Carlos G. Román-Zúñiga

Context. O2 stars define the hottest end of the normal O-star sequence, but their identification requires blue-optical nitrogen diagnostics, which are absent in many surveys. Aims. We reassess three luminous Large Magellanic Cloud (LMC) sources with multi-epoch Sloan Digital Sky Survey V spectroscopy obtained with the Baryon Oscillation Spectroscopic Survey (BOSS) spectrographs: [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. Methods. We measured visit-resolved radial velocities and H, He, and N equivalent widths (EWs); compared robust combinations with LMC O2 templates at a common effective resolution; and used OSTAR2002 spectral energy distribution fits and a Gaia colour–magnitude diagram (CMD) as broadband checks. Results. The weighted log|EW(N IV λ 4058)/EW(N III λ 4640)| values are 0.64 ± 0.29, 0.48 ± 0.23, and 0.34 ± 0.44 for [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. The detection of N V λλ 4604, 4620 supports the O2 If ^* and O2 V–III classifications adopted for [WSI2008] 778 and MGSD LH 117-43A/140, respectively. For Sk −69 135, the uncertain N IV/N III ratio and non-detection of N V motivate O2–3 V–III. Weak He I λ 4471 may indicate unresolved later-type contamination in [WSI2008] 778 and Sk −69 135. The radial velocities of Sk −69 135 show suggestive but non-significant variability. OSTAR2002 fits over 45–55 kK and the Gaia CMD place all three sources in the luminous very early O regime; [WSI2008] 778 shows a strong mid-infrared excess, MGSD LH 117-43A/140 is nearly photospheric, and Sk −69 135 shows only a modest 3–8 μm excess. Conclusions. The BOSS spectra place [WSI2008] 778 and MGSD LH 117-43A/140 in the LMC very early O population and identify Sk −69 135 as an additional O2–3 source. These cases show how incomplete blue coverage and ambiguous catalogue associations can conceal the hottest massive stars.

We develop a convolutional neural network classifier that performs spectroscopic identification of stellar youth (T _eff and log g , providing a reliable means to substantially reduce contamination in photometrically selected young-stellar-object candidates. In addition to constructing a classifier for APOGEE spectra, we also perform classification of young stars in optical Baryon Oscillation Spectroscopic Survey spectra.

We present a new spectroscopic view of the brown dwarf population in the young star-forming region IC 1396 and investigate the impact of environment on low-mass star formation. We use deep optical photometry from Subaru-HSC to identify the candidate low-mass stars and brown dwarfs in the region. Our follow-up low-resolution spectroscopic survey with GTC-EMIR and IRTF-SpeX has identified 32 new members in the region with spectral types between M3 and M9, among which 25 are brown dwarfs with spectral types M6 or later. We use the BT-Settl atmospheric models to derive the effective temperatures of the members. Using a comprehensive catalogue of known members and candidates, we estimate the star to brown dwarf ratio for IC 1396 to be 5.0±0.4, for a mass range between 1-0.03 M⊙. This ratio is largely consistent with measurements in other young clusters spanning a range of UV radiation fields and stellar densities, supporting formation scenarios in which the relative abundance of brown dwarfs is not strongly influenced by the local environmental conditions.

Mapping the local and distant Universe is key to our understanding of it. For decades, the Sloan Digital Sky Survey (SDSS) has made a concerted effort to map millions of celestial objects to constrain the physical processes that govern our Universe. The most recent and fifth generation of SDSS (SDSS-V) is organized into three scientific “mappers”: the Milky Way Mapper, which aims to chart the various components of the Milky Way and constrain its formation and assembly; the Black Hole Mapper, which focuses on understanding supermassive black holes in distant galaxies across the Universe; and the Local Volume Mapper, which uses integral field spectroscopy to map the ionized interstellar medium in the Local Group. This paper describes the scope and content for the nineteenth data release (DR19) of SDSS, which is the most substantial to date in SDSS-V. DR19 is the first to contain data from all three mappers. Additionally, we also describe nine value-added catalogs that enhance the science that can be conducted with the SDSS-V data. Finally, we discuss how to access SDSS DR19 and provide illustrative examples and tutorials.

The so-called “Galactic center lobe” (GCL) is an extended (~1°) radio continuum feature situated above the Galactic plane, the nature and location of which have been subject to varying claims in the literature. Using new optical integral field spectroscopic observations from the SDSS-V Local Volume Mapper, we confirm the characterization of the GCL as a foreground photoionized H II region, not associated with the Galactic center. We present a new analysis of the ionized gas morphology, line ratio diagnostics, and kinematics. From our [S III ] λ 9532 emission line map, which suffers the least extinction, we identify ionized gas emission throughout a closed outer loop, which does not fill the GCL interior. All optical line ratio diagnostics are consistent with photoionization. By comparing the ionized gas reddening from the Balmer decrement with 3D dust maps, we directly constrain the distance to the GCL to ~2 kpc. [N II ] λ 6583 line kinematics show a uniform velocity structure across the GCL, further confirming that the entire bubble is one structure. The size and emission line morphology is strongly reminiscent of that seen in the nearby Barnard’s Loop, providing a possible analog to explain how this outer shell may be photoionized by a more distant and off-center embedded young cluster. We suggest that the acronym GCL be repurposed to instead abbreviate the name “greatly confused loop”.

In this paper we present a simple analysis around scaling relations derived from the Schmidt conjecture for star-forming molecular clouds, at the intra-cloud scale. Using a hierarchical tree (dendrograms) above a constant threshold (A_V = 7 mag), we separate individual gas structures in a column density map of the nearby Giant Molecular Cloud Orion A, constructed from Herschel far-infrared maps. These structures define regions of dense molecular gas that can actively form stars. We also estimate their current embedded population using a list of known young stars. From the combined analysis of the column density map and the young star catalog, we construct a series of plots that show the intra-cloud level behavior of three well-known scaling relations: N_yso vs. M_gas, Σ_SFR vs. Σ_gas and R_eq vs. M_gas. Our dataset, along with other sets from literature, show the validity of a linear relation for N_yso vs. M_gas, from intra-cloud to inter-cloud scales, over three orders of magnitude. We also especulate on the possibility that the relation could be valid over an even larger scale range. Additionally, our data are consistent with the R_eq vs. M_gas discussed in previous studies. However, our data is not quite in agreement with previously proposed fits for the Σ_SFR vs. Σ_gas relation, and we discuss the implications of using the free-fall timescale as the main parameter defining the star-forming efficiency in dense gas regions.

We present observations of the W51A region, including the massive protoclusters W51-E and W51-IRS2, with JWST in 10 NIRCam and 5 MIRI filters. In this work, we highlight the most novel features apparent in these images and compare them with other multiwavelength images. The broad view of the NIRCam/MIRI images of the W51A region shows that areas dominated by warm dust and ionized gas are distinct from those dominated by polycyclic aromatic hydrocarbons. The high angular resolution of the JWST images resolves dust filaments in high contrast, revealing geometrically converging features feeding W51-E and a cavity around W51-IRS2. This picture adds support to the hypothesis that feedback from W51-IRS2 is suppressing further gas infall onto the protocluster, while by contrast, gas is still accreting onto W51-E. Comparing the NIRCam and MIRI images to Atacama Large Millimeter/submillimeter Array (ALMA) data, we find 24 sources detected by both JWST and ALMA, accounting for only ∼10% of the ALMA sources; the rest are too embedded or too cool to be detected by JWST. A knot of [Fe ii ] and H ₂ emission north of W51-IRS2, previously detected in ground-based images, reveals peculiarly bright and compact peaks detected in all JWST bands. The knot is likely the most energetic example of a protostellar jet driven by a massive star impacting dense interstellar medium. The new images provide a complementary view to the previous long-wavelength perspective on this 4 × 8 pc area of one of the most active star-forming regions in our Galaxy, revealing new mysteries to be further explored.

We present a spatially resolved spectroscopic analysis of the Helix Nebula (NGC 7293) using data from the SDSS-V Local Volume Mapper (LVM), by applying the recently developed LVM Data Analysis Pipeline (LVM-DAP). Covering the full optical range (3600-9800 angstroms) over a contiguous ~0.5 degree field, the LVM data provide the first hexagonally sampled, wide-field emission-line maps of all major ionic species in this archetypal planetary nebula. The resulting flux, kinematic, and line-ratio maps reveal the well-known ionization stratification of the nebula, from the compact He++ core to the bright [O III] ring and the extended low-ionization envelope, enabling a detailed comparison with classical aperture spectroscopy. Owing to the sensitivity and uniform spatial sampling of the LVM, numerous faint auroral and diagnostic lines are detected across the nebula, including [O III] 4363, [N II] 5755, and He I lines, allowing precise measurements of weak-line morphology. The derived radial trends confirm the remarkably low dust content and the overall homogeneity of electron temperature and density across the main ring. Ionized-gas kinematics traced by H-alpha further support the scenario of a slowly expanding, limb-brightened shell consistent with previous studies. This work demonstrates the diagnostic power of LVM spectroscopy for extended nebulae and highlights its capability to recover both global and spatially resolved physical conditions across complex ionized structures.