Journal of Astrophysics and Astronomy最新文献

In recent times, 3.6m Devasthal Optical Telescope (DOT) has been installed with an optical to near infra-red spectrograph, TANSPEC, which provides spectral coverage from 0.55 to 2.5 microns. Using TANSPEC, we have obtained a single epoch spectrum of a set, containing nine FUors and EXors. We have analysed line profiles of the sources and compared them with the previously published spectra of these objects. Comparing the line profile shapes with the existing theoretical predictions, we have tried to interpret the physical processes that are responsible for the current disc evolution and the present accretion dynamics. Our study has shown the importance of time-evolved spectroscopic studies for a better understanding of the evolution of the accretion a mechanisms. This in turn can help in the better characterization of the young stars displaying episodic accretion behavior.

A systematic method is used to study the problem of propagation of planar, cylindrically symmetric and spherically symmetric shock waves of the one-dimensional motion of an inviscid, self-gravitating, non-ideal interstellar gas cloud. The analytic solution of the problem is resolved, which specifies non-linear behavior in the physical plane. The transport equation, which describes the evolution of weak discontinuity in non-ideal gas is derived. It is observed that the nature of the solution completely depends on the net volumetric cooling rate and self-gravitating parameter. It is observed that an increase in the value of self-gravitating parameter results in delay of process of shock formation and shock forms early when heating dominates cooling in the system. Also, expansive waves take less time to decay in planar geometry as compared to cylindrical and spherical geometries and compressive waves take more time to develop shocks for cylindrical and spherical geometries as compared to planar geometry.

Currently, the orbit determination accuracy of Navigation with Indian Constellation (NavIC) satellites is better than 20 m using the measurements from one-way ranging network of stations. The NavIC ephemerides along with time are continuously transmitted from the NavIC satellites. The transmitted ephemerides are also known as the broadcast ephemerides. These ephemerides are functions of precise orbit determination and dynamical model used for the orbit propagation and are one of the major error sources in the NavIC receiver position. In this paper, a study has been carried out to improve the orbit determination. This is performed through the precise orbit determination using extended tracking network with a sufficiently long baseline tracking stations (LBTS). We first attempted to fix a suitable number of LBTS based on several criteria, such as geometry, elevation cut-off angle, minimum tracking duration, etc. LBTS are optimally selected to reduce the along-track, in-track and cross-track errors in the estimated orbit. We also address the precise orbit determination of NavIC satellites by employing a weighed least square differential correction method. In the orbit determination process, the numerical integration orbit propagator using Cowell’s method, a step-by-step algorithm, is utilized. The simulated one-way range measurements are used for the precise orbit determination with a white noise sigma of 10 cm. We found that the observed position accuracy improvements are better than 10 m.

Mid-infrared emission spectra, obtained from ISO archive, of thirteen astrophysical objects as well as computed spectra of 27 polycyclic aromatic hydrocarbon (PAH) molecules are studied. All the objects show strong aromatic infrared band (AIB) features with variations that correlate with object type. Based on AIB peak positions, the features for IRC (+)10216, Monoceros R2, and IC 5117 and PN-SwSt 1 are classified as type ‘A’, ‘B’ or ‘C’ for the first time. The AIBs at 6.2, 7.7 and 11.2 (mu hbox {m}) are used to obtain band intensity ratios for 6.2/7.7 and 11.2/6.2, which respectively indicate PAH size as number of carbon atoms and the ionization conditions of the medium. The smaller value of 6.2/7.7 points towards the presence of large PAH molecules, while higher value of 11.2/6.2 ratio relates to harsh conditions around the object. In general, for star-forming regions, the 6.2/7.7 band ratio obtained is >1 and the 11.2/6.2 ratio is >2, while for late type carbon stars, these values are <1 and <2. This indicates that small/medium-sized ionized PAHs are likely in star-forming regions and large PAHs in evolved stars. For each of the 27 plain PAH molecules, the integrated intensity in these bands is obtained from the computed infrared spectra and the band ratios are calculated. The ratio 6.2/7.7 in several computed medium and large sized PAH cations is in the range of observed ratio in most objects, but some molecules show large variations in band ratios, indicating that PAHs possible in interstellar medium could be more complex and with irregular structures.

A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms of Fanaroff–Riley (FR) classification. In HYMORS, one radio lobe appears edge-darkened (FR I), while the other shows a well-defined emission peak near its outer edge (edge-brightened, FR II). Such sources are rare, but critical for constraining the mechanism responsible for FR dichotomy, a widely debated issue in extragalactic astrophysics. Here, we highlight the need for caution in assigning FR type, in view of some upcoming observational campaigns to confirm HYMORS among the candidates. To illustrate this, we highlight the cases of three radio sources, which have been perceived to be HYMORS, including the radio galaxy 0500(+)630 (4C (+)63.07), which has been claimed to be a good, original example of a HYMORS, with a FR I western lobe and a FR II eastern lobe marked by a prominent terminal hot spot. However, its recent VLASS map at 3 GHz has revealed that the western lobe actually extends much farther out than reported and terminates in a well-defined emission peak. This implies that the source is a regular FR II radio galaxy and not a HYMORS. We also provide a brief perspective of the HYMORS phenomenon and underscore the need to confirm a FR I classification by ruling out additional FR II characteristics, such as an inward lobe-widening and spectral steepening, as well as a lack of prominent radio jet within the lobe.

We study the distribution of fundamental-mode Cepheids in the Magellanic Clouds as a function of their positions and ages using the data from the OGLE IV survey. The ages of the Cepheids are determined through well known period–age relations for the LMC and SMC Cepheids, which are used to understand the star-formation scenario in the Magellanic Clouds. The age distributions of the Cepheids in LMC and SMC show prominent peaks around (158^{+46}_{-35}) Myr and (219^{+63}_{-49}) Myr, respectively. This indicates that a major star-formation event took place in the Magellanic Clouds about 200 Myr ago. It is believed that this episode of enhanced star formation might have been triggered by a close encounter between the two components of the Magellanic Clouds or due to a possible tidal interaction between the Magellanic Clouds and Milky Way galaxy during one of its pericentric passages around the Milky Way. Cepheids are found to be asymmetrically distributed in both the LMC and SMC in an elongated manner. A high spatial density clumpy structure is found to be located towards the eastern side of the LMC and the south-west direction of the SMC from their respective galactic centers.

We have performed a detailed analysis on the Teutsch 76 (T76) open cluster using the deep near-infrared (NIR) observations taken with the TANSPEC instrument mounted on the 3.6m Devasthal optical telescope along with the recently available high quality proper motion data from the Gaia data release 3 and deep photometric data from Pan-STARRS1 survey. We have found that the T76 cluster is having a central density concentration with circular morphology, probably due to the star-formation processes. The radius of the T76 cluster is found to be 45(^{prime prime }) (1.24 pc) and 28 stars within this radius were marked as highly probable cluster members. We have found that the cluster is located at a distance of (5.7pm 1.0) kpc and is having an age of (50pm 10) Myr. The mass function slope ((Gamma )) in the cluster region in the mass range of (sim ) (0.75<M/M_odot <5.8) is estimated as (-1.3pm 0.2), which is similar to the value of −1.35 given by Salpeter (1955). The cluster is not showing any signatures of mass-segregation and is currently undergoing dynamical relaxation.

The low-frequency radio telescope of the Square Kilometre Array (SKA) is being built by the international radio astronomical community to (i) have orders of magnitude higher sensitivity and (ii) be able to map the sky several hundred times faster, than any other existing facilities over the frequency range of 50–350 MHz. The sensitivity of a radio telescope array is in general, dependent upon the number of electromagnetic sensors used to receive the sky signal. The total number of them is further constrained by the effects of mutual coupling between the sensor elements, allowable grating lobes in their radiation patterns, etc. The operating frequency band is governed by the desired spatial and spectral responses, acceptable sidelobe and backlobe levels, radiation efficiency, polarization purity and calibratability of sensors’ response. This paper presents a brief review of several broadband antennas considered as potential candidates by various engineering groups across the globe, for the low-frequency radio telescope of SKA covering the frequency range of 50–350 MHz, on the basis of their suitability for conducting primary scientific objectives.

Actively accreting young stellar objects show H(alpha ) emission line in their spectra. We present the results of survey for H(alpha ) emission-line stars in the star-forming region IC 5070 taken with 2-m Himalyan Chandra Telescope. Based on the H(alpha ) slitless spectroscopy data, we identified 131 emission-line stars in (sim )0.29 square degrees area of the IC 5070 region. Using Gaia early data release 3, we estimated the mean proper motion and parallax of the emission-line stars. We also estimated the mean distance and reddening toward the region using the emission-line stars, which are (sim )833 pc and (sim )2 mag, respectively. By examining the locations of these stars in the color–magnitude diagrams constructed using Gaia and PanSTARRS1 data, we found that a majority of the H(alpha ) emitters are young low-mass (({<}1.5, M_odot )) stars. We also compared our catalog of emission-line stars with the available young stellar catalogs and found that most of them are class ii/flat spectrum sources with the spectral type ranging from K to M. Based on the proper motion/parallax values and locations on the color–magnitude diagrams, about 20 emission-line stars are flagged as non-members. The relative proper motion of the emission-line stars with respect to the ionizing source suggest the possibility of the ‘rocket effect’ scenario in the remnant cloud (BRC 31).

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens.