Star formation in the region of young open cluster - NGC 225

NGC 225 is believed to be a 120 Myr old open cluster located at ∼ 650 pc. Eight stars with Hα emission are found to be located around the cluster, of which two are probable Herbig Be stars, indicating a very young age for the cluster. To explore whether the Herbig Be stars, which are premain sequence (PMS) stars are part of this cluster, we re-estimated the cluster parameters using optical (UBV)pg and 2MASS JHK photometry. We combined the above data to detect the presence of any possible PMS stars in the cluster region. Among the identified 28 proper motion members, 15 stars were found to have near-infrared (NIR) excess indicating that they are PMS stars. Also, most of the upper MS stars were found to show NIR excess suggesting that the brighter proper motion member stars have not yet reached the MS. PMS isochrones were used to estimate the age of stars with NIR excess and is found to be between 0.5–10 Myr. Thus, the cluster NGC 225 is a very young cluster, younger than 10 Myr and its age is not 120 Myr as previously believed. We propose that a recent star formation has resulted in the formation of NGC 225, two Herbig Be stars, stars with Hα emission, dust lanes and nebulosity in the vicinity of the cluster

Spectroscopic study of Herbig Ae/Be stars in the Galactic Anti-center region from LAMOST DR5

We study a sample of 119 Herbig Ae/Be stars in the Galactic anti-center direction using the spectroscopic data from Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey program. Emission lines of hydrogen belonging to the Balmer and Paschen series, and metallic lines of species such as FeII, OI, CaII triplet are identified. A moderate correlation is observed between the emission strengths of H$\alpha$ and FeII 5169 \r{A}, suggesting a possible common emission region for FeII lines and one of the components of H$\alpha$. We explored a technique for the extinction correction of the HAeBe stars using diffuse interstellar bands present in the spectrum. We estimated the stellar parameters such as age and mass of these HAeBe stars, which are found to be in the range 0.1 -- 10 Myr and 1.5 -- 10 $M_{\odot}$, respectively. We found that the mass accretion rate of the HAeBe stars in the Galactic anti-center direction follows the relation $\dot{M}_{acc}$ $\propto$ $M_{*}^{3.12^{+0.21}_{-0.34}}$, which is similar to the relation derived for HAeBe stars in other regions of the Galaxy. The mass accretion rate of HAeBe stars is found to have a functional form of $\dot{M}_{acc} \propto t^{-1.1 \pm 0.2}$ with age, in agreement with previous studies.Comment: 18 pages, 11 figures, 3 tables, accepted for publication in MNRA

Gaia: Surveying Heavens

In this paper we attempt to study an ongoing astrometry mission of the European Space Agency (ESA), named Gaia, whose aim is to make the largest and most precise three-dimensional map of our Galaxy. We present the scientific goals of Gaia and give a brief description of the spacecraft. We also present a preliminary analysis of comparing distance estimates of Be stars from the first Gaia data release, Gaia DR1, and Hipparcos mission. From our analysis, we confirm that Gaia stands out as a promising mission in terms of the distance measurements when compared to Hipparcos, particularly for distances greater than 1 kpc

Are Narrow-line Seyfert 1 Galaxies Powered by Low-mass Black Holes?

Narrow-line Seyfert 1 galaxies (NLS1s) are believed to be powered by the accretion of matter onto low-mass black holes (BHs) in spiral host galaxies with BH masses MBH ∼ 106–108 Msun. However, the broadband spectral energy distribution of the γ-ray-emitting NLS1s are found to be similar to flat-spectrum radio quasars. This challenges our current notion of NLS1s having low MBH. To resolve this tension of low MBH values in NLS1s, we fitted the observed optical spectrum of a sample of radio-loud NLS1s (RL-NLS1s), radio-quiet NLS1s (RQ-NLS1s), and radio-quiet broad-line Seyfert 1 galaxies (RQ-BLS1s) of ∼500 each with the standard Shakura–Sunyaev accretion disk (AD) model. For RL-NLS1s we found a mean log(MBH,AD/Msun) of 7.98 ± 0.54. For RQ-NLS1s and RQ-BLS1s we found mean log(MBH,AD/Msun) of 8.00 ± 0.43 and 7.90 ± 0.57, respectively. While the derived MBH,AD values of RQ-BLS1s are similar to their virial masses, for NLS1s the derived MBH,AD values are about an order of magnitude larger than their virial estimates. Our analysis thus indicates that NLS1s have MBH similar to RQ-BLS1s and their available virial MBH values are underestimated, influenced by their observed relatively small emission line widths. Considering the Eddington ratio as an estimation of the accretion rate and using MBH,AD, we found the mean accretion rate of our RQ-NLS1s, RL-NLS1s, and RQ-BLS1s as 0.06, 0.05 and 0.05, respectively. Our results, therefore, suggest that NLS1s have BH masses and accretion rates that are similar to BLS1s.</p

Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/GSC observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 10^(37) erg/s (MAXI), which got reduced to 10^(32) erg/s (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to H-alpha emission, we found emission lines of He I in the optical spectrum of this star. By comparing our spectrum of the object with those from literature we found that He I lines show variability. Using photometric study we estimate that the star is having effective temperature of 20000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favours it to be evolving off the main sequence in the Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.Comment: 9 pages, 5 figures, 1 table, accepted for publication in ApJ

Identifying the population of T-Tauri stars in Taurus: UV-optical synergy

With the third data release of the Gaia mission $Gaia$ DR3 with its precise photometry and astrometry, it is now possible to study the behaviour 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 CMDs by combining the GALEX and Gaia surveys. We found 19 TTS candidates and 5 of them are newly identified TTS in the Taurus Molecular Cloud (TMC), not catalogued 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 the 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 show that the two populations have similar ages and proper motion distribution. Using the $Gaia$ DR3 colour-magnitude diagram, we show that the age of Taurus is consistent with 1 Myr.Comment: 13 pages, 10 figure