39 research outputs found
A new approach to distant solar system object detection in large survey data sets
The recently postulated existence of a giant ninth planet in our solar system
has sparked search efforts for distant solar system objects (SSOs) both via new
observations and archival data analysis. Due to the likely faintness of the
object in the optical and infrared regime, it has so far eluded detection. We
set out to re-analyze data acquired by the Wide-Field Infrared Survey Explorer
(WISE), an all-sky survey well suited for the detection of SSOs. We present a
new approach to SSO detection via parallactic fitting. Using the heliocentric
distance as a fit parameter, our code transforms groups of three or more single
observation point sources to heliocentric coordinates under the assumption that
all data stem from an object. The fact that the orbit of a distant SSO is
approximately linear in heliocentric coordinates over long time-scales can be
utilized to produce candidates, which can then be confirmed with follow-up
observations. We demonstrate the feasibility of the approach by a posteriori
detecting the outer SSO Makemake within WISE data. An all-sky search for Planet
Nine yielded no detection. While the postulated Planet Nine eluded detection by
our algorithm, we tentatively predict that this new approach to moving-object
analysis will enable the discovery of new distant SSOs that cannot be
discovered by other algorithms. Especially in cases of sparse data observed
over long time spans, our approach is unique and robust due to the use of only
one fit parameter.Comment: 9 pages, 10 figure
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
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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&
Hybrid star HD 81817 accompanied by brown dwarf or substellar companion
HD 81817 is known as a hybrid star. Hybrid stars have both cool stellar wind
properties and Ultraviolet (UV) or even X-ray emission features of highly
ionized atoms in their spectra. A white dwarf companion has been suggested as
the source of UV or X-ray features. HD 81817 has been observed since 2004 as a
part of a radial velocity (RV) survey program to search for exoplanets around K
giant stars using the Bohyunsan Observatory Echelle Spectrograph at the 1.8 m
telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained 85
RV measurements between 2004 and 2019 for HD 81817 and found two periodic RV
variations. The amplitudes of RV variations are around 200 m s^-1, which are
significantly lower than that expected from a closely orbiting white dwarf
companion. Photometric data and relevant spectral lines were also analyzed to
help determine the origin of the periodic RV variations. We conclude that
627.4-day RV variations are caused by intrinsic stellar activities such as
long-term pulsations or rotational modulations of surface activities based on
H{\alpha} equivalent width (EW) variations of a similar period. On the other
hand, 1047.1-day periodic RV variations are likely to be caused by a brown
dwarf or substellar companion, which is corroborated by a recent GAIA proper
motion anomaly for HD 81817. The Keplerian fit yields a minimum mass of 27.1
M_Jup, a semimajor axis of 3.3 AU, and an eccentricity of 0.17 for the stellar
mass of 4.3 M_sun for HD 81817. The inferred mass puts HD 81817 b in the brown
dwarf desert
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 M⊙ and 2:13±0:01 M⊙ and radii 1:87±0:04 R⊙ and 1:85±0:04 R⊙.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 M⊙ stars. V402 Lac is composed of two stars with masses 2:80 ± 0:05 M⊙ and 2:78 ± 0:05 M⊙ and radii 2:38 ± 0:03 R⊙ and 2:36 ± 0:03 R⊙. 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
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 ΔΩ = −0.039 ± 0.003 rad d−1 and α = ΔΩ/Ωeq = −0.030 ± 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
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&