141 research outputs found
Phase shifts between RVs and stellar variability indicators
Detailed characterisation of small exoplanets is currently hindered by radial velocity signals arising from the star's intrinsic variability. In extracting the precise radial velocities, we can also extract spectral line profile information. These are useful indicators for the stellar variability processes. However, for them to be optimally used in a global model, it is essential to bear in mind that these indicators are not directly correlated with the radial velocities, but rather shifted by several days. In this talk I will give an overview of this effect as studied on Solar data from the HARPS-N spectrograph, which has been continuously gathering data for over six years
Confronting compositional confusion through the characterization of the sub-Neptune orbiting HD 77946
We report on the detailed characterization of the HD 77946 planetary system. HD 77946 is an F5 (M* = 1.17 M⊙, R* = 1.31 R⊙) star, which hosts a transiting planet recently discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and radial velocity observations, and find that the planet, HD 77946 b, orbits with period Pb = 6.527282+0.000015−0.000020 d, has a mass of Mb = 8.38 ± 1.32 M⊕, and a radius of Rb=2.705+0.086−0.081 R⊕. From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a ~1 per cent H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with Teq =1248+40−38 K, water may be in a supercritical state. The characterization of HD 77946 b, adding to the small sample of well-characterized sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways. Furthermore, HD 77946 b has one of the highest transmission spectroscopic metrics for small planets orbiting hot stars, thus transmission spectroscopy of this key planet could prove vital for constraining the compositional confusion that currently surrounds small exoplanets.Peer reviewe
Modelling stellar variability in archival HARPS data: I - rotation and activity properties with multi-dimensional Gaussian processes
Although instruments for measuring the radial velocities (RVs) of stars now routinely reach sub-meter per second accuracy, the detection of low-mass planets is still very challenging. The rotational modulation and evolution of spots and/or faculae can induce variations in the RVs at the level of a few m/s in Sun-like stars. To overcome this, a multi-dimensional Gaussian Process framework has been developed to model the stellar activity signal using spectroscopic activity indicators together with the RVs. A recently published computationally efficient implementation of this framework, S+LEAF 2, enables the rapid analysis of large samples of targets with sizeable data sets. In this work, we apply this framework to HARPS observations of 268 well-observed targets with precisely determined stellar parameters. Our long-term goal is to quantify the effectiveness of this framework to model and mitigate activity signals for stars of different spectral types and activity levels. In this first paper in the series, we initially focus on the activity indicators (S-index and Bisector Inverse Slope), and use them to a) measure rotation periods for 49 slow rotators in our sample, b) explore the impact of these results on the spin-down of middle-aged late F, G & K stars, and c) explore indirectly how the spot to facular ratio varies across our sample. Our results should provide valuable clues for planning future RV planet surveys such as the Terra Hunting Experiment or the PLATO ground-based follow-up observations program, and help fine-tune current stellar structure and evolution models
Modelling stellar variability in archival HARPS data: I -- Rotation and activity properties with multi-dimensional Gaussian Processes
Although instruments for measuring the radial velocities (RVs) of stars now
routinely reach sub-meter per second accuracy, the detection of low-mass
planets is still very challenging. The rotational modulation and evolution of
spots and/or faculae can induce variations in the RVs at the level of a few m/s
in Sun-like stars. To overcome this, a multi-dimensional Gaussian Process
framework has been developed to model the stellar activity signal using
spectroscopic activity indicators together with the RVs. A recently published
computationally efficient implementation of this framework, S+LEAF 2, enables
the rapid analysis of large samples of targets with sizeable data sets. In this
work, we apply this framework to HARPS observations of 268 well-observed
targets with precisely determined stellar parameters. Our long-term goal is to
quantify the effectiveness of this framework to model and mitigate activity
signals for stars of different spectral types and activity levels. In this
first paper in the series, we initially focus on the activity indicators
(S-index and Bisector Inverse Slope), and use them to a) measure rotation
periods for 49 slow rotators in our sample, b) explore the impact of these
results on the spin-down of middle-aged late F, G & K stars, and c) explore
indirectly how the spot to facular ratio varies across our sample. Our results
should provide valuable clues for planning future RV planet surveys such as the
Terra Hunting Experiment or the PLATO ground-based follow-up observations
program, and help fine-tune current stellar structure and evolution models.Comment: Accepted for publication in MNRA
Modelling stellar variability in archival HARPS data:I -- Rotation and activity properties with multi-dimensional Gaussian Processes
Although instruments for measuring the radial velocities (RVs) of stars now routinely reach sub-meter per second accuracy, the detection of low-mass planets is still very challenging. The rotational modulation and evolution of spots and/or faculae can induce variations in the RVs at the level of a few m/s in Sun-like stars. To overcome this, a multi-dimensional Gaussian Process framework has been developed to model the stellar activity signal using spectroscopic activity indicators together with the RVs. A recently published computationally efficient implementation of this framework, S+LEAF 2, enables the rapid analysis of large samples of targets with sizeable data sets. In this work, we apply this framework to HARPS observations of 268 well-observed targets with precisely determined stellar parameters. Our long-term goal is to quantify the effectiveness of this framework to model and mitigate activity signals for stars of different spectral types and activity levels. In this first paper in the series, we initially focus on the activity indicators (S-index and Bisector Inverse Slope), and use them to a) measure rotation periods for 49 slow rotators in our sample, b) explore the impact of these results on the spin-down of middle-aged late F, G & K stars, and c) explore indirectly how the spot to facular ratio varies across our sample. Our results should provide valuable clues for planning future RV planet surveys such as the Terra Hunting Experiment or the PLATO ground-based follow-up observations program, and help fine-tune current stellar structure and evolution models
ABORAS: polarimetric, 10cm/s RV observations of the Sun as a star
We present a description of A dual-Beam pOlarimetric Robotic Aperture for the
Sun (ABORAS), to serve as a Solar input with a dedicated Stokes V polarimeter
for the HARPS3 high-resolution spectrograph. ABORAS has three main science
drivers: trying to understand the physics behind stellar variability, tracking
the long-term stability of HARPS3, and serve as a benchmark for Earth-sized
exoplanet detection with HARPS3 by injecting an Earth RV signal into the data.
By design, ABORAS will (together with the HARPS3 instrument) be able to measure
10cm/s variations in RV of the integrated Solar disk and detect integrated
magnetic field levels at sub 1 Gauss level through circularly polarized light.Comment: 8 pages, 5 figures, SPIE Proceedings pre-print, draft version
Sub-m s upper limits from a deep HARPS-N radial-velocity search for planets orbiting HD 166620 and HD 144579
Minimising the impact of stellar variability in Radial Velocity (RV)
measurements is a critical challenge in achieving the 10 cm s precision
needed to hunt for Earth twins. Since 2012, a dedicated programme has been
underway with HARPS-N, to conduct a blind RV Rocky Planets Search (RPS) around
bright stars in the Northern Hemisphere. Here we describe the results of a
comprehensive search for planetary systems in two RPS targets, HD 166620 and HD
144579. Using wavelength-domain line-profile decorrelation vectors to mitigate
the stellar activity and performing a deep search for planetary reflex motions
using a trans-dimensional nested sampler, we found no significant planetary
signals in the data sets of either of the stars. We validated the results via
data-splitting and injection recovery tests. Additionally, we obtained the 95th
percentile detection limits on the HARPS-N RVs. We found that the likelihood of
finding a low-mass planet increases noticeably across a wide period range when
the inherent stellar variability is corrected for using scalpels U-vectors. We
are able to detect planet signals with M for orbital
periods shorter than 10 days. We demonstrate that with our decorrelation
technique, we are able to detect signals as low as 54 cm s, which brings
us closer to the calibration limit of 50 cm s demonstrated by HARPS-N.
Therefore, we show that we can push down towards the RV precision required to
find Earth analogues using high-precision radial velocity data with novel
data-analysis techniques.Comment: 7 tables, 24 figures (including those in appendix
Characteristics and spread to the native population of HIV-1 non-B subtypes in two European countries with high migration rate
Background: Non-B subtypes account for at least 50 % of HIV-1 infections diagnosed in Belgium and Luxembourg. They are considered to be acquired through heterosexual contacts and infect primarily individuals of foreign origin. Information on the extent to which non-B subtypes spread to the local population is incomplete.
Methods: Pol and env gene sequences were collected from 410 non-subtype B infections. Profound subtyping was performed using 5 subtyping tools and sequences of both pol and env. Demographic information, disease markers (viral load, CD4 count) and viral characteristics (co-receptor tropism) were compared between subtypes. Maximum likelihood phylogenetic trees were constructed and examined for clustering.
Results: The majority of non-B infections were diagnosed in patients originating from Africa (55.8 %), individuals born in Western Europe represented 30.5 %. Heterosexual transmission was the most frequently reported transmission route (79.9 %), MSM transmission accounted for 12.2 % and was significantly more frequently reported for Western Europeans (25.7 % versus 4.3 % for individuals originating from other regions; p < 0.001). Subtypes A and C and the circulating recombinant forms CRF01_AE and CRF02_AG were the most represented and were included in the comparative analysis. Native Western Europeans were underrepresented for subtype A (14.5 %) and overrepresented for CRF01_AE (38.6 %). The frequency of MSM transmission was the highest for CRF01_AE (18.2 %) and the lowest for subtype A (0 %). No differences in age, gender, viral load or CD4 count were observed. Prevalence of CXCR4-use differed between subtypes but largely depended on the tropism prediction algorithm applied. Indications for novel intersubtype recombinants were found in 20 patients (6.3 %). Phylogenetic analysis revealed only few and small clusters of local transmission but could document one cluster of CRF02_AG transmission among Belgian MSM.
Conclusions: The extent to which non-B subtypes spread in the native Belgian-Luxembourg population is higher than expected, with 30.5 % of the non-B infections diagnosed in native Western Europeans. These infections resulted from hetero-as well as homosexual transmission. Introduction of non-B variants in the local high at risk population of MSM may lead to new sub-epidemics and/or increased genetic variability and is an evolution that needs to be closely monitored
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