38 research outputs found
X-ray and UV emission of the ultrashort-period, low-mass eclipsing binary system BX Tri
Close binary systems provide an excellent tool to determine stellar
parameters such as radii and masses with a high degree of precision. Due to the
high rotational velocities, most of these systems exhibit strong signs of
magnetic activity, which has been postulated to be the underlying reason for
radius inflation in many of the components. We aim to extend the sample of
low-mass binary systems with well-known X-ray properties. For this, we analyze
data from a singular XMM-Newton pointing of the close, low-mass eclipsing
binary system BX Tri. The UV light curve is modeled with the eclipsing binary
modeling tool PHOEBE and data acquired with the EPIC cameras is analyzed to
search for hints of orbital modulation. We find clear evidence of orbital
modulation in the UV light curve and show that PHOEBE is fully capable of
modeling data within this wavelength range. Comparison to a theoretical flux
prediction based on PHOENIX models shows that the majority of UV emission is of
photospheric origin. While the X-ray light curve does exhibit strong
variations, the signal-to-noise ratio of the observation is insufficient for a
clear detection of signs of orbital modulation. There is evidence of a
Neupert-like correlation between UV and X-ray data.Comment: 6 pages, 8 figures; Received 22 August 2018 / Accepted 4 September
201
Chromospheric activity and photospheric variation of Ori during the great dimming event in 2020
The so-called great dimming event of alpha Ori in late 2019 and early 2020
sparked our interest in the behaviour of chromospheric activity during this
period. To study the timeline of chromospheric activity, we derive a S_MWO time
series of TIGRE and Mount Wilson values, and we compare this long time series
with photometric data from the AAVSO database. In addition, we determine the
absolute and normalised excess flux of the Ca II H&K lines. To do so, we
estimate the changing effective temperature from TIGRE spectra and find a clear
drop of about 80 K between November 2019 and February 2020, which coincides
with the minimum of visual brightness. During the same period, the S-index
increased significantly, yet this is a mere contrast effect, because the
normalised excess flux of the Ca II H&K lines did not change significantly.
However, the latter dropped immediately after this episode. Comparing the
combined S_MWO values and visual magnitude time series, we find a similar
increase in the S-index during another noticeable decrease in the visual
magnitude of alpha Ori, which took place in 1984 and 1985. To also probe the
dynamics of the upper photosphere, we analysed the lines in 6251-6263 A and
found core distance varies which shows a relation with the great dimming event.
This type of variation could be caused by rising and sinking cool plumes as a
temporary spill-over of convection on alpha Ori. Based on our study, we
conclude that the cause for the great dimming is located in the photosphere.
Furthermore, the long-term spectroscopic and photometric time series suggests
that this great dimming does not appear to be a unique phenomenon, but rather
that such dimmings do occur more frequently, which motivates further monitoring
of alpha Ori with facilities such as TIGRE.Comment: 18 pages, 27 figures, accepted for publication in A&
The first Doppler imaging of the active binary prototype RS Canum Venaticorum
We present the first Doppler images of the prototypical active binary star RS Canum Venaticorum, derived from high-resolution spectra observed in 2004, 2016 and 2017, using three different telescopes and observing sites. We apply the least-squares deconvolution technique to all observed spectra to obtain high signal-to-noise line profiles, which are used to derive the surface images of the active K-type component. Our images show a complex spot pattern on the K star, distributed widely in longitude. All star-spots revealed by our Doppler images are located below a latitude of about 70°. In accordance with previous light-curve modelling studies, we find no indication of a polar spot on the K star. Using Doppler images derived from two consecutive rotational cycles, we estimate a surface differential rotation rate of ÎΩ = â0.039 ± 0.003 rad dâ1 and α = ÎΩ/Ωeq = â0.030 ± 0.002 for the K star. Given the limited phase coverage during those two rotations, the uncertainty of our differential rotation estimate is presumably higher
Absolute dimensions and apsidal motion of the eclipsing binaries V889 Aquilae and V402 Lacertae
Context. Double-lined eclipsing binaries allow the direct determination of masses and radii, which are key for testing stellar models. With the launch of the TESS mission, many well-known eclipsing binaries have been observed at higher photometric precision, permitting the improvement of the absolute dimension determinations. Aims. Using TESS data and newly obtained spectroscopic observations, we aim to determine the masses and radii of the eccentric eclipsing binary systems V889 Aql and V402 Lac, together with their apsidal motion parameters. Methods. We simultaneously modelled radial velocity curves and times of eclipse for each target to precisely determine the orbital parameters of the systems, which we used to analyse the light curves and then obtain their absolute dimensions. We compared the obtained values with those predicted by theoretical models. Results. We determined masses and radii of the components of both systems with relative uncertainties lower than 2%. V889 Aql is composed of two stars with masses 2:17±0:02 Mâ and 2:13±0:01 Mâ and radii 1:87±0:04 Râ and 1:85±0:04 Râ.We find conclusive evidence of the presence of a third body orbiting V889 Aql with a period of 67 yr. Based on the detected third light and the absence of signal in the spectra, we suggest that this third body could in turn be a binary composed of two ±1.4 Mâ stars. V402 Lac is composed of two stars with masses 2:80 ± 0:05 Mâ and 2:78 ± 0:05 Mâ and radii 2:38 ± 0:03 Râ and 2:36 ± 0:03 Râ. The times of minimum light are compatible with the presence of a third body for this system too, although its period is not yet fully sampled. In both cases we have found a good agreement between the observed apsidal motion rates and the model predictions
Long-term variations in the X-ray activity of HR 1099
Although timing variations in close binary systems have been studied for a
long time, their underlying causes are still unclear. A possible explanation is
the so-called Applegate mechanism, where a strong, variable magnetic field can
periodically change the gravitational quadrupole moment of a stellar component,
thus causing observable period changes. One of the systems exhibiting such
strong orbital variations is the RS CVn binary HR 1099, whose activity cycle
has been studied by various authors via photospheric and chromospheric activity
indicators, resulting in contradicting periods. We aim at independently
determining the magnetic activity cycle of HR 1099 using archival X-ray data to
allow for a comparison to orbital period variations. Archival X-ray data from
80 different observations of HR 1099 acquired with 12 different X-ray
facilities and covering almost four decades were used to determine X-ray fluxes
in the energy range of 2-10 keV via spectral fitting and flux conversion. Via
the Lomb-Scargle periodogram we analyze the resulting long-term X-ray light
curve to search for periodicities. We do not detect any statistically
significant periodicities within the X-ray data. An analysis of optical data of
HR 1099 shows that the derivation of such periods is strongly dependent on the
time coverage of available data, since the observed optical variations strongly
deviate from a pure sine wave. We argue that this offers an explanation as to
why other authors derive such a wide range of activity cycle periods based on
optical data. We conclude that our analysis constitutes the longest stellar
X-ray activity light curve acquired to date, yet the still rather sparse
sampling of the X-ray data, along with stochastic flaring activity, does not
allow for the independent determination of an X-ray activity cycle.Comment: 8 pages, 6 figures, 2 tables accepted for publication in A&
On the nature of the candidate T-Tauri star V501 Aurigae
We report new multi-colour photometry and high-resolution spectroscopic
observations of the long-period variable V501 Aur, previously considered to be
a weak-lined T-Tauri star belonging to the Taurus-Auriga star-forming region.
The spectroscopic observations reveal that V501 Aur is a single-lined
spectroscopic binary system with a 68.8-day orbital period, a slightly
eccentric orbit (e ~ 0.03), and a systemic velocity discrepant from the mean of
Taurus-Auriga. The photometry shows quasi-periodic variations on a different,
~55-day timescale that we attribute to rotational modulation by spots. No
eclipses are seen. The visible object is a rapidly rotating (vsini ~ 25 km/s)
early K star, which along with the rotation period implies it must be large (R
> 26.3 Rsun), as suggested also by spectroscopic estimates indicating a low
surface gravity. The parallax from the Gaia mission and other independent
estimates imply a distance much greater than the Taurus-Auriga region,
consistent with the giant interpretation. Taken together, this evidence
together with a re-evaluation of the LiI~6707 and H lines
shows that V501 Aur is not a T-Tauri star, but is instead a field binary with a
giant primary far behind the Taurus-Auriga star-forming region. The large mass
function from the spectroscopic orbit and a comparison with stellar evolution
models suggest the secondary may be an early-type main-sequence star.Comment: 13 pages, 7 figures. Accepted to MNRA
Time evolution of magnetic activity cycles in young suns: the curious case of ÎșCeti
Stars and planetary system
The CARMENES search for exoplanets around M dwarfs, Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star
We present the discovery of an Earth-mass planet () on a 15.6d orbit of a relatively nearby (9.6pc)
and low-mass () M5.0V star, Wolf 1069. Sitting at a
separation of au away from the host star puts Wolf 1069b in
the habitable zone (HZ), receiving an incident flux of
. The planetary signal was detected using
telluric-corrected radial-velocity (RV) data from the CARMENES spectrograph,
amounting to a total of 262 spectroscopic observations covering almost four
years. There are additional long-period signals in the RVs, one of which we
attribute to the stellar rotation period. This is possible thanks to our
photometric analysis including new, well-sampled monitoring campaigns undergone
with the OSN and TJO facilities that supplement archival photometry (i.e., from
MEarth and SuperWASP), and this yielded an updated rotational period range of
d, with a likely value at d. The stellar
activity indicators provided by the CARMENES spectra likewise demonstrate
evidence for the slow rotation period, though not as accurately due to possible
factors such as signal aliasing or spot evolution. Our detectability limits
indicate that additional planets more massive than one Earth mass with orbital
periods of less than 10 days can be ruled out, suggesting that perhaps Wolf
1069 b had a violent formation history. This planet is also the 6th closest
Earth-mass planet situated in the conservative HZ, after Proxima Centauri b, GJ
1061d, Teegarden's Star c, and GJ 1002 b and c. Despite not transiting, Wolf
1069b is nonetheless a very promising target for future three-dimensional
climate models to investigate various habitability cases as well as for
sub-ms RV campaigns to search for potential inner sub-Earth-mass planets
in order to test planet formation theories.Comment: 26 pages, 15 figure