Population Of Young Stars Research Articles

The accretion history of the Milky Way

Dwarf spheroidal galaxies are known to be dominated by old stellar populations. This has led to the assumption that their gas-rich progenitors lost their gas during their infall in the Milky Way (MW) halo at distant look-back times. Here, we report a discovery of a tiny but robustly detected population of possibly young (∼1 Gyr old) and intermediate-mass (1.8 M⊙ ≤ M < 3 M⊙) stars in MW dwarf spheroidal galaxies. This was established on the basis of their positions in color–magnitude diagrams, after filtering out the bulk of the foreground MW using Gaia DR3 proper motions. We have considered the possibility that this population is made of evolved blue stragglers. For Sculptor, it seems unlikely, because 95.5% of its stars are older than 8 Gyr, leading to masses smaller than 0.9 M⊙. This would only allow blue straggler masses of less than 1.8 M⊙, which is much lower than what we observed. Alternatively, it would require the merger of three turnoff stars, which appears even more unlikely. On the other hand, the recent Gaia proper motion measurements of MW dwarf galaxies infer their low binding energies and large angular momenta, pointing to a more recent, ≤3 Gyr, infall. Although the nature of the newly discovered stars still needs further confirmation, we find that they are consistent with the recent infall of the dwarf galaxies into the MW halo, when star formation occurred from the ram pressurization of their gas content before its removal by the hot Galactic corona. The abundance of this plausibly young population of stars is similar to the expectations drawn from hydrodynamical simulations. These results point to a novel origin for MW dwarf spheroidal galaxies.

A JWST/MIRI and NIRCam Analysis of the Young Stellar Object Population in the Spitzer I Region of NGC 6822

We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.

An Estimate of the Binary Star Fraction among Young Stars at the Galactic Center: Possible Evidence of a Radial Dependence

We present the first estimate of the intrinsic binary fraction of young stars across the central ≈0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 spectroscopically confirmed young stars, using 119 nights of 10″ wide adaptive optics imaging observations taken at W. M. Keck Observatory over 16 yr in the K′ -[2.1 μm] and H-[1.6 μm] bands. We photometrically detected three binary stars, all of which are situated more than 1″ (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. All are contact binaries or have photometric variability originating from stellar irradiation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined the experimental sensitivity through detailed light-curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population, and underlying binary system parameters (periods, mass ratios, and eccentricities) similar to those of local massive stars. As might be expected, our experimental sensitivity decreases for eclipses narrower in phase. The detections and simulations imply that the young, massive stars in the GC have a stellar binary fraction ≥71% (68% confidence), or ≥42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. based on radial velocity measurements for stars ≲1″ of the SMBH. Constrained with these two studies, the probability that the same underlying young star binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction, and therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH.

Evidence of a Decreased Binary Fraction for Massive Stars within 20 milliparsecs of the Supermassive Black Hole at the Galactic Center

We present the results of the first systematic search for spectroscopic binaries within the central 2 × 3 arcsec2 around the supermassive black hole at the center of the Milky Way galaxy. This survey is based primarily on over a decade of adaptive optics-fed integral-field spectroscopy (R ∼ 4000), obtained as part of the Galactic Center Orbits Initiative at Keck Observatory, and it has a limiting K’-band magnitude of 15.8, which is at least 4 mag deeper than previous spectroscopic searches for binaries at larger radii within the central nuclear star cluster. From this primary data set, over 600 new radial velocities are extracted and reported, increasing by a factor of 3 the number of such measurements. We find no significant periodic signals in our sample of 28 stars, of which 16 are massive, young (main-sequence B) stars and 12 are low-mass, old (M and K giant) stars. Using Monte Carlo simulations, we derive upper limits on the intrinsic binary star fraction for the young star population at 47% (at 95% confidence) located ∼20 mpc from the black hole. The young star binary fraction is significantly lower than that observed in the field (70%). This result is consistent with a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted, and it may have important implications for the production of gravitational waves and hypervelocity stars.

The most probable host of CHIME FRB 190425A, associated with binary neutron star merger GW190425, and a late-time transient search

ABSTRACT The identification and localization of fast radio bursts (FRBs) to their host galaxies have revealed important details about the progenitors of these mysterious, millisecond-long bursts of coherent radio emission. In this work, we study the most probable host galaxy of the apparently non-repeating CHIME/FRB event FRB 20190425A – a particularly high-luminosity, low-dispersion measure event that was demonstrated in a recent paper to be temporally and spatially coincident with the LIGO-Virgo-KAGRA binary neutron star merger GW190425, suggesting an astrophysical association (p-value 0.0052). In this paper, we remain agnostic to this result, and we confirm UGC10667 as the most probable host galaxy of FRB 20190425A, demonstrating that the host galaxies of low-dispersion measure, one-off CHIME FRBs can be plausibly identified. We then perform multiwavelength observations to characterize the galaxy and search for any afterglow emission associated with the FRB and its putative GW counterpart. We find no radio or optical transient emission in our observations $2.5\, \mathrm{yr}$ post-burst. UGC10667 is a spiral galaxy at z ∼ 0.03, dominated by an old stellar population. We find no evidence of a large population of young stars, with nebular emission dominated by star formation at a rate of $1\!-\!2\, ~\mathrm{M_\odot \, yr^{-1}}$. While we cannot rule out a young magnetar as the origin of FRB 20190425A, our observations are consistent with an origin in a long delay-time neutron star binary merger.

Follow-up of Young Stars Identified with BANYAN Σ: New Low-mass Members of Nearby Moving Groups

The characterization of moving groups offers a powerful means to identify large populations of young stars. In this paper, we present a sample of follow-up observations for 56 systems that have previously been proposed as members of young stellar associations through the application of the BANYAN Σ kinematic classification tool. Our measurements, which probe seven different associations, provide a sample of 39 stellar systems that either are confirmed or appear consistent with being young members of their respective associations. Nineteen of these are single M dwarfs. This sample expands our knowledge of Upper Centaurus Lupus, Coma Berenices, and AB Doradus Major to cooler temperatures and also significantly increases the known population of the Carina-Near association. The young systems present excellent targets for future planet searches and would also be valuable for studies of star formation and evolution. Additionally, we find two stellar systems that show indications of being rare instances of late-stage circumstellar accretion. Lastly, our follow-up measurements serve as a test of BANYAN Σ, finding an overall contamination rate that is consistent with previous findings (29% for systems with RV measurements, 37% without).

Tucana B: A Potentially Isolated and Quenched Ultra-faint Dwarf Galaxy at D ≈ 1.4 Mpc* *This paper includes data gathered with the 6.5 m Magellan Telescope at Las Campanas Observatory, Chile.

We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy at a distance of D = 1.4 Mpc. Tucana B was found during a search for ultra-faint satellite companions to the known dwarfs in the outskirts of the Local Group, although its sky position and distance indicate the nearest galaxy to be ∼500 kpc distant. Deep ground-based imaging resolves Tucana B into stars, and it displays a sparse red giant branch consistent with an old, metal-poor stellar population analogous to that seen in the ultra-faint dwarf galaxies of the Milky Way, albeit at fainter apparent magnitudes. Tucana B has a half-light radius of 80 ± 40 pc and an absolute magnitude of mag ( L ⊙), which is again comparable to the Milky Way’s ultra-faint satellites. There is no evidence for a population of young stars, either in the optical color–magnitude diagram or in GALEX archival ultraviolet imaging, with the GALEX data indicating for star formation on ≲100 Myr timescales. Given its isolation and physical properties, Tucana B may be a definitive example of an ultra-faint dwarf that has been quenched by reionization, providing strong confirmation of a key driver of galaxy formation and evolution at the lowest mass scales. It also signals a new era of ultra-faint dwarf galaxy discovery at the extreme edges of the Local Group.

Lupus DANCe

Context. Lupus is recognised as one of the closest star-forming regions, but the lack of trigonometric parallaxes in the pre-Gaia era hampered many studies on the kinematic properties of this region and led to incomplete censuses of its stellar population. Aims. We use the second data release of the Gaia space mission combined with published ancillary radial velocity data to revise the census of stars and investigate the 6D structure of the Lupus complex. Methods. We performed a new membership analysis of the Lupus association based on astrometric and photometric data over a field of 160 deg2 around the main molecular clouds of the complex and compared the properties of the various subgroups in this region. Results. We identified 137 high-probability members of the Lupus association of young stars, including 47 stars that had never been reported as members before. Many of the historically known stars associated with the Lupus region identified in previous studies are more likely to be field stars or members of the adjacent Scorpius-Centaurus association. Our new sample of members covers the magnitude and mass range from G ≃ 8 to G ≃ 18 mag and from 0.03 to 2.4 M⊙, respectively. We compared the kinematic properties of the stars projected towards the molecular clouds Lupus 1–6 and showed that these subgroups are located at roughly the same distance (about 160 pc) and move with the same spatial velocity. Our age estimates inferred from stellar models show that the Lupus subgroups are coeval (with median ages ranging from about 1 to 3 Myr). The Lupus association appears to be younger than the population of young stars in the Corona-Australis star-forming region recently investigated by our team using a similar methodology. The initial mass function of the Lupus association inferred from the distribution of spectral types shows little variation compared to other star-forming regions. Conclusions. In this paper, we provide an updated sample of cluster members based on Gaia data and construct the most complete picture of the 3D structure and 3D space motion of the Lupus complex.

A wide survey for circumstellar disks in the Lupus complex

Previous star formation studies have, out of necessity, often defined a population of young stars confined to the proximity of a molecular cloud. Gaia allows us to examine a wider, three-dimensional structure of nearby star forming regions, leading to a new understanding of their history. We present a wide-area survey covering 494 deg2 of the Lupus complex, a prototypical low-mass star forming region. Our survey includes all known molecular clouds in this region as well as parts of the Upper Scorpius and Upper Centaurus Lupus (UCL) groups of the Sco-Cen complex. We combine Gaia DR2 proper motions and parallaxes as well as ALLWISE mid-infrared photometry to select young stellar objects (YSOs) with disks. The YSO ages are inferred from Gaia color-magnitude diagrams, and their evolutionary stages from the slope of the spectral energy distributions. We find 98 new disk-bearing sources. Our new sample includes objects with ages ranging from 1 to 15 Myr and masses ranging from 0.05 to 0.5 M⊙, and consists of 56 sources with thick disks and 42 sources with anemic disks. While the youngest members are concentrated in the clouds and at distances of 160 pc, there is a distributed population of slightly older stars that overlap in proper motion, spatial distribution, distance, and age with the Lupus and UCL groups. The spatial and kinematic properties of the new disk-bearing YSOs indicate that Lupus and UCL are not distinct groups. Our new sample comprises some of the nearest disks to Earth at these ages, and thus provides an important target for follow-up studies of disks and accretion in very low mass stars, for example with ALMA and ESO-VLT X-shooter.

Wide binaries are rare in open clusters

ABSTRACT The population statistics of binary stars are an important output of star formation models. However, populations of wide binaries evolve over time due to interactions within a system’s birth environment and the unfolding of wide, hierarchical triple systems. Hence, the wide binary populations observed in star-forming regions or OB associations may not accurately reflect the wide binary populations that will eventually reach the field. We use Gaia DR2 data to select members of three open clusters, Alpha Per, the Pleiades, and Praesepe and to flag cluster members that are likely unresolved binaries due to overluminosity or elevated astrometric noise. We then identify the resolved wide binary population in each cluster, separating it from coincident pairings of unrelated cluster members. We find that these clusters have an average wide binary fraction in the 300–3000 au projected separation range of 2.1$\pm ^{0.4}_{0.2}$ per cent increasing to 3.0$\pm ^{0.8}_{0.7}$ per cent for primaries with masses in the 0.5–1.5 M⊙ range. This is significantly below the observed field wide binary fraction, but shows some wide binaries survive in these dynamically highly processed environments. We compare our results with another open cluster (the Hyades) and two populations of young stars that likely originated in looser associations (young moving groups and the Pisces-Eridanus stream). We find that the Hyades also has a deficit of wide binaries while the products of looser associations have wide binary fractions at or above field level.

Discovery of new members of the nearby young stellar association in Cepheus

Context. Young field stars are hardly distinguishable from older ones because their space motion rapidly mixes them with the stellar population of the Galactic plane. Nevertheless, a careful target selection allows for young stars to be spotted throughout the sky. Aims. We aim to identify additional sources associated with the four young comoving stars that we discovered towards the CO Cepheus void and to provide a comprehensive view of the Cepheus association. Methods. Based on multivariate analysis methods, we have built an extended sample of 193 young star candidates, which are the optical and infrared counterparts of ROSAT All-Sky Survey and XMM-Newton X-ray sources. From optical spectroscopic observations, we measured their radial velocity with the cross-correlation technique. We derived their atmospheric parameters and projected rotational velocity with the code ROTFIT. We applied the subtraction of inactive templates to measure the lithium equivalent width, from which we infer their lithium abundance and age. Finally, we studied their kinematics using the second Gaia data release. Results. Our sample is mainly composed of young or active stars and multiple systems. We identify two distinct populations of young stars that are spatially and kinematically separated. Those with an age between 100 and 300 Myr are mostly projected towards the Galactic plane. In contrast, 23 of the 37 sources younger than 30 Myr are located in the CO Cepheus void, and 21 of them belong to the stellar kinematic group that we previously reported in this sky area. We report a total of 32 bona fide members and nine candidates for this nearby (distance = 157 ± 10 pc) young (age = 10–20 Myr) stellar association. According to the spatial distribution of its members, the original cluster is already dispersed and partially mixed with the local population of the Galactic plane.

A kinematically hot population of young stars in the solar neighbourhood

ABSTRACT In the last three decades several hundred nearby members of young stellar moving groups (MGs) have been identified, but there has been less systematic effort to quantify or characterize young stars that do not belong to previously identified MGs. Using a kinematically unbiased sample of 225 lithium-rich stars within 100 pc, we find that only 50 ± 10 per cent of young (≲125 Myr), low-mass (0.5 &amp;lt; M/M⊙ &amp;lt; 1.0) stars, are kinematically associated with known MGs. Whilst we find some evidence that five of the non-MG stars may be connected with the Lower Centaurus–Crux association, the rest form a kinematically ‘hotter’ population, much more broadly dispersed in velocity, and with no obvious concentrations in space. The mass distributions of the MG members and non-MG stars are similar, but the non-MG stars may be older on average. We briefly discuss several explanations for the origin of the non-MG population.

Corona-Australis DANCe

Context. Corona-Australis is one of the nearest regions to the Sun with recent and ongoing star formation, but the current picture of its stellar (and substellar) content is not complete yet. Aims. We take advantage of the second data release of the Gaia space mission to revisit the stellar census and search for additional members of the young stellar association in Corona-Australis. Methods. We applied a probabilistic method to infer membership probabilities based on a multidimensional astrometric and photometric data set over a field of 128 deg2 around the dark clouds of the region. Results. We identify 313 high-probability candidate members to the Corona-Australis association, 262 of which had never been reported as members before. Our sample of members covers the magnitude range between G ≳ 5 mag and G ≲ 20 mag, and it reveals the existence of two kinematically and spatially distinct subgroups. There is a distributed “off-cloud” population of stars located in the north of the dark clouds that is twice as numerous as the historically known “on-cloud” population that is concentrated around the densest cores. By comparing the location of the stars in the HR-diagram with evolutionary models, we show that these two populations are younger than 10 Myr. Based on their infrared excess emission, we identify 28 Class II and 215 Class III stars among the sources with available infrared photometry, and we conclude that the frequency of Class II stars (i.e. “disc-bearing” stars) in the on-cloud region is twice as large as compared to the off-cloud population. The distance derived for the Corona-Australis region based on this updated census is d = 149.4 +0.4−0.4 pc, which exceeds previous estimates by about 20 pc. Conclusions. In this paper we provide the most complete census of stars in Corona-Australis available to date that can be confirmed with Gaia data. Furthermore, we report on the discovery of an extended and more evolved population of young stars beyond the region of the dark clouds, which was extensively surveyed in the past.

X-shaped Radio Galaxies: Optical Properties, Large-scale Environment, and Relationship to Radio Structure

Abstract In pursuit of clues to the origin of “winged” or X-shaped radio galaxies (XRGs) we investigate the parent galaxies of a large sample of 106 XRGs for optical–radio axes alignment, interstellar medium, black hole mass, and large-scale environment. For 41 of the XRGs it was possible to determine the optical major axis and the primary radio axis and the strong tendency for the two axes to be fairly close is confirmed. However, several counter-examples were also found and these could challenge the widely discussed backflow diversion model for the origin of the radio wings. Comparison with a well-defined large sample of normal FR II radio galaxies has revealed that: (i) XRGs possess slightly less massive central black holes than the normal radio galaxies (average masses being logM BH ∼ 8.81 M ⊙ and 9.07 M ⊙, respectively); (ii) a much higher fraction of XRGs (∼80%) exhibits red mid-IR colors (W2 − W3 &gt; 1.5), indicating a population of young stars and/or an enhanced dust mass, probably due to relatively recent galaxy merger(s). A comparison of the large-scale environment (i.e., within ∼1 Mpc) shows that both XRGs and FRII radio galaxies inhabit similarly poor galaxy clustering environments (medium richness being 8.94 and 11.87, respectively). Overall, the origin of XRGs seems difficult to reconcile with a single dominant physical mechanism and competing mechanisms seem prevalent.

A kinematically unbiased, all-sky search for nearby, young, low-mass stars

ABSTRACT The past two decades have seen dramatic progress in our knowledge of the population of young stars of age $\lt \!200\,$ Myr that lie within $150\,$ pc of the Sun. These nearby, young stars, most of which are found in loose, comoving groups, provide the opportunity to explore (among many other things) the dissolution of stellar clusters and their diffusion into the field star population. Here, we exploit the combination of astrometric and photometric data from Gaia and photometric data from GALEX (UV) and 2MASS (near-IR) in an attempt to identify additional nearby, young, late-type stars. Specifically, we present a sample of 146 GALEX UV-selected late-type (predominantly K-type) field stars with Gaia-based distances $\lt \!125\,$ pc (based on Gaia Data Release 1) that have isochronal ages $\lt \!80\,$ Myr even if equal-components binaries. We investigate the spectroscopic and kinematic properties of this sample. Despite their young isochronal ages, only ∼10 per cent of stars among this sample can be confidently associated with established nearby, young moving groups (MGs). These candidate MG members include five stars newly identified in this study. The vast majority of our sample of 146 nearby young star candidates have anomalous kinematics relative to the known MGs. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population. We discuss the implications and caveats of such a hypothesis – including the intriguing fact that, in addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within $50\,$ pc appear surprisingly X-ray faint.

The multiphase gas structure and kinematics in the circumnuclear region of NGC 5728

ABSTRACT We report on our combined analysis of HST, VLT/MUSE, VLT/SINFONI, and ALMA observations of the local Seyfert 2 galaxy, NGC 5728 to investigate in detail the feeding and feedback of the active galactic nucleus (AGN). The data sets simultaneously probe the morphology, excitation, and kinematics of the stars, ionized gas, and molecular gas over a large range of spatial scales (10 pc to 10 kpc). NGC 5728 contains a large stellar bar that is driving gas along prominent dust lanes to the inner 1 kpc where the gas settles into a circumnuclear ring. The ring is strongly star forming and contains a substantial population of young stars as indicated by the lowered stellar velocity dispersion and gas excitation consistent with H ii regions. We model the kinematics of the ring using the velocity field of the CO (2–1) emission and stars and find it is consistent with a rotating disc. The outer regions of the disc, where the dust lanes meet the ring, show signatures of inflow at a rate of 1 M$\odot$ yr−1. Inside the ring, we observe three molecular gas components corresponding to the circular rotation of the outer ring, a warped disc, and the nuclear stellar bar. The AGN is driving an ionized gas outflow that reaches a radius of 250 pc with a mass outflow rate of 0.08 M$\odot$ yr−1 consistent with its luminosity and scaling relations from previous studies. While we observe distinct holes in CO emission which could be signs of molecular gas removal, we find that largely the AGN is not disrupting the structure of the circumnuclear region.

An Initial Overview of the Extent and Structure of Recent Star Formation within the Serpens Molecular Cloud Using Gaia Data Release 2

Abstract The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure ∼1167 kinematic members of Serpens, few of which have been previously identified, to evaluate the star formation history of the complex. The optical members of Serpens are concentrated in three distinct groups located at 380–480 pc; the densest clusters are still highly obscured by optically thick dust and have few optical members. The total population of young stars and protostars in Serpens is at least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may be much higher. Distances to dark clouds measured from deficits in star counts are consistent with the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila Rift, dark clouds located at 600–700 pc, and behind patchy extinction, here called the Serpens Cirrus, located at ∼250 pc. Based on the lack of a distributed population of older stars, the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of 20 within the past ∼5 Myr. The optically bright stars in Serpens Northeast are visible because their natal molecular cloud has been eroded, not because they were flung outwards from a central factory of star formation. The separation between subclusters of 20–100 pc and the absence of an older population together lead to speculation that an external forcing was needed to trigger the active star formation.

The young star population of Lynds 1188

We present new results on the young star population of the Lynds~1188 molecular cloud, associated with the Cepheus Bubble, a giant interstellar shell around the association Cep~OB\,2. In order to reveal the star-forming scenario of the molecular cloud located on the supershell, and understand the history of star formation in the region, we identified and characterized young star candidates based on an H$\alpha$ emission survey and various published photometric datasets. Using Gaia DR2 astrometry we studied the spatial distribution of the young star candidates and isolated three groups based on their distances. We constructed spectral energy distributions of our target stars, based on Pan-STARRS, 2MASS, {\it Spitzer} and {\it WISE} photometric data, estimating their spectral types, extinctions, and luminosities. We estimated masses by means of pre-main-sequence evolutionary models, and derived accretion rates from the equivalent width of the H$\alpha$ line. We studied the structure of the cloud by constructing a new extinction map, based on Pan-STARRS data. Our results show that the distribution of low-mass young stars in L1188 is well correlated with that of the dust and mole\-cular gas. We identified two small, compact clusters and a loose aggregate of young stars. We found that star formation in L1188 started about 5 million years ago. The apparent age gradient of young stars across the cloud and the ammonia cores located to the east of the optically visible young stellar groups support the scenario of star formation propagating away from the centre of the Cepheus Bubble.

Evidence of Hubble Flow-like Motion of Young Stellar Populations away from the Perseus Arm

Abstract In this Letter we present evidence of the coherent outward motion of a sample of young stars (t &lt; 30 Myr) in the Perseus Arm, whose apparent origin is located in the vicinity of the W3/W4/W5 complex. Using astrometric and photometric data from the Gaia Data Release 2 catalog of an 8◦ radius field centered near W4, we selected a sample of young intermediate- to high-mass star candidates. The sample is limited to sources with parallax uncertainties below 20% and Bayesian distance estimates within 1800 and 3100 pc. The selection includes embedded stellar populations as well as young open clusters. Projected velocities derived from perspective-corrected proper motions clearly suggest that the young star population emerged from the Perseus Arm, with a possible convergence zone near W3/W4/W5 region, tracing a front that expands away at a rate of about .

Sgr A* envelope explosion and the young stars in the centre of the Milky Way

ABSTRACT Sgr A* is the supermassive black hole residing in the centre of the Milky Way. There is plenty of observational evidence that a massive gas cloud fell into the central parsec of the Milky Way ∼6 Myr ago, triggering formation of a disc of young stars and activating Sgr A*. In addition to the disc, there is an unexplained population of young stars on randomly oriented orbits. Here we hypothesize that these young stars were formed by fragmentation of a massive quasi-spherical gas shell driven out from Sgr A* potential well by an energetic outflow. To account for the properties of the observed stars, the shell must be more massive than 105 solar masses, be launched from inside ∼0.01 pc, and the feedback outflow has to be highly super-Eddington albeit for a brief period of time, producing kinetic energy of at least 1055 erg. The young stars in the central parsec of the Galaxy may be a unique example of stars formed from atomic rather than molecular hydrogen, and forged by extreme pressure of black hole outflows.