Young Low-mass Stars Research Articles

A dusty magnetospheric stream explaining the light curves of the dipper objects: Finding a new inclination threshold to produce dippers

Context. The so-called “dippers” are young stellar objects that exhibit dimming episodes in their optical light curves. The common interpretation for the occurrence of these dips is that dusty regions periodically or quasi-periodically cross the line of sight toward the object. Aims. We develop a model where we assume that these regions are located at the intersection of the magnetospheric stream with the disk. The stream is fed by gas and dust coming from the disk. As the material follows the magnetic field lines above the disk plane, it forms an opaque screen that partially blocks the stellar emission. The amount of extinction caused by the material crossing the line of sight depends on the abundance and location of the dust along the stream, which depends on the degree of dust evaporation due to the heating by the star. Methods. We run hydrodynamical simulations of dusty accretion streams to produce synthetic dipper light curves for a sample of low-mass young stars still accreting from their disk according to evolutionary models. We compare the distribution of the light curve amplitudes between the synthetic sample and observed samples of dippers from various star-forming regions. Results. Dust evaporation along the accretion column drives the distribution of photometric amplitudes. Our results suggest that most of the observed dippers correspond to systems seen at high inclination. However, dust survival within accretion columns may also produce dippers at lower inclination, down to about 45°. We find that the dust temperature arising from stellar irradiation should be increased by a factor 1.6 to find consistency between the fraction of dippers our model predicts in star-forming regions and the observed fraction of 20–30%. Conclusions. Transient dust survival in accretion columns appear as an alternative (or complementary) mechanism to inner disk warp occultation in order to account for low-inclination dippers in star-forming regions.

Localizing Stellar Activity on Low-mass Stars with the Transiting Exoplanet Survey Satellite

Magnetic reconnection is the underlying cause of stellar flares which are linked to regions of high magnetic activity, like star spots. To understand trends in stellar activity, we need to study how it is heightened by rapid rotation and deep convection in young low-mass stars. We analyze light curves of such stars observed by the Transiting Exoplanet Survey Satellite to investigate trends in flare timing with starspot modulation signals. Trends in flare properties and spot modulation can provide a means to “localize” which face of a star flares more frequently and better understand their association with active regions. We present an analysis of light curves from M and K dwarfs with no companions from five nearby and young moving groups spanning ages ∼20–150 Myr. We discuss a technique to analyze the distribution of flares and star spots and describe our results, which reveal a tentative correlation.

The population of young low-mass stars in Trumpler 14

The population of young low-mass stars in Trumpler 14

The jet of BP Tau

Context. A strong global magnetic field of young low-mass stars and a high accretion rate are the necessary conditions for the formation of collimated outflows (jets) from these objects. But it is still unclear whether these conditions are also sufficient. Aims. We aim to check whether BP Tau, an actively accreting young star with a strong magnetic field, has a jet. Methods. We carried out narrowband [S II] 672 nm imaging and spectroscopic observations of BP Tau and its vicinity. Results. We find that BP Tau is a source of a Herbig-Haro flow (assigned number HH 1181), which includes two HH objects moving from the star in opposite directions and a micro- (counter-) jet of ∼1″ projected length. The flow is oriented along position angle 59 ± 1°.

Dynamical Mass of the Ophiuchus Intermediate-mass Stellar System S1 with DYNAMO-VLBA

We report dynamical mass measurements of the individual stars in the most luminous and massive stellar member of the nearby Ophiuchus star-forming region, the young tight binary system S1. We combine 28 archival data sets with seven recent proprietary Very Long Baseline Array (VLBA) observations obtained as part of the Dynamical Masses of Young Stellar Multiple Systems with the VLBA project (DYNAMO–VLBA), to constrain the astrometric and orbital parameters of the system, and recover high-accuracy dynamical masses. The primary component, S1A, is found to have a mass of 4.11 ± 0.10 M ⊙, significantly lower than the typical value ∼6 M ⊙ previously reported in the literature. We show that the spectral energy distribution (SED) of S1A can be reproduced by a reddened blackbody with a temperature between roughly 14,000 and 17,000 K. According to evolutionary models, this temperature range corresponds to stellar masses between 4 M ⊙ and 6 M ⊙, so the SED is not a priori inconsistent with the dynamical mass of S1A. The luminosity of S1 derived from SED fitting, however, is only consistent with models for stellar masses above 5 M ⊙. Thus, we cannot reconcile the evolutionary models with the dynamical mass measurement of S1A: The models consistent with the location of S1A in the Hertzsprung-Russel diagram correspond to masses higher by 25% at least than the dynamical mass. For the secondary component, S1B, a mass of 0.831 ± 0.014 M ⊙ is determined, consistent with a low-mass young star. While the radio flux of S1A remains roughly constant throughout the orbit, the flux of S1B is found to be higher near apastron.

Optical and Infrared Counterparts of the X-Ray Sources Detected in the Chandra Cygnus OB2 Legacy Survey

The young massive OB association Cygnus OB2, in the Cygnus X complex, is the closest (∼1400 pc) star-forming region to the Sun hosting thousands of young low-mass stars and up to 1000 OB stars, among which are some of the most massive stars known in our Galaxy. This region holds great importance for several fields of modern astrophysics, such as the study of the physical properties of massive and young low-mass stars and the feedback provided by massive stars on star and planet formation processes. Cyg OB2 has been recently observed with Chandra/ACIS-I as part of the 1.08 Ms Chandra Cygnus OB2 Legacy Project. This survey detected 7924 X-ray sources in a square degree area centered on Cyg OB2. Since a proper classification and study of the observed X-ray sources also requires the analysis of their optical and infrared counterparts, we combined a large and deep set of optical and infrared catalogs available for this region with our new X-ray catalog. In this paper we describe the matching procedure and present the combined catalog containing 5703 sources. We also briefly discuss the nature of the X-ray sources with optical and infrared counterparts using their position in the color–magnitude and color–color diagrams.

X-Ray Spectral Characterization of the Young Cygnus OB2 Population

We analyze the X-ray spectra of the ∼8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (this focus issue), with the goals of characterizing the coronal plasma of the young low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample we constrain the spectral parameters to characteristic ranges defined from the brightest stars. For X-ray sources with <20 net photons we adopt a conversion factor from detected photon flux to intrinsic flux. This was defined, building on the results for the previous sample, as a function of the 20% quantile of the detected photon energy distributions, which we prove to also correlate well with extinction. We then use the X-ray extinction from the spectral fits to constrain the ratio between optical and X-ray extinction toward Cyg OB2, finding it consistent with standard “Galactic” values, when properly accounting for systematics. Finally, we exploit the large number of sources to constrain the average coronal abundances of several elements, through two different ensemble analyses of the X-ray spectra of low-mass Cyg OB2 members. We find the pattern of abundances to be largely consistent with that derived for the young stellar coronae in the Orion Nebula Cluster.

Imaging the warped dusty disk wind environment of SU Aurigae with MIRC-X

Context. T Tauri stars are low-mass young stars whose disks provide the setting for planet formation, which is one of the most fundamental processes in astronomy. Yet the mechanisms of this are still poorly understood. SU Aurigae is a widely studied T Tauri star and here we present original state-of-the-art interferometric observations with better uv and baseline coverage than previous studies. Aims. We aim to investigate the characteristics of the circumstellar material around SU Aur, and constrain the disk geometry, composition and inner dust rim structure. Methods. The MIRC-X instrument at CHARA is a six-telescope optical beam combiner offering baselines up to 331 m. We undertook image reconstruction for model-independent analysis, and fitted geometric models such as Gaussian and ring distributions. Additionally, the fitting of radiative transfer models constrained the physical parameters of the disk. Results. Image reconstruction reveals a highly inclined disk with a slight asymmetry consistent with inclination effects obscuring the inner disk rim through absorption of incident star light on the near side and thermal re-emission/scattering of the far side. Geometric models find that the underlying brightness distribution is best modelled as a Gaussian with a Full-Width Half-Maximum of 1.53 ± 0.01 mas at an inclination of 56.9 ± 0.4° and a minor axis position angle of 55.9 ± 0.5°. Radiative transfer modelling shows a flared disk with an inner radius at 0.16 au which implies a grain size of 0.14 μm assuming astronomical silicates and a scale height of 9.0 au at 100 au. In agreement with the literature, only the dusty disk wind successfully accounts for the near infrared excess by introducing dust above the mid-plane. Conclusions. Our results confirm and provide better constraints than previous inner disk studies of SU Aurigae. We confirm the presence of a dusty disk wind in the cicumstellar environment, the strength of which is enhanced by a late infall event which also causes very strong misalignments between the inner and outer disks.

Mass Accretion, Spectral, and Photometric Properties of T Tauri Stars in Taurus Based on TESS and LAMOST

We present the analysis of 16 classical T Tauri stars (CTTSs) using LAMOST and TESS data, investigating spectral properties, photometric variations, and mass accretion rates. All 16 stars exhibit emissions in Hα lines, from which the average mass accretion rate of 1.76 × 10−9 M ⊙ yr−1 is derived. Two of the stars, DL Tau and Haro 6-13, show mass accretion bursts simultaneously in TESS, ASAS-SN, and/or the ZTF survey. Based on these observations, we find that the mass accretion rates of DL Tau and Haro 6-13 reach their maxima of 2.5 × 10−8 M ⊙ yr−1 and 2 × 10−10 M ⊙ yr−1, respectively, during the TESS observation. We detect 13 flares among these stars. The flare frequency distribution shows that the CTTSs’ flare activity is not only dominated by strong flares with high energy but also much more active than those of solar-type and young low-mass stars. By comparing the variability classes reported in the literature, we find that the transition timescale between different classes of variability in CTTSs, such as from stochastic (S) to bursting (B) or from quasi-periodic symmetric to quasi-periodic dipping, may range from 1.6 to 4 yr. We observe no significant correlation between inclination and mass accretion rates derived from the emission indicators. This suggests that inner disk properties may be more important than those of outer disks. Finally, we find a relatively significant positive correlation between the asymmetric metric M and the cold disk inclination compared to the literature. A weak negative correlation between the periodicity metric Q value and inclination has also been found.

The post-disk (or primordial) spin distribution of M dwarf stars

Context. The rotation periods of young low-mass stars after disks have dissipated (≲-pagination10 Myr) but before magnetized winds have removed significant angular momentum is an important launch point for gyrochronology and models of stellar rotational evolution; the rotation of these stars also regulates the magnetic activity and the intensity of high-energy emission that affects any close-in planets. A recent analysis of young M dwarf stars suggests a distribution of specific angular momentum (SAM) that is mass-independent, but the physical basis of this observation is unclear. Aims. We investigate the influence of an accretion disk on the angular momentum (AM) evolution of young M dwarfs, whose parameters govern the AM distribution after the disk phase, and whether this leads to a mass-independent distribution of SAM. Methods. We used a combination of protostellar spin and implicit hydrodynamic disk evolution models to model the innermost disk (∼0.01 AU), including a self-consistent calculation of the accretion rate onto the star, non-Keplerian disk rotation, and the influence of stellar magnetic torques over the entire disk lifetime. We executed and analyzed over 500 long-term simulations of the combined stellar and disk evolution. Results. We find that above an initial rate of Ṁcrit ∼ 10−8 M⊙ yr−1, accretion “erases” the initial SAM of M dwarfs during the disk lifetime, and stellar rotation converges to values of SAM that are largely independent of initial conditions. For stellar masses &gt; 0.3 M⊙, we find that observed initial accretion rates Ṁinit are comparable to or exceed Ṁcrit. Furthermore, stellar SAM after the disk phase scales with the stellar magnetic field strength as a power law with an exponent of −1.1. For lower stellar masses, Ṁinit is predicted to be smaller than Ṁcrit and the initial conditions are imprinted in the stellar SAM after the disk phase. Conclusions. To explain the observed mass-independent distribution of SAM, the stellar magnetic field strength has to range between 20 G and 500 G (700 G and 1500 G) for a 0.1 M⊙ (0.6 M⊙) star. These values match observed large-scale magnetic field measurements of young M dwarfs and the positive relation between stellar mass and magnetic field strength agrees with a theoretically motivated scaling relation. The scaling law between stellar SAM, mass, and the magnetic field strength is consistent for young stars, where these parameters are constrained by observations. Due to the very limited number of available data, we advocate for efforts to obtain more such measurements. Our results provide new constraints on the relation between stellar mass and magnetic field strength and they can be used as initial conditions for future stellar spin models, starting after the disk phase.

Spectral classification of young stars using conditional invertible neural networks

Aims. We introduce a new deep-learning tool that estimates stellar parameters (e.g. effective temperature, surface gravity, and extinction) of young low-mass stars by coupling the Phoenix stellar atmosphere model with a conditional invertible neural network (cINN). Our networks allow us to infer the posterior distribution of each stellar parameter from the optical spectrum. Methods. We discuss cINNs trained on three different Phoenix grids: Settl, NextGen, and Dusty. We evaluate the performance of these cINNs on unlearned Phoenix synthetic spectra and on the spectra of 36 class III template stars with well-characterised stellar parameters. Results. We confirm that the cINNs estimate the considered stellar parameters almost perfectly when tested on unlearned Phoenix synthetic spectra. Applying our networks to class III stars, we find good agreement with deviations of 5–10% at most. The cINNs perform slightly better for earlier-type stars than for later-type stars such as late M-type stars, but we conclude that estimates of effective temperature and surface gravity are reliable for all spectral types within the training range of the network. Conclusions. Our networks are time-efficient tools that are applicable to large numbers of observations. Among the three networks, we recommend using the cINN trained on the Settl library (Settl-Net) because it provides the best performance across the widest range of temperature and gravity.

Survey of $$\hbox {H}{\varvec{\alpha }}$$ emission-line stars in the star-forming region IC 5070

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 {\it 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, that 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).

Enhanced YSO population in Serpens

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.

How tidal waves interact with convective vortices in rapidly rotating planets and stars

Context. The dissipation of tidal inertial waves in planetary and stellar convective regions is one of the key mechanisms that drive the evolution of star–planet and planet–moon systems. This dissipation is particularly efficient for young low-mass stars and gaseous giant planets, which are rapid rotators. In this context, the interaction between tidal inertial waves and turbulent convective flows must be modelled in a realistic and robust way. In the state-of-the-art simulations, the friction applied by convection on tidal waves is commonly modeled as an effective eddy viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of the tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices such as those observed at the poles of Jupiter and Saturn. The large-scale vortices are potentially triggered by convection in rapidly-rotating bodies in which the Coriolis acceleration forms the flow in columnar vortical structures along the direction of the rotation axis. Aims. We investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. Methods. We used a quasi-geostrophic semi-analytical model of a convective columnar vortex, which is validated by numerical simulations. First, we carried out linear stability analysis using both numerical and asymptotic Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) methods. We then conducted linear numerical simulations of the interactions between a convective columnar vortex and an incoming tidal inertial wave. Results. The vortex we consider is found to be centrifugally stable in the range –Ωp ≤ Ω0 ≤ 3.62Ωp and unstable outside this range, where Ω0 is the local rotation rate of the vortex at its center and Ωp is the global planetary (stellar) rotation rate. From the linear stability analysis, we find that this vortex is prone to centrifugal instability with perturbations with azimuthal wavenumbers m = {0,1, 2}, which potentially correspond to eccentricity, obliquity, and asynchronous tides, respectively. The modes with m &gt; 2 are found to be neutral or stable. The WKBJ analysis provides analytic expressions of the dispersion relations for neutral and unstable modes when the axial (vertical) wavenumber is sufficiently large. We verify that in the unstable regime, an incoming tidal inertial wave triggers the growth of the most unstable mode of the vortex. This would lead to turbulent dissipation. For stable convective columns, the wave-vortex interaction leads to the mixing of momentum for tidal inertial waves while it creates a low-velocity region around the vortex core and a new wave-like perturbation in the form of a progressive wave radiating in the far field. The emission of this secondary wave is the strongest when the wavelength of the incoming wave is close to the characteristic size (radius) of the vortex. Incoming tidal waves can also experience complex angular momentum exchanges locally at critical layers of stable vortices. Conclusions. The interaction between tidal inertial waves and large-scale coherent convective vortices in rapidly-rotating planets (stars) leads to turbulent dissipation in the unstable regime and complex behaviors such as mixing of momentum and radiation of new waves in the far field or wave-vortex angular momentum exchanges in the stable regime. These phenomena cannot be modeled using a simple effective eddy viscosity.

A survey for variable young stars with small telescopes – VII. Spot properties on YSOs in IC 5070

ABSTRACTWe present measurements of spot properties on 31 young stellar objects, based on multiband data from the HOYS (Hunting Outbursting Young Stars) project. On average the analysis for each object is based on 270 data points during 80 d in at least three bands. All the young low-mass stars in our sample show periodic photometric variations. We determine spot temperatures and coverage by comparing the measured photometric amplitudes in optical bands with simulated amplitudes based on atmosphere models, including a complete error propagation. 21 objects in our sample feature cool spots, with spot temperatures 500–2500 K below the stellar effective temperature (Teff), and a coverage of 0.05–0.4. Six more have hot spots, with temperatures up to 3000 K above Teff and coverage below 0.15. The remaining four stars have ambiguous solutions or are AA Tau-type contaminants. All of the stars with large spots (i.e. high coverage &amp;gt;0.1) are relatively cool with Teff &amp;lt; 4500 K, which could be a result of having deeper convection zones. Apart from that, spot properties show no significant trends with rotation period, infrared excess, or stellar properties. Most notably, we find hot spots in stars that do not show K − W2 infrared excess, indicating the possibility of accretion across an inner disc cavity or the presence of plage.

Low-mass young stars in the Milky Way unveiled by DBSCAN and Gaia EDR3: Mapping the star forming regions within 1.5 kpc

Context. With an unprecedented astrometric and photometric data precision, Gaia EDR3 provides, for the first time, the opportunity to systematically detect and map, in the optical bands, the low-mass populations of the star forming regions (SFRs) in the Milky Way. Aims. We aim to provide a catalogue of the Gαiα EDR3 data (photometry, proper motions and parallaxes) of the young stellar objects (YSOs) identified in the Galactic plane |b| &lt; 30°) within about 1.5 kpc. The catalogue of the SFRs to which they belong is also provided to study the properties of the very young clusters and put them in the context of the Galaxy structure. Methods. We applied the machine learning unsupervised clustering algorithm density-based spatial clustering of applications with noise (DBSCAN) to a sample of Gaia EDR3 data photometrically selected on the region where very young stars (t ≲ 10 Myr) are expected to be found, with the aim of identifying co-moving and spatially consistent stellar clusters. A sub-sample of 52 clusters, selected among the 7 323 found with DBSCAN, has been used as template data set to identify very young clusters from the pattern of the observed colour-absolute magnitude diagrams through a pattern-match process. Results. We find 124440 candidate YSOs clustered in 354 SFRs and stellar clusters younger than 10 Myr and within ≲ 1.5 kpc. In addition, 65 863 low-mass members of 322 stellar clusters located within ~500pc and with ages 10Myr ≲ t ≲ 100 Myr were also found. Conclusions. The selected YSOs are spatially correlated with the well-known SFRs. Most of them are associated with well-concentrated regions or complex structures of the Galaxy, and a substantial number of them have been recognised for the first time. The massive SFRs, such as, for example, Orion, Sco-Cen, and Vela, located within 600–700 pc trace a very complex three-dimensional pattern, while the farthest ones seem to follow a more regular pattern along the Galactic plane.

The Gaia-ESO survey: Constraining evolutionary models and ages for young low mass stars with measurements of lithium depletion and rotation

Abstract A growing disquiet has emerged in recent years that standard stellar models are at odds with observations of the colour-magnitude diagrams (CMDs) and lithium depletion patterns of pre main sequence (PMS) stars in clusters. In this work we select 1,246 high probability K/M-type constituent members of 5 young open clusters (5–125 Myr) in the Gaia-ESO Survey to test a series of models that use standard input physics and others that incorporate surface magnetic fields or cool starspots. We find that: standard models provide systematically under-luminous isochrones for low-mass stars in the CMD and fail to predict Li-depletion of the right strength at the right colour; magnetic models provide better CMD fits with isochrones that are ∼1.5 − 2 times older, and provide better matches to Li depletion patterns. We investigate how rotation periods, most of which are determined here for the first time from Transiting Exoplanet Survey Satellite data, correlate with CMD position and Li. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic ‘Li-rotation connection’ for the first time at ∼35 Myr in NGC 2547, and finding some evidence that it exists in the early M-stars of NGC 2264 at &amp;lt;10 Myr. However, the wide dispersion in Li depletion observed in fully-convective M-dwarfs in the γ Vel cluster at ∼20 Myr appears not to be correlated with rotation and is challenging to explain without a very large (&amp;gt;10 Myr) age spread.

Massive Compact Dust Disk with a Gap around CW Tau Revealed by ALMA Multiband Observations

Compact protoplanetary disks with a radius of ≲50 au are common around young low-mass stars. We report high-resolution ALMA dust continuum observations toward a compact disk around CW Tau at Bands 4 (λ = 2.2 mm), 6 (1.3 mm), 7 (0.89 mm), and 8 (0.75 mm). The spectral energy distribution shows the spectral slope of 2.0 ± 0.24 between 0.75 and 1.3 mm, while it is 3.7 ± 0.29 between 2.17 and 3.56 mm. The steep slope between 2.17 and 3.56 mm is consistent with that of optically thin emission from small grains (≲350 μm). We perform parametric fitting of the ALMA data to characterize the dust disk. Interestingly, if the dust-to-gas mass ratio is 0.01, the Toomre Q parameter reaches ∼1–3, suggesting that the CW Tau disk might be marginally gravitationally unstable. The total dust mass is estimated as ∼250 M ⊕ for the maximum dust size of 140 μm that is inferred from the previous Band 7 polarimetric observation and at least 80 M ⊕ even for larger grain sizes. This result shows that the CW Tau disk is quite massive in spite of its smallness. Furthermore, we clearly identify a gap structure located at ∼20 au, which might be induced by a giant planet. In spite of these interesting characteristics, the CW Tau disk has normal disk luminosity, size, and spectral index at ALMA Band 6, which could be a clue to the mass budget problem in Class II disks.

Deep V and I CCD photometry of young star cluster NGC 1893 with the 3.6m DOT

Young star clusters consisting of massive stars are the ideal sites to study the star formation processes and influence of massive stars on the subsequent star formation. NGC 1893 is a young star cluster associated with the Hii region Sh2-236. It contains about five ‘O’-type stars and several early ‘B’-type stars. It is located at a moderate distance of $$\sim $$ 3.25 kpc and has a reddening, $$E(B-V) \sim 0.4$$ mag. To characterize the young low-mass stellar population in the central portion of the cluster, we carried out deep VI band observations of the region using the $$4\mathrm{K} \times 4$$ K CCD IMAGER mounted on the 3.6-m Devasthal Optical Telescope. Our analysis shows that the present data are deep enough to detect stars below $$V\sim 24$$ mag. We found optical counterparts of $$\sim $$ 220 candidates, including young stars and unclassified cluster members from Caramazza et al. (2008). We estimated the membership probabilities of the Gaia sources (mostly bright stars with $$G<19$$ mag) located within the cluster radius using the Gaia EDR3. Toward the fainter end, we used the optical color-magnitude diagram (CMD) to select the cluster members from a sample of young stars. The locations of young stars on the CMD show that a majority of them are low-mass stars with age <10 Myr. The unclassified candidates and X-ray sources from Caramazza et al. (2012) are also found to be young low-mass stars. In total, we identified $$\sim $$ 425 young stars with age <10 Myr, and 110 of these are new. Most of these stars appear as kinematic members of the cluster. By examining the CMD for the stars in the cluster region, we suggest that the cluster has insignificant contamination due to field stars in the pre-main-sequence zone of the CMD. The slope of the mass function in the mass range $$0.2\le M/M_\odot \le 2.5$$ is found to be $$\Gamma = -\,1.43 \pm 0.15$$ , consistent with those of other star-forming complexes. The spatial distribution of the young stars as a function of mass suggests that toward the cluster center, most of the stars are massive.

Compact object mergers in hierarchical triples from low-mass young star clusters

Abstract A binary star orbited by an outer companion constitutes a hierarchical triple system. The outer body may excite the eccentricity of the inner binary through the von Zeipel-Lidov-Kozai (ZLK) mechanism, triggering the gravitational wave (GW) coalescence of the inner binary when its members are compact objects. Here, we study a sample of hierarchical triples with an inner black hole (BH) – BH binary, BH – neutron star (NS) binary, and BH – white dwarf (WD) binary, formed via dynamical interactions in low-mass young star clusters. Our sample of triples was obtained self-consistently from direct N-body simulations of star clusters which included up-to-date stellar evolution. We find that the inner binaries in our triples cannot merge via GW radiation alone, and the ZLK mechanism is essential to trigger their coalescence. Contrary to binaries assembled dynamically in young star clusters, binary BHs merging in triples have preferentially low mass ratios (q ≃ 0.3) and higher primary masses ($m_{\rm p} \gtrsim 40 {\, \rm M_\odot }$). We derive a local merger rate density of 0.60, 0.11 and $0.5 {-1} \, \rm Gpc^{-3}$ for BH-BH, BH-NS and BH-WD binaries, respectively. Additionally, we find that merging binaries have high eccentricities across the GW spectrum, including the LIGO-Virgo-KAGRA (LVK), LISA, and DECIGO frequencies. About 7 per cent of BH-BH and 60 per cent of BH-NS binaries will have detectable eccentricities in the LVK band. Our results indicate that the eccentricity and the mass spectrum of merging binaries are the strongest features for the identification of GW mergers from triples.