In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over five magnitudes for a period of 7 months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter we present new optical and near-IR high resolution spectroscopy of EX~Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the energetics and kinematics observed. Emission line strengths, widths, and profiles changed significantly between the two observations. Also, modeling of the 2.2935 um CO overtone bandhead emission suggests that an inner gap in the circumstellar gas disk around the star may be present and it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ~2+-0.5~Lsun and ~2+-0.5x10^-7 Msun yr^-1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.
EX Lupi is the prototype for a class of young, pre-main sequence stars which are observed to undergo irregular, presumably accretion-generated, optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra ACIS ToO observations shortly thereafter. We find very strong evidence that most of the X-ray emission in the first few months after the optical outburst is generated by accretion of circumstellar material onto the stellar photosphere. Specifically, we find a strong correlation between the decreasing…
We report monitoring observations of the T Tauri star EX Lupi during its outburst in 2008 in the CO fundamental band at 4.6-5.0 um. The observations were carried out at the VLT and the Subaru Telescope at six epochs from April to August 2008, covering the plateau of the outburst and the fading phase to a quiescent state. The line flux of CO emission declines with the visual brightness of the star and the continuum flux at 5 um, but composed of two subcomponents that decay with different rates. The narrow line emission (50 km s-1 in FWHM) is near the systemic velocity of EX Lupi. These emission…
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables, during its quiescence and outburst phases. We analyse high-resolution optical spectra taken before, during, and after its 2008 outburst. In quiescence and outburst, the star presents many permitted emission lines, including typical CTTS lines and numerous neutral and ionized metallic lines. During the outburst, the number of emission lines increases to over a thousand, with narrow plus broad component structure (NC+BC). The BC profile is highly variable on short timescales (24-72h). An active chromosphere can explain …
Extreme outbursts in young stars may be a common stage of pre-main-sequence stellar evolution. These outbursts, caused by enhanced accretion and accompanied by increased luminosity, can also strongly impact the evolution of the circumstellar environment. We present ALMA and VLA observations of EX Lupi, a prototypical outburst system, at 100 GHz, 45 GHz, and 15 GHz. We use these data, along with archival ALMA 232 GHz data, to fit radiative transfer models to EX Lupi's circumstellar disk in its quiescent state following the extreme outburst in 2008. The best fit models show a compact disk with a…
We discuss ASASSN-13db, an EX Lupi-type ("EXor") accretion event on the young stellar object (YSO) SDSS J051011.01$-$032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution (SED) and find that it is consistent with a low-mass class II YSO near the Orion star forming region ($d \sim 420$ pc). We present follow-up photometric and spectroscopic observations of the source after the $ΔV \sim-$5.4 magnitude outburst that began in September 2013 and ended i…
EX Lup is the prototype of the EXor class of young eruptive stars: objects showing repetitive brightenings due to increased accretion from the circumstellar disk to the star. In this paper, we report on medium-resolution near-infrared spectroscopy of EX\,Lup taken during its extreme outburst in 2008, as well as numerical modeling with the aim of determining the physical conditions around the star. We detect emission lines from atomic hydrogen, helium, and metals, as well as first overtone bandhead emission from carbon monoxide. Our results indicate that the emission lines are originating from …
We present a comparison of archival Spitzer spectra of the strongly variable T Tauri EX Lupi, observed before and during its 2008 outburst. We analyze the mid-infrared emission from gas-phase molecules thought to originate in a circumstellar disk. In quiescence the emission shows a forest of H2O lines, highly excited OH lines, and the Q branches of the organics C2H2, HCN, and CO2, similar to the emission observed toward several T Tauri systems. The outburst emission shows instead remarkable changes: H2O and OH line fluxes increase, new OH, H2, and HI transitions are detected, and organics are …
The FU Orionis (FUor) or EX Orionis (EXor) phenomenon has attracted increasing attention in recent years and is now accepted as a crucial element in the early evolution of low-mass stars. FUor and EXor eruptions of young stellar objects (YSOs) are caused by strongly enhanced accretion from the surrounding disk. FUors display optical outbursts of $\sim$ 4 mag or more and last for several decades, whereas EXors show smaller outbursts ($Δ$m $\sim$ 2 - 3 mag) that last from a few months to a few years and may occur repeatedly. Therefore, FUor/EXor eruptions represent a rare but very important phen…
Context: It is hypothesized that low-mass young stellar objects undergo eruptive phases during their early evolution. The outburst of V1647 Ori between 2003 and 2006 offered a rare opportunity to investigate such an accretion event. Aims: By means of our interferometry observing campaign during this outburst, supplemented by other observations, we investigate the temporal evolution of the inner circumstellar structure of V1647 Ori We also study the role of the changing extinction in the brightening of the object and separate it from the accretional brightening. Methods: We observed V1647 Ori w…
We present follow-up photometric and spectroscopic observations, and subsequent analysis of Gaia20eae. This source triggered photometric alerts during 2020 after showing a $\sim$3 mag increase in its brightness. Its Gaia Alert light curve showed the shape of a typical eruptive young star. We carried out observations to confirm Gaia20eae as an eruptive young star and classify it. Its pre-outburst spectral energy distribution shows that Gaia20eae is a moderately embedded Class II object with $L_\mathrm{bol} = 7.22$ L$_\odot$. The color-color and color-magnitude diagrams indicate that the evoluti…
V899 Mon is an eruptive young star showing 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 multi-filter 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, u…
FU Orionis-type objects (FUors) are young low-mass stars undergoing powerful accretion outbursts. The increased accretion is often accompanied by collimated jets and energetic, large-scale molecular outflows. The extra heating during the outburst may also induce detectable geometrical, chemical, and mineralogical changes in the circumstellar material, affecting possible planet formation around these objects. V346 Nor is a southern FUor with peculiar spectral characteristics. Decades after the beginning of its outburst, it unexpectedly underwent a fading event around 2010 due to a decrease in t…
EX Lupi-type objects (EXors) form a sub-class of T Tauri stars, defined by sudden sporadic flare-ups of 1-5 magnitudes at optical wavelengths. These eruptions are attributed to enhanced mass accretion from the circumstellar disk to the star, and may constitute important events in shaping the structure of the inner disk and the forming planetary system. Although disk properties must play a fundamental role in driving the outbursts, they are surprisingly poorly known. In order to characterize the dust and gas components of EXor disks, here we report on observations of the $^{12}$CO J=3-2 and 4-3…
Context. In August 2010, the sudden optical brightening of two young stellar objects, located in the North America/Pelican Nebula Complex, was announced. Early observations indicated that these objects may belong to the FUor or EXor class of young eruptive stars. The eruptions of FUors and EXors are often explained by enhanced accretion of material from the circumstellar disk to the protostar. Aims. In order to determine the true nature of these two objects, we started an optical and near-infrared monitoring program, and complemented our data with archival observations and data from the litera…
V582 Aur is a pre-main sequence FU Orionis type eruptive star, which entered a brightness minimum in 2016 March due to changes in the line-of-sight extinction. Here, we present and analyze new optical $B$, $V$, $R_C$ and $I_C$ band multiepoch observations and new near-infrared $J$, $H$ and $K_S$ band photometric measurements from 2018 January$-$2019 February, as well as publicly available mid-infrared WISE data. We found that the source shows a significant optical$-$near-infrared variability, and the current brightness minimum has not completely finished yet. If the present dimming originates …
We present here new results of two-dimensional hydrodynamical simulations of the eruptive events of the 1840s (the great) and the 1890s (the minor) eruptions suffered by the massive star $η$ Car. The two bipolar nebulae commonly known as the Homunculus and the little Homunculus were formed from the interaction of these eruptive events with the underlying stellar wind. As in previous work (Gonzalez et al. 2004a, 2004b), we assume here an interacting, nonspherical multiple-phase wind scenario to explain the shape and the kinematics of both Homunculi, but adopt a more realistic parametrization of…
We present results from an optical imaging and spectroscopic monitoring campaign on the young, low-mass eruptive variable star V1647 Orionis. The star and associated nebulosity (McNeil's Nebula) were observed over the period February 2004 to February 2006 with observations commencing a few months after the original outburst event occurred. Using the Gemini North telescope, we obtained multi-band optical imaging photometry and medium-resolution long-slit spectroscopy of V1647 Ori on an approximately monthly interval. During this period, V1647 Ori remained at, or close to, peak brightness and th…
V1647 Ori is a young eruptive variable star, illuminating a reflection nebula (McNeil's Nebula). It underwent an outburst in 2003 before fading back to its pre-outburst brightness in 2006. In 2008, V1647 Ori underwent a new outburst. The observed properties of the 2003-2006 event are different in several respects from both the EXor and FUor type outbursts, and suggest that this star might represent a new class of eruptive young stars, younger and more deeply embedded than EXors, and exhibiting variations on shorter time scales than FUors. In outburst, the star lights up the otherwise invisible…
Gaia19fct is one of the Gaia-alerted eruptive young stars that has undergone several brightening events. We conducted monitoring observations using multi-filter optical and near-infrared photometry, as well as near-infrared spectroscopy, to understand the physical properties of Gaia19fct and investigate whether it fits into the historically defined two classes. We present the analyses of light curves, color variations, spectral lines, and CO modeling. The light curves show at least five brightening events since 2015, and the multi-filter color evolutions are mostly gray. The gray evolution ind…
Only a dozen confirmed FU Orionis-type young outbursting stars (FUors) are known today; this explains the interest in the recent FUor eruption of 2MASS J06593158-0405277. Its outburst and expected decline will be subject to numerous studies in the future. Almost equally important for the understanding of the eruption mechanism, however, is the physical characterization of the FUor's precursor. Here we analyze unpublished archival data and summarize - and partly revise - all relevant photometry from optical to submillimeter wavelengths. Our analysis implies that the FUor is possibly associated …
We report on the discovery of three candidate eruptive young stars, found during our comprehensive multi-wavelength study of the young stellar population of the dark cloud L1340. These stars are as follows. (1) IRAS 02224+7227 (2MASS 02270555+7241167, HH 487 S) exhibited FUor-like spectrum in our low-resolution optical spectra. The available photometric data restrict its luminosity to 23 L_sun < L_bol < 59 L_sun. (2) 2MASS 02263797+7304575, identified as a classical T Tauri star during our H alpha survey, exhibited an EXor type brightening in 2005 November, at the time of the SDSS observations…
Young stars exhibit variability due to changes in the gas accretion rate onto them, an effect that should be quite significant in the early stages of their formation. As protostars are embedded within their natal cloud, this variability may only be inferred through long wavelength observations. We perform radiative transfer simulations of young stellar objects (YSOs) formed in hydrodynamical simulations, varying the structure and luminosity properties in order to estimate the long-wavelength, sub-mm and mm, variations of their flux. We find that the flux increase due to an outburst event depen…
V582 Aur is an FU Ori-type young eruptive star in outburst since $\sim$1985. The eruption is currently in a relatively constant plateau phase, with photometric and spectroscopic variability superimposed. Here we will characterize the progenitor of the outbursting object, explore its environment, and analyse the temporal evolution of the eruption. We are particularly interested in the physical origin of the two deep photometric dips, one occurred in 2012, and one is ongoing since 2016. We collected archival photographic plates, and carried out new optical, infrared, and millimeter wave photomet…
Aims. We report the discovery of a strong outburst of the embedded young stellar object (YSO), namely UKIDSS-J185318.36+012454.5, located in the star-forming region associated with IRAS 18507+0121 source and GAL 034.4+00.23 HII region. Methods. Using the archival photometric data and images we determined the amplitude and the epoch of the outburst, as well as the evolution stage and the basic parameters of the object. Results. According to the near and mid-infrared colors and spectral energy distribution, we classify the object as an intermediate-mass young stellar object (YSO) with Class 0/I …
EX Lup is the prototype of a class of pre-main sequence eruptive stars defined by their repetitive outbursts lasting several months. In 2008 January-September EX Lup underwent its historically largest outburst, brightening by about 4 magnitudes in visual light. In previous studies we discovered on-going silicate crystal formation in the inner disk during the outburst, but also noticed that the measured crystallinity fraction started decreasing after the source returned to the quiescent phase. Here we present new observations of the 10 $μ$m silicate feature, obtained with the MIDI and VISIR ins…
We present Gemini-North optical and near-IR observations of a young eruptive star in Cygnus, designated as V2492 Cyg in the General Catalog of Variable Stars. This object is one of two young stars, located within 2 degrees of each other, that recently brightened by around 5 mags and were reported as possible new FU Orionis-type variables. The outburst spectrum of V2492 Cyg shows atomic emission features throughout the optical and near-infrared. In the optical, Halpha is in emission and has an associated blue-shifted absorption component. The far-red CaII triplet lines are also in emission and,…
Eruptive variables are young stars that show episodic variations of brightness: EXors/FUors variations are commonly associated with enhanced accretion outbursts occurring at intermittent cadence of months/years (EXors) and decades/centuries (FUors). Variations that can be ascribed to a variable extinction along their line of sight are instead classified as UXors. We aim at investigating the long-term photometric behaviour of three sources classified as eruptive variables. We present data from the archival plates of the Asiago Observatory relative to the fields where the targets are located. Fo…
Our Solar System originated in interstellar gas and dust; the latter is in the form of amorphous silicate particles and carbonaceous dust. The composition of cometary material shows that a significant fraction of the amorphous silicates was transformed into crystalline form during the early evolution of the protosolar nebula. How and when this transformation happened has been controversial, with the main options being heating by the young Sun or shock heating. Here we report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that were not present in quiescence.…
We present optical and near-IR observations of the young eruptive variable star V1647 Orionis which illuminates McNeil's Nebula. In late 2003, V1647 Ori was observed to brightened by around 5 mag to r'=17.7. In early 2006 the star faded back to its quiescent brightness of r'~23, however, in mid-2008 it brightened yet again by ~5 mag. Our new observations, taken in early 2011, show V1647 Ori to be in an elevated photometric state with an optical brightness similar to the value found at the start of the 2003 and 2008 outbursts. Optical images taken between 2008 and 2011 suggest that the star has…
EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by outbursts attributed to highly enhanced disk accretion. The trigger mechanism of EXor outbursts is still debated. One theory requires a close (sub)stellar companion that perturbs the inner disk and triggers the onset of the outburst. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of EXors. We conducted a 5-year RV survey with HARPS and FEROS. We analyzed the activity of EX Lup by checking the bisector and several other activity indicators. We complemented the RV data with photometri…
EX Lup is the prototype of EXors, a subclass of low-mass pre-main sequence stars whose episodic eruptions are attributed to temporarily increased accretion. In quiescence the optical and near-infrared properties of EX Lup cannot be distinguished from those of normal T Tau stars. Here we investigate whether it is the circumstellar disk structure which makes EX Lup an atypical Class II object. During outburst the disk might undergo structural changes. Our characterization of the quiescent disk is intended to serve as a reference to study the physical changes related to one of EX Lupi's strongest…
FUors are young stellar objects experiencing large optical outbursts due to highly enhanced accretion from the circumstellar disk onto the star. FUors are often surrounded by massive envelopes, which play a significant role in the outburst mechanism. Conversely, the subsequent eruptions might gradually clear up the obscuring envelope material and drive the protostar on its way to become a disk-only T Tauri star. Here we present an APEX $^{12}$CO and $^{13}$CO survey of eight southern and equatorial FUors. We measure the mass of the gaseous material surrounding our targets. We locate the source…
Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 μm and 2.2 μm from the ground and between 3.6 μm and …
V1118 Ori is an eruptive variable belonging to the EXor class of Pre-Main Sequence stars whose episodic outbursts are attributed to disk accretion events. Since 2006, V1118 Ori is in the longest quiescence stage ever observed between two subsequent outbursts of its recent history. We present near-infrared photometry of V1118 Ori carried out during the last eight years, along with a complete spectroscopic coverage from 0.35 to 2.5 um. A longterm sampling of V1118 Ori in quiescence has never been done, hence we can benefit from the current circumstance to determine the lowest values (i.e. the ze…
We present an analysis of the optical observations of Herbig AeBe star V1686 Cyg, which is associated with a small isolated star-forming region around HAeBe star BD+40$^{\circ}$4124. We observed this star as a part of our project of young eruptive stars investigation. Observations were held on 2.6m telescope of Byurakan Observatory from 2015 to 2017. For this period we obtained V1686 Cyg direct images and 14 medium- and low-resolution spectra. In the course of observations we noticed that this star underwent a not-typical brightness outburst. After data reduction we found that the full rise an…
We present the discovery of 816 high amplitude infrared variable stars ($ΔK_{\rm s} >$ 1 mag) in 119 deg$^2$ of the Galactic midplane covered by the Vista Variables in the Via Lactea (VVV) survey. Almost all are new discoveries and about 50$\%$ are YSOs. This provides further evidence that YSOs are the commonest high amplitude infrared variable stars in the Galactic plane. In the 2010-2014 time series of likely YSOs we find that the amplitude of variability increases towards younger evolutionary classes (class I and flat-spectrum sources) except on short timescales ($<$25 days) where this tren…
We present an optical/near-IR survey of 11 variable young stars (EXors and EXor candidates) aimed at deriving and monitoring their accretion properties. About 30 optical and near-infrared spectra ($\Re$ $\sim$ 1500-2000) were collected between 2014-2019 with the Large Binocular Telescope (LBT). From the spectral analysis we have derived the accretion luminosity and mass accretion rate, the visual extinction, the temperature and density of the permitted line formation region, and the signature of the outflowing matter. Two sources (ASASSN-13db and iPTF15afq) have been observed in outburst and q…
During 2017, when the Sun was moving toward the minimum phase of solar cycle 24, an exceptionally eruptive active region (AR) NOAA 12673 emerged on the Sun during August 28-September 10. During the highest activity level, the AR turned into a delta-type sunspot region, which manifests the most complex configuration of magnetic fields from the photosphere to the coronal heights. The AR 12673 produced four X-class and 27 M-class flares, along with numerous C-class flares, making it one of the most powerful ARs of solar cycle 24. Notably, it produced the largest flare of solar cycle 24, namely, t…
The known FU Ori-type young eruptive stars are exceedingly rare (a dozen or so confirmed objects) and 2MASS J06593158-0405277, with its 2014 outburst, is likely the latest addition to the family. All members have displayed just one such eruption in their recorded history, an event lasting for decades. To test the FU Ori nature of 2MASS J06593158-0405277, we have reconstructed its photometric history by measuring its brightness on Harvard photographic plates spanning the time interval 1899-1989. No previous large amplitude eruption similar to that initiated in 2014 has been found, as in bona fi…
We present the results of the monitoring campaign of the Type IIn supernova (SN) 2018cnf (aka ASASSN-18mr). It was discovered about 10 days before the maximum light (on MJD = 58293.4+-5.7 in the V band, with MV = -18.13+-0.15 mag). The multiband light curves show an immediate post-peak decline with some minor luminosity fluctuations, followed by a flattening starting about 40 days after maximum. The early spectra are relatively blue and show narrow Balmer lines with P Cygni profiles. Additionally, Fe II, O I, He I and Ca II are detected. The spectra show little evolution with time, with interm…
EX Lup is a low-mass pre-main sequence star that occasionally shows accretion-related outbursts. Here, we present JWST/MIRI medium resolution spectroscopy obtained for EX Lup fourteen years after its powerful outburst. EX Lup is now in quiescence and displays a Class II spectrum. We detect a forest of emission lines from molecules previously identified in infrared spectra of classical T Tauri disks: H2O, OH, H2, HCN, C2H2, and CO2. The detection of organic molecules demonstrates that they are back after disappearing during the large outburst. Spectral lines from water and OH are for the first …
Decades after the beginning of its FU Orionis-type outburst, V346 Nor unexpectedly underwent a fading event of $ΔK$ = 4.6 mag around 2010. We obtained near-infrared observations and re-analysed data from the VISTA/VVV survey to outline the brightness evolution. In our VLT/NaCO images, we discovered a halo of scattered light around V346~Nor with a size of about 0.04 arcsec (30 au). The VISTA data outlined a well-defined minimum in the light curve at late 2010/early 2011, and tentatively revealed a small-amplitude periodic modulation of 58 days. Our latest data points from 2016 demonstrate that …
The decade-long Vista Variables in the Via Lactea (VVV) survey has detected numerous highly variable young stellar objects (YSOs). We present a study of 61 highly variable VVV YSOs ($ΔK_s$ = 1-5 mag), combining near infrared spectra from Magellan and VLT with VVV and NEOWISE light curves to investigate physical mechanisms behind eruptive events. Most sources are spectroscopically confirmed as eruptive variables (typically Class I YSOs) but variable extinction is also seen. Among them, magnetically controlled accretion, identified by H{\sc i} recombination emission (usually accompanied by CO em…
CK Vulpeculae was observed in outburst in 1670-16721, but no counterpart was seen until 1982, when a bipolar nebula was found at its location. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but a similarity to 'red transients', which are more luminous than classical nova and thought to be the result of stellar collisions, has re-opened the question of CK Vul's status. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust. No stellar source has been seen in CK Vul, though a radio continuum source was identified…
M31-RV was an extraordinarily luminous (~10^6 Lsun) eruptive variable, displaying very cool temperatures (roughly 1000 Kelvins) as it faded. The photometric behavior of M31-RV (and several other very red novae, i.e. luminous eruptive red variables) has led to several models of this apparently new class of astrophysical object. One of the most detailed models is that of "mergebursts": hypothetical mergers of close binary stars. These are predicted to rival or exceed the brightest classical novae in luminosity, but to be much cooler and redder than classical novae, and to become slowly hotter an…
I review recent progress on understanding eruptions of unstable massive stars, with particular attention to the diversity of observed behavior in extragalatic optical transient sources that are generally associated with giant eruptions of luminous blue variables (LBVs). These eruptions are thought to represent key mass loss episodes in the lives of massive stars. I discuss the possibility of dormant LBVs and implications for the duration of the greater LBV phase and its role in stellar evolution. These eruptive variables show a wide range of peak luminosity, decay time, expansion speeds, and p…
An interferometric mid-infrared study of the eruptive star binary Z CMa with MATISSE/VLTI. I. Imaging the protoplanetary disks during the 2023 outburst
