EC 03089-6421: a new, very rapidly pulsating sdO star

Abstract

EC 03089−6421, classified sdO in the Edinburgh-Cape (EC) blue object survey, is shown to have unusually rapid pulsations with a dominant frequency near 32 mHz (amplitude ∼0.02 mag; period 31.1 s)-which appears to be strongly variable in amplitude on timescales of hours and days-and a generally weaker frequency near 29 mHz (amplitude ∼0.004 mag; period 34.2 s), which is also variable in amplitude. This star varies at twice the frequency of any known hot subdwarf pulsator. Although the low-resolution EC spectrogram appears very similar to those of DAO stars, our analysis derives T eff = 40 200 ± 1600 K; log g = 6.25 ± 0.23 and log N(He)/N(H) = −1.63 ± 0.55; more recent spectrograms give T eff = 37 400 ± 1000 K; log g = 5.70 ± 0.13 and log N(He)/N(H) = −2.02 ± 0.17, both of which indicate that the gravity is too low for a white dwarf star, although the low temperature derived from the Balmer lines is at odds with the absence of neutral Helium and the strength of He II 4686. It is possible that EC 03089−6421 is a field analogue of the ω Cen sdO variables.

New hot subdwarf variables from the EC survey

“…2. The two frequencies found by Kilkenny et al (2017) are recovered in the two SHOC runs from 2017 Jan 14/15 Table 2. The upper two periodograms are from SHOC observations, the lower is from STE3 and is truncated at the Nyquist frequency for those data (equivalent to 22 s).…”

“…EC 03089-6421 was classified sdO in Zone 3 of the EC survey (Kilkenny et al 2015), but was picked up much earlier in the BPS survey (BPS CS 31064-0017; Beers, Preston & Schectman 1992) with photometry V = 14.75, (B − V) = −0.34, and (U − B) = −1.18 by Norris, Ryan & Beers (1999;cf Table 1) but with no spectral type assigned. Kilkenny, Worters & Østensen (2017) found the star to be variable with extremely short periods of 34.2 and 31.1 s (frequencies of 29.2 and 32.1 mHz, respectively). They gave sample light curves for EC 03089-6421 and noted that the pulsation amplitudes appeared to be variable on rather short time-scalesdeclining visibly during the course of a single night and appearing to decline and recover more slowly over several nights.…”

“…The EC survey classification spectrum of EC 21281-5010 (Kilkenny et al 2015) was no longer available and its T eff was determined by Remie and Lamers (1982) iteration. EC 03089-6421 is known (Kilkenny et al 2017) to be an sdO pulsator and is therefore a special case discussed below.…”

An Observer's View on the Future of Asteroseismology

“…The search for members of the new instability strip on the Extreme Horizontal Branch (EHB) continued and five sdO pulsators were discovered in ω Cen (Randall et al, 2011(Randall et al, , 2016. A very rapidly pulsating sdO star was discovered in the Edinburgh-Cape (EC) survey with strongly variable amplitude, which may be a field analogue of the ω Cen sdO variables (Kilkenny et al, 2017). The harvest for the sdB stars based on space data started with the clear identification of nine compact pulsators and a number of interesting binary stars (Østensen et al, 2010).…”

Summary of Eighth Meeting on Hot Subdwarfs and Related Objects

“…Latour et al (2017b) use N IV/N V ionisation ratios to determine T e = 60000 ± 5000 K for the five sdO pulsators Randall et al (2016) find in ω Cen, placing them within the predicted instability strip. Randall reported new observations made to establish whether the five ω Cen pulsators are unique objects, without counterparts among field pulsating sdO stars (Woudt et al 2006;Østensen 2012;Kilkenny et al 2017) or the six pulsators Brown et al (2013) find in the core of the globular cluster NGC 2808.…”