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491. Active extragalactic objects II.
ID:
ivo://CDS.VizieR/J/AJ/87/1429
Title:
Active extragalactic objects II.
Short Name:
J/AJ/87/1429
Date:
21 Oct 2021
Publisher:
CDS
Description:
A single primary-beam area centered on alpha=08h52m16s, Delta=+17d16' (1950.0) was surveyed with the Very Large Array at 1.452 GHz. The rms map noise is 0.030 + 0.001 mJy, and the resulting source list is complete to a map peak flux density S_p = 0.2 mJy within 25 arcmin of the field center. Source counts between 0.2 and 5 mJy were determined. Our best-fit model for the counts suggests that the faintest sources have absolute radio luminosities L < 10^24 W/Hz and do not evolve significantly. Sources stronger than 0.2 mJy appear to account for most of the radio-source background. The rms sky-temperature fluctuation in the central 10'x10' of the map is less than 0.029 K (>99% confidence) on an angular scale ~19 arcsec. This limit constrains theories which attribute the cosmic microwave background to thermalized radiation from massive pregalactic stars or bursts of star formation in young galaxies.
492. Active Extragalactic Objects III
ID:
ivo://CDS.VizieR/J/AJ/90/1437
Title:
Active Extragalactic Objects III
Short Name:
J/AJ/90/1437
Date:
21 Oct 2021
Publisher:
CDS
Description:
Three selected fields were observed with the Very Large Array (VLA) at 1411MHz as part of an ongoing project to compile complete samples of active extragalactic objects. The radio-source list for the field "A" centered on alpha=08h54m, delta=+17d30' is presented. 67 maps cover the area 3x3deg so that every point lies within at least one primary half-power beamwidth. The median map noise is 0.5mJy and the 6{sigma} completeness level is less than 6mJy over half the field. The synthesized point-source response is an elliptical Gaussian approximately 20x17" between half-intensity points. The areal density of sources with flux densities in the range 5<S<150mJy was determined. Optical identifications based on position coincidences with 90 objects brighter than B=~21.5 in the A and B survey fields are reported.
493. Active galactic nuclei from HeII
ID:
ivo://CDS.VizieR/J/MNRAS/466/2879
Title:
Active galactic nuclei from HeII
Short Name:
J/MNRAS/466/2879
Date:
21 Oct 2021
Publisher:
CDS
Description:
In order to perform a more complete census of active galactic nuclei (AGN) in the local Universe, we investigate the use of the HeII {lambda}4685 emission line diagnostic diagram by Shirazi & Brinchmann (2012, Cat. J/MNRAS/421/1043) in addition to the standard methods based on other optical emission lines. The HeII-based diagnostics is more sensitive to AGN ionization in the presence of strong star formation than conventional line diagnostics. We survey a magnitude-limited sample of 63915 galaxies from the Sloan Digital Sky Survey Data Release 7 at 0.02<z<0.05 and use both the conventional BPT emission line diagnostic diagrams, as well as the HeII diagram to identify AGN. In this sample, 1075 galaxies are selected as AGN using the BPT diagram, while additional 234 galaxies are identified as AGN using the HeII diagnostic diagram, representing a 22 per cent increase of AGN in the parent galaxy sample. We explore the host galaxy properties of these new HeII-selected AGN candidates and find that they are most common in star-forming galaxies on the blue cloud and on the main sequence where ionization from star formation is most likely to mask AGN emission in the BPT lines. We note in particular a high He II AGN fraction in galaxies above the high-mass end of the main sequence where quenching is expected to occur. We use archival Chandra observations to confirm the AGN nature of candidates selected through He II-based diagnostic. Finally, we discuss how this technique can help inform galaxy/black hole coevolution scenarios.
494. Active Kepler stars differential rotation
ID:
ivo://CDS.VizieR/J/A+A/583/A65
Title:
Active Kepler stars differential rotation
Short Name:
J/A+A/583/A65
Date:
21 Oct 2021
Publisher:
CDS
Description:
In addition to the discovery of hundreds of exoplanets, the high-precision photometry from the CoRoT and Kepler satellites has led to measurements of surface rotation periods for tens of thousands of stars, which can potentially be used to infer stellar ages via gyrochronology. Our main goal is to derive ages of thousands of field stars using consistent rotation period measurements derived by different methods. Multiple rotation periods are interpreted as surface differential rotation (DR). We study the dependence of DR with rotation period and effective temperature. We reanalyze a previously studied sample of 24,124 Kepler stars using different approaches based on the Lomb- Scargle periodogram. Each quarter (Q1-Q14) is treated individually using a prewhitening approach. Additionally, the full time series and their different segments are analyzed.
495. Active radio galaxies in cluster A569
ID:
ivo://CDS.VizieR/J/AZh/79/963
Title:
Active radio galaxies in cluster A569
Short Name:
J/AZh/79/963
Date:
21 Oct 2021
Publisher:
CDS
Description:
The nature of bright radio sources with known radio spectra in the direction of the nearby cluster of galaxies A569 (z=0.0193) is investigated. The optical identifications of the sources were provided. 45% of these radio sources are associated with compact galaxies. A substantial fraction of these galaxies have active nuclei. Some of them have radio halos.
496. Active red giants asteroseismic & rotation param.
ID:
ivo://CDS.VizieR/J/A+A/639/A63
Title:
Active red giants asteroseismic & rotation param.
Short Name:
J/A+A/639/A63
Date:
08 Feb 2022
Publisher:
CDS
Description:
Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, and access to their fundamental properties enable detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, standing upon the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We select a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (~8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 non-oscillators are in this group of binaries. To the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones less so. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measure are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets not available in the Sun.
497. Activity and accretion in {gamma} Vel and Cha I
ID:
ivo://CDS.VizieR/J/A+A/575/A4
Title:
Activity and accretion in {gamma} Vel and Cha I
Short Name:
J/A+A/575/A4
Date:
21 Oct 2021
Publisher:
CDS
Description:
We use the fundamental parameters (effective temperature, surface gravity, lithium abundance, and radial velocity) delivered by the GES consortium in the first internal data release to select the members of Gamma Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 Gamma Vel members and 74 Cha I members were studied. The procedure adopted by the GES to derive stellar fundamental parameters provided also measures of the projected rotational velocity (vsini). We calculated stellar luminosities through spectral energy distributions, while stellar masses were derived by comparison with evolutionary tracks. The spectral subtraction of low-activity and slowly rotating templates, which are rotationally broadened to match the vsini of the targets, enabled us to measure the equivalent widths (EWs) and the fluxes in the H{alpha} and H{beta} lines. The H{alpha} line was also used for identifying accreting objects, on the basis of its equivalent width and the width at the 10% of the line peak (10%W), and for evaluating the mass accretion rate (M_acc_). The distribution of vsini for the members of Gamma Vel displays a peak at about 10km/s with a tail toward faster rotators. There is also some indication of a different vsini distribution for the members of its two kinematical populations. Most of these stars have H{alpha} fluxes corresponding to a saturated activity regime. We find a similar distribution, but with a narrower peak, for Cha I. Only a handful of stars in Gamma Vel display signatures of accretion, while many more accretors were detected in the younger Cha I, where the highest H{alpha} fluxes are mostly due to accretion, rather than to chromospheric activity. Accreting and active stars occupy two different regions in a T_eff-flux diagram and we propose a criterion for distinguishing them. We derive M_acc in the ranges 10^-11^-10^-9^M_{sun}_/yr and 10^-10^-10^-7^M_{sun}_/yr for Gamma Vel and Cha I accretors, respectively.
498. Activity and kinematics of ultracool dwarfs
ID:
ivo://CDS.VizieR/J/AJ/133/2258
Title:
Activity and kinematics of ultracool dwarfs
Short Name:
J/AJ/133/2258
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the activity and kinematics of a representative volume-limited (20pc) sample of 152 late-M and L dwarfs (M7-L8) photometrically selected from the Two Micron All Sky Survey (2MASS). Using new proper-motion measurements and spectrophotometric distance estimates, we calculate tangential velocities. The sample has a mean tangential velocity of <V_tan_>=31.5km/s, a velocity dispersion of {sigma}_tan_=20.7km/s, and a maximum tangential velocity of V_tan_=138.8km/s. These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood. H{alpha} emission, an indicator of chromospheric activity, was detected in 63 of 81 late-M dwarfs and in 16 of 69 L dwarfs examined. We find a lack of correlation between activity strength, measured by log(FH{alpha}/Fbol), and V_tan_, although velocity distributions suggest that the active dwarfs in our sample are slightly younger than the inactive dwarfs. Consistent with previous studies of activity in ultracool dwarfs, we find that the fraction of H-emitting objects per spectral type peaks at spectral type M7 and declines through mid-L dwarfs. Activity strength is similarly correlated with spectral type for types later than M7. Eleven dwarfs out of 150 show evidence of variability, ranging from small fluctuations to large flare events. We estimate a flare cycle of 5% for late-M dwarfs and 2% for L dwarfs. Observations of strong, variable activity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event on the M7 dwarf 2MASS J1028404-143843 are discussed.
499. Activity and rotation in Praesepe and the Hyades
ID:
ivo://CDS.VizieR/J/ApJ/795/161
Title:
Activity and rotation in Praesepe and the Hyades
Short Name:
J/ApJ/795/161
Date:
21 Oct 2021
Publisher:
CDS
Description:
Open clusters are collections of stars with a single, well-determined age, and can be used to investigate the connections between angular-momentum evolution and magnetic activity over a star's lifetime. We present the results of a comparative study of the relationship between stellar rotation and activity in two benchmark open clusters: Praesepe and the Hyades. As they have the same age and roughly solar metallicity, these clusters serve as an ideal laboratory for testing the agreement between theoretical and empirical rotation-activity relations at ~600 Myr. We have compiled a sample of 720 spectra - more than half of which are new observations - for 516 high-confidence members of Praesepe; we have also obtained 139 new spectra for 130 high-confidence Hyads. We have also collected rotation periods (P_rot_) for 135 Praesepe members and 87 Hyads. To compare H{alpha} emission, an indicator of chromospheric activity, as a function of color, mass, and Rossby number R_o_, we first calculate an expanded set of {chi} values, with which we can obtain the H{alpha} to bolometric luminosity ratio, L_H{alpha}_/L_bol_, even when spectra are not flux-calibrated and/or stars lack reliable distances. Our {chi} values cover a broader range of stellar masses and colors (roughly equivalent to spectral types from K0 to M9), and exhibit better agreement between independent calculations, than existing values. Unlike previous authors, we find no difference between the two clusters in their H{alpha} equivalent width or L_H{alpha}_/L_bol_ distributions, and therefore take the merged H{alpha} and P_rot_ data to be representative of 600 Myr old stars. Our analysis shows that H{alpha} activity in these stars is saturated for R_O_<=0.11\-0.03_^+0.02^. Above that value activity declines as a power-law with slope {beta}=0.73_-0.12_^+0.16^, before dropping off rapidly at R_o_{approx} 0.4. These data provide a useful anchor for calibrating the age-activity-rotation relation beyond 600 Myr.
500. Activity cycles in 3203 Kepler stars
ID:
ivo://CDS.VizieR/J/A+A/603/A52
Title:
Activity cycles in 3203 Kepler stars
Short Name:
J/A+A/603/A52
Date:
21 Oct 2021
Publisher:
CDS
Description:
In recent years it has been claimed that the length of stellar activity cycles is determined by the stellar rotation rate. It has been observed that the cycle period increases with rotation period along two distinct sequences, known as the active and inactive sequences. In this picture the Sun occupies a solitary position between the two sequences. Whether the Sun might undergo a transitional evolutionary stage is currently under debate. Our goal is to measure cyclic variations of the stellar light curve amplitude and the rotation period using four years of Kepler data. Periodic changes in the light curve amplitude or the stellar rotation period are associated with an underlying activity cycle. Using a recent sample of active stars we compute the rotation period and the variability amplitude for each individual Kepler quarter and search for periodic variations of both time series. To test for periodicity in each stellar time series we consider Lomb-Scargle periodograms and use a selection based on a false alarm probability (FAP). We detect amplitude periodicities in 3203 stars between 0.5-6 years covering rotation periods between 1-40 days. Given our sample size of 23,601 stars and our selection criteria that the FAP is less than 5%, this number is almost three times higher than that expected from pure noise. We do not detect periodicities in the rotation period beyond those expected from noise. Our measurements reveal that the cycle period shows a weak dependence on rotation rate, slightly increasing for longer rotation periods. We further show that the shape of the variability deviates from a pure sine curve, consistent with observations of the solar cycle. The cycle shape does not show a statistically significant dependence on effective temperature. We detect activity cycles in more than 13% of our final sample with a FAP of 5% (calculated by randomly shuffling the measured 90-day variability measurements for each star). Our measurements do not support the existence of distinct sequences in the Prot-Pcyc plane, although there is some evidence for the inactive sequence for rotation periods between 5-25 days. Unfortunately,the total observing time is too short to draw sound conclusions on activity cycles with similar lengths to that of the solar cycle.

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