24 research outputs found
Reconstructing the XUV Spectra of Active Sun-like Stars Using Solar Scaling Relations with Magnetic Flux
Kepler Space Telescope and Transiting Exoplanet Survey Satellite unveiled
that Sun-like stars frequently host exoplanets. These exoplanets are subject to
fluxes of ionizing radiation in the form of X-ray and extreme-ultraviolet (EUV)
radiation that may cause changes in their atmospheric dynamics and chemistry.
While X-ray fluxes can be observed directly, EUV fluxes cannot be observed
because of severe interstellar medium absorption. Here, we present a new
empirical method to estimate the whole stellar XUV (X-ray plus EUV) and FUV
spectra as a function of total unsigned magnetic fluxes of stars. The response
of the solar XUV and FUV spectrum (0.1-180 nm) to the solar total unsigned
magnetic flux is investigated by using the long-term Sun-as-a-star dataset over
10 yrs, and the power-law relation is obtained for each wavelength with a
spectral resolution of 0.1-1 nm. We applied the scaling relations to active
young Sun-like stars (G-dwarfs), EK Dra (G1.5V), Uma (G1.5V) and
Ceti (G5V), and found that the observed spectra (except for the
unobservable longward EUV wavelength) are roughly consistent with the extension
of the derived power-law relations with errors of an order of magnitude. This
suggests that our model is a valuable method to derive the XUV/FUV fluxes of
Sun-like stars including the EUV band mostly absorbed at wavelengths longward
of 36 nm. We also discuss differences between the solar extensions and stellar
observations at the wavelength in the 2-30 nm band and concluded that
simultaneous observations of magnetic and XUV/FUV fluxes are necessary for
further validations.Comment: 29 pages, 10 figures, 8 tables. Accepted for publication in The
Astrophysical Journa
Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
We report the latest view of Kepler solar-type (G-type main-sequence)
superflare stars, including recent updates with Apache Point Observatory (APO)
3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly
conducted APO3.5m spectroscopic observations of 18 superflare stars found from
Kepler 1-min time cadence data. More than half (43 stars) are confirmed to be
"single" stars, among 64 superflare stars in total that have been
spectroscopically investigated so far in this APO3.5m and our previous
Subaru/HDS observations. The measurements of (projected rotational
velocity) and chromospheric lines (Ca II H\&K and Ca II 8542\AA) support the
brightness variation of superflare stars is caused by the rotation of a star
with large starspots. We then investigated the statistical properties of Kepler
solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates.
As a result, the maximum superflare energy continuously decreases as the
rotation period increases. Superflares with energies
erg occur on old, slowly-rotating Sun-like stars
(25 days) approximately once every 2000--3000 years,
while young rapidly-rotating stars with a few days have
superflares up to erg. The maximum starspot area does not depend on
the rotation period when the star is young, but as the rotation slows down, it
starts to steeply decrease at 12 days for Sun-like
stars. These two decreasing trends are consistent since the magnetic energy
stored around starspots explains the flare energy, but other factors like spot
magnetic structure should also be considered.Comment: 71 pages, 31 figures, 10 tables. Accepted for publication in The
Astrophysical Journal (on March 29, 2019
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Universal Scaling Laws for Solar and Stellar Atmospheric Heating: Catalog of Power-law Index between Solar Activity Proxies and Various Spectral Irradiances
The formation of extremely hot outer atmospheres is one of the most prominent manifestations of magnetic activity common to late-type dwarf stars, including the Sun. It is widely believed that these atmospheric layers, the corona, transition region, and chromosphere, are heated by the dissipation of energy transported upwards from the stellar surface by the magnetic field. This is signified by the spectral line fluxes at various wavelengths, scaled with power-law relationships against the surface magnetic flux over a wide range of formation temperatures, which are universal to the Sun and Sunlike stars of different ages and activity levels. This study describes a catalog of power-law indices between solar activity proxies and various spectral line fluxes. Compared to previous studies, we expanded the number of proxies, which now includes the total magnetic flux, total sunspot number, total sunspot area, and the F10.7 cm radio flux, and further enhanced the number of spectral lines by a factor of 2. This provides the data to study in detail the flux–flux scaling laws from the regions specified by the temperatures of the corona (log(T/K) = 6–7) to those of the chromosphere (log(T/K) ∼ 4), as well as the reconstruction of various spectral line fluxes of the Sun in the past, F-, G-, and K-type dwarfs, and the modeled stars.
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A Superflare on YZ Canis Minoris Observed by Seimei Telescope and TESS: Red Asymmetry of H Emission Associated with White-Light Emission
Active M-type stars are known to often produce superflares on the surface.
Radiation from stellar (super-)flares is important for the exoplanet
habitability, but the mechanisms are not well understood. In this paper, we
report simultaneous optical spectroscopic and photometric observations of a
stellar superflare on an active M dwarf YZ CMi with the 3.8-m Seimei telescope
and the . The flare bolometric
energy was and H energy
was . The H emission line
profile showed red asymmetry throughout the flare with a duration of . The velocity of the red asymmetry was and line width of H was broadened up to
. The redshifted velocity and line width of H line decayed more
rapidly than the equivalent width, and their time evolutions are correlated
with that of the white-light emission. This indicates a possibility that the
white light, H red asymmetry, and H line broadening originate
from nearly the same site, i.e., the dense chromospheric condensation region
heated by non-thermal electrons. On the other hand, the flux ratio of the
redshifted excess components to the central components is enhanced one hour
after the flare onset. This may be due to the change of the main source of the
red asymmetry to the post-flare loops in the later phase of the flare.Comment: 13 pages, 5 figures. Accepted for publication in The Astrophysical
Journa
Starspot Mapping with Adaptive Parallel Tempering. II. Application to TESS Data for M-dwarf Flare Stars AU Microscopii, YZ Canis Minoris, and EV Lacertae
Starspots and stellar flares are indicators of stellar magnetic activity. The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely understood. To investigate the relation between spot locations deduced from light curves and the occurrence of flares therein, we perform starspot modeling for the TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables us to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We find that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that any spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons why there is no or low correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude
Detection of a high-velocity prominence eruption leading to a CME associated with a superflare on the RS CVn-type star V1355 Orionis
Stellar coronal mass ejections (CMEs) have recently received much attention
for their impacts on exoplanets and stellar evolution. Detecting prominence
eruptions, the initial phase of CMEs, as the blue-shifted excess component of
Balmer lines is a technique to capture stellar CMEs. However, most of
prominence eruptions identified thus far have been slow and less than the
surface escape velocity. Therefore, whether these eruptions were developing
into CMEs remained unknown. In this study, we conducted simultaneous optical
photometric observations with Transiting Exoplanet Survey Satellite and optical
spectroscopic observations with the 3.8m Seimei Telescope for the RS CVn-type
star V1355 Orionis that frequently produces large-scale superflares. We
detected a superflare releasing . In the
early stage of this flare, a blue-shifted excess component of extending its velocity up to was
observed and thought to originate from prominence eruptions. The velocity
greatly exceeds the escape velocity (i.e., ), which provides important evidence that stellar prominence eruptions
can develop into CMEs. Furthermore, we found that the prominence is very
massive (). These data will clarify whether such events follow existing
theories and scaling laws on solar flares and CMEs even when the energy scale
far exceeds solar cases.Comment: 16 pages, 8 figures. Accepted for publication in The Astrophysical
Journa
Starspot mapping with adaptive parallel tempering I: Implementation of computational code
Starspots are thought to be regions of locally strong magnetic fields,
similar to sunspots, and they can generate photometric brightness modulations.
To deduce stellar and spot properties, such as spot emergence and decay rates,
we implement computational code for starspot modeling. It is implemented with
an adaptive parallel tempering algorithm and an importance sampling algorithm
for parameter estimation and model selection in the Bayesian framework. For
evaluating the performance of the code, we apply it to synthetic light curves
produced with 3 spots. The light curves are specified in the spot parameters,
such as the radii, intensities, latitudes, longitudes, and emergence/decay
durations. The spots are circular with specified radii and intensities relative
to the photosphere, and the stellar differential rotation coefficient is also
included in the light curves. As a result, stellar and spot parameters are
uniquely deduced. The number of spots is correctly determined: the 3-spot model
is preferable because the model evidence is much greater than that of 2-spot
models by orders of magnitude and more than that of 4-spot model by a more
modest factor, whereas the light curves are produced to have 2 or 1 local
minimum during one equatorial rotation period by adjusting the values of
longitude. The spot emergence and decay rates can be estimated with error less
than an order of magnitude, considering the difference of the number of spots.Comment: 27 pages, 14 figures, 2 tables, accepted for publication in Ap