Fernando Comeron

The ‘Diamond Ring’ within Cygnus X, south-west of the DR21 ridge, stands out as a prominent, nearly circular structure in infrared (IR) and far-infrared (FIR) emission, spanning approximately 6 pc in diameter. It is enclosed by clumpy molecular clouds seen in CO lines and contains an H II region, visible in cm emission. It resembles a classical H II region associated with an expanding bubble seen routinely in the 158 μm line of ionised carbon ([C II ]). However, our recent observations utilising the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) under the FEEDBACK program for the spectrally resolved [C II ] line have revealed a slightly tilted ring with a mass of ~10 ³ M _⊙ , advancing at a velocity of ~1.3 km s ⁻¹ . The bulk emission of the gas has a line-of-sight (LOS) velocity around −2 km s ⁻¹ . The [C II ] data revealed that the ‘Diamond’ of the Diamond Ring is an unrelated, dense gas clump at a LOS velocity of ~7 km s ⁻¹ . The driving source, which is also responsible for powering the associated H II region, is a B0.5e star, classified by our IR spectroscopy. This observation marks the first instance where we observe only a slowly expanding ring of [C II ] emission and not an expanding 3D shell. We suggest that the H II region (along with its associated [C II ] bubble), initially formed by a massive star, expanded outwards from a flat slab of molecular gas nearly in the plane of the sky. Presently, the [C II ] ring is confined by the swept-up gas of the slab, while the parts of the shell moving in the directions perpendicular to the shell along the LOS have dissipated, resulting in a notable decrease in the expansion of the remaining ring. This scenario is supported by dedicated simulations that trace the evolution of the [C II ] bubble. Our observations support the scenario of H II region evolution in ‘flat’ molecular clouds, reported earlier in the literature. In this geometry, we propose that the Diamond Ring represents the terminal phase of an expanding [C II ] bubble driven by stellar winds and thermal pressure.

Context. Luminous blue variables (LBVs) are an elusive stage of the post-main sequence evolution of massive stars. Wray 15−0751 is one of the few LBVs known to belong to a cluster, FSR 1570, to which we can constrain the distance and kinematics thanks to Gaia, providing additional valuable information of the star. Aims. We jointly consider the Gaia astrometric information on Wray 15−0751 and its cluster, plus its spectral classification and that of the brightest cluster members, to assess the evolutionary path leading to the current stage of the star and infer some properties of the likely binary system that originated it. Methods. In addition to the astrometry from Gaia DR3 we present classification spectroscopy in the visible of Wray 15−0751 and the 18 brightest cluster members of FSR 1570, plus new photometric and spectroscopic observations of Wray 15−0751, which we analyzed jointly with published evolutionary tracks. Results. Astrometry confirms the membership of Wray 15−0751 in its host cluster, without displaying any evidence of a kick velocity. The current cluster turnoff at late O types implies an age of 10.2 ^+6.3 _−3.2 Myr. Together with the results of existing far-infrared observations pointing to it having recently undergone the red supergiant phase, we conclude that Wray 15−0751 is most likely a blue straggler now in the blue loop, resulting from a complete merger of two members of a close binary system with similar initial masses, not much in excess of ∼20 solar masses each.

Context. The new census of massive stars in the Carina Nebula reveals the presence of 54 apparently single O-type stars in the Car OB1 association, an extremely active star-forming region that hosts some of the most luminous stars of the Milky Way. A detailed spectroscopic analysis of the current most complete sample of O-type stars in the association can be used to inspect the main physical properties of cluster members and test evolutionary and stellar atmosphere models. Aims. Our aim is to carry out a spectroscopic characterization of the census of the apparently single (actual single or SB1) O-type stars in Car OB1, obtaining a reliable distribution of rotational velocities and stellar parameters from high-resolution spectra from the Gaia-ESO survey (GES) and the LiLiMaRlin library database, which is itself fed by spectroscopic surveys such as the high-resolution spectroscopic monitoring survey of Southern Galactic O- and WN-type stars (OWN), the high-resolution, multi-epoch project of Galactic OB stars (IACOB), the northern massive dim stars survey (NoMaDS), and the Calar Alto fiber-fed echelle – binary evolution Andalusian northern survey (CAFÉ-BEANS). Methods. To derive rotational velocities, we used the semi-automatized tool for the line-broadening characterization of OB stars (iacob-broad) which is based on a combined Fourier transform and the goodness-of-fit methodology. To derive the stellar parameters, we used the iacob-gbat tool, FASTWIND stellar models, and astrometry provided by the Gaia third data release. The BONNSAI tool was used to compute evolutionary masses and ages. Results. We performed quantitative spectroscopic analysis for the most complete sample of apparently single O-type stars in Car OB1 with available spectroscopic data. From the high-resolution GES and OWN spectra, we obtain a reliable distribution of rotational velocities for a sample of 37 O-type stars. It shows a bimodal structure with a low velocity peak at 60 km s⁻¹ and a short tail of fast rotators reaching 320 km s⁻¹. We also performed a quantitative spectroscopic analysis and derived effective temperature, surface gravity, and He abundance for a sample of 47 O-type stars, now including further stars from the GOSSS database. Radii, luminosities, and spectroscopic masses were also determined using Gaia astrometry. We created the Hertzsprung-Russell diagram to inspect the evolutionary status of the region and confirm the lack of stars close to the zero-age main sequence (ZAMS) from ∼35–55 M_⊙. We confirm a very young population with an age distribution peaking at 1 Myr, with some stars close to or even on the ZAMS, and a secondary peak at 4–5 Myr in the age distribution. We confirm the youth of Trumpler 14, which is also the only cluster not showing the secondary peak. We also find a clear trend of evolutionary masses higher than derived spectroscopic masses for stars with an evolutionary mass below 40 M_⊙ .

Context. Characterising stellar jet asymmetries is key to setting robust constraints on jet launching models and improving our understanding of the underlying mechanisms behind jet launching. Aims. We aim to characterise the asymmetric properties of the bipolar jet coming from the Classical T Tauri Star Th 28. Methods. We combined data from integral field spectroscopy with VLT/MUSE and high-resolution spectra from VLT/X-shooter to map the optical emission line ratios in both jet lobes. We carried out a diagnostic analysis of these ratios to compare the density, electron temperature, and ionisation fraction within both lobes. The mass accretion rate was derived from the emission lines at the source and compared with the mass outflow rate derived for both lobes, using the estimated densities and measured [O I]λ6300 and [S II]λ6731 luminosities. Results. The blue-shifted jet exhibits a significantly higher electron temperature and moderately higher ionisation fraction than the red-shifted jet. In contrast to previous studies, we also estimated higher densities, denoted as n_H , in the blue-shifted jet by a factor of ~2. These asymmetries are traced to within 1″ (160 au) of the source in the line ratio maps. We find Ṁ_acc = 2.4 × 10⁻⁷ M_⊙ yr⁻¹, with an estimated obscuration factor of ~54 due to grey scattering around the star. Estimated values of Ṁ_out range between 0.66 and 13.7 × 10⁻⁹ M_⊙ yr⁻¹ in the blue-shifted jet and 5–9 × 10⁻⁹ M_⊙ yr⁻¹ in the red-shifted jet. Conclusions. The emission line maps and diagnostic results suggest that the jet asymmetries originate close to the source and are likely to be intrinsic to the jet. Furthermore, the combined dataset offers access to a broad array of accretion tracers. In turn, this enables a more accurate estimation of the mass accretion rate, revealing a value of Ṁ_acc that is higher by a factor >350 than would otherwise be determined.

The HH 24 complex harbors five collimated jets emanating from a small protostellar multiple system. We have carried out a multiwavelength study of the jets, their driving sources, and the cloud core hosting the embedded stellar system, based on data from the Hubble Space Telescope, Gemini, Subaru, Apache Point Observatory 3.5 m, Karl G. Jansky Very Large Array, and Atacama Large Millimeter/submillimeter Array (ALMA) telescopes. The data show that the multiple system, SSV 63, contains at least 7 sources, ranging in mass from the hydrogen-burning limit to proto-Herbig Ae stars. The stars are in an unstable nonhierarchical configuration, and one member, a borderline brown dwarf, is moving away from the protostellar system with 25 km s⁻¹, after being ejected ∼5800 yr ago as an orphaned protostar. Five of the embedded sources are surrounded by small, possibly truncated, disks resolved at 1.3 mm with ALMA. Proper motions and radial velocities imply jet speeds of 200–300 km s⁻¹. The two main HH 24 jets, E and C, form a bipolar jet system that traces the innermost portions of parsec-scale chains of Herbig–Haro and H₂ shocks with a total extent of at least 3 pc. H₂CO and C¹⁸O observations show that the core has been churned and continuously fed by an infalling streamer. ¹³CO and ¹²CO trace compact, low-velocity, cavity walls carved by the jets and an ultracompact molecular outflow from the most embedded object. Chaotic N-body dynamics likely will eject several more of these objects. The ejection of stars from their feeding zones sets their masses. Dynamical decay of nonhierarchical systems can thus be a major contributor to establishing the initial mass function.

The HH 24 complex harbors five collimated jets emanating from a small protostellar multiple system. We have carried out a multi-wavelength study of the jets, their driving sources, and the cloud core hosting the embedded stellar system, based on data from the HST, Gemini, Subaru, APO 3.5m, VLA, and ALMA telescopes. The data show that the multiple system, SSV 63, contains at least 7 sources, ranging in mass from the hydrogen-burning limit to proto-Herbig Ae stars. The stars are in an unstable non-hierarchical configuration, and one member, a borderline brown dwarf, is moving away from the protostellar system with 25 km/s, after being ejected about 5,800 yr ago as an orphaned protostar. Five of the embedded sources are surrounded by small, possibly truncated, disks resolved at 1.3 mm with ALMA. Proper motions and radial velocities imply jet speeds of 200-300 km/s. The two main HH 24 jets, E and C, form a bipolar jet system which traces the innermost portions of parsec-scale chains of Herbig-Haro and H2 shocks with a total extent of at least 3 parsec. H2CO and C18O observations show that the core has been churned and continuously fed by an infalling streamer. 13CO and 12CO trace compact, low-velocity, cavity walls carved by the jets and an ultra-compact molecular outflow from the most embedded object. Chaotic N-body dynamics likely will eject several more of these objects. The ejection of stars from their feeding zones sets their masses. Dynamical decay of non-hierarchical systems can thus be a major contributor to establishing the initial mass function.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. The ultraviolet irradiation and the action of stellar winds of newly formed massive stars on their parental molecular environment often produces isolated or small clouds, some of which become sites of star formation themselves.Aims. We investigate the young stellar populations associated with DR 18 and ECX 6-21, which are two isolated globules irradiated by the O-type stars of the Cygnus OB2 association. Both are HII regions containing obvious tracers of recent and ongoing star formation. We also study smaller isolated molecular structures in their surroundings.Methods. We combined near-infrared broad- and narrow-band imaging with broad-band imaging in the visible and with archive images obtained with the Spitzer Space Telescope. We used the joint photometry to select young stellar objects (YSOs), simultaneously estimating their intrinsic properties and classifying them according to the characteristics of their infrared excess. We also present low-resolution visible spectroscopy of selected sources.Results. We reproduce previous findings of an extended population of YSOs around both globules, dominated by the more evolved classes, associated with the general Cygnus OB2 population. Both globules contain their own embedded populations, with a higher fraction of the less-evolved classes. Masses and temperatures are estimated under the assumption of a common age of 1 Myr, which has been found to appropriately represent the general Cygnus OB2 YSO population but is most probably an overestimate for both globules, especially ECX 6-21. The early-B star responsible for the erosion of DR 18 is found to be part of a small aggregate of intermediate-mass stars still embedded in the cloud, which probably contains a second site of recent star formation, also with intermediate-mass stars. We confirm the two main star forming sites embedded in ECX 6-21 described in previous works, with the southern site being more evolved than the northern site. We also discuss the small globule ECX 6-21-W (=G79.8 + 1.2), and propose that its non thermal radio spectrum is due to synchrotron emission from an embedded jet, whose existence is suggested by our observations.Conclusions. The extreme youth of some of the YSOs suggests that star formation in both globules started after they became externally irradiated. The populations of both globules are not found to be particularly rich, but they contain stars with estimated masses similar or above that of the Sun in numbers that hint at some differences with respect to the star formation process taking place in more quiescent regions where low-mass stars dominate, which deeper observations may confirm.

Context. Th 28 is a Classical T Tauri star in the Lupus 3 cloud that drives an extended bipolar jet. Previous studies of the inner jet identified signatures of rotation around the outflow axis, a key result for theories of jet launching. Thus this is an important source in which to investigate the poorly understood jet launching mechanism. Aims. In this study we investigate the morphology and kinematics of the Th 28 micro-jets, with the aim of characterising their structure and outflow activity, using optical integral-field spectroscopy observations obtained with VLT/MUSE. Methods. We use spectro-imaging and position–velocity maps to investigate the kinematic and morphological features of the jet and to obtain a catalogue of emission lines in which the jet is visible. A Lucy-Richardson deconvolution procedure is used to differentiate the structure of the inner micro-jet region in selected emission lines. Spatial profiles extracted perpendicular to the jet axis are fitted to investigate the jet width, opening angle, and the evolution of the jet axis. Results. We confirm the previously identified knot HHW₂ within the red-shifted jet and identify three additional knots in each lobe for the first time. We also find [O III]λ5007 emission from the blue-shifted micro-jet, including the knot closest to the star. Proper motions for the innermost knots on each side are estimated to be 0′′.35 yr⁻¹ and 0′′.47 yr⁻¹ for the red- and blue-shifted jets, respectively. Based on this we show that new knots are ejected on an approximate timescale of 10–15 yr. Gaussian fitting to the jet axis centroids shows a point-symmetric wiggle within the inner portion of both micro-jets, indicating precession of the jet. We use the jet shape to measure a precession period of 8 yr, with a half-opening angle β < 0.6°. This precession may provide an alternative explanation for the rotation signatures previously reported. Conclusions. We find that these parameters are compatible with precession due to a brown dwarf companion orbiting at a separation of ≤0.3 au. Further observations with higher spatial resolution may help to clarify the source of this precession.

Context. The dominant mechanism leading to the formation of brown dwarfs (BDs) remains uncertain. While the census of Class II analogs in the substellar domain continues to grow, the most direct keys to formation, which are obtained from younger objects (pre-BD cores and proto-BDs), are limited by the very low number statistics available. Aims. We aim to identify and characterize a set of pre- and proto-BDs as well as Class II BDs in the Lupus 1 and 3 molecular clouds to test their formation mechanism. Methods. We performed ALMA band 6 (1.3 mm) continuum observations of a selection of 64 cores previously identified from AzTEC/ASTE data (1.1 mm), along with previously known Class II BDs in the Lupus 1 and 3 molecular clouds. Surveyed archival data in the optical and infrared were used to complement these observations. We expect these ALMA observations prove efficient in detecting the youngest sources in these regions, since they probe the frequency domain at which these sources emit most of their radiation. Results. We detected 19 sources from 15 ALMA fields. Considering all the pointings in our observing setup, the ALMA detection rate was ∼23% and the derived masses of the detected sources were between ∼0.18 and 124 M_Jup. We classified these sources according to their spectral energy distribution as 5 Class II sources, 2 new Class I/0 candidates, and 12 new possible pre-BD or deeply embedded protostellar candidates. We detected a promising candidate for a Class 0/I proto-BD source (ALMA J154229.778−334241.86) and inferred the disk dust mass of a bona fide Class II BD. The pre-BD cores might be the byproduct of an ongoing process of large-scale collapse. The Class II BD disks follow the correlation between disk mass and the mass of the central object that is observed at the low-mass stellar regime. Conclusions. We conclude that it is highly probable that the sources in the sample are formed as a scaled-down version of low-mass star formation, although disk fragmentation may be responsible for a considerable fraction of BDs.