Alexandre Roman-Lopes

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.

The Sloan Digital Sky Survey (SDSS)-V Local Volume Mapper (LVM) is a wide-field integral field spectroscopic survey of the southern Milky Way plane, the Magellanic Clouds, and nearby Local Group galaxies. We used early science observations of the whole body of the nearest nuclear cluster, Omega Centauri, to extend the LVM beyond its primary interstellar-medium science case. The wide LVM field allowed us to precisely map ω Cen’s line-of-sight rotation out to ~3 r _HL or 15′, reaching a maximum value of (8.4 ± 0.8) km s ⁻¹ at r ≈ 4.7′. Within the central region, comparisons with existing VLT MUSE oMEGACat data explicitly show that the unresolved light signal is dominated by a small number of bright stars, with an effective sample size of only ~12 per resolution element. Using Gaia DR3 as an external reference, we verified that the SDSS-V’s LVM reduction pipeline recovers integrated stellar fluxes to 1–4% across six magnitudes of surface brightness. Our resulting total spectrum of ω Cen is one of the highest S/N integrated spectra for any globular or nuclear star cluster. We used it to test four widely used SSP template libraries against the resolved age-metallicity ground truth from oMEGACat. All templates recovered an old, metal-poor population. However, even at signal to noise ratio of ~1300, the inferred mean ages and mean [Fe/H] vary by ~7 Gyr and ~0.4 dex, respectively, across libraries and wavelength ranges, reflecting a systematic floor for integrated-light studies of old multi-population systems.

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.

Studies of the solar neighborhood require spectroscopic follow-up of stars identified in astrometric surveys to fully characterize their physical properties. The SDSS-V Solar Neighborhood Census (SNC) is a dedicated program to observe stars within 100 pc. However, due to competing observing programs and fiber assignment constraints, the resulting sample carries severe and complex selection effects. A framework is presented for characterizing the selection function of the SDSS-V SNC relative to the Gaia Catalog of Nearby Stars (GCNS), along with a forward-modeling method to infer the properties of stellar subpopulations across the GCNS-defined 100 pc sample. The selection function is based on a method that models the selection probability as a function of sky position, Gaia G magnitude, and BP − RP. The resulting detection probabilities faithfully reproduce the known survey planning logic. This work further introduces the concept of a “subpopulation probability”—a grid of posterior estimates across the Hertzsprung–Russell (H-R) diagram representing the likelihood that a GCNS member belongs to a given SDSS-V defined subpopulation. The framework is validated with a mock dataset, and its scientific utility is demonstrated through two applications using data from Data Release 19: mapping H α emission across the H-R diagram and measuring the variation of stellar density with mass and metallicity. These results illustrate how statistically robust population studies can be conducted with an incomplete spectroscopic survey when the selection function is well characterized. The code is made publicly available with this work, which will serve as an important tool for future studies.

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”.

The abundance discrepancy problem refers to the systematic differences observed between chemical abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) of heavy ions. It remains a major unsolved problem in the study of ionized nebulae and is quantified by the abundance discrepancy factor (ADF). In this work, we present a deep integral field spectroscopic data set of the entire Lagoon Nebula (M8), obtained by the Sloan Digital Sky Survey V Local Volume Mapper project, at a spatial resolution of 0.21 pc spaxel ⁻¹ . This unique data set allows us, for the first time, to investigate spatially resolved maps of oxygen RL intensities (O ii V1), together with maps of H i RLs, heavy-ion CELs, and dust attenuation across a whole H ii region. We map the electron temperature using CELs and RLs of O ²⁺ and CELs of N ⁺ , and we map the electron density using CELs of S ⁺ . We derive CEL-based ionic and elemental oxygen abundances and, for the first time, a spatially resolved map of the RL-based O ²⁺ abundance in an H ii region. These measurements enable construction of the first spatially resolved ADF(O ²⁺ ) map of an H ii region and yield a global mean ADF of ∼0.47 ± 0.02 dex. Focusing on the central region of M8, where ionization is dominated by the O-type star Her 36, we find radial variations in the ADF, ranging between ∼0.35 and 0.50 dex. Our findings provide novel constraints on the spatial behavior and origin of the abundance discrepancy in the H ii 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.

The supernova remnant (SNR) RCW86 is among the few SNRs with Balmer-emission lines containing broad and narrow spectral components that trace fast, nonradiative shocks in partially ionized gas. These are invaluable laboratories for collisionless shock physics, especially for poorly understood phenomena like electron-ion equilibration, and shock precursors. Here we present the first ∼0.3 pc spatial scale integral field unit observations of the southwestern RCW86 shock, obtained as part of the Sloan Digital Sky Survey-V Local Volume Mapper (LVM). The forward shock, clearly visible as thin filaments in narrowband images, exhibits broad component H α emission, indicating shock velocities varying from 500–900 km s ⁻¹ in the south to 1000–1500 km s ⁻¹ in the north. The varying velocity widths and broad-to-narrow intensity ratios show that electrons and ions have lower equilibration ( T _e / T _p → 0.1) in faster (>800 km s ⁻¹ ) shocks, in line with previous studies. The broad components are generally redshifted from the narrow components by ≲100 km s ⁻¹ , likely due to shock-obliquity or non-Maxwellian post-shock distributions. We observe high extinction-corrected Balmer-decrements of 3–5 in the narrow components, indicating that conversion of Ly β photons to H α is more efficient than Ly γ to H β . Upper limits on the He II λ 4686 in the southern shock are consistent with a moderate-to-high (30%–100%) neutral fraction in the preshock gas. We also find the first evidence of an intermediate H α component in RCW86, with ΔV(FWHM) = 193–207 km s ⁻¹ , likely due to a fast neutral precursor. We also briefly discuss the southwestern radiative shock, and lay out the exciting future of studying astrophysical shocks with LVM.