Abstract
ABSTRACT The presence of planetary material in white dwarf atmospheres, thought to be accreted from a dusty debris disc produced via the tidal disruption of a planetesimal, is common. Approximately 5 per cent of these discs host a co-orbital gaseous component detectable via emission from atomic transitions – usually the 8600 Å Ca ii triplet. These emission profiles can be highly variable in both morphology and strength. Furthermore, the morphological variations in a few systems have been shown to be periodic, likely produced by an apsidally precessing asymmetric disc. Of the known gaseous debris discs, that around HE 1349–2305 has the most rapidly evolving emission-line morphology, and we present updated spectroscopy of the Ca ii triplet of this system. The additional observations show that the emission-line morphologies vary periodically and consistently, and we constrain the period to two aliases of 459 ± 3 and 502 ± 3 d. We produce images of the Ca ii triplet emission from the disc in velocity space using Doppler tomography – only the second such imaging of a white dwarf debris disc. We suggest that the asymmetric nature of these velocity images is generated by gas moving on eccentric orbits with radially dependent excitation conditions via photoionization from the white dwarf. We also obtained short-cadence (≃4 min) spectroscopy to search for variability on the time-scale of the disc’s orbital period (≃hours) due to the presence of a planetesimal, and rule out variability at a level of ≃1.4 per cent.
Highlights
The observational evidence of planetary systems around white dwarfs is abundant (Zuckerman & Becklin 1987; Graham et al 1990; Aannestad et al 1993; Vanderburg et al 2015; Gänsicke et al 2019; Manser et al 2019; Vanderbosch et al 2020; Vanderburg et al 2020; Guidry et al 2021; Vanderbosch et al 2021)
This is further corroborated by the discovery that debris discs with a gaseous component in emission appear to show the largest amounts of infrared variability (Swan et al 2020), indicating they are the most dynamically active white dwarf debris discs
We note that the gaseous material around the white dwarf WD 1145+017 has been modelled as an eccentric disc that precesses with a period of 4.6 yr (Cauley et al 2018; Fortin-Archambault et al 2020), this gaseous component is only detected via absorption and shows no evidence of emission. In this manuscript we extend the 0.95 yr of spectroscopy of the gaseous debris disc around HE 1349–2305 presented by Dennihy et al (2018) by 1.84 yr, covering a total of 2 cycles, in addition to two archival observation obtained in 2009 and 2011
Summary
The observational evidence of planetary systems around white dwarfs is abundant (Zuckerman & Becklin 1987; Graham et al 1990; Aannestad et al 1993; Vanderburg et al 2015; Gänsicke et al 2019; Manser et al 2019; Vanderbosch et al 2020; Vanderburg et al 2020; Guidry et al 2021; Vanderbosch et al 2021). The planetesimal is ripped apart, forming a highly eccentric debris stream that is assumed to circularise within the tidal disruption radius to form a disc of dusty debris (Jura 2003; Debes et al 2012; Veras et al 2014, 2015; Malamud & Perets 2020a,b) These debris discs are usually detected via their infrared emission which is in excess of the white dwarf continuum (Zuckerman & Becklin 1987; Rocchetto et al 2015; Farihi 2016; Chen et al 2020; Dennihy et al 2020a,b; Gentile Fusillo et al 2021; Xu et al 2020). We discuss the results and the prospects of using Doppler maps as key probes in understanding the structure and evolution of gaseous debris discs
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