3,372 research outputs found
Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars
The connection between stellar rotation, stellar activity, and convective
turnover time is revisited with a focus on the sole contribution of magnetic
activity to the Ca II H&K emission, the so-called excess flux, and its
dimensionless indicator R in relation to other stellar
parameters and activity indicators. Our study is based on a sample of 169
main-sequence stars with directly measured Mount Wilson S-indices and rotation
periods. The R values are derived from the respective S-indices
and related to the rotation periods in various -colour intervals. First,
we show that stars with vanishing magnetic activity, i.e. stars whose excess
flux index R approaches zero, have a well-defined,
colour-dependent rotation period distribution; we also show that this rotation
period distribution applies to large samples of cool stars for which rotation
periods have recently become available. Second, we use empirical arguments to
equate this rotation period distribution with the global convective turnover
time, which is an approach that allows us to obtain clear relations between the
magnetic activity related excess flux index R, rotation
periods, and Rossby numbers. Third, we show that the activity versus Rossby
number relations are very similar in the different activity indicators. As a
consequence of our study, we emphasize that our Rossby number based on the
global convective turnover time approaches but does not exceed unity even for
entirely inactive stars. Furthermore, the rotation-activity relations might be
universal for different activity indicators once the proper scalings are used.Comment: 13 pages, 7 figures, accepted for publication in A&
Coronal X-ray emission and planetary irradiation in HD 209458
HD 209458 is one of the benchmark objects in the study of hot Jupiter
atmospheres and their evaporation through planetary winds. The expansion of the
planetary atmosphere is thought to be driven by high-energy EUV and X-ray
irradiation. We obtained new Chandra HRC-I data, which unequivocally show that
HD 209458 is an X-ray source. Combining these data with archival XMM-Newton
observations, we find that the corona of HD 209458 is characterized by a
temperature of about 1 MK and an emission measure of 7e49 cm^-3, yielding an
X-ray luminosity of 1.6e27 erg/s in the 0.124-2.48 keV band. HD 209458 is an
inactive star with a coronal temperature comparable to that of the inactive Sun
but a larger emission measure. At this level of activity, the planetary
high-energy emission is sufficient to support mass-loss at a rate of a few
times 1e10 g/s.Comment: Accepted for publication in A&
Fifteen years in the high-energy life of the solar-type star HD 81809. XMM-Newton observations of a stellar activity cycle
Aims. The data set of the long-term XMM-Newton monitoring program of HD 81809
is analyzed to study its X-ray cycle, to investigate if the latter is related
to the chromospheric one, to infer the structure of the corona of HD 81809, and
to explore if the coronal activity of HD 81809 can be ascribed to phenomena
similar to the solar ones and, therefore, considered an extension of the solar
case. Methods. We analyze the observations of HD 81809 performed with
XMM-Newton with a regular cadence of 6 months from 2001 to 2016 and
representing one of the longest available observational baseline (~yr)
for a solar-like star with a well-studied chromospheric cycle (with a period of
~yr). We investigate the modulation of coronal luminosity and
temperature and its relation with the chromospheric cycle. We interpret the
data in terms of a mixture of solar-like coronal regions, adopting a
methodology originally proposed to study the Sun as an X-ray star. Results. The
observations show a well-defined regular cyclic modulation of the X-ray
luminosity that reflects the activity level of HD 81809. The data covers
approximately two cycles of coronal activity; the modulation has an amplitude
of a factor of (excluding evident flares, as in the June 2002
observation) and a period of ~yr, consistent with that of the
chromospheric cycle. We demonstrate that the corona of HD 81809 can be
interpreted as an extension of the solar case and it can be modeled with a
mixture of solar-like coronal regions along the whole cycle. The activity level
is mainly determined by a varying coverage of very bright active regions,
similar to cores of active regions observed in the Sun. Evidence of unresolved
significant flaring activity is present especially in proximity of cycle
maxima.Comment: 11 pages, 5 Figures, A&A accepte
The Ca II infrared triplet's performance as an activity indicator compared to Ca II H and K
Aims. A large number of Calcium Infrared Triplet (IRT) spectra are expected
from the GAIA- and CARMENES missions. Conversion of these spectra into known
activity indicators will allow analysis of their temporal evolution to a better
degree. We set out to find such a conversion formula and to determine its
robustness.
Methods. We have compared 2274 Ca II IRT spectra of active main-sequence F to
K stars taken by the TIGRE telescope with those of inactive stars of the same
spectral type. After normalizing and applying rotational broadening, we
subtracted the comparison spectra to find the chromospheric excess flux caused
by activity. We obtained the total excess flux, and compared it to established
activity indices derived from the Ca II H & K lines, the spectra of which were
obtained simultaneously to the infrared spectra.
Results. The excess flux in the Ca II IRT is found to correlate well with
and , as well as , if the
-dependency is taken into account. We find an empirical conversion formula
to calculate the corresponding value of one activity indicator from the
measurement of another, by comparing groups of datapoints of stars with similar
B-V.Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
pres
Time series of high resolution spectra of SN 2014J observed with the TIGRE telescope
We present a time series of high resolution spectra of the Type Ia supernova
2014J, which exploded in the nearby galaxy M82. The spectra were obtained with
the HEROS echelle spectrograph installed at the 1.2 m TIGRE telescope. We
present a series of 33 spectra with a resolution of R = 20, 000, which covers
the important bright phases in the evolution of SN 2014J during the period from
January 24 to April 1 of 2014. The spectral evolution of SN 2014J is derived
empirically. The expansion velocities of the Si II P-Cygni features were
measured and show the expected decreasing behaviour, beginning with a high
velocity of 14,000 km/s on January 24. The Ca II infrared triplet feature shows
a high velocity component with expansion velocities of > 20, 000 km/s during
the early evolution apart from the normal component showing similar velocities
as Si II. Further broad P-Cygni profiles are exhibited by the principal lines
of Ca II, Mg II and Fe II. The TIGRE SN 2014J spectra also resolve several very
sharp Na I D doublet absorption components. Our analysis suggests interesting
substructures in the interstellar medium of the host galaxy M82, as well as in
our Milky Way, confirming other work on this SN. We were able to identify the
interstellar absorption of M82 in the lines of Ca II H & K at 3933 and 3968 A
as well as K I at 7664 and 7698 A. Furthermore, we confirm several Diffuse
Interstellar Bands, at wavelengths of 6196, 6283, 6376, 6379 and 6613 A and
give their measured equivalent widths.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Revisiting the cycle-rotation connection for late-type stars
We analyse the relation between the activity cycle length and the Rossby
number and collected a sample of 44 main sequence stars with well-known
activity cycle periods and rotation periods. We find a linear behaviour in the
double-logarithmic relation between the Rossby number and cycle period. The
bifurcation into a long and a short period branch is clearly real but it
depends, empirically, on the colour index B-V, indicating a physical dependence
on effective temperature and position on the main sequence. Furthermore, there
is also a correlation between cycle length and convective turnover time with
the relative depth of the convection zone. Based on this, we derive empirical
relations between cycle period and Rossby number, and for the short period
cycle branch relations, we estimate a scatter of the relative deviation between
14% and 28% on the long-period cycle branch. With these relations, we obtain a
good match with the 10.3 yr period for the well known 11-year solar Schwabe
cycle and a long-period branch value of 104 yr for the Gleissberg cycle of the
Sun. Finally, we suggest that the cycles on the short-period branch appear to
be generated in the deeper layers of the convective zone, while long-period
branch cycles seem to be related to fewer deep layers in that zone. We show
that for a broader B-V range, the Rossby number is a more suitable parameter
for universal relation with cycle-rotation than just the rotation period alone.
As proof, we demonstrate that our empirical stellar relations are consistent
with the 11-year solar Schwabe cycle, in contrast to earlier studies using just
the rotation period in their relations. Previous studies have tried to explain
the cycle position of the Sun in the cycle-rotation presentation via other
kinds of dynamo, however, in our study, no evidence is found that would suggest
another type of dynamo for the Sun and other stars.Comment: 18 pages, 18 figures, accepted for publication in A&
Simultaneous X-ray and optical spectroscopy of the Oef supergiant lambda Cep
Probing the structures of stellar winds is of prime importance for the
understanding of massive stars. Based on their optical spectral morphology and
variability, the stars of the Oef class have been suggested to feature
large-scale structures in their wind. High-resolution X-ray spectroscopy and
time-series of X-ray observations of presumably-single O-type stars can help us
understand the physics of their stellar winds. We have collected XMM-Newton
observations and coordinated optical spectroscopy of the O6Ief star lambda Cep
to study its X-ray and optical variability and to analyse its high-resolution
X-ray spectrum. We investigate the line profile variability of the He II 4686
and H-alpha emission lines in our time series of optical spectra, including a
search for periodicities. We further discuss the variability of the broadband
X-ray flux and analyse the high-resolution spectrum of lambda Cep using
line-by-line fits as well as a code designed to fit the full high-resolution
X-ray spectrum consistently. During our observing campaign, the He II 4686 line
varies on a timescale of ~18 hours. On the contrary, the H-alpha line profile
displays a modulation on a timescale of 4.1 days which is likely the rotation
period of the star. The X-ray flux varies on time-scales of days and could in
fact be modulated by the same 4.1 days period as H-alpha, although both
variations are shifted in phase. The high-resolution X-ray spectrum reveals
broad and skewed emission lines as expected for the X-ray emission from a
distribution of wind-embedded shocks. Most of the X-ray emission arises within
less than 2R* above the photosphere.Comment: Accepted for publication in Astronomy & Astrophysic
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