The 2021 superoutburst of the dwarf nova LL Andromedae

2023 August 10

Jeremy Shears

We present photometry of the 2021 outburst of LL And, which was only the fourth confirmed on record, with a mean interval between outbursts of 14.0 years. The outburst amplitude was ~7.8 mag, reaching magnitude 12.8 at maximum, and the main part lasted about 30 days, although the object was still slightly above quiescence some 70 days after the outburst started. Superhumps appeared nine days into the outburst, confirming that this was a superoutburst. The superhumps grew to a maximum peak-to-peak amplitude of 0.32 mag and a period of P_sh= 0.056633(180)d. We estimate the mass ratio of the secondary to the white-dwarf primary as q = 0.13. The photometric behaviour, short orbital period and long interval between outbursts are consistent with LL And being a member of the WZ Sge subclass of dwarf novae.

Introduction

Dwarf novae are a type of cataclysmic variable (CV) star, in which a cool main-sequence secondary star loses mass to a white dwarf primary. Material from the secondary falls through the inner Lagrangian point, L1, and, because it carries substantial angular momentum, does not settle on the primary immediately but forms an accretion disc. From time to time, as material builds up in the disc, thermal instability drives the disc into a hotter, brighter state, causing an outburst in which the star brightens by several magnitudes.

Dwarf novae of the SU UMa family occasionally exhibit superoutbursts, which last several times longer than normal outbursts and may be up to a magnitude brighter. During a superoutburst, the light curve of a SU UMa star is characterised by superhumps. These are modulations which are a few percent longer than the orbital period.^1,2 They are thought to arise from the interaction of the secondary-star orbit with a slowly precessing, eccentric accretion disc. The eccentricity of the disc arises because of a 3:1 resonance between the secondary-star orbit and the motion of matter in the outer part of the accretion disc.

WZ Sge stars are a subclass of the SU UMa family, which are more highly evolved systems. WZ Sge systems have very short orbital periods, long intervals between outbursts (typically decades), and exceptionally large outburst amplitudes, usually exceeding six magnitudes. For a full account of dwarf novae, and specifically SU UMa and WZ Sge stars, the reader is directed to Hellier (2001) and Warner (1995).^1,2

LL Andromedae was identified as an eruptive variable by Wild in 1979,³ when it was seen to brighten to mag_pg ~13 and appeared to fade after about 10 days, although it was very poorly sampled.

The first outburst observed in detail was in 1993 December, when it reached magnitude 14.3–14.4V and during which superhumps were observed,^4,5 confirming that LL And is a dwarf nova of the SU UMa type. Kato (2004) reported a superhump period P_sh = 0.05697(3)d and proposed that it was a member of the WZ Sge subclass, due to its very short orbital period, very large outburst amplitude and long interval between outbursts.⁵

The object underwent another superoutburst in 2004 May–June but was poorly observed due to its unfavourable position. Kato et al. (2009) reported the presence of superhumps with P_sh = 0.05658(2)d.⁶ No other confirmed outbursts have been reported since, in spite of intensive monitoring.

Patterson et al. (2003) proposed an orbital period P_orb = 0.0550553(6)d,⁷ based on a photometric period they measured in 1997 December. Howell et al. (2002) used spectroscopy from the Hubble Space Telescope to estimate the white dwarf at 0.6 ± 0.3 solar masses and a distance of 760 ± 100pc.⁸

This paper presents the results of photometric and spectroscopic observations made during the fourth confirmed outburst of LL And in 2021 October and November.

Detection & outburst light curve

The 2021 outburst of LL And was detected by the author at magnitude 12.8C (CCD, clear filter) on September 14.902 and reported to the Variable Star Section e-mailing list,⁹ as well as several other e-mail alert systems, which allowed follow-up photometry to be performed by observers around the world.

Patrick Schmeer subsequently noted that ATLAS (Asteroid Terrestrial-impact Last Alert System) photometry placed the object already in outburst on September 12.432 at magnitude 12.3 (o-band, orange filter, 5600–8200Å), which we take as the start of the outburst in subsequent discussions. The outburst must have started less than 48h before that time, as ATLAS had LL And at 19.8cy (cy-band, cyan filter covering 4200–6500Å) on September 10.436. An image of LL And in outburst is shown in Figure 1.

The outburst light curve is shown in Figure 2, which comprises data from the BAA Variable Star Section (VSS) and the American Association of Variable Star Observers (AAVSO), All Sky Automated Survey for SuperNovae (ASAS-SN), ATLAS and Zwicky Transient Facility (ZTF) databases; observers are listed in Table 1. The first 25 days of the outburst corresponded to the plateau phase, during which the object gradually faded from magnitude 12.3 to 14.9 at ~0.1mag/d. There was then a rapid decline at ~1.1mag/d over the next three days, to 18.5C. There was a brief recovery in brightness to 17.7C, after which it gradually approached quiescence.

SDSS-DR13 lists LL And at g = 20.07 in quiescence,⁹ which means that the outburst amplitude was about 7.8 magnitudes. The main part of the outburst lasted about 30 days, but the light curve in Figure 1 shows that it was at 19.0C, still slightly above quiescence, some 70 days after the outburst started.

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