CO Bandhead Research Articles

Unveiling accretion in the massive young stellar object G033.3891

Context. The inner parts of the hot discs surrounding massive young stellar objects (MYSOs) are still barely explored due to observational limitations in terms of angular resolution, scarcity of diagnostic lines, and the embedded and rare nature of these targets. Aims. We present the first K-band spectro-interferometric observations towards the MYSO G033.3891, which based on former kinematic evidence via the CO bandhead emission is known to host an accreting disc. Methods. Using the high spectral resolution mode (R∼4000) of the GRAVITY/VLTI, we spatially resolved the emission of the inner dusty disc and the crucial gaseous interface between the star and the dusty disc. Using detailed modelling on the K-band dust continuum and tracers known to be associated with the ionised and molecular gaseous interface (Brγ, CO), we report on the smallest scales of accretion and ejection. Results. The new observations in combination with our geometric and kinematic models employed to fit former high spectral resolution observations on the source (R∼30 000; CRIRES/VLTI) allowed us to constrain the size of the inner gaseous disc both spatially and kinematically via the CO overtone emission at only 2 au. Our models reveal that both Brγ and CO emissions are located well within the dust sublimation radius (5 au) as traced by the hot 2.2 µm dust continuum. Conclusions. Our paper provides the first case study where the tiniest scales of gaseous accretion around the MYSO G033.3891 are probed both kinematically and spatially via the CO bandhead emission. This analysis of G033.3891 stands as only the second instance of such an investigation within MYSOs, underscoring the gradual accumulation of knowledge regarding how massive young stars gain their mass while further solidifying the disc nature of accretion at the smallest scales of MYSOs.

Exploring fluorine chemical evolution in the Galactic disk: The open cluster perspective

Open clusters are ideal tools for tracing the abundances of different elements because their stars are expected to have the same age, distance, and metallicity. Therefore, they serve as powerful tracers for investigating the cosmic origins of elements. This paper expands on a recent study by us, in which the element fluorine was studied in seven open clusters; here we add six open clusters and eight field stars. The primary objective is to determine the abundance of fluorine (F) to gain insight into its production and evolution. The magnesium (Mg) abundances were derived to categorize the field stars into high and low alpha disk populations. Additionally, cerium (Ce) abundances were determined to better understand the interplay between F and s-process elements. Our goal is to analyze the trend of F abundances across the Galactic disk based on metallicity and age. By comparing observational data with Galactic chemical evolution models, the origin of F can be better understood. The spectra were obtained from the high-resolution near-infrared GIANO-B instrument at the Telescopio Nazionale Galileo (TNG). For the derivation of the stellar parameters and abundances, the Python version of Spectroscopy Made Easy ( PySME ) was used. OH, CN, and CO molecular lines and band heads along with Fe I lines were used to determine the stellar parameters in the H-band region. Two HF lines in the K band ($ 2.28, and 2.33 mu m), three K-band Mg I lines ($ 2.10, 2.11, and 2.15 mu m), and two Ce II lines in the H band ($ 1.66, and 1.71 mu m) were used to derive the abundances of F, Mg, and Ce, respectively. F, Mg, and Ce abundances were derived for 14 stars from 6 OCs, as well as for 8 field stars. The F and Ce abundances were investigated as a function of metallicity, age, and galactocentric distance. We also compared our findings with different Galactic chemical evolution models. Our results indicate that asymptotic giant branch stars and massive stars, including a subset of fast rotators (whose rotation speed likely increases as metallicity decreases), are necessary to explain the cosmic origin of F. This finding is consistent with and, with the large sample size, reinforces the conclusion of our previous study.

JWST/NIRSpec and MIRI observations of an expanding, jet-driven bubble of warm H2 in the radio galaxy 3C 326 N

The physical link between AGN activity and the suppression of star formation in their host galaxies is one of the major open questions of the AGN feedback scenario. The Spitzer space mission revealed a subset of powerful nearby radio galaxies with unusually bright line emission from warm ($T 100$ K) molecular hydrogen, while typical star-formation tracers such as polycyclic aromatic hydrocarbons (PAHs) or a dust continuum have been exceptionally faint or undetected. Here, we present JWST NIRSpec and MIRI MRS IFU observations of one of the best studied galaxies of this class, 3C 326 N at z=0.09. We identified a total of 19 lines of the S, O, and Q series of ro-vibrational H$_2$ emission with NIRSpec at a 0.11 spatial resolution, probing a small quantity odot $) of gas at temperatures of $T 1000$ K. We also mapped the rotational mid-infrared lines of H$_2$ 0--0 S(3), S(5), and S(6) at a spatial resolution of 0.4 with MIRI/MRS, probing most of the $2 odot $ of warm H$_2$ in this galaxy. The CO band heads show a stellar component consistent with a 'slow-rotator' that is typical of a massive ($3 $\,M$_ galaxy, offering a reliable systemic redshift of $z=0.08979 0.0003$. The extended line emission shows a bipolar bubble expanding through the molecular disk at velocities of up to $, delineated by several bright clumps along the northern outer rim, potentially coming from gas fragmentation. Throughout the disk, H$_2$ is very broadly dispersed, with an FWHM of $ 100-1300$ km $ and complex, dual-component Gaussian line profiles. The extended FeII lambda 1.644 and Paalpha follow the same morphology, however NeIII lambda 15.56 is more symmetric about the nucleus. We show that most of the gas (with the exception of NeIII lambda 15.56) is predominantly heated by shocks driven by the radio jets into the gas, both for the ro-vibrational and rotational H$_2$ lines. In addition, the accompanying line broadening is sufficient to suppress star formation in the molecular gas. We also compared the morphology and kinematics of the rotational and ro-vibrational lines, finding the latter to be a good proxy to the global morphology and kinematic properties of the former in strongly turbulent environments. This demonstrates the potential of using the higher frequency ro-vibrational lines in studying turbulent molecular gas. Provided they are bright enough, they would allow us to examine turbulence in galaxies during the early phases of cosmic history, while most rotational lines are red-shifted out of the MIRI bandpass for $z ge1.5$

GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

Massive pre-main-sequence stars in M17

Context. Recently much progress has been made in probing the embedded stages of massive star formation, pointing to formation scenarios that are reminiscent of a scaled-up version of low-mass star formation. However, the latest stages of massive-star formation have rarely been observed, as young massive stars are assumed to reveal their photospheres only when they are fully formed. Aims. Using first and second overtone CO bandhead emission and near- to mid-infrared photometry, we aim to characterize the remnant formation disks around five unique pre-main-sequence (PMS) stars with masses 6–12 M⊙ that have constrained stellar parameters thanks to their detectable photospheres. We seek to understand this emission and the disks from which it originates in the context of the evolutionary stage of the studied sources. Methods. We used an analytic disk model, and adopted local thermodynamical equilibrium, to fit the CO bandhead and the dust emission, assumed to originate in different disk regions. For the first time, we modeled the second overtone emission, which helped us to put tighter constraints on the density of the CO gas. Furthermore, we fit continuum normalized bandheads, using models for stellar and dust continuum, and show the importance of this in constraining the emission region. We also included 13CO in our models as an additional probe of the young nature of the studied objects. Results. We find that the CO emission originates in a narrow region close to the star (<1 AU) and under very similar disk conditions (temperatures and densities) for the different objects. This is consistent with previous modeling of this emission in a diverse range of young stellar objects and identifies CO emission as an indicator of the presence of a gaseous inner disk reaching close to the stellar surface. From constraining the location of the inner edge of the dust emission, we find that all but one of the objects have undisrupted inner dust disks. Conclusions. We discuss these results in the context of the positions of these PMS stars in the Hertzsprung-Russel diagram and the CO emission’s association with an early age and high accretion rates in (massive) young stellar objects. We conclude, considering their mass range and the fact that their photospheres are detected, that the M17 PMS stars are observed in a relatively early formation stage. They are therefore excellent candidates for longer wavelength studies to further constrain the end stages of massive star formation.

Imaging the innermost gaseous layers of the Mira star R Car with GRAVITY-VLTI

Context. The mass-loss mechanisms in M-type asymptotic giant branch (AGB) stars are still not well understood; these include, in particular, the formation of dust-driven winds from the innermost gaseous layers around these stars. One way to understand the gas-dust interaction in these regions and its impact on the mass-loss mechanisms is through the analysis of high-resolution observations of the stellar surface and its closest environment. Aims. We aim to characterize the inner circumstellar environment (~3 R*) of the M-type Mira star R Car in the near-infrared at different phases of a pulsation period. Methods. We used GRAVITY interferometric observations in the K band obtained during two different epochs over 2018. Those data were analyzed using parametric models and image reconstruction of both the pseudo-continuum and the CO band heads observed. The reported data are the highest angular resolution observations on the source in the K band. Results. We determined sizes of R Car’s stellar disk of 16.67 ± 0.05 mas (3.03 au) in January 2018 and 14.84 ± 0.06 mas (2.70 au) in February, 2018, respectively. From our physical model, we determined temperatures and size ranges for the innermost CO layer detected around R Car. The derived column density of the CO is in the ~9.18×1018–1×1019 cm−2 range, which is sufficient to permit dust nucleation and the formation of stable dust-driven winds. We find that magnesium composites, Mg2SiO4 and MgSiO3, have temperatures and condensation distances consistent with the ones obtained for the CO layer model and pure-line reconstructed images, which are the dust types most likely to be responsible for wind formation. Our reconstructed images show evidence of asymmetrical and inhomogeneous structures, which might trace a complex and perhaps clumpy structure of the CO molecule distribution. Conclusions. Our work demonstrates that the conditions for dust nucleation and thus for initialising dust-driven winds in M-type AGB stars are met in R Car, and we identify magnesium composites as the most probable candidates. We find structural changes between two observing epochs (which are separated by ~10% of the full pulsation period of the star) and evidence of the effects of asymmetries and clumpiness. This observational evidence is crucial to constraining the role of convection and pulsation in M-type stars.

Near-infrared spectroscopy of embedded protostars in the massive metal-poor star-forming region NGC 346

ABSTRACT We present medium-resolution (R ∼ 4000) YJ-, H-, and K-band spectroscopy of candidate young stellar objects (YSOs) in NGC 346, the most active star-formation region in the metal-poor (Z = 1/5 Z⊙) Small Magellanic Cloud. The spectra were obtained with the KMOS (K-Band Multi-Object Spectrograph) integral field instrument on the Very Large Telescope. From our initial sample of 18 candidate high-mass YSOs previously identified from mid-IR photometry and radiative transfer model fits to their spectral energy distributions, approximately half were resolved into multiple components by our integral-field data. In total, we detect 30 continuum sources and extract reliable spectra for 12 of these objects. The spectra show various features including hydrogen recombination lines, and lines from H2, He i, and [Fe ii], which are indicative of accretion, discs, and outflowing material in massive YSOs. We spectroscopically confirm the youthful nature of nine YSO candidates, and identify two others as OB stars. All of the confirmed YSOs have Br γ in emission, but no emission is seen from the CO bandhead, despite other disc tracers present in the spectra. He i 1.083 $\mu$m emission is also detected at appreciably higher rates than for the Galaxy.

A Multi-epoch, Multiwavelength Study of the Classical FUor V1515 Cyg Approaching Quiescence

Historically, FU Orionis-type stars are low-mass, pre-main-sequence stars. The members of this class experience powerful accretion outbursts and remain in an enhanced accretion state for decades or centuries. V1515 Cyg, a classical FUor, started brightening in the 1940s and reached its peak brightness in the late 1970s. Following a sudden decrease in brightness, it stayed in a minimum state for a few months, then started brightening for several years. We present the results of our ground-based photometric monitoring complemented with optical/near-infrared spectroscopic monitoring. Our light curves show a long-term fading with strong variability on weekly and monthly timescales. The optical spectra show P Cygni profiles and broad blueshifted absorption lines, common properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca ii 8498 Å line, a part of the Ca infrared triplet, formed by an outflowing wind, suggesting that the absorbing gas in the wind is optically thin. The newly obtained near-infrared spectrum shows the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler since the last spectroscopic observation in 2015. The current luminosity of the accretion disk dropped from the peak value of 138 L ⊙ to about 45 L ⊙, suggesting that the long-term fading is also partly caused by the dropping of the accretion rate.

Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes

To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of 109 red point sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and mid-infrared photometry, including 12 objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample (100 objects) show CO band-head absorption at 2.3 μm, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with L-band spectra (9/82 = 0.11) exhibit large H2O ice column densities (N > 2 × 1018 cm−2), and six of them also reveal CH3OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate (≲0.02 M ⊙ yr−1), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong “shoulder” CO2 ice absorption at 15.4 μm observed in YSO candidates in the previous paper arises from CH3OH-rich ice grains having a large CO2 concentration [N(CO2)/N(CH3OH) ≈ 1/3].

V899 Mon: A Peculiar Eruptive Young Star Close to the End of Its Outburst

Abstract The eruptive young star V899 Mon shows characteristics of both FUors and EXors. It reached a peak brightness in 2010, then briefly faded in 2011, followed by a second outburst. We conducted multifilter optical photometric monitoring, as well as optical and near-infrared spectroscopic observations, of V899 Mon. The light curves and color–magnitude diagrams show that V899 Mon has been gradually fading after its second outburst peak in 2018, but smaller accretion bursts are still happening. Our spectroscopic observations taken with Gemini/IGRINS and VLT/MUSE show a number of emission lines, unlike during the outbursting stage. We used the emission line fluxes to estimate the accretion rate and found that it has significantly decreased compared to the outbursting stage. The mass-loss rate is also weakening. Our 2D spectroastrometric analysis of emission lines recovered jet and disk emission of V899 Mon. We found that the emission from permitted metallic lines and the CO bandheads can be modeled well with a disk in Keplerian rotation, which also gives a tight constraint for the dynamical stellar mass of 2 M ⊙. After a discussion of the physical changes that led to the changes in the observed properties of V899 Mon, we suggest that this object is finishing its second outburst.

Resolved Nuclear Kinematics Link the Formation and Growth of Nuclear Star Clusters with the Evolution of Their Early- and Late-type Hosts

Abstract We present parsec-scale kinematics of 11 nearby galactic nuclei, derived from adaptive-optics assisted integral-field spectroscopy at (near-infrared) CO band-head wavelengths. We focus our analysis on the balance between ordered rotation and random motions, which can provide insights into the dominant formation mechanism of nuclear star clusters (NSCs). We divide our target sample into late- and early-type galaxies, and discuss the nuclear kinematics of the two subsamples, aiming at probing any link between NSC formation and host galaxy evolution. The results suggest that the dominant formation mechanism of NSCs is indeed affected by the different evolutionary paths of their hosts across the Hubble sequence. More specifically, nuclear regions in late-type galaxies are on average more rotation dominated, and the formation of nuclear stellar structures is potentially linked to the presence of gas funneled to the center. Early-type galaxies, in contrast, tend to display slowly rotating NSCs with lower ellipticity. However, some exceptions suggest that in specific cases, early-type hosts can form NSCs in a way similar to spirals.

MOLsphere and pulsations of the Galactic Center’s red supergiant GCIRS 7 from VLTI/GRAVITY

Context. GCIRS 7, the brightest star in the Galactic central parsec, formed 6 ± 2 Myr ago together with dozens of massive stars in a disk orbiting the central black-hole. It has been argued that GCIRS 7 is a pulsating body, on the basis of photometric variability. Aims. Our goal is to confirm photospheric pulsations based on interferometric size measurements to better understand how the mass loss from these massive stars enriches the local interstellar medium. Methods. We present the first medium-resolution (R = 500), K-band spectro-interferometric observations of GCIRS 7, using the GRAVITY instrument with the four auxiliary telescopes of the ESO VLTI. We looked for variations using two epochs, namely 2017 and 2019. Results. We find GCIRS 7 to be moderately resolved with a uniform-disk photospheric diameter of θUD* = 1.55 ± 0.03 mas (RUD* = 1368 ± 26 R⊙) in the K-band continuum. The narrow-band uniform-disk diameter increases above 2.3 μm, with a clear correlation with the CO band heads in the spectrum. This correlation is aptly modeled by a hot (TL = 2368 ± 37 K), geometrically thin molecular shell with a diameter of θL = 1.74 ± 0.03 mas, as measured in 2017. The shell diameter increased (θL = 1.89 ± 0.03 mas), while its temperature decreased (TL = 2140 ± 42 K) in 2019. In contrast, the photospheric diameter θUD* and the extinction up to the photosphere of GCIRS 7 (AKS = 3.18 ± 0.16) have the same value within uncertainties at the two epochs. Conclusions. In the context of previous interferometric and photo-spectrometric measurements, the GRAVITY data allow for an interpretation in terms of photospheric pulsations. The photospheric diameter measured in 2017 and 2019 is significantly larger than previously reported using the PIONIER instrument (θ* = 1.076 ± 0.093 mas in 2013 in the H band). The parameters of the photosphere and molecular shell of GCIRS 7 are comparable to those of other red supergiants that have previously been studied using interferometry. The extinction we measured here is lower than previous estimates in the direction of GCIRS 7 but typical for the central parsec region.

The GRAVITY young stellar object survey

Context. 51 Oph is a Herbig Ae/Be star that exhibits strong near-infrared CO ro-vibrational emission at 2.3 μm, most likely originating in the innermost regions of a circumstellar disc. Aims. We aim to obtain the physical and geometrical properties of the system by spatially resolving the circumstellar environment of the inner gaseous disc. Methods. We used the second-generation Very Large Telescope Interferometer instrument GRAVITY to spatially resolve the continuum and the CO overtone emission. We obtained data over 12 baselines with the auxiliary telescopes and derive visibilities, and the differential and closure phases as a function of wavelength. We used a simple local thermal equilibrium ring model of the CO emission to reproduce the spectrum and CO line displacements. Results. Our interferometric data show that the star is marginally resolved at our spatial resolution, with a radius of ~10.58 ± 2.65R⊙. The K-band continuum emission from the disc is inclined by 63° ± 1°, with a position angle of 116° ± 1°, and 4 ± 0.8 mas (0.5 ± 0.1 au) across. The visibilities increase within the CO line emission, indicating that the CO is emitted within the dust-sublimation radius. By modelling the CO bandhead spectrum, we derive that the CO is emitted from a hot (T = 1900–2800 K) and dense (NCO = (0.9–9) × 1021 cm−2) gas. The analysis of the CO line displacement with respect to the continuum allows us to infer that the CO is emitted from a region 0.10 ± 0.02 au across, well within the dust-sublimation radius. The inclination and position angle of the CO line emitting region is consistent with that of the dusty disc. Conclusions. Our spatially resolved interferometric observations confirm the CO ro-vibrational emission within the dust-free region of the inner disc. Conventional disc models exclude the presence of CO in the dust-depleted regions of Herbig AeBe stars. Ad hoc models of the innermost disc regions, that can compute the properties of the dust-free inner disc, are therefore required.

The Outburst of the Young Star Gaia19bey

Abstract We report photometry and spectroscopy of the outburst of the young stellar object Gaia19bey. We have established the outburst light curve with archival Gaia “G,” ATLAS “Orange,” Zwicky Transient Facility r-band, and Pan-STARRS “rizy”-filter photometry, showing an outburst of ~4 yr duration, longer than typical EX Lupi objects but shorter than FU Orionis objects. Its pre-outburst spectral energy distribution shows a flat far-infrared spectrum, confirming the early evolutionary state of Gaia19bey and its similarity to other deeply embedded young stars experiencing outbursts. A lower limit to the peak outburst luminosity is ~182 at an assumed distance of 1.4 kpc, the minimum plausible distance. Infrared and optical spectroscopy near maximum light show an emission line spectrum, including H i lines, strong red Ca ii emission, other metal emission lines, infrared CO bandhead emission, and a strong infrared continuum. Toward the end of the outburst, the emission lines have all but disappeared and the spectrum has changed into an almost pure continuum spectrum. This indicates a cessation of magnetospheric accretion activity. The near-infrared colors have become redder as Gaia19bey has faded, indicating a cooling of the continuum component. Near the end of the outburst, the only remaining strong emission lines are forbidden shock-excited emission lines. Adaptive optics integral-field spectroscopy shows the H2 1–0 S(1) emission with the morphology of an outflow cavity and the extended emission in the [Fe ii] line at 1644 nm with the morphology of an edge-on disk. However, we do not detect any large-scale jet from Gaia19bey.

VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud

The process of massive star (M ≥ 8 M⊙) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud. We took advantage of the spectral resolution and wavelength coverage of X-shooter (300−2500 nm), which is mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect Hα, β, [O I] 630.0 nm, Ca II, infrared triplet [Fe II] 1643.5 nm, fluorescent Fe II1687.8 nm, H22121.8 nm, Brγ, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of ten candidates as bona fide MYSOs. We compared our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We computed lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Brγluminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features, such as fluorescent Fe IIand CO bandheads, which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.

The GRAVITY young stellar object survey

Context. The inner regions of the discs of high-mass young stellar objects (HMYSOs) are still poorly known due to the small angular scales and the high visual extinction involved. Aims. We deploy near-infrared spectro-interferometry to probe the inner gaseous disc in HMYSOs and investigate the origin and physical characteristics of the CO bandhead emission (2.3–2.4 μm). Methods. We present the first GRAVITY/VLTI observations at high spectral (ℛ = 4000) and spatial (mas) resolution of the CO overtone transitions in NGC 2024 IRS 2. Results. The continuum emission is resolved in all baselines and is slightly asymmetric, displaying small closure phases (≤8°). Our best ellipsoid model provides a disc inclination of 34° ±1°, a disc major axis position angle (PA) of 166° ± 1°, and a disc diameter of 3.99 ± 0.09 mas (or 1.69 ± 0.04 au, at a distance of 423 pc). The small closure phase signals in the continuum are modelled with a skewed rim, originating from a pure inclination effect. For the first time, our observations spatially and spectrally resolve the first four CO bandheads. Changes in visibility, as well as differential and closure phases across the bandheads are detected. Both the size and geometry of the CO-emitting region are determined by fitting a bidimensional Gaussian to the continuum-compensated CO bandhead visibilities. The CO-emitting region has a diameter of 2.74±0.070.08 mas (1.16 ± 0.03 au), and is located in the inner gaseous disc, well within the dusty rim, with inclination and PA matching the dusty disc geometry, which indicates that both dusty and gaseous discs are coplanar. Physical and dynamical gas conditions are inferred by modelling the CO spectrum. Finally, we derive a direct measurement of the stellar mass of M* ∼ 14.7−3.6+2 M⊙ by combining our interferometric and spectral modelling results.

The New EXor Outburst of ESO-Hα 99 Observed by Gaia ATLAS and TESS

Abstract We report photometry and spectroscopy of the outburst of the young stellar object ESO-Hα99. The outburst was first noticed in Gaia alert Gaia18dvc and later by the Asteroid Terrestrial-impact Last Alert System (ATLAS). We have established the outburst light curve with archival ATLAS orange filter photometry, Gaia data, new V-band photometry, and J, H, and K s photometry from the Infrared Imaging System (IRIS) and the United Kingdom Infrared Telescope (UKIRT). The brightness has fluctuated several times near the light curve maximum. The Transiting Exoplanet Survey Satellite (TESS) satellite observed ESO-Hα 99 with high cadence during one of these minor minima and found brightness fluctuations on timescales of days and hours. Imaging with UKIRT shows the outline of an outflow cavity, and we find one knot of emission, now named MHO 1520, on the symmetry axis of this nebula, indicating recent collimated outflow activity from ESO-Hα 99. Its pre-outburst SED shows a flat far-infrared spectrum, confirming its early evolutionary state and its similarity to other deeply embedded objects in the broader EXor class. The pre-outburst luminosity is 34 L ⊙, a much higher luminosity than typical EXors, indicating that ESO-Hα 99 may be a star of intermediate mass. Infrared and optical spectroscopy show a rich emission-line spectrum, including H i lines, strong red Ca ii emission, as well as infrared CO bandhead emission, all characteristic EXors in the broadest sense. Comparison of the present spectra with an optical spectrum obtained in 1993, presumably in the quiescent state of the object, shows that during the present outburst the continuum component of the spectrum has increased notably more than the emission lines. The Hα equivalent width during the outburst is down to one-half of its 1993 level, and shock-excited emission lines are much less prominent.

Exploring the dimming event of RW Aurigae A through multi-epoch VLT/X-shooter spectroscopy

Context. RW Aur A is a classical T Tauri star that has suddenly undergone three major dimming events since 2010. The reason for these dimming events is still not clear. Aims. We aim to understand the dimming properties, examine accretion variability, and derive the physical properties of the inner disc traced by the CO ro-vibrational emission at near-infrared wavelengths (2.3 μm). Methods. We compared two epochs of X-shooter observations, during and after the dimming. We modelled the rarely detected CO bandhead emission in both epochs to examine whether the inner disc properties had changed. The spectral energy distribution was used to derive the extinction properties of the dimmed spectrum and compare the infrared excess between the two epochs. Lines tracing accretion were used to derive the mass accretion rate in both states. Results. The CO originates from a region with physical properties of T = 3000 K, NCO = 1 × 1021 cm−2 and vk sin i = 113 km s−1. The extinction properties of the dimming layer were derived with the effective optical depth ranging from τeff ~2.5−1.5 from the UV to the near-IR. The inferred mass accretion rate Ṁacc is ~1.5 × 10−8 M⊙ yr−1 and ~2 × 10−8 M⊙ yr−1 after and during the dimming respectively. By fitting the spectral energy distribution, additional emission is observed in the infrared during the dimming event from dust grains with temperatures of 500–700 K. Conclusions. The physical conditions traced by the CO are similar for both epochs, indicating that the inner gaseous disc properties do not change during the dimming events. The extinction curve is flatter than that of the interstellar medium, and large grains of a few hundred microns are thus required. When we correct for the observed extinction, the mass accretion rate is constant in the two epochs, suggesting that the accretion is stable and therefore does not cause the dimming. The additional hot emission in the near-IR is located at about 0.5 au from the star and is not consistent with an occulting body located in the outer regions of the disc. The dimming events could be due to a dust-laden wind, a severe puffing-up of the inner rim, or a perturbation caused by the recent star-disc encounter.

Unveiling the nature of candidate high-mass young stellar objects in the Magellanic Clouds with near-IR spectroscopy

As nearby neighbors to the Milky Way, the Large and Small Magellanic Clouds (LMC and SMC) provide a unique opportunity to study star formation in the context of their galactic ecosystems. Thousands of young stellar objects (YSOs) have been characterized with large-scale Spitzer and Herschel surveys. In this paper, we present new near-IR spectroscopy of five high-mass YSOs in the LMC and one in the SMC. We detect multiple hydrogen recombination lines, as well as He I 2.058 $\mu$m, H$_2$, [Fe II], and [S III] in these highly excited sources. We estimate the internal extinction of each source and find that it is highest for sources with the youngest evolutionary classifications. Using line ratios, we assess the dominant excitation mechanism in the three sources where we detect both H$_2$ 2.12 $\mu$m and [Fe II] 1.64 $\mu$m. In each case, photoexcitation dominates over shock excitation. Finally, we detect CO bandhead absorption in one of our LMC sources. While this feature is often associated with evolved stars, this object is likely young with strong PAH and fine-structure emission lines tracing an H II region detected at longer wavelengths. Compared to high-mass YSOs in the Galaxy, our sources have higher bolometric and line luminosities, consistent with their selection as some of the brightest sources in the LMC and SMC.

High-resolution infrared spectroscopy of field Red Horizontal Branch stars

We obtained the surface abundances of more than 20 elements from H- and K-band observations of three Red Horizontal Branch (RHB) stars. High-resolution (R ≃ 45,000) infrared spectra were obtained using IGRINS (the Immersion Grating Infrared Spectrograph) on the 2.7 m Smith telescope at McDonald Observatory. IGRINS has the capacity of making simultaneous observations with a wavelength coverage of essentially the entire H and K-bands (1.5–2.4 μm). High S/N (>200) IGRINS data serve as a laboratory of molecular bands. A large number of OH, CO and CN molecular bands provided us invaluable information on the abundances of C, N and O, which can be altered as stars evolve from birth to death. OH bands confirmed and greatly strengthened the oxygen abundances previously measured from visible [O I] lines. Carbon and nitrogen abundances were determined from CO and CN molecular bands, respectively. IGRINS also made it possible to achieve more robust abundances of light elements such as Na, Mg, Al, and Si than obtained from their visible counterparts, and allowed S, P and K abundance determination mostly for the first time. Most importantly, we were able to detect 12C/13C isotopic ratios from the CO (2–0) and CO (3–1) band-heads present in the K-band, which give us important clues about the evolutionary status of the stars and mixing processes that take place in the interiors of these RHB He core-burning stars.