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MERLIN observations of bipolar outflow
from HM Sagittae
By S t ew a r t Ey r e sy, H a r o l d K e nnyz, J i m C oh e ny,
Huw L l o yd--, S e a n D o u g h e r t y--, R i c h a r d D a v i sy
AND M i ke B o d e--
e­mail: spe@jb.man.ac.uk
y University of Manchester, NRAL, Jodrell Bank, Macclesfield, Cheshire SK11 9DL, UNITED
KINGDOM
z Royal Military College of Canada, Kingston, Ontario K7K 5LO, CANADA
-- School of Chemical and Physical Sciences, Liverpool John Moores University, Byrom Street,
Liverpool, L3 3AF, UNITED KINGDOM
MERLIN maps of HM Sagittae at 6 cm and 18 cm wavelengths are presented. At 6 cm, the
inner nebula has been mapped at a resolution of 60 milli­arcsec, revealing two parallel ridge
structures to the north and south of a deep central minimum. In addition, these central features
are surrounded by a complex, weak structure. The ridges and many of the weak features are
correlated with ultra­violet (UV) structure observed with the Hubble Space Telescope (HST).
The 18 cm map reveals elongated structure to the east and west of the star's position. There
is strong evidence that the east and west edges of this extension are non­thermal. The E--W
and N--S features are associated with components detected with optical spectroscopy. The radial
velocities of these components suggest that the E--W features are a high velocity bipolar outflow.
1. Introduction
Symbiotic stars are a group of around 130 stars, defined by the features apparent in
their optical spectra (Kenyon (1986), Allen (1984)). In the red part, the underlying
spectrum is typical of a red giant, with a cool continuum, and prominent absorption
features, such as TiO. However, superimposed on this are strong emission lines, extending
into the violet and UV, and consistent with those from the ionised nebulae associated
with hot stars. The most commonly accepted explanation for the peculiar spectra of
symbiotic stars is that they are binaries, with a cool component, usually considered to
be a red giant or a Mira­type variable, and a hot component, either a white dwarf or a
main sequence star. Given the broadness of this definition, objects have occasionally been
misclassified as symbiotics. On the other hand, many symbiotic stars have previously
been misclassified as planetary nebulae (PNe). HM Sagittae is one such object, and has
been proposed as a candidate for a proto­planetary nebula. Many, although by no means
all, have been shown to be binaries, which is consistent both with the explanation of the
spectra, and with some theories for the formation of PNe.
HM Sagittae is an example of the small subclass of symbiotic novae, which show
weak nova­like optical outbursts lasting in excess of decades. As with classical novae,
these outbursts are accompanied by radio emission, and HM Sagittae has been proven
to be one of the brightest symbiotic radio emitters. Since its discovery by Dokuchaeva
(1976), following an optical outburst, it has been well studied in the radio. Some of the
earliest measurements were made in 1977 at the Algonquin Radio Observatory (Purton
et al. (1983)). Initial work by Kwok & Purton (1979) (see also Purton et al. (1983))
found the emission to be thermal, while Kwok et al. (1984) pointed out that the radio
brightness was increasing. This trend stopped in 1984 Kwok (1988), and it seems likely
1

2 S.P.S. Eyres et al.: MERLIN observations of HM Sagittae
that the brightening was associated with the effects of the optical outburst. Kwok et al.
(1984) attributed the increase in flux to expansion of the ionised nebula, as the emission
remained thermal and optically thick for ten years following outburst. It was suggested
that the `halo' around HM Sagittae was the wind of the Mira­type cool component, swept­
up by the hot component white dwarf wind, caused by the onset of a thermonuclear flash
at outburst.
To date, only a few attempts have been made to map the radio emission from HM
Sagittae. Purton et al. (1983) published two VLA maps, finding the overall angular size,
in 1980, to be 0 00 3 at 6 cm and 0 00 :2 at 2 cm. The first hint of structure within the nebula
was found by Kwok et al. (1984) in their 1.3 cm VLA map. This shows an elongated
central peak, which in later images separates into two peaks (Li (1993)). Kenny et al.
(1993) have published the most up to date VLA map, made at 6 cm. Although this
shows no new structure, the angular extent has increased to ¸ 0 00 :4, and the integrated
flux to 56 mJy.
Here, we report radio observations of HM Sagittae at 6 cm and 18 cm made with
the Multi Element Radio Linked Interferometer Network (MERLIN). The resolution of
MERLIN at these wavelengths allows us to trace the structure and spectral behaviour
of the star's circumstellar matter at very small scales. We have resolved new structures,
and have found definite evidence of a non­thermal component to the radio emission.
2. Observations
HM Sagittae was observed at both 6 cm and 18 cm with MERLIN. Observations at
6 cm were made on 1992 December 26, using six telescopes, at Cambridge, Knockin,
Defford, Darnhall, Tabley, and the Mark II Telescope at Jodrell Bank. The observing
frequency was 5.008 GHz, with a bandwidth of 15 MHz. Both right­ and left­hand
circular polarisations were recorded. We observed in phase referencing mode, spending
¸ 4 minutes on the source and then switching regularly to the phase calibrator 1932+204
for ¸ 2 minutes. The total observing time was 9 hours 18 minutes.
The 18 cm observations were carried out on 1993 October 9 using all eight telescopes
of MERLIN (those listed above, along with Wardle, and the Lovell Telescope at Jodrell
Bank). The total observing time was 15 hours 10 minutes. Again, we observed in the
phase referencing mode, spending ¸ 5 to 8 minutes on the source and switching regularly
to the phase calibrator 1932+204 for ¸ 1 to 2 minutes. However, the Lovell Telescope was
only switched to the phase calibrator approximately every 17 to 28 minutes, i.e. every
third calibration observation, due to that telescope's slow slew­rate. The calibration
interpolation was then carried out over a smaller number of calibration points for the
Lovell Telescope. The observing frequency was 1.658 GHz, with a bandwidth of 15 MHz;
both right and left hand circular polarisations were recorded.
At both wavelengths, the flux calibration was carried out using 0552 + 398, and this
was itself calibrated against 3C286, using the flux scale of Baars et al. (1977). The
calibrated flux for 0552 + 398 was 7:37 \Sigma 0:91 Jy at 6 cm, and 2:19 \Sigma 0:23 Jy at 18 cm.
Initial calibration was carried out using the MERLIN--Specific software procedure dproc,
with which the absolute flux scale for the observations was determined. At this stage, the
right and left hand polarisations were added. Subsequent calibration and mapping were
carried out using the AIPS package. The phase calibrator 1932 + 204 was mapped with
three passes of self­calibration over a 512 \Theta 512 pixel field, and the data were reweighted
to account for the differing telescope sensitivities. As HM Sagittae is a relatively weak
radio source, we used the gain and phase solutions from the phase calibrator to correct
the data, without any further self­calibration.

S.P.S. Eyres et al.: MERLIN observations of HM Sagittae 3
Figure 1. Radio map of HM Sagittae made using MERLIN at 6 cm. The rms noise is
90¯ Jy beam \Gamma1 . The ridges called N and S in the text are prominent in red. The contours
overlaid are the HST UV image of Hack & Paresce (1993). The UV contours are \Gamma1, 1, 2, 3, 4,
5, 6, 8, 12, 16, 20, 26, 32, 38, 44 times 4 \Theta 10 \Gamma15 ergs cm \Gamma2 s \Gamma1 š A \Gamma1 .
The 6 cm map was made using a pixel size of 15 mas (milli­arcsec), which resulted in
a final CLEANed field of size 3 00 :84 \Theta 3 00 :84. The data were naturally weighted to optimise
the signal to noise ratio, and finally reconvolved with a 60 mas gaussian beam. The rms
noise of the final map was 90¯ Jy beam \Gamma1 . The 18 cm map was made with a pixel size
of 45 mas, giving a 23 00 \Theta 23 00 field. Again, the data were naturally weighted to optimise
noise, and the resultant beam was 280 \Theta 250 mas, at position angle \Gamma87 ffi (north through
east). The rms noise of the final map was 240¯ Jy beam \Gamma1 .
3. Results
At both observing wavelengths, we have resolved new structure. The 6 cm image is
shown in Figure 1. The most important features in this image are the two ridges to
the north and south of a central minimum (called N and S respectively hereafter). The
ridges are clearly symmetric about the minimum, and are aligned approximately parallel.
These features may be associated with those found in previous VLA images made at 1.3
cm (Kwok et al. (1984), Li (1993)), but the resolution of MERLIN has allowed us
to discern two distinct ridges, separated by a central minimum. This is the first time
these features have been resolved at 6 cm. Each ridge has a flux density indicating a
brightness temperature of around 45,000 K, while in the central minimum the brightness
temperature drops to 6,400 K.
The MERLIN observations did not detect the extended halo emission found by Kenny
et al. (1993) at 6 cm. This is to be expected, as the extended emission in the VLA map
drops to only a few ¯ Jy beam \Gamma1 when observed with MERLIN at the same wavelength,
because of the much smaller beam. The MERLIN map includes around 85% of the total
flux of the source at 6 cm, as measured by Kenny et al. (1993).
The 18 cm map is shown in Figure 2. This image is elongated in the E--W direction,
with two new components (called E and W respectively hereafter). The direction and
extent of this structure are similar to that of the optical outflow discovered by Solf (1984)
and Solf (1993) with high­resolution long­slit spectroscopy of [N II] at –6583. The radio
emission of E and W is clearly non­thermal, as we discuss in more detail below. Prior to
this work, the radio emission from HM Sagittae had been thought to be entirely thermal
in character.

4 S.P.S. Eyres et al.: MERLIN observations of HM Sagittae
Figure 2. Radio map of HM Sagittae made using MERLIN at 18 cm. The rms noise is
240¯ Jy beam \Gamma1 at 18 cm. The regions referred to as E and W in the text are coloured green
at the east and west extremes of the structure.
Figure 3. Variation of mean spectral index ¯
ff with Right Ascension. The error bars are
determined in the usual manner for a weighted mean. See text for explanation.
Spectral indices ff (following the S š / š ff convention) were calculated using the 18 cm
MERLIN map (Figure 2) and a VLA image at 6 cm made on 1991 August 19 (Kenny
et al. (1993)). The VLA map was used to ensure all the flux at 6 cm was included.
This required interpolation of the MERLIN map to the VLA 0: 00 :1 pixel grid, and then
smoothing the result to the resolution of the VLA map. Spectral indices were then
calculated on a pixel­by­pixel basis. Figure 3 shows the variation of mean spectral index
¯
ff from east to west along the major axis of the bipolar structure in Figure 2. Each point
is a weighted mean of the four pixels between Declination at each 0 00 :1 interval of Right
Ascension. The weight chosen was the inverse square of the error in the value of ff for
each pixel.
The spectral indices vary systematically from east to west across the source. Feature
E was found to have a mean spectral index as low as ¯
ff = \Gamma2:11 \Sigma 0:30, while feature W
has a minimum at ¯
ff = \Gamma1:14 \Sigma 0:23. This is an unambiguous signature of non­thermal
emission. The central region reaches a maximum of ¯
ff = 1:71 \Sigma 0:12, while in the area
of the VLA peak (Kenny et al. (1993)) ¯
ff = 1:98 \Sigma 0:12. This indicates emission from
optically thick thermal plasma. We believe this to be the radio emission closest to the
central stars. We also find that the spectral index returns to near­zero values eastward
of Right Ascension 19 h 41 m 57 s :14, at the extreme eastern edge of E, suggesting optically

S.P.S. Eyres et al.: MERLIN observations of HM Sagittae 5
thin thermal emission. Unfortunately, no reliable conclusions regarding ff to the extreme
west of W can be obtained, as the emission there is too weak.
4. Comparisons with Observations at other Wavelengths
4.1. Ultra­Violet Continuum
On 1992 December 1, only 25 days before our 6 cm observations, Hack & Paresce (1993)
observed HM Sagittae with the Hubble Space Telescope (HST). Using the Faint Object
Camera (FOC), they obtained an image with approximately 67 mas resolution in the
band ––1843 \Gamma 2117 š A.
This HST image bears a remarkable similarity to our MERLIN 6 cm image. The image
is reproduced in contour form in Figure 1, The HST used the position of 19 h 41 m 57 s :075,
+16 ffi 44 0 39 00 :73 (J2000), with absolute pointing accuracies of only \Sigma0 00 :5 (Hack (1993)).
Thus, the features have been aligned with those of the 6 cm MERLIN map by eye. The
correspondence between the two images (see Figure 1) is striking.
The major discrepancy between our MERLIN map and HST image of Hack & Paresce
(1993) is the presence of a UV peak at the location of our radio minimum. We suggest
that this is due primarily to the presence of the hot component, which emits strongly in
the UV, but has negligible emission at radio wavelengths. Modelling the hot component
as a blackbody, of temperature 200,000 K and angular diameter 2:9 \Theta 10 \Gamma4 mas M¨urset
et al. (1991), we find the calculated approximate UV emission to be commensurate with
the observed peak UV emission. Hack & Paresce (1993) suggest that the UV emission
from the nebula is dominated by emission lines, particularly the CIII] –1909 line and the
SiIII] –1892 line. Conversely, the central UV peak is dominated by the hot component
continuum. As the hot component will not contribute significantly to the radio emission,
this results in the central minimum apparent in Figure 1, as the surrounding nebula
becomes dominant.
4.2. Optical Spectroscopy
The observations of Solf (1984) and Solf (1993) show extended optical emission from HM
Sagittae in [N II] at –6583 using long­slit spectroscopy. His decomposition of the results
gave information about the sub­arcsecond structure. Figure 4 of Solf (1984) shows the
locations of five separate radial velocity features. The emission is dominated by a broad
component, centred on the systemic velocity, which has approximately the same angular
extent as the 6 cm radio emission shown in Figure 1. In addition to this component,
there are high velocity features to the east and west, at \Sigma46 km s \Gamma1 . These correspond
spatially with the E--W extension seen in Figure 2, suggesting that E and W are the
result of bipolar outflow from the central stars.
From Figure 2, we measure the maximum east­west extent of the radio emission to be
¸ 1 00 :8. At a distance of D (kpc), this corresponds to ¸ 1800D AU. If we assume that
this region expanded from a point following the outburst in 1975, the components have
separated at a mean transverse velocity of ¸ 475D km s \Gamma1 .
As indicated in Section 3, N and S may be associated with peaks found with the VLA
at 1.3 cm at several epochs between 1983 and 1988 (Li (1993)). These VLA images show
the peaks to be separating at a rate of 8:76 \Sigma 2:56 mas per year. This corresponds to a
separation velocity of ¸ (41:6 \Sigma 12:2)D km s \Gamma1 . Fitting gaussian components to N and
S, we find the angular separation of N and S to be ¸ 0 00 :16 \Sigma 0 00 :02, which corresponds to
a transverse velocity of ¸ (42:3 \Sigma 5:3)D km s \Gamma1 , assuming uniform expansion following
the outburst in 1975. This is in close agreement with the result from the VLA maps.
Solf also found two radial velocity features A1 and A2, at \Gamma9:5 and +9:5 km s \Gamma1

6 S.P.S. Eyres et al.: MERLIN observations of HM Sagittae
respectively. His analysis suggested an angular separation of ¸ 0 00 :2 \Sigma 0 00 :08 in the north­
south direction for A1 and A2. It seems sensible to associate A1 and A2 directly with N
and S respectively. However, the radial velocities of these features need to be reconciled
with the separation velocity we have determined above.
5. Conclusions
We have used MERLIN to map the circumstellar gas associated with HM Sagittae at 6
cm and 18 cm, and found a number of new aspects to both the structure and the emission.
Spatial correlations between the 18 cm elongated structure and optical features found by
Solf (1984) and Solf (1993) have led us to conclude that we have directly observed bipolar
outflow from the central stars. Spectral indices for components E and W unequivocally
point to a non­thermal emission mechanism. This is the first time this has been found
in HM Sagittae.
At 6 cm we have resolved two distinct, parallel ridges to the north and south of a
central minimum. An ultra­violet HST image (Hack (1993)) shows a similar structure in
the inner nebula. Observing at radio wavelengths has allowed us to avoid contamination
by the hot component, which is dominant in the ultra­violet. The north and south radio
components have brightness temperatures of 45,000 K, which suggests a non­radiative
heating mechanism. For further interpretation see Eyres et al. (1994).
MERLIN is a national facility operated by the University of Manchester on behalf of
PPARC. HML and SMD are supported by Research Assistantships from PPARC. SPSE
is supported by a Research Studentship Award from PPARC.
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