108 research outputs found
Planetesimals to brown dwarfs: What is a planet?
The past 15 years have brought about a revolution in our understanding of our Solar System and other planetary systems. During this time, discoveries include the first Kuiper belt objects (KBOs), the first brown dwarfs, and the first extrasolar planets. Although discoveries continue apace, they have called into question our previous perspectives on planets, both here and elsewhere. The result has been a debate about the meaning of the word "planet" itself. It is clear that scientists do not have a widely accepted or clear definition of what a planet is, and both scientists and the public are confused (and sometimes annoyed) by its use in various contexts. Because "planet" is a very widely used term, it seems worth the attempt to resolve this problem. In this essay, we try to cover all the issues that have come to the fore and bring clarity (if not resolution) to the debate
Rotation and differential rotation of active Kepler stars
We present rotation periods for thousands of active stars in the Kepler field
derived from Q3 data. In most cases a second period close to the rotation
period was detected, which we interpreted as surface differential rotation
(DR). Active stars were selected from the whole sample using the range of the
variability amplitude. To detect different periods in the light curves we used
the Lomb-Scargle periodogram in a pre-whitening approach to achieve parameters
for a global sine fit. The most dominant periods from the fit were ascribed to
different surface rotation periods, but spot evolution could also play a role.
Due to the large number of stars the period errors were estimated in a
statistical way. We thus cannot exclude the existence of false positives among
our periods. In our sample of 40.661 active stars we found 24.124 rotation
periods between 0.5-45 days. The distribution of stars with 0.5 < B-V <
1.0 and ages derived from angular momentum evolution that are younger than 300
Myr is consistent with a constant star-formation rate. A second period
within % of the rotation period was found in 18.619 stars (77.2%).
Attributing these two periods to DR we found that the relative shear
increases with rotation period, and slightly
decreases with effective temperature. The absolute shear
slightly increases between K. Above 6000 K
shows much larger scatter. We found weak dependence of on
rotation period. Latitudinal differential rotation measured for the first time
in more than 18.000 stars provides a comprehensive picture of stellar surface
shear, consistent with major predictions from mean-field theory. To what extent
our observations are prone to false positives and selection bias is not fully
explored, and needs to be addressed using more Kepler data.Comment: 19 pages, 18 figures, accepted by A&A. A table containing all
periods, KIC number, etc. can be found here:
http://www.astro.physik.uni-goettingen.de/~reinhold/period_table.te
Temperature determinations of hot DA white dwarfs using IUE continuum fluxes
Effective temperatures of 15 DA white dwarfs hotter than 20,000 K were derived from low-dispersion far ultraviolet spectra obtained with IUE. The analysis was carried out by comparing the observed far ultraviolet fluxes with model fluxes scaled to the V-band flux. Accurate calibration of the IUE spectra is critical for this analysis. Observations at all epochs were corrected to the 1980 IUE calibration using the time-dependent corrections of Bohlin (1988). Taking advantage of the smooth and well-defined continuum fluxes provided by DA white dwarfs, seven white dwarfs for which accurate, independent temperature determinations were made from line profile analyses were used to improve the accuracy of the IUE flux calibration. The correction to the original calibration is as great as 20 percent in individual 5 A wavelength bins, while the average over the IUE wavelength range is 5 percent. The final calibration correction and the temperatures for the hot white dwarfs are presented
Double-dipping to refine stellar rotation periods
We present a refined analysis of 15038 Kepler main sequence light curves to
determine the stellar rotation periods. The initial period estimates come from
an autocorrelation function, as has been done before. We then measure the
duration of every intensity dip in the light curve, expressed as fractions of
the initial rotation period estimate. These dip duration distributions are
subdivided into several regions whose relation to each other helps determine
which harmonic of the initial rotation period is most physically plausible. We
compare our final rotation periods to those from McQuillan, Mazeh, & Aigrain
(2014) and find that the great majority agree, but about 10% of their periods
are doubtful (usually twice as long as is most plausible). We are still
refining our method, and will later extend it to more stars to substantially
increase the sample of reliable stellar rotation periods.Comment: Published in AN on 3 August 2020. 6 pages, 6 figures, preprint
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