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Mapping accretion and its variability in the young open cluster NGC 2264: a study based on <i>u</i> -band photometry
摘要： Context. The accretion process has a central role in the formation of stars and planets. Aims. We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). Methods. We performed a deep ugri mapping as well as a simultaneous u-band+r-band monitoring of the star-forming region with CFHT/MegaCam in order to directly probe the accretion process onto the star from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range ～0.1–2 M(cid:4). About 40% of the sample are classical (accreting) T Tauri stars, based on various diagnostics (Hα, UV and IR excesses). The remaining non-accreting members de?ne the (photospheric + chromospheric) reference UV emission level over which ?ux excess is detected and measured. Results. We revise the membership status of cluster members based on UV accretion signatures, and report a new population of 50 classical T Tauri star (CTTS) candidates. A large range of UV excess is measured for the CTTS population, varying from a few times 0.1 to ～3 mag. We convert these values to accretion luminosities and accretion rates, via a phenomenological description of the accretion shock emission. We thus obtain mass accretion rates ranging from a few 10?10 to ～10?7 M(cid:4)/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we ?nd a >6σ correlation between mass accretion rate and stellar mass. A power-law ?t, properly accounting for censored data (upper limits), yields ˙Macc ∝ M1.4±0.3. At any given stellar mass, we ?nd a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, i.e., much smaller than the observed spread in accretion rates. We suggest that a non-negligible age spread across the star-forming region may e?ectively contribute to the observed spread in accretion rates at a given mass. In addition, di?erent accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-?ow accretion) may be associated to di?erent ˙Macc regimes. Conclusions. A huge variety of accretion properties is observed for young stellar objects in the NGC 2264 cluster. While a de?nite correlation seems to hold between mass accretion rate and stellar mass over the mass range probed here, the origin of the large intrinsic spread observed in mass accretion rates at any given mass remains to be explored.
关键词： accretion,stars: pre-main sequence,stars: formation,stars: low-mass,ultraviolet: stars,open clusters and associations: individual: NGC 2264,accretion disks
更新于2025-09-04 15:30:14
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2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds
摘要： We present spectral classifications from optical spectroscopy of 263 massive stars in the north-eastern region of the Large Magellanic Cloud. The observed two-degree field includes the massive 30 Doradus star-forming region, the environs of SN1987A, and a number of star-forming complexes to the south of 30 Dor. These are the first classifications for the majority (203) of the stars and include eleven double-lined spectroscopic binaries. The sample also includes the first examples of early OC-type spectra (AA? 30 Dor 248 and 280), distinguished by the weakness of their nitrogen spectra and by C IV λ4658 emission. We propose that these stars have relatively unprocessed CNO abundances compared to morphologically normal O-type stars, indicative of an earlier evolutionary phase. From analysis of observations obtained on two consecutive nights, we present radial-velocity estimates for 233 stars, finding one apparent single-lined binary and nine (>3σ) outliers compared to the systemic velocity; the latter objects could be runaway stars or large-amplitude binary systems and further spectroscopy is required to investigate their nature.
关键词： stars: fundamental parameters,Magellanic Clouds,stars: early-type,open clusters and associations: general
更新于2025-09-04 15:30:14
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Photoelectron Spectroscopy and Theoretical Study of Cr <sub/><i>n</i> </sub> Si <sub/> 15– <i>n</i> </sub><sup>–</sup> ( <i>n</i> = 1–3): Effects of Doping Cr Atoms on the Structural and Magnetic Properties
摘要： CrnSi15?n? (n = 1?3) clusters were investigated by using size-selected anion photoelectron spectroscopy combined with density functional theory calculations. The results show that the most stable structure of CrSi14? is of C2v symmetry with the Cr atom encapsulated in a Si14 cage which can be viewed as a boat-shaped Si10 unit capped by four additional silicon atoms. A large HOMO?LUMO gap of neutral CrSi14 is confirmed based on the photoelectron spectrum of CrSi14?. Cr2Si13? has two isomers nearly degenerate in energy: one can be characterized as one Si atom interacting with a Cr2Si12 hexagonal prism while the other can be viewed as one Si atom capping a distorted Cr2Si12 hexagonal antiprism. Cr3Si12? has a D6d symmetric wheel structure in which three Cr atoms form an axle surrounded by 12 Si atoms. The magnetic moments of CrSi14?, Cr2Si13?, and Cr3Si12? increase from 1 to 3 μB and then to 7 μB with the increasing number of Cr atoms in the clusters. The magnetic moments of Cr2Si13? and Cr3Si12? are mainly contributed by the surface Cr atoms.
关键词： Photoelectron spectroscopy,Silicon clusters,Chromium doping,Density functional theory,Magnetic properties
更新于2025-09-04 15:30:14
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Using Lyman- <i>α</i> to detect galaxies that leak Lyman continuum
摘要： Aims. We propose to infer the output of the ionising continuum-leaking properties of galaxies based upon their Lyα line pro?les. Methods. We carried out Lyα radiation transfer calculations in two models of H ii regions. These models are porous to ionising continuum escape: 1) we de?ne Lyman-continuum (LyC) optically thin star clusters, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, in other words, almost totally ionised; and 2) we de?ne riddled ionisation-bounded media that are surrounded by neutral interstellar medium, but have holes, which results in a covering fraction lower than unity. Results. The Lyα spectra that emerge from these con?gurations have distinctive features: 1) a classical asymmetric redshifted pro?le in the ?rst case, but with a small shift of the pro?le maximum compared to the systemic redshift (vpeak ≤ 150 km s?1); 2) a main peak at the systemic redshift in the second case (vpeak = 0), with a non-zero Lyα ?ux bluewards of the systemic redshift as a consequence. If in a galaxy that leaks ionising photons the Lyα component that emerges from the leaking star cluster(s) is assumed to dominate the total Lyα spectrum, the Lyα shape may be used as a pre-selection tool for detecting LyC-leaking galaxies in objects with high spectral resolution Lyα spectra (R ≥ 4000). Our predictions are corroborated by examination of a sample of ten local starbursts with high-resolution HST/COS Lyα spectra that are known in the literature as LyC leakers or leaking candidates. Conclusions. Observations of Lyα pro?les at high resolution are expected to show de?nite signatures revealing the escape of Lyman-continuum photons from star-forming galaxies.
关键词： line: pro?les – radiative transfer – galaxies: ISM – ultraviolet: galaxies – galaxies: kinematics and dynamics – galaxies: star clusters: general
更新于2025-09-04 15:30:14
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= 1,...,10)
摘要： Using fixed-node diffusion quantum Monte Carlo (DMC) simulation we investigate the structural properties and energetics of the linear and cyclic carbon clusters Cn for n (cid:2) 10. We calculate the binding energy, the electron correlation energy, the dissociation energy, and the second difference in energy. We also present an analysis of the structural properties of the clusters. It is found that the bond lengths, binding energies, and dissociation energies obtained from the DMC calculations are in excellent agreement with the available experimental results. The electron-correlation contribution to the binding energy indicates that in the case of the linear isomers, the clusters of odd-number size are relatively more favored than their neighbors of even-number size, whereas for the cyclic isomers, we do not observe the oscillation pattern. In the range of cluster size under investigation, we find that the electron-correlation impact in the binding energy of the cyclic clusters is larger than that of the corresponding linear ones, varying from 30% to 40% of the binding energy values. The electron correlation is also essential to the stability of the clusters enhancing by up to 52% their dissociation energy. A comparative analysis of the dissociation energy and second difference in energy indicates that the linear isomers C3 and C5 are the most stable ones.
关键词： dissociation energy,electron correlation,quantum Monte Carlo,carbon clusters,binding energy
更新于2025-09-04 15:30:14
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Three-Dimensional Superlattices Based on Unusual Chalcogenide Supertetrahedral In–Sn–S Nanoclusters
摘要： Reported here are two novel metal chalcogenide superlattices built from unusual supertetrahedral TO2-InSnS clusters. With regard to only one previously reported case of a TO2-InS-based 2D-layered structure, such a combination of In?Sn?S components is leading to the first observation of 3D superlattices based on TO2-InSnS clusters. Besides, these title semiconducting materials also display good performance on the electrocatalytic oxygen reduction reaction.
关键词： 3D superlattices,metal chalcogenide,electrocatalytic oxygen reduction reaction,supertetrahedral clusters
更新于2025-09-04 15:30:14
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Heterometallic Organic Frameworks Built from Trinuclear Indium and Cuprous Halide Clusters: Ligand-Oriented Assemblies and Iodine Adsorption Behavior
摘要： Two novel heterometallic organic frameworks built from trinuclear indium and cuprous halide clusters, [(In3O)2(Cu2I2)3(ina)12(H2O)6](NO3)2·7DMA·10H2O (1) and [NH2(CH3)2][In3(OH)2(H2O)2(ina)8(Cu4I4)2]·5DMA·8H2O (2), where Hina = isonicotinic acid, have been successfully constructed with the orientation of the ina ligand. 1 shows a fascinating highly porous honeycomb-like 3D cationic framework with a trigonal-bipyramid-type cage based on a planar [In3O(CO2)6]+ trimer and a rhombohedral Cu2I2 cluster. Comparably, 2 displays a 3D negative network with irregular hexagonal channels constructed from a [In3(OH)2(CO2)8]? trimer and a cubane-like Cu4I4 cluster. Especially, 1 displays a reversible I2 adsorption/release performance with high adsorption capacity, whose mechanism has been disclosed by theoretical simulation. Also, the green/red emission of 2 stems from iodocuprate centers with quenched indium-centered emission.
关键词： I2 adsorption,cuprous halide clusters,luminescence properties,heterometallic organic frameworks,trinuclear indium
更新于2025-09-04 15:30:14