---
author:
- 'Á. Kóspál'
- 'P. Ábrahám'
- 'J. A. Acosta-Pulido'
- 'M. J. Arévalo Morales'
- 'M. I. Carnerero'
- 'E. Elek'
- 'J. Kelemen'
- 'M. Kun'
- 'A. Pál'
- 'R. Szakáts'
- 'K. Vida'
bibliography:
- 'paper.bib'
date: 'Received date; accepted date'
title: |
    The outburst and nature of two young eruptive stars\
    in the North America/Pelican Nebula Complex
---

[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 literature.]{} [We plot and analyze pre-outburst and outburst spectral energy distributions (SEDs), multi-filter light curves, and color-color diagrams.]{} [The quiescent SED of HBC722 is consistent with that of a slightly reddened normal TTauri-type star. The source brightened monotonically in about two months, and the SED obtained during maximum brightness indicates the appearance of a hot, single-temperature blackbody. The current fading rate implies that the star will return to quiescence in about a year, questioning its classification as a bone fide FUor. The quiescent SED of VSXJ205126.1+440523 looks like that of a highly embedded Class I source. The outburst of this source happened more gradually, but reached an unprecedentedly high amplitude. At 2.5 months after the peak, its light curves show a deep minimum, when the object was close to its pre-outburst optical brightness. Further monitoring indicates that it is still far from being quiescent.]{} [The shape of the light curves, as well as the bolometric luminosities and accretion rates suggest that these objects do not fit into the classic FUor group. Although HBC722 exhibit all spectral characteristics of a bona fide FUor, its luminosity and accretion rate is too low, and its timescale is too fast compared to classical FUors. VSXJ205126.1+440523 seems to be an example where quick extinction changes modulate the light curve.]{}

Introduction
============

In August 2010, two new young eruptive star candidates were discovered in the North America/Pelican Nebula Complex (distance: 550pc, @straizys1989). HBC722 (also known as LkH$\alpha$188G4 and PTF10qpf) brightened by $\Delta$R=3.3mag between 2010 May and August [@semkov2010a]. VSXJ205126.1+440523 (also known as IRAS20496+4354 and PTF10nvg) brightened by 1.8mag in unfiltered light between 2009 December and 2010 June, but Digitized Sky Survey plates show that it had been several magnitudes fainter in quiescence [@itagaki2010; @munari2010]. @semkov2010 and @miller2010 provided light curves and spectroscopy for HBC722 and concluded that we witness a bona fide outburst of a FUor-type object. FUors, named after the prototype object FUOrionis, brighten by up to 5mag at optical wavelengths and may stay in the high state for decades. @covey2010 presented light curves and spectroscopy for VSXJ205126.1+440523, and found that in many respects this object is different from FUors or EXors (the latter being another class of eruptive YSOs, named after the prototype EXLup, which flares up by 1-5mag in every few years and stay bright for several months). Currently only about two dozens of young eruptive stars (FUors and EXors) are known, thus the two new outbursts announced in August 2010 are noteworthy events. Should they turn out to be accretion-powered eruptions, their detailed study may contribute to the understanding of these important phases of early stellar evolution.

In this paper we present an optical and infrared view of the two eruptive star candidates. Using archival and new data, we characterize their circumstellar environment and compare them with those of some better studied FUors and EXors. We present new optical and near-infrared photometric data points taken during the outburst, which indicate that HBC722 already passed its peak brightness and started a monotonous fading with a steady fading rate, while neither the brightening, nor the fading of VSXJ205126.1+440523 was monotonous.

Observations and data reduction
===============================

#### Optical observations.

(B)VRI-band images were obtained between 19 September 2010 and 2 January 2011 with three telescopes: the 60/90/180cm (aperture diameter/primary mirror diameter/focal length) Schmidt telescope of the Konkoly Observatory (Hungary), the 1m (primary mirror diameter) RCC telescope of the Konkoly Observatory, and the 80cm (primary mirror diameter) IAC-80 telescope of the Teide Observatory in the Canary Islands (Spain). The Konkoly Schmidt telescope is equipped with a 4096$\times$4096 pixel Apogee Alta U16 CCD camera (pixel scale: 1.03$''$), and a Bessel BV(RI)$_{\rm C}$ filter set. The 1m RCC is equipped with a 1300$\times$1340 pixel Roper Scientific WersArray:1300B CCD camera (pixel scale: 0.306$''$), and a Bessel UBV(RI)$_{\rm C}$ filter set. The Teide IAC-80 telescope is equipped with a 2048$\times$2048 pixel Spectral Instruments E2V 42-40 back-illuminated CCD camera ‘CAMELOT’ (pixel scale: 0.304$''$), and a Johnson-Bessel UBV(RI)$_{\rm J}$ filter set. The images were reduced in IDL following the standard processing steps of bias subtraction and flat-fielding. On each night, for each target, images were obtained in blocks of 3 or 5 frames per filter. Aperture photometry for the target and other field stars were performed on each image using IDL’s *cntrd* and *aper* procedures. Since HBC722 is surrounded by a reflection nebula, in order to be consistent with the photometry of @semkov2010, we used the same apertures: an aperture radius of 4$''$ and a sky annulus between 13$''$ and 19$''$. For HBC722, instrumental magnitudes were transformed to the standard system using the 8 brightest stars (from star ‘A’ to star ‘H’) from the comparison sequence given in @semkov2010. For each image we fitted the difference of the instrumental and the standard magnitudes of the comparison stars as a function of the V$-$I color, and used this relationship to convert the instrumental magnitudes of our target to standard magnitudes. For VSXJ205126.1+440523, we observed the standard field NGC6823 with the Schmidt telescope during the photometric night 23/24 September 2010, and calibrated 6 comparison stars in the vicinity of our target. A finding chart and the standard magnitudes of the comparison stars can be seen in Fig. \[fig:compstars\] and in Tab. \[tab:compstars\] in the Online Material. The conversion of instrumental to standard magnitudes was done the same way as for HBC722. Similarly to the comparison stars of HBC722, we cannot exclude that the comparison stars of VSXJ205126.1+440523 might be variables on longer timescales, although they were constant within the measurement uncertainties during our observing period. The resulting photometry for our two targets is presented in Tabs. \[tab:phot\_vsxj20\] and \[tab:phot\_hbc722\] in the Online Material. We note that the R and I filters on the two telescopes are different, which may introduce a systematic difference in the magnitudes obtained with the different telescopes. However, in our experience, this difference is less than 0.05mag [@kospal2010]. Since the observed brightness variations of our targets are much larger than 0.05mag, this possible difference in the filter systems does not affect our analysis and conclusions.

#### Near-infrared observations.

JHK$_{\rm S}$ images were obtained with the 1.52m Telescopio Carlos Sanchez (TCS, Teide Observatory, Spain) between 19 September and 19 November 2010, using the 256$\,{\times}\,$256 pixel Nicmos 3 detector CAIN-3 with the wide field optics (pixel scale: 1$''$). Observations were performed in a 5-point dither pattern in order to enable proper sky subtraction. The images were reduced using a modified version of [*caindr*]{}, an iraf data reduction package written by J. Acosta-Pulido[^1]. Data reduction steps included sky subtraction, flat-fielding, and the co-addition of all frames taken with the same filter. The sky image was obtained as the median combination of all frames, masking regions occupied by bright sources. The final image was produced using the standard “shift-and-add” technique, including rejection of outlier pixels. The instrumental magnitudes of the target and all good-quality 2MASS stars in the field were extracted using aperture photometry in IDL. For the photometric calibration we used the Two Micron All Sky Survey (2MASS) catalog [@cutri2003]. We determined the offset between the instrumental and the 2MASS magnitudes by averaging typically 20-30 stars, using [*biweight\_mean*]{}, an outlier-resistant averaging method. The resulting photometry of our two targets is presented in Tabs. \[tab:phot\_vsxj20\] and \[tab:phot\_hbc722\] in the Online Material. We obtained additional near-infrared photometry using archival images from the UKIRT InfraRed Deep Sky Surveys (UKIDSS). These images were taken with the Wide Field Camera on the 3.8m diameter UKIRT in 2006, and they are part of Data Release 8. Aperture photometry and calibration were executed in the same way as for the TCS data.

Results and analysis
====================

HBC722
------

HBC722 is part of a small cluster of young stars called “LkH$\alpha$188 group” by @cohen1979. In their naming convention, HBC722 is called LkH$\alpha$ 188 G4. The whole cluster is located in a dark cloud separating the North America and the Pelican Nebulae [@straizys1989]. In outburst, the star is surrounded by a compact, asymmetric reflection nebula, which is well visible at optical wavelengths [@miller2010], but also discernible in our J band images.

#### Light curve.

The brightening of HBC722 is well documented in @semkov2010 and @miller2010. Between August 2009 and July 2010, the star gradually brightened by 1mag in the R band, then between July and August 2010, it brightened by another 3mag, reaching a maximum brightness at the end of August 2010. We have been monitoring the star since September 2010. Our data confirm that the star reached is maximal brightness, and is currently gradually fading (Fig. \[fig:light\_hbc722\]). Between 20 September 2010 and 3 December 2010, HBC722 decreased its brightness by 0.55, 0.54, 0.47, 0.37, 0.32, and 0.21mag in the B, V, R, I, J, H, and K$_{\rm S}$ bands, respectively, indicating that the source has become slightly redder. Fitting a line to the data points between these two epochs gives fading rates of 0.34, 0.25, 0.25, 0.21, 0.16, 0.13, and 0.07 mag/month in the B, V, R, I, J, H, and K$_{\rm S}$ band, respectively. Our last optical data points taken on 2 January 2011 fit into this trend. Assuming that the linear fading continues with these rates, and taking into account the pre-outburst optical fluxes observed by @semkov2010 around July 2009, we estimate that the source will return to quiescence some time between fall 2011 and spring 2012. The two pre-outburst JHK$_{\rm S}$ data sets (2MASS from June 2000 and UKIDSS from July 2006) agree within 0.1mag, indicating a rather constant pre-outburst near-infrared brightness. Considering the relatively slow near-infrared fading rates, HBC722 may exhibit higher than quiescent near-infrared fluxes even until fall 2013. Thus, current fading rates indicate that the outburst of HBC722 would last approximately 2 or 3 years.

In order to put into context the outburst history of HBC722, in Fig. \[fig:context\] we compare its light curve with those of other young eruptive objects. The recent extreme outburst of EXLup exhibited a faster onset, more peaked maximum, and a fading which was initially very similar to that of HBC722. However, EXLup showed two deep minima before it went back to quiescence some 8 months after its peak brightness. The light curve of V1057Cyg, the FUor having the fastest known outburst and fading, is still much slower than that of HBC722. The light curve of V1647Ori, a source often classified as an intermediate-type between FUors and EXors, initially displayed a relatively slow fading, then suddenly went back to quiescence, thus had an approximately 2.5-year-long outburst. If HBC722 continues the linear fading it currently displays, the predicted outburst length will be remarkably similar to that of V1647Ori. This comparison implies that HBC722 is different from the classical FUors and is more similar to EXors, or intermediate objects between FUors and EXors.

![Light curves of HBC722. For better visibility, the B, V, I, J, H, and K$_{\rm S}$ light curves were shifted along the y axis by the values indicated in the figure. Filled dots are from this work, plus signs are from @semkov2010 and @miller2010.[]{data-label="fig:light_hbc722"}](light_hbc722.ps){width="\columnwidth"}

![Light curves of our targets and those of different young eruptive stars. [*Triangles:*]{} photographic light curve of V1057Cyg from @gieseking1973; [*stars:*]{} I$_{\rm C}$ light curve of V1647Ori from @acosta2007; [*squares:*]{} visual light curve of EXLup during its extreme outburst in 2008 [@abraham2009]. For HBC722 and VSXJ205126.1+440523 we plotted the R-band light curves. The data for V1057Cyg, V1647Ori, and EXLup were shifted along the y axis for better visibility and also along the x axis so that the peak brightness is approximately at the same position for all stars.[]{data-label="fig:context"}](ilight_long.ps){width="\columnwidth"}

#### Spectral energy distribution.

Fig. \[fig:sed\_hbc722\] shows the pre-outburst and outburst SEDs of HBC722. Pre-outburst data are from @miller2010 and references therein, while the outburst data are from this work. In this figure, we also overplotted with gray shading the typical SED of a TTauri-type star in the Taurus star-forming region [@dalessio1999; @furlan2006], scaled to the H-band data point, and reddened by A$_{\rm V}$=3.36mag [@cohen1979]. The outburst photometry indicates that a hot continuum is added to the quiescent SED. The B, V, R, I, J, H, and K$_{\rm S}$ points indicate a blackbody-like spectrum at all epochs during the outburst. We could fit these points with a single temperature blackbody and obtained a temperature of 4000K (using A$_{\rm V}$=3.36mag). With the assumption that the SED is similar to the Taurus median above 24$\,\mu$m, we calculated a pre-outburst bolometric luminosity of 0.85L$_{\odot}$ by integrating the de-reddened SED between 0.44 and 200$\,\mu$m. The outburst bolometric luminosity can be similarly calculated, but due to the lack of mid-infrared data points, we can either assume a blackbody shape until 10$\,\mu$m and assume that the SED did not change above 10$\,\mu$m, or assume that the SED changed self-similarly in the whole 2$-$200$\,\mu$m range. The former approach give L$_{\rm
  bol}$=8.7L$_{\odot}$, the latter, L$_{\rm
  bol}$=12L$_{\odot}$. The true outburst luminosity is probably between these two values.

![Spectral energy distribution of HBC722. Filled dots are pre-outburst data from @miller2010, while open circles are outburst data (this work). The gray shaded area indicates the median SED of TTauri stars in the Taurus star-forming region with spectral types between K5 and M2 (data below 1.25$\,\mu$m and above 40$\,\mu$m are from @dalessio1999, data between 1.25 and 40$\,\mu$m are from @furlan2006).[]{data-label="fig:sed_hbc722"}](sed_hbc722.ps){width="\columnwidth"}

VSXJ205126.1+440523
-------------------

VSXJ205126.1+440523 is situated in an isolated molecular cloud located about 15$'$ southeast of the Pelican Nebula molecular cloud [@bally2003]. The eastern rim of this small cloud is well visible in the \[SII\] and H$\alpha$ images of @bally2003. This morphology suggests that VSXJ205126.1+440523 sits on the tip of a column of dense material, out of which it had been born. Apart from the H$\alpha$ emission from the rim, no extended emission seems to be associated with the source, not even in outburst. @bally2003 discovered several Herbig-Haro objects in this area, and claim that one of them, HH569, is possibly driven by VSXJ205126.1+440523.

#### Light curve.

In Fig. \[fig:light\_vsxj20\] we plotted the light curves of VSXJ205126.1+440523. @covey2010 reported the source to be between R=18–19.25mag in mid-2009. After that, it brightened by $\approx$6mag, reaching its maximal brightness in August 2010. Since then, the source started fading, and by November 2010, it has nearly reached its mid-2009 optical brightness, then it brightened again. The light curves show that neither the brightening, nor the fading was monotonous. Although the near-infrared light curves are not as well-sampled as the optical ones, they delineate similar trends but with smaller amplitudes. We note that the R=19.25mag reported by @covey2010 may not be the true quiescent brightness of the source, since the source was $\approx$20mag in the POSS2 red plate taken in 1990 [@itagaki2010]. The comparison of the UKIDSS and TCS photometry indicates that the source brightened by $\Delta$J=7.9mag, $\Delta$H=6.7mag, and $\Delta$K$_{\rm
  S}$=4.8mag between July 2006 and September 2010. We note that the source was K$_{\rm S}$=13.15mag in 2006, but it was not visible in the K$_{\rm S}$ band in the 2MASS images taken in October 2000. Since the 2MASS PSC is complete down to K$_{\rm S}$=14.3 [@cutri2003], the source must have brightened at least 1.15mag between 2000 and 2006, making the true K$_{\rm S}$-band magnitude change at least 5.8mag.

The comparison of the light curve of VSXJ205126.1+440523 with other young eruptive stars in Fig. \[fig:context\] indicates that this source is different from all the other sources plotted, although the brightening and fading rates are most similar to those of EXLup. Especially remarkable is the deep minimum of VSXJ205126.1+440523 in November 2010, which is similar to the minima displayed by EXLup shortly before the end of the eruption.

![Light curves of VSXJ205126.1+440523. Filled dots are from this work, plus signs are from @covey2010 and from Seiichiro Kiyota and Hiroyuki Maehara (vsnet, http://tech.dir.groups.yahoo.com/group/vsnet-recent-fuori/messages), crosses are visual estimates by @itagaki2010.[]{data-label="fig:light_vsxj20"}](light_vsxj20.ps){width="\columnwidth"}

#### Spectral energy distribution.

In Fig. \[fig:sed\_vsxj20\] we compiled a pre-outburst SED using data from the UKIDSS database (this work), the MSX6C Infrared Point Source Catalog [@egan2003], the AKARI/IRC mid-infrared all-sky survey [@ishihara2010], and Spitzer data [@covey2010 and references therein]. Out of these data points, the UKIDSS and the Spitzer are quasi-simultaneous (all obtained between June and August 2006), while the MSX data are from 1996-1997, and the AKARI from 2006-2007. This SED should be analysed with caution, considering the K$_{\rm S}$-band variability mentioned above. The outburst SED contains optical and near-infrared photometry we obtained on 20/23 September 2010 and 16/17 November 2010. We note that by September, the source was already $\approx$2mag fainter in R-band than at maximal brightness some 20 days earlier. The shape of the SED and the fact that the source in quiescence was practically invisible (the only pre-outburst image where the source is visible at optical wavelengths is the POSS2 red plate in Fig. \[fig:map\_vsxj20\]) suggest that the source is highly extincted. However, dereddening its colors does not make it fall onto the TTauri locus (Fig. \[fig:tcd\]). Correcting for a reddening of A$_{\rm V}$=17...20mag would result in a J$-$H color typical for TTauri stars, but its H$-$K$_{\rm S}$ color would still be too red. The reason for the strange near-infrared colors of VSXJ205126.1+440523 may be partly interstellar reddening caused by the small cloud in which the source is embedded and whose outlines are visible in Fig. \[fig:map\_vsxj20\], partly circumstellar reddening by an envelope or thick disk. The relative importance of these two effects is not known, thus we do not attempt to correct for interstellar reddening, and calculate a bolometric luminosity of 14.7L$_{\odot}$ by simply integrating the quiescent SED from 1.25 to 200$\,\mu$m. We calculate an outburst luminosity of 22L$_{\odot}$ similarly, assuming that the SED did not change above 10$\,\mu$m, and using the September 2010 data points in Fig. \[fig:sed\_vsxj20\].

![Spectral energy distribution of VSXJ205126.1+440523. Filled dots are pre-outburst data from various dates, while open circles are outburst data (see text).[]{data-label="fig:sed_vsxj20"}](sed_vsxj20.ps){width="\columnwidth"}

![Near-infrared color-color diagram. The main-sequence is marked by a thick solid line, the giant branch with dotted line [@koornneef1983], the reddening path with dashed lines [@cardelli1989], and the TTauri locus with dash-dotted line [@meyer1997]. [*Open symbols:*]{} quiescent colors; [*filled symbols:*]{} outburst colors. Source of data: @kenyon1991 for V1057Cyg, @acosta2007 for V1647Ori, 2MASS PSC and @juhasz2010 for EXLup, and this work for HBC722 and VSXJ205126.1+440523.[]{data-label="fig:tcd"}](tcd.ps){width="\columnwidth"}

Discussion
==========

The nature of the sources in quiescence
---------------------------------------

The SED of HBC722 seems to be consistent with that of a slightly reddened TTauri star, both regarding the optical–near-infrared part of the SED and the 24$\,\mu$m photometric point. This conclusion is in accordance with the claim of @miller2010 that HBC722 is a Class II object where the central star is a K7-type star. It is noteworthy, however, that between 3.6 and 8$\,\mu$m there is an excess emission in the SED compared to the Taurus median. In this respect, the source somewhat resembles DRTau, a highly accreting TTauri star, suggesting that HBC722 might be a highly accreting TTauri star even in quiescence. The fact that the quiescent optical spectrum, taken by @cohen1979 exhibits an unusually prominent H$\alpha$ emission with an equivalent width of 100Å supports this idea.

VSXJ205126.1+440523 seems to be a much more reddened source. Its quiescent bolometric luminosity ($\approx$15L$_{\odot}$) is significantly larger than that of HBC722, indicating a somewhat higher mass. The quiescent SED in Fig. \[fig:sed\_vsxj20\] probably represents a moderately reddened Class I source. The findings of @covey2010, who determined 6mag$<$A$_{V}{<}$12.4mag from the ratio of H emission lines observed in the outburst spectrum, supports this idea. This scenario requires a dense envelope in the system, but the lack of a reflection nebula around this source suggests the lack of an extended envelope. However, high interstellar extinction may render the scattered light invisible. The presence of a related Herbig-Haro object also advocates for the Class I scenario.

The outburst mechanism
----------------------

The comparison of the quiescent and the outburst SEDs (Fig. \[fig:sed\_hbc722\]) suggests that the brightening of HBC722 can be interpreted as the appearance of a hot continuum. The data points can be fitted with a temperature of $\approx$4000K, somewhat less than what one would expect from an accretion outburst, where ionized material is present. Note however that the temperature may be higher if the extinction towards the system is higher. It is noteworthy that the outburst excess can be described in the optical–near-infrared regime by a single temperature blackbody rather than a disk-like emission reflecting a radial (usually outwards decreasing) temperature profile. The fading of HBC722 is mostly color-independent in the near-infrared regime (Fig. \[fig:tcd\]), while optical colors are becoming slightly redder. This may indicate that the hot continuum is both fading and cooling.

The reason for the brightening of VSXJ205126.1+440523 is more enigmatic. In this case the excess emission in outburst is not a single-temperature radiation, but seems to have a temperature distribution at all epochs (Fig. \[fig:light\_vsxj20\]). The amplitude of the outburst in the J, H and K$_{\rm S}$ bands is larger than any brightening observed so far for YSOs. At first glance, one may think that the brightening was due to suddenly decreased extinction. However, in this case the source should have moved along the reddening path in the J$-$H vs. H$-$K$_{\rm S}$ diagram (Fig. \[fig:tcd\]) which was not the case during the rising part of the lightcurve. JHK$_{\rm S}$ photometry obtained after maximum brightness indicate that the fading of VSXJ205126.1+440523 initially happened along the same path as the brightening, suggesting that whatever was the cause of the flux changes, it was a reversible process. Changing accretion is an appealing idea because it would explain the presence of the Herbig-Haro object possibly ejected from the source during a previous outburst (enhanced accretion is often accompanied by enhanced mass outflow). It also suggests that the outburst activity of VSXJ205126.1+440523 might be repetitive. However, photometry obtained in November 2010 indicates a deep minimum in Fig. \[fig:light\_vsxj20\]. At the same time, the source moved along the reddening path in Fig. \[fig:tcd\]. Both the color changes and the brightness changes in the J, H, and K$_{\rm S}$ bands are consistent with an extinction increase of A$_{\rm
  V}$=9mag. Thus, it may be possible that the deep minimum in November 2010 was caused by a dust condensation effect (similar to what happened to V1515Cyg in 1980 [@kenyon1991], or an eclipse by dust clumps in an almost edge-on disk system (similarly to what causes deep optical minima of the UX Orionis-type stars, see e.g. @eaton1995).

Classification as a FUor outburst
---------------------------------

When discovered, both sources were announced as FUor candidates. Extensive analyses presented in @semkov2010 and @miller2010 indicated that HBC722 can be considered as a bona fide FUor. On the other hand, @covey2010 concluded that VSXJ205126.1+440523 does not appear to belong to either the FUor or the EXor class. FUors are usually bright objects in outburst with luminosities of a few hundred L$_{\odot}$ [@hk96]. The immense radiation is related to the increased accretion rate, which can reach values up to 10$^{-4}$M$_{\odot}$/yr. However, the outburst luminosities of our objects are only in the order of 10-20L$_{\odot}$. Assuming that the luminosity excess in eruption is all due to the release of accretion energy, the computed accretion rate for HBC722 (assuming a stellar mass of 0.5M$_{\odot}$ and a radius of 3R$_{\odot}$) is 10$^{-6}$M$_{\odot}$/yr. @covey2010 determined an accretion rate of 2.5$\times$10$^{-7}$M$_{\odot}$/yr for VSXJ205126.1+440523. Both of these values are well below the typical value for classical FUors.

The brightening and fading rates for both of our sources are also too fast compared to classical FUor light curves. In the case of HBC722 we made a comparison with several young eruptive stars and found a mismatch with the prototype FUor V1057Cyg but more similarities with the light curves of EXLup, the prototype of EXors, and V1647Ori, an object often classified as an intermediate object between FUors and EXors. The slow brightening of VSXJ205126.1+440523 is not unheard of (both FUOri and V1057Cyg had a rise-time of about 1 year, for other sources the estimates range between 3 and 20 years, @bell1994), but the fading is far too rapid. According to our light curve (Fig. \[fig:light\_vsxj20\]), 2.5 months after peak brightness VSXJ205126.1+440523 dimmed by about 5mag in V-band. This is closer to the typical timescales of EXor flare-ups than that of bona fide FUor outbursts (decades to centuries). The non-monotonous fading of this source also resembles the light curve of the recent outburst of EXLup (Fig. \[fig:context\]). The moderate resolution near-infrared spectrum obtained in outburst by @covey2010 also shows similarities to that of EXLup [@kospal2011].

It is remarkable that HBC722 possesses all spectral characteristics of bona fide FUors but its luminosity is an order of magnitude lower, and its fading timescale is much faster. This suggests that the physical mechanism which is behind the FUor-type eruptions should also work with lower accretion rates although probably producing shorter outbursts. This conclusion questions the thermal instability model of @bell1994, who suggested the existence of a threshold mass accretion rate from the outer to the inner circumstellar disk. Matter can pile up at the inner edge of the disk and fall onto the stellar surface following a sudden thermal instability only if the quiescent accretion rate is higher than this threshold value. Thus, it seems that in the regime of low luminosity outbursts ($\approx$10L$_{\odot}$), both FUor-like eruptions (when the source exhibits all spectral characteristics of FUors like HBC722) and EXor-like events (when the source exhibits a typical TTauri spectrum with emission lines and CO bandhead emission, somewhat similar to VSXJ205126.1+440523) can occur. If both HBC722 and VSXJ205126.1+440523 are indeed young eruptive stars, one might conclude that the class of young eruptive stars is even more diverse than what was thought before.

This work is based in part on observations made with the Telescopio Carlos Sanchez operated on the island of Tenerife by the Instituto de Astrofísica de Canarias in the Observatorio del Teide. The authors wish to thank the telescope manager A. Oscoz, support astronomer P. Montañes, and telescope operators R. Martí, and M. Díaz for their help during the observations. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This work is based in part on data obtained as part of the UKIRT Infrared Deep Sky Survey. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the NASA and the National Science Foundation. The research of Á.K. is supported by the Nederlands Organization for Scientific Research.

[^1]: for more details on the [*caindr*]{} package, see http://www.iac.es/telescopes/cain/cain\_eng.html.
