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Register a new userThe outburst of the eruptive young star OO Serpentis between 1995 and 2006
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\\'Agnes K\\'osp\\'al
Publication date
2007
fields
Physics
and research's language is
English
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OO Serpentis is a deeply embedded pre-main sequence star that went into outburst in 1995 and gradually faded afterwards. Its eruption resembled the well-known FU Orionis-type or EX Lupi-type outbursts. Since very few such events have ever been documented at infrared wavelengths, our aim is to study the temporal evolution of OO Ser in the infrared. OO Ser was monitored with the Infrared Space Observatory starting 4 months after peak brightness and covering 20 months. In 2004-2006 we again observed OO Ser from the ground and complemented this dataset with archival Spitzer obsevations also from 2004. We analysed these data with special attention to source confusion and constructed light curves at 10 different wavelengths as well as spectral energy distributions. The outburst caused brightening in the whole infrared regime. According to the infrared light curves, OO Ser started a wavelength-independent fading after peak brightness. Later the flux decay became slower but stayed wavelength-independent. The fading is still ongoing, and current fading rates indicate that OO Ser will not return to quiescent state before 2011. The outburst timescale of OO Ser seems to be shorter than that of FUors, but longer than that of EXors. The outburst timescale and the moderate luminosity suggest that OO Ser is different from both FUors and EXors, and shows similarities to the recently erupted young star V1647 Ori. Based on its spectral energy distribution and bolometric temperature, OO Ser seems to be an early class I object, with an age of < 10^5 yr. The object is probably surrounded by an accretion disc and a dense envelope. Due to the shorter outburst timescales, the viscosity in the circumstellar disc of OO Ser is probably an order of magnitude higher than usual for FUors.
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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 with MIDI/VLTI at two epochs in this outburst. First, during the slowly fading plateau phase (2005 March) and second, just before the rapid fading of the object (2005 September), which ended the outburst. We used the radiative transfer code MC3D to fit the interferometry data and the spectral energy distributions from five different epochs at different stages of the outburst. The comparison of these models allowed us to trace structural changes in the system on AU-scales. We also considered qualitative alternatives for the interpretation of our data. Results: We found that the disk and the envelope are similar to those of non-eruptive young stars and that the accretion rate varied during the outburst. We also found evidence for the increase of the inner radii of the circumstellar disk and envelope at the beginning of the outburst. Furthermore, the change of the interferometric visibilities indicates structural changes in the circumstellar material. We test a few scenarios to interpret these data. We also speculate that the changes are caused by the fading of the central source, which is not immediately followed by the fading of the outer regions. However, if the delay in the fading of the disk is responsible for the changes seen in the MIDI data, the effect should be confirmed by dynamical modeling.
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 from the same orbiting dust clump that caused a similar brightness variation in 2012, than our results suggest a viscous spreading of the dust particles along the orbit. Another scenario is that the current minimum is caused by a dust structure, that is entering and leaving the inner part of the system. The WISE measurements could be consistent with this scenario. Our long-term data, as well as an accretion disk modeling hint at a general fading of V582 Aur, suggesting that the source will reach the quiescent level in $sim$80 years.
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 the mass accretion rate onto the star by at least two orders of magnitude. Here we present optical and near-infrared photometry and spectroscopy obtained after the minimum. Our light curves show a gradual re-brightening of V346 Nor, with its Ks-band brightness only 1.5 mag below the outburst brightness level. Our VLT/XSHOOTER spectroscopic observations display several strong forbidden emission lines towards the source from various metals and molecular hydrogen, suggesting the launch of a new jet. Our N-band spectrum obtained with VLT/VISIR outlines a deeper silicate absorption feature than before, indicating that the geometry of the circumstellar medium has changed in the post-outburst period compared to peak brightness.
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, ZTF r-band and Pan-STARRS rizy-filter photometry, showing an outburst of approximately 4 years duration, longer than typical EXors but shorter than FUors. Its pre-outburst SED 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 approximately 182 L_sun at an assumed distance of 1.4 kpc, the minimum plausible distance. Infrared and optical spectroscopy near maximum light showed an emission line spectrum, including HI lines, strong red CaII emission, other metal emission lines, infrared CO bandhead emission, and a strong infrared continuum. Towards 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 H_2 1--0 S(1) emission with the morphology of an outflow cavity and the extended emission in the [FeII] line at 1644 nm with the morphology of an edge-on disk. However, we do not detect any large-scale jet from Gaia19bey.
The occurrence of new FU Orionis-like objects is fundamental to understand the outburst mechanism in young stars and their role in star formation and disk evolution. Our work is aimed at investigating the properties of the recent outburst of V1647 Ori. Using optical and mid infrared long slit spectroscopy we monitored V1647 Ori in outburst between February 2004 and January 2006. The optical spectrum is characterized by Halpha and Hbeta in P-Cygni profile and by many weak FeI and FeII emission lines. Short timescale variability was measured in the continuum and line emission. On January 2006 we detected for the first time forbidden emission lines ([OI], [SII] and [FeII]). These lines are likely produced by an Herbig-Haro object driven by V1647 Ori. The mid infrared the spectrum of V1647 Ori is flat and featureless at all epochs. The SED changed drastically: the source was much redder in the early outburst than in the final phase. The magnitude rise and the SED of V1647 Ori resembles that of a FUor while the duration and recurrence of the outburst resemble that of a EXor. The optical spectrum is clearly distinct from either the absorption line spectrum of a FUor or the T Tauri-like spectrum of an EXor. Our data are consistent with a disk instability event which led to an increase of the mass accretion rate. The data also suggest the presence of a circumstellar envelope around the star+disk system. The peculiar N band spectrum might be explained by dust sublimation in the outer layers of the disk. The presence of the envelope and the outburst statistics suggest that these instability events occur only in a specific stage of a Class I source (e.g. in the transition phase to an optically visible star surrounded by a protoplanetary disk). We discuss the outburst mechanisms in term of the thermal instability model.
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