Journal of Astrophysics and Astronomy最新文献

Incomplete sky analysis of cosmic microwave background (CMB) polarization spectra poses a major problem of leakage between E- and B-modes. We present a machine learning approach to remove this E-to-B leakage using a convolutional neural network (CNN) in presence of detector noise. The CNN predicts the full sky E- and B-modes spectra for multipoles (2 le ell le 384) from the partial sky spectra for (N_textrm{side} = 256). We use tensor-to-scalar ratio (r=0.001) to simulate the CMB polarization maps. We train our CNN using (10^5) full sky target spectra and an equal number of noise contaminated partial sky spectra obtained from the simulated maps. The CNN works well for two masks covering the sky area of (sim )80% and (sim )10%, respectively after training separately for each mask. For the assumed theoretical E- and B-modes spectra, predicted full sky E- and B-modes spectra agree well with the corresponding target spectra and their means agree with theoretical spectra. The CNN preserves the cosmic variances at each multipole, effectively removes correlations of the partial sky E- and B-modes spectra, and retains the entire statistical properties of the targets avoiding the problem of so-called E-to-B leakage for the chosen theoretical model.

With the third data release of the Gaia mission, Gaia DR3 with its precise photometry and astrometry, it is now possible to study the behavior of stars at a scale never seen before. In this paper, we developed new criteria to identify T-Tauri stars (TTS) candidates using UV and optical color-magnitude diagrams (CMDs) by combining the GALEX and Gaia surveys. We found 19 TTS candidates and five of them are newly identified TTS in the Taurus molecular cloud (TMC), not cataloged before as TMC members. For some of the TTS candidates, we also obtained optical spectra from several Indian telescopes. We also present the analysis of distance and proper motion of young stars in the Taurus using data from Gaia DR3. We found that the stars in Taurus show a bimodal distribution with distance, having peaks at (130.17_{-1.24}^{1.31}) pc and (156.25_{-5.00}^{1.86}) pc. The reason for this bimodality, we think, is due to the fact that different clouds in the TMC region are at different distances. We further showed that the two populations have similar ages and proper motion distribution. Using the Gaia DR3 CMD, we showed that the age of Taurus is consistent with 1 Myr.

We have modeled the diffuse background at the Galactic Poles in the far-ultraviolet (FUV: 1536 Å) and the near-ultraviolet (NUV: 2316 Å). The background is well-fit using a single-scattering dust model with an offset representing the extragalactic light plus any other contribution to the diffuse background. We have found a dust albedo of 0.35–0.40 (FUV) and 0.11–0.19 in the NGP ((b > 70^{circ })) and 0.46–0.56 (FUV) and 0.31–0.33 (NUV) in the SGP ((b < 70^{circ })). The differences in the albedo may reflect changes in the dust-to-gas ratio over the sky or in the dust distribution. We find offsets at zero-reddening of 273–286 and 553–581 photons (hbox {cm}^{-2}) (hbox {s}^{-1}) (hbox {sr}^{-1}) Å(^{-1}) in the FUV and NUV, respectively, in the NGP with similar values in the SGP.

In general, the initial designs of periodic orbits around the Lagrangian points and transfers to them from the Earth are generated under the Circular Restricted Three Body Problem (CRTBP) framework and used as reference designs to generate realistic designs under Ephemeris models. The realistic ephemeris designs are quasi-periodic multi-revolution orbits. So, the use of periodic orbits, which are multi-revolution in nature can be explored as reference designs as an alternative. Although, the periodic orbits generated in the Elliptical Restricted Three Body Problem (ERTBP) framework are multi-revolution in nature, in the Sun–Earth system, these orbits have large in-plane and out-of-plane amplitudes and hence, these are not suitable for scientific missions. For viable smaller amplitudes, only quasi-halo orbits do exist around the Sun–Earth Lagrangian point L₁ in the ERTBP framework. Hence, as an alternative to the periodic orbits under the CRTBP framework, the quasi-halo orbits under the ERTBP framework are designed and used as reference designs to generate ephemeris designs. These quasi-halo orbits are generated using a methodology, proposed in this paper, which involves differential evolution, an evolutionary optimization technique. Using a similar approach based on the differential evolution technique, the quasi-halo orbits are designed in the Ephemeris model also. The methodology could generate quasi-halo orbits that do not require any theoretical velocity corrections for about five years (10 revolutions). The pros and cons of reference designs in generating ephemeris design are analyzed. It is found that the design in the Ephemeris model is close to both the initial designs in CRTBP and ERTBP frameworks. However, the use of ERTBP design as the reference design does not result in a reduction in computational time. Furthermore, optimal two-impulse transfers to the quasi-halo orbit from an Earth parking orbit, are generated under the ERTBP framework and the Ephemeris model. The location of insertion and the components of orbit insertion velocity are treated as unknowns and determined using differential evolution. The transfer cost in the Ephemeris model is found to be less compared to transfers in CRTBP and ERTBP frameworks.

The planetary conjunctions have always procured a very prominent place in astronomy texts from India. The calculations aim at the determination of exact instant of conjunction by method of iteration and prediction of the possibility of occultation, grazing or otherwise. We discuss details of the procedure from a text of 17th century and offer two examples from texts of 16th century devoted to worked examples based on different methods. One of them gives an angular separation of 1′, which would have been a challenge to observe. The possible sources of error in the estimates of longitude and speed are discussed. We also infer that the approximations in the estimation of angular diameter and node led to errors in the prediction of type of occultation.

Age distribution of the open cluster system is a key piece of information to decipher the star-formation history of the milky way disk. Recently, a remarkable earlier drop of its older end was found, which caught our attention. Precisely, we analysed in detail the population of open clusters older than 1 Gyr located inside a circle of 2.0 kpc from the Sun contained in the milky way star-cluster catalog, using the data release 3.0 of the Gaia survey, and found that it contains a slightly larger old open-cluster population with respect to that witnessing the earlier drop age distribution. However, there are still some aspects that deserve further attention to undoubtedly handle a statistically complete cluster sample, that allows us to comprehensively know the older end of the open-cluster age-distribution function. We discuss some reasons that affect such a completeness, among them, the photometric depth of the database employed, the performance of machine-learning techniques used to recognize open clusters, the cleaning of cluster color–magnitude diagrams from field star contamination, etc.

We have investigated the five-dimensional Bianchi type-III string cosmological models with dark energy using the Saez–Ballester scalar-tensor theory of gravitation. To solve the field equations, we applied the laws of volumetric expansions and assumed a scaling relation between the shear scalar (sigma ) and the expansion scalar (theta ), which leads to a relationship between the metric potentials, i.e., ( D=C^{r} ) (where r is a non-zero constant). We have considered both power-law model and exponential model and have discussed the physical and kinematical parameters of these models.

Understanding the evolution and dissipation of protoplanetary disks are crucial in star and planet formation studies. We report the protoplanetary disk population in the nearby young (sigma ) Orionis cluster ((dsim 408) pc; (textrm{age}sim 1.8) Myr) and analyse the disk properties, such as dependence on stellar mass and disk evolution. We utilize the comprehensive census of 170 spectroscopic members of the region refined using astrometry from Gaia DR3 for a wide mass range of (sim )19–0.004 (M_odot ). Using the near-infrared (2MASS) and mid-infrared (WISE) photometries, we classify the sources based on the spectral index, into class I, class II, flat spectrum and class III young stellar objects. The frequency of sources hosting a disk with stellar mass <2 (M_odot ) in this region is (41pm 7)%, which is consistent with the disk fraction estimated in previous studies. We see that there is no significant dependence of disk fraction on stellar mass among T Tauri stars (<2 (M_odot )), but we propose rapid disk depletion around higher mass stars (>2 (M_odot )). Furthermore, we found the lowest mass of a disk-bearing object to be (sim )20 (M_{textrm{Jup}}) and the pronounced disk-fraction among the brown dwarf population hints at the formation scenario that brown dwarfs form similar to low-mass stars.

Using multi-wavelength data sets, we studied the star-formation activity in H ii region Sh 2-61 (hereafter S61). We identified a clustering in the region and estimated the membership using the Gaia proper motion data. The physical environment of S61 is inspected using infrared to radio wavelength images. We also determined the Lyman continuum flux associated with the H ii region and found that the H ii region is formed by at least two massive stars (S1 and S2). We also analyzed the (^{12})CO ((J =3)–2) JCMT data of S61, and a shell structure accompanying three molecular clumps are observed towards S61. We found that the ionized gas in S61 is surrounded by dust and a molecular shell. Many young stellar objects and three molecular clumps are observed at the interface of the ionized gas and surrounding gas. The pressure at the interface is higher than in a typical cool molecular cloud.

Classical T Tauri stars (TTS) are low-mass pre-main sequence stars with an active circumstellar environment. In this work, we present the identification and study of 260 classical TTS using LAMOST Data Release 6, among which 104 stars are newly identified. We distinguish classical TTS from giants and main-sequence dwarfs based on the (log {g}) values, and the presence of H(alpha ) emission line and infrared excess that arises from the circumstellar accretion disk. We estimated the mass and age of 210 stars using the Gaia color–magnitude diagram. The age is from 0.1 to 20 Myr, where 90% of the stars have age <10 Myr and the mass ranges between 0.11 and 1.9 (M_{odot }). From the measured H(alpha ) equivalent widths, we homogeneously estimated the mass accretion rates for 172 stars, with most values ranging from (10^{-7}) to (10^{-10}) (M_{odot }) yr(^{-1}). The mass accretion rates are found to follow a power law distribution with the mass of the star, having a relation of the form (dot{M}_textrm{acc}propto M_{*} ^ {1.43 pm 0.26}), in agreement with previous studies.