Covariations of chromospheric and photometric variability of the young Sun analogue HD 30495: evidence for and interpretation of mid-term periodicities

This study reports the synchronization between the chromospheric and photometric variability at time-scale of about 1.6–1.8 yr as observed for the young, rapidly rotating solar analogue HD 30495. In addition, HD 30495 may be presenting evidence of surface differential rotation at time-scales of about 11 d and 21 d, as well as the sunspot-like decadal cycles at 11–12 yr or so. We apply a new gapped wavelet method of time–frequency analysis for studying the variability in a new composite of the chromospheric S-index (1967–2018) and the longest photometric Δ(b + y)/2 index (1993–2018). We discuss and interpret our results in relation to other observed mid-term periodicities roughly of the same time-scales that had been found recently from not only chromospheric and photospheric activity indices but also from coronal X-ray emissions as observed in a considerably large set of stellar samples including those young Sun analogues from the Kepler satellite project. Thus, there is an apparent universality of such mid-term activity modulation time-scales as this solar-stellar magnetic phenomenon is well observed directly for a host of solar activity related indices covering the photopsheric, chromospheric, coronal, and even the heliospheric (utilizing the measures of incoming galactic cosmic rays as a probe of activity variations) activity records. This is why we made a further attempt to interpret the results in search of a realistic generation mechanism as well as spatio-temporal persistency of the phenomenon under a wide scenario of dynamo simulations

A composite sea surface temperature record of the northern South ChinaSea for the past 2500 years: A unique look into seasonality and seasonalclimate changes during warm and cold periods

High-resolution late Holocene climate records that can resolve seasonality are essential for confirming past climatic dynamics, understanding the late 20th century global warming and predicting future climate. Here a new composite record of the sea surface temperature, SST, variation in the northern South China Sea (SCS) during the late Holocene is constructed by combining seven seasonally-resolved coral and Tridacna gigas Sr/Ca-based SST time-windows with the instrumental SST record from modern interval between 1990 and 2000. This composite multi-proxy marine record, together with the reconstructions from mainland China and tropical Western Pacific, indicates that the late Holocene warm periods, the Roman Warm Period (RWP) and Medieval Warm Period (MWP), were prominently imprinted and documented in the climatic and environmental history of the East Asia&ndash;Western Pacific region. Meanwhile, substantial and significant SST seasonality variations during the late Holocene were observed in the composite record. The observed increase in seasonality (or amplitude of seasonal cycles) during the cold periods around our study area was probably caused by the different amplitudes between winter versus summer SST variations in northern SCS, with much larger SST variation during winters than during summers for the late Holocene. In addition, the distinctive warm, cold and neutral climatic episodes identified in our northern SCS composite SST record correspond well with other paleo reconstructions from mainland China and especially well with the Northern Hemisphere-wide composites by Moberg et al. (2005) and Ljungqvist (2010). The overall agreement however also calls for more information and insights on how seasonal temperatures and their ranges vary on decadal&ndash;centennial timescales.</p

Latitudinal insolation gradients throughout the Holocene II: High frequency variations

In a first paper, we presented an in-depth discussion and a computational method (free of the calendar problem) to reckon any kind of latitudinal insolation gradients, LIGs, throughout the Holocene and up to CE 3000. One of the main insights from this exact definition of LIGs is that, unlike what is argued in prior works, during the Holocene, a general classification of LIGs in terms of obliquity signal or climatic precession variations is much more complex, even in summertime. It is especially evident in the Southern Hemisphere, where summer half-year LIGs evolve under the relatively stronger modulation by climatic precession. In this work, the short-term periodicities (high frequency variations in time domain) of all these LIGs are studied by means of the multitaper spectral analysis. The goal is to get more insights on the competing effects between obliquity and climatic precession during the Holocene, and to know how the relative intensity of the obliquity's periodicities is when compared to the main spectral peaks produced by the climatic precession effects at short time scales (e.g., from annual to decadal bands). Our main result is the clarification of the role of the 18.63 yr periodicity originated in the well known retrograding cycle of the Moons’ orbit. We found that this lunar cycle is always present at a 99% significance level in all the analysed LIGs, even in winter with solar cycle included. The conceptual explanation of this persistence is based on the fact that all accurate short-term orbital forcing calculation must include the lunar nodal cycle even in climatic precession variations. We propose to use more specific definitions when short-term orbital variations are taken into account in describing Milanković forcing.Fil: Cionco, Rodolfo Gustavo. Universidad Tecnológica Nacional. Facultad Regional San Nicolás. Grupo de Estudios Ambientales; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; ArgentinaFil: Soon, Willie W. H.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Elias, Ana Georgina. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Quaranta, Nancy Esther. Universidad Tecnológica Nacional. Facultad Regional San Nicolás. Grupo de Estudios Ambientales; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; Argentin

Properties of Sun-like Stars with Planets: ρ1 Cancri, τ Bootis, and υ Andromedae

Planets have been reported orbiting the Sun-like stars ρ1 Cnc, τ Boo, and υ And based on low-amplitude radial velocity variations. We have derived information on the first two stars from analysis of spectra, as well as parallel records of high-precision Strömgren b and y photometry and Ca II H + K fluxes. In the case of ρ1 Cnc, the upper limit (peak to peak) of nondetection of photometric variability at the orbital period is Δy ~ 0.0004 mag. The possibility of a planetary transit cannot be ruled out completely from the photometric data. Variations of the Ca II fluxes suggest a rotational period of ~42 days, in agreement with the inferred v sin i ~ 2 km s-1. The age of ρ1 Cnc is ~5 Gyr, based on its average Ca II flux and a relation between Ca II flux and age. The star τ Boo, unlike the other reported solar-type stars with planets, is relatively young (~2 Gyr). Despite its young age, it is photometrically nonvariable at the orbital period with an amplitude of Δ(b + y)/2 ~ 0.0004 mag (peak to peak); however, small-amplitude interseasonal variability is seen. No planetary transits were found in the photometry, which limits the inclination of the planet\u27s orbital plane to Earth\u27s line of sight to less than 83° (where 90° is coplanar). The Ca II record shows a weakly significant rotational period near 3.3 days, coincident with the orbital period of the companion. The Ca II record also shows a period of 116 days that has persisted for 30 years and is not seen in the photometric record. The persistence and timescale of this Ca II variation mean that it has no counterpart in Sun-like magnetic activity. The amplitude of the reflex velocity of the parent star (~450 m s-1) is much larger than the radial velocity perturbations expected from the presence of either surface inhomogeneities or line-bisector variations. Thus the anticipated perturbations from those stellar effects do not refute the inference of reflex velocities. We have few Ca II flux measurements for υ And. Its age and rotational period are estimated to be ~5 Gyr and 12 days, respectively. Our results for ρ1 Cnc and τ Boo are consistent with the explanation of planets as the cause of the velocity variations

Photometric and Ca II H and K Spectroscopic Variations in Nearby Sun-like Stars with Planets. III.

We present the results of an analysis of time-series photometry, Ca II H and K spectrophotometry, and high-dispersion visible spectra of nine nearby Sun-like stars recently identified as having planets. For the six stars whose presumed planets have orbital periods of less than 4 months (τ Boo, 51 Peg, υ And, ρ1 Cnc, ρ CrB, and 70 Vir), sine-curve fits to the photometric data show no variations with semiamplitude greater than 1 or 2 parts in 104. Photometric variations in 47 UMa are similarly small, although our photometric data of this star are slightly affected by variability of the comparison star. Nonvariability at this level of precision is sufficient to rule out surface magnetic activity as the cause of the observed radial-velocity variations in these seven stars and makes nonradial pulsations unlikely as well. Thus, our photometry provides indirect but strong support for true reflex motions—planets—in these seven stars, but cannot yet so support the planetary hypothesis for the two additional stars, 16 Cyg B and Gl 411. Continued photometric monitoring of the short-period systems may soon result in the direct detection of these planets in reflected light. We have used our photometric fluxes to search for possible transits of the extrasolar planets. Transits definitely do not occur in τ Boo, 51 Peg, υ And, and ρ1 Cnc, and probably do not occur in ρ CrB and 70 Vir. Our transit-search results are inconclusive for 47 UMa, and we cannot address the issue for 16 Cyg B and Gl 411. The precision of our photometry is sufficient to detect transits of planets even if they are not gas giants, as currently assumed, but much smaller objects with rocky compositions. The chance of finding at least one transit in the six stars is ~40%. We find significant year-to-year photometric variability only in τ Boo, which is not only the youngest star in the sample but also the star with the shallowest convective zone. The interseasonal range in its yearly mean photometric flux is ~0.002 mag, roughly twice the 0.0008 mag decadal variation in the Sun\u27s total irradiance. Monitoring of the relative Ca II H and K fluxes began between 1966 and 1968 for 51 Peg, τ Boo, ρ CrB, and Gl 411, between 1990 and 1993 for 47 UMa, 70 Vir, 16 Cyg B, and ρ1 Cnc, and in 1996 for υ And. The data have been newly recalibrated for improved long-term instrumental stability, resulting in better precision of the Ca II records. Five of the nine stars in this study have little or no detectable year-to-year variation in Ca II flux. The remaining four show moderate or pronounced variability: τ Boo, whose radial-velocity and photometric variations have comparatively high amplitudes; Gl 411, whose planetary companion was inferred astrometrically, not spectroscopically; ρ1 Cnc, which may undergo decadal cyclic activity; and υ And, which shows moderate year-to-year variability. Except for 47 UMa, intraseasonal variability consistent with rotation was detected in the Ca II records of all stars. However, the rotation periods determined for υ And, 70 Vir, and 16 Cyg B are of low confidence. An examination of the recalibrated Ca II records for 51 Peg finds a rotation period of 22 days, in contrast to our previous result of 37 days. Ages have been estimated from the mean Ca II flux and, where possible, the rotation period. We find general consistency with the ages determined by others comparing properties determined from high-resolution spectroscopy to evolutionary models, although the uncertainties are, in general, large

Properties of Sun-like Stars with Planets: 51 Pegasi, 47 Ursae Majoris, 70 Virginis, and HD 114762

Radial velocity variations have revealed planets orbiting 51 Peg, 47 UMa, and 70 Vir, and a low-mass companion orbiting HD 114762. We analyze parallel records of photometric measurements in Strömgren b and y and Johnson V, R, and I passbands and Ca II H and K fluxes in those stars. In the case of 51 Peg, the high precision of the differential photometric measurements made by the 0.75 m Automatic Photoelectric Telescope and the nonvariability of the star would allow the detection of a transit of a planet as small as Earth (corresponding to an amplitude of 0.0001 mag) if its orbit were nearly coplanar with our line of sight. No transits were observed