Astrophysical Masers and Their Environments
Proceedings IAU Symposium No. 242, 2007
J. Chapman & W. Baan, eds.
c© 2007 International Astronomical Union
DOI: 00.0000/X000000000000000X
SiO maser observations of a wide
dust-temperature range sample
Jun-ichi Nakashima1 and Shuji Deguchi2
1Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617,
Taiwan
email: junichi@asiaa.sinica.edu.tw
2Nobeyama Radio Observatory, National Astronomical Observatory, Minamimaki,
Minamisaku, Nagano 384-1305, Japan
Abstract. We present the results of SiO line observations of a sample of known SiO maser
sources covering a wide dust-temperature range. The aim of the present research is to investigate
the causes of the correlation between infrared colors and SiO maser intensity ratios among
different transition lines. We observed in total 75 SiO maser sources with the Nobeyama 45m
telescope quasi-simultaneously in the SiO J = 1−0 v = 0, 1, 2, 3, 4 and J = 2−1 v = 1, 2
lines. We also observed the sample in the 29SiO J = 1−0 v = 0 and J = 2−1 v = 0, and
30SiO J = 1−0 v = 0 lines, and the H2O 61,6−52,3 line. As reported in previous papers, we
confirmed that the intensity ratios of the SiO J = 1−0 v = 2 to v = 1 lines clearly correlate
with infrared colors. In addition, we found possible correlation between infrared colors and the
intensity ratios of the SiO J = 1−0 v = 3 to v = 1&2 lines.
Keywords. masers — stars: AGB and post-AGB — stars: late-type — stars: mass loss — stars:
statistics
1. Introduction
An important problem in the studies on the SiO maser was that SiO maser sources ever
known were considerably biased. Specifically, the dust (effective) temperature of known
SiO maser sources, which was calculated from mid-infrared flux densities (such as the
IRAS andMSX flux densities), was limited roughly in a range of 250 K. Tdust . 2000 K.
This is because the previous SiO maser surveys have been limited to relatively warm
dust-temperature ranges. Consequently a non-negligible number of potential SiO maser
sources (especially with a low dust-temperature) have been slipped from the previous
SiO maser surveys.
Nyman et al.(1993) first realized the importance of SiO maser sources exhibiting a
low dust-temperature. They investigated how SiO maser emission behaves in a low
dust-temperature range by observing OH/IR stars in the SiO J = 1−0 v = 1&2 and
J =2−1 v = 1 lines. The OH/IR stars often exhibit a low dust-temperature less than
Tdust = 250 K. In their observation cold objects clearly show a larger intensity ratio of the
SiO J = 1−0 v = 2 to v = 1 lines. Both collisional and radiative schemes cannot fully ex-
plain this observational properties of the SiO masers (Bujarrabal 1994; Deol et al. 1995).
Nyman et al.(1993) suggested that an infrared H2O line (116,6 ν2 = 1→127,5 ν2 = 0)
overlapping with the SiO J = 0 v = 1→J = 1 v = 2 transition might play an important
role. However, in early 1990s the number of cold SiO maser sources (like OH/IR stars)
was quite limited, and it was difficult to statistically investigate the relation between
infrared colors and intensity ratios of SiO maser lines.
Nakashima & Deguchi(2003b) recently extended the Nyman’s study by surveying the
SiO maser emission in cold, dusty IRAS sources exhibiting low dust temperature less
http://arxiv.org/abs/0704.1063v1
2 J. Nakashima & S. Deguchi
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
corr: 0.59 A
log(F25/F12)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
corr: 0.40
log(F25/F12)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
log(F25/F12)
corr: 0.35
Figure 1. Infrared colors versus intensity ratio of SiO maser lines. The horizontal axes represent
infrared colors. F25 and F12 denote the IRAS flux densities at λ = 25 and 12 µm, respectively.
The filled dots (•), upward triangles (△) and downward triangles (▽) respectively represent the
intensity ratios of the SiO maser lines, lower limits of the ratio and upper limits of the ratio.
Correlation coefficients are given in the upper-left corners of each panel. The dashed lines are
the results of least-square fitting of a first order polynomial.
than 250 K. They found roughly 40 new SiO maser sources in the cold dusty objects,
and in conjunction with the results of another SiO maser survey of relatively warm IRAS
objects (Nakashima & Deguchi 2003a) they clearly demonstrated that the intensity ratio
of the SiO J = 1−0 v = 2 to v = 1 lines increases in inversely proportional to the
dust temperature. Nakashima & Deguchi(2003b) again suggested that the overlap line
of H2O might explain this correlation if the overlap line becomes stronger with decrease
of the dust temperature. To consider further this problems, we need to confirm whether
properties of the SiO lines other than J = 1−0 v = 1 and 2 lines are consistent with
the existence of the H2O overlap line. In this contributed paper we present the result
of quasi-simultaneous observations in the multiple different SiO rotational lines with the
Nobeyama 45m telescope. The main aim of the observation is to check the behavior of
SiO maser intensity ratios including lines other than the J = 1−0 v = 1 and 2 lines.
2. Observations and Results
The observing targets were selected from Nakashima & Deguchi(2003a, b) and the
Nobeyama SiO maser source catalog (Gorny et al. in preparation) in terms of the IRAS
colors and flux densities. The targets are distributed roughly in the right ascension range
between 18h and 22h, because the cold SiO maser sources found by Nakashima & Deguchi(2003b)
are distributed roughly in this range. We selected the observing targets basically in order
of the brightness at λ = 12 µm, but we also paid attention to the source distribution in
the IRAS two-color diagram so that the observing targets continuously cover the entire
color range.
SiO line observations with the Nobeyama 45m telescope were made in two separated
periods: May 11–19, 2004 and February 15–19, 2006. In the first period we observed, in
total, 38 objects. The observed SiO transitions in the first period were J = 1−0 v =1, 2, 3
and J = 2−1 v =1, 2. We also observed in the 29SiO J = 1−0 v =0 and J = 2−1 v =0
lines. In addition, we observed 27 objects in the H2O maser line at 22 GHz (61,6−52,3)
as a backup observation under rainy/heavy cloudy condition. In the second period we
observed, in total, 53 objects. The observed transitions in the second period were SiO
J = 1−0 v =0, 1, 2, 3, 4, 29SiO J = 1−0 v =0 and 30SiO J = 1−0 v =0. The technical
details of the observations will be presented in our future paper (Nakashima & Deguchi,
in preparation).
In this paper, we focus on the properties of the SiO J = 1−0 v = 1, 2 and 3 lines, in
SiO maser intensity ratios 3
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
log(F25/F12)
8 micron flux (MSX)
SiO J=1-0 v=1
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
SiO J=1-0 v=2
log(F
SiO J=1-0 v=3
C1.71+0.26 2.47+0.25
2.12+0.20
Figure 2. [Panels A, B and C]: Relation between infrared colors and absolute intensity of SiO
maser lines. The notation of the infrared colors is same with that used in Figure 1. The intensity
of SiO maser lines is standardized at the distance of 1 kpc. The thick dashed lines represent the
results of least-square-fitting of a first order polynomial. The inclinations of the fitted lines (thick
dashed lines) are given at the lower-right corners of each panel with statistical uncertainty. In
the panel A, only the data points below log(F25/F12) = 0.5 were fitted by the polynomial. The
data points above log(F25/F12) = 0.5 are independently fitted by a first order polynomial, and
the results of the fitting is given as the chain line. [Panel D]: Relation between 8µm absolute
flux density and infrared color. The 8µm flux is standardized at the distance of 1 kpc.
which we detected an enough number of objects for statistical analysis. Figure 1 shows
the relations between infrared colors and intensity ratios among the SiO maser lines. The
line intensities used to calculate the intensity ratios are velocity-integrated intensities.
In the panel A of Figure 1 we can clearly confirm the positive correlation between the
log(F25/F12) color and the intensity ratio of the SiO J = 1−0 v = 2 to v = 1 lines
as reported by Nakashima & Deguchi(2003b). Interestingly, the intensity ratios of the
J = 1−0 v = 3 to v = 2 lines and of the J = 1−0 v = 3 to v = 1 lines seem to also
correlate with the log(F25/F12) color (see, panels B and C) even though the correlation
coefficients are slightly smaller than that of panel A.
Figure 2 shows the relations between infrared colors and absolute intensities of the SiO
maser lines. The intensity of the SiO maser lines is standardized at the distance of 1 kpc
using the luminosity distances. The panels A, B and C of Figure 2 show the relations
between the log(F25/F12) color and the absolute intensity of the SiO J = 1−0 v = 1,
2 and 3 lines. A notable feature seen in these panels is that the SiO maser absolute
intensities undoubtedly correlate with the log(F25/F12) color. Another clear feature is
that the higher the vibrational transitions, the steeper the inclination of the dashed lines
representing the results of least-square-fitting of a first order polynomial. This tendency
is consistent with the correlation seen in Figure 1. In the panel A in Figure 2, the values
of the absolute intensity of SiO maser emission seem to maximize at log(F25/F12) ∼
0.5, and the values tend to decrease with increase of the color in the red region above
log(F25/F12) = 0.5. The log(F25/F12) color of 0.5 corresponds the boundary between
distributions of AGB and post-AGB stars in the log(F25/F12) color. In fact, the panel A
4 J. Nakashima & S. Deguchi
in Figure 1 (and Figure 8 in Nakashima & Deguchi 2003b) shows a sudden change of the
feature at log(F25/F12) ∼ 0.5. No such change is seen in the panels B and C in Figure
2, simply because the SiO J = 1−0 v = 2 and 3 lines have not been detected above
log(F25/F12) = 0.5 in the present observations.
A possible reason for the correlation seen in panels A, B and C in Figure 2 is that
the energy input to the SiO maser region increases with the infrared colors. To confirm
this possibility, in panel D in Figure 2 we plotted the 8µm flux densities as a function of
the infrared colors. The values of the 8µm flux densities were taken from the MSX point
source catalog. If we rely on the radiative scheme the 8µm flux should well represent
the energy input to the SiO maser region, because the λ = 8µm corresponds to the
∆v = 1 SiO transition (e.g., Deguchi & Iguchi 1976). In panel D in Figure 2 the 8µm flux
densities are standardized at the distance of 1kpc using the luminosity distances. The
distribution of the data points seen in panel D is, in fact, strikingly similar with those
seen in panels A, B and C, supporting that the 8µm flux tightly correlates with the SiO
maser intensity as suggested by Bujarrabal et al.(1987).
3. Discussion
In this section we discuss the possible explanation for the correlation between infrared
colors and SiO maser intensity ratios among the v = 1, 2 and 3 lines at 43 GHz. One
possible explanation is to introduce the overlap line of H2O (116,6 ν2 = 1 → 127,5 ν2 = 0),
which has been first suggested by Olofsson et al.(1981) to explain the anomalous, weak
intensity of the SiO J = 2−1 v = 2 line in oxygen-rich (O-rich) stars. This H2O line
overlaps with the SiO J = 0 v = 1→J = 1 v = 2 transition with a velocity difference of
1 km s−1. With this line overlap, the J = 1 v = 2 level is overpopulated, and the weakness
of the SiO J = 2−1 v = 2 line is explained by this overpopulation. The overpopulation
at the J = 1 v = 2 level is also consistent with the strong intensity of the J = 1−0 v = 2
line. Thus, the correlation between the infrared colors and the intensity ratio of the SiO
J = 1−0 v = 2 to v = 1 lines may be explained if this overlap line of H2O becomes
stronger with increase of the infrared colors. One problem in this interpretation is that
the intensity ratios of the SiO J = 1−0 v = 3 to v = 1&2 lines cannot be explained only
by the H2O 116,6 ν2 = 1 → 127,5 ν2 = 0 line. However Cho et al.(2007) recently reported
an interesting detection of the SiO J = 2−1 v = 3 line toward an S-type star, χ Cyg.
They also confirmed that the SiO J = 2−1 v = 3 line is weak in O-rich stars. The S-
type stars have almost same amount of oxygen and carbon atoms in their envelopes, and
consequently they have few H2O molecules in the envelopes. These results potentially
suggest that another overlap line of H2O affects on the population distribution of SiO in
O-rich stars, and Cho et al.(2007) have suggested that the H2O 50,5 ν2 = 2→63,4 ν2 = 1
line overlapping with the SiO J = 0 v = 2→J = 1 v = 3 line (with a velocity difference
of about 1.5 km s−1) acts on the population distribution of SiO. Thus, if both H2O
116,6 ν2 = 1→127,5 ν2 = 0 and 50,5 ν2 = 2→63,4 ν2 = 1 lines becomes stronger with
increase of infrared colors, all correlations between infrared colors and the SiO maser
intensity ratios among the J = 1−0 v = 1, 2 and 3 lines might be explained. The line
intensity of the H2O 50,5 ν2 = 2→63,4 ν2 = 1 line is usually weaker than that of the
116,6 ν2 = 1→127,5 ν2 = 0line. This fact also seems to be consistent with the relatively
weak intensity of the SiO J = 1−0 v = 3 line.
However, there are some other problems on the explanation with the overlap line of
H2O. First, we have to explain how the H2O infrared lines overlapping with the SiO
lines become stronger with increase of infrared colors. The relative abundance of H2O
molecules possibly increases with infrared colors, but this is not conclusive. Second, the
SiO maser intensity ratios 5
correlation between infrared colors and the intensity ratios of the SiO J = 1−0 v = 2 to
v = 1 lines might be explained without the overlap line of H2O. In the envelopes of very
cold objects, strong 8µm emission comes from every direction to the SiO masing region,
causing ineffective pumping through the SiO ∆v = 1 transition. On the other hand, 4µm
emission corresponding to the SiO ∆v = 2 transition is more effectively pump the SiO
population instead of the 8µm. These processes might explain the correlation seen in
Figure 3 (e.g., Doel et al. 1995). Third, a recent theoretical calculation predicted that if
we introduce the overlap line of H2O the spatial distribution of the maser spots cannot
be theoretically reproduced (Soria-Ruiz et al. 2004). Thus, this problem will be remain
controversial for some more time.
4. Summary
In this research we observed 75 known SiO maser sources quasi-simultaneously in the
SiO J = 1−0, v = 0, 1, 2, 3 and 4 lines, SiO J = 2−1 v = 1 and 2, 29SiO J = 1−0 v = 0
and J = 2−1 v = 0, and 30SiO J = 1−0 v = 0 lines. We also observed the targets in the
H2O 61,6−52,3 line under rainy/heavy cloudy condition. The sample continuously covers
a very wide dust-temperature range from 150 K to 2000 K. The correlation between
infrared colors and the intensity ratio of the SiO J = 1−0 v = 2 to v = 1 lines is
confirmed as reported by Nakashima & Deguchi(2003b). The intensity rations of SiO
J = 1−0 v = 3 to v = 1&2 lines possibly correlate with infrared colors. The overlap lines
of H2O might explain the correlations between the infrared colors and the SiO maser
intensity ratios among the J = 1−0 v = 1, 2 and 3 lines, although there are alternative
ways to interpret the phenomena.
Acknowledgements
The present research has been supported by the Academia Sinica Institute of Astron-
omy & Astrophysics in Taiwan.
References
Bujarrabal, V. 1994, A&A 285, 953
Bujarrabal, V., Planesas, P., & del Romero, A. 1987, A&A 175, 164
Cho, S.-H., Lee, C. W., & Park, Y.-S. 2007, ApJ 657, 482
Deguchi, S., & Iguchi, T. 1976, PASJ 28, 307
Doel, R. C., et al. 1995, A&A 302, 797
Nakashima, J., & Deguchi, S. 2003a, PASJ 55, 203
Nakashima, J., & Deguchi, S. 2003b, PASJ 55, 229
Nyman, L.-Å., Hall, P. J., & Le Bertre, T. 1993, A&A 280, 551
Olofsson, H., Rydbeck, O. E. H., Lane, A. P., & Predmore, C. R. 1981, ApJ 247, L81
Olofsson, H., Rydbeck, O. E. H., & Nyman, L.-A. A&A 150, 169
Soria-Ruiz, R., et al. 2004, A&A 426, 131
Discussion
Elitzur: In our recent work, infrared intensity and colors are uniquely determined by
the optical depth of the dust shell. SiO maser intensity is also correlated to the optical
depth. Therefore, what you are finding is somehow related to the effect of the activity of
the photosphere such as the variation of mass loss rates.
Nakashima: Thanks for useful comments. (We took account of Elitzur’s comments in
the text.)
	Introduction
	Observations and Results
	Discussion
	Summary

  We present the results of SiO line observations of a sample of known SiO
maser sources covering a wide dust-temperature range. The aim of the present
research is to investigate the causes of the correlation between infrared
colors and SiO maser intensity ratios among different transition lines. We
observed in total 75 SiO maser sources with the Nobeyama 45m telescope
quasi-simultaneously in the SiO J=1-0 v=0, 1, 2, 3, 4 and J=2-1 v=1, 2 lines.
We also observed the sample in the 29SiO J=1-0 v=0 and J=2-1 v=0, and 30SiO
J=1-0 v=0 lines, and the H2O 6(1,6)-5(2,3) line. As reported in previous
papers, we confirmed that the intensity ratios of the SiO J=1-0 v=2 to v=1
lines clearly correlate with infrared colors. In addition, we found possible
correlation between infrared colors and the intensity ratios of the SiO J=1-0
v=3 to v=1&2 lines.

Introduction
An important problem in the studies on the SiO maser was that SiO maser sources ever
known were considerably biased. Specifically, the dust (effective) temperature of known
SiO maser sources, which was calculated from mid-infrared flux densities (such as the
IRAS andMSX flux densities), was limited roughly in a range of 250 K. Tdust . 2000 K.
This is because the previous SiO maser surveys have been limited to relatively warm
dust-temperature ranges. Consequently a non-negligible number of potential SiO maser
sources (especially with a low dust-temperature) have been slipped from the previous
SiO maser surveys.
Nyman et al.(1993) first realized the importance of SiO maser sources exhibiting a
low dust-temperature. They investigated how SiO maser emission behaves in a low
dust-temperature range by observing OH/IR stars in the SiO J = 1−0 v = 1&2 and
J =2−1 v = 1 lines. The OH/IR stars often exhibit a low dust-temperature less than
Tdust = 250 K. In their observation cold objects clearly show a larger intensity ratio of the
SiO J = 1−0 v = 2 to v = 1 lines. Both collisional and radiative schemes cannot fully ex-
plain this observational properties of the SiO masers (Bujarrabal 1994; Deol et al. 1995).
Nyman et al.(1993) suggested that an infrared H2O line (116,6 ν2 = 1→127,5 ν2 = 0)
overlapping with the SiO J = 0 v = 1→J = 1 v = 2 transition might play an important
role. However, in early 1990s the number of cold SiO maser sources (like OH/IR stars)
was quite limited, and it was difficult to statistically investigate the relation between
infrared colors and intensity ratios of SiO maser lines.
Nakashima & Deguchi(2003b) recently extended the Nyman’s study by surveying the
SiO maser emission in cold, dusty IRAS sources exhibiting low dust temperature less
http://arxiv.org/abs/0704.1063v1
2 J. Nakashima & S. Deguchi
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
corr: 0.59 A
log(F25/F12)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
corr: 0.40
log(F25/F12)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
log(F25/F12)
corr: 0.35
Figure 1. Infrared colors versus intensity ratio of SiO maser lines. The horizontal axes represent
infrared colors. F25 and F12 denote the IRAS flux densities at λ = 25 and 12 µm, respectively.
The filled dots (•), upward triangles (△) and downward triangles (▽) respectively represent the
intensity ratios of the SiO maser lines, lower limits of the ratio and upper limits of the ratio.
Correlation coefficients are given in the upper-left corners of each panel. The dashed lines are
the results of least-square fitting of a first order polynomial.
than 250 K. They found roughly 40 new SiO maser sources in the cold dusty objects,
and in conjunction with the results of another SiO maser survey of relatively warm IRAS
objects (Nakashima & Deguchi 2003a) they clearly demonstrated that the intensity ratio
of the SiO J = 1−0 v = 2 to v = 1 lines increases in inversely proportional to the
dust temperature. Nakashima & Deguchi(2003b) again suggested that the overlap line
of H2O might explain this correlation if the overlap line becomes stronger with decrease
of the dust temperature. To consider further this problems, we need to confirm whether
properties of the SiO lines other than J = 1−0 v = 1 and 2 lines are consistent with
the existence of the H2O overlap line. In this contributed paper we present the result
of quasi-simultaneous observations in the multiple different SiO rotational lines with the
Nobeyama 45m telescope. The main aim of the observation is to check the behavior of
SiO maser intensity ratios including lines other than the J = 1−0 v = 1 and 2 lines.
2. Observations and Results
The observing targets were selected from Nakashima & Deguchi(2003a, b) and the
Nobeyama SiO maser source catalog (Gorny et al. in preparation) in terms of the IRAS
colors and flux densities. The targets are distributed roughly in the right ascension range
between 18h and 22h, because the cold SiO maser sources found by Nakashima & Deguchi(2003b)
are distributed roughly in this range. We selected the observing targets basically in order
of the brightness at λ = 12 µm, but we also paid attention to the source distribution in
the IRAS two-color diagram so that the observing targets continuously cover the entire
color range.
SiO line observations with the Nobeyama 45m telescope were made in two separated
periods: May 11–19, 2004 and February 15–19, 2006. In the first period we observed, in
total, 38 objects. The observed SiO transitions in the first period were J = 1−0 v =1, 2, 3
and J = 2−1 v =1, 2. We also observed in the 29SiO J = 1−0 v =0 and J = 2−1 v =0
lines. In addition, we observed 27 objects in the H2O maser line at 22 GHz (61,6−52,3)
as a backup observation under rainy/heavy cloudy condition. In the second period we
observed, in total, 53 objects. The observed transitions in the second period were SiO
J = 1−0 v =0, 1, 2, 3, 4, 29SiO J = 1−0 v =0 and 30SiO J = 1−0 v =0. The technical
details of the observations will be presented in our future paper (Nakashima & Deguchi,
in preparation).
In this paper, we focus on the properties of the SiO J = 1−0 v = 1, 2 and 3 lines, in
SiO maser intensity ratios 3
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
log(F25/F12)
8 micron flux (MSX)
SiO J=1-0 v=1
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
SiO J=1-0 v=2
log(F
SiO J=1-0 v=3
C1.71+0.26 2.47+0.25
2.12+0.20
Figure 2. [Panels A, B and C]: Relation between infrared colors and absolute intensity of SiO
maser lines. The notation of the infrared colors is same with that used in Figure 1. The intensity
of SiO maser lines is standardized at the distance of 1 kpc. The thick dashed lines represent the
results of least-square-fitting of a first order polynomial. The inclinations of the fitted lines (thick
dashed lines) are given at the lower-right corners of each panel with statistical uncertainty. In
the panel A, only the data points below log(F25/F12) = 0.5 were fitted by the polynomial. The
data points above log(F25/F12) = 0.5 are independently fitted by a first order polynomial, and
the results of the fitting is given as the chain line. [Panel D]: Relation between 8µm absolute
flux density and infrared color. The 8µm flux is standardized at the distance of 1 kpc.
which we detected an enough number of objects for statistical analysis. Figure 1 shows
the relations between infrared colors and intensity ratios among the SiO maser lines. The
line intensities used to calculate the intensity ratios are velocity-integrated intensities.
In the panel A of Figure 1 we can clearly confirm the positive correlation between the
log(F25/F12) color and the intensity ratio of the SiO J = 1−0 v = 2 to v = 1 lines
as reported by Nakashima & Deguchi(2003b). Interestingly, the intensity ratios of the
J = 1−0 v = 3 to v = 2 lines and of the J = 1−0 v = 3 to v = 1 lines seem to also
correlate with the log(F25/F12) color (see, panels B and C) even though the correlation
coefficients are slightly smaller than that of panel A.
Figure 2 shows the relations between infrared colors and absolute intensities of the SiO
maser lines. The intensity of the SiO maser lines is standardized at the distance of 1 kpc
using the luminosity distances. The panels A, B and C of Figure 2 show the relations
between the log(F25/F12) color and the absolute intensity of the SiO J = 1−0 v = 1,
2 and 3 lines. A notable feature seen in these panels is that the SiO maser absolute
intensities undoubtedly correlate with the log(F25/F12) color. Another clear feature is
that the higher the vibrational transitions, the steeper the inclination of the dashed lines
representing the results of least-square-fitting of a first order polynomial. This tendency
is consistent with the correlation seen in Figure 1. In the panel A in Figure 2, the values
of the absolute intensity of SiO maser emission seem to maximize at log(F25/F12) ∼
0.5, and the values tend to decrease with increase of the color in the red region above
log(F25/F12) = 0.5. The log(F25/F12) color of 0.5 corresponds the boundary between
distributions of AGB and post-AGB stars in the log(F25/F12) color. In fact, the panel A
4 J. Nakashima & S. Deguchi
in Figure 1 (and Figure 8 in Nakashima & Deguchi 2003b) shows a sudden change of the
feature at log(F25/F12) ∼ 0.5. No such change is seen in the panels B and C in Figure
2, simply because the SiO J = 1−0 v = 2 and 3 lines have not been detected above
log(F25/F12) = 0.5 in the present observations.
A possible reason for the correlation seen in panels A, B and C in Figure 2 is that
the energy input to the SiO maser region increases with the infrared colors. To confirm
this possibility, in panel D in Figure 2 we plotted the 8µm flux densities as a function of
the infrared colors. The values of the 8µm flux densities were taken from the MSX point
source catalog. If we rely on the radiative scheme the 8µm flux should well represent
the energy input to the SiO maser region, because the λ = 8µm corresponds to the
∆v = 1 SiO transition (e.g., Deguchi & Iguchi 1976). In panel D in Figure 2 the 8µm flux
densities are standardized at the distance of 1kpc using the luminosity distances. The
distribution of the data points seen in panel D is, in fact, strikingly similar with those
seen in panels A, B and C, supporting that the 8µm flux tightly correlates with the SiO
maser intensity as suggested by Bujarrabal et al.(1987).
3. Discussion
In this section we discuss the possible explanation for the correlation between infrared
colors and SiO maser intensity ratios among the v = 1, 2 and 3 lines at 43 GHz. One
possible explanation is to introduce the overlap line of H2O (116,6 ν2 = 1 → 127,5 ν2 = 0),
which has been first suggested by Olofsson et al.(1981) to explain the anomalous, weak
intensity of the SiO J = 2−1 v = 2 line in oxygen-rich (O-rich) stars. This H2O line
overlaps with the SiO J = 0 v = 1→J = 1 v = 2 transition with a velocity difference of
1 km s−1. With this line overlap, the J = 1 v = 2 level is overpopulated, and the weakness
of the SiO J = 2−1 v = 2 line is explained by this overpopulation. The overpopulation
at the J = 1 v = 2 level is also consistent with the strong intensity of the J = 1−0 v = 2
line. Thus, the correlation between the infrared colors and the intensity ratio of the SiO
J = 1−0 v = 2 to v = 1 lines may be explained if this overlap line of H2O becomes
stronger with increase of the infrared colors. One problem in this interpretation is that
the intensity ratios of the SiO J = 1−0 v = 3 to v = 1&2 lines cannot be explained only
by the H2O 116,6 ν2 = 1 → 127,5 ν2 = 0 line. However Cho et al.(2007) recently reported
an interesting detection of the SiO J = 2−1 v = 3 line toward an S-type star, χ Cyg.
They also confirmed that the SiO J = 2−1 v = 3 line is weak in O-rich stars. The S-
type stars have almost same amount of oxygen and carbon atoms in their envelopes, and
consequently they have few H2O molecules in the envelopes. These results potentially
suggest that another overlap line of H2O affects on the population distribution of SiO in
O-rich stars, and Cho et al.(2007) have suggested that the H2O 50,5 ν2 = 2→63,4 ν2 = 1
line overlapping with the SiO J = 0 v = 2→J = 1 v = 3 line (with a velocity difference
of about 1.5 km s−1) acts on the population distribution of SiO. Thus, if both H2O
116,6 ν2 = 1→127,5 ν2 = 0 and 50,5 ν2 = 2→63,4 ν2 = 1 lines becomes stronger with
increase of infrared colors, all correlations between infrared colors and the SiO maser
intensity ratios among the J = 1−0 v = 1, 2 and 3 lines might be explained. The line
intensity of the H2O 50,5 ν2 = 2→63,4 ν2 = 1 line is usually weaker than that of the
116,6 ν2 = 1→127,5 ν2 = 0line. This fact also seems to be consistent with the relatively
weak intensity of the SiO J = 1−0 v = 3 line.
However, there are some other problems on the explanation with the overlap line of
H2O. First, we have to explain how the H2O infrared lines overlapping with the SiO
lines become stronger with increase of infrared colors. The relative abundance of H2O
molecules possibly increases with infrared colors, but this is not conclusive. Second, the
SiO maser intensity ratios 5
correlation between infrared colors and the intensity ratios of the SiO J = 1−0 v = 2 to
v = 1 lines might be explained without the overlap line of H2O. In the envelopes of very
cold objects, strong 8µm emission comes from every direction to the SiO masing region,
causing ineffective pumping through the SiO ∆v = 1 transition. On the other hand, 4µm
emission corresponding to the SiO ∆v = 2 transition is more effectively pump the SiO
population instead of the 8µm. These processes might explain the correlation seen in
Figure 3 (e.g., Doel et al. 1995). Third, a recent theoretical calculation predicted that if
we introduce the overlap line of H2O the spatial distribution of the maser spots cannot
be theoretically reproduced (Soria-Ruiz et al. 2004). Thus, this problem will be remain
controversial for some more time.
4. Summary
In this research we observed 75 known SiO maser sources quasi-simultaneously in the
SiO J = 1−0, v = 0, 1, 2, 3 and 4 lines, SiO J = 2−1 v = 1 and 2, 29SiO J = 1−0 v = 0
and J = 2−1 v = 0, and 30SiO J = 1−0 v = 0 lines. We also observed the targets in the
H2O 61,6−52,3 line under rainy/heavy cloudy condition. The sample continuously covers
a very wide dust-temperature range from 150 K to 2000 K. The correlation between
infrared colors and the intensity ratio of the SiO J = 1−0 v = 2 to v = 1 lines is
confirmed as reported by Nakashima & Deguchi(2003b). The intensity rations of SiO
J = 1−0 v = 3 to v = 1&2 lines possibly correlate with infrared colors. The overlap lines
of H2O might explain the correlations between the infrared colors and the SiO maser
intensity ratios among the J = 1−0 v = 1, 2 and 3 lines, although there are alternative
ways to interpret the phenomena.
Acknowledgements
The present research has been supported by the Academia Sinica Institute of Astron-
omy & Astrophysics in Taiwan.
References
Bujarrabal, V. 1994, A&A 285, 953
Bujarrabal, V., Planesas, P., & del Romero, A. 1987, A&A 175, 164
Cho, S.-H., Lee, C. W., & Park, Y.-S. 2007, ApJ 657, 482
Deguchi, S., & Iguchi, T. 1976, PASJ 28, 307
Doel, R. C., et al. 1995, A&A 302, 797
Nakashima, J., & Deguchi, S. 2003a, PASJ 55, 203
Nakashima, J., & Deguchi, S. 2003b, PASJ 55, 229
Nyman, L.-Å., Hall, P. J., & Le Bertre, T. 1993, A&A 280, 551
Olofsson, H., Rydbeck, O. E. H., Lane, A. P., & Predmore, C. R. 1981, ApJ 247, L81
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Discussion
Elitzur: In our recent work, infrared intensity and colors are uniquely determined by
the optical depth of the dust shell. SiO maser intensity is also correlated to the optical
depth. Therefore, what you are finding is somehow related to the effect of the activity of
the photosphere such as the variation of mass loss rates.
Nakashima: Thanks for useful comments. (We took account of Elitzur’s comments in
the text.)
	Introduction
	Observations and Results
	Discussion
	Summary
