PASJ: Publ. Astron. Soc. Japan 64, 27, 2012 April 25
c 2012. Astronomical Society of Japan.
Near-Infrared High-Dispersion Spectroscopic Observations of Water
in Comet 81P/Wild 2 with Subaru/IRCS
Mio H ASHIMOTO
Saga Prefecture Space and Science Museum, 16351 Nagashima, Takeo, Takeo, Saga 843-0021
[email protected]
and
Hitomi KOBAYASHI and Hideyo K AWAKITA
Department of Physics, Faculty of Science,
Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555
(Received 2011 May 30; accepted 2011 October 11)
Abstract
Near-infrared high-dispersion spectroscopic observations of comet 81P/Wild 2 were carried out on 2010
January 31 and February 1 by the Subaru telescope with IRCS. We detected two hot-band emission lines of H2 O:
X(1,0,1) 202 –X(1,0,0) 303 at 3526.5 cm1 and X(1,0,1) 211 –X(1,0,0) 312 at 3514.4 cm1 . The water production rate
(QH2 O ) was determined as (1.21˙0.05) 1028 molecules s1 based on the spherical coma model assuming the rotational temperature of 30 K on February 1. We found that water production rates determined by de Val-Borro et al.
(2010, A&A, 521, L50) with Herschel/HIFI were consistent with ours within 3 error-levels. Furthermore, our
result is consistent with previous water production rates determined in the 1997 apparition.
Key words: comets: individual (81P/Wild 2)
1. Introduction
Water is the most abundant volatile in cometary ice, and thus
water vapor largely controls the physico-chemical conditions in
the coma. Collisions of other minor molecules with water may
lead to chemical reactions and collision transitions of those
molecules. The products of water photolysis by solar UV, such
as OH and H, are kinetically energetic, and their subsequent
collisions with coma gases warm the coma. Thus, the gas
production rate of water QH2 O is an important indicator of not
only cometary outgassing, but also the physical conditions of
the coma.
Water production rates have been determined through observations of photo-dissociation products of water, OH and
oxygen atom excited to the metastable states (1 D or 1 S states)
in some comets (Feldman et al. 2004). The OH radical can
be observed in near-UV (300 nm), in near-infrared (3 m),
and in radio ( 18 cm), while forbidden oxygen lines from the
metastable states can be observed in optical (557.7 nm, 630 nm,
and 636.4 nm). Emission lines from water itself are recognized
in infrared (as ro–vibrational transitions) and in the sub-mm
or radio domain (as rotational transitions) (Bockelée-Morvan
et al. 2004). For a decade following its first direct detection in
1985, cometary water could be detected only from the Kuiper
Airborne Observatory. In 1995, the detection of cometary H2 O
from the ground using hot-band transitions was first reported,
and the appearance of the bright comet C/1996 B2 Hyakutake
in the following spring marked the first application of the hotband methodology to a bright comet. Since then, the hot-band
method has been greatly expanded, and today it is the standard method for quantifying cometary water. Commissioning
of the Infrared Space Observatory in 1996, demonstrated
the power of space observations by quantifying water in
C/1995 O1 Hale–Bopp and several later comets, and the
Spitzer Space Telescope and Akari later continued this work.
However, their application to trace molecules was limited by
the low spectral resolving power of the on-board spectrometers. Fortunately, rapid progress in high-dispersion spectrographs since 1995 at ground-based observatories, allows
us to determine the water production rates in comets with
direct measurements of water (DiSanti & Mumma 2008, and
references therein).
In this article, we present near-infrared high-dispersion spectroscopic observations of water in comet 81P/Wild 2 during
the 2010 apparition. This comet was the target of the
NASA/Stardust sample-return mission. The Stardust spacecraft was launched in 1999, and flew by comet 81P/Wild 2
on 2004 January 2. Many observations of the comet were
carried out in the 1997 apparition from ground-based observatories, as supporting observations to follow the Stardust
mission. However, there have been no direct measurements of
water in comet 81P/Wild 2 from a ground-based observatory.
2. Observations
The observations of comet 81P/Wild 2 were carried out on
2010 January 31 and February 1 UT by the Subaru telescope.
We used a near-infrared high-dispersion spectrograph (IRCS:
Kobayashi et al. 2000). We observed the comet in the L-band;
the grating settings were set to take spectra from 2.83 m to
3.62 m with gaps in the wavelength. We used a 0:0054 9:00 5
slit (/Δ is 5000) for both the comet and a photometric
standard star (HR 5072, its spectral type is G2.5 V). The comet
was 10th magnitude in the V -band,1 and was observed just
1
hhttp://www.aerith.net/comet/catalog/0081P/2010-j.htmli.
M. Hashimoto, H. Kobayashi, and H. Kawakita
before its perihelion passage.
Here, we concentrate on data taken on 2010 February 1 UT,
because the S/N ratio of the spectra taken on 2010
January 31 UT was poor due to severe absorption by the
telluric atmosphere and inaccurate non-sidereal tracking for the
comet. The heliocentric (Rh ), geocentric distances (Δ), and
the velocity of the comet relative to the telescope (Δ-dot) are
listed in table 1.
The targets for both the comet and the standard star were put
on the slit at two different positions, “A” and “B”, separated
by 500 in order to cancel the sky emissions by subtracting these
frames taken at different positions from each other. We used
14 ABBA sequences for the analysis, and the total integration
time was 56 minutes on source for the comet. We used IRAF
software distributed by NOAO for data reduction. We calculated the (“image A” “image B”) (“image B” “image A”)
= (2 “image A”) (2 “image B”) for cancellation of the
sky background emission (dark components were also canceled
out by the subtraction), and then all results were flat-fielded.
Wavelength calibration was performed by comparing background sky emission lines. The comet signal was extracted
within a small area, 0:0054 0:0049 (369 km 332 km at the
comet), for the one-dimensional spectrum.
Modeled telluric absorption spectra of the standard star were
calculated by the LBLRTM code (Clough et al. 2005). We evaluated the efficiency of both the telescope and the IRCS based
on the modeled transmittance. The flux loss of the standard
star at the slit was estimated from the spatial brightness profile
of the star along the slit. We calibrated the cometary spectra
based on the efficiencies determined for the standard star. The
difference in airmass between the comet and the standard star
was taken into account by using the LBLRTM.
The signal from a comet includes both sunlight reflected
3. Result
The single-generation Haser model was applied to the observations (Haser 1957). The parent molecule (e.g., H2 O) would
photo-dissociate into daughter species (e.g., OH and H) with
an exponential decay (the lifetime of 83000 s at 1 AU from
the Sun: Huebner et al. 1992). The number density of H2 O,
-19
1.0x10
H 2 O( )
*
*
-20
8.0x10
-20
6.0x10
*
-20
4.0x10
-20
2.0x10
0
-2.0x10
-4.0x10
Table 1. Observation log.
[Vol. 64,
by cometary dust grains and emission lines from gaseous
species in the coma. The dust component has to be subtracted
in order to extract gas emission lines only. The cometary
continuum component was modeled and removed as a smooth
continuum multiplied by the modeled transmittance spectrum.
The obtained spectrum was flux-calibrated by comparing with
the spectrum of a standard star; the result was Doppler-shifted
by the topocentric velocity of the comet at the observations.
The obtained one-dimensional spectrum is shown in figure 1.
We detected two hot-band emission lines of ortho-H2O in
our spectrum: X(1,0,1) 202 –X(1,0,0) 303 at 3526.5 cm1 and
X(1,0,1) 211 –X(1,0,0) 312 at 3514.4cm1. We list our measurements of fluxes and corresponding transitions of these emission
lines in table 2.
Flux density [W/m2 ]
27-2
-6.0x10
-20
-20
-20
28400
28500
1
UT Date
Rh (AU)
Δ(AU)
Δ-dot (km s )
2010 Jan 31
2010 Feb 1
1.61
1.61
0.95
0.94
13.6
13.4
28600
28700
28800
28900
Wavelength [angstroms]
Fig. 1. One-dimensional spectrum of comet 81P/Wild 2. The 0 s mark
the water emission lines. Lines which are upper and lower sides of zero
flux density show the ˙1 errors.
Table 2. Measured water emission lines and relevant g-factors.
Line assignment
Wavenumber
[cm1 ]
Flux
[10 W m2 ]
g-factor [W molecule1 ]
F=g [109 molecules m2 ]
18
Trot = 20 K
Trot = 30 K
Trot = 40 K
27
28
X (1,0,1) 202 –X (1,0,0) 303
3526.5
1.40 ˙ 0.21
1.18 10
1.18 ˙ 0.1
9.95 10
1.4 ˙ 0.12
8.62 1028
1.62 ˙ 0.14
X (1,0,1) 211 –X (1,0,0) 312
3514.4
0.46 ˙ 0.04
2.14 1028
2.16 ˙ 0.16
2.95 1028
1.56 ˙ 0.12
3.34 1028
1.38 ˙ 0.1
X (1,0,1) 303 –X (1,0,0) 404
3507.3
1.13 1029
< 4.47
4.26 1029
< 1.18
7.08 1029
< 0.71
< 0.05
Wavenumbers in the rest.
At the top of the atmosphere.
This value is proportional to water production rate (F and g denote observed flux and g-factor, respectively, see the section 3 in the text). The error-bars
correspond to ˙1 levels.
3 upper limit.
Water production rate Q(H2 O) [molecules s -1 ]
No. 2]
Near-Infrared Spectroscopy of Comet 81P/Wild 2
27-3
4. Discussion
1.6x10
28
1.2 x10
28
8.0 x10 27
4.0 x10 27
0
100
400
200
300
Nucleocentric distance [km]
500
Fig. 2. Q-curve analysis. Horizontal axis showing the nucleocentric
distance at the tangent point, for each extract. QH2 O is (1.21 ˙ 0.05)
1028 moleculess1 as a terminal value at Trot = 30 K, and the growth
factor was 1.65˙ 0.10 in our analysis (see text).
nH2 O .r/ [molecules cm3 ] at a distance of r [m] from the
nucleus was derived as follows:
QH2 O exp vHr O
2
e
nH2 O .r/ =
;
(1)
4 r 2 v
where QH2 O denotes the water production rate [moleculess1 ],
v denotes the gas expansion velocity of H2 O, assumed to be
800 Rh0:5 [m s1 ] at Rh [AU] from the Sun, H2 O [s] denotes
the water photo-dissociation lifetime, which is 83000 Rh2 [s].
We also assumed optically thin conditions in the coma.
The fluorescence efficiencies (g-factors) for the water hotband emission lines were calculated based on our fluorescence
excitation model of water. This excitation model is basically
the same as the model by Dello Russo et al. (2004), but with
updates for the transitions and the solar spectrum. We assumed
that the population distribution in the vibrational ground
state follows the Boltzmann distribution at a given rotational
excitation temperature (Trot ).
If we use a correct Trot , the F=g (F and g denote the flux at
the top of atmosphere and g-factor, respectively, for each line,
and the F=g is proportional to QH2 O ) values are consistent
among all emission lines. As listed in table 2, the F=g values
are consistent among all lines for Trot = 30 K. We thus adopted
Trot as 30 K for the analysis (we assumed that ortho-to-para
abundance ratio of water equals to 3.0, high-temperature limit).
We also performed a “Q-curve” analysis to derive QH2 O ,
because the photons from the center of coma could not
be collected completely by the very narrow slit (DiSanti &
Mumma 2008). We derived an H2 O production rate of comet
81P/Wild 2 of QH2 O = (1.21 ˙ 0.05) 1028 molecules s1
by assuming Trot = 30 K based on the “Q-curve” analysis
(shown in figure 2); the growth factor is 1.65 ˙ 0.10 with
respect to the flux within 9 rows (spanning ˙4.5 rows from
the nucleus), corresponding to ˙168 km at the comet used for
the 1D-spectrum shown in figure 1. The errors of the water
production rate are 1 statistical uncertainties.
The water production rate determined with 3 uncertainties
in this study is (1.21 ˙ 0.15) 1028 molecules s1 , which is
consistent with results determined from observations of H2 O
in the sub-mm wavelength region by Herschel/HIFI and from
radio observations of OH (both were reported by de Val-Borro
et al. 2010) performed during the same apparition (figure 3a).
The water production rate, QH2 O in comet 81P/Wild 2 was
determined by the Herschel space telescope with the HIFI
instrument through rotational transitions of water, while the
OH radical (a photo-dissociation product) was observed at
a radio wavelength of 18 cm, and QH2 O was derived from
observations of OH.
Regarding Trot , de Val-Borro et al. (2010) assumed that the
kinetic gas temperature was 40 K on 2010 February 1–4 UT in
their data analysis for the sub-mm observations of 81P/Wild 2
taken by Herschel/HIFI. If the kinetic temperature is consistent
with Trot , collisions may be dominant in the coma. However,
Trot is usually lower than the kinetic gas temperature, if radiative cooling is dominant in the coma. If we assume Trot to
be 40 K, the same as de Val-Borro et al. (2010), QH2 O is
(1.22 ˙ 0.05) 1028 molecules s1 . This is almost the same as
QH2 O assumed as Trot = 30 K. Furthermore, the water production rates of our result and de Val-Borro et al. (2010) are consistent with each other within 3 errors, as shown in figure 3.
Although their observations were performed at different wavelengths and their results were based on a different excitation
model (fluorescence excitation for our data and collisional
excitation for de Val-Borro et al. 2010) and a different
treatment of optical thickness, our result is consistent with
theirs. This fact indicates that our assumption for the model
is probably reasonable.
We also compared our result with the water production rates
determined during the previous apparition in 1997 (Farnham &
Schleicher 2005; Fink et al. 1999; Combi et al. 2011). Panel (a)
of figure 3 shows all results observed (in both 2010 and 1997
apparitions) at 1.57–1.65 AU from the Sun, while panel (b)
shows the data for 1.57–1.92 AU from the Sun. In the panel (b)
QH2 O determined by Combi et al. (2011) is averaged every a
0.01 AU interval for readability. There are no significant differences among those results, except for the data from Farnham
and Schleicher (2005). Those data are systematically lower
than the others (probably due to the difference in the model to
calculate QH2 O ). We conclude that the water production rates
around the perihelion passage remained unchanged, at least,
from 1997 to 2010 for this comet 81P. This result suggests
that the activity near perihelion has not changed in the ten-year
interval separating successive perihelion passages, as seen in
the case of comet 46P/Wirtanen (Kobayashi & Kawakita 2010).
27-4
M. Hashimoto, H. Kobayashi, and H. Kawakita
[Vol. 64,
Fig. 3. Comparison of QH2 O with different apparitions. Panel (a) shows the water production rates for Rh = 1.57–1.65 AU (Rh denotes a heliocentric
distance), panel (b) shows those for 1.57–1.92 AU. Error-bars correspond to ˙1 levels. The data from de Val-Borro et al. (2010) and our result were
taken in 2010 apparition, and other works were carried out in 1997 apparition (see text).
No. 2]
Near-Infrared Spectroscopy of Comet 81P/Wild 2
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