The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex

Abstract

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 literature. Methods. We plot
and analyze pre-outburst and outburst spectral energy distributions (SEDs),
multi-filter light curves, and color-color diagrams. Results. HBC 722
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 outburst of VSX J205126.1+440523 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. Conclusions. The shape of the light curves, the bolometric
luminosities and accretion rates suggest that these objects do not fit into the
classic FUor group. Although HBC 722 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. VSX J205126.1+440523 seems to be an
example where quick extinction changes modulate the light curve.

Full text

Page 1

arXiv:1011.4009v2 [astro-ph.SR] 17 Jan 2011
Astronomy & Astrophysics manuscript no. paper
January 18, 2011
c ? ESO 2011
The outburst and nature of two young eruptive stars
in the North America/Pelican Nebula Complex
´A. K´ osp´ al1, P.´Abrah´ am2, J. A. Acosta-Pulido3,4, M. J. Ar´ evalo Morales3,4, M. I. Carnerero3,4, E. Elek2, J. Kelemen2,
M. Kun2, A. P´ al2,5, R. Szak´ ats6, and K. Vida2
1Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: kospal@strw.leidenuniv.nl
2Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary
3Instituto de Astrof´ ısica de Canarias, 38200 La Laguna, Tenerife, Spain
4Departamento de Astrof´ ısica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
5Department of Astronomy, Lor´ and E¨ otv¨ os University, P´ azm´ any P. st. 1/A, Budapest H-1117, Hungary
6Baja Astronomical Observatory of B´ acs-Kiskun County, PO Box 766, 6500 Baja, Hungary
Received date; accepted date
ABSTRACT
Context. In August 2010, the sudden optical brightening of two young stellar objects, HBC722 and VSXJ205126.1+440523, located
in the North America/Pelican Nebula Complex, was announced. Early photometric and spectroscopic observations of these objects
indicated that they may belong to the FUor or EXor class of young eruptive stars. The eruptions of FUors and EXors are often ex-
plained 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 literature.
Methods. Weplot and analyze pre-outburst and outburst spectral energy distributions (SEDs), multi-filter light curves, and color-color
diagrams.
Results. The quiescent SED of HBC722 is consistent with that of a slightly reddened normal TTauri-type star. The source bright-
ened 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.
Conclusions. 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.
Key words. stars: formation – stars: circumstellar matter – infrared: stars – stars: individual: HBC 722 – stars: individual: VSX
J205126.1+440523
1. Introduction
In August 2010, two new young eruptive star candidates were
discovered in the North America/Pelican Nebula Complex
(distance: 550pc, Straizys et al. 1989). HBC722 (also known
as LkHα188G4 and PTF10qpf) brightened by ∆R=3.3mag
between 2010 May and August (Semkov & Peneva 2010).
VSXJ205126.1+440523(also known as IRAS20496+4354and
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
(Itagaki & Yamaoka 2010; Munari et al. 2010). Semkov et al.
(2010) and Miller et al. (2010) provided light curves and spec-
troscopy 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 op-
tical wavelengths and may stay in the high state for decades.
Covey et al. (2011) 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 an-
other class of eruptiveYSOs, named after the prototypeEXLup,
which flares up by1-5mag in everyfew years and stay bright for
severalmonths).Currentlyonlyabouttwodozensofyoungerup-
tive stars (FUors and EXors) are known, thus the two new out-
bursts announcedin August 2010 are noteworthyevents. 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 fadingrate,while neitherthe brightening,northe fading
of VSXJ205126.1+440523was monotonous.
1

Page 2

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex
2. Observations and data reduction
Optical observations. (B)VRI-band images were obtained be-
tween 19 September 2010 and 2 January 2011 with three
telescopes: the 60/90/180cm (aperture diameter/primary mir-
ror 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 4096×4096 pixel Apogee Alta U16 CCD
camera (pixel scale: 1.03′′), and a Bessel BV(RI)C filter set.
The 1m RCC is equipped with a 1300×1340 pixel Roper
Scientific WersArray:1300B CCD camera (pixel scale: 0.306′′),
and a Bessel UBV(RI)Cfilter set. The Teide IAC-80 telescope
is equipped with a 2048×2048 pixel Spectral Instruments E2V
42-40 back-illuminated CCD camera ‘CAMELOT’ (pixel scale:
0.304′′), and a Johnson-Bessel UBV(RI)Jfilter set. The images
were reduced in IDL following the standard processing steps of
bias subtraction and flat-fielding. On each night, for each tar-
get, images were obtained in blocks of 3 or 5 frames per filter.
Aperturephotometryforthe targetandotherfield stars were per-
formed 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 Semkov et al. (2010), we
used the same apertures: an aperture radius of 4′′and a sky an-
nulus between 13′′and 19′′. For HBC722, instrumental mag-
nitudes were transformed to the standard system using the 8
brightest stars (from star ‘A’ to star ‘H’) from the comparison
sequence given in Semkov et al. (2010). For each image we fit-
ted the difference of the instrumental and the standard magni-
tudes 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 compari-
son stars can be seen in Fig. 1 and in Tab. 1 in the Online
Material.Theconversionofinstrumentalto standardmagnitudes
wasdonethesamewayas forHBC722.Similarlytothecompar-
ison stars of HBC722, we cannot exclude that the comparison
stars of VSXJ205126.1+440523 might be variables on longer
timescales, althoughthey were constant within the measurement
uncertainties during our observing period. The resulting pho-
tometry for our two targets is presented in Tabs. 2 and 3 in the
Online Material. We note that the R and I filters on the two tele-
scopes are different, which may introduce a systematic differ-
ence in the magnitudes obtained with the different telescopes.
However, in our experience, this difference is less than 0.05mag
(K´ osp´ al et al. 2010). Since the observed brightness variations of
our targets are much larger than 0.05mag, this possible differ-
ence in the filter systems does not affect our analysis and con-
clusions.
Near-infrared observations. JHKSimages were obtained with
the 1.52m Telescopio Carlos Sanchez (TCS, Teide Observatory,
Spain) between 19 September and 19 November2010, using the
256×256 pixel Nicmos 3 detector CAIN-3 with the wide field
optics (pixel scale: 1′′). Observations were performed in a 5-
pointditherpatterninordertoenableproperskysubtraction.The
images were reduced using a modified version of caindr, an iraf
data reduction package written by J. Acosta-Pulido1. Data re-
duction 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 pro-
duced 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 ex-
tracted using aperture photometry in IDL. For the photometric
calibration we used the Two Micron All Sky Survey (2MASS)
catalog (Cutri et al. 2003). We determined the offset between
the instrumental and the 2MASS magnitudes by averaging typ-
ically 20-30 stars, using biweight mean, an outlier-resistant av-
eraging method. The resulting photometry of our two targets is
presented in Tabs. 2 and 3 in the Online Material. We obtained
additional near-infrared photometry using archival images from
the UKIRT InfraRed Deep Sky Surveys (UKIDSS). These im-
ages were taken with the Wide Field Camera on the 3.8m di-
ameter 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.
3. Results and analysis
3.1. HBC722
HBC722 is part of a small cluster of young stars called
“LkHα188 group” by Cohen & Kuhi (1979). In their naming
convention, HBC722 is called LkHα 188 G4. The whole clus-
ter is located in a dark cloud separating the North America and
the Pelican Nebulae (Straizys et al. 1989). In outburst, the star is
surroundedbyacompact,asymmetricreflectionnebula,whichis
well visible at optical wavelengths (Miller et al. 2010), but also
discernible in our J band images.
Light curve. ThebrighteningofHBC722is welldocumentedin
Semkov et al. (2010) and Miller et al. (2010). 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 monitoringthe star since September
2010. Our data confirm that the star reached is maximal bright-
ness, and is currently gradually fading (Fig. 1). 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 KSbands, respectively, indicating that the
source has become slightly redder. Fitting a line to the data
points betweenthese two epochsgives fadingrates of 0.34,0.25,
0.25,0.21,0.16,0.13,and 0.07mag/monthin the B, V, R, I, J, H,
and KSband, respectively. Our last optical data points taken on
2 January 2011 fit into this trend. Assuming that the linear fad-
ing continues with these rates, and taking into account the pre-
outburst optical fluxes observed by Semkov et al. (2010) around
July 2009, we estimate that the source will return to quiescence
some time between fall 2011 and spring 2012. The two pre-
outburst JHKSdata sets (2MASS from June 2000 and UKIDSS
from July 2006) agree within 0.1mag, indicating a rather con-
stant pre-outburst near-infrared brightness. Considering the rel-
atively slow near-infrared fading rates, HBC722 may exhibit
higher than quiescent near-infrared fluxes even until fall 2013.
1for
http://www.iac.es/telescopes/cain/cain eng.html.
moredetails onthe
caindr
package,see
2

Page 3

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex
22
20
18
16
14
12
10
8
Magnitude
300350400
JD (2,455,000 + )
450 500550

Apr MayJun JulAugSepOct Nov Dec
K-1
H-1
J-1
I-1
R
V+1
B+2
HBC 722
literature
this work
Fig.1. Light curves of HBC722. For better visibility, the B, V,
I, J, H, and KSlight curves were shifted along the y axis by the
valuesindicatedinthe figure.Filleddots arefromthis work,plus
signs are from Semkov et al. (2010) and Miller et al. (2010).
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. 2 we compare its light curve with those of other young
eruptive objects. The recent extreme outburst of EXLup exhib-
ited 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 knownoutburstand fad-
ing, 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 ap-
proximately 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 compari-
son implies that HBC722 is different from the classical FUors
and is more similar to EXors, or intermediate objects between
FUors and EXors.
Spectral energy distribution. Fig. 3 shows the pre-outburst
and outburst SEDs of HBC722. Pre-outburst data are from
Miller et al. (2010) 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-typestar in the Taurus
star-forming region (D’Alessio et al. 1999; Furlan et al. 2006),
scaled to the H-band data point, and reddened by AV=3.36mag
(Cohen & Kuhi 1979). The outburst photometry indicates that
a hot continuum is added to the quiescent SED. The B, V, R,
I, J, H, and KS 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 AV=3.36mag).With the assumptionthat the SED
0 200 400600
JD (2,455,000 + )
80010001200 1400
25
20
15
10
Magnitude
V1647 Ori
0.065 mag/month
HBC 722
V1057 Cyg
0.035 mag/month
EX Lup
0.25 mag/month
VSX J205126.1+440523
Fig.2. Light curves of our targets and those of different young
eruptivestars. Triangles:photographiclight curveofV1057Cyg
from Gieseking (1973); stars: IC light curve of V1647Ori
from Acosta-Pulido et al. (2007); squares: visual light curve
of EXLup during its extreme outburst in 2008 (´Abrah´ am et al.
2009).ForHBC722andVSXJ205126.1+440523weplottedthe
R-band light curves. The data for V1057Cyg, V1647Ori, and
EXLupwereshiftedalongtheyaxisforbettervisibilityandalso
along the x axis so that the peak brightness is approximately at
the same position for all stars.
is similar to the Taurus median above 24µm, we calculated a
pre-outburst bolometric luminosity of 0.85L⊙by integrating the
de-reddened SED between 0.44 and 200µm. The outburst bolo-
metric luminosity can be similarly calculated,but due to the lack
of mid-infrared data points, we can either assume a blackbody
shapeuntil10µmandassumethatthe SED didnotchangeabove
10µm, or assume that the SED changed self-similarly in the
whole 2−200µm range. The former approach give Lbol=8.7L⊙,
the latter, Lbol=12L⊙. The true outburst luminosity is probably
between these two values.
3.2. VSXJ205126.1+440523
VSXJ205126.1+440523 is situated in an isolated molecu-
lar cloud located about 15′southeast of the Pelican Nebula
molecular cloud (Bally & Reipurth 2003). The eastern rim of
this small cloud is well visible in the [SII] and Hα images
of Bally & Reipurth (2003). This morphology suggests that
VSXJ205126.1+440523sits onthe tip ofa columnofdensema-
terial,outofwhichit hadbeenborn.ApartfromtheHα emission
from the rim, no extended emission seems to be associated with
thesource,noteveninoutburst.Bally & Reipurth(2003)discov-
ered several Herbig-Haro objects in this area, and claim that one
ofthem,HH569,is possiblydrivenbyVSXJ205126.1+440523.
Light curve. In Fig. 4 we plotted the light curves of
VSXJ205126.1+440523. Covey et al. (2011) reported the
3

Page 4

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex
1 10 100
Wavelength [µm]
10-15
10-14
10-13
10-12
νFν [W m-2]
HBC 722
Taurus median
AV = 3.36 mag
10-15
10-14
10-13
10-12
2010 Sep 19/20
2010 Dec 3
Miller et al.
Schmidt+TCS
Fig.3. Spectral energy distribution of HBC722. Filled dots are
pre-outburst data from Miller et al. (2010), 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 1.25µm
and above 40µm are from D’Alessio et al. 1999, data between
1.25 and 40µm are from Furlan et al. 2006).
source to be between R=18–19.25mag in mid-2009. After that,
it brightened by ≈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
Covey et al. (2011) may not be the true quiescent brightness of
the source,since the source was ≈20mag in the POSS2 red plate
takenin 1990(Itagaki & Yamaoka2010).The comparisonof the
UKIDSS and TCS photometry indicates that the source bright-
ened by ∆J=7.9mag, ∆H=6.7mag, and ∆KS=4.8mag between
July 2006 and September 2010. We note that the source was
KS=13.15mag in 2006, but it was not visible in the KSband
in the 2MASS images taken in October 2000. Since the 2MASS
PSC is completedown to KS=14.3(Cutri et al. 2003), the source
must have brightened at least 1.15mag between 2000 and 2006,
making the true KS-band magnitude change at least 5.8mag.
Thecomparison of
VSXJ205126.1+440523 with other young eruptive stars
in Fig. 2 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.
the light curveof
Spectral energy distribution. In Fig. 5 we compiled a pre-
outburst SED using data from the UKIDSS database (this work),
the MSX6C Infrared Point Source Catalog (Egan et al. 2003),
the AKARI/IRC mid-infrared all-sky survey (Ishihara et al.
2010), and Spitzer data (Covey et al. 2011, 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 from1996-1997,andthe AKARI
from 2006-2007. This SED should be analysed with caution,
considering the KS-band variability mentioned above. The out-
22
20
18
16
14
12
10
8
Magnitude
300 350400
JD (2,455,000 + )
450 500550

Apr May Jun JulAugSep Oct Nov Dec
K-0.5
H-1
J-1.5
I-1
R
V+1
VSX J205126.1+440523
literature
this work
Fig.4. Light curves of VSXJ205126.1+440523. Filled dots
are from this work, plus signs are from Covey et al. (2011)
and from Seiichiro Kiyota and Hiroyuki Maehara (vsnet,
http://tech.dir.groups.yahoo.com/group/vsnet-recent-fuori/messages),
crosses are visual estimates by Itagaki & Yamaoka (2010).
burst SED contains optical and near-infrared photometrywe ob-
tained on 20/23 September 2010 and 16/17 November2010. We
note that by September, the source was already ≈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. 2) suggest that the source is highly extincted. However,
dereddeningits colorsdoesnotmakeitfallontotheTTaurilocus
(Fig. 6). Correcting for a reddening of AV=17...20mag would
result in a J−H color typical for TTauri stars, but its H−KScolor
would still be too red. The reason for the strange near-infrared
colorsof VSXJ205126.1+440523maybe partlyinterstellar red-
dening caused by the small cloud in which the source is embed-
ded and whose outlines are visible in Fig. 2, 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 cor-
rect for interstellar reddening, and calculate a bolometric lumi-
nosity of 14.7L⊙by simply integrating the quiescent SED from
1.25 to 200µm. We calculate an outburst luminosity of 22L⊙
similarly, assuming that the SED did not change above 10µm,
and using the September 2010 data points in Fig. 5.
4. Discussion
4.1. 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 24µm photometric point. This
conclusionis in accordancewith the claimof Miller et al. (2010)
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
8µm there is an excess emission in the SED compared to the
4

Page 5

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex
1 10 100
Wavelength [µm]
10-17
10-16
10-15
10-14
10-13
10-12
νFν [W m-2]
VSX J205126.1+440523
10-17
10-16
10-15
10-14
10-13
10-12
2010
Sep 20/23
2010
Nov 16/17
UKIDSS
Spitzer
MSX
Akari
Schmidt+TCS
Fig.5. Spectral energy distribution of VSXJ205126.1+440523.
Filled dots are pre-outburst data from various dates, while open
circles are outburst data (see text).
0.0 0.51.0 1.52.02.5 3.03.5
H - Ks
0.0
0.5
1.0
1.5
2.0
2.5
3.0
J - H
HBC 722
VSX J205126.1+440523
V1057 Cyg
V1647 Ori
EX Lup
Fig.6. Near-infrared color-color diagram. The main-sequence
is marked by a thick solid line, the giant branch with dotted
line (Koornneef 1983), the reddening path with dashed lines
(Cardelli et al.1989), andthe TTauri locuswith dash-dottedline
(Meyer et al. 1997). Open symbols: quiescent colors; filled sym-
bols: outburst colors. Source of data: Kenyon et al. (1991) for
V1057Cyg, Acosta-Pulido et al. (2007) for V1647Ori, 2MASS
PSC and Juh´ asz et al. (2011) for EXLup, and this work for
HBC722 and VSXJ205126.1+440523.
Taurus median. In this respect, the source somewhat resembles
DRTau,a highlyaccretingTTauristar,suggestingthatHBC722
might be a highly accreting TTauri star even in quiescence. The
fact that the quiescent optical spectrum, taken by Cohen & Kuhi
(1979) exhibits an unusually prominent Hα emission with an
equivalent width of 100Å supports this idea.
VSXJ205126.1+440523seems to be a much more reddened
source. Its quiescent bolometric luminosity (≈15L⊙) is signifi-
cantlylargerthanthatofHBC722,indicatingasomewhathigher
mass. The quiescent SED in Fig. 5 probably represents a mod-
erately reddened Class I source. The findings of Covey et al.
(2011), who determined 6mag<AV<12.4mag from the ratio of
H emissionlines observedintheoutburstspectrum,supportsthis
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 extinc-
tion may render the scattered light invisible. The presence of a
related Herbig-Haro object also advocates for the Class I sce-
nario.
4.2. The outburst mechanism
The comparison of the quiescent and the outburst SEDs (Fig. 3)
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 ≈4000K, somewhat less than what one
would expect from an accretion outburst, where ionized mate-
rial 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 decreas-
ing)temperatureprofile.The fadingof HBC722is mostly color-
independent in the near-infrared regime (Fig. 6), 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 tem-
perature distribution at all epochs (Fig. 4). The amplitude of the
outburst in the J, H and KSbands is larger than any brightening
observed so far for YSOs. At first glance, one may think that the
brighteningwas due to suddenly decreased extinction. However,
in this case the source should have moved along the reddening
path in the J−H vs. H−KSdiagram (Fig. 6) which was not the
case during the rising part of the lightcurve. JHKSphotometry
obtained after maximum brightness indicate that the fading of
VSXJ205126.1+440523initially happened along the same path
as the brightening,suggesting that whateverwas 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 ac-
tivity of VSXJ205126.1+440523might be repetitive. However,
photometry obtained in November 2010 indicates a deep mini-
mum in Fig. 4. At the same time, the source moved along the
reddening path in Fig. 6. Both the color changes and the bright-
ness changes in the J, H, and KSbands are consistent with an
extinction increase of AV=9mag. Thus, it may be possible that
the deep minimum in November2010 was caused by a dust con-
densationeffect(similartowhathappenedtoV1515Cygin1980
(Kenyon et al. 1991), or an eclipse by dust clumps in an almost
edge-on disk system (similarly to what causes deep optical min-
ima ofthe UX Orionis-typestars, see e.g.Eaton & Herbst 1995).
4.3. Classification as a FUor outburst
When discovered, both sources were announced as FUor can-
didates. Extensive analyses presented in Semkov et al. (2010)
and Miller et al. (2010) indicated that HBC722 can be con-
sidered as a bona fide FUor. On the other hand, Covey et al.
(2011) concluded that VSXJ205126.1+440523 does not ap-
5

Page 6

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex
pear to belong to either the FUor or the EXor class. FUors are
usually bright objects in outburst with luminosities of a few
hundred L⊙ (Hartmann & Kenyon 1996). The immense radia-
tion is related to the increased accretion rate, which can reach
values up to 10−4M⊙/yr. However, the outburst luminosities
of our objects are only in the order of 10-20L⊙. 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⊙ and a radius of 3R⊙) is
10−6M⊙/yr. Covey et al. (2011) determined an accretion rate of
2.5×10−7M⊙/yr for VSXJ205126.1+440523.Both of these val-
ues 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 pro-
totype 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,
Bell & Lin 1994), but the fading is far too rapid. According
to our light curve (Fig. 4), 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 bonafide FUor outbursts (decades to centuries).The non-
monotonous fading of this source also resembles the light curve
of the recent outburst of EXLup (Fig. 2). The moderate resolu-
tion near-infrared spectrum obtained in outburst by Covey et al.
(2011) also shows similarities to that of EXLup (K´ osp´ al et al.
in prep.).
It is remarkable that HBC722 possesses all spectral charac-
teristics of bona fide FUors but its luminosityis an order of mag-
nitude lower, and its fading timescale is much faster. This sug-
geststhatthephysicalmechanismwhichis behindtheFUor-type
eruptions should also work with lower accretion rates although
probablyproducing shorter outbursts. This conclusion questions
the thermal instability model of Bell & Lin (1994), who sug-
gested the existence of a threshold mass accretion rate from the
outer to the inner circumstellar disk. Matter can pile up at the in-
ner edge of the disk and fall onto the stellar surface following a
sudden thermal instability only if the quiescent accretion rate is
higherthanthisthresholdvalue.Thus,it seems thatin theregime
of low luminosity outbursts (≈10L⊙), 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,somewhatsimilar to VSXJ205126.1+440523)canoc-
cur. If both HBC722 and VSXJ205126.1+440523 are indeed
young eruptive stars, one might conclude that the class of young
eruptivestars is evenmorediversethanwhatwas thoughtbefore.
Acknowledgements. This work is based in part on observations made with the
Telescopio Carlos Sanchez operated on the island of Tenerife by the Instituto
de Astrof´ ısica de Canarias in the Observatorio del Teide. The authors wish
to thank the telescope manager A. Oscoz, support astronomer P. Monta˜ nes,
and telescope operators R. Mart´ ı, and M. D´ ıaz for their help during the ob-
servations. 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 publi-
cation 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´A.K. is supported by the
Nederlands Organization for Scientific Research.
References
´Abrah´ am, P., Juh´ asz, A., Dullemond, C. P., et al. 2009, Nature, 459, 224
Acosta-Pulido, J. A., Kun, M.,´Abrah´ am, P., et al. 2007, AJ, 133, 2020
Bally, J. & Reipurth, B. 2003, AJ, 126, 893
Bell, K. R. & Lin, D. N. C. 1994, ApJ, 427, 987
Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245
Cohen, M. & Kuhi, L. V. 1979, ApJS, 41, 743
Covey, K. R., Hillenbrand, L. A., Miller, A. A., et al. 2011, AJ, 141, 40
Cutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003, 2MASS All Sky Catalog
of point sources.
D’Alessio, P., Calvet, N., Hartmann, L., Lizano, S., & Cant´ o, J. 1999, ApJ, 527,
893
Eaton, N. L. & Herbst, W. 1995, AJ, 110, 2369
Egan, M. P., Price, S. D., Kraemer, K. E., et al. 2003, VizieR Online Data
Catalog, 5114, 0
Furlan, E., Hartmann, L., Calvet, N., et al. 2006, ApJS, 165, 568
Gieseking, F. 1973, Information Bulletin on Variable Stars, 806, 1
Hartmann, L. & Kenyon, S. J. 1996, ARA&A, 34, 207
Ishihara, D., Onaka, T., Kataza, H., et al. 2010, A&A, 514, A1
Itagaki, K. & Yamaoka, H. 2010, Central Bureau Electronic Telegrams, 2426, 1
Juh´ asz, A., Dullemond, C. P., van Boekel, R., et al. 2011, ApJ, submitted
Kenyon, S. J., Hartmann, L. W., & Kolotilov, E. A. 1991, PASP, 103, 1069
Koornneef, J. 1983, A&A, 128, 84
K´ osp´ al,´A.,´Abrah´ am, P., Reg´ aly, Z., et al. in prep., ApJ
K´ osp´ al,´A., Salter, D. M., Hogerheijde, M. R., Mo´ or, A., & Blake, G. A. 2010,
ArXiv e-prints
Meyer, M. R., Calvet, N., & Hillenbrand, L. A. 1997, AJ, 114, 288
Miller, A. A., Hillenbrand, L. A., Covey, K. R., et al. 2010, ApJ, submitted
Munari, U., Valisa, P., Dallaporta, S., & Itagaki, K. 2010, Central Bureau
Electronic Telegrams, 2428, 1
Semkov, E. & Peneva, S. 2010, The Astronomer’s Telegram, 2801, 1
Semkov, E. H., Peneva, S. P., Munari, U., Milani, A., & Valisa, P. 2010, A&A,
523, L3
Straizys, V., Meistas, E., Vansevicius, V., & Goldberg, E. P. 1989, A&A, 222, 82
6

Page 7

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex, Online Material p 1
Online Material

Page 8

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex, Online Material p 2
Fig.1. Finding chart for VSXJ205126.1+440523and its comparison stars. North is up and east is to the left.
Table 1. V(RI)Cphotometry for the comparison stars used for VSXJ205126.1+440523.Typical uncertainties are 0.04mag.
Star
A
B
C
D
E
F
2MASS idVRC
IC
2MASS J20510393+4411406
2MASS J20511195+4407213
2MASS J20510294+4409277
2MASS J20511769+4402412
2MASS J20514142+4403032
2MASS J20513057+4403449
14.091
14.268
14.684
14.981
15.300
15.924
13.509
12.400
13.856
14.146
14.790
14.864
12.985
10.968
13.198
13.307
14.236
13.816

Page 9

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex, Online Material p 3
Table 2. Photometry for VSXJ205126.1+440523.
Date JD−2,400,000
53936.95
55459.47
55460.31
55460.46
55461.32
55463.36
55464.27
55478.47
55479.34
55480.34
55481.47
55482.31
55483.30
55505.32
55505.32
55506.35
55506.35
55507.35
55507.35
55508.34
55508.40
55514.40
55515.37
55516.20
55517.42
55518.42
55519.42
55524.20
55531.24
55541.27
VRIJHKS
Telescope
UKIRT
TCS
Schmidt
TCS
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
TCS
IAC-80
TCS
IAC-80
TCS
IAC-80
TCS
IAC-80
Schmidt
Schmidt
Schmidt
TCS
TCS
TCS
Schmidt
Schmidt
Schmidt
2006-Jul-20
2010-Sep-19
2010-Sep-20
2010-Sep-20
2010-Sep-21
2010-Sep-23
2010-Sep-24
2010-Oct-09
2010-Oct-10
2010-Oct-11
2010-Oct-12
2010-Oct-13
2010-Oct-14
2010-Nov-04
2010-Nov-04
2010-Nov-05
2010-Nov-05
2010-Nov-06
2010-Nov-06
2010-Nov-07
2010-Nov-07
2010-Nov-13
2010-Nov-14
2010-Nov-15
2010-Nov-16
2010-Nov-17
2010-Nov-18
2010-Nov-18
2010-Nov-30
2010-Dec-10
19.34(7)
11.41(1)
16.50(7)
9.85(2)
13.15(11)
8.35(4)
16.76(5) 15.19(4)13.71(4)
11.46(1) 9.80(1)8.25(2)
16.74(4)
16.79(5)
15.16(4)
15.30(4)
15.12(8)
14.80(4)
14.83(4)
14.69(4)
14.94(4)
15.08(5)
14.99(4)
13.69(3)
13.82(4)
13.75(4)
13.41(4)
13.40(4)
13.38(4)
13.56(4)
13.67(4)
13.86(6)
16.35(4)
16.39(4)
16.33(4)
16.56(4)
16.67(4)
16.67(5)
12.75(3)10.82(2)8.90(1)
18.15(5) 16.51(6)15.08(5)
12.60(3)10.78(2)8.98(3)
17.98(5)16.40(4)14.84(4)
13.13(1)11.08(1) 9.18(3)
17.14(10)15.43(11)
13.04(1)11.05(1)9.20(3)
18.87(11)
19.55(13)
20.46(30)
20.05(25)
17.21(10)
18.01(17)
18.46(7)
18.72(7)
15.56(9)
16.47(11)
16.76(4)
16.84(5)
15.97(4)
16.05(4)
14.43(4)
19.71(14)
17.79(5)
17.93(8)
16.06(5)
10 -1

-1
0
1
DEC offset [arcmin]



10 -1
RA offset [arcmin]




10 -1
RA offset [arcmin]







10 -1
RA offset [arcmin]

-1
0
1
DEC offset [arcmin]
-1
0
1
10 -1
RA offset [arcmin]




Fig.2. VSXJ205126.1+440523 and its surroundings. The field of view is 3′×3′. The upper row shows pre-outburst images (left:
POSS2 red, middle: UKIDSS J, right: Spitzer/IRAC 8µm), while the bottom row shows outburst images (left: Konkoly Schmidt,
middle: Teide TCS).

Page 10

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex, Online Material p 4
Table 3. Photometry for HBC722.
DateJD−2,400,000
53936.99
55459.44
55460.28
55460.44
55461.29
55463.37
55464.32
55478.43
55483.44
55505.35
55505.35
55506.37
55506.39
55507.35
55507.37
55508.36
55508.38
55514.38
55515.41
55516.25
55517.41
55517.42
55518.42
55519.44
55519.44
55520.40
55520.40
55524.33
55531.19
55532.31
55534.42
55541.31
55543.34
55564.32
BVRIJHKS
Telescope
UKIRT
TCS
Schmidt
TCS
Schmidt
Schmidt
Schmidt
Schmidt
Schmidt
TCS
IAC-80
TCS
IAC-80
IAC-80
TCS
TCS
IAC-80
Schmidt
Schmidt
Schmidt
IAC-80
TCS
TCS
TCS
IAC-80
TCS
IAC-80
RCC
Schmidt
IAC-80
IAC-80
Schmidt
IAC-80
IAC-80
2006-Jul-20
2010-Sep-19
2010-Sep-20
2010-Sep-20
2010-Sep-21
2010-Sep-23
2010-Sep-24
2010-Oct-09
2010-Oct-14
2010-Nov-04
2010-Nov-04
2010-Nov-05
2010-Nov-05
2010-Nov-06
2010-Nov-06
2010-Nov-07
2010-Nov-07
2010-Nov-13
2010-Nov-14
2010-Nov-15
2010-Nov-16
2010-Nov-16
2010-Nov-17
2010-Nov-18
2010-Nov-18
2010-Nov-19
2010-Nov-19
2010-Nov-23
2010-Nov-30
2010-Dec-01
2010-Dec-03
2010-Dec-10
2010-Dec-12
2011-Jan-02
13.23(8)
9.96(5)
12.21(6)
9.12(1)
11.32(7)
8.68(3)
13.71(3) 12.68(1)11.67(1)
9.99(1) 9.09(1) 8.70(2)
13.76(2)
13.70(11)
13.62(9)
13.70(2)
13.85(1)
12.71(2)
12.59(2)
12.59(5)
12.68(2)
12.80(1)
11.70(1)
11.60(1)
11.65(4)
11.67(1)
11.76(1)
15.23(2)
10.16(3) 9.28(1)8.75(1)
15.46(5) 13.96(3) 12.89(2)11.86(1)
10.18(3) 9.22(3) 8.75(6)
15.47(2)
15.50(5)
13.96(1)
13.97(5)
12.91(2)
12.92(6)
11.85(6)
10.16(1)
10.21(1)
9.26(1)
9.27(1)
8.71(1)
8.78(1)
15.56(5)
15.57(5)
15.68(4)
15.66(3)
15.69(3)
14.02(2)
14.07(1)
14.07(2)
14.07(1)
14.13(2)
13.01(3)
13.00(1)
13.02(1)
13.05(2)
13.10(1)
11.97(3)
11.98(1)
12.00(1)
10.28(1)
10.31(1)
10.31(1)
9.38(1)
9.37(1)
9.36(1)
8.81(3)
8.86(1)
8.82(1)
15.73(5) 14.16(5)13.16(5)
10.24(1)9.34(1)8.81(1)
15.69(9) 14.15(3)13.10(3)
12.09(5)
12.12(1)
12.14(1)
12.14(2)
12.23(2)
12.23(2)
12.40(2)
15.79(5)
15.79(4)
15.80(3)
15.93(4)
15.98(5)
16.17(4)
14.23(2)
14.26(2)
14.26(2)
14.39(2)
14.40(2)
14.59(2)
13.18(1)
13.20(1)
13.22(1)
13.30(2)
13.33(2)
13.52(2)

Page 11

´A. K´ osp´ al et al.: The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex, Online Material p 5
1 0 -1

-1
0
1
DEC offset [arcmin]



1 0 -1




10 -1
RA offset [arcmin]







10 -1
RA offset [arcmin]
-1
0
1
DEC offset [arcmin]
-1
0
1
10 -1
RA offset [arcmin]



Fig.3. HBC722 and its surroundings. The field of view is 3′×3′. The uppper row shows pre-outburst images (left: POSS2 red,
middle: UKIDSS J, right: Spitzer/IRAC 8µm), while the bottom row shows outburst images (left: Konkoly Schmidt, middle: Teide
TCS).