Explore the vast range of titles available.

For several decades we have been performing photometric monitoring of some of the star formation regions using the telescopes in Rozhen Observatory. We consider that the study of photometric variability of pre-main sequence (PMS) stars is of great importance in understanding stellar evolution. A number of young stellar objects show brightness variability with large amplitudes that can be registered with small and medium-sized telescopes. But in recent years, bright traces of satellites have increasingly appeared in our CCD images. So far they are not much of a problem since most of our objects and the standard stars around them are point sources of small apparent size. But in the future, the possible increase in the number and brightness of satellite tracks may affect the quality of the photometric data we receive.

We investigated chromospheric activities of pre-main-sequence (PMS) stars. First, we studied the Ca II infrared triplet emission lines with Subaru/HDS and other spectroscopic instruments. Most PMS stars have narrow Ca II lines whose intensities are as large as the maximum of the zero-age main-sequence (ZAMS) stars. The chromosphere of PMS stars is suggested to be filled by the Ca II emitting region. Second, we found many faint chromospheric emission lines such as Mg I and Fe I for more than half of the ZAMS stars. Third, we searched the periodic light variation caused by a starspot for the 26 PMS stars. Their TESS light variations and Ca II emission line strengths show the positive correlation, and are located on the extensions of the superflare stars. In summary, PMS stars have very active chromosphere driven by strong dynamo process due to the fast rotation and the long convection timescale.

Although planets are being discovered around stars more massive than the Sun, information about the proto-planetary disks where such planets have built up is sparse. We have imaged mid-infrared emission from polycyclic aromatic hydrocarbons at the surface of the disk surrounding the young intermediate-mass star HD 97048 and characterized the disk. The disk is in an early stage of evolution, as indicated by its large content of dust and its hydrostatic flared geometry, indicative of the presence of a large amount of gas that is well mixed with dust and gravitationally stable. The disk is a precursor of debris disks found around more-evolved A stars such as β-Pictoris and provides the rare opportunity to witness the conditions prevailing before (or during) planet formation.

The process of accretion through circumstellar disks in young stellar objects is an integral part of star formation and the H α emission line is a prominent signature of accretion in low-mass stars. We present the detection and characterization of H α emission line sources in the central region of a distant, low-metallicity young stellar cluster Dolidze 25 (at ∼  4.5 kpc) using medium-resolution optical spectra (4750–9350 Å) obtained with the multi-unit spectroscopic explorer (MUSE) at the VLT. We have identified 14 potential accreting sources within a rectangular region of ( 2 ′ × 1 ′ ) towards the center of the cluster based on the detection of strong and broad emissions in H α as well as the presence of other emission lines, such as [OI] and H β . Based on their positions in both photometric color–magnitude and color–color diagrams, we have also confirmed that these objects belong to the pre-main sequence phase of star formation. Our results were compared with the disk and diskless members of the cluster previously identified by Guarcello et al. ( 2021 ) using near-IR colors, and all sources they had identified as disks were confirmed to be accreting based on the spectroscopic characteristics.

The FU Orionis (FUor) and EX Lupi (EXor) type objects are rare pre-main sequence low-mass stars undergoing accretion outbursts. Maser emission is widespread and is a powerful probe of mass accretion and ejection on small scales in star forming region. However, very little is known about the overall prevalence of water masers towards FUors/Exors. We present results from our survey using the Effelsberg 100-m telescope to observe the largest sample of FUors and EXors, plus additional Gaia alerted sources (with the potential nature of being eruptive stars), a total of 51 targets, observing the 22.2 GHz H2O maser, while simultaneously covering the NH3 23 GHz.

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.

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 ∼ 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 ∼ 5 . 4 ′ ′ .

Dusty circumstellar disks in orbit around main-sequence stars were discovered in 1983 by the infrared astronomical satellite. It was the first time material that was not another star had been seen in orbit around a main-sequence star other than our Sun. Since that time, analyses of data from the infrared astronomical satellite, the infrared space observatory, and ground-based telescopes have enabled astronomers to paint a picture of dusty disks around numerous main-sequence and post-main-sequence stars. This review describes, primarily in an evolutionary framework, the properties of some dusty disks orbiting, first, pre-main-sequence stars, then main-sequence and post-main-sequence stars, and ending with white dwarfs.

We calculate the spectral energy distribution of the first galaxies which contain pre-main-sequence stars by using the stellar evolution code Modules for Experiments in Stellar Astrophysics, the spectra model BT-Settl, and the stellar population synthesis code PEGASE. We calculate the galaxy spectral energy distribution for Salpeter Initial Mass Function. We find that very young first galaxies are bright also in mid-infrared, and the contribution of pre-main-sequence stars can be significant over 0.1 Myr after a star-formation episode.

Our aim is to present a new and so far most complete catalog of optically selected young stars. The basis of this work is an extensive literature search for young stars in all the known nearby (< 2 kpc) star forming regions, included in the Handbook of Star Forming Regions [4, 5], and in 67 additional catalogs. We collected data on known young, pre-main-sequence stars detected in optical bands. The catalog contains the celestial coordinates, object names, names of the enclosing star forming region, identification methods, distances, and other information (e.g., references, binarity) for 15208 young stellar objects. It is already in use by the Gaia Photometric Science Alerts Team to identify variable young stars in the Gaia data. Our catalog was cross-correlated with the Gaia DR2 and we obtained flux and distance estimations for 86% of the stars.