1,685 research outputs found
Periodic photometric variability of the brown dwarf Kelu-1
We have detected a strong periodicity of 1.80+/-0.05 hours in photometric
observations of the brown dwarf Kelu-1. The peak-to-peak amplitude of the
variation is ~1.1% (11.9+/-0.8 mmag) in a 41nm wide filter centred on 857nm and
including the dust/temperature sensitive TiO & CrH bands. We have identified
two plausible causes of variability: surface features rotating into- and
out-of-view and so modulating the light curve at the rotation period; or,
elliposidal variability caused by an orbiting companion. In the first scenario,
we combine the observed vsin(i) of Kelu-1 and standard model radius to
determine that the axis of rotation is inclined at 65+/-12 degrees to the line
of sight.Comment: 7 pages, 9 figures. Accepted for publication in MNRA
Global environmental effects of impact-generated aerosols: Results from a general circulation model
Cooling and darkening at Earth's surface are expected to result from the interception of sunlight by the high altitude worldwide dust cloud generated by impact of a large asteroid or comet, according to the one-dimensional radioactive-convective atmospheric model (RCM) of Pollack et al. An analogous three-dimensional general circulation model (GCM) simulation obtains the same basic result as the RCM but there are important differences in detail. In the GCM simulation the heat capacity of the oceans, not included in the RCM, substantially mitigates land surface cooling. On the other hand, the GCM's low heat capacity surface allows surface temperatures to drop much more rapidly than reported by Pollack et al. These two differences between RCM and GCM simulations were noted previously in studies of nuclear winter; GCM results for comet/asteroid winter, however, are much more severe than for nuclear winter because the assumed aerosol amount is large enough to intercept all sunlight falling on Earth. In the simulation the global average of land surface temperature drops to the freezing point in just 4.5 days, one-tenth the time required in the Pollack et al. simulation. In addition to the standard case of Pollack et al., which represents the collision of a 10-km diameter asteroid with Earth, additional scenarios are considered ranging from the statistically more frequent impacts of smaller asteroids to the collision of Halley's comet with Earth. In the latter case the kinetic energy of impact is extremely large due to the head-on collision resulting from Halley's retrograde orbit
The Mass-Radius Relation Of Young Stars. I. Usco 5, An M4.5 Eclipsing Binary In Upper Scorpius Observed By K2
We present the discovery that UScoCTIO 5, a known spectroscopic binary in the Upper Scorpius star-forming region (P = 34 days, M-tot sin(i) = 0.64M(circle dot)), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M-A = 0.329 +/- 0.002 M-circle dot, R-A = 0.834 +/- 0.006 R-circle dot, M-B = 0.317 +/- 0.002 M-circle dot, and R-B = 0.810 +/- 0.006 R-circle dot. The radii are broadly consistent with pre-main-sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25%-50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications to mass-dependent outcomes of star and planet formation. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-T-eff scale. The simplest modification to the models (changing T-eff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (tau similar to 11 Myr).NASA Science Mission directorateW. M. Keck FoundationAstronom
M Dwarfs in SDSS Stripe 82: Photometric Light Curves and Flare Rate Analysis
We present a flare rate analysis of 50,130 M dwarf light curves in SDSS
Stripe 82. We identified 271 flares using a customized variability index to
search ~2.5 million photometric observations for flux increases in the u- and
g-bands. Every image of a flaring observation was examined by eye and with a
PSF-matching and image subtraction tool to guard against false positives.
Flaring is found to be strongly correlated with the appearance of H-alpha in
emission in the quiet spectrum. Of the 99 flare stars that have spectra, we
classify 8 as relatively inactive. The flaring fraction is found to increase
strongly in stars with redder colors during quiescence, which can be attributed
to the increasing flare visibility and increasing active fraction for redder
stars. The flaring fraction is strongly correlated with |Z| distance such that
most stars that flare are within 300 pc of the Galactic plane. We derive flare
u-band luminosities and find that the most luminous flares occur on the
earlier-type M dwarfs. Our best estimate of the lower limit on the flaring rate
(averaged over Stripe 82) for flares with \Delta u \ge 0.7 magnitudes on stars
with u < 22 is 1.3 flares hour^-1 square degree^-1 but can vary significantly
with the line-of-sight.Comment: 44 pages, 13 figure
The Factory and The Beehive II. Activity and Rotation in Praesepe and the Hyades
Open clusters are collections of stars with a single, well-determined age,
and can be used to investigate the connections between angular-momentum
evolution and magnetic activity over a star's lifetime. We present the results
of a comparative study of the relationship between stellar rotation and
activity in two benchmark open clusters: Praesepe and the Hyades. As they have
the same age and roughly solar metallicity, these clusters serve as an ideal
laboratory for testing the agreement between theoretical and empirical
rotation-activity relations at 600 Myr. We have compiled a sample of
720 spectra --- more than half of which are new observations --- for 516
high-confidence members of Praesepe; we have also obtained 139 new spectra for
130 high-confidence Hyads. We have collected rotation periods () for
135 Praesepe members and 87 Hyads. To compare emission, an indicator
of chromospheric activity, as a function of color, mass, and Rossby number
, we first calculate an expanded set of values, with which we can
obtain the to bolometric luminosity ratio, ,
even when spectra are not flux-calibrated and/or stars lack reliable distances.
Our values cover a broader range of stellar masses and colors (roughly
equivalent to spectral types from K0 to M9), and exhibit better agreement
between independent calculations, than existing values. We find no difference
between the two clusters in their equivalent width or
distributions, and therefore take the merged
and data to be representative of 600-Myr-old stars. Our analysis
shows that activity in these stars is saturated for
. Above that value activity declines as a
power-law with slope , before dropping off rapidly
at ...Comment: 17 pages, 15 figures, Accepted by Ap
Exploring the Local Milky Way: M Dwarfs as Tracers of Galactic Populations
We have assembled a spectroscopic sample of low-mass dwarfs observed as part
of the Sloan Digital Sky Survey along one Galactic sightline, designed to
investigate the observable properties of the thin and thick disks. This sample
of ~7400 K and M stars also has measured ugriz photometry, proper motions, and
radial velocities. We have computed UVW space motion distributions, and
investigate their structure with respect to vertical distance from the Galactic
Plane. We place constraints on the velocity dispersions of the thin and thick
disks, using two-component Gaussian fits. We also compare these kinematic
distributions to a leading Galactic model. Finally, we investigate other
possible observable differences between the thin and thick disks, such as
color, active fraction and metallicity.Comment: 11 pages, 12 figures, Accepted by A
The Angular Momentum Content and Evolution of Class I and Flat-Spectrum Protostars
We report on the angular momentum content of heavily embedded protostars
based on our analysis of the projected rotation velocities (v sin i s) of 38
Class I/flat spectrum young stellar objects presented by Doppmann et al (2005).
After correcting for projection effects, we find that infrared-selected Class
I/flat spectrum objects rotate significantly more quickly (median equatorial
rotation velocity ~ 38 km/sec) than Classical T Tauri stars (CTTSs; median
equatorial rotation velocity ~ 18 km/sec) in the Rho Ophiuchi and
Taurus-Aurigae regions. The detected difference in rotation speeds between
Class I/flat spectrum sources and CTTSs proves difficult to explain without
some mechanism which transfers angular momentum out of the protostar between
the two phases. Assuming Class I/flat spectrum sources possess physical
characteristics (M_*,R_*,B_*) typical of pre-main sequence stars, fully disk
locked Class I objects should have co-rotation radii within their protostellar
disks that match well (within 30%) with the predicted magnetic coupling radii
of Shu et al (1994). The factor of two difference in rotation rates between
Class I/flat spectrum and CTTS sources, when interpreted in the context of disk
locking models, also imply a factor of 5 or greater difference in mass
accretion rates between the two phases.Comment: 13 pages, 6 figures. Accepted for publication in the Astronomical
Journal (tentatively for June 2005 edition
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A comparative analysis of Simplified General Circulation Models of the atmosphere of Venus
Within the context of a working group supported by ISSI (Bern, Switzerland), we have made an intercomparison work between Global Circulation Models using simpli?ed parameterizations for radiative forcing and other physical processes. Even with similar schemes and parameters, the different GCMs produce different circulations, illustrating interesting differences between dynamical model cores
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