599 research outputs found
The Faint Young Sun Paradox: An Observational Test of an Alternative Solar Model
We report the results of deep observations at radio (3.6 cm) wavelengths of the nearby solarâtype star Ï^01 Ursa Majoris with the Very Large Array (VLA) intended to test an alternative theory of solar luminosity evolution. The standard model predicts a solar luminosity only 75% of the present value and surface temperatures below freezing on Earth and Mars at 4 Ga, seemingly in conflict with geologic evidence for liquid water on these planets. An alternative model invokes a compensatory mass loss through a declining solar wind that results in a more consistent early luminosity. The freeâfree emission from an enhanced wind around nearby young Sunâlike stars should be detectable at microwave frequencies. Our observations of Ï^01 UMa, a 300 million yearâold solarâmass star, place an upper limit on the mass loss rate of 4â5 Ă 10^(â11) M_â yr^(â1). Total mass loss from such a star over 4 Gyr would be less than 6%. If this star is indeed an analog of the early Sun, it casts doubt on the alternative model as a solution to the faint young Sun paradox, particularly for Mars
Testing the Metal of Late-Type Kepler Planet Hosts with Iron-Clad Methods
It has been shown that F, G, and early K dwarf hosts of Neptune-sized planets
are not preferentially metal-rich. However, it is less clear whether the same
holds for late K and M dwarf planet hosts. We report metallicities of Kepler
targets and candidate transiting planet hosts with effective temperatures below
4500 K. We use new metallicity calibrations to determine [Fe/H] from visible
and near-infrared spectra. We find that the metallicity distribution of late K
and M dwarfs monitored by Kepler is consistent with that of the solar
neighborhood. Further, we show that hosts of Earth- to Neptune-sized planets
have metallicities consistent with those lacking detected planets and rule out
a previously claimed 0.2 dex offset between the two distributions at 6sigma
confidence. We also demonstrate that the metallicities of late K and M dwarfs
hosting multiple detected planets are consistent with those lacking detected
planets. Our results indicate that multiple terrestrial and Neptune-sized
planets can form around late K and M dwarfs with metallicities as low as 0.25
of the solar value. The presence of Neptune-sized planets orbiting such
low-metallicity M dwarfs suggests that accreting planets collect most or all of
the solids from the disk and that the potential cores of giant planets can
readily form around M dwarfs. The paucity of giant planets around M dwarfs
compared to solar-type stars must be due to relatively rapid disk evaporation
or a slower rate of core accretion, rather than insufficient solids to form a
core.Comment: 9 pages, 5 figures. Accepted to Ap
Mars: The Next Steps
In 1993, the National Aeronautics and Space Administration (NASA) began the Mars Surveyor program to fly small, focused missions to Mars at every launch opportunity (2-year intervals) using the âfaster, better, cheaperâ approach championed by administrator Dan Goldin. NASA's plans are now under intense scrutiny because of the loss of all spacecraft arriving at Mars within the past six months. Panels commissioned to study the failures have released reports enumerating technical, managerial, and program problems and made recommendations to address these lapses. Recent events also provide an opportunity to reconsider the larger picture of Mars exploration: What are the program's goals within the context of both the scientific community and the society that supports it, and what is the best strategy to achieve those goals
Spectroscopy and Photometry of Nearby Young Solar Analogs
We present new photometry and spectroscopy of 34 stars from a catalog of 38 nearby (d \u3c 25 pc) G and K dwarfs selected as analogs to the early Sun. We report that the least active star in our sample is also slowly rotating and probably of solar age. Two other stars appear to be evolved objects that have recently acquired angular momentum. A fourth star may be a spectroscopic binary. Many of the other stars belong to previously identified common proper-motion groups. Space motions, lithium abundances, and Ca II emission of these stars suggest ages between 70 and 800 Myr
The continuous period search method and its application to the young solar analogue HD 116956
We formulate an improved time series analysis method for the analysis of
photometry of active stars. This new Continuous Period Search (CPS) method is
applied to 12 years of V band photometry of the young solar analogue HD 116956
(NQ UMa). The new method is developed from the previous Three Stage Period
Analysis (TSPA) method. Our improvements are the use of a sliding window in
choosing the modelled datasets, a criterion applied to select the best model
for each dataset and the computation of the time scale of change of the light
curve. We test the performance of CPS with simulated and real data. The CPS has
a much improved time resolution which allows us to better investigate fast
evolution of stellar light curves. We can also separate between the cases when
the data is best described by periodic and aperiodic (e.g. constant brightness)
models. We find, however, that the performance of the CPS has certain
limitations. It does not determine the correct model complexity in all cases,
especially when the underlying light curve is constant and the number of
observations too small. Also the sensitivity in detecting two close light curve
minima is limited and it has a certain amount of intrinsic instability in its
period estimation. Using the CPS, we find persistent active longitudes in the
star HD 116956 and a "flip-flop" event that occurred during the year 1999.
Assuming that the surface differential rotation of the star causes observable
period variations in the stellar light curve, we determine the differential
rotation coefficient to be |k|>0.11. The mean timescale of change of the light
curve during the whole 12 year observing period was T_C=44.1 d, which is of the
same order as the predicted convective turnover time of the star. We also
investigate the presence of activity cycles on the star, but do not find any
conclusive evidence supporting them.Comment: 14 pages, 11 figures, 3 table
New Worlds on the Horizon: Earth-Sized Planets Close to Other Stars
The search for habitable planets like Earth around other stars fulfils an
ancient imperative to understand our origins and place in the cosmos. The past
decade has seen the discovery of hundreds of planets, but nearly all are gas
giants like Jupiter and Saturn. Recent advances in instrumentation and new
missions are extending searches to planets the size of the Earth, but closer to
their host stars. There are several possible ways such planets could form, and
future observations will soon test those theories. Many of these planets we
discover may be quite unlike Earth in their surface temperature and
composition, but their study will nonetheless inform us about the process of
planet formation and the frequency of Earth-like planets around other stars.Comment: to appear in Science, October 12, 200
- âŠ