11 research outputs found
The Ultraviolet Radiation Environment Around M dwarf Exoplanet Host Stars
The spectral and temporal behavior of exoplanet host stars is a critical
input to models of the chemistry and evolution of planetary atmospheres. At
present, little observational or theoretical basis exists for understanding the
ultraviolet spectra of M dwarfs, despite their critical importance to
predicting and interpreting the spectra of potentially habitable planets as
they are obtained in the coming decades. Using observations from the Hubble
Space Telescope, we present a study of the UV radiation fields around nearby M
dwarf planet hosts that covers both FUV and NUV wavelengths. The combined
FUV+NUV spectra are publically available in machine-readable format. We find
that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ
832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition
region UV emission. No "UV quiet" M dwarfs are observed. The bright stellar
Ly-alpha emission lines are reconstructed, and we find that the Ly-alpha line
fluxes comprise ~37-75% of the total 1150-3100A flux from most M dwarfs; >
10^{3} times the solar value. The F(FUV)/F(NUV) flux ratio, a driver for
abiotic production of the suggested biomarkers O2 and O3, is shown to be ~0.5-3
for all M dwarfs in our sample, > 10^{3} times the solar ratio. For the four
stars with moderate signal-to-noise COS time-resolved spectra, we find UV
emission line variability with amplitudes of 50-500% on 10^{2} - 10^{3} s
timescales. Finally, we observe relatively bright H2 fluorescent emission from
four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832).
Additional modeling work is needed to differentiate between a stellar
photospheric or possible exoplanetary origin for the hot (T(H2) \approx
2000-4000 K) molecular gas observed in these objects.Comment: ApJ, accepted. 16 pages, 10 figures. On-line data at:
http://cos.colorado.edu/~kevinf/muscles.htm
Ultraviolet Spectroscopy of Rapidly-Rotating Solar-Mass Stars: Emission Line Redshifts as a Test of the Solar-Stellar Connection
We compare high-resolution ultraviolet spectra of the Sun and thirteen
solarmass main sequence stars with different rotational periods that serve as
proxies for their different ages and magnetic field structures. In this the
second paper in the series, we study the dependence of ultraviolet
emission-line centroid velocities on stellar rotation period, as rotation rates
decrease from that of the Pleiades star HII314 (Prot = 1.47 days) to Alpha Cen
A (Prot = 28 days). Our stellar sample of F9 V to G5 V stars consists of six
stars observed with the Cosmic Origins 1Guest Observer, NASA/ESA Hubble Space
Telescope and User of the Data Archive at the Space Telescope Science
Institute. Spectrograph on HST and eight stars observed with the Space
Telescope Imaging Spectrograph on HST. We find a systematic trend of increasing
redshift with more rapid rotation (decreasing rotation period) that is similar
to the increase in line red shift between quiet and plage regions on the Sun.
The fastest-rotating solar-mass star in our study, HII314, shows significantly
enhanced redshifts at all temperatures above log T = 4.6, including the corona,
which is very different from the redshift pattern observed in the more
slowly-rotating stars. This difference in the redshift pattern suggests that a
qualitative change in the magnetic-heating process occurs near Prot = 2 days.
We propose that HII314 is an example of a solar-mass star with a magnetic
heating rate too large for the physical processes responsible for the redshift
pattern to operate in the same way as for the more slowly rotating stars.
HII314 may therefore lie above the high activity end of the set of solar-like
phenomena that is often called the "solar-stellar connection".Comment: 36 pages, 7 figures, 6 tables, to appear in the Astrophysical Journal
July 201
Far Ultraviolet Continuum Emission: Applying this Diagnostic to the Chromospheres of Solar-Mass Stars
The far ultraviolet (FUV) continuum flux is recognized as a very sensitive
diag- nostic of the temperature structure of the Sun's lower chromosphere.
Until now analysis of the available stellar FUV data has shown that solar-type
stars must also have chromospheres, but quantitative analyses of stellar FUV
continua require far higher quality spectra and comparison with new non-LTE
chromosphere models. We present accurate far ultraviolet (FUV,
1150-1500^{\circ}) continuum flux measurements for solar-mass stars, made
feasible by the high throughput and very low detector background of the Cosmic
Origins Spectrograph (COS) on the Hubbble Space Telescope. We show that the
continuum flux can be measured above the detector background even for the
faintest star in our sample. We find a clear trend of increasing continuum
brightness temperature at all FUV wavelengths with decreasing rotational
period, which provides an important measure of magnetic heating rates in
stellar chromospheres. Comparison with semiempirical solar flux models shows
that the most rapidly rotating solar-mass stars have FUV continuum brightness
temperatures similar to the brightest faculae seen on the Sun. The thermal
structure of the brightest solar faculae therefore provides a first-order
estimate of the thermal structure and heating rate for the most rapidly
rotating solar-mass stars in our sample.Comment: 21 pages, 5 figures, to appear in Ap
Global Oceans
Global Oceans is one chapter from the State of the Climate in 2019 annual report and is avail-able from https://doi.org/10.1175/BAMS-D-20-0105.1. Compiled by NOAA’s National Centers for Environmental Information, State of the Climate in 2019 is based on contr1ibutions from scien-tists from around the world. It provides a detailed update on global climate indicators, notable weather events, and other data collected by environmental monitoring stations and instru-ments located on land, water, ice, and in space. The full report is available from https://doi.org /10.1175/2020BAMSStateoftheClimate.1