A multi-wavelength study of the young star V1118 Orionis in outburst

Abstract

Abriged version for astroph: The young late-type star V1118 Orionis was in outburst from 2005 to 2006. We followed the outburst with optical and near-infrared photometry; the X-ray emission was further probed with observations taken with XMM-Newton and Chandra during and after the outburst. In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer at the peak of the outburst and in the post-outburst phase. The spectral energy distribution of V1118 Ori varied significantly over the course of the outburst. The optical flux showed the largest variations, most likely due to enhanced emission by a hot spot. The latter dominated the optical and near-infrared emission at the peak of the outburst, while the disk emission dominated in the mid-infrared. The X-ray flux correlated with the optical and infrared fluxes, indicating that accretion affected the magnetically active corona and the stellar magnetosphere. The thermal structure of the corona was variable with some indication of a cooling of the coronal temperature in the early phase of the outburst with a gradual return to normal values. Color-color diagrams in the optical and infrared showed variations during the outburst, with no obvious signature of reddening due to circumstellar matter. Using MC realizations of star+disk+hotspot models to fit the SED in ``quiescence'' and at the peak of the outburst, we determined that the mass accretion rate varied from about 2.5E-7 Msun/yr to 1E-6 Msun/yr; in addition the fractional area of the hotspot increased significantly as well. The multi-wavelength study of the V1118 Ori outburst helped us to understand the variations in spectral energy distributions and demonstrated the interplay between the disk and the stellar magnetosphere in a young, strongly accreting star. Comment: Accepted in A&A, Tables will be published online

Full text

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arXiv:0912.3224v1 [astro-ph.SR] 16 Dec 2009
Astronomy & Astrophysics manuscript no. aa
December 16, 2009
c ? ESO 2009
A multi-wavelength study of the young star V1118 Orionis in
outburst⋆
M. Audard1,2, G. S. Stringfellow3, M. G¨ udel4, S. L. Skinner3, F. M. Walter5, E. F. Guinan6, R. T. Hamilton6,7,
K. R. Briggs4, C. Baldovin-Saavedra1,2
1ISDC Data Center for Astrophysics, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzerland
2Observatoire de Gen` eve, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland
3Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA
4Institut f¨ ur Astronomie, ETH Z¨ urich, 8093 Z¨ urich, Switzerland
5Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
6Department of Astronomy and Astrophysics, Villanova University, Villanova 19085, PA, USA
7Department of Astronomy, New Mexico State University, 320 East Union Ave, Apt. 1434, Las Cruces, NM 88001, USA
Received 2009 July 31; accepted 2009 December 16
ABSTRACT
Context. The accretion history of low-mass young stars is not smooth but shows spikes of accretion that can last from months and
years to decades and centuries.
Aims. Observations of young stars in outbursts can help us understand the temporal evolution of accreting stars and the interplay
between the accretion disk and the stellar magnetosphere.
Methods. The young late-type star V1118 Orionis was in outburst from 2005 to 2006. We followed the outburst with optical and
near-infrared photometry; the X-ray emission was further probed with observations taken with XMM-Newton and Chandra during
and after the outburst. In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer at the peak of the outburst and
in the post-outburst phase.
Results. The spectral energy distribution of V1118 Ori varied significantly over the course of the outburst. The optical flux showed
the largest variations, most likely due to enhanced emission by a hot spot. The latter dominated the optical and near-infrared emission
at the peak of the outburst, while the disk emission dominated in the mid-infrared. The emission silicate feature in V1118 Ori is
flat and does not vary in shape, but was slightly brighter at the peak of the outburst compared to the post-outburst spectrum. The
X-ray flux correlated with the optical and infrared fluxes, indicating that accretion affected the magnetically active corona and the
stellar magnetosphere. The thermal structure of the corona was variable with some indication of a cooling of the coronal temperature
in the early phase of the outburst with a gradual return to normal values. Color–color diagrams in the optical and infrared showed
variations during the outburst, with no obvious signature of reddening due to circumstellar matter. Using Monte-Carlo realizations of
star+disk+hotspot models to fit the spectral energy distributions in “quiescence” and at the peak of the outburst, we determined that
the mass accretion rate varied from about 2.5×10−7M⊙yr−1to 1.0×10−6M⊙yr−1; in addition the fractional area of the hotspot
increased significantly as well.
Conclusions. The multi-wavelength study of the V1118 Ori outburst helped us to understand the variations in spectral energy distri-
butions and demonstrated the interplay between the disk and the stellar magnetosphere in a young, strongly accreting star.
Key words. accretion, accretion disks – Infrared: stars – stars: circumstellar matter – stars: coronae – stars: pre-main-sequence –
stars: individual (V1118 Ori) – X-rays: stars
1. Introduction
The star formation process involves strong accretion of circum-
stellar matter onto the protostar. The time evolution of the mass
accretion rate is of deep interest to understand the timescale
of stellar growth and lifetime of proto-planetary disks. While
the mass accretion rate in young stars overall decreases with
increasing stellar age (Hartmann et al. 1998), it can also show
significant and rapid changes over time (e.g., Hartmann et al.
1993).A handfulofaccretingyoungstarsdisplaypowerfulerup-
tive events with flux increases in the optical regime of a few
magnitudes. Two classes have emerged: FUors, which display
outbursts of 4 magnitudes and more, last several decades and,
Send offprint requests to: e-mail: Marc.Audard@unige.ch
⋆Tables A.2 and A.3, and Figure B.1 are only available in electronic
format at http://www.aanda.org
therefore, show a low recurrence rate. EXors (named after the
prototype EX Lup), in contrast, show somewhat smaller out-
bursts (∆V = 2 − 3 mag) on a much shorter timescale, from
a few months to a few years, and may occur repeatedly (see
review by Hartmann & Kenyon 1996; also Herbig 2008). Such
outbursts are believed to originate during a rapid increase of the
disk accretion rate over a short period of time, from values of
10−7M⊙yr−1to 10−4M⊙yr−1, althoughtheunderlyingcause
of such increase in mass accretion rate is unclear (thermal disk
instabilities, Lin & Papaloizou 1985; Bell & Lin 1994; close
companions, Bonnell & Bastien 1992; cluster-induced encoun-
ters,Pfalzner et al.2008; giantplanetsinthedisk,Clarke & Syer
1996; Lodato & Clarke 2004; a combination of gravitational in-
stability and the triggering of the magnetorotational instability,
Armitage et al. 2001; accretion of clumps in a gravitationally
unstable disk, Vorobyov & Basu 2005, 2006). The limited num-
ber of eruptive young stars and the long recurrence time (espe-

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2 M. Audard et al.: The 2005 Outburst of V1118 Ori
cially for FUor-type objects) make it difficult to test models. It
is, therefore,important to study in as much detail as possible the
evolution of outbursts and to understand the place of outburst-
ing young stars in the evolutionary scheme from highly accret-
ing, embeddedprotostars to “normal”accretingclassical T Tauri
stars (CTTS).
The optically revealed CTTS can make excellent compar-
ison stars to outbursting young stars: they have been studied
extensively in the past: optical and infrared observations have
given clues about the presence of accretion disks, on the mass
accretion rates, and disk winds, while millimeter observations
provided constraints on the dust disk mass and on CO outflows.
IntheX-rays,theoriginofemissioninyoung,accretingstars has
gained significant attention in the last few years. Kastner et al.
(2002) obtained the first X-ray grating spectrum of TW Hya, a
moderatelyaccretingCTTS,andobservedunexpectednewspec-
tral features: the O VII and Ne IX He-like triplets showed very
low forbidden-to-intercombinationline ratios and indicated high
electron densities of the order of 1012− 1013cm−3. The effect
of UV photoexcitation on the observed line ratio was deemed
negligible. In addition, the X-ray spectrum was consistent
with a quasi-isothermal plasma of about 3 MK. Kastner et al.
(2002) concluded that the X-ray emission in TW Hya was due
to accretion, a result further supported by Stelzer & Schmitt
(2004) and Ness & Schmitt (2005). Additional high-resolution
X-ray spectra of CTTS also indicated that accretion may play a
significant role for X-rays in accreting stars (Schmitt et al. 2005;
G¨ unther et al. 2006; Robrade & Schmitt 2006; Argiroffi et al.
2007; G¨ unther et al. 2007; Sacco et al. 2008). Soft X-ray
emission from shocks in jets may also be detected (G¨ udel et al.
2005, 2008; Kastner et al. 2005; Robrade & Schmitt 2007;
Schneider & Schmitt2008;
Telleschi et al. (2007) and G¨ udel & Telleschi (2007) showed
that CTTS display a soft X-ray excess from plasma at low
temperature. Such a plasma (mostly detected in the O VII
lines) is difficult to detect with CCD spectroscopy but can
easily be revealed with high-resolution X-ray spectra when the
absorbing column density is not too high. The soft X-ray excess
can co-exist with the hot plasma observed in the vast majority
of young stars, which is due to scaled-up solar-like magnetic
activity (Smith et al. 2005; Audard et al. 2005a; Preibisch et al.
2005; Telleschi et al. 2007). The origin of such an excess is
unclear, but it could be due in part to accretion onto the stellar
photosphere. It is crucial to understand how the X-ray emission
in young stars, in particular those accreting matter actively, can
be influenced by the accretion process. FUor and EXor stars are,
therefore, the ideal cases to understand the physical mechanism
common to both CTTS and outbursting stars.
In the recent years, several young low-mass stars were
observed in outburst, in the optical and infrared, but also
in the X-rays: V1647 Ori in 2003–2005 (e.g., Kastner et al.
2004, Grosso et al. 2005, Kastner et al. 2006a; see also
Aspin et al. 2008 and references therein) and recently in 2008–
2009 (Itagaki 2008; Aspin 2008; Venkat & Anandarao 2008;
Aspin et al. 2009), V1118 Ori in 2005-2006 (Audard et al.
2005b; Lorenzetti et al. 2006, 2007; Herbig 2008). Kastner et al.
(2006b) also observed with Chandra a young stellar object in
eruptioninLDN1415butfailedtodetectit.EXLupalsoshowed
an extreme outburst in early 2008 (Jones 2008a,b; Kospal et al.
2008).´Abrah´ am et al. (2009) detected crystalline features in the
silicate feature of EX Lup in outburst which were not present in
the pre-outburst spectrum, suggesting crystallization by thermal
annealing in the surface layer of the inner disk. X-ray obser-
vations were obtained with Chandra (PI: Weintraub) and Swift
G¨ unther & Schmitt2009).
(PI: Stringfellow)butare notyet published.We notealso that the
FUorobjectsFUOri,V1735Cyg,andZCMa werealso detected
in X-rays (Skinner et al. 2006, 2009a; Stelzer et al. 2009).
The V1647 Ori campaign showed an increase of the X-ray
flux up to a factor of 200 from its pre-outburst flux, in line with
the flux increase in the infrared. The X-ray flux then followed
the optical outburst flux and returned to its pre-outburst level
after the outburst ended (Kastner et al. 2006a). The initial ob-
servations of V1118 Ori in X-rays showed a different behavior,
with little flux enhancement (Audard et al. 2005b), but also in-
dicated that the rapid increase of accretion rate in outbursts can
impact the X-ray emission of young accreting stars. The present
paper aims to present the remainder of the X-ray data of V1118
Ori taken duringand after the 2005–2006outburst,togetherwith
contemporaneousoptical and infrared data.
2. The young erupting star V1118 Ori
The outburst of V1118 Ori, a young low-mass M1e star in the
Orion Nebula (d = 400 pc; Muench et al. 2008 for a discus-
sion), was reported in early January 2005 by Williams et al.
(2005). Hillenbrand (1997) and Stassun et al. (1999) provide
details about its physical properties: M⋆ = 0.41M⊙; R⋆ =
1.29R⊙; Prot= 2.23 ± 0.04 d; Lbol≥ 0.25 L⊙; logTeff[K] =
3.562; logt[yr] = 6.28. Recently, Reipurth et al. (2007) re-
solved V1118 Ori into a close binary separated by 0.′′18 with
a position angle of 329◦and a magnitude difference of ∆m =
0.4 mag in the Hα-band image. The observation was obtained
on 2004 Jan 29, thus before the 2005–2006outburst (see Fig. 6).
The binarity of V1118 Ori leaves unclear which star actually
erupted. However, we note that the small magnitude difference
indicates that the components are similar, suggesting similar ef-
fectivetemperatures.We mayalso assumethattheyshowsimilar
disk evolution. While the binarity of V1118 Ori complicates the
interpretation of the combined photometry and spectroscopy in
quiescence, this should have little impact on the interpretation
of the outburst data, as only one star+disk componentdominates
the emission. In this paper, we have used the above stellar prop-
erties that assumed a single star.
V1118 Ori has shown frequent outbursts (e.g., 1983-84,
1988-90, 1992-94, 1997-98; see Garcia & Parsamian 2000 and
Herbig 2008 for details). In fact, after returning in quiescence
in mid-2006, V1118 Ori had another outburst in late 2007
(Garcia & Parsamian 2008). We focus here on our monitoring
campaign to study the 2005–2006 outburst of V1118 Ori in the
X-rays, optical and infrared. Additional properties in the opti-
cal and infrared were obtained independently during and after
the outburst by Lorenzetti et al. (2006, 2007) and Herbig(2008).
Using wind models, Lorenzetti et al. (2006) derived a mass loss
rate of 4 × 10−8M⊙yr−1from the H I recombination line, and
(3−8)×10−7M⊙yr−1fromthe CO emission at 2.3 µm for the
neutral molecular gas; they also found no evidence of infrared
cooling from a collimated jet or outflow. Lorenzetti et al. (2007)
alsofoundevidenceofintrinsicpolarizationintheI band,andir-
regular fluctuations during the outburst. Herbig (2008) obtained
Keck/HIRES spectra of V1118 Ori in the decaying phase of
the outburst and in the post-outburst phase. He noted the de-
tection of Li I λ6707 in emission during the outburst, in contrast
with the absorbed feature in CTTS spectra. The feature, how-
ever, returned in absorption after the outburst. A similar behav-
ior occurred in the K I lines at λλ 7664 and 7698 ˚ A. A P Cyg
profile was also found in Hα during the outburst, which disap-
peared thereafter. In the near-infrared, Lorenzetti et al. (2007)
also found that the emission lines detected during the outburst

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M. Audard et al.: The 2005 Outburst of V1118 Ori3
Table 1. X-ray observation log.
ParameterSep 2002 Jan 2005Feb 2005 Mar 2005Sep 2005
Satellite.....................
ObsID ......................
Duration (ks) . ...............
Observation date. ............
UT .........................
Average JD - 2,450,000. ......
Chandra
2548
48
2002 Sep 6–7
12:57–02:54
2,524.3
Chandra
6204
5
2005 Jan 26
03:07–05:04
3,396.7
XMM-Newton
0212480301
20
2005 Feb 18-19
22:36–04:50
3,420.6
XMM-Newton
0212480401
20
2005 Mar 21
16:20–22:08
3,451.3
XMM-Newton
0212481101
20
2005 Sep 8
12:43–18:51
3,622.2
Jan 2006Feb 2006 Mar 2006Apr 2006 Jul 2006Dec 2007
Satellite.....................
ObsID ......................
Duration (ks) . ...............
Observation date. ............
UT .........................
Average JD - 2,450,000. ......
Chandra
6416
30
2006 Jan 4–5
18:33–03:09
3,740.5
Chandra
6417
30
2006 Feb 23
09:41–17:47
3,790.2
XMM-Newton
0403200101
100
2006 Mar 2–3
19:02–21:06
3,797.8
Chandra
6418
30
2006 Apr 23
14:42–23:31
3,849.3
Chandra
6419
30
2006 Jul 24
07:50–16:49
3,941.0
Chandra
8936
37.5
2007 Dec 14–15
14:49–01:13
4,449.3
(H I, He I, CO 2.3 µm band, a few neutral metals) disappeared
about a year later.
In the X-ray regime, the initial Jan-Mar 2005 data were pub-
lished by Audard et al. (2005b). In brief, the X-ray data of early
2005 indicated that the X-ray flux and luminosity stayed simi-
lar within a factor of two during the outburst, and at the same
level as in a pre-outburst observation in 2002. The fluxes in
the optical and near-infrared varied more significantly, within
factors of 2 − 10. The hydrogen column density showed no
evidence for variation from its modest pre-outburst value of
NH≈ 3 × 1021cm−2. However, there was evidence of a spec-
tral change from a dominant hot plasma (≈ 25 MK) in 2002
and in January 2005 to a cooler plasma (≈ 8 MK) in February
2005 and probably in March 2005. We hypothesizedthat the hot
magnetic loops high in the corona were disrupted by the clos-
ing in of the accretion disk due to the increased accretion rate
during the outburst, whereas the lower cooler loops were proba-
bly less affected and became the dominant coronal component
(Audard et al. 2005b). We argued that the cool component in
V1118 Ori could not originate from shocks because free-fall ve-
locities of matter falling from the truncation radius are too low
(see Audard et al. 2005b for further discussion). In a subsequent
paper, Lorenzetti et al. (2006) independently analyzed our pub-
lic datasets ofearly 2005,and also includedthe September2005
XMM-Newton observation,togetherwith their near-infrareddata
sets. The September 2005 observation showed a decrease in X-
ray flux at the start of the decay phase.
Theinitial paperbyAudard et al.(2005b)presentedandana-
lyzed the outburstdata throughMarch2005.We present here the
XMM-Newton and Chandra data obtained from September 2005
on, which are analyzed in the context of our multiwavelength
analysis. We also include a post-outburst data set obtained by
Chandra in December 2007. The latter was taken during a mi-
nor outburst detected in the optical (Garcia & Parsamian 2008).
Extensive optical and near-infrared photometry are presented
andanalyzed,obtainedfromourteam’simagingdata,alongwith
mid-infrared photometry and spectroscopy with Spitzer.
3. Observations and data reduction
3.1. X-ray
Table 1 provides the observation log of our 2005–2006monitor-
ing campaign of the outburst of V1118 Ori with XMM-Newton
(Jansen et al. 2001) and Chandra (Weisskopf et al. 1996). We
also providethe informationabout the 2002serendipitousobser-
vation of V1118 Ori (an observationin 2001 with XMM-Newton
was reported in Audard et al. 2005b but is not mentioned here
since the star was not detected). A deep XMM-Newton obser-
vation was obtained in early 2006 that complements the short
monitoring observations. Finally, we include the post-outburst
Chandra observation in December 2007 as well. Note that the
angularresolutionsofbothX-raysatellites werenothighenough
to separate the V1118 Ori binary.
The XMM-Newton data were processed with SAS 7.0.
Standard procedures were applied. We used an extraction cir-
cle of radius 20′′for the source and a nearby background cir-
cular region of 60′′radius (40′′for Sep 2005 and 35′′for Mar
2006). Event patterns lower than 4 and 12 were used only for
the European Photon Imaging Cameras (EPIC) pn and MOS
(Str¨ uder et al. 2001; Turner et al. 2001), respectively. The back-
ground flux levels were high during all XMM-Newton observa-
tions, and in particular in March and September 2005. As de-
scribed in Audard et al. (2005b), we used only the MOS1 and
MOS2 data for the March 2005 observation. The September
2005 observationwas so affected by the backgroundthat we lost
about half the exposure time in the EPIC pn, while there was
no MOS data available. The deep March 2006 observation was
affected by a system failure at the Mission Operations Centre
and the EPIC pn experienced full scientific buffer in the last part
of the observation, explaining the reduced exposure time in the
EPIC pn (78ks) compared to the EPIC MOS (90 ks). No Optical
Monitor data were taken with the XMM-Newton X-ray observa-
tion due to the presence of the nearby bright Trapezium stars.
The Chandra data were processed with CIAO 3.3 and
CALDB 3.2.31. The task psextract was used to extract the spec-
tra for V1118 Ori and the nearby background. We used a circle
of 2′′radius (≈ 4 pixels) for the star and an annulus centered
at the position of the star but with radii of 10 and 60 pixels for
the background; our background area was, therefore, 212 times
larger than the source extraction area. Our extraction radius for
the star includes 95% of the encircled energy at 1.5 keV and
90% at 4.5 keV. Note that for the January 2005 observation in
sub-array mode, we used an outer radius of 40 pixels for the
backgroundannulus. For the 2002 observation, we used two cir-
cles of radii of 15′′and 45′′for the source and the background,
respectively (see Audard et al. 2005b).
1We used the pipeline data for the December 2007 observation,
which was calibrated with CALDB 3.4.2.

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4M. Audard et al.: The 2005 Outburst of V1118 Ori
Table 2. Spitzer IRAC and MIPS flux density measurements (units of mJy).
Date
3.6 µm
4.5 µm
5.8 µm
8.0 µm
24 µm
2004 Mar 09 ................
2004 Mar 20 ................
2004 Oct 12. ................
2004 Oct 27. ................
2005 Feb 20. ................
2005 Mar 28 ................
2007 Oct 21. ................
28.6 ± 0.1
···
46.5 ± 0.2
···
143.3 ± 1.7
173.7 ± 1.8
42.4 ± 1.7
36.3 ± 0.1
···
50.6 ± 0.2
43.4 ± 0.1
169.7 ± 2.0
203.9 ± 2.0
48.7 ± 1.9
39.2 ± 1.3
···
53.3 ± 0.3
···
148.2 ± 4.9
194.1 ± 5.0
52.2 ± 4.9
54.8 ± 0.5
···
55.9 ± 0.5
57.5 ± 0.3
144.1 ± 1.9
167.6 ± 1.9
53.1 ± 1.8
···
74.2 ± 0.5
···
···
···
···
···
3.2. Optical and near-infrared
3.2.1. SMARTS
We observed V1118 Ori with the ANDICAM dual-channel im-
agerontheSMARTS/CTIO21.3mtelescopeon115nights.Such
dense coverage is possible by the service mode operations em-
ployed by the SMARTS consortium.We have 5 pre-outburst ob-
servations (2004 Feb 2 through 2004 Apr 8) that can be used
to set the quiescent flux levels. The intensive monitoring began
2005 Jan 10, shortly after the outburst was reported. There were
no observations from 2005 Apr 4 through 2005 Jul 31 due to
proximity to the Sun. We observed with cadences between 1
per day and 1 per week, with observations generally every 3-
4 days. Our intensive monitoringterminatedon 2006 May 6. We
also followed the decay of the second outburst from 2007 Dec
27 through 2008 Mar 27. Details about the data reduction are
given in AppendixA. Table A.2 (available online only)gives the
nightly average photometry for SMARTS.
3.2.2. Villanova
Photometric coverage in the optical (standard Bessel V RI) was
obtained with the Celestron 14” optical tube assembly with
a Paramount ME German equatorial mount at the Villanova
UniversityObservatorylocatedonthecampusnearPhiladelphia,
PA. Observations were carried out with a SBIG ST7-XME de-
tectorthermoelectricallycooledto-25degreesCelsius.Darkand
flat field frames were collected at the end of each nights obser-
vations. A standard error of 0.05 mag was estimated from the
signal-to-noise ratio and seeing conditions. Table A.3 (available
online only) lists the magnitudes obtained at Villanova.
3.2.3. Additional data
We have used published optical and near-infrared photometric
data from Lorenzetti et al. (2007) (IJHK), Garcia et al. (2006)
and Garcia & Parsamian (2008) (V ).
3.3. Spitzer
In addition to our observations of V1118 Ori in outburst (pro-
gram ID 3716, PI: G. Stringfellow) and in post-outburst (pro-
gram ID 41019, PI: M. Audard),V1118 Ori was serendipitously
observed with the InfraRed Array Camera (IRAC; Fazio et al.
2004) by the Spitzer Space Telescope (Werner et al. 2004) in
March 2004 and twice in October 2004 (program IDs 43 and
50, PI: G. Fazio). We show the IRAC images taken before the
outburst in March 2004 centered on V1118 Ori (Fig. 1). We
2SMARTS, the Small and Medium Aperture Research Telescope
Facility, is a consortium of universities and research institutions that
operate the small telescopes at Cerro Tololo under contract withAURA.
provide the IRAC fluxes for all observations in Table 2 (see
also Lorenzetti et al. 2007 for the IRAC data of October 27,
2004). Details about the Spitzer IRAC data reduction are given
in Appendix B.
V1118 Ori was also observed with the Multiband Imaging
Photometer for Spitzer (MIPS; Rieke et al. 2004) at 24 µm
before the outburst, on March 20, 2004 (program ID 58, PI:
G. Rieke). Again, Appendix B provides the details of the data
reduction. No MIPS photometry is available for V1118 Ori dur-
ing or after the outburst. Our programs (3716 and 41019) in-
cluded MIPS spectral energy distribution data (R ≈ 20) in the
70 µm band; however, the on-time exposure (180 s) did not al-
low us to detect V1118Ori, even duringthe outburst.Indeed,the
background level (due to diffuse emission in the Orion nebula)
produced a much higher flux (of order 40 Jy at 70 µm) than the
expected signal from V1118 Ori (of the order 0.1 Jy if extrapo-
lating the SED at 70 µm, see Fig. 12).
Spitzer also observed V1118 Ori with the InfraRed
Spectrograph(IRS; Houck et al. 2004) twice during the outburst
(PID3716)withtheShort-Low(SL: 5.2-14.7µm,R = λ/∆λ ≈
64), Short-High (SH: 9.9-19.6 µm, R = λ/∆λ ≈ 600), and
Long-High (LH: 18.7-37.2 µm, R = λ/∆λ ≈ 600) modules
30"
IRAC1
N
E
V1118 Ori
IRAC2
IRAC3
IRAC4
Fig.1. Spitzer IRAC images centered on V1118 Ori, taken pre-
outburst in March 2004. The images are shown with a linear
scale from 0 to 200 MJy sr−1. The Herbig Ae star V372 Ori is
seen near V1118 Ori.

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M. Audard et al.: The 2005 Outburst of V1118 Ori5
10 203040
Wavelength (µm)
0.1
1.0
Fν (Jy)
(d) 2008−11−14

0.1
1.0
Fν (Jy)
(c) 2008−11−14
H2 0−0 S(2)
H2 0−0 S(1)
H2 0−0 S(0)
[Ne II] [S III][S III]
[Si II]

0.1
1.0
Fν (Jy)
(a) 2005−02−18
SL
SH
LH

0.1
1.0
Fν (Jy)
(b) 2005−03−11
Fig.2. Spitzer IRS background-subtracted spectra obtained near the peak of the outburst (panels a and b) and after the outburst
(panels c and d). Panel d shows the SL and LL module data. Panels a, b, and c show the SL spectra as thick black lines, whereas the
SH and LH spectra are shown as red and blue lines. Detected emission lines are labeled (while other lines well-subtracted by the
background spectrum are shown in italics).
Table 3. Tentative Spitzer IRS line fluxes derived from the SH and LH spectra.
Line
λ (µm) Flux (10−18W m−2)
Feb 2005 Mar 2005 Nov 2008
H20 − 0 S(2)......
[Neii]2P1/2–2P3/2
[Siii]3P2–3P1....
H20 − 0 S(0)......
[Siii]3P1–3P0....
12.2786
12.8136
18.7130
28.2188
33.4810
······
7.8 ± 0.8
42.7 ± 0.9
34.6 ± 1.2
9.5 ± 1.6
183.4 ± 4.7
41.1 ± 0.7
46.5 ± 1.4
15.2 ± 2.2
463.3 ± 9.6
39.7 ± 1.1
42.5 ± 1.5
18.9 ± 3.4
374.4 ± 7.9
(2005 February 18 and 2005 March 11). No background ob-
servations were taken with the high-resolution modules. On the
other hand, the post-outburst data (PID 41019; 2008 November
14) were taken with background spectra for the high-resolution
modules and also included the Long-Low (LL: 14.0-38.0 µm,
R = λ/∆λ ≈ 64) module.
Figure 2 shows all IRS spectra after background subtrac-
tion. We describe in Appendix B our methodology to derive
the background-subtracted spectra of V1118 Ori. We also dis-
cuss in the Appendix the validity to use the post-outburst back-
ground observation for our outburst IRS SH spectra. In brief,
the post-outburst, background-subtracted high-resolution spec-
trum is well-subtracted for the continuum, as the flux is con-
sistent with the low-resolution spectrum. In the case of the out-
burst SH spectra, the continuum is also accurate below 14 µm
since they are consistent with the low-resolution SL spectra.
However, we prefer to stay on the safe side and claim that the
observed increase in continuum flux for λ > 14 µm during
the outburst is unreliable, since we have no MIPS photometry
or low-resolution spectra to confirm the increase. We also urge
caution with regards to the detection of lines in the background-
subtractedSH andLHspectra.Thestrongbackgroundline emis-
sion of the Orion nebula, and its inhomogeneity (see Fig. B.2)
make the background subtraction difficult, although not impos-