- ID:
- ivo://CDS.VizieR/J/A+A/597/A52
- Title:
- K and G dwarfs stellar granulation variability
- Short Name:
- J/A+A/597/A52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In solar-type stars, the attenuation of convective blueshift by stellar magnetic activity dominates the RV (radial velocity) variations over the low amplitude signal induced by low mass planets. Models of stars that differ from the Sun will require a good knowledge of the attenuation of the convective blueshift to estimate its impact on the variations. It is therefore crucial to precisely determine not only the amplitude of the convective blueshift for different types of stars, but also the dependence of this convective blueshift on magnetic activity, as these are key factors in our model producing the RV. We studied a sample of main sequence stars with spectral types from G0 to K2 and focused on their temporally averaged properties: the activity level and a criterion allowing to characterise the amplitude of the convective blueshift. This criterion is derived from the dependence of the convective blueshift with the intensity at the bottom of a large set of selected spectral lines.
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- ID:
- ivo://CDS.VizieR/J/A+A/607/A124
- Title:
- K-G-F dwarfs stellar granulation variability
- Short Name:
- J/A+A/607/A124
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The inhibition of small-scale convection in the Sun dominates the long-term radial velocity (RV) variability: it therefore has a critical effect on light exoplanet detectability using RV techniques. We here extend our previous analysis of stellar convective blueshift and its dependence on magnetic activity to a larger sample of stars in order to extend the Teff range, to study the impact of other stellar properties, and finally to improve the comparison between observed RV jitter and expected RV variations. Methods. We estimate a differential velocity shift for Fe and Ti lines of different depths and derive an absolute convective blueshift using the Sun as a reference for a sample of 360 F7-K4 stars with different properties (age, Teff, metallicity). We confirm the strong variation in convective blueshift with Teff and its dependence on (as shown in the line list in Paper I) activity level. Although we do not observe a significant effect of age or cyclic activity, stars with a higher metallicity tend to have a lower convective blueshift, with a larger effect than expected from numerical simulations. Finally, we estimate that for 71% of the stars in our sample the RV and LogR'_HK_ variations are compatible with the effect of activity on convection, as observed in the solar case, while for the other stars, other sources (such as binarity or companions) must be invoked to explain the large RV variations. We also confirm a relationship between LogR'_HK_ and metallicity, which may affect discussions of the possible relationship between metallicity and exoplanets, as RV surveys are biased toward low LogR'_HK_ and possibly toward high-metallicity stars. We conclude that activity and metallicity strongly affect the small-scale convection levels in stars in the F7-K4 range, with a lower amplitude for the lower mass stars and a larger amplitude for low-metallicity stars.
- ID:
- ivo://CDS.VizieR/J/A+A/598/A62
- Title:
- KIC red giants showing depressed mixed modes
- Short Name:
- J/A+A/598/A62
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Seismic observations with the space-borne Kepler mission have shown that a number of evolved stars exhibit low-amplitude dipole modes, which is referred to as depressed modes. Recently, these low amplitudes have been attributed to the presence of a strong magnetic field in the stellar core of those stars. Subsequently, and based on this scenario, the prevalence of high magnetic fields in evolved stars has been inferred. It should be noted, however, that this conclusion remains indirect. We intend to study the properties of mode depression in evolved stars, which is a necessary condition before reaching conclusions about the physical nature of the mechanism responsible for the reduction of the dipole mode amplitudes. We perform a thorough characterization of the global seismic parameters of depressed dipole modes and show that these modes have a mixed character. The observation of stars showing dipole mixed modes that are depressed is especially useful for deriving model-independent conclusions on the dipole mode damping. We use a simple model to explain how mode visibilities are connected to the extra damping seen in depressed modes. Results. Observations prove that depressed dipole modes in red giants are not pure pressure modes but mixed modes. This result, observed in more than 90% of the bright stars (m_V_<=11), invalidates the hypothesis that depressed dipole modes result from the suppression of the oscillation in the radiative core of the stars. Observations also show that, except for visibility, seismic properties of the stars with depressed modes are equivalent to those of normal stars. The measurement of the extra damping that is responsible for the reduction of mode amplitudes, without any prior on its physical nature, potentially provides an efficient tool for elucidating the mechanism responsible for the mode depression. The mixed nature of the depressed modes in red giants and their unperturbed global seismic parameters carry strong constraints on the physical mechanism responsible for the damping of the oscillation in the core. This mechanism is able to damp the oscillation in the core but cannot fully suppress it. Moreover, it cannot modify the radiative cavity probed by the gravity component of the mixed modes. The recent mechanism involving high magnetic fields proposed for explaining depressed modes is not compliant with the observations and cannot be used to infer the strength and prevalence of high magnetic fields in red giants.
- ID:
- ivo://CDS.VizieR/J/ApJS/141/469
- Title:
- Linear polarization catalog toward HD 62542
- Short Name:
- J/ApJS/141/469
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have obtained linear polarization measurements of stars along the western side of the IRAS Vela Shell toward HD 62542. From 16 CCD fields distributed along the ionization front (I-front) we have built a catalog of 856 objects with polarization signal-to-noise ratio larger than 10. We detect very significant levels of polarization and hence an appreciable magnetic field throughout the region. Composite polarization maps around the I-front are shown. In some regions the polarization vectors are parallel to the I-front, but a perpendicular trend is also evident along parts of the front. In addition, the polarization pattern seems to be affected by gas streaming inside the cloud. The observations were made using a CCD imaging polarimeter attached to the 1.5m Telescope of Cerro Tololo Inter-American Observatory (CTIO), Chile. The data were collected in 1991 December and 1992 December.
- ID:
- ivo://CDS.VizieR/J/A+A/653/A155
- Title:
- Lockman Hole Polarised Sources at 1.4GHz
- Short Name:
- J/A+A/653/A155
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the context of structure formation and galaxy evolution, the contribution of magnetic fields is not well understood. Feedback processes originating from active galactic nucleus (AGN) activity and star formation can be actively influenced by magnetic fields, depending on their strength and morphology. One of the best tracers of magnetic fields is polarised radio emission. Tracing this emission over a broad redshift range therefore allows an investigation of these fields and their evolution. We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. We use a 15-pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the Westerbork Synthesis Radio Telescope (WSRT). The data were analysed using the rotation measure synthesis technique. We achieved a noise of 7uJy/beam in polarised intensity, with a resolution of 15". Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one-third of our detected polarised sources. Results. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4%. No source was found with a fractional polarisation higher than 21%. With a total area of 6.5deg^2^ and a detection threshold of 6.25{sigma}, we find 23 polarised sources per deg^2^. Based on our multi-wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance (i.e. large-scale structures). Considering the absolute radio luminosity, we find a general trend of increased probability of detecting weak sources at low redshift and strong sources at high redshift. We attribute this trend to a selection bias. Further, we find an anti-correlation between fractional polarisation and redshift for our strong-source sample at z>=0.6. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales; however, the changing properties of cluster environments over cosmic time may play an important role. Disentangling these two effects requires deeper and wider polarisation observations as well as better models of the morphology and strength of cosmic magnetic fields.
- ID:
- ivo://CDS.VizieR/J/A+A/583/A137
- Title:
- LOFAR-HBA 3C196 field RM cubes
- Short Name:
- J/A+A/583/A137
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project. We have used the high band antennas (HBA) of LOFAR to image this region and Rotation Measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth. The brightness temperature of the detected Galactic emission is 5-15K in polarized intensity and covers the range from -3 to +8 rad/m^2^ in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad/m^2^ running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto-ionic medium. The mean line-of-sight magnetic field component is determined to be 0.3-/+0.1uG and its spatial variation across the 3C196 field is 0.1uG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n_e_*B_||_>6.2cm^-3^uG) compared to its surroundings.
- ID:
- ivo://CDS.VizieR/J/A+A/658/A4
- Title:
- LOFAR imaging of Arp299 at 150MHz
- Short Name:
- J/A+A/658/A4
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- We present the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4GHz Very Large Array (VLA) observations. Nowadays, LOFAR allows to compare its resolution with the VLA ones. These will allow us to obtain pixel-by-pixel spectral imdex maps and local radio SEDs. With this information we recover the nature of the ISM around the main structures (either clumpy or continuous), as well as magnetic fields. The 150MHz image also shows structures never seen before, pointing out that these radio-frequencies observations are a perfect tool to detect them.
- ID:
- ivo://CDS.VizieR/J/MNRAS/484/3646
- Title:
- LOFAR PSR low-frequency Faraday rotation measures
- Short Name:
- J/MNRAS/484/3646
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We determined Faraday rotation measures (RMs) towards 137 pulsars in the northern sky, using Low-Frequency Array (LOFAR) observations at 110-190MHz. This low-frequency RM catalogue, the largest to date, improves the precision of existing RM measurements on average by a factor of 20 - due to the low frequency and wide bandwidth of the data, aided by the RM synthesis method. We report RMs towards 25 pulsars for the first time. The RMs were corrected for ionospheric Faraday rotation to increase the accuracy of our catalogue to approximately 0.1rad/m^2^. The ionospheric RM correction is currently the largest contributor to the measurement uncertainty. In addition, we find that the Faraday dispersion functions towards pulsars are extremely Faraday thin - mostly less than 0.001rad/m^2^. We use these new precise RM measurements (in combination with existing RMs, dispersion measures, and distance estimates) to estimate the scale height of the Galactic halo magnetic field: 2.0(3)kpc for Galactic quadrants I and II above and below the Galactic plane (we also evaluate the scale height for these regions individually). Overall, our initial low-frequency catalogue provides valuable information about the 3-D structure of the Galactic magnetic field.
- ID:
- ivo://CDS.VizieR/J/MNRAS/499/5379
- Title:
- Longitudinal magnetic field of 6 B stars
- Short Name:
- J/MNRAS/499/5379
- Date:
- 02 Feb 2022 09:52:06
- Publisher:
- CDS
- Description:
- Rapidly rotating early-type stars with strong magnetic fields frequently show H{alpha} emission originating in centrifugal magnetospheres (CMs), circumstellar structures in which centrifugal support due to magnetically enforced corotation of the magnetically confined plasma enables it to accumulate to high densities. It is not currently known whether the CM plasma escapes via centrifugal breakout (CB), or by an unidentified leakage mechanism. We have conducted the first comprehensive examination of the H{alpha} emission properties of all stars currently known to display CM-pattern emission. We find that the onset of emission is dependent primarily on the area of the CM, which can be predicted simply by the value B_K_ of the magnetic field at the Kepler corotation radius R_K_. Emission strength is strongly sensitive to both CM area and B_K_. Emission onset and strength are not dependent on effective temperature, luminosity, or mass-loss rate. These results all favour a CB scenario; however, the lack of intrinsic variability in any CM diagnostics indicates that CB must be an essentially continuous process, i.e. it effectively acts as a leakage mechanism. We also show that the emission profile shapes are approximately scale-invariant, i.e. they are broadly similar across a wide range of emission strengths and stellar parameters. While the radius of maximum emission correlates closely as expected to R_K_, it is always larger, contradicting models that predict that emission should peak at R_K_.
- ID:
- ivo://CDS.VizieR/J/A+A/622/A23
- Title:
- LoTSS compact groups of galaxies
- Short Name:
- J/A+A/622/A23
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use the LOFAR Two-metre Sky Survey (LoTSS) Data Release I to identify the groups of galaxies (and individual galaxies) from the Hickson compact groups (HCG) and magnitude-limited compact groups (MLCG) samples that emit at the frequency of 150MHz, characterise their radio emission (extended or limited to the galaxies), and compare new results to earlier observations and theoretical predictions. The detection of 73 systems (and 7 more - probably) out of 120, of which as many as 17 show the presence of extended radio structures, confirms the previous hypothesis of the common character of the magnetic field inside galaxy groups and its detectability. In order to investigate the future potential of low-frequency radio studies of galaxy groups, we also present a more detailed insight into four radio-emitting systems, for which the strength of the magnetic field inside their intergalactic medium (IGM) is calculated. The estimated values are comparable to that found inside star-forming galaxies, suggesting a dynamical and evolutionary importance of the magnetic field in galaxy groups.
