The OmegaWhite survey for short-period variable stars – III: follow-up photometric and spectroscopic observations

Abstract

We present photometric and spectroscopic follow-up observations of short-period variables discovered in the OmegaWhite survey: a wide-field high-cadence g-band synoptic survey targeting the Galactic Plane. We have used fast photometry on the SAAO 1.0-m and 1.9-m telescopes to obtain light curves of 27 variables, and use these results to validate the period and amplitude estimates from the OmegaWhite processing pipeline. Furthermore, 57 sources (44 unique, 13 also with new light curves) were selected for spectroscopic follow-up using either the SAAO 1.9-m telescope or the Southern African Large Telescope. We find many of these variables have spectra which are consistent with being δ Scuti type pulsating stars. At higher amplitudes, we detect four possible pulsating white dwarf/subdwarf sources and an eclipsing cataclysmic variable. Due to their rarity, these targets are ideal candidates for detailed follow-up studies. From spectroscopy, we confirm the symbiotic binary star nature of two variables identified as such in the SIMBAD database. We also report what could possibly be the first detection of the 'Bump Cepheid' phenomena in a δ Scuti star, with OW J175848.21-271653.7 showing a pronounced 22% amplitude dip lasting 3 minutes during each pulsational cycle peak. However, the precise nature of this target is still uncertain as it exhibits the spectral features of a B-type star.

Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf B-star pulsators

“…Another more recent survey is the OmegaWhite (OW) survey, which covers a total of 400 deg 2 at low Galactic latitudes (|b| < 10 • ) as well as in the Galactic Bulge using high-cadence optical observations. Two neighbouring 1 deg 2 fields are alternatingly observed in 39-s exposures over an observing duration of 2 h, with an observational median cadence of ≈2.7 min per field (Macfarlane et al 2015(Macfarlane et al , 2017aToma et al 2016;Kupfer et al 2017) As part of the Zwicky Transient Facility (ZTF), the Palomar 48inch (P48) telescope images the sky every clear night conducting several surveys including a Northern Sky Survey with a 3-d cadence, as well as smaller surveys such as a 1-d Galactic plane survey and simultaneous observations of the Northern TESS sectors (Graham et al 2019;van Roestel et al 2019;Bellm et al 2019a,b). As part of the partnership share of ZTF, we conducted a dedicated high-cadence Galactic plane survey with a cadence of 40 sec at low Galactic latitudes aiming to find UCBs and compact pulsators.…”

The DECam minute cadence survey – II. 49 variables but no planetary transits of a white dwarf

“…We compare the equivalent width measurements of these features to the spectral templates from Pickles (1998) and identify this object as a ≈G0V star. Macfarlane et al (2017) identify two δ Scuti pulsators with dominant periods of 9.0-9.6 min, including the G5 type star OW J075531.59-315058.2. Hence, J0906-0407 appears to be similar to other G-type δ Scuti stars with relatively short pulsation periods.…”

“…The OmegaWhite Survey (Macfarlane et al 2015(Macfarlane et al , 2017Toma et al 2016) is observing 400 deg 2 of the sky along the Galactic Plane with a median cadence of 2.7 min, with the goal of finding short period (P orb < 80 min) variables, especially ultracompact binaries as these are predicted to be the strongest emitters of gravitational waves in the milli-Hertz frequency range (Amaro-Seoane et al 2013;Roelofs et al 2007). However, the 2 hour duration of their observations is not ideal for finding transits around white dwarfs as planets with such short periods would be tidally disrupted (Agol 2011).…”

Summary of Eighth Meeting on Hot Subdwarfs and Related Objects

“…CD −30 ∘ 11223 (Vennes et al 2012) is one such object, as is PTF1 J082340.04+081936.5 (Kupfer et al 2017b). Macfarlane et al (2015Macfarlane et al ( , 2017 identify OW J074106.1-294811.0 as a binary having a white dwarf and pulsating sdO star; the orbital period is 44-minutes. A more detailed study should become possible as Kupfer expected the Zwicky Transit Facility (Bellm 2014) to find many more ultracompact hot subdwarf binaries.…”