 Subjects -> METEOROLOGY (Total: 106 journals)
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- Magnetic fields of low-mass main sequences stars: non-linear dynamo theory
and mean-field numerical simulations-
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Pages: 1601 - 1612
Abstract: ABSTRACTOur theoretical and numerical analysis have suggested that for low-mass main sequences stars (of the spectral classes from M5 to G0) rotating much faster than the Sun, the generated large-scale magnetic field is caused by the mean-field α2Ω dynamo, whereby the α2 dynamo is modified by a weak differential rotation. Even for a weak differential rotation, the behaviour of the magnetic activity is changed drastically from aperiodic regime to non-linear oscillations and appearance of a chaotic behaviour with increase of the differential rotation. Periods of the magnetic cycles decrease with increase of the differential rotation, and they vary from tens to thousand years. This long-term behaviour of the magnetic cycles may be related to the characteristic time of the evolution of the magnetic helicity density of the small-scale field. The performed analysis is based on the mean-field simulations (MFS) of the α2Ω and α2 dynamos and a developed non-linear theory of α2 dynamo. The applied MFS model was calibrated using turbulent parameters typical for the solar convective zone.
PubDate: Wed, 27 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2708
Issue No: Vol. 526, No. 2 (2023)
- The SAMI galaxy survey: impact of black hole activity on galaxy
spin–filament alignments-
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Pages: 1613 - 1632
Abstract: ABSTRACTThe activity of central supermassive black holes might affect the alignment of galaxy spin axes with respect to the closest cosmic filaments. We exploit the Sydney–AAO Multi-object Integral-field Galaxy Survey to study possible relations between black hole activity and the spin–filament alignments of stars and ionized gas separately. To explore the impact of instantaneous black hole activity, active galaxies are selected according to emission-line diagnostics. Central stellar velocity dispersion (σc) is used as a proxy for black hole mass and its integrated activity. We find evidence for the gas spin–filament alignments to be influenced by AGN, with Seyfert galaxies showing a stronger perpendicular alignment at fixed bulge mass with respect to galaxies, where ionization is consequence of low-ionization nuclear emission-line regions (LINERs) or old stellar populations (retired galaxies). On the other hand, the greater perpendicular tendency for the stellar spin–filament alignments of high-bulge mass galaxies is dominated by retired galaxies. Stellar alignments show a stronger correlation with σc compared to the gas alignments. We confirm that bulge mass (Mbulge) is the primary parameter of correlation for both stellar and gas spin–filament alignments (with no residual dependency left for σc), while σc is the most important property for secular star formation quenching (with no residual dependency left for Mbulge). These findings indicate that Mbulge and σc are the most predictive parameters of two different galaxy evolution processes, suggesting mergers trigger spin–filament alignment flips and integrated black hole activity drives star formation quenching.
PubDate: Tue, 12 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2728
Issue No: Vol. 526, No. 2 (2023)
- On the triple pulsar profiles generated by ordinary mode
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Pages: 1633 - 1645
Abstract: ABSTRACTA detailed study of the refraction of an ordinary wave in the magnetosphere of radio pulsars was carried out. For this, a consistent theory of the generation of secondary particles was constructed, which essentially takes into account the dependence of the number density and the energy spectrum of secondary particles on the distance from the magnetic axis. This made it possible to determine with high accuracy the refraction of the ordinary O-mode in the central region of the outflowing plasma, which makes it possible to explain the central peak of three-humped mean radio profiles. As shown by detailed numerical calculations, in most cases it is possible to reproduce quite well the observed mean profiles of radio pulsars.
PubDate: Tue, 12 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2731
Issue No: Vol. 526, No. 2 (2023)
- Heating and cooling in stellar coronae: coronal rain on a young Sun
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Pages: 1646 - 1656
Abstract: ABSTRACTRecent observations of rapidly rotating cool dwarfs have revealed H α line asymmetries indicative of clumps of cool, dense plasma in the stars’ coronae. These clumps may be either long-lived (persisting for more than one stellar rotation) or dynamic. The fastest dynamic features show velocities greater than the escape speed, suggesting that they may be centrifugally ejected from the star, contributing to the stellar angular momentum loss. Many, however, show lower velocities, similar to coronal rain observed on the Sun. We present 2.5D magnetohydrodynamic simulations of the formation and dynamics of these condensations in a rapidly rotating (Prot = 1 d) young Sun. Formation is triggered by excess surface heating. This pushes the system out of thermal equilibrium and triggers a thermal instability. The resulting condensations fall back towards the surface. They exhibit quasi-periodic behaviour, with periods longer than typical periods for solar coronal rain. We find line-of-sight velocities for these clumps in the range of 50 km s−1 (blueshifted) to 250 km s−1 (redshifted). These are typical of those inferred from stellar H α line asymmetries, but the inferred clump masses of 3.6 × 1014 g are significantly smaller. We find that a maximum of ${\simeq}3~{{ \rm per\ cent}}$ of the coronal mass is cool clumps. We conclude that coronal rain may be common in solar-like stars, but may appear on much larger scales in rapid rotators.
PubDate: Tue, 12 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2752
Issue No: Vol. 526, No. 2 (2023)
- JWST/NIRSpec spectroscopy of z = 7–9 star-forming galaxies with CEERS:
new insight into bright Lyα emitters in ionized bubbles-
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Pages: 1657 - 1686
Abstract: ABSTRACTWe describe new JWST/NIRSpec observations of galaxies at z ≳ 7 taken from the CEERS survey. Previous observations of this area have revealed associations of Lyα emitters at redshifts (z = 7.5, 7.7, and 8.7) where the intergalactic medium (IGM) is thought to be mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 z ≳ 7 galaxies with robust redshifts in the CEERS data set, including 10 in the Lyα associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 = 17.84 and Ne3O2 = 0.89, linear scale) and metallicity ($12+\log {(\rm {O/H})}=7.84$) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six z ≳ 7 Lyα emitters in the sample. Each has a hard ionizing spectrum indicating that their visibility is likely enhanced by efficient ionizing photon production. Lyα velocity offsets are found to be very large (≳300 km s−1), likely also contributing to their detectability. We find that Lyα in z ≳ 7 galaxies is 6–12 × weaker than in lower redshift samples with matched rest-optical spectral properties. If the bubbles around the Lyα emitters are relatively small (≲0.5–1 pMpc), we may expect such significant attenuation of Lyα in these ionized regions. We discuss several other effects that may contribute to weaker Lyα emission at z ≳ 7. Deep spectroscopy of fainter galaxies in the vicinity of the Lyα emitters will better characterize the physical scale of the ionized bubbles in this field.
PubDate: Tue, 12 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2763
Issue No: Vol. 526, No. 2 (2023)
- Can diffusion model conditionally generate astrophysical images'
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Pages: 1699 - 1712
Abstract: ABSTRACTGenerative adversarial networks (GANs) are frequently utilized in astronomy to construct an emulator of numerical simulations. Nevertheless, training GANs can prove to be a precarious task, as they are prone to instability and often lead to mode collapse problems. Conversely, the diffusion model also has the ability to generate high-quality data without adversarial training. It has shown superiority over GANs with regard to several natural image data sets. In this study, we undertake a quantitative comparison between the denoising diffusion probabilistic model (DDPM) and StyleGAN2 (one of the most robust types of GANs) via a set of robust summary statistics from scattering transform. In particular, we utilize both models to generate the images of 21 cm brightness temperature mapping, as a case study, conditionally based on astrophysical parameters that govern the process of cosmic reionization. Using our new Fréchet Scattering Distance (FSD) as the evaluation metric to quantitatively compare the sample distribution between generative models and simulations, we demonstrate that DDPM outperforms StyleGAN2 on varied sizes of training sets. Through Fisher forecasts, we demonstrate that on our data sets, StyleGAN2 exhibits mode collapses in varied ways, while DDPM yields a more robust generation. We also explore the role of classifier-free guidance in DDPM and show the preference for a non-zero guidance scale only when the training data is limited. Our findings indicate that the diffusion model presents a promising alternative to GANs in the generation of accurate images. These images can subsequently provide reliable parameter constraints, particularly in the realm of astrophysics.
PubDate: Tue, 12 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2778
Issue No: Vol. 526, No. 2 (2023)
- Why should models of dwarf galaxy evolution care about the initial mass
function at low star-formation rates'-
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Pages: 1713 - 1727
Abstract: ABSTRACTWhen star clusters are formed at low star-formation rates (SFRs), their stellar initial mass function (IMF) can hardly be filled continuously with stars at each mass. This lack holds for massive stars and is verified observationally by the correlation between star-cluster mass and its most massive cluster star. Since galaxy evolution is strongly affected by massive stars, numerical models should account for this lack. Because a filled IMF is mostly applied even when only fractions of massive stars form, here we investigate, by means of 3D chemo-dynamical simulations of isolated dwarf galaxies, how deviations from a standard IMF in star clusters affect the evolution. We compare two different IMF recipes, a filled IMF and one truncated at a maximum mass at which a single complete star forms. Attention is given to energetic and chemical feedback by massive stars. Since their energy release is mass-dependent but steeper than the negative IMF slope, the energetic feedback retains a positive mass dependence, so that a filled IMF regulates star formation (SF) more strongly than truncated IMFs, though only stellar number fractions exist. The higher SFR of the truncated IMF in the simulation leads to more Type II supernovae (SNeII), driving galactic winds. Whether this results from the model-inherent larger SFR is questioned and therefore explored analytically. This shows the expected result for the Lyman continuum, but that the total SNII energy release is equal for both IMF modes, while the power is smaller for the truncated IMF. Reasonably, the different IMFs leave fingerprints in the abundance ratios of massive to intermediate-mass star elements.
PubDate: Fri, 15 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2787
Issue No: Vol. 526, No. 2 (2023)
- A search for magnetic δ Scuti stars in Kepler hybrid candidates
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Pages: 1728 - 1741
Abstract: ABSTRACTNumerous candidate hybrid stars of type δ Scuti–γ Doradus have been identified with the Kepler satellite. However, many of them lie outside the theoretically expected instability strip for hybrid pulsation, where δ Sct and γ Dor pulsations can be simultaneously excited. We postulate that some of these pulsating stars may not be genuine hybrid pulsators but rather magnetic δ Sct stars, for which the rotational modulation from spots on the surface associated with the magnetic field produces frequencies in the same domain as γ Dor pulsations. We search for the presence of a magnetic field in a small sample of selected hybrid δ Sct–γ Dor stars using spectropolarimetry. At the time of observations, the only δ Sct star known to have a magnetic field was HD 188774 with a field strength of a few hundred Gauss. Our observations were thus tailored to detect fields of this typical strength. We find no magnetic field in the hybrid candidate stars we observed. However, two of the three other magnetic δ Sct stars discovered since these observations have much weaker fields than HD 188774, and are of dynamo origin rather than fossil fields. It is likely that our observations are not sensitive enough to detect such dynamo magnetic fields in the cooler stars of our sample if they are present. This work nevertheless provides reliable upper limits on possible fossil fields in the hotter stars, pointing towards typically weaker fields in δ Sct stars than in OBA stars in general.
PubDate: Fri, 15 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2798
Issue No: Vol. 526, No. 2 (2023)
- CzSL: Learning from citizen science, experts, and unlabelled data in
astronomical image classification-
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Pages: 1742 - 1756
Abstract: ABSTRACTCitizen science is gaining popularity as a valuable tool for labelling large collections of astronomical images by the general public. This is often achieved at the cost of poorer quality classifications made by amateur participants, which are usually verified by employing smaller data sets labelled by professional astronomers. Despite its success, citizen science alone will not be able to handle the classification of current and upcoming surveys. To alleviate this issue, citizen science projects have been coupled with machine learning techniques in pursuit of a more robust automated classification. However, existing approaches have neglected the fact that, apart from the data labelled by amateurs, (limited) expert knowledge of the problem is also available along with vast amounts of unlabelled data that have not yet been exploited within a unified learning framework. This paper presents an innovative learning methodology for citizen science capable of taking advantage of expert- and amateur-labelled data, featuring a transfer of labels between experts and amateurs. The proposed approach first learns from unlabelled data with a convolutional auto-encoder and then exploits amateur and expert labels via the pre-training and fine-tuning of a convolutional neural network, respectively. We focus on the classification of galaxy images from the Galaxy Zoo project, from which we test binary, multiclass, and imbalanced classification scenarios. The results demonstrate that our solution is able to improve classification performance compared to a set of baseline approaches, deploying a promising methodology for learning from different confidence levels in data labelling.
PubDate: Tue, 19 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2852
Issue No: Vol. 526, No. 2 (2023)
- Planetesimal formation via the streaming instability with multiple grain
sizes-
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Pages: 1757 - 1772
Abstract: ABSTRACTKilometre-sized planetesimals form from pebbles of a range of sizes. We present the first simulations of the streaming instability (SI) that begin with a realistic, peaked size distribution, as expected from grain growth predictions. Our 3D numerical simulations directly form planetesimals via the gravitational collapse of pebble clouds. Models with multiple grain sizes show spatially distinct dust populations. The smallest grains in the size distribution do not participate in the formation of filaments or the planetesimals that are formed by the remaining ∼80 per cent of the dust mass. This implies a size cutoff for pebbles incorporated into asteroids and comets. Disc observations cannot resolve this dust clumping. However, we show that clumping, combined with optical depth effects, can cause significant underestimates of the dust mass, with 20–80 per cent more dust being present even at moderate optical depths if the SI is active.
PubDate: Tue, 19 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2855
Issue No: Vol. 526, No. 2 (2023)
- Cosmography from well-localized fast radio bursts
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Pages: 1773 - 1782
Abstract: ABSTRACTFast radio bursts (FRBs) are millisecond-duration pulses occurring at cosmological distances that have emerged as prominent cosmological probes due to their dispersion measure (DM) evolution with redshift. In this work, we use cosmography, a model-independent approach to describe the evolution of the Universe, to introduce the cosmographic expansion of the DM– z relation. By fitting two different models for the intergalactic medium and host contributions to a sample of 23 well-localized FRBs, we estimate the kinematic parameters $q_0=-0.59^{+0.20}_{-0.17}$, $j_0=1.08^{+0.62}_{-0.56}$, s0 = −2.1 ± 7.0, and H0 = 69.4 ± 4.7 achieving a precision of 6 per cent and 7 per cent for the Hubble constant depending on the models used for contributions. Furthermore, we demonstrate that this approach can be used as an alternative and complementary cosmological model-independent method to revisit the long-standing ‘Missing Baryons’ problem in astrophysics by estimating that 82 per cent of the baryonic content of the Universe resides in the intergalactic medium, within 7 per cent and 8 per cent precision, according to the contribution models considered here. Our findings highlight the potential of FRBs as a valuable tool in cosmological research and underscore the importance of ongoing efforts to improve our understanding of these enigmatic events.
PubDate: Tue, 19 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2856
Issue No: Vol. 526, No. 2 (2023)
- An HST survey of 33 T8 to Y1 brown dwarfs: NIR photometry and multiplicity
of the coldest isolated objects-
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Pages: 1783 - 1798
Abstract: ABSTRACTWe present results from a Hubble Space Telescope imaging search for low-mass binary and planetary companions to 33 nearby brown dwarfs with spectral types of T8–Y1. Our survey provides new photometric information for these faint systems, from which we obtained model-derived luminosities, masses, and temperatures. Despite achieving a deep sensitivity to faint companions beyond 0.2–0.5 arcsec, down to mass ratios of 0.4–0.7 outside ∼5 au, we find no companions to our substellar primaries. From our derived survey completeness, we place an upper limit of $f \lt 4.9~{{\ \rm per\ cent}}$ at the 1σ level (<13.0 per cent at the 2σ level) on the binary frequency of these objects over the separation range 1–1000 au and for mass ratios above q = 0.4. Our results confirm that companions are extremely rare around the lowest mass and coldest isolated brown dwarfs, continuing the marginal trend of decreasing binary fraction with primary mass observed throughout the stellar and substellar regimes. These findings support the idea that if a significant population of binaries exist around such low-mass objects, it should lie primarily below 2–3 au separations, with a true peak possibly located at even tighter orbital separations for Y dwarfs.
PubDate: Fri, 22 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2870
Issue No: Vol. 526, No. 2 (2023)
- Analysing mixed modes of the four solar-like oscillating subgiant stars
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Pages: 1799 - 1808
Abstract: ABSTRACTThe observation of an unprecedented number of solar-like oscillating subgiant (SG) stars by the Kepler and TESS missions is crucial for the asteroseismic characterization of these stars, stellar population studies, and the study of stellar evolution theories. Owing to these missions, the fundamental parameters of the solar-like oscillating stars are precisely calculated from the evolution codes using the observed oscillation frequencies. Herein, we considered four solar-like oscillating SG stars. We obtained the fundamental parameters of the SG stars by constructing interior models using asteroseismic and non-asteroseismic observed parameters. The interior models of the four SG stars are constructed using the Modules for Experiments in Stellar Astrophysics code to effectively determine the fundamental properties. Using this method, the four solar-like oscillating SG stars are found to have masses and radii within the ranges of 1.16–1.75 M⊙ and 2.26–3.17 R⊙, respectively. The estimation accuracy of the typical asteroseismic radius, mass, and age is increased by fitting the observed and model reference frequencies. The typical uncertainties of the mass and radius are 3–4 ${{\ \rm per\ cent}}$ and 1–2 ${{\ \rm per\ cent}}$, respectively. Furthermore, the observed l = 1 frequencies, which showed a mixed mode for the first time, were also fitted to the models. Information regarding the gravity and density of the helium core was obtained by examining the mixed modes. Moreover, new asteroseismic methods for determining the age of SG stars are developed for the first time in this study.
PubDate: Thu, 21 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2876
Issue No: Vol. 526, No. 2 (2023)
- An optimized transient detection pipeline for the ASKAP Variables and Slow
Transients (VAST) survey-
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Pages: 1809 - 1821
Abstract: ABSTRACTIn this paper, we present an optimized version of the detection pipeline for the ASKAP Variables and Slow Transients (VAST) survey, offering significant performance improvement. The key to this optimization is the replacement of the original w-projection algorithm integrated in the Common Astronomy Software Applications package with the w-stacking algorithm implemented in the WSClean software. Our experiments demonstrate that this optimization improves the overall processing efficiency of the pipeline by approximately a factor of 3. Moreover, the residual images generated by the optimized pipeline exhibit lower noise levels and fewer artefact sources, suggesting that our optimized pipeline not only enhances detection accuracy but also improves imaging fidelity. This optimized VAST detection pipeline is integrated into the Data Activated Liu Graph Engine (DALiuGE) execution framework, specifically designed for SKA-scale big data processing. Experimental results show that the performance and scalability advantages of the pipeline using DALiuGE over traditional MPI or BASH techniques increase with the data size. In summary, the optimized transient detection pipeline significantly reduces runtime, increases operational efficiency, and decreases implementation costs, offering a practical optimization solution for other ASKAP imaging pipelines as well.
PubDate: Fri, 29 Sep 2023 00:00:00 GMT
DOI: 10.1093/mnras/stad2809
Issue No: Vol. 526, No. 2 (2023)
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