We present the results of a blind exercise to test the recoverability of stellar rotation and differential rotation in Kepler light curves. The simulated light curves lasted 1000 d and included ...activity cycles, Sun-like butterfly patterns, differential rotation and spot evolution. The range of rotation periods, activity levels and spot lifetime were chosen to be representative of the Kepler data of solar-like stars. Of the 1000 simulated light curves, 770 were injected into actual quiescent Kepler light curves to simulate Kepler noise. The test also included five 1000-d segments of the Sun's total irradiance variations at different points in the Sun's activity cycle. Five teams took part in the blind exercise, plus two teams who participated after the content of the light curves had been released. The methods used included Lomb–Scargle periodograms and variants thereof, autocorrelation function and wavelet-based analyses, plus spot modelling to search for differential rotation. The results show that the ‘overall’ period is well recovered for stars exhibiting low and moderate activity levels. Most teams reported values within 10 per cent of the true value in 70 per cent of the cases. There was, however, little correlation between the reported and simulated values of the differential rotation shear, suggesting that differential rotation studies based on full-disc light curves alone need to be treated with caution, at least for solar-type stars. The simulated light curves and associated parameters are available online for the community to test their own methods.
Magnetic activity on stars manifests itself in the form of dark spots on the stellar surface, which cause modulations of a few percent in the light curve of the star as it rotates. When a planet ...eclipses its host star, it might cross in front of one of these spots, creating a "bump" in the transit light curve. By modeling these spot signatures, it is possible to determine the physical properties of the spots such as size, temperature, and location. In turn, monitoring of the spots' longitude provides estimates of the stellar rotation and differential rotation. This technique was applied to the star Kepler-17, a solar-type star orbited by a hot Jupiter. The model yields the following spot characteristics: average radius of 49 10 Mm, temperatures of 5100 300 K, and surface area coverage of 6 4%. The rotation period at the transit latitude, , occulted by the planet was found to be 11.92 0.05 day, slightly smaller than the out-of-transit average period of 12.4 0.1 day. Adopting a solar-like differential rotation, we estimated the differential rotation of Kepler-17 to be rd day−1, which is close to the solar value of 0.050 rd day−1, and a relative differential rotation of . Because Kepler-17 is much more active than our Sun, it appears that, for this star, larger rotation rate is more effective in the generation of magnetic fields than shear.
Abstract
We investigate the magnetic nanoparticles hyperthermia in a non-adiabatic and radiating process through the calorimetric method. Specifically, we propose a theoretical approach to magnetic ...hyperthermia from a thermodynamic point of view. To test the robustness of the approach, we perform hyperthermia experiments and analyse the thermal behavior of magnetite and magnesium ferrite magnetic nanoparticles dispersed in water submitted to an alternating magnetic field. From our findings, besides estimating the specific loss power value from a non-adiabatic and radiating process, thus enhancing the accuracy in the determination of this quantity, we provide physical meaning to a parameter found in literature that still remained not fully understood, the effective thermal conductance, and bring to light how it can be obtained from experiment. In addition, we show our approach brings a correction to the estimated experimental results for specific loss power and effective thermal conductance, thus demonstrating the importance of the heat loss rate due to the thermal radiation in magnetic hyperthermia.
Context.
The study of the distribution, morphology, and kinematics of cold molecular gas in the nuclear and circumnuclear regions of active galactic nuclei (AGNs) helps to characterise and hence to ...quantify the impact of the AGNs on the host galaxy over its lifetime.
Aims.
We present the analysis of the molecular gas in the nuclear regions of three Seyfert galaxies, NGC 4968, NGC 4845, and MCG-06-30-15, using Atacama Large sub-Millimetre Array (ALMA) observations of the CO(2−1) emission line. The aim is to determine the kinematics of the gas in the central (∼1 kpc) region and thereby to probe nuclear fueling and feedback of AGNs.
Methods.
We used two different softwares, namely the 3D-Based Analysis of Rotating Object via Line Observations and DiskFit, to model the kinematics of the gas in the molecular disc, and thereby to determine the gas rotation and any kinematical perturbations.
Results.
Circular motions dominate the kinematics of the molecular gas in the central discs, mainly in NGC 4845 and MCG-06-30-15; however there is clear evidence of non-circular motions in the central (∼1 kpc) region of NGC 4845 and NGC 4968. The strongest non-circular motion is detected in the inner disc of NGC 4968, mainly along the minor kinematic axis, with a velocity ∼115 km s
−1
. Of all DiskFit models, the bisymmetric model is found to give the best fit for NGC 4968 and NGC 4845, indicating that the observed non-circular motions in the inner disc of these galaxies could result from the nuclear barred structure, where the gas streams in elliptical orbits aligned along the bar. If the dynamics of NGC 4968 is modelled as a corotation pattern just outside of the bar, the bar pattern speed becomes Ω
b
= 52 km s
−1
kpc
−1
; the corotation is set at 3.5 kpc; and the inner Lindblad resonance (ILR) ring is
R
= 300 pc, corresponding to the CO emission ring. In the NGC 4968 galaxy, the torques exerted on the gas by the bar are positive in the centre, within the gas nuclear ring, and negative outside. This shows that the gas is transiently trapped in the ILR. The comparison of the CO intensity maps with the map of the cold dust emission shows an absence of CO in the centre of NGC 4968; also the dust distribution and CO emission in and around the centre of NGC 4845 have similar extensions. The 1.2 mm ALMA continuum is peaked and compact in NGC 4968 and MCG-06-30-15, but their CO(2−1) emissions have extended distributions. Allowing the CO-to-H
2
conversion factor
α
CO
between 0.8 and 3.2, which is typical of nearby galaxies of the same type, the molecular mass
M
(H
2
) is estimated to be ∼3 − 12 × 10
7
M
⊙
(NGC 4968), ∼9 − 36 × 10
7
M
⊙
(NGC 4845), and ∼1 − 4 × 10
7
M
⊙
(MCG-06-30-15).
Conclusions.
We conclude that the observed non-circular motions in the molecular disc of NGC 4968 and likely those seen in NGC 4845 are due to the presence of the bar in the nuclear region. We discuss the possibility that the observed pattern in the kinematics might be a consequence of the presence of AGNs, and this might be the case for NGC 4845. At the current spectral and spatial resolution and sensitivity, we cannot claim any strong evidence in these sources of the long sought feedback or feeding effect resulting from the presence of AGNs.
The high-quality light curves from the Transiting Exoplanet Survey Satellite (TESS) represent a unique laboratory for the study of stellar rotation, which is a fundamental observable driving stellar ...and planetary evolution, including planetary atmospheres and impacting habitability conditions and the genesis of life around stars. As of 2020 April 14, this mission delivered public light curves for 1000 TESS objects of interest (TOIs), observed with a 2 minute cadence during the first 20 months of the mission. Here, we present a search for rotation signatures in these TOIs, using fast Fourier transform, Lomb-Scargle, and wavelet techniques, accompanied by a rigorous visual inspection. This effort revealed 163 targets with rotation signatures, 131 of which present unambiguous rotation periods ranging from 0.321 and 13.219 days, whereas 32 of them present dubious rotation periodicities. Of these stars, 109 show flux fluctuations whose root cause is not clearly identified. For 714 TOIs, the light curves show a noisy behavior, corresponding to typically low-amplitude signals. Our analysis has also revealed 10 TOI stars with pulsation periodicities ranging from 0.049 to 2.995 days and four eclipsing binaries. With upcoming TESS data releases, our periodicity analysis will be expanded to almost all TOI stars, thereby contributing in defining criteria for follow-up strategy itself, and the study of star-planet interactions, surface dynamic of host stars, and habitability conditions in planets, among other aspects. In this context, a living catalog is maintained on the Filtergraph visualization portal at https://filtergraph.com/tess_rotation_tois.
Magnetohydrodynamic turbulence displays velocity anisotropies that reflect the direction of the magnetic field. This anisotropy has led to the development of a number of statistical techniques for ...studying magnetic fields in the interstellar medium. In this paper, we review and compare three techniques that use radio position-position-velocity data to determine magnetic field strength and morphology: the correlation function anisotropy (CFA), principal component analysis of anisotropies (PCAA), and the more recent velocity gradient technique (VGT). We compare these three techniques and suggest improvements to the CFA and PCAA techniques to increase their accuracy and versatility. In particular, we suggest and successfully implement a much faster way to calculate nonperiodic correlation functions for the CFA. We discuss possible improvements to the current implementation of the PCAA. We show the advantages of the VGT in terms of magnetic field tracing and stress the complementary nature with the other two techniques.
We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the ...reference abundance, we determine the Fe/H for the whole sample. These metallicities, together with the Teff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, $(B\!-\!V)_{0}$ and $\log g$) using theoretical isochrones and a Bayesian estimation method. The $(B\!-\!V)_{0}$ values so estimated turn out to be in excellent agreement (to within ~0.05 mag) with the observed $(B\!-\!V)$, confirming the reliability of the Teff-$(B\!-\!V)_{0}$ relation used in the isochrones. On the other hand, the estimated $\log g$ values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on Fe/H determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the $1<\theta<10$ mas interval (or, alternatively, finding mean differences of just 6%). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ~4 Gyr, and by a large Fe/H spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the Fe/H dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius.
The dynamics of the interstellar medium (ISM) are strongly affected by turbulence, which shows increased anisotropy in the presence of a magnetic field. We expand upon the Esquivel & Lazarian method ...to estimate the Alfven Mach number using the structure function anisotropy in velocity centroid data from Position-Position-Velocity maps. We utilize three-dimensional magnetohydrodynamic simulations of fully developed turbulence, with a large range of sonic and Alfvenic Mach numbers, to produce synthetic observations of velocity centroids with observational characteristics such as thermal broadening, cloud boundaries, noise, and radiative transfer effects of carbon monoxide. In addition, we investigate how the resulting anisotropy-Alfven Mach number dependency found in Esquivel & Lazarian might change when taking the second moment of the Position-Position-Velocity cube or when using different expressions to calculate the velocity centroids. We find that the degree of anisotropy is related primarily to the magnetic field strength (i.e., Alfven Mach number) and the line-of-sight orientation, with a secondary effect on sonic Mach number. If the line of sight is parallel to up to approximately 45 deg off of the mean field direction, the velocity centroid anisotropy is not prominent enough to distinguish different Alfvenic regimes. The observed anisotropy is not strongly affected by including radiative transfer, although future studies should include additional tests for opacity effects. These results open up the possibility of studying the magnetic nature of the ISM using statistical methods in addition to existing observational techniques.
Pulsation in TESS Objects of Interest Gomes, R. L.; Martins, B. L. Canto; Fontinele, D. O. ...
Astrophysical journal/The Astrophysical journal,
01/2024, Letnik:
961, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
We report the discovery of three Transiting Exoplanet Survey Satellite Objects of Interest (TOI) with signatures of pulsation, observed in more than one sector. Our main goal is to explore ...how large is the variety of classical pulsators such as
δ
Sct,
γ
Dor, RR Lyrae and Cepheid among TOI pulsators. The analysis reveals two stars with signatures of
δ
Sct and one of
γ
Dor, out of a sample of 3901 TOIs with available light curves (LCs). To date, there is a very scarce number of known pulsating stars hosting planets. The present finding also emerges as an exciting laboratory for studying different astrophysical phenomena, including the effects of star–planet interaction on pulsation and timing detection of planetary companions. We have also identified 16 TOI stars with periodicities and LCs morphology compatible with different classical pulsating classes, but for most of them, the dominant frequency signals originate from contaminating sources.
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