Low-mass Objects Research Articles

Tìm kiếm đĩa tàn dư xung quanh các sao lùn kiểu phổ M-trễ nằm trong vùng lân cận Mặt Trời

In this paper, we present our search for debris disks in a sample of nearby late-M dwarfs based on infrared data of the Wide Infrared Survey Explorer. Using archival data, we constructed spectral energy distributions of these targets to detect their infrared excess. We detected infrared excess only in one target. This late-M dwarf is an excellent benchmark for further study of disks around very low-mass objects.

Warm gas in protostellar outflows

Context. OMC-2/3 is one of the nearest embedded cluster-forming regions that includes intermediate-mass protostars at early stages of evolution. A previous CO (3–2) mapping survey towards this region revealed outflow activity related to sources at different evolutionary phases. Aims. The present work presents a study of the warm gas in the high-velocity emission from several outflows found in CO (3–2) emission by previous observations, determines their physical conditions, and makes a comparison with previous results in low-mass star-forming regions. Methods. We used the CHAMP+ heterodyne array on the APEX telescope to map the CO (6–5) and CO (7–6) emission in the OMC-2 FIR 6 and OMC-3 MMS 1-6 regions, and to observe 13CO (6–5) at selected positions. We analyzed these data together with previous CO (3–2) observations. In addition, we mapped the SiO (5–4) emission in OMC-2 FIR 6. Results. The CO (6–5) emission was detected in most of the outflow lobes in the mapped regions, while the CO (7–6) was found mostly in the OMC-3 outflows. In the OMC-3 MMS 5 outflow, a previously undetected extremely high-velocity gas was found in CO (6–5). This extremely high-velocity emission arises from the regions close to the central object MMS 5. Radiative transfer models revealed that the high-velocity gas from MMS 5 outflow consists of gas with nH2 = 104–105 cm−3 and T > 200 K, similar to what is observed in young Class 0 low-mass protostars. For the other outflows, values of nH2 > 104 cm−3 were found. Conclusions. The physical conditions and kinematic properties of the young intermediate-mass outflows presented here are similar to those found in outflows from Class 0 low-mass objects. Due to their excitation requirements, mid − J CO lines are good tracers of extremely high-velocity gas in young outflows likely related to jets.

Constraints on the formation of brown dwarfs by turbulent compression

ABSTRACT We perform radiation hydrodynamical simulations in spherical symmetry in order to investigate the formation of very low mass objects, i.e. brown dwarfs, by external compression. According to the Jeans stability criterion, a very low mass molecular cloud core must reach a very high density in order to become gravitationally unstable. One possibility to create such a high density is the compression by turbulent flows within the larger molecular cloud. Using our self-developed radiation hydrodynamics code, we aim to test the validity of this scenario, and to constrain the strength of the turbulence that is needed. We find that the probability for sufficiently strong and long-lived turbulence is very low under typical conditions even when using very optimistic assumptions, and therefore conclude that turbulent compression is unlikely to be the dominant mechanism for creating brown dwarfs. We also investigate the properties of objects formed by this turbulent compression process. Specifically, we compare the lifetime of the first core stage for the cases with and without external compression. We confirm our previous findings that the first core lifetime increases by about an order of magnitude at the extremely low-mass end, but this increase is somewhat less dramatic and occurs at even lower masses than in our previous work, in which no external compression was present.

Do evolved stars in the LMC show dual dust chemistry?

ABSTRACTWe study a group of evolved M-stars in the Large Magellanic Cloud, characterized by a peculiar spectral energy distribution. While the 9.7 $\mu$m feature arises from silicate particles, the whole infrared data seem to suggest the presence of an additional featureless dust species. We propose that the circumstellar envelopes of these sources are characterized by a dual dust chemistry, with an internal region, harbouring carbonaceous particles, and an external zone, populated by silicate, iron, and alumina dust grains. Based on the comparison with results from stellar modelling that describe the dust formation process, we deduce that these stars descend from low-mass (M < 2 M⊙) objects, formed 1–4 Gyr ago, currently evolving either in the post-AGB phase or through an after-pulse phase, when the shell CNO nuclear activity is temporarily extinguished. Possible observations able to confirm or disregard the present hypothesis are discussed.

ISPY – NaCo Imaging Survey for Planets around Young stars

Context. The interaction between low-mass companions and the debris discs they reside in is still not fully understood. A debris disc can evolve due to self-stirring, a process in which planetesimals can excite their neighbours to the point of destructive collisions. In addition, the presence of a companion could further stir the disc (companion-stirring). Additional information is necessary to understand this fundamental step in the formation and evolution of a planetary system, and at the moment of writing only a handful of systems are known where a companion and a debris disc have both been detected and studied at the same time. Aims. Our primary goal is to augment the sample of these systems and to understand the relative importance between self-stirring and companion-stirring. Methods. In the course of the VLT/NaCo Imaging Survey for Planets around Young stars (ISPY), we observed HD 193571, an A0 debris disc hosting star at a distance of 68 pc with an age between ∼60 and 170 Myr. We obtained two sets of observations in L′ band and a third epoch in H band using the GPI instrument at Gemini-South. Results. A companion was detected in all three epochs at a projected separation of ∼11 au (∼0.17″), and co-motion was confirmed through proper motion analysis. Given the inferred disc size of 120 au, the companion appears to reside within the gap between the host star and the disc. Comparison between the L′ and H band magnitude and evolutionary tracks suggests a mass of ∼0.31 − 0.39 M⊙. Conclusions. We discovered a previously unknown M-dwarf companion around HD 193571, making it the third low-mass stellar object discovered within a debris disc. A comparison to self- and companion-stirring models suggests that the companion is likely responsible for the stirring of the disc.

Spectral characterization of newly detected young substellar binaries with SINFONI

We observe 14 young low-mass substellar objects using the VLT/SINFONI integral field spectrograph with laser guide star adaptive optics to detect and characterize three candidate binary systems. All three binary candidates show strong signs of youth, with two of them likely belonging to young moving groups. Together with the adopted young-moving-group ages we employ isochrones from the BT-Settle CIFIST substellar evolutionary models to estimate individual masses for the binary components. We find 2MASS J15104786–2818174 to be part of the ≈30−50 Myr Argus moving group and to be composed of a 34−48 MJup primary brown dwarf with spectral type M9γ and a fainter 15−22 MJup companion, separated by ≈100 mas. 2MASS J22025794–5605087 is identified as an almost equal-mass binary in the AB Dor moving group, with a projected separation of ≈60 mas. Both components share spectral type M9γ/β, which with the adopted age of 120−200 Myr yields masses in the range of 50−68 MJup for each component individually. The observations of 2MASS J15474719–2423493 are of lower quality and we obtain no spectral characterization for the target, but resolve two components separated by ≈170 mas which with the predicted young field age of 30−50 Myr yields individual masses below 20 MJup. Out of the three candidate binary systems, 2MASS J22025794–5605087 has unambiguous spectroscopic signs of being a bona-fide binary, while the other two will require second-epoch confirmation. The small projected separations between the binary components correspond to physical separations of ≈4−7 AU, meaning that astrometric monitoring of just a few years would be adequate to generate constrained orbital fits and dynamical masses for the systems. In combination with their young ages, these binaries will prove to be excellent benchmarks for calibrating substellar evolutionary models down to a very low-mass regime.

A centrally concentrated sub-solar-mass starless core in the Taurus L1495 filamentary complex

Abstract The formation scenario of brown dwarfs is still unclear because observational studies to investigate its initial condition are quite limited. Our systematic survey of nearby low-mass star-forming regions using the Atacama Compact Array (aka the Morita array) and the IRAM 30-m telescope in 1.2 mm continuum has identified a centrally concentrated starless condensation with a central H2 volume density of ∼106 cm−3, MC5-N, connected to a narrow (width ∼0.03 pc) filamentary cloud in the Taurus L1495 region. The mass of the core is $\sim {0.2\!-\!0.4}\, M_{\odot }$, which is an order of magnitude smaller than typical low-mass pre-stellar cores. Taking into account a typical core to star formation efficiency for pre-stellar cores (∼20%–40%) in nearby molecular clouds, brown dwarf(s) or very low-mass star(s) may be going to be formed in this core. We have found possible substructures at the high-density portion of the core, although much higher angular resolution observation is needed to clearly confirm them. The subsequent N2H+ and N2D+ observations using the Nobeyama 45-m telescope have confirmed the high-deuterium fractionation (∼30%). These dynamically and chemically evolved features indicate that this core is on the verge of proto-brown dwarf or very low-mass star formation and is an ideal source to investigate the initial conditions of such low-mass objects via gravitational collapse and/or fragmentation of the filamentary cloud complex.

Dark-ages Reionization and Galaxy Formation Simulation – XV. Stellar evolution and feedback in dwarf galaxies at high redshift

We directly compare predictions of dwarf galaxy properties in a semi-analytic model (SAM) with those extracted from a high-resolution hydrodynamic simulation. We focus on galaxies with halo masses of 1e9<Mvir/Msol<1e11 at high redshift ($z\ge5$). We find that, with the modifications previously proposed in Qin et al. (2018), including to suppress the halo mass and baryon fraction, as well as to modulate gas cooling and star formation efficiencies, the SAM can reproduce the cosmic evolution of galaxy properties predicted by the hydrodynamic simulation. These include the galaxy stellar mass function, total baryonic mass, star-forming gas mass and star formation rate at $z\sim5-11$. However, this agreement is only possible by reducing the star formation threshold relative to that suggested by local observations. Otherwise, too much star-forming gas is trapped in quenched dwarf galaxies. We further find that dwarf galaxies rapidly build up their star-forming reservoirs in the early universe ($z>10$), with the relevant time-scale becoming significantly longer towards lower redshifts. This indicates efficient accretion in cold mode in these low-mass objects at high redshift. Note that the improved SAM, which has been calibrated against hydrodynamic simulations, can provide more accurate predictions of high-redshift dwarf galaxy properties that are essential for reionization study.

A Multimass Velocity Dispersion Model of 47 Tucanae Indicates No Evidence for an Intermediate-mass Black Hole

Abstract In this paper, we analyze stellar proper motions in the core of the globular cluster 47 Tucanae to explore the possibility of an intermediate-mass black hole (IMBH) influence on the stellar dynamics. Our use of short-wavelength photometry affords us an exceedingly clear view of stellar motions into the very center of the crowded core, yielding proper motions for &gt;50,000 stars in the central 2′. We model the velocity dispersion profile of the cluster using an isotropic Jeans model. The density distribution is taken as a central IMBH point mass added to a combination of King templates. We individually model the general low-mass cluster objects (main sequence/giant stars), as well as the concentrated populations of heavy binary systems and dark stellar remnants. Using unbinned likelihood model fitting, we find that the inclusion of the concentrated populations in our model plays a crucial role in fitting for an IMBH mass. The concentrated binaries and stellar-mass black holes (BHs) produce a sufficient velocity dispersion signal in the core so as to make an IMBH unnecessary to fit the observations. We additionally determine that a stellar-mass BH retention fraction of ≳8.5% becomes incompatible with our observed velocities in the core.

New view of the corona of classical T Tauri stars: Effects of flaring activity in circumstellar disks

Context. Classical T Tauri stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. They are characterized by high levels of coronal activity, as revealed by X-ray observations. This activity may affect the disk stability and the circumstellar environment. Aims. Here we investigate if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and might even trigger accretion phenomena with rates comparable with those observed. Methods. We modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodinamic simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases characterized by a dipole plus an octupole stellar magnetic field configuration and different density of the disk or by different levels of flaring activity. Results. As a result of the simulated intense flaring activity, we observe the formation of several loops that link the star to the disk; all these loops build up a hot extended corona with an X-ray luminosity comparable with typical values observed in CTTSs. The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trigger overpressure waves that travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity and thus contribute to the mass accretion rate of the star. Accretion rates synthesized from the simulations are in a range between 10−10 and 10−9 M⊙ yr−1. The accretion columns can be perturbed by the flares, and they can interact with each other; they might merge into larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density structure, and clumped.

Unveiling a cluster of protostellar disks around the massive protostar GGD 27 MM1

Context. Most stars form in clusters and thus it is important to characterize the protostellar disk population in dense environments to assess whether the environment plays a role in the subsequent evolution. Specifically, it is critical to evaluate whether planet formation is altered with respect to more isolated stars formed in dark clouds. Aims. We seek to investigate the properties of the protostellar disks in the GGD 27 cluster and compare these with those obtained from disks formed in nearby regions. Methods. We used ALMA to observe the star-forming region GGD 27 at 1.14 mm with an unprecedented angular resolution, 40 mas (∼56 au), and sensitivity (∼0.002 M⊙). Results. We detected a cluster of 25 continuum sources, most of which likely trace disks around Class 0/I protostars. Excluding the two most massive objects, disks masses are in the range 0.003–0.05 M⊙. The analysis of the cluster properties indicates that GGD 27 displays moderate subclustering. This result, combined with the dynamical timescale of the radio jet (∼104 years), suggests the youthfulness of the cluster. The lack of disk mass segregation signatures may support this as well. We found a clear paucity of disks with Rdisk &gt; 100 au. The median value of the radius is 34 au; this value is smaller than the median of 92 au for Taurus but comparable to the value found in Ophiuchus and in the Orion Nebula Cluster. In GGD 27 there is no evidence of a distance-dependent disk mass distribution (i.e., disk mass depletion due to external photoevaporation), most likely due to the cluster youth. There is a clear deficit of disks for distances &lt;0.02 pc. Stars can only form larger and more massive disksfor distances &gt;0.04 pc. This suggests that dynamical interactions far from the cluster center are weaker, although the small disks found could be the result of disk truncation. This work demonstrates the potential to characterize disks from low-mass young stellar objects in distant and massive (still deeply embedded) clustered environments.

The SOMA Radio Survey. I. Comprehensive SEDs of High-mass Protostars from Infrared to Radio and the Emergence of Ionization Feedback

Abstract We study centimeter continuum emission of eight high- and intermediate-mass protostars that are part of the SOFIA Massive Star Formation Survey, thus building extended spectral energy distributions (SEDs) from the radio to the infrared. We discuss the morphology seen in the centimeter continuum images, which are mostly derived from archival Very Large Array data, and the relation to infrared morphology. We use the SEDs to test new models of high-mass star formation including radiative and disk-wind feedback and associated free–free and dust continuum emission. We show that interferometric data of the centimeter continuum flux densities provide additional, stringent tests of the models by constraining the ionizing luminosity of the source; they also help to break degeneracies encountered when modeling the infrared-only SEDs, especially for the protostellar mass. Our derived parameters are consistent with physical parameters estimated by other methods, such as dynamical protostellar masses. We find a few examples of additional stellar sources in the vicinity of the high-mass protostars, which may be low-mass young stellar objects. However, the stellar multiplicity of the regions, at least as traced by radio continuum emission, appears to be relatively low.

Preliminary Trigonometric Parallaxes of 184 Late-T and Y Dwarfs and an Analysis of the Field Substellar Mass Function into the “Planetary” Mass Regime

We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfs using observations from Spitzer (143), USNO (18), NTT (14), and UKIRT (9). To complete the 20-pc census of $\ge$T6 dwarfs, we combine these measurements with previously published trigonometric parallaxes for an additional 44 objects and spectrophotometric distance estimates for another 7. For these 235 objects, we estimate temperatures, sift into five 150K-wide $T_{\rm eff}$ bins covering the range 300-1050K, determine the completeness limit for each, and compute space densities. To anchor the high-mass end of the brown dwarf mass spectrum, we compile a list of early- to mid-L dwarfs within 20 pc. We run simulations using various functional forms of the mass function passed through two different sets of evolutionary code to compute predicted distributions in $T_{\rm eff}$. The best fit of these predictions to our L, T, and Y observations is a simple power-law model with $\alpha \approx 0.6$ (where $dN/dM \propto M^{-\alpha}$), meaning that the slope of the field substellar mass function is in rough agreement with that found for brown dwarfs in nearby star forming regions and young clusters. Furthermore, we find that published versions of the log-normal form do not predict the steady rise seen in the space densities from 1050K to 350K. We also find that the low-mass cutoff to formation, if one exists, is lower than $\sim$5 $M_{Jup}$, which corroborates findings in young, nearby moving groups and implies that extremely low-mass objects have been forming over the lifetime of the Milky Way.

DECam survey for low-mass stars and substellar objects in the UCL and LCC subgroups of the Sco-Cen OB Association (SCOCENSUS)

Using images taken with the Dark Energy Camera (DECam), the first extensive survey of low mass and substellar objects is made in the 15-20 Myr Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC) subgroups of the Scorpius Centaurus OB Association (Sco-Cen). Due to the size of our dataset (>2Tb) we developed an extensive open source set of python libraries to reduce our images, including astrometry, coaddition, and PSF photometry. Our survey consists of 29$\times$3 deg$^2$ fields in the UCL and LCC subgroups of Sco-Cen and the creation of a catalog with over 11 million point sources. We create a prioritized list of candidate for members in UCL and LCC, with 118 \emph{best} and another 348 \emph{good} candidates. We show that the luminosity and mass functions of our low mass and substellar candidates are consistent with measurements for the younger Upper Scorpius subgroup and estimates of a universal IMF, with spectral types ranging from M1 down to L1.

Accelerating N-Body Simulation of Self-Gravitating Systems with Limited First-Order Post-Newtonian Approximation

In this study, an $N$-body simulation code was developed for self-gravitating systems with a limited first-order post-Newtonian approximation. The code was applied to a special case in which the system consists of one massive object and many low-mass objects. Therefore, the behavior of stars around the massive black hole could be analyzed. A graphics processing unit (GPU) was used to accelerate the code execution, and it could be accelerated by several tens of times compared to a single-core CPU for $N \simeq 10^4$ objects.

Extreme fragmentation and complex kinematics at the center of the L1287 cloud

Aims. The filamentary ~10-pc-scale infrared dark cloud L1287 located at a parallax distance of ~929 pc is actively forming a dense cluster of low-mass young stellar objects (YSOs) at its inner ~0.1 pc region. To help understand the origin of this low-mass YSO cluster, the present work aims at resolving the gas structures and kinematics with high angular resolution. Methods. We performed ~1′′ angular resolution (~930 AU) observations at ~1.3 mm wavelengths using the Submillimeter Array (SMA), which simultaneously cover the dust continuum emission and various molecular line tracers for dense gas, warm gas, shocks, and outflows. Results. From a 1.3-mm continuum image with a resolution of ~2′′ we identified six dense cores, namely SMA1-6. Their gas masses are in the range of ~0.4–4 M⊙. From a 1.3-mm continuum image with a resolution of ~1′′, we find a high fragmentation level, with 14 compact millimeter sources within 0.1 pc: SMA3 contains at least nine internal condensations; SMA5 and SMA6 are also resolved with two internal condensations. Intriguingly, one condensation in SMA3 and another in SMA5 appear associated with the known accretion outburst YSOs RNO 1C and RNO 1B. The dense gas tracer DCN (3–2) well traces the dust continuum emission and shows a clear velocity gradient along the NW-SE direction centered at SMA3. There is another velocity gradient with opposite direction around the most luminous YSO, IRAS 00338 + 6312. Conclusions. The fragmentation within 0.1 pc in L1287 is very high compared to other regions at the same spatial scales. The incoherent motions of dense gas flows are sometimes interpreted by being influenced by (proto)stellar feedback (e.g., outflows), which is not yet ruled out in this particular target source. On the other hand, the velocities (with respect to the systemic velocity) traced by DCN are small, and the directions of the velocity gradients traced by DCN are approximately perpendicular to those of the dominant CO outflow(s). Therefore, we alternatively hypothesize that the velocity gradients revealed by DCN trace the convergence from the ≳0.1 pc scales infalling motion towards the rotational motions around the more compact (~0.02 pc) sources. This global molecular gas converging flow may feed the formation of the dense low-mass YSO cluster. Finally, we also found that IRAS 00338 + 6312 is the most likely powering source of the dominant CO outflow. A compact blue-shifted outflow from RNO 1C is also identified.

Optical characterization and Radial velocity monitoring of Exoplanet and Eclipsing Binary

In recent times, many new exoplanet and eclipsing binary candidates are reported, particularly from the archival data produced by the Kepler space mission destined to detect exoplanets in the Cygnus-Lyra region of the sky. In the framework of Belgo-Indian Network for Astronomy &amp; Astrophysics (BINA) project, we also initiated a long-term programme ”Optical characterization and Radial velocity monitoring with Belgian and Indian Telescopes (ORBIT)” which aims at collecting ground-based photometric and high-resolution spectroscopic observations of exoplanet candidates as well as low-mass eclipsing binary candidates using the Indo-Belgian telescopes. We initially focus our study on a few bright candidates for which high-precision radial velocity and photometry could be possible from the available facilities within the BINA network. Our aim is to determine the physical parameters of these candidate stars in order to derive their true nature. Having a large enough sample of low-mass objects from this survey as well as from those reported in the earlier surveys, we intend to understand the mass-radius relation which is still debated for the low-mass regime of the main-sequence stars.

The outbursting protostar 2MASS 22352345 + 7517076 and its environment

We studied the Class I protostar 2MASS 22352345+7517076 whose dramatic brightening between the IRAS, Akari, and WISE surveys was reported by Onozato et al. (2015). 2MASS 22352345+7517076 is a member of a small group of low-mass young stellar objects, associated with IRAS 22343+7501 in the molecular cloud Lynds 1251. The IRAS, ISO, Spitzer, Akari, Herschel, and WISE missions observed different stages of its outburst. Supplemented these data with archival and our own near-infrared observations, and considering the contributions of neighbouring sources to the mid-infrared fluxes we studied the nature and environment of the outbursting object, and its photometric variations from 1983 to 2017. The low-state bolometric luminosity Lbol ~ 32 Lsun is indicative of a 100000-200000 years old protostar of 1.6-1.8 solar masses. Its 2-micron brightness started rising between 1993 and 1998, reached a peak in 2009-2011, and started declining in 2015. Changes in the spectral energy distribution suggest that the outburst was preceded by a decade-long, slow brightening in the near-infrared. The actual accretion burst occurred between 2004 and 2007. We fitted the spectral energy distribution in the bright phases with simple accretion disc models. The modelling suggested an increase of the disc accretion rate from some 3.5\times 10^{-7} Msun yr^{-1} to 1.1 \times 10^{-4} Msun yr^{-1}. The central star accreted nearly 10^{-3} solar masses, about a Jupiter mass during the ten years of the outburst. We observed H_2 emission lines in the K-band spectrum during the fading phase in 2017. The associated optical nebulosity RNO 144 and the Herbig-Haro object HH 149 have not exhibited significant variation in shape and brightness during the outburst.

Radiation-hydrodynamic Simulations of Spherical Protostellar Collapse for Very Low-mass Objects

Abstract We perform radiation-hydrodynamical simulations of protostellar collapse in spherical symmetry, with a special focus on very low-mass objects, i.e., brown dwarfs and sub-brown dwarfs. The inclusion of a realistic equation of state, which includes the effect of hydrogen dissociation, allows for a modeling of the complete process from the beginning of the collapse until the formation of the protostar. We solve the frequency-dependent radiative transfer equation without any diffusion approximation, using realistic dust and gas opacities. Our results show that the properties of the protostar are essentially independent of the initial conditions, which had previously only been confirmed for higher mass ranges. For very low-mass initial conditions, however, we find that the first core phase of the collapse shows some significant differences in the time evolution, with the first core lifetime increasing dramatically because of the reduced accretion rate from the surrounding envelope. We consider the observational implications of this. We also investigate the opposite case of a collapse without any first core phase, which may occur for very unstable initial conditions. In the Appendix, we describe a severe numerical problem that causes an unphysical expansion after the formation of the protostar, which may affect other attempts at similar calculations of self-gravitational collapse. We explain the origin of the unphysical behavior and present a solution that can be used in similar investigations.

A Large Moving Group within the Lower Centaurus Crux Association

Abstract Scorpius–Centaurus is the nearest OB association, and its hundreds of members are divided into subgroups, including the Lower Centaurus Crux (LCC). Here we study the dynamics of the LCC area. We report the revelation of a large moving group containing more than 1800 intermediate- and low-mass young stellar objects and brown dwarfs that escaped identification until Gaia DR2 allowed a kinematic and photometric selection to be performed. We investigate the stellar and substellar content of this moving group using the Gaia DR2 astrometric and photometric measurements. The median distance of the members is 114.5 pc, and 80% lie between 102 and 135 pc from the Sun. Our new members cover a mass range of 0.02–5 M ⊙ and add up to a total mass of about 700 M ⊙. The present-day mass function follows a log-normal law with m c = 0.22 M ⊙ and σ = 0.64. We find more than 200 brown dwarfs in our sample. The star formation rate had its maximum of about 9 Myr ago. We grouped the new members into four denser subgroups, which have increasing age from 7 to 10 Myr, surrounded by “free-floating” young stars with mixed ages. Our isochronal ages, now based on accurate parallaxes, are compatible with several earlier studies of the region. The whole complex is presently expanding, and the expansion started between 8 and 10 Myr ago. Two hundred members show infrared excess compatible with circumstellar disks from full to debris disks. This discovery provides a large sample of nearby young stellar and substellar objects for disk and exoplanet studies.