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1. The IGRINS YSO Survey. III. Stellar Parameters of Pre-main-sequence Stars in Ophiuchus and Upper Scorpius

   https://doi.org/10.3847/1538-4357/acab04  

   López-Valdivia, Ricardo; Mace, Gregory N.; Han, Eunkyu; Sawczynec, Erica; Hernández, Jesús; Prato, L.; Johns-Krull, Christopher M.; Oh, Heeyoung; Lee, Jae-Joon; Kraus, Adam; et al (January 2023, The Astrophysical Journal)

   Abstract We used the Immersion GRating Infrared Spectrometer (IGRINS) to determine fundamental parameters for 61 K- and M-type young stellar objects (YSOs) located in the Ophiuchus and Upper Scorpius star-forming regions. We employed synthetic spectra and a Markov chain Monte Carlo approach to fit specificK-band spectral regions and determine the photospheric temperature (T), surface gravity ( $\log g$), magnetic field strength (B), projected rotational velocity ( $v\sin i$), andK-band veiling (r_K). We determinedBfor ∼46% of our sample. Stellar parameters were compared to the results from Taurus-Auriga and the TW Hydrae association presented in Paper I of this series. We classified all the YSOs in the IGRINS survey with infrared spectral indices from Two Micron All Sky Survey and Wide-field Infrared Survey Explorer photometry between 2 and 24μm. We found that Class II YSOs typically have lower $\log g$and $v\sin i$, similarB, and higherK-band veiling than their Class III counterparts. Additionally, we determined the stellar parameters for a sample of K and M field stars also observed with IGRINS. We have identified intrinsic similarities and differences at different evolutionary stages with our homogeneous determination of stellar parameters in the IGRINS YSO survey. Considering $\log g$as a proxy for age, we found that the Ophiuchus and Taurus samples have a similar age. We also find that Upper Scorpius and TWA YSOs have similar ages, and are more evolved than Ophiuchus/Taurus YSOs. 

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2. Viscous heating in the disk of the outbursting star FU Orionis

   https://doi.org/10.1051/0004-6361/202039370  

   Labdon, Aaron; Kraus, Stefan; Davies, Claire L.; Kreplin, Alexander; Monnier, John D.; Le Bouquin, Jean-Baptiste; Anugu, Narsireddy; ten Brummelaar, Theo; Setterholm, Benjamin; Gardner, Tyler; et al (February 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. FU Orionis is the archetypal FUor star, a subclass of young stellar objects (YSOs) that undergo rapid brightening events, often gaining between four and six magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion, which is one of the most ubiquitous processes in astrophysics for bodies ranging from planets and stars to super-massive black holes. We present multi-band interferometric observations of the FU Ori circumstellar environment, including the first J -band interferometric observations of a YSO. Aims. We investigate the morphology and temperature gradient of the innermost regions of the accretion disk around FU Orionis. We aim to characterise the heating mechanisms of the disk and comment on potential outburst-triggering processes. Methods. Recent upgrades to the MIRC-X instrument at the CHARA array have allowed for the first dual-band J and H observations of YSOs. Using baselines up to 331 m, we present high-angular-resolution data of a YSO covering the near-infrared bands J , H , and K . The unprecedented spectral range of the data allowed us to apply temperature gradient models to the innermost regions of FU Ori. Results. We spatially resolved the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to T ∝ r −0.74 ± 0.02 . This agrees with theoretical works that predict T ∝ r −0.75 for actively accreting, steady-state disks, which is a value only obtainable through viscous heating within the disk. We found a disk that extends down to the stellar surface at 0.015 ± 0.007 au, where the temperature is found to be 5800 ± 700 K. We found a disk inclined at 32 ± 4° with a minor-axis position angle of 34 ± 11°. Conclusions. We demonstrate that J -band interferometric observations of YSOs are feasible with the MIRC-X instrument at CHARA. The temperature gradient power-law derived for the inner disk is consistent with theoretical predictions for steady-state, optically thick, viciously heated accretion disks. 

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3. Effective Temperature Estimations from Line Depth Ratios in the H- and K-band Spectra of IGRINS

   https://doi.org/10.3847/1538-4357/acc946  

   Afşar, Melike; Bozkurt, Zeynep; Topcu, Gamze Böcek; Özdemir, Sergen; Sneden, Christopher; Mace, Gregory N.; Jaffe, Daniel T.; López-Valdivia, Ricardo (June 2023, The Astrophysical Journal)

   Abstract Determining accurate effective temperatures of stars buried in the dust-obscured Galactic regions is extremely difficult from photometry. Fortunately, high-resolution infrared spectroscopy is a powerful tool for determining the temperatures of stars with no dependence on interstellar extinction. It has long been known that the depth ratios of temperature-sensitive and relatively insensitive spectral lines are excellent temperature indices. In this work, we provide the first extensive line depth ratio (LDR) method application in the infrared region that encompasses both the H and K bands (1.48 μ m − 2.48 μ m). We applied the LDR method to high-resolution ( R ≃ 45,000) H- and K -band spectra of 110 stars obtained with the Immersion Grating Infrared Spectrograph. Our sample contained stars with 3200 < T eff (K) < 5500, 0.20 ≤ log g < 4.6, and −1.5 < [M/H] < 0.5. The application of this method in the K band yielded 21 new LDR– T eff relations. We also report five new LDR– T eff relations found in the H -band region, augmenting the relations already published by other groups. The temperatures found from our calibrations provide reliable temperatures within ∼70 K accuracy compared to spectral  T eff values from the literature. 

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4. Measuring the Spot Variability of T Tauri Stars Using Near-infrared Atomic Fe and Molecular OH Lines

   https://doi.org/10.3847/1538-4357/ad5e7f  

   Tang, Shih-Yun; Johns-Krull, Christopher_M; Prato, L.; Stahl, Asa_G (September 2024, The Astrophysical Journal)

   Abstract As part of the Young Exoplanets Spectroscopic Survey, this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infraredHband, using spectra from the Immersion GRating INfrared Spectrometer. By analyzing effective temperature (T_eff) sensitive lines of atomic Feiat ∼1.56259μm and ∼1.56362μm, and molecular OH at ∼1.56310 and ∼1.56317μm, we develop an empirical equivalent width ratio (EWR) relationship forT_effin the range of 3400–5000 K. This relationship allows for precise relativeT_effestimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of theT_eff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibitT_effvariations exceeding 150 K over a span of 2 yr. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 ± 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation betweenT_effvariation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars. 

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5. GOALS-JWST: NIRCam and MIRI Imaging of the Circumnuclear Starburst Ring in NGC 7469

   https://doi.org/10.3847/2041-8213/acab61  

   Bohn, Thomas; Inami, Hanae; Diaz-Santos, Tanio; Armus, Lee; Linden, S. T.; U, Vivian; Surace, Jason; Larson, Kirsten L.; Evans, Aaron S.; Hoshioka, Shunshi; et al (January 2023, The Astrophysical Journal Letters)

   Abstract We present James Webb Space Telescope (JWST) imaging of NGC 7469 with the Near-Infrared Camera and the Mid-InfraRed Instrument. NGC 7469 is a nearby, z = 0.01627, luminous infrared galaxy that hosts both a Seyfert Type-1.5 nucleus and a circumnuclear starburst ring with a radius of ∼0.5 kpc. The new near-infrared (NIR) JWST imaging reveals 66 star-forming regions, 37 of which were not detected by Hubble Space Telescope (HST) observations. Twenty-eight of the 37 sources have very red NIR colors that indicate obscurations up to A v ∼ 7 and a contribution of at least 25% from hot dust emission to the 4.4 μ m band. Their NIR colors are also consistent with young (<5 Myr) stellar populations and more than half of them are coincident with the mid-infrared (MIR) emission peaks. These younger, dusty star-forming regions account for ∼6% and ∼17% of the total 1.5 and 4.4 μ m luminosity of the starburst ring, respectively. Thanks to JWST, we find a significant number of young dusty sources that were previously unseen due to dust extinction. The newly identified 28 young sources are a significant increase compared to the number of HST-detected young sources (4–5). This makes the total percentage of the young population rise from ∼15% to 48%. These results illustrate the effectiveness of JWST in identifying and characterizing previously hidden star formation in the densest star-forming environments around active galactic nuclei (AGN). 

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