Low-mass Young Stellar Objects Research Articles

Submillimeter and Mid-Infrared Variability of Young Stellar Objects in the M17SWex Intermediate-mass Star-forming Region

We present a comprehensive analysis of young stellar object (YSO) variability within the M17 Southwest Extension (M17 SWex), using 3.5 yr of monitoring data from the James Clerk Maxwell Telescope (JCMT) Transient Survey at submillimeter and 9 yr from the NEOWISE mission at mid-infrared (mid-IR). Our study encompasses observations of 147 bright submillimeter peaks identified within our deep JCMT coadded map as well as 156 YSOs in NEOWISE W1 and 179 in W2 that were previously identified in Spitzer surveys. We find three robust submillimeter variables: two are candidate YSOs and one is a likely extragalactic source. At mid-IR wavelengths, our analysis reveals secular and stochastic variability in 47 YSOs, with the highest fraction of secular variability occurring at the earliest evolutionary stage. This is similar to what has previously been observed for low-mass YSO variability within the Gould Belt. However, we observe less overall variability in M17 SWex at both the submillimeter and mid-IR. We suspect that this lower fraction is due to the greater distance to M17 SWex. Our findings showcase the utility of multiwavelength observations to better capture the complex variability phenomena inherent to star formation processes and demonstrate the importance of years-long monitoring of a diverse selection of star-forming environments.

The accretion burst of the massive young stellar object G323.46−0.08

Context. Accretion bursts from low-mass young stellar objects (YSOs) have been known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) have been observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (hereafter masers), which are caused by an increase in exciting mid-infrared (MIR) emission. They can lead to long-lasting thermal afterglows of the dust continuum radiation visible at infrared (IR) and (sub)millimeter (hereafter (sub)mm) wavelengths. Furthermore, they might cause a scattered light echo. The G323.46−0.08 (hereafter G323) event, which shows all these features, extends the small sample of known MYSO bursts. Aims. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling timescales for eruptive MYSOs and to infer the main burst parameters. Methods. Burst light curves, as well as the pre-burst spectral energy distribution (SED) were established from archival IR data. The properties of the MYSO, including its circumstellar disk and envelope, were derived by using static radiative transfer modeling of pre-burst data. For the first time, TDRT was used to predict the temporal evolution of the SED. Observations with SOFIA/HAWC+ were performed to constrain the burst energy from the strength of the thermal afterglow. Image subtraction and ratioing were applied to reveal the light echo. Results. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with a Ks-band increase of ∼2.5 mag. Both Ks-band and integrated maser flux densities follow an exponential decay. TDRT indicates that the duration of the thermal afterglow in the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a flux increase of (14.2 ± 4.6)% at 70 μm and (8.5 ± 6.1)% at 160 μm in 2022 (2 yr after the burst ended). A one-sided light echo emerged that was propagating into the interstellar medium. Conclusions. The burst origin of the G323 maser flare has been verified. TDRT simulations revealed the strong influence of the burst energetics and the local dust distribution on the strength and duration of the afterglow. The G323 burst is probably the most energetic MYSO burst that has been observed so far. Within 8.4 yr, an energy of (0.9−0.8+2.5) × 1047 erg was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least (7−6+20) MJup and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity. The associated IR light echo is the second observed from a MYSO burst.

Accretion Funnel Reconfiguration during an Outburst in a Young Stellar Object: EX Lupi

EX Lupi, a low-mass young stellar object, went into an accretion-driven outburst in 2022 March. The outburst caused a sudden phase change of ∼112° ± 5° in periodically oscillating multiband lightcurves. Our high-resolution spectra obtained with the High Resolution Spectrograph (HRS) on board the Southern African Large Telescope also revealed a consistent phase change in the periodically varying radial velocities (RVs), along with an increase in the RV amplitude of various emission lines. The phase change and increase in RV amplitude morphologically translates to a change in the azimuthal and latitudinal location of the accretion hotspot over the stellar surface, which indicates a reconfiguration of the accretion funnel geometry. Our three-dimensional magnetohydrodynamic simulations reproduce the phase change for EX Lupi. To explain the observations, we explored the possibility of forward shifting of the dipolar accretion funnel as well as the possibility of the emergence of a new accretion funnel. During the outburst, we also found evidence of the hotspot’s morphology extending azimuthally asymmetrically with a leading hot edge and cold tail along the stellar rotation. Further, our high-cadence photometry showed that the accretion flow has clumps. We also detected possible clumpy accretion events in the HRS spectra that showed episodically highly blueshifted wings in the Ca ii IR triplet and Balmer H lines.

Sample and Statistical Analysis on NEOWISE Variability of ATLASGAL Sources

This study focuses on the analysis of mid-infrared variability in a sample of high-mass young stellar objects (YSOs) associated with the cataloged sources from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The Near-Earth Object Wide-field Infrared Survey Explorer Reactivation Mission (NEOWISE) database was used to explore the long-term mid-infrared variability of these high-mass YSOs at a half-year scale. After matching with NEOWISE photometric measurements, a total of 2230 ATLASGAL sources were selected for the variability analysis, out of which 717 were identified as variables. The derived proportions of variables at different evolutionary stages show that the variability rate of high-mass YSOs is highest during the YSO stage and decreases with evolution toward the H ii region stage, resembling the behavior of low-mass YSOs. The variables can be classified into six types based on their light curves, divided into two categories: secular (linear, sin, sin+linear) and stochastic variables (burst, drop, and irregular). The magnitude–color variations observed in ∼160 secular variables can be mainly divided into “bluer when brighter/redder when dimming” and “redder when brighter/bluer when dimming,” likely originating from changes in accretion rate or the effect of extinction due to obscuration. Moreover, several episodic accretion candidates were selected for further observational studies.

New Herbig–Haro objects associated with embedded sources

ABSTRACT We continue to present the results of the Byurakan Narrow Band Imaging Survey (BNBIS). The main goal of this survey is to search for Herbig–Haro (HH) objects and jets in Galactic dark clouds. In this work we present the results of the search in the vicinity of infrared sources that are bright in the WISE survey and embedded in the dark clouds. The survey is performed with the 1-m Schmidt telescope of Byurakan Observatory, lately equipped with a new CCD detector, which allows to obtain one square degree images of the sky in various filters. Narrow-band filters were used to obtain H α and [S ii] images, and a medium-width filter was used for the continuum imaging. New HH flows and knots were found near six embedded IR sources, which constitutes a significant proportion of the objects observed. At least two of the newly found HH flows (HH 1226 and HH 1227) lie in isolated dark clouds, thus pointing to active star formation in these regions. Other flows are also located in detached and dense globules or filaments. The length of the HH 1228 flow is about 1 pc; it has also a molecular hydrogen counterpart of the same extension. Coordinates, charts, detailed descriptions and distance estimates are provided. The lower limits of bolometric luminosities of the source stars are typical for low-mass young stellar objects.

Star Formation in the H ii Region Sh2-87: Evidence of Global Hierarchical Collapse

We present a detailed study of the Sh2-87 H ii region using a multiwavelength data set in optical to radio bands. A Herschel column density map revealed that the host cloud is filamentary in nature, and together they formed a central dense hub. The extinction map generated using near-infrared photometric data also signifies the nonuniform distribution of the cloud and reveals its filamentary nature. We estimated a sizable variable extinction over the region up to A V = 34.4 mag, with an average value of A V = 3.4 mag. Using the various infrared color–color criteria, we identified 13 Class I and 202 Class II young stellar objects (YSOs) and 22 Hα-emitting sources toward this region. Further analysis showed that the cluster is mainly composed of low-mass YSOs with a typical age of ∼3 Myr having masses in the range of 0.1–6.0 M ⊙. The identified evolved YSOs (i.e., Class II YSOs) are primarily distributed along the filaments and in the outer parts of the cloud, while the recent star formation, inferred by the presence of Class I YSOs, ionized gas, and star-forming clumps, is observed in the hub region. The overall star formation scenario in the Sh2-87 region resembles the global hierarchical collapse model of star formation, where younger massive star formation activity is expected at the central hub along with the distribution of evolved low-mass YSOs in the filaments and the outer parts of the cloud.

Multiwavelength detection of an ongoing FUOr-type outburst on a low-mass YSO

ABSTRACT During the pre-main-sequence evolution, Young Stellar Objects (YSOs) assemble most of their mass during the episodic accretion process. The rarely seen FUOr-type events are valuable laboratories to investigate the outbursting nature of YSOs. Here, we present multiwavelength detection of a high-amplitude eruptive source in the young open cluster VdBH 221 with an ongoing outburst, including optical to mid-infrared time series and near-infrared spectra. The initial outburst has an exceptional amplitude of >6.3 mag in Gaia and 4.6 mag in Ks, with a peak luminosity up to 16 L⊙ and a peak mass accretion rate of 1.4 × 10−5 M⊙ yr−1. The optical to infrared spectral energy distribution of this object is consistent with a low-mass star (0.2 M⊙) with a modest extinction (AV < 2 mag). A 100-d delay between optical and infrared rising stages is detected, suggesting an outside-in origin of the instability. The spectroscopic features of this object reveal a self-luminous accretion disc, very similar to FU Orionis, with a low line-of-sight extinction. Most recently, there has been a gradual increase in brightness throughout the wavelength range, possibly suggesting an enhancement of the mass accretion rate.

A probable Keplerian disk feeding an optically revealed massive young star.

The canonical picture of star formation involves disk-mediated accretion, with Keplerian accretion disks and associated bipolar jets primarily observed in nearby, low-mass young stellar objects (YSOs). Recently, rotating gaseous structures and Keplerian disks have been detected around several massive (M > 8 M⊙) YSOs (MYSOs)1-4, including several disk-jet systems5-7. All the known MYSO systems are in the Milky Way, and all are embedded in their natal material. Here we report the detection of a rotating gaseous structure around an extragalactic MYSO in the Large Magellanic Cloud. The gas motion indicates that there is a radial flow of material falling from larger scales onto a central disk-like structure. The latter exhibits signs of Keplerian rotation, so that there is a rotating toroid feeding an accretion disk and thus the growth of the central star. The system is in almost all aspects comparable to Milky Way high-mass YSOs accreting gas from a Keplerian disk. The key difference between this source and its Galactic counterparts is that it is optically revealed rather than being deeply embedded in its natal material as is expected of such a massive young star. We suggest that this is the consequence of the star having formed in a low-metallicity and low-dust content environment. Thus, these results provide important constraints for models of the formation and evolution of massive stars and their circumstellar disks.

A resolved rotating disk wind from a young T Tauri star in the Bok globule CB 26

Context. The disk-outflow connection plays a key role in extracting excess angular momentum from a forming protostar. Although indications of jet rotation have been reported for a few objects, observational constraints of outflow rotation are still very scarce. We have previously reported the discovery of a small collimated molecular outflow from the edge-on T Tauri star-disk system in the Bok globule CB 26 that shows a peculiar velocity pattern, reminiscent of an outflow that corotates with the Keplerian disk. However, we could not ultimately exclude possible alternative explanations for the origin of the observed velocity field. Aims. We report new, high angular resolution millimeter-interferometric observations of CB 26 with the aim of revealing the morphology and kinematics of the outflow at the disk-outflow interface to unambiguously discriminate between the possible alternative explanations for the observed peculiar velocity pattern. Methods. The IRAM PdBI array and the 30 m telescope were used to observe HCO+(1–0) and H13CO+(1–0) at 3.3 mm and 12CO(2–1) at 1.3 mm in three configurations plus zerospacing, resulting in spectral line maps with angular resolutions of 3.″5 and 0.″5, respectively. The SMA was used to observe the HCO+(3–2) line at 1.1 mm with an angular resolution of 1.″35. Additional earlier observations of 13CO(1-0) at 2.7mm with an angular resolution of 1.″0, obtained with OVRO, are also used for the analysis. Using a physical model of the disk, which was derived from the dust continuum emission, we employed chemo-dynamical modeling combined with line radiative transfer calculations to constrain kinematic parameters of the system and to construct a model of the CO emission from the disk that allowed us to separate the emission of the disk from that of the outflow. Results. Our observations confirm the disk-wind nature of the rotating molecular outflow from CB 26 - YSO 1. The new high-resolution data reveal an X-shaped morphology of the CO emission close to the disk, and vertical streaks extending from the disk surface with a small half-opening angle of ≈7°, which can be traced out to vertical heights of ≈500 au. We interpret this emission as the combination of the disk atmosphere and a well-collimated disk wind, of which we mainly see the outer walls of the outflow cone. The decomposition of this emission into a contribution from the disk atmosphere and the disk wind allowed us to trace the disk wind down to vertical heights of ≈40 au, where it is launched from the surface of the flared disk at radii of RL ≈ 20–45 au. The disk wind is rotating with the same orientation and speed as the Keplerian disk and the velocity structure of the cone walls along the flow is consistent with angular momentum conservation. The observed CO outflow has a total gas mass of ≈ 10−3 M⊙, a dynamical age of τdyn ≈ 740 yr, and a total momentum flux of ṖCO ≈ 1.0 × 10−5 M⊙ km s−1 yr−1, which is nearly three orders of magnitude larger than the maximum thrust that can be provided by the luminosity of the central star. Conclusions. We conclude that photoevaporation cannot be the main driving mechanism for this outflow, but it must be predominantly a magnetohydrodynamic disk wind. It is thus far the best-resolved rotating disk wind observed to be launched from a circumstellar disk in Keplerian rotation around a low-mass young stellar object (YSO), albeit also the one with the largest launch radius. It confirms the observed trend that disk winds from Class I YSOs with transitional disks have much larger launch radii than jets ejected from Class 0 protostars.

Investigating the Impact of Metallicity on Star Formation in the Outer Galaxy. I. VLT/KMOS Survey of Young Stellar Objects in Canis Major

The effects of metallicity on the evolution of protoplanetary disks may be studied in the outer Galaxy where the metallicity is lower than in the solar neighborhood. We present the VLT/KMOS integral field spectroscopy in the near-infrared of ∼120 candidate young stellar objects (YSOs) in the CMa-ℓ224 star-forming region located at a Galactocentric distance of 9.1 kpc. We characterize the YSO accretion luminosities and accretion rates using the hydrogen Brγ emission and find a median accretion luminosity of . Based on the measured accretion luminosities, we investigate the hypothesis of star formation history in the CMa-ℓ224. Their median values suggest that Cluster C, where most of YSO candidates have been identified, might be the most evolved part of the region. The accretion luminosities are similar to those observed toward low-mass YSOs in the Perseus and Orion molecular clouds, and they do not reveal the impact of lower metallicity. Similar studies in other outer Galaxy clouds covering a wide range of metallicities are critical to gain a complete picture of star formation in the Galaxy.

Quiescence of an outburst of a low-mass young stellar object: LDN1415-IRS

Quiescence of an outburst of a low-mass young stellar object: LDN1415-IRS

A runaway T Tauri star leaving an extended trail

Aims.We address the problem of young stellar objects that are found too far away from possible star formation sites. Different mechanisms have been proposed before to explain this unexpected circumstance. The idea of high-velocity protostars is one of these mechanisms, although observational support is not always easy to obtain. We aim to shed light on this issue after the serendipitous discovery of a related stellar system.Methods.Following the inspection of archival infrared data, a peculiar anonymous star was found that apparently heads a long tail that resembles a wake-like feature. We conducted a multiwavelength analysis including photometry, astrometry, and spectroscopy. Together with theoretical physical considerations, this approach provided a reasonable knowledge of the stellar age and kinematic properties, together with compelling indications that the extended feature is indeed the signature of a high-velocity, or runaway, newborn star.Results.Our main result is the discovery of a low-mass young stellar object that fits the concept of a runaway T Tauri star that was hypothesized several decades ago. In this peculiar star, nicknamed UJT-1, the interaction of the stellar wind with the surrounding medium becomes extreme. Under reasonable assumptions, this unusual degree of interaction has the potential to encode the mass-loss history of the star on timescales of several ∼105years.

Evidence of high-mass star formation through multiscale mass accretion in hub-filament-system clouds

ABSTRACT We present a statistical study of a sample of 17 hub-filament-system (HFS) clouds of high-mass star formation using high-angular resolution (∼1–2 arcsec) ALMA 1.3 and 3 mm continuum data. The sample includes eight infrared (IR)-dark and nine IR-bright types, which correspond to an evolutionary sequence from the IR-dark to IR-bright stage. The central massive clumps and their associated most massive cores are observed to follow a trend of increasing mass (M) and mass surface density (Σ) with evolution from the IR-dark to IR-bright stage. In addition, a mass-segregated cluster of young stellar objects (YSOs) are revealed in both IR-dark and IR-bright HFSs with massive YSOs located in the hub and the population of low-mass YSOs distributed over larger areas. Moreover, outflow feedback in all HFSs are found to escape preferentially through the inter-filamentary diffuse cavities, suggesting that outflows would render a limited effect on the disruption of the HFSs and ongoing high-mass star formation therein. From the above observations, we suggest that high-mass star formation in the HFSs can be described by a multiscale mass accretion/transfer scenario, from hub-composing filaments through clumps down to cores, which can naturally lead to a mass-segregated cluster of stars.

Molecules in the peculiar age-defying source IRAS 19312+1950

Context. IRAS 19312+1950 is an isolated infrared source that exhibits a characteristic quasi-point-symmetric morphology in the near-and mid-infrared images and is also very bright in molecular radio lines. Because of its unique observational characteristics, various observational studies have been conducted and several hypotheses have been proposed regarding its origin, which is still unclear. So far, it has been suggested that it could be a peculiar evolved star, a young stellar object, or even a red nova remnant. Regardless of which type of object it is ultimately classified as, IRAS 19312+1950 is exceptionally bright in the infrared and molecular radio lines and therefore will undoubtedly be crucial as a prototype of this kind of object having a peculiar nature or unusual evolutionary phase. Aims. This study aims to reveal the molecular composition of the central part of IRAS 19312+1950 by performing an unbiased molecular radio line survey and discussing the origin of the object from a molecular chemical point of view. Methods. We carried out a spectral line survey with the Institut de Radioastronomie Millimtrique 30 m telescope towards the centre of IRAS 19312+1950 in the 3 and 1.3 mm windows with a frequency coverage of 83.9–91.8 and 218.2–226.0 GHz, respectively. Results. In total, 28 transition lines of 22 molecular species and those isotopologues are detected towards IRAS 19312+1950, some of which exhibit a broad and a narrow component. Seventeen thermal lines and one maser line are newly detected. The molecular species of C17O, 30SiO, HN13C, HC18O+, H2CO, and c-C3H2 are detected for the first time in this object. We calculated the optical depths of the transition lines of 13CO, C18O, HCN, H13CN, and C2H, and determined the rotational temperatures, column densities, and fractional abundances of the detected molecules. We got the isotopic ratios of 12C/13C, 14N/15N, 16O/18O, 16O/17O, 18O/17O, 28Si/30Si, and 29Si/30Si in IRAS 19312+1950 and the values were compared to those of evolved stars, red novae, young stellar objects, and the Sun. The intensities of the molecular radio lines of IRAS 19312+1950 were compared with those of different categories of objects, indicating that the spectral pattern of the broad-line region is similar to that of a red nova or a low-mass young stellar object, while the narrow-line region behaves like an envelope of the asymptotic giant branch star. Conclusions. Our results, in combination with previous studies, favour the hypothesis that IRAS 19312+1950 might be a red nova remnant, in which the progenitors that merged to become a red nova may have contained at least two evolved stars with oxygen-rich and carbon-rich chemistry, respectively.

Survey of Ices toward Massive Young Stellar Objects. I. OCS, CO, OCN−, and CH3OH

An important tracer of the origin and evolution of cometary ices is the comparison with ices found in dense clouds and toward young stellar objects (YSOs). We present a survey of ices in the 2–5 μm spectra of 23 massive YSOs, taken with the NASA InfraRed Telescope Facility SpeX spectrometer. The 4.90 μm absorption band of OCS ice is detected in 20 sight lines, more than 5 times the previously known detections. The absorption profile shows little variation and is consistent with OCS embedded in CH3OH-rich ices, and proton-irradiated H2S or SO2-containing ices. The OCS column densities correlate well with those of CH3OH and OCN−, but not with H2O and apolar CO ice. This association of OCS with CH3OH and OCN− firmly establishes their formation location deep inside dense clouds or protostellar envelopes. The median composition of this ice phase toward massive YSOs, as a percentage of H2O, is CO:CH3OH:OCN−:OCS = 24:20:1.53:0.15. CS, due to its low abundance, is likely not the main precursor to OCS. Sulfurization of CO is likely needed, although the source of this sulfur is not well constrained. Compared to massive YSOs, low-mass YSOs and dense clouds have similar or somewhat lower CO and CH3OH ice abundances, but less OCN− and more apolar CO, while OCS awaits detection. Comets tend to be underabundant in carbon-bearing species, but this does not appear to be the case for OCS, perhaps signalling OCS production in protoplanetary disks.

Infrared spectra of complex organic molecules in astronomically relevant ice mixtures

Context. The increasing sensitivity and resolution of ground-based telescopes have enabled the detection of gas-phase complex organic molecules (COMs) across a variety of environments. Many of the detected species are expected to form on the icy surface of interstellar grains and transfer later into the gas phase. Therefore, icy material is regarded as a primordial source of complex molecules in the interstellar medium. Upcoming James Webb Space Telescope (JWST) observations of interstellar ices in star-forming regions will reveal infrared (IR) features of frozen molecules with unprecedented resolution and sensitivity. To identify COM features in the JWST data, laboratory IR spectra of ices for conditions that simulate interstellar environments are needed. Aims. This work provides laboratory mid-IR spectra of methyl cyanide (CH3CN, also known as acetonitrile) ice in its pure form and mixed with known interstellar molecules at cryogenic temperatures. The spectroscopic data presented in this work will support the interpretation of JWST ice observations and are made available to the community through the Leiden Ice Database for Astrochemistry (LIDA). Methods. Fourier transform IR spectroscopy is used to record the mid-IR spectra (500–4000 cm−1/20–2.5 µm, with a resolution of 1 cm−1 ) of methyl cyanide (acetonitrile, CH3 CN) mixed with H2O, CO, CO2, CH4, NH3, H2O:CO2, and H2O:CH4:NH3, at temperatures ranging from 15 to 150 K. The refractive index (at 632.8 nm) of pure amorphous CH3CN ice at 15 K and the band strength of selected IR transitions are also measured. Results. We present a variety of reference mid-IR spectra of frozen CH3CN that can be compared to astronomical ice observations. The peak position and full width at half maximum (FWHM) of six absorption bands of frozen methyl cyanide in its pure form and mixed ices, at temperatures between 15–150 K, are characterized. These bands are the following: the CH3 symmetric stretching at 2940.9 cm−1 (3.400 µm), the CN stretching at 2252.2 cm−1 (4.440 µm), a peak resulting from a combination of different vibrational modes at 1448.3 cm−1 (6.905 µm), the CH3 antisymmetric deformation at 1410 cm−1 (7.092 µm), the CH3 symmetric deformation at 1374.5 cm−1 (7.275 um), and the CH3 rock vibration at 1041.6 cm−1 (9.600 um). Additionally, the apparent band strength of these vibrational modes in mixed ices is derived. The laboratory spectra of CH3CN are compared to observations of interstellar ices toward W33A and three low-mass Young Stellar Objects (YSO). Since an unambiguous identification of solid methyl cyanide toward these objects is not possible, upper limits for the CH3CN column density are determined as ≤2.4 × 1017 molecules cm−2 for W33A and 5.2 × 1016, 1.9 × 1017, and 3.8 × 1016 molecules cm−2 for EC92, IRAS 03235, and L1455 IRS3, respectively. With respect to solid H2O, these values correspond to relative abundances of 1.9, 3.1, 1.3, and 4.1%, for W33A, EC92, IRAS 03235, and L1455 IRS3, respectively.

Accretion variability from minute to decade timescales in the classical T Tauri star CR Cha

Context. Classical T Tauri stars are pre-main-sequence stars that are surrounded by a circumstellar disk from which they accrete material. The mass accretion process is essential in the formation of Sun-like stars. Although often described with simple and static models, the accretion process is inherently time variable. Aims. We examine the accretion process of the low-mass young stellar object CR Cha on a wide range of timescales from minutes to a decade by analyzing both photometric and spectroscopic observations from 2006, 2018, and 2019. Methods. We carried out a period analysis of the light curves of CR Cha from the TESS mission and the ASAS-SN and the ASAS-3 databases. We studied the color variations of the system using I, J, H, K-band photometry obtained contemporaneously with the TESS observing window. We analyzed the amplitude, timescale, and the morphology of the accretion tracers found in a series of high-resolution spectra obtained in 2006 with the AAT/UCLES, in 2018 with the HARPS, and in 2019 with the ESPRESSO and the FEROS spectrographs. Results. All photometric data reveal periodic variations compatible with a 2.327-day rotational period. In addition, the ASAS-SN and ASAS-3 data indicate a long-term brightening by 0.2 mag between 2001 and 2008, and a slightly lower brightening than 0.1 mag in the 2015–2018 period. The near-infrared photometry indicates a short-term brightening trend during the observations in 2019. The corresponding color variations can be explained either by a changing accretion rate or changes in the inner disk structure. The Hα line profile variability suggests that the amplitude variations of the central peak, likely due to accretion, are most significant on daily or hourly timescales. On yearly timescales, the line morphology also changes significantly. Conclusions. The photometric variability shows that the period of about 2.3 days is stable in the system over decades. Our results show that the amplitude of the variations in the Hα emission increases on timescales from hours to days or weeks, after which it remains similar even at decadal timescales. On the other hand, we found significant morphological variations on yearly or decadal timescales, indicating that the different physical mechanisms responsible for the line profile changes, such as accretion or wind, are present to varying degrees at different times.

Accretion and outflows in young stars with CUBES

The science case on studies of accretion and outflows in low-mass (<1.5M_{odot }) young stellar objects (YSOs) with the new CUBES instrument is presented. We show the need for a high-sensitivity, near-ultraviolet (NUV) spectrograph like CUBES, with a resolving power at least four times that of X-Shooter and combined with UVES via a fibrelink for simultaneous observations. Simulations with the CUBES exposure time calculator and the end-to-end software show that a significant gain in signal-to-noise can be achieved compared to current instruments, for both the spectral continuum and emission lines, including for relatively embedded YSOs. Our simulations also show that the low-resolution mode of CUBES will be able to observe much fainter YSOs (V sim22 mag) in the NUV than we can today, allowing us extend studies to YSOs with background-limited magnitudes. The performance of CUBES in terms of sensitivity in the NUV will provide important new insights into the evolution of circumstellar disks, by studying the accretion, jets/winds and photo-evaporation processes, down to the low-mass brown dwarf regime. CUBES will also open-up new science as it will be able to observe targets that are several magnitudes fainter than those reachable with current instruments, facilitating studies of YSOs at distances of sim kpc scale. This means a step-change in the field of low-mass star formation, as it will be possible to expand the science case from relatively local star-forming regions to a large swathe of distances within the Milky Way.

Tracing episodic accretion with NOEMA: FU Orionis-type stars and their millimeter environment

The earliest phases of star formation are characterised by intense mass accretion from the circumstellar disk to the central star. One group of lowmass young stellar objects, FU Orionis-type stars (FUors) exhibit accretion rate peaks accompanied by bright eruptions in the optical and infrared regime. The occurance of these outbursts might solve the luminosity problem of protostars, play a key role in accumulating the final stellar mass, and have a significant effiect on the parameters of the envelope and the disk. We are performing a systematic investigation of FUors with millimeter interferometry using NOEMA and ALMA to study the outburst events and examine whether FUors represent normal young stars in exceptional times or are unusual objects. The targeted FUors show very diverse circumstellar morphologies with envelope parameters similar to those of both Class I and Class II systems, but their disks are more massive and more compact than T Tauri disks. To shed light onto the process of disk-formation, accretion, and to what role FUors play in low-mass starformation, we require the identification and light curve monitoring of as many of these stars as possible, together with the multi-wavelength and multi-scale mapping of their circumstellar environment.

Tying the Geometrical Traits of Massive Young Stellar Objects and Their Disks to a Potential Evolutionary Sequence Using Infrared Observations

Abstract Young massive stars influence their surroundings from local to galactic scales, but the observational challenges associated with their distance and embedded nature has, until the recent decade, made high-resolution studies of these objects difficult. In particular, comparative analyses of massive young stellar object (MYSO) disks are currently lacking and our understanding of their evolution is limited. Here, we combine the results of two studies with the aim to attribute geometrical features to an evolutionary sequence for a sample of seven MYSOs. The time evolution is based on near-infrared spectral features, while the geometry is determined from a multi-size-scale study of MYSOs. We find that MYSO disks with determined geometrical substructure turn out to be also spectroscopically more evolved. This implies that disk evolution and dispersal occur within MYSOs similarly to low-mass young stellar object (YSO) disk evolution, despite their faster formation timescales.