High-resolution radio observations of HM Sge — II. Two decades after outburst

Abstract

HM Sge is a symbiotic binary star that underwent a nova‐like outburst in 1975. Its radio emission has been monitored using MERLIN and the VLA, and maps from 1992 to 1997 are presented. Thermal emission within 0.4 arcsec of the optical peak is elongated north‐‐south and hotspots appear to be moving around an anticlockwise ellipse. If this is due to motion in an inclined disc, the period is ∼ 90 yr. Eyres et al. predicted that if this is due to Mach shocks in the post‐nova wind, proper motions should be seen in expansion. This has not been detected since 1992, so an alternative model is adopted, based on the work of Eyres et al. and Kenny et al., of colliding winds in a binary system following the nova‐like outburst. The emission peaks appear to be corotating with the binary orbit as the ionization front and the hot wind from the white dwarf interact with the Mira wind. The positions of the stars are estimated, at a separation of ∼ 25 au. A distance of ∼ 1 kpc is most consistent with the observations reported here. On arcsecond scales the emission is extended east‐‐west, consistent with a biconical outflow arising from the collimation of the nova outburst through interaction with the pre‐existing cool wind. The presence of non‐thermal emission at a separation of ∼700 mas from the stars is confirmed. This is very unusual at such a distance from low‐mass stars, and could arise from synchrotron emission in a μT magnetic field. A model for this is developed, which shows that non‐thermal emission is expected to decline within decades as the nova wind decelerates. This could explain the non‐detection of non‐thermal emission in other symbiotic stars.

Infrared Spectroscopy of Symbiotic Stars. Ix. D-Type Symbiotic Novae

“…This agrees with the polarization position angle. Requiring consistency with a component separation of 50 AU (Richards et al 1999) results in a distance of 1250 ± 280 pc. Eyres et al (2001) found that the nebula shows two distinct regions.…”

“…Wallerstein et al (1984) had previously demonstrated that the optical emission line profiles in HM Sge required an origin in a conical nebula resulting from interacting winds. Richards et al (1999) continued monitoring the radio emission of HM Sge through the 1990s. They found hot spots that appear to be rotating in an inclined disk with a period of 90 yr.…”

A catalogue of symbiotic stars

“…169=HM Sge Mean K magnitude during outburst. A complex nebula with possible jet-like features resolved at optical and radio wavelengths ( [53] and references therein, [253]). The nebula is aligned with the binary orientation deduced from spectropolarimetry of the Raman scattered O vi lines [264].…”

The spectral energy distribution of D-type symbiotic stars: the role of dust shells

“… References: Belczyński et al (2000); BI Cru: Schwarz & Corradi (1992), Contini et al (2009c); SS73 38: Munari (1992), Gromadzki et al (2009); V835 Cen: Whitelock (1987a), Feast et al (1983b); H1‐36: Allen (1983), Angeloni et al (2007b); HM Sge: Whitelock (1987a), Richards et al (1999), Bogdanov & Taranova (2001), Schild et al (2001); V1016 Cyg: Parimucha (2003), Schild & Schmid (1996), Lee et al (2003), Taranova & Shenavrin (2000); RR Tel: Feast et al (1983b), Kotnik‐Karuza, Friedjung & Selvelli (2002), Gromadzki et al (2009); V627 Cas: Bergner et al (1988); R Aqr: Gromadzki et al (2009), Hollis, Pedelty & Lyon (1997), Contini & Formiggini (2003), Dougherty et al (1995). …”