0.03
day when the dimensionless diffusivity factors and c_\chi vary
between 1 and 0.1 (standard value c_\nu \simeq c_\chi \simeq 0.3, see Table 1.)
In all cases the related temperature differences between pole and equator at
the surface are unobservably small.
The (clockwise) meridional circulation which we obtain flows opposite to the
(counterclockwise) circulation appearing as a byproduct in the \Lambda-theory
of the nonuniform rotation in outer convection zones. The consequences of this
situation for those dynamo theories of stellar activity are discussed which
work with the meridional circulation as the dominant magnetic-advection effect
in latitude to produce the solar-like form of the butterfly diagram.
Key words: Hydrodynamics, Star: rotation, Stars: pre-main sequence, Stellar
activityComment: 4 pages, 3 figures, Astronomy and Astrophysics (subm.
Effect of Binary Source Companions on the Microlensing Optical Depth Determination toward the Galactic Bulge Field
Currently, gravitational microlensing survey experiments toward the Galactic
bulge field utilize two different methods of minimizing blending effect for the
accurate determination of the optical depth \tau. One is measuring \tau based
on clump giant (CG) source stars and the other is using `Difference Image
Analysis (DIA)' photometry to measure the unblended source flux variation.
Despite the expectation that the two estimates should be the same assuming that
blending is properly considered, the estimates based on CG stars systematically
fall below the DIA results based on all events with source stars down to the
detection limit. Prompted by the gap, we investigate the previously
unconsidered effect of companion-associated events on determination.
Although the image of a companion is blended with that of its primary star and
thus not resolved, the event associated with the companion can be detected if
the companion flux is highly magnified. Therefore, companions work effectively
as source stars to microlensing and thus neglect of them in the source star
count could result in wrong \tau estimation. By carrying out simulations based
on the assumption that companions follow the same luminosity function of
primary stars, we estimate that the contribution of the companion-associated
events to the total event rate is ~5f_{bi}% for current surveys and can reach
up to ~6f_{bi}% for future surveys monitoring fainter stars, where f_{bi} is
the binary frequency. Therefore, we conclude that the companion-associated
events comprise a non-negligible fraction of all events. However, their
contribution to the optical depth is not large enough to explain the systematic
difference between the optical depth estimates based on the two different
methods.Comment: 4 pages, 1 figure, 1 table, ApJ, submitte
AD Leonis: Radial Velocity Signal of Stellar Rotation or Spin–Orbit Resonance?
AD Leonis is a nearby magnetically active M dwarf. We find Doppler variability with a period of 2.23 days, as well as photometric signals: (1) a short-period signal, which is similar to the radial velocity signal, albeit with considerable variability; and (2) a long-term activity cycle of 4070 ± 120 days. We examine the short-term photometric signal in the available All-Sky Automated Survey and Microvariability and Oscillations of STars (MOST) photometry and find that the signal is not consistently present and varies considerably as a function of time. This signal undergoes a phase change of roughly 0.8 rad when considering the first and second halves of the MOST data set, which are separated in median time by 3.38 days. In contrast, the Doppler signal is stable in the combined High-Accuracy Radial velocity Planet Searcher and High Resolution Echelle Spectrometer radial velocities for over 4700 days and does not appear to vary in time in amplitude, phase, period, or as a function of extracted wavelength. We consider a variety of starspot scenarios and find it challenging to simultaneously explain the rapidly varying photometric signal and the stable radial velocity signal as being caused by starspots corotating on the stellar surface. This suggests that the origin of the Doppler periodicity might be the gravitational tug of a planet orbiting the star in spin–orbit resonance. For such a scenario and no spin–orbit misalignment, the measured v sin i indicates an inclination angle of 15°̣5 ± 2°̣5 and a planetary companion mass of 0.237 ± 0.047 M Jup
Gravity Waves in the Sun
We present numerical simulations of penetrative convection and gravity wave
excitation in the Sun. Gravity waves are self-consistently generated by a
convective zone overlying a radiative interior. We produce power spectra for
gravity waves in the radiative region as well as estimates for the energy flux
of gravity waves below the convection zone. We calculate a peak energy flux in
waves below the convection zone to be three orders of magnitude smaller than
previous estimates for m=1. The simulations show that the linear dispersion
relation is a good approximation only deep below the convective-radiative
boundary. Both low frequency propagating gravity waves as well as higher
frequency standing modes are generated; although we find that convection does
not continually drive the standing g-mode frequencies.Comment: 22 pages, 14 figures, submitted to MNRA
Calculations of periodicity from H<i>α</i> profiles of Proxima Centauri
We investigate retrieval of the stellar rotation signal for Proxima Centauri. We make use of high-resolution spectra taken with UVES and HARPS of Proxima Centauri over a 13-yr period as well as photometric observations of Proxima Centauri from ASAS and HST. We measure the Hα equivalent width and Hα index, skewness and kurtosis and introduce a method that investigates the symmetry of the line, the peak ratio, which appears to return better results than the other measurements. Our investigations return a most significant period of 82.6 ± 0.1 days, confirming earlier photometric results and ruling out a more recent result of 116.6 days which we conclude to be an alias induced by the specific HARPS observation times. We conclude that whilst spectroscopic Hα measurements can be used for period recovery, in the case of Proxima Centauri the available photometric measurements are more reliable. We make 2D models of Proxima Centauri to generate simulated Hα, finding that reasonable distributions of plage and chromospheric features are able to reproduce the equivalent width variations in observed data and recover the rotation period, including after the addition of simulated noise and flares. However the 2D models used fail to generate the observed variety of line shapes measured by the peak ratio. We conclude that only 3D models which incorporate vertical motions in the chromosphere can achieve this
The Optical Gravitational Lensing Experiment. Catalog of stellar proper motions in the OGLE-II Galactic bulge fields
We present a proper motion (\mu) catalogue of 5,080,236 stars in 49 Optical
Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields,
covering a range of -11 deg. <l< 11 deg. and -6 deg. <b<3 deg., the total area
close to 11 square degrees. The proper motion measurements are based on 138 -
555 I-band images taken during four observing seasons: 1997-2000. The catalogue
stars are in the magnitude range 11 < I < 18 mag. In particular, the catalogue
includes Red Clump Giants (RCGs) and Red Giants in the GB, and main sequence
stars in the Galactic disc. The proper motions up to \mu = 500 mas/yr were
measured with the mean accuracy of 0.8-3.5 mas/yr, depending on the brightness
of a star. This catalogue may be useful for studying the kinematic of stars in
the GB and the Galactic disk.Comment: 13 pages, 16 figures, MNRAS in pres
Evidence for Magnetic Flux Saturation in Rapidly Rotating M Stars
We present magnetic flux measurements in seven rapidly rotating M dwarfs. Our
sample stars have X-ray and H-alpha emission indicative of saturated emission,
i.e., emission at a high level independent of rotation rate. Our measurements
are made using near-infrared FeH molecular spectra observed with HIRES at Keck.
Because of their large convective overturn times, the rotation velocity of M
stars with small Rossby numbers is relatively slow and does not hamper the
measurement of Zeeman splitting. The Rossby numbers of our sample stars are as
small as 0.01. All our sample stars exhibit magnetic flux of kilo-Gauss
strength. We find that the magnetic flux saturates in the same regime as
saturation of coronal and chromospheric emission, at a critical Rossby number
of around 0.1. The filling factors of both field and emission are near unity by
then. We conclude that the strength of surface magnetic fields remains
independent of rotation rate below that; making the Rossby number yet smaller
by a factor of ten has little effect. These saturated M-star dynamos generate
an integrated magnetic flux of roughly 3 kG, with a scatter of about 1 kG. The
relation between emission and flux also has substantial scatter.Comment: 10 pages, accepted for publication in Ap
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