A&A 470, 211-219 (2007)
DOI: 10.1051/0004-6361:20066108
Á. Kóspál1 - P. Ábrahám1 - T. Prusti2 - J. Acosta-Pulido3 - S. Hony4 - A. Moór1 - R. Siebenmorgen5
1 - Konkoly Observatory of the Hungarian Academy of Sciences, PO Box
67, 1525 Budapest, Hungary
2 -
ESTEC/SCI-SAF, Postbus 299, 2200 AG Noordwijk, The
Netherlands
3 -
Instituto de Astrofísica de Canarias, 38205 La Laguna,
Tenerife, Canary Islands, Spain
4 - Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001
Leuven, Belgium
5 -
European Southern Observatory, Karl-Schwarzschild-Strasse 2,
85748 Garching, Germany
Received 25 July 2006 / Accepted 22 April 2007
Abstract
Aims. OO Serpentis is a deeply embedded pre-main sequence star in the Serpens NW star-forming region. The star went into outburst in 1995 and gradually faded afterwards. In many respects its eruption resembled the well-known FU Orionis-type (FUor) or EX Lupi-type (EXor) 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.
Methods. OO Ser was monitored with the Infrared Space Observatory in the
m wavelength range, starting 4 months after peak brightness and covering a period of 20 months. Eight years later, in 2004-2006 we again observed OO Ser at 2.2 and
m from the ground and complemented this dataset with archival Spitzer observations 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.
Results. The outburst caused brightening in the whole infrared regime. According to the infrared light curves, OO Ser started a wavelength-independent fading after the peak brightness. Later the flux decay became slower but stayed practically 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.
Conclusions. The outburst timescale and the moderate luminosity suggest that OO Ser is different from both FUors and EXors, and shows some similarities to the recently erupted young star V1647 Ori. Based on its SED and bolometric temperature, OO Ser seems to be an early class I object, with an age of <105 yr. As proposed by outburst models, the object is probably surrounded by an accretion disc and a dense envelope. This picture is also supported by the wavelength-independence of the fading. Due to the shorter outburst timescales, models developed for FUors can only work for OO Ser if the viscosity parameter in the circumstellar disc, ,
is set to an order of magnitude higher value than usual for FUors.
Key words: stars: pre-main sequence - stars: circumstellar matter - infrared: stars - stars: individual: OO Serpentis
OO Serpentis (
,
)
is a
deeply embedded pre-main sequence star in the Serpens NW star-forming
region at a distance of 311 pc. In 1995 OO Ser underwent a
large increase in K-band flux in less than 1 year, reaching its
maximum brightness in 1995 October (Hodapp et al. 1996b). The object was
not visible, even at peak brightness, in the J-band or at shorter
wavelengths, therefore it is also known as Serpens Deeply Embedded
Outburst Star (DEOS). In the H and K bands the object was observable
but the emission is dominated by scattered light. In the K-band
Hodapp (1999) monitored the outburst until 1998 October, and found
that after the peak OO Ser gradually faded at a rate faster than the
typical fading rate of FU Orionis-type objects (FUors), but slower
than that of EX Lupi-type stars (EXors). Its K-band spectrum (a
steeplyrising, smooth continuum) also differed from both FUor and
EXor spectra, which usually exhibit absorption or emission features.
Eruptions of pre-main sequence stars are rare events, thus a new outburst is always noteworthy. The eruption mechanism is thought to be caused by enhanced accretion from the circumstellar disc onto the star (e.g. Hartmann & Kenyon 1996). The close link between the eruption phenomenon and the circumstellar material makes it crucial to document the outburst also at infrared wavelengths, where the circumstellar dust radiates. However, such observing programmes are constrained by the availability of active infrared satellite missions. The physical analysis of the phenomenon is limited due to the lack of preoutburst data. The eruption of OO Ser in 1995 provided a unique opportunity to collect such a dataset and carry out a multiwavelength infrared study of the whole outburst event for the first time.
Triggered by the news on the outburst of OO Ser we activated a Target
of Opportunity programme on the Infrared Space Observatory
(ISO, Kessler et al. 1996). The infrared monitoring started 4 months after
the maximum brightness of OO Ser, and continued for 20 months. The
ISO-SWS measurements from this programme were published separately by
Larsson et al. (2000). They found that OO Ser changed its infrared fluxes
in the
m range and estimated an extinction of
mag from the optical depth of the
m silicate absorption
feature. In an independent programme the Serpens core was surveyed by the
ISOCAM instrument providing 6.7 and
m photometry on
OO Ser for a single epoch (Kaas et al. 2004).
In this paper we analyse our ISOPHOT and ISOCAM observations from the
monitoring programme. In addition to ISO measurements, we observed
OO Ser from the ground at m in 2004 and 2006 as well as at
m in 2004. We complemented this database with archival
Spitzer data also from 2004, as well as with previously published
measurements on OO Ser from the literature.
ISOPHOT, the photometer on-board ISO (Lemke et al. 1996), carried out
multi-filter photometry with 9 different filters in the
m wavelength range and spectrophotometry in the
m range, at 8 different epochs between 1996
February and 1997 September. Table 1 shows the log of
the observations. In most cases small raster maps were obtained except
in 1997 September, when at
m and shortwards the source and
background positions were observed separately. The typical integration
time was 64 s. Aperture sizes varied according to the filters:
at
m, 18'' at
m,
at
12, 15 and
m. At 60 and
m the C100 camera
(
pixel,
per pixel), while at
170 and
m the C200 camera (
pixel,
per pixel) was utilised. Some
far-infrared observations were performed in the PHT 32 mode, which
provided higher spatial resolution (for a description of this
mode see Tuffs & Gabriel 2003).
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Figure 1: OO Ser and its surroundings at different infrared wavelengths. V370 Ser (EC 37), V371 Ser (EC 53), EC 38, SMM 9 and SMM 1 are also marked. In the middle panel, the spots below the brightest sources are instrumental artifacts ("bandwidth effects''). The white star in the right panel indicates the position of IRAS 18272+0114 as given in the IRAS Catalog of Point Sources. |
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The 6.7 and m photometry was obtained with the ISOCAM
instrument (Cesarsky et al. 1996) on 1997 September 22 (see
Table 1). The data were reduced with the CAM
Interactive Analysis Software V5.0 (CIA, Ott et al. 1997). A dark current
correction was applied following the "VilSpa'' method. Glitches were
removed using the "multiresolution median transform''. This efficiently
removes glitches based on the fact that glitches in general are much
shorter than the signature of a real source. The data were stabilised
using the Fouks-Schubert model. Next the individual frames were
averaged to four mean images, one for each sky position. After this, a
second deglitching was applied based on the overlapping projected sky
positions of these mean images. Finally, the images were flat-fielded
and combined into the final mosaic. The pixel values were converted to mJy/arcsec2 using the tabulated conversion factors available in
CIA.
Photometry of the sources was obtained using the IRAF tool "xphot''. We
used a small aperture photometry of
and
radius
for the 6.7 and
m mosaic, respectively. These values were
then corrected for the flux falling in the wings of the point spread
function outside the chosen aperture. The small aperture was selected
because of a nearby source (V370 Ser at a distance of
)
which otherwise would have contributed to the
measured flux. The results are shown in Table 1. The
insignificant background level of 1 and 3 mJy/arcsec2 does not
contribute to the measurement error, which should be dominated by the
absolute flux calibration uncertainty of 3.3% at
m and 4.8% at
m (Blommaert et al. 2003).
-band images were obtained by the LIRIS instrument on 2004 June 11 (as part of the LIRIS Guaranteed Time programme) and on 2006 May 6. LIRIS is an infrared camera/spectrograph built at the Instituto de
Astrofísica de Canarias (Manchado et al. 2004; Acosta-Pulido et al. 2003), and is mounted on the
4.2 m William Herschel Telescope at the Observatorio del Roque de los
Muchachos (Canary Islands). The detector used is a
HAWAII-1 detector (Hodapp et al. 1996a), which
provides a plate scale of
/pixel and a total area of
.
In 2004 a three point dither pattern
was used with a total exposure time of 140 s, while in 2006 a
five-point dither pattern was used and the total exposure time was 250 s. The images were reduced using the IRAF package "liris_ql''
developed by the LIRIS team within the IRAF environment. The reduction
steps consist of flat-fielding, sky subtraction and image
co-addition.
In Fig. 1 (left) a part of our -band LIRIS image
from 2004 can be seen showing the surroundings of OO Ser. The
eruptive star was clearly detected in
and a faint nebulosity
around the star can be seen as well (see also
Sect. 3.1). Because of this nebulosity, photometry should
be done carefully. In order to be able to compare our measurements
with those of Hodapp (1999), we used an aperture diameter of
,
the same as Hodapp (1999). Conversion from
instrumental magnitudes to real magnitudes was done using the 2MASS
colour versus
(the difference between the
instrumental and the 2MASS
magnitudes) relationship for 20 comparison stars in the field. The resulting values are
mag in 2004 and
mag in 2006. We note that the colours of the 20 comparison stars used in this
calibration procedure covered a large enough range to include also the
colour of OO Ser itself. Similar photometry was derived for some
nearby sources (similarly to Hodapp 1999, we used a larger,
diameter aperture for the two extended objects: OO Ser
and V371 Ser, and a smaller
diameter aperture with
the corresponding aperture correction for the point-like objects:
V370 Ser, EC 38 and SMM 9); the results can be seen in
Table 2.
A K-band spectrum of OO Ser was obtained using LIRIS on 2006 May 6. The observation was performed following an ABBA telescope nodding
pattern. The total exposure time was 2400 s, split in 4 individual
exposures of 600 s. In order to reduce the readout noise, the
measurements were done using multiple correlated readout mode, with 4 readouts before and after the integration. We used a slit width of 1
and a medium resolution sapphire grism which yielded a
spectral resolution of 2500. The wavelength calibration was provided
by observations of an Argon lamp available in the calibration unit at
the A&G box of the telescope. In order to obtain the telluric
correction, the nearby A0V star HIP 90123 was observed with the same
configuration as the object. The data were reduced and calibrated
using the package "liris_ql''. Consecutive pairs of AB two-dimensional images were subtracted to remove the sky background,
then the resulting images were wavelength calibrated and flat-fielded
before registering and coadding all frames to provide the final
combined spectrum. A one-dimensional spectrum was extracted with the
IRAF "apall'' task. The extracted spectrum was divided by a composite
to eliminate telluric contamination. This composite spectrum was
generated from the observed spectrum of the calibration star, divided
by a stellar model and convolved to our spectral resolution. The
resulting normalised K-band spectrum is shown in Fig. 2.
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Figure 2: Normalised K-band spectra of OO Ser. Continuous line: this work, dashed and dotted lines: Hodapp (1999). For clarity, we multiplied the spectrum from 1998 by 10 and the spectrum from 1995 by 100. |
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OO Ser was imaged with TIMMI2 mounted on the ESO 3.6 m telescope at
La Silla on 2004 October 21, under clear and stable conditions. The
N11.9-OCLI filter was used which has a central wavelength of
m and a FWHM of
m. The total integration time
was 4 min. Both chopping and nodding amplitudes were
,
and the
pixel scale of the
Raytheon
detector was set. This resulted in a chop-nod corrected image with
two negative and two positive beams captured on the detector. The four
beams were used for independent determination of the source flux and
error. The resulting fluxes are 0.64 Jy for OO Ser and 0.18 Jy for
V370 Ser (a nearby young star also visible in the field, see also
Table 2). The flux calibrator was the photometric
standard HD 96171 which was observed with the same
set-up. Other photometric standard star observations of the same night
were inspected to estimate the total absolute flux uncertainty of
.
OO Ser was measured with the IRAC and MIPS instruments of the Spitzer
Space Telescope on 2004 April 4 and 5, respectively, as part of the
legacy programme "c2d'' (PI: Neal J. Evans II). The third data
release of this programme containing enhanced data products and
catalogues can be downloaded from the Spitzer
website.
As an example we plotted parts of the
m IRAC and the
m MIPS maps in Fig. 1.
The "c2d'' catalogue contains IRAC fluxes for OO Ser, but it is
treated as a point source. The nebulosity seen in the K-band images,
however, are still visible at 3.6 and 4.5m too. In order to be
able to compare the IRAC fluxes with the ISO measurements, we decided
to extract fluxes from the IRAC images for OO Ser using the same
apertures as ISOPHOT used (diameter of
at
m and 18'' at
m)
The current version of the "c2d'' catalogue does not contain MIPS
fluxes yet. In order to obtain photometry in the MIPS bands, we
downloaded the enhanced MIPS images. At m we selected 7 isolated stars to construct the point spread function. Since
OO Ser was saturated, this profile was then fitted to the
non-saturated wings of OO Ser. At
m OO Ser and a nearby
young star, V370 Ser could not be fully separated. Thus an aperture
of 20'' was utilized, which included both objects. The measured flux
was then distributed between OO Ser and V370 Ser in the same ratio
as their peak brightnesses (measured in a 5'' aperture at the
position of the two sources). We also extracted
m photometry
for some other sources in the vicinity of OO Ser, since they possibly
cause source confusion in IRAS and ISO far-infrared measurements. In
these cases, a 20'' aperture was utilized with a fixed 0.142 Jy
background.
Colour corrections were applied to each measurement for each source by convolving the observed SED with the IRAC and MIPS filter profiles in an iterative way. The resulting fluxes and estimated uncertainties can be seen in Table 2.
Figure 1 shows that several infrared and submillimetre
sources are present in the vicinity of OO Ser: V370 Ser (also known
as EC 37), V371 Ser (also known as EC 53 or SMM 5),
EC 38, SMM 9 and SMM 1. At shorter wavelengths
(2.2, 3.6, 4.5 and m) an extended nebulosity around
OO Ser can also be seen. Thus, when comparing the fluxes of OO Ser
measured with different instruments one has to keep in mind that - to
some extent - the nebulosity and some of the abovementioned sources may
contribute to the observed flux at a particular wavelength. At
m and with the ISOPHOT/ISOCAM at 3.6, 4.8, 6.7 and
m the beams included OO Ser only. At 12, 15 and
m the ISOPHOT beams included OO Ser and V370 Ser. At 60,
100, 170 and
m the fluxes extracted for OO Ser include
also contributions from V370 Ser, V371 Ser, EC 38 and
SMM 9. Recent infrared photometry for these sources can be seen in
Table 2. It should be noted that fluxes presented in
Table 1 contain the contributions of nearby sources as
discussed above. In Sect. 3.4 we give a detailed
description of how we corrected the ISOPHOT measurements for the
effects of source confusion.
According to Hodapp (1999) before the outburst a triangle-shaped
nebula west of OO Ser and a small elongated nebula east of OO Ser
could be seen. During the outburst these - presumably
reflection - nebulae became much brighter. In this phase
Hodapp et al. (1996b) observed OO Ser in the
and
bands, as well as at 11.7 and
m. They found that the
nebulosity can be seen in the
but not at longer
wavelengths. Our recent
-band images reveal that the nebulae
still exist and look very similar to the preoutburst image of
Hodapp (1999, see his Fig. 1 left). The nebulosity around OO Ser
is also visible in the Spitzer/IRAC maps from 2004 at 3.6, 4.5 and
m and some extended emission can be suspected even at
m.
In order to study the consequences of the outburst in the whole
infrared regime, one has to compile first the SED of OO Ser in the
quiescent phase, i.e. estimate the preoutburst fluxes. At m
there exists a preoutburst measurement from 1994 (Hodapp et al. 1996b). At
longer wavelengths, only the IRAS measurements are available from 1983. Analysing high-resolution IRAS maps, Hurt & Barsony (1996) derived
fluxes of 0.63, 4.5, 24 and 111 Jy at 12, 25, 60 and 100
m,
respectively. The authors claim that these values represent the total
fluxes from a region encompassing three confused sources: OO Ser,
V371 Ser and SMM 9, and they give the one third of the
abovementioned values as upper limits for the brightness of
OO Ser. The position of IRAS18272+0114 from the IRAS Catalog of
Point Sources (marked by a white star in Fig. 1 right)
is located halfway between the sources, which may indicate that there
was no dominant source, but all sources had comparable contributions
to the IRAS flux.
With the help of Spitzer/MIPS measurements, it is possible to check
the validity of the assumption of Hurt & Barsony (1996) via estimating
the preoutburst fluxes of OO Ser at 25 and m. Assuming
that EC 38 and SMM 9 have non-variable far-infrared fluxes, we
subtracted the contribution of these sources from the IRAS values
cited above. Another nearby source, V371 Ser, exhibits
near-infrared variability of
1.5 mag and shares many
characteristics with EXors (Hodapp 1999). At far-infrared
wavelengths, however, eruptive young stars do not typically show
significant flux changes (Ábrahám et al. 2004a). Thus we assumed that V371 Ser
is also non-variable at 25 and
m, at least within our
measurement uncertainties, and we subtracted its flux from the IRAS
values. In practice, we estimated
m fluxes for these nearby
sources via interpolating from the
m MIPS values in
Table 2, and subtracted the sum of these (3.6 Jy) from
the value given by Hurt & Barsony (1996) (4.5 Jy). The result is 0.9 Jy, which is indeed of the order of one third of 4.5 Jy. The
result at
m is also roughly consistent with the one third
value. Due to the lack of recent 12 or
m data for all
nearby sources, the same test cannot be done at these
wavelengths. Therefore, for homogeneity, at all four IRAS wavelengths
we adopted as preoutburst fluxes the one third values within a factor
of 2 uncertainty:
0.21+0.21-0.10 Jy at
m,
1.5+1.5-0.8 Jy at
m,
8+8-4 Jy at
m,
37+37-19 Jy at
m. We note that OO Ser
is the first eruptive young star where preoutburst fluxes are
available in the whole infrared wavelength regime. Similar preoutburst
data can be found only for one other young eruptive star, V1647 Ori
(Ábrahám et al. 2004b).
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Figure 3:
Spectral energy distribution of OO Ser. Squares:
preoutburst fluxes measured with IRAS in 1983 (Hurt & Barsony 1996), and
K-band photometry from 1994 August, (Hodapp et al. 1996b); Dots and
line: outburst fluxes from 1996 September, measured with ISOPHOT;
Stars: current fluxes from 2004, measured with LIRIS,
TIMMI2 and Spitzer. Error bars smaller than the symbol size are not
plotted. ISOPHOT beams at 100 and ![]() |
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Figure 4:
Ice features in the spectrum of OO Ser. Data below
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Figure 5:
Light curves of OO Ser at different wavelengths. Open
circles: measurements of Hodapp (1999); Hodapp et al. (1996b); Stars:
LIRIS; Filled circles: ISOPHOT; Triangle: TIMMI2;
Squares: Spitzer; Dashed lines: preoutburst fluxes from 1994
at ![]() ![]() |
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In Fig. 3 the SED of OO Ser in the high (outburst) state
from 1996 September is plotted with filled symbols. We attempted to
correct for the effects of source confusion as described in details in
Sect. 3.4. Thus, the resulting SED in Fig. 3
represents the flux from OO Ser alone at
m. Due to high extinction the star is
invisible at optical wavelengths and is very faint in the
near-infrared regime. In the mid-infrared (
m) the SED is
rising towards longer wavelengths. This wavelength regime is zoomed in
Fig. 4, showing a broad silicate absorption feature at
m, as well as several ice features, which also indicate high
extinction. Measuring the optical depth of the
m feature,
and converting it to visual extinction assuming
(Draine 2003), we obtained
mag. This value is twice as large as the one measured
by Larsson et al. (2000); the difference is probably related to the low
signal-to-noise of his SWS spectra around the
m absorption
feature. The measured high extinction indicates that OO Ser is more
deeply embedded than most FUors. At far-infrared wavelengths
(
m) the SED is flat, but it should be noted that data
points at 100 and
m are contaminated by source confusion
(see Sect. 2.7).
In Fig. 3 the preoutburst SED (see Sect. 3.2) and the present SED (based on measurements from 2004) are also plotted. They will be discussed in Sects. 3.4.2 and 3.4.6.
ISOPHOT, ISOCAM, LIRIS, TIMMI2, Spitzer, IRAS (Hurt & Barsony 1996) and K-band measurements (Hodapp 1999; Hodapp et al. 1996b) were combined to construct the light curves of OO Ser at different wavelengths between 1995 and 2006.
For the subsequent light curve analyses, we attempted to correct for the effects of source confusion. One possibility would be to smooth all data to the same resolution, in most cases defined by the ISOPHOT aperture. In doing so, however, several additional sources would be included in the beam, falsifying the fading rate calculations for OO Ser. Instead, we decided to use the higher spatial resolution images to correct for the contribution of additional, unrelated sources in the large ISOPHOT beams.
At m TIMMI2 could resolve OO Ser and V370 Ser. Assuming
that V370 Ser is not variable at this wavelength, we subtracted its
contribution of 0.18 Jy (see Table 2) from each ISOPHOT
m measurement. The MIPS camera of Spitzer at
m could
separate OO Ser from V370 Ser giving a flux of 1.56 Jy for the
latter source, which we interpolated to
m (1.71 Jy) and
subtracted from the ISOPHOT points at
m. At
m, MIPS
m measurements could be utilized. Using the SEDs presented in
Table 2, we interpolated
m fluxes for V370 Ser,
V371 Ser, EC 38 and SMM 9, and subtracted the sum of these values
(24 Jy) from the ISOPHOT
m data points. Due to the lack of
m fluxes for the nearby sources, a similar correction was
not possible in the case of the
m ISOPHOT light curve.
In Fig. 5 six representative light curves between
2.2 and m are shown. All these light curves are corrected
for source confusion and represent the brightness evolution of OO Ser
alone. In the following, we describe these light curves in detail.
FUor and EXor ourbursts were historically monitored at optical
wavelengths. Since OO Ser is invisible in the optical,
is the
shortest available band where the outburst could have been followed.
The top left panel of Fig. 5 shows the
m
light curve of OO Ser. The star brightened by 4.6 mag between 1994 August and 1995 July. After reaching peak brightness, it started an
approximately exponential fading with a rate of 1.00 mag/year in the
first 350 days and 0.34 mag/year afterwards, as the data between 1995 and 1999 indicate (Hodapp 1999). The change in fading rate
divides this period of the outburst into a first and a second
part. Our measurements from 2004-2006 prove that the fading
continued, although at a slightly different rate, representing a third
part of the fading. Comparing the preoutburst flux with the new
observations, one can conclude that at
m OO Ser is still
above the preoutburst flux level.
In the following, we describe the lightcurves of OO Ser at longer wavelengths, following the abovementioned division: the initial rise (until peak brightness); the first part of the fading (until mid-1996); the second part of the fading (until mid-1997); and the third part of the fading (until 2004).
Comparison of the preoutburst fluxes with the SED in outburst
(Fig. 3) shows that the eruption caused brightening in the
whole near- to far-infrared spectrum. Although the fluxes at
m are contaminated by nearby sources, there is a flux change
at this wavelength, too. The shape of the SED changed significantly:
the outburst SED is flatter.
At m the light curve reached its peak in 1995 October. At 3.6, 4.8 and
m, the exact date of the peak is not known,
but it happened not later than 1995 October, thus it was probably
simultaneous with the K-band peak. At
m, however, the peak
took place in 1996 April, i.e. some 200 days later than at
m. The shape of the peak at
m is also different, it
is broader than at
m, and has a triangle-like shape. At 60and
m, the photometry is more uncertain, thus the peak dates
cannot be determined. Nevertheless, the time-shift at
m gives
a hint that the peak brightness happened gradually later at longer
wavelengths. To our knowledge, such a time-shift has not been observed
for other eruptive stars.
In order to analyse the wavelength dependence of the brightening, in
Fig. 6 we plotted the ratio of the peak flux to
the preoutburst value at 2.2, 12, 25, 60 and m.
Since at
m both the preoutburst IRAS flux and the peak
ISOPHOT flux include contributions from several nearby sources, the
ratio derived from these numbers represents a lower limit for the
brightening of OO Ser itself. Figure 6 suggests
that the amplitude of the flux increase has a characteristic
wavelength dependence: the flux ratio is lower at longer wavelengths,
with an approximately linear dependence on
.
Although the
peak brightness did not occur simultaneously at different wavelengths,
this trend can also be seen if one compares in Fig. 3 the
preoutburst SED with the SED from 1996 September (close to peak
brightness). We note that this result is different from the case of
another young eruptive star, V1647 Ori, where the initial rise was
practically wavelength-independent (Muzerolle et al. 2005; Ábrahám et al. 2004b).
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Figure 6: Initial rise: flux ratio of peak brightness to preoutburst brightness at each wavelength. (See discussion in Sect. 3.4.2.) |
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Figure 7: Fading rates of OO Ser at different wavelengths, a) between 1996 April and 1996 September, b) between 1996 October and 1997 March, c) between 1997 April and 2004. |
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During the first year after peak brightness, OO Ser exhibited similar
exponential fading at all wavelengths. This can be quantified by
fitting a linear relationship to the first part of the light curves
plotted logarithmically in Fig. 5, using data
taken only after peak brightness at a given wavelength. The derived
fading rates are displayed in Fig. 7a.
This graph shows that the fading rate during this period was
approximately mag/year irrespectively of wavelength
in the whole mid-infrared regime.
The second part of the light curves (between 1996 October and 1997 September) can be characterised by fading rates of
mag/year, similar at all wavelengths from 2.2 to
m
(Fig. 7b). This is approximately three times slower
than the fading during the first part. In this period ISOPHOT-S
spectra were also obtained, which give the opportunity to analyse the
fading with high wavelength resolution. In order to increase the
signal-to-noise ratio, the fitted fading rates of ISOPHOT-S were
binned (in the case of the short wavelength channels) or smoothed (in
the case of the long-wavelength channels). The resulting values are
overplotted with small open or filled dots in
Fig. 7b, respectively. These values also support a
wavelength-independent fading, characteristic of the second part of
the outburst (except where the emission of OO Ser is strongly reduced
by the silicate absorption around
m, introducing a large
uncertainty in the fit).
New measurements from 2004 revealed that the fading monitored by
ISOPHOT continued at all wavelengths after 1997. We calculated fading
rates for the period 1997-2004. The resulting numbers, which are
plotted in Fig. 7c, are even lower than
those for the second part and they also show no
wavelength dependence. The values are in the range of
mag/year.
In Fig. 5 the preoutburst fluxes (marked by dashed lines) can be compared with the latest measurements from 2004-2006. This comparison reveals that in 2004 OO Ser was still above the preoutburst state in the whole infrared wavelength regime, indicating that the outburst had not finished yet. The same conclusion can be drawn from Fig. 3 if one compares the SED from 1996 with that from 2004.
In Fig. 5 the last K-band photometric point
from 2006 seems to deviate from a linear extrapolation of the
preceding fading, possibly indicating a fourth phase of the outburst
with very slow flux change. Extrapolating the lightcurves using the
rates calculated for the third part of the fading (1997-2004),
the expected end date of the outburst is 2011 from both
the
m and the
m lightcurve. The slowing down
of the fading process, however, may delay this event even well beyond
2011.
Figure 2 displays our K-band spectrum from 2006, together
with the outburst spectra from 1995 and 1998 (Hodapp 1999). All
three spectra are taken with comparable slit widths and were
normalised to their value at m. In general, all spectra show
a steep continuum, rising towards longer wavelengths. No individual
absorption or emission lines can be seen. The
overall shape of the three spectra is very similar, though there is a
tendency that later spectra are less steep.
OO Ser offers a unique possibility to investigate the long-term behaviour of an eruptive young stellar object. The rise time of OO Ser was 8.5 months at the longest (Hodapp et al. 1996b), similar to the typical timescale of "fast'' FUor outbursts (e.g. the outburst of FU Ori or V1057 Cyg, which was modelled by a triggered eruption, see Bell et al. 1995).
As discussed in Sect. 3.4.1, the fading rate abruptly decreased
350 days after the maximum brightness. A similar event was observed in
the case of V1057 Cyg (Kenyon & Hartmann 1991) too, but there the transition
happened later, about 1300 days after the maximum. Using the light
curves of Kenyon & Hartmann (1991) we computed fading rates for V1057 Cyg and
compared them with corresponding values of OO Ser. We found that both
before and after the transition, the fading of OO Ser (1.00 mag/yr
before, 0.35 mag/yr after) was significantly faster than that of
V1057 Cyg (
0.09...0.40 mag/yr before,
0.04...0.14 mag/yr after,
between 0.44 and m).
From the m light curve in Fig. 5 we can
conclude that the object will return to the quiescent phase some
time after 2011. This implies that the duration of the outburst of
OO Ser will be at least 16 years. This timescale differs both
from that of FUors (being several decades or a century) and that of
EXors (being some weeks or months), suggesting that OO Ser is a young
eruptive object that differs from both FUors and EXors.
The magnitude of the luminosity change during outburst is another
argument in favour of OO Ser being different from FUors or
EXors. From Fig. 3 we could estimate a bolometric
preoutburst luminosity of
for OO Ser and from the ISOPHOT data we also computed luminosities
for OO Ser for the different epochs during the outburst. Since the
far-infrared fluxes are contaminated by nearby sources, we calculated
an upper and a lower limit for the luminosity, by including or
neglecting the
m data points, respectively. Adopting
the 1996 February values of
as a representative outburst luminosity range, OO Ser changed its
luminosity by a factor of about
.
Thus one may conclude
that both the peak brightness and the amplitude of the luminosity
increase was significantly lower than the corresponding values of
classical FUors (
and a factor of 100, see Hartmann & Kenyon 1996).
We note that there exists another star, V1647 Ori, which seems to
share some characteristics of OO Ser. Based on the amplitude of
its brightening, V1647 Ori was classified as an intermediate-type
object between FUors and EXors by Muzerolle et al. (2005). Preoutburst and
outburst luminosities of V1647 Ori are
(Ábrahám et al. 2004b) and
(Muzerolle et al. 2005),
respectively, thus its luminosity changed by a factor of about 8,
similarly to OO Ser. The timescales of their outbursts are somewhat
different, because the eruption of V1647 Ori was only 2 years long
(Kóspál et al. 2005) and it also produced an outburst in the 1960s
(Aspin et al. 2006).
We speculate, following Hodapp et al. (1996b), that OO Ser (and in some respects V1647 Ori) may be the representative of a new class of young eruptive stars ("Deeply Embedded Outburst Star'' or DEOS in Hodapp et al. 1996b). Members of this class may be defined by their relatively short timescales compared to FUors, possibly recurrent outbursts, modest increase in bolometric luminosity and accretion rate, and an evolutionary state earlier than that of typical FUors or EXors (see Sect. 4.2).
Based on optical to millimetre measurements available at that time,
Hodapp et al. (1996b) claimed that the SED of OO Ser appeared consistent
with that of a class I source and assumed its age to be 105 yr,
indicating that OO Ser is in a very early evolutionary phase. The new
data presented in this paper make it possible to reestablish the
evolutionary stage of the source. We followed the method of
Chen et al. (1995) and computed the bolometric temperature
according to their Eq. (1) for the preoutburst,
outburst and present SEDs. Due to the uncertainty of the far-infrared
data points (see Sect. 2.7 about source confusion), we
calculated
in two different ways, with and without
data points affected by source confusion
(
m). The resulting values are in the
range of 50-120 K for all three SEDs. Within this interval, the
bolometric temperature slightly increased, and the luminosity changed
by a factor of 7 during the outburst. We compared these values with
the distribution of corresponding values among young stellar objects
in the Taurus and
Ophiuchus star forming regions
(Chen et al. 1995). From this check we can conclude that OO Ser seems
to be an early class I object, and its age is <105 yr.
The circumstellar environment of young eruptive stars is usually
modelled with a flat or flared accretion disc and an extended
infalling envelope (e.g. Kenyon & Hartmann 1991; Turner et al. 1997). In these models the
emission of the central source (the star and the innermost part of the
accretion disc) dominates the emission at optical and near-infrared
wavelengths. Between 3 and m the origin of the emission is
the release of accretion energy in the disc, and also starlight
reprocessed in the same part of the disc and also in the envelope. The
emission at
m is starlight reprocessed in the
envelope. The outburst occurs when, due to thermal instability in the
inner edge of the disc, the accretion rate suddenly increases. After
peak brightness, the accretion slowly relaxes to its quiescent
value. The decreasing accretion rate causes the fading of the central
source and consequently leads to the simultaneous fading of the
reprocessing envelope. Thus, this model predicts a
wavelength-independent fading of the source in the whole optical to
mid-infrared regime (see e.g. V1057 Cyg in Ábrahám et al. 2004a and
V1647 Ori in Acosta-Pulido et al. 2007).
The circumstellar environment of OO Ser probably shares many properties with the abovementioned models, except that the unusually high extinction indicates a larger and/or denser envelope. As it can be seen in Fig. 7, the fading of OO Ser was indeed wavelength-independent in the whole near- to mid-infrared wavelength regime, in agreement with the model predictions. This, together with the overall shape of its SED and the ice features in its mid-infrared spectrum indicates that the circumstellar structure of OO Ser is similar to those of other young eruptive stars, i.e. possesses a circumstellar accretion disc and is embedded in a dense circumstellar envelope.
Bell & Lin (1994) modelled FUor outbursts as self-regulated accretion
events in protostellar accretion discs. In this model the risetime of
the outburst, the subsequent high state and the time between
successive outbursts are dependent on ,
the viscosity
parameter in the model of Shakura & Sunyaev (1973). Fitting their model to the
observed FUor timescales, Bell & Lin (1994) derived
for the
value in the cool, neutral
state, and
in the hot, ionised state. Since
in many respects OO Ser is similar to FUors, the model of
Bell & Lin (1994) might be applicable, although the timescales of the
OO Ser outburst are remarkably shorter than that of FUors. Thus,
applying this model to OO Ser requires different parameters than
those for FUors. Bell & Lin (1994) give in their Table 2 the dependence of
different timescales on
,
from which we can estimate
for OO Ser. This is one order of magnitude
higher than the usual value for FUors, which implies that OO Ser may
differ from classical FUors in a way that its disc has different, one
order of magnitude higher viscosity.
In this paper we presented an infrared monitoring programme on OO Ser, a deeply embedded young eruptive star in the Serpens NW star-forming region. OO Ser went into outburst in 1995 and has been gradually fading since then. Our infrared photometric data obtained between 1996 and 2006 revealed that the fading of the source is still ongoing in the whole infrared wavelength regime, and that OO Ser will probably not return to quiescent state before 2011. The flux decay has become slower since the outburst peak and has been practically wavelength-independent.
From these results we draw the following conclusions:
Acknowledgements
The ISOPHOT data presented in this paper were reduced using the ISOPHOT Interactive Analisys package PIA, which is a joint development by the ESA Astrophysics Division and the ISOPHOT Consortium, lead by the Max-Planck-Institut für Astronomie (MPIA). We thank Gaspare Lo Curto for kindly providing us with the TIMMI2 data on OO Ser. We also thank the referee, Klaus Hodapp, for useful suggestions that greatly improved the paper. The work was partly supported by the grant OTKA K 62304 of the Hungarian Scientific Research Fund. J.A.P. acknowledge support from grant AYA 2001-1658, financed by the Spanish Dirección General de Investigación.
Table 1:
Log of ISOPHOT and ISOCAM observations. SP stands for
spectrophotometry. "Map size'' indicates the sizes of the final
maps.
denotes the increment between adjacent pixel
positions in the map (mapping) or the separation between source and
background positions (ON/OFF). At certain wavelengths the beam
contained nearby sources; for a detailed discussion see
Sect. 2.7. All fluxes are colour corrected. The last two
columns give the uncertainties of the absolute and the relative flux
calibration (Sect. 2.2).
Table 2:
Log of observations from 2004-2006. All fluxes are presented
in mJy. Spitzer fluxes are from the third delivery of data from the
"c2d'' legacy project, except the m data of all sources and
the 3.6, 4.5, 5.8, 8 and
m data of OO Ser, which
were extracted by us from MIPS and IRAC images improved and
published by the "c2d'' legacy team. All Spitzer fluxes are colour
corrected.