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The Sloan Digital Sky Survey IV APOGEE-2 primary science goal was to observe red giant stars throughout the Galaxy to study its dynamics, morphology, and chemical evolution. The APOGEE instrument, a high-resolution 300-fiber H -band (1.55–1.71 μ m) spectrograph, is also ideal to study other stellar populations in the Galaxy, among which are a number of star-forming regions and young open clusters. We present the results of the determination of six stellar properties ( T _eff , $\mathrm{log}\,g$ , [Fe/H], L / L _⊙ , M / M _⊙ , and age) for a sample that is composed of 3360 young stars, of subsolar to supersolar types, in 16 Galactic star formation and young open cluster regions. Those sources were selected by using a clustering method that removes most of the field contamination. Samples were also refined by removing targets affected by various systematic effects of the parameter determination. The final samples are presented in a comprehensive catalog that includes all six estimated parameters. This overview study also includes parameter spatial distribution maps for all regions and Hertzsprung–Russell ( $\mathrm{log}L/{L}_{\odot }$ vs. T _eff ) diagrams. This study serves as a guide for detailed studies on individual regions and paves the way for the future studies on the global properties of stars in the pre-main-sequence phase of stellar evolution using more robust samples.

Very young ( t ≲ 10 Myr) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar disks to the surface of the star. Upon impacting the stellar surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11–20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main-sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of n _e = 10 ^11 –10 ^12 cm ^−3 , and excitation temperatures that are inversely correlated with electron density (from T ∼ 5000 K for n _e ∼ 10 ^12 cm ^−3 to T ∼ 12,500 K at n _e ∼ 10 ^11 cm ^−3 ). These physical parameters are in good agreement with predictions from modeling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission-line sources in the APOGEE Data Release 17 data set.

In this paper, we describe the results obtained in our study of Galactic Wolf–Rayet stars of the WN type, using the APOGEE2 spectrographs. The APOGEE2 WN star near-infrared spectra presented here were selected through a careful examination of the APOGEE2 DR16 sample, being compound of at least one exemplar of each Wolf–Rayet star from WN4 to WN11 types. Besides the APOGEE2 WN sample, we also identified three new Galactic exemplars of the WN type. The first is WR 147–1, a narrow-lined Wolf–Rayet star of the WN4-5 type located in the direction of the Cygnus-X North complex. The two remaining objects, WR 48–11 and WR 48–12, are found at projected angular distances of several arcminutes southwest from the Danks 1 cluster. WR 48–12 has an estimated heliocentric distance of kpc, which is not compatible (within the quoted uncertainties) with that of about 3.8 ± 0.6 kpc for the G305 complex. On the other hand, WR 48–11 has a probable heliocentric distance of kpc, which is about three times that of the former, implying that it is probably placed well behind the G305 complex, perhaps at the outermost portion of the Perseus arm. [ABSTRACT FROM AUTHOR]

In this work, we have applied a semi-empirical spectral classification method for OB-stars using the APOGEE spectrograph to a sample of candidates in the W3–W4–W5 (W345) complexes. These massive star-forming regions span over 200 pc across the Perseus arm and have a notorious population of massive stars, from which a large fraction are members of various embedded and young open clusters. From 288 APOGEE spectra showing H-band spectral features typical of O- and B-type sources, 46 probably correspond to previously unknown O-type stars. Therefore, we confirm that Br11–Br13 together with He ii λ16923 (7–12) and He ii λ15723 (7–13) lines contained in the APOGEE spectral bands are useful in providing spectral classification down to one spectral sub-class for massive stars in regions as distant as d ≈ 2 kpc. The large number of newly found O-type stars as well as the numerous intermediate-mass population confirm that W345 is a very efficient massive star factory, with an integral stellar population probably amounting several thousand solar masses. [ABSTRACT FROM AUTHOR]