Post on 21-Oct-2020
arX
iv:1
709.
0819
7v1
[as
tro-
ph.G
A]
24
Sep
2017
Superflares of H2O Maser Emission
in the Protostellar Object IRAS 18316−0602
c©2017 E.E. Lekht1, M.I. Pashchenko1, G.M. Rudnitskij1,and A.M. Tolmachev2
1M.V. Lomonosov Moscow State University, Sternberg Astronomical Institute,
13 Universitetskii prospekt, Moscow, 119234 Russia2Pushchino Radio Astronomy Observatory, Astrospace Center
of the Lebedev Institute of Physics, Russian Academy of Sciences,
Pushchino, Moscow Region, 142290 Russia
e-mail: lekht@sai.msu.ru eelekht@mail.ru
The results of the study of the maser emission source IRAS 18316−0602 in the H2O
line at λ = 1.35 cm are reported. The observations have been carried out at the RT-
22 radio telescope of the Pushchino Radio Astronomy Observatory (Russia) since June
2002 until March 2017. Three superflares have been detected, in 2002, 2010, and 2016,
with peak flux densities of 3400, 19,000, and 46,000 Jy, respectively. The results of the
analysis of the superflares are given. The flares took place during periods of high maser
activity in a narrow interval of radial velocities (40.5–42.5 km/s) and could be associated
with the passage of a strong shock wave. During our monitoring the emission of three
groups of features at radial velocities of about 41, 42, and 43 km/s dominated. The
flare of 2016 was accompanied by a considerable increase in the flux densities of several
features in an interval of 35–56 km/s.
Key words: star formation, masers, molecular outflows, individual objects
(IRAS18316−0602)
1
http://arxiv.org/abs/1709.08197v1
1 Introduction
The source IRAS18316−0602 is located in a region of active star formation. It is asso-
ciated with the ultracompact HII region G25.65+1.05 (Kurtz et al. [1]; Jenness et al.
[2]; Walsh et al. [3]) and a molecular outflow (McCutcheon et al. [4]). The radio source
coincides spatially with an unresolved IR source and submillimeter emission at 350, 450,
and 850 µm (Hunter et al. [5]; Walsh et al. [6]). Shepherd and Churchwell [7] showed
that the CO bipolar outflow is centered on the radio source. Sánchez-Monge et al. [8]
mapped the molecular outflow in the SiO J = 2 − 1 line and found that the redshifted
(+48.5 < Vred < 88.1 km/s) and blueshifted (+5.9 < Vblue < +39.5 km/s) maxima are
offset in declination by 40′′.
The adopted distance to the source is kinematic. Molinari et al. [9] give 3.17 kpc,
and Sunada et al. [10], 2.7 kpc. The value 3.17 kpc is considered as preferable.
Palla et al. [11] observed toward IRAS18316−0602 strong maser emission in the H2O
at a radial velocity of 45.17 km/s with a peak flux density of 725 Jy. In July 1994 it was
109 Jy (Jenness et al. [2]). In 1995 the H2O emission was observed in a broad interval
of radial velocities; the main emission took place at 41.30 and 45.26 km/s with peak flux
densities of 452 and 260 Jy [12].
OH maser emission in the main lines 1665 and 1667 MHz was detected by Edris
et al. [13]. In the 1612-MHz satellite line emission is thermal, and the 1720-MHz line is
in absorption. In addition, maser emission in methanol CH3OH lines was observed; e.g.,
Walsh et al. [3], Szymczak et al. [14], Surcis et al. [15]. It is associated with the radio
source, probably, with the disk, but not with the bipolar outflow.
The positions of the ultracompact HII region, OH and H2O, near- and far-infrared
sources are sufficiently close (e.g., Jenness et al. [2]).
2 Observations and Data Presentation
The maser source IRAS 18316−0602 was included into the program of our 1.35-cm water
vapor line monitoring at the RT-22 radio telescope in Pushchino in 2002. The system
noise temperature was 130–250 K depending on weather conditions. The half-power
beamwidth at λ = 1.35 cm is 2.6′. The antenna sensitivity is 25 Jy/K. The signal was
analyzed by a 2048-channel autocorrelation spectrometer with a resolution of 6.1 kHz
(0.0822 km/s). All spectra were corrected for the absorption in the Earth’s atmosphere.
Figures 1–7 present the results of our observations. The horizontal axis is the radial
velocity with respect to the Local Standard of Rest in km/s, and the vertical axis is
the flux density in janskys. Because of wide flux variations, the graphs are plotted in
different scales of the vertical axis. In Figure 3 at the left spectra are clipped at some
levels, and central parts of the spectra are shown at the right. This is done in order
to show both faint emission features and the powerful flare feature. The double arrow
shows the scale of the vertical axis. The epochs of the observations are indicated.
2
In Figure 6 the feature at 41.8 km/s is also clipped to show fainter features. In
another scale this feature is shown completely in panel (14). Each maximum is labeled
with the date of the observations: at the right for the ascending branch and at the left
for the descending. We have also used the kindly supplied results obtained by Sergio
Poppi [16] at the 64-meter radio telescope in Sardinia and by Simona Righini [17] on
March 13, 2017, at the 32-meter radio telescope in Medicina. The data were processed
by Sergio Poppi. Figure 7 presents the results; it also shows the spectrum obtained at
RT-22 in Pushchino on March 22, 2017.
Figure 8 shows the evolution of individual components throughout our monitoring.
Velocity variations are shown in Figure 8(a). The data are plotted with different sym-
bols depending on the flux magnitude. Variations of the radial velocities of four main
features are approximated with dashed straight lines. Figures 8(b) and (c) show the flux
variability of individual features. For flux maxima the corresponding radial velocities are
given. Secondary maxima of the main emission feature are marked with vertical arrows.
3 Discussion
We observed main H2Omaser emission of IRAS 18316−0602 in a narrow interval of radial
velocities, from 39 to 44 km/s. Faint high-velocity emission was occasionally observed.
This differs considerably from the structure of the spectra obtained in 1989 by Palla et al.
[11] and in 1994 by Jenness et al. [2]. In their spectra the main maser emission was at
a velocity of about 45 km/s with flux densities of 725 and 109 Jy. Meanwhile, there is
much in common with the spectrum obtained by Kurtz and Hofner [18] in September
1995, in the first turn that the main emission was observed at 41.6 km/s.
From 2002 to 2016 we have observed three superflares and weaker flares that can be
described as periods of high maser activity. The strongest flares took place in the velocity
interval 40.5–42.5 km/s and were associated with the emission features whose points in
the graph are linked with straight line 2. The maxima of the superflare emission tend
to shift toward higher velocities (Figure 8(a)). This can be related to structural changes
of the maser sources.
Thus, the most powerful emission was associated with feature (or cluster) 3. Most
stable in time was the emission of feature 3.
We now consider the evolution of the maser emission flares in the chronological order.
3.1 Superflare of 2002
The beginning of our monitoring of IRAS 18316−0602 fell onto the descending branch
of very high H2O maser activity. In July 2002 the flux density of the main feature at
41 km/s was 3500 Jy. This means that at the flare maximum F was > 3500 Jy. During
the evolution of this flare the linewidth did not depend on the flux density; it remained
3
constant at 0.64 km/s. The line was well approximated with a gaussian, though it was
slightly asymmetric: the right wing of the line is somewhat shallower than the left one.
During the flare of this feature the variability of the features at 39.4 and 43.3 km/s
correlated with it. Then we observed sufficiently high activity of several features in the
velocity interval 39.0–44.5 km/s with flux densities of up to 130 Jy. During 2004 we
observed a feature at 40.6 km/s with a peak flux density of 340 Jy (May 2005). After
that a period of low maser activity began.
3.2 Superflare of 2010
The evolution of this emission had a complicated character (see Figures 3 and 8). The
superflare was preceded by a period of high maser activity. In the emission two features
dominated. The emission of the fainter one was stable in the velocity (40.7 km/s),
and the flux density did not exceed 370 Jy. The second feature (2b in Figure 8) was
much stronger than the first one, and during 2009 its flux density was varying within
550–1630 Jy. At the same time its radial velocity was slowly decreasing from 41.9 to
41.6 km/s. This feature may be considered as a precursor of the flare.
In three days (from January 26 to 29, 2010) the flux density at 41.6 km/s increased
from 1640 to 7330 Jy. The analysis of the variation of all parameters of the superflare
emission (flux density, radial velocity, width of the line and its shape) has shown that
powerful emission consecutively appeared in two features with peak flux densities 19,060
and 6300 Jy. The time interval between the maxima was 5 months. The flux density
peak of the former one (2b) was drifting in the radial velocity from 41.9 to 40.8 km/s (the
superflare precursor taken into account). At the epoch of the maximum of the emission
of this feature the line was symmetric with a width at half maximum of 0.67 km/s; it
was well approximated with a gaussian.
The velocity of the other feature (2c in Figure 8 was 41.8 km/s, and it did not
change appreciably. The observed velocity drift of the features could be related to the
accelerated motion of maser condensations under the action of a shock wave.
We come to the conclusion that the most powerful flare of 2010 is associated with
feature 2b (Figure 8), which was in the active state for about two years and was not
associated with 2c. Thus the flares of 2010 and 2016 (this one discussed below) are
associated with different features (maser condensations).
Since the end of 2014 emission features began to appear at VLSR > 45 km/s; they
were mostly short-lived.
3.3 Superflare of 2016
This has been the strongest flare in this source. We detected it at the ascending branch of
its evolution. During a one-day time interval the flux density increased by a factor of 1.5.
The peak flux density reached 46,000 Jy. The flare was short-lived. Its duration at the 0.5
level was about one month. The linewidth and flux density correlated: the line narrowed
4
with growing flux, and when the flux decreased the line broadened again. Figure 9 shows
this dependence in the coordinates (ln F ), (∆V )−2, where F is the peak flux density in
janskys and ∆V is the linewidth at half maximum in km/s. The experimental data are
plotted with circles. The graph is fitted with a straight line. At the maximum activity
the line is strictly symmetric and it is ideally fitted with a gaussian. At other epochs
of observations (both at the ascending and descending branches of the evolution) the
flux density was below 15,000 Jy, and the line right wing was slightly shallower than the
left one. The symmetry of the line at the activity maximum and its small width testify
that the emission is related to a single maser condensation. The slight asymmetry at
other epochs and a small line shift in radial velocity may occur if the condensation is
inhomogeneous. The same line shape was observed in the flare of 2002. At that time the
line velocity was 41.05 km/s, which is only by 0.75 km/s lower than in 2016. Possibly
this is the same feature whose radial-velocity drift for 14 years was 0.75 km/s. Powerful
superflares were observed earlier in other sources, e.g., in Orion KL at a velocity of
8 km/s, see Matveenko [19], Matveenko et al. [20]. According to Garay et al. [21],
enhanced activity of the source persisted in 1979–1987. At that time the source flux
density exceeded 106 Jy. With a difference in the distances to Orion KL (500 pc) and
IRAS 18316−0602 (3.3 kpc) superflares in these two sources are comparable, together
with similar linewidths (∼ 0.6 km/s). The difference is only in the activity duration.
At the descending branch of the 2016 powerful flare in IRAS18316−0602 we observed
a considerably enhanced activity of the maser source in a broad interval of radial veloc-
ities (35–56 km/s). Flux densities of some features reached 500 Jy. As in the case of
the main feature, their flux densities were varying very rapidly: during 1–2 days they
changed by a factor of 1.5–2. No organized structures were noted. Probably the activity
was enhanced in individual features or clusters of features with similar radial velocities
within ∼ 2 km/s.
Thus the flare had global character for the maser source in IRAS 18316−0602 and
most likely was associated with a strong shock wave from the central source. Nearly
correlated variations of features’ flux densities and fast emission decrease can take place
in the case of a compact cluster of maser condensations and of their small sizes.
The lack of VLA maps and the fact that the strong emission comes only in a narrow
interval of radial velocities do not allow us to reveal organized structures, which, as a rule,
exist in the form of extended filaments or chains. It is interesting that the linewidth of the
strongest emission at all epochs of observations including 1990 [11] was 0.60–0.67 km/s.
4 Results
We report the results of monitoring in the water vapor line at λ = 1.35 cm of the source
IRAS18316−0602 associated with a region of active star formation. The observations
have been carried out on the RT-22 radio telescope of the Pushchino Observatory (Rus-
sia) in 2002–2017.
5
We observed three superflares in 2002, 2010, and 2016 with peak flux densities 3400,
19,000, and 46,000 Jy, respectively. They took place within a narrow interval of radial
velocities (40.5–42.5 km/s) and might be associated with a passage of a strong shock
wave. We have found correlation between flux density variations and linewidth for the
strongest flare of 2016 indicating that the maser was unsaturated.
The emission of three main groups of features was dominating. We observed a small
radial-velocity drift of this emission. In 2016 their velocities were sufficiently close,
about 41, 42, and 43 km/s. Probably, they were localized nearly in the sky plane or in
a compact group.
Faint emission was occasionally observed at VLSR < 37 and VLSR > 45 km/s.
Meanwhile, the superflare of 2016 was accompanied by rather intense emission (up to
500 Jy) in a velocity interval of 35–56 km/s. No organized structures were revealed.
Most probably, the emission came from individual features or a cluster of features with
close radial velocities.
Acknowledgments
This work was supported by the Russian Foundation for Basic Research (project code
15-02-07676).The authors are grateful to the staff of the Pushchino Radio Astronomy
Observatory for the great help with the observations, to Jan Brand, Sergio Poppi, and
Simona Righini for the results of H2O observations at the 64-m and 32-m radio telescopes
in Sardinia and Medicina.
References
[1] S. Kurtz, E. Churchwell, and D.O.S. Wood, Astrophys. J. Suppl. Ser. 91, 659 (1994).
[2] T. Jennesss, P.F. Scott, and R. Padman, Montly Not. Roy. Astron. Soc. 276, 1024
(1995).
[3] A.J. Walsh, M.G. Burton, A.R. Hyland, and G. Robinson, Mon. Not. Roy. Astron.
Soc. 301, 640 (1998).
[4] W.H. McCutcheon, P.E. Dewdney, C.R. Purton, and T. Sato, Astron. J. 101, 1435
(1991).
[5] T.R. Hunter, E. Churchwell, C. Watson, C. Cox, D.J. Benford, and P.R. Roelfsema,
Astron. J. 119, 2711 (2000).
[6] A.J. Walsh, G.H. Macdonald, N.D.S. Alvey, M.G. Burton, and J.-K. Lee, Astron.
and Astrophys. 410, 597 (2003).
[7] D.S. Shepherd and E. Churchwell, Astrophys. J. 472, 225 (1996).
6
[8] Á. Sánchez-Monge, A. López-Sepulcre, R. Cesaroni, C.M. Walmsley, C. Codella,
M.T. Beltrán, M. Pestalozzi, and S. Molinari, Astron. and Astrophys. 557, A94
(2013).
[9] A. Molinari, J. Brand, R. Cesaroni, and F. Palla, Astron. and Astrophys. 308, 573
(1996).
[10] K. Sunada, T. Nakazato, N. Ikeda, S. Hongo, Y. Kitamura, and J. Yang, Publ.
Astron. Soc. Japan 59, 185 (2007).
[11] F. Palla, J. Brand, R. Cesaroni, G. Comoretto, and M. Felli, Astron. and Astrophys.
246, 249 (1991).
[12] S. Kurtz and P. Hofner, Astron. J., 130 711 (2005).
[13] K.A. Edris, G.A. Fuller, and R.J. Cohen, Astron. and Astrophys. 465, 865 (2007).
[14] M. Szymczak, G. Hrynek, and A.J. Kus, Astron. and Astrophys. Suppl. 143, 269
(2000).
[15] G. Surcis, W.H.T. Vlemmings, H.J. van Langevelde, B. Hutawarakorn Kramer,
A. Bartkiewicz, and M.G. Blasi, Astron. and Astrophys. 578, A102 (2015)
[16] S. Poppi, private communication (2017).
[17] S. Righini, private communication (2017).
[18] S. Kurtz and P. Hofner, Astron. J. 130, 720 (2005).
[19] L.I. Matveenko, Astron. Lett. 7, 54 (1981).
[20] L.I. Matveenko, P.J. Diamond, and D.A. Graham, Astron. Rep. 44, 592 (2000).
[21] G. Garay, J.M. Moran, and A.D. Haschick, Astrophys. J. 338, 224 (1989).
7
36 40 44 48 36 40 44 48 36 40 44 48
F
lux d
ensi
ty, Jy
13.06.2002
18.11.2002
19.11.2002
24.04.2003
30.01.2003
1000 Jy
(1)
100 Jy
25.05.2004
27.05.2003
26.06.2003
5.08.2003
23.09.2003
27.10.2003
5.12.2003
28.01.2004
21.04.2004
Radial velocity, km/s
(2)
100 Jy
20.07.2004
27.09.2004
4.11.2004
24.12.2004
1.02.2005
17.03.2005
14.04.2005
28.06.2005
22.08.2005
26.09.2005
13.12.2005
9.11.2005
(3)
Figure 1: H2O λ = 1.35 cm maser emission spectra of IRAS18316−0602 in 2002–2005. Doublevertical arrows show the scale in janskys. The epochs of the observations are given.
8
30 35 40 45 50 55 30 35 40 45 50 55
F
lux d
ensi
ty, Jy
Radial velocity, km/s
100 Jy
15.04.2009
(4)
17.06.2008
15.05.2008
9.04.2008
26.04.2006
27.03.2006
31.01.2006
16.06.2009
1.07.2009
25.08.2009
9.11.2009
14.12.2009
26.01.2010
(5)
500 Jy
Figure 2: H2O maser emission spectra of IRAS 18316−0602 in 2006–2010.
9
30 35 40 45 50 55 40 42 44
F
lux d
ensi
ty, Jy
Radial velocity, km/s
28.04.2010
26.08.2010
1.11.2010
15.12.2010
5.02.2011
2.03.2011
1.04.2011
26.05.2011
15.06.2011
(6)
26.02.2010100 Jy
29.01.2010
5000 Jy
15.06.2011
26.05.2011
1.04.2011
2.03.2011
5.02.2011
15.12.2010
1.11.2010
26.08.2010
28.04.2010
26.02.2010
(7)
29.01.2010
Figure 3: H2O maser emission spectra of IRAS 18316−0602 in 2010–2011.
10
30 35 40 45 50 55 30 35 40 45 50 55
F
lux d
ensi
ty, Jy
Radial velocity, km/s
29.06.2011
31.01.2012
(8)
22.08.2011
24.09.2011
24.10.2011
24.11.2011
14.12.2011
200 Jy
26.07.2011
28.03.2012
24.04.2012
21.11.2012
(9)
200 Jy
29.05.2012
2.07.2012
30.07.2012
29.08.2012
29.10.2012
Figure 4: H2O maser emission spectra of IRAS 18316−0602 in 2011–2012.
11
30 35 40 45 50 55 30 35 40 45 50 55
F
lux d
ensi
ty, Jy
Radial velocity, km/s
27.02.2013
18.07.2013
19.08.2013
17.09.2013
28.10.2013
25.11.2013
15.12.2013
28.01.2014
25.02.2014
25.03.2014
22.04.2014
(10)
200 Jy
600 Jy
20.05.2014
16.06.2014
28.07.2014
25.08.2014
22.09.2014
29.10.2014
21.12.2014
30.01.2015
26.02.2015
24.03.2015
20.05.2015
19.06.2015
(11)
200 Jy
25.11.2014
Figure 5: H2O maser emission spectra of IRAS 18316−0602 in 2013–2015.
12
30 35 40 45 50 55
30 35 40 45 50 55
41 42 43
F
lux d
ensi
ty, Jy
Radial velocity, km/s
29.07.2015
25.09.2015
21.10.2015
17.11.2015
14.12.2015
18.02.2016
23.03.2016
(12)
200 Jy
1150 Jy
(13)
17.05.2016
16.06.2016
29.08.2016
22.11.2016
23.11.2016
19.04.2046
12.12.2016
14.12.2016
23.12.2016
200 Jy
28.12.2016
22.11.2016
23.11.2016
10000 Jy
(14)
22.12.2016
12.12.2016
14.12.2016
28.12.2016
Figure 6: H2O maser emission spectra of IRAS18316−0602 in 2015–2016. The central featureis shown in full in panel (14). Each maximum is labeled with the date of the observations; left:
for the ascending branch, right: for the descending.
13
30 35 40 45 50 55 60
16.01.2017
18.01.2017
4000 Jy
16.01.2017
18.01.2017
21.03.2017
Radial velocity, km/s
24.02.2017
13.03.2017
200 Jy
Flu
x d
ensi
ty, Jy
Figure 7: H2O maser emission spectra of IRAS 18316−0602 in 2017.
14
35
40
45
50
55
0
10000
20000
30000
40000
50000
2002 2004 2006 2008 2010 2012 2014 2016 2018
0
200
400
600
800
Rad
ial
vel
oci
ty, km
/s
< 500 Jy
500 - 1000 Jy
1000 - 10000 Jy
> 10000 Jy
Flu
x d
ensi
ty, km
/s
1
2
3
2a2b 2c
4
41.05
41.29
41.8
Years
V=41.8 km/s
40.6442.87
40.58 40.78 40.55
43.14 43.24
47.8
39.4
40.63
38.7
Figure 8: Variability of the radial velocity (a) and flux density (b, c) of the main spectralfeatures. Radial-velocity variations of three main features are approximated with straight
(dashed) lines and are numbered. Flux density maxima are labeled with the corresponding
radial velocities. Fainter maxima of the main emission feature are marked with vertical arrows.
15
9.0 9.5 10.0 10.5 11.0
-0.40
-0.38
-0.36
-0.34
-0.32
-0.30
Y A
xis
Title
lnF
V=41.8 km/s
(∆V)-2
Figure 9: Connection between variations of the flux density and linewidth for the 41.8-km/semission feature.
16
1 Introduction2 Observations and Data Presentation3 Discussion3.1 Superflare of 20023.2 Superflare of 20103.3 Superflare of 2016
4 Results