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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 K s, 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 (A V < 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. [ABSTRACT FROM AUTHOR]

We have performed a comprehensive search of a VISTA Variables in the Via Lactea (VVV) data base of 9.5 yr light curves for variable sources with Δ Ks ≥ 4 mag, aiming to provide a large sample of high amplitude eruptive young stellar objects (YSOs) and detect unusual or new types of infrared variable source. We find 222 variable or transient sources in the Galactic bulge and disc, most of which are new discoveries. The sample mainly comprises novae, YSOs, microlensing events, Long Period Variable stars (LPVs), and a few rare or unclassified sources. Additionally, we report the discovery of a significant population of aperiodic late-type giant stars suffering deep extinction events, strongly clustered in the Nuclear Disc of the Milky Way. We suggest that these are metal-rich stars in which radiatively driven mass loss has been enhanced by super-solar metallicity. Among the YSOs, 32/40 appear to be undergoing episodic accretion. Long-lasting YSO eruptions have a typical rise time of ∼2 yr, somewhat slower than the 6–12 month time-scale seen in the few historical events observed on the rise. The outburst durations are usually at least 5 yr, somewhat longer than many lower amplitude VVV events detected previously. The light curves are diverse in nature, suggesting that multiple types of disc instability may occur. Eight long-duration extinction events are seen wherein the YSO dims for a year or more, attributable to inner disc structure. One binary YSO in NGC 6530 displays periodic extinction events (P =59 d) similar to KH 15D. [ABSTRACT FROM AUTHOR]

During the pre-main-sequence (pre-MS) evolution stage of a star, significant amounts of stellar mass are accreted during episodic accretion events, such as multidecade FUor-type outbursts. Here, we present a near-infrared spectroscopic follow-up study of 33 high-amplitude (most with Δ Ks > 4 mag) variable sources discovered by the Vista Variables in the Via Lactea (VVV) survey. Based on the spectral features, 25 sources are classified as eruptive young stellar objects (YSOs), including 15 newly identified FUors, six with long-lasting, but EXor-like bursts of magnetospheric accretion and four displaying outflow-dominated spectra. By examining the photometric behaviours of eruptive YSOs, we found most FUor-type outbursts have higher amplitudes (Δ Ks and Δ W 2), faster eruptive time-scales and bluer infrared colours than the other outburst types. In addition, we identified seven post-MS variables apparently associated with deep dipping events and an eruptive star with deep aluminium monoxide absorption bands resembling those seen in the V838 Mon stellar merger. [ABSTRACT FROM AUTHOR]

The growing number of multi-epoch optical and infrared sky surveys are uncovering unprecedented numbers of new variable stars, of an increasing number of types. The short interval between observations in adjacent near-infrared filters in the UKIDSS Galactic Plane Survey (UGPS) allows for the discovery of variability on the time-scale of minutes. We report on the nature of one such object, through the use of optical spectroscopy, time series photometry, and targeted X-ray observations. We propose that UGPS J194310.32+183851.8 is a magnetic cataclysmic variable star of novel character, probably featuring a longer than average spin period and an orbital period likely to be shorter than the period gap (i.e. P orb < 2 h). We reason that the star is likely a member of the short-period intermediate-polar subclass that exists below this period boundary, but with the additional feature that system's spectral energy distribution is fainter and redder than other members of the group. [ABSTRACT FROM AUTHOR]

We present a catalog of 167 newly discovered, irregular variables spanning a ∼7 deg2 area that encompasses the G 305 star-forming complex, one of the most luminous giant H ii regions in the Galaxy. We aim to unveil and characterize the young stellar object (YSO) population of the region by analyzing the Ks-band variability and JHKs infrared colors from the VISTA Variables in the Vía Láctea survey. Additionally, SDSS-IV APOGEE-2 infrared spectra of selected objects are analyzed. The sample shows relatively high amplitudes (0.661 mag < ΔKS < 3.521 mag). Most of them resemble sources with outbursts with amplitude >1 mag and duration longer than a few days, typically at least a year, known as eruptive variables. About 60% are likely to be Class II/flat/I objects. This is also confirmed by the spectral index α when available. From the analysis of APOGEE-2 near-infrared spectra of sources in the region, another 122 stars are classified as YSOs and display some infrared variability. The measured effective temperature Teff peak is around 4000 K, and they are slightly supersolar in metal abundance. The modal radial velocity is approximately −41 km s−1. Combining available catalogs of YSOs in the region with our data, we investigate the spatial distributions of 700 YSOs. They are clearly concentrated within the central cavity formed by the massive clusters Danks 1 and 2. The calculated surface density for the entire catalog is 0.025 YSOs pc−2, while the central cavity contains 10 times more objects per area (0.238 YSOs pc−2). [ABSTRACT FROM AUTHOR]

We report the discovery of a mid-infrared outburst in a young stellar object (YSO) with an amplitude close to 8 mag at λ ≈ 4.6 μm. WISEA J142238.82−611553.7 is one of 23 highly variable Wide-field Infrared Survey Explorer (WISE) sources discovered in a search of infrared dark clouds (IRDCs). It lies within the small IRDC G313.671−0.309 (d ≈2.6 kpc), seen by the Herschel /Hi-Gal survey as a compact massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest it is a class I YSO, a view supported by observation of weak 2.12 μm H2 emission in an otherwise featureless red continuum spectrum in 2019 (6 mag below the peak in K s). Spitzer, WISE , and VISTA Variables in the Via Lactea (VVV) data show that the outburst began by 2006 and has a duration >13 yr, with a fairly flat peak from 2010 to 2014. The low pre-outburst luminosity implies a low-mass progenitor. The outburst luminosity of a few × 102 L⊙ is consistent with an accretion rate |$\dot{M} \approx 10^{-4}$|  M⊙yr−1, comparable to a classical FU Orionis event. The 4.6 μm peak in 2010 implies T = 800–1000 K and a disc radial location R ≈ 4.5 au for the emitting region. The colour evolution suggests subsequent progression outwards. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear. [ABSTRACT FROM AUTHOR]

High extinction and crowding create a natural limitation for optical surveys towards the central regions of the Milky Way, where the gas and dust are mainly confined. Large-scale near-infrared (IR) surveys of the Galactic plane and bulge are a good opportunity to explore open scientific questions as well as to test our capability to explore future data sets efficiently. Thanks to the VISTA Variables in the Vía Láctea (VVV) ESO public survey, it is now possible to explore a large number of objects in those regions. This paper addresses the variability analysis of all VVV point sources having more than 10 observations in VVVDR4 using a novel approach. In total, the near-IR light curves of 288 378 769 sources were analysed using methods developed in the New Insight Into Time Series Analysis project. As a result, we present a complete sample having 44 998 752 variable star candidates (VVV-CVSC), which include accurate individual coordinates, near-IR magnitudes (Z,Y,J, and Hs), extinctions A (Ks), variability indices, periods, amplitudes, among other parameters to assess the science. Unfortunately, a side effect of having a highly complete sample, is also having a high level of contamination by non-variable (contamination ratio of non-variables to variables is slightly over 10:1). To deal with this, we also provide some flags and parameters that can be used by the community to decrease the number of variable candidates without heavily decreasing the completeness of the sample. In particular, we cross-identified 339 601 of our sources with Simbad and AAVSO data bases, which provide us with information for these objects at other wavelengths. This subsample constitutes a unique resource to study the corresponding near-IR variability of known sources as well as to assess the IR variability related with X-ray and gamma-ray sources. On the other hand, the other |${\sim }99.5{{\ \rm per\ cent}}$| sources in our sample constitutes a number of potentially new objects with variability information for the heavily crowded and reddened regions of the Galactic plane and bulge. The present results also provide an important queryable resource to perform variability analysis and to characterize ongoing and future surveys like TESS and LSST. [ABSTRACT FROM AUTHOR]

We report the detection of variable stars within a |$11.5\, \text{arcmin}\times 11.5\, \text{arcmin}$| region near the Galactic Centre that includes the Arches and Quintuplet clusters, as revealed by the VISTA Variables in the Vía Láctea survey. There are 353 sources that show K S-band variability, of which the large majority (81 per cent) correspond to red giant stars, mostly in the asymptotic giant branch phase. We analyse a population of 52 red giants with long-term trends that cannot be classified into the typical pulsating star categories. Distances and extinctions are calculated for nine Mira variables, and we discuss the impact of the chosen extinction law on the derived distances. We also report the presence of 48 new identified young stellar object candidates in the region. [ABSTRACT FROM AUTHOR]

We present a catalogue of 618 high-amplitude infrared variable stars (1 < ΔK < 5mag) detected by the two widely separated epochs of 2.2 μm data in the UKIDSS Galactic plane survey, from searches covering ~1470 deg2. Most were discovered by a search of all fields at 30 < l < 230°. Sources include new dusty Mira variables, three new cataclysmic variable candidates, a blazar and a peculiar source that may be an interacting binary system. However, ~60 per cent are young stellar obbjects (YSOs), based on spatial association with star-forming regions at distances ranging from 300 pc to over 10 kpc. This confirms our initial result in Contreras Peña et al. (Paper I) that YSOs dominate the high-amplitude infrared variable sky in the Galactic disc. It is also supported by recently published VISTA Variables in the Via Lactea (VVV) results at 295 < l < 350°. The spectral energy distributions of the YSOs indicate class I or flat-spectrum systems in most cases, as in the VVV sample. A large number of variable YSOs are associated with the Cygnus X complex and other groups are associated with the North America/Pelican nebula, the Gemini OB1 molecular cloud, the Rosette complex, the Cone nebula, theW51 star-forming region and the S86 and S236 HII regions. Most of the YSO variability is likely due to variable/episodic accretion on time-scales of years, albeit usually less extreme than classical FUors and EXors. Luminosities at the 2010 Wide-field Infrared Survey Explorer epoch range from ~0.1 to 103 L☉ but only rarely exceed 102.5 L☉. [ABSTRACT FROM AUTHOR]

The Quintuplet cluster is one of the most massive star clusters in the Milky Way, situated very close to the Galactic Centre. We present a new search for variable stars in the vicinity of the cluster, using the five-year data base of the Vista Variables in the Via Lactea (VVV) ESO Public Survey in the near-infrared. A total of 7586 objects were identified in the zone around 2 arcmin from the cluster centre, using 55 KS-band epochs. 33 stars show KS-band variability, 24 of them being previously undiscovered. Most of the variable stars found are slow/semiregular variables, long-period variables of the Mira type, and OH/IR stars. In addition, a good number of our candidates show variations in a rather short time-scale. We also propose four young stellar object candidates, which could be cluster members. [ABSTRACT FROM AUTHOR]