Journal of Astrophysics and Astronomy最新文献

In the present paper, we performed the optical, astrometric and spectroscopic studies for the open star cluster NGC 6475 using Gaia DR3 data. Using the radial density profile, we estimated the radius of the cluster to be equal to 1.44°. It is located at a distance of 279 ± 17 pc and has a service life of 224 ± 26 Myr. The mean proper motions in RA are 268.50 ± 0.80 mas yr⁻¹ and in DEC, they are −34.83 ± 0.70 mas yr⁻¹. Using spectroscopic radial velocity data of Gaia DR3, the median radial velocity is −14.47 ± 1.92 km s⁻¹. We determined the physical parameters (T_eff, log g and V sin i) for 11 possible members of the B-type stars in the open cluster NGC 6475, using the Barbier–Chalonge–Divan (BCD) spectrophotometry system. We also determined the projected rotational velocity of the stars by matching them with the theoretical LTE model and located our studied stars over the HR diagram. From the 11 studied B-type stars, we found six members from them.

TIFR Near Infrared Imaging Camera-II (TIRCAM2) is being used at the 3.6-m Devasthal Optical Telescope (DOT) operated by Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, Uttarakhand, India. Earlier, the TIRCAM2 was used at the main port of the DOT on time-shared basis. It has now been installed at the side port of the telescope. Side port installation allows near simultaneous observations with the main port instrument as well as longer operating periods. Thus, the TIRCAM2 serves the astronomical community for a variety of observations ranging from lunar occultations, transient events and normal scheduled observations.

Herbig Ae/Be stars are intermediate-mass pre-main sequence stars undergoing accretion through their circumstellar disk. The optical and infrared (IR) spectra of HAeBe stars show Hi emission lines belonging to Balmer, Paschen and Brackett series. We used the archival X-Shooter spectra available for 109 HAeBe stars from Vioque et al. (2018) and analysed the various Hi lines present in them. We segregated the stars into different classes based on the presence of higher-order lines in different Hi series. We discussed the dependence of the appearance of higher-order lines on the stellar parameters. We found that most massive and younger stars show all the higher-order lines in emission. The stars showing only lower-order lines have (T_textrm{eff} < {12{,}000}) K and an age range of 5–10 Myr. We performed a case B line ratio analysis for a sub-sample of stars showing most of the Hi lines in emission. We noted that all but four stars belonging to the sub-sample show lower Hi line ratios than theoretical values, owing to the emitting medium being optically thick. The Hi line flux ratios do not depend on the star’s spectral type. Further, from the line ratios of lower-order lines and Paschen higher-order lines, we note that line ratios of most HAeBe stars match with electron density value in the range of (10^9)–(10^{11}) cm(^{-3}). The electron temperature, however, could not be ascertained with confidence using the line ratios studied in this work.

The nature of equation of state for the matter in the neutron star plays an important role in determining its maximal mass. In addition, it must comply with the condition of causality. Noting that the central density of a maximally massive neutron star is well above the nuclear saturation density, a deconfined quark core in the central region is motivated in this paper. We analyze this scenario by employing the MIT bag model to represent the core region and one of the unified equations of state for the region outside the core. Such a combination is found to solve the problem of causality violation. In each case of the combined equations of state, the radial profile of (rho r^2) displays a peak and the dominant contribution to the total mass of the star comes from the region around the peak value of (rho r^2), whereas the contribution is small from the regions near the center and the surface. This peak occurs in the region of hadronic matter for the combinations considered in this paper. Importantly, we find that the position of the peak in (rho r^2) is well-correlated with the maximal mass—the highest value of 1.98 (M_odot ) obtains for the case with the peak occurring farthest from the center. This gravitational threshold being obtained for a non-rotating neutron star, we expect the threshold to lie well above 2 (M_odot ) for a rapidly rotating neutron star, that may explain the existence of massive pulsars from recent astronomical observations.

We study the timing and spectral properties of the X-ray pulsar 2S 1553–542 using the NuSTAR and NICER during the outburst in January–February 2021. During the outburst, the spin period of the neutron star was (Psim 9.2822) s based on NuSTAR data. The pulse profiles are studied using different NICER observations, which implies that the profile is more or less sinusoidal with a single peak and the beaming patterns are dominated mainly by the pencil beam. The NICER spectra of the source are studied for different days of the outburst. They can be well described by a model consisting of a blackbody emission, power law and photoelectric absorption component. The variation of spectral parameters with luminosity is studied over the outburst. The photon index shows anti-correlation with luminosity below the critical luminosity, which implies that the source was accreting in the sub-critical accretion regime during the NICER observations. We also report the anti-correlation between pulsed fraction (PF) and luminosity of the 2S 1553–542 using NICER observations. The evolution of spin-up rate with luminosity is studied during the outburst, which implies that both are strongly correlated. The torque-luminosity model is applied to estimate the magnetic field at different spin-up rates. The magnetic field is estimated to be ({simeq }2.56 times 10^{12}) G from the torque-luminosity model using the source distance of 20 kpc. The magnetic field is also estimated using the critical luminosity, consistent with our findings.

We present photometric and kinematic analyses of an intermediate-age open cluster NGC 1027 using ({UBV(RI)}_c) and Gaia Early Data Release 3 (EDR3) data. Structural and fundamental parameters, such as cluster center, cluster extent, reddening, age and distance are estimated in this study. Cluster center is found about 4 arcmin away from the center reported earlier. Radius has been estimated to be about 8.00 arcmin (2.65 pc). Using proper motion Gaia EDR3 data, membership probabilities have been derived for the stars in the region of cluster radius. We found mean proper motion of the cluster to be (sim )(−0.84, 2.04) mas yr(^{-1}) in (RA, DEC). We found 217 most probable ((P_mu>) 70%) cluster members with mean parallax (0.892 pm 0.088) mas. Out of these, 160 members have counterparts in our optical observations. Few stars having (P_mu > 70)%, are found out of the cluster radius showing imprints of dynamical evolution. The color–color and color–magnitude diagrams for the cluster members found within 8.00 arcmin have been constructed using ({UBV(RI)}_c) photometry and Gaia EDR3 data. This yields a reddening (E(B-V) sim 0.36) mag, age (sim )130 Myr and distance (sim )1.14 kpc. The mass function slope in the cluster region is (Gamma sim -1.46 pm 0.15), which is similar to other Galactic open clusters. The dynamical study shows lack of faint stars in its inner region leading to mass segregation effect. A comparison of dynamical age with cluster age indicates that NGC 1027 is a dynamically relaxed cluster suggesting that mass segregation may be imprint of its dynamical relaxation.

Fragmentation is a key step in the process of transforming clouds (and their substructures, such as filaments, clumps and cores) into protostars. The thermal gas pressure and gravitational collapse are believed to be the primary agents governing this process, referred as thermal Jeans fragmentation. However, the contributions of other factors (such as magnetic fields and turbulence) to the fragmentation process remain less explored. In this work, we have tested possible fragmentation mechanisms by estimating the mean core mass and mean inter-core separation of the B213 filament. We have used the (sim )14(^circ ) resolution James Clerk Maxwell Telescope (JCMT) Submillimeter Common-User Bolometer Array 2 (SCUBA-2)/POL-2 850 (mu )m dust continuum map and combined it with a Planck 850 (mu )m map and Herschel data. We find that in addition to the thermal contribution, the presence of ordered magnetic fields could be important in the fragmentation of the B213 filament.

LDN1415-IRS, a low-mass young stellar object (YSO), went into an outburst between 2001 and 2006, illuminating a surrounding nebula, LDN1415-Neb. LDN1415-Neb was found to have brightened by (I=3.77) mag by April 2006. The optical light curve covering (sim )15.5 years, starting from October 2006 to January 2022, is presented in this study. The initial optical spectrum indicated the presence of winds in the system, but the subsequent spectra have no wind indicators. The declining light curve and the absence of the P-Cygni profile in later epoch spectra indicate that the star and nebula system is retrieving back from its outburst state. Two Herbig–Haro objects (HHOs) are positioned linearly with respect to the optical brightness peak of the nebula, probably indicating the circumstellar disk being viewed edge-on. Our recent deep near-infrared (NIR) imaging using TANSPEC has revealed the presence of a nearby star-like source, south of the LDN1415-IRS, at an angular distance of (sim !5.4'').

Protostellar jets are one of the primary signposts of star formation. A handful of protostellar objects exhibit radio emission from ionized jets, of which a few display negative spectral indices, indicating the presence of synchrotron emission. In this study, we characterize the radio spectra of HH80-81 jet with the help of a numerical model that we have developed earlier, which takes into account both thermal free–free and non-thermal synchrotron emission mechanisms. For modeling the HH80-81 jet, we consider jet emission towards the central region close to the driving source along with two Herbig-Haro objects, HH80 and HH81. We have obtained the best-fit parameters for each of these sources by fitting the model to radio observational data corresponding to two frequency windows taken across two epochs. Considering an electron number density in the range of (10^3)–(10^5) cm(^{-3}), we obtained the thickness of the jet edges and fraction of relativistic electrons that contribute to non-thermal emission in the range of (0.01^{circ })–(0.1^{circ }) and (10^{-7})–(10^{-4}), respectively. For the best-fit parameter sets, the model spectral indices lie in the range of − 0.15 to (+)0.11 within the observed frequency windows.

The Solar Mean Magnetic Field (SMMF) is the mean value of the line-of-sight (LOS) component of the solar vector magnetic field averaged over the visible hemisphere of the Sun. So far, the studies on SMMF have mostly been confined to the magnetic field measurements at the photosphere. In this study, we calculate and analyse the SMMF using magnetic field measurements at the chromosphere, in conjunction with that of photospheric measurements. For this purpose, we have used full-disk LOS magnetograms derived from spectropolarimetric observations carried out in Fe i 6301.5 Å and Ca ii 8542 Å by the Synoptic Optical Long-term Investigations of the Sun (SOLIS)/Vector Spectromagnetograph (VSM) instrument during 2010–2017. It is found from this study that the SMMF at the chromosphere is weaker by a factor of 0.60 compared to the SMMF at the upper-photosphere. The correlation analysis between them gives a Pearson correlation coefficient of 0.80. The similarity and reduced intensity of the chromospheric SMMF with respect to the photospheric SMMF corroborate the idea that it is the source of the Interplanetary Magnetic Field (IMF).