Spectroscopic observations of AM CVn binary candidates MGAB-248 and MGAB-249

ATel #13048; Gavin Ramsay (Armagh Observatory & Planetarium), Mark Kennedy (U Manchester), Pasi Hakala (FINCA, U Turku), C. Simon Jeffery (Armagh Observatory & Planetarium)
on 29 Aug 2019; 12:57 UT
Credential Certification: Gavin Ramsay (gavin.ramsay@armagh.ac.uk)

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Rivera Sandoval, Maccarone & Murawski (ATEL #12847) reported Swift observations of two objects, MGAB-V248 (RA=294.69313 DEC=+58.69828) and MGAB-V249 (RA=322.73629 DEC=+44.34622), which they had previously identified as having 'orbital' periods of 27.9 min and 39.3 min respectively. Based on their period, blue colours and lack of X-ray emission, Rivera Sandoval et al suggested both objects were AM CVn binaries (double degenerate interacting binaries with orbital periods in the range ~5-70 min).

We obtained low resolution spectroscopic observations of both sources using the 4.2m William Herschel Telescope and the ACAM instrument on the night of 2019 Aug 22. We used the V400 grating (giving a resolution of ~13A at 5000A) and took 4x600 sec and 4x240 sec exposures of MGAB-V248 and MGAB-V249 respectively. The data were reduced in the standard manner and we obtained co-added spectra for each source. A pdf of each co-added spectrum can be downloaded from:

https://armaghop.sharepoint.com/:b:/s/GRamsey/Ea8x0UJJGiNMnOGuEtEMoAcBWdx-yRWE98NcRbZTxuBAIQ?e=56INKM
https://armaghop.sharepoint.com/:b:/s/GRamsey/EZ1KNpY1QpZEmA82YdDrH7cBBWmEIOWrpSDklj4BOirqLQ?e=Rmjd85

The instrumental response has not be removed and should be considered as preliminary.

We conclude that neither object is an AM CVn binary. MGAB-248 shows hydrogen lines in absorption together with weaker HeI and HeII absorption lines and further absorption lines between 5100-5300A. MGAB-249 shows HeI and HeII absorption lines, with a contribution from hydrogen. We tentatively suggest that MGAB-248 may be a DAB white dwarf and MGAB-249 a He sdOB star.

DAB white dwarfs are rare: only 0.3% of the 15716 DA white dwarfs reported by Kepler et al (2019, MNRAS, 486, 2169) are contaminated by weak helium lines. Around 10% of hot subdwarf stars reported by Geier et al (2017, A&A, 600, A50) are classed as He-sdB/OB/O stars. The period quoted for MGAB-249 by Sandoval et al. would be consistent with variability observed in the He-sdOB star V366 Aqr (= LS IV-14 116: Ahmad & Jeffery 2005, AA 437, 51) and attributed to g-mode oscillations driven by C/O opacity (Saio & Jeffery 2019: MN 482, 758). The variability seen in MGAB-248 is too long for pulsation, but could be consistent with rotation as seen in the DAB white dwarf GD 323 (Pereira et al. 2005, ApJ 623, 1076).

Both stars are in the Gaia DR2 release:
Source           G   Dist     MG
MGAB-248 18.0 4.4 kpc 4.8
MGAB-249 15.5 1.2 kpc 5.1

where we have assumed a scale length of L=1.35 kpc. Both stars are also in the Panstarrs DR2.

Our results are based on service observations (SW2019a10) made using the WHT operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We thank Ovidio Vaduvescu for carrying out the observations.